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Nervous System


General organization

The nervous system allows us both to detect and respond to our external environment and to monitor and control our viscera. The brain and spinal cord are central in position and are wired to the rest of the body by peripheral nerves.

The nervous system can be separated into parts based on function and on structure:

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    functionally, it can be divided into somatic and visceral parts;

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    structurally, it can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS) ( Fig. 6.1 ).

    Fig. 6.1, CNS and PNS.

Functional subdivisions of the nervous system

Functionally, the nervous system can be divided into somatic and visceral parts.

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    The somatic part ( soma, from the Greek for “body”) innervates structures (skin and most skeletal muscle) derived from somites in the embryo and is mainly involved with receiving and responding to information from the external environment.

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    The visceral part ( viscera, from the Greek for “guts”) innervates organ systems in the body and other visceral elements, such as smooth muscle and glands, in peripheral regions of the body. It is concerned mainly with detecting and responding to information from the internal environment.

Somatic part of the nervous system

The somatic part of the nervous system consists of:□

  • nerves that carry conscious sensations from peripheral regions back to the CNS, and

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    nerves that innervate voluntary muscles.

Somatic nerves arise segmentally along the developing CNS in association with somites, which are themselves arranged segmentally along each side of the neural tube ( Fig. 6.2 ). Part of each somite (the dermatomyotome) gives rise to skeletal muscle and the dermis of the skin. As cells of the dermatomyotome differentiate, they migrate into posterior (dorsal) and anterior (ventral) areas of the developing body:

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    Cells that migrate anteriorly give rise to muscles of the limbs and trunk (hypaxial muscles) and to the associated dermis.

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    Cells that migrate posteriorly give rise to the intrinsic muscles of the back (epaxial muscles) and the associated dermis.

Fig. 6.2, Differentiation of somites in a “tubular” embryo.

Developing nerve cells within anterior regions of the neural tube extend processes peripherally into posterior and anterior regions of the differentiating dermatomyotome of each somite.

Simultaneously, derivatives of neural crest cells (cells derived from neural folds during formation of the neural tube) differentiate into neurons on each side of the neural tube and extend processes both medially and laterally ( Fig. 6.3 ):

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    Medial processes pass into the posterior aspect of the neural tube.

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    Lateral processes pass into the differentiating regions of the adjacent dermatomyotome.

Fig. 6.3, Somatic sensory and motor neurons. Blue lines indicate motor nerves and red lines indicate sensory nerves.

Neurons that develop from cells within the spinal cord are motor neurons and those that develop from neural crest cells are sensory neurons.

Somatic sensory and somatic motor fibers that are organized segmentally along the neural tube become parts of all spinal nerves and some cranial nerves.

The clusters of sensory nerve cell bodies derived from neural crest cells and located outside the CNS form sensory ganglia.

Generally, all sensory information passes into the posterior aspect of the spinal cord, and all motor fibers leave anteriorly.

Somatic sensory neurons carry information from the periphery into the CNS and are also called somatic sensory afferents or general somatic afferents (GSAs). The modalities carried by these nerves include temperature, pain, touch, and proprioception. Proprioception is the sense of determining the position and movement of the musculoskeletal system detected by special receptors in muscles and tendons.

Somatic motor fibers carry information away from the CNS to skeletal muscles and are also called somatic motor efferents or general somatic efferents (GSEs). Like somatic sensory fibers that come from the periphery, somatic motor fibers can be very long. They extend from cell bodies in the spinal cord to the muscle cells they innervate.

Dermatomes

Because cells from a specific somite develop into the dermis of the skin in a precise location, somatic sensory fibers originally associated with that somite enter the posterior region of the spinal cord at a specific level and become part of one specific spinal nerve ( Fig. 6.4 ). Each spinal nerve therefore carries somatic sensory information from a specific area of skin on the surface of the body. A dermatome is that area of skin supplied by a single spinal cord level, or on one side, by a single spinal nerve.

Fig. 6.4, Dermatomes.

There is overlap in the distribution of dermatomes, but usually a specific region within each dermatome can be identified as an area supplied by a single spinal cord level. Testing touch in these autonomous zones in a conscious patient can be used to localize lesions to a specific spinal nerve or to a specific level in the spinal cord.

Myotomes

Somatic motor nerves that were originally associated with a specific somite emerge from the anterior region of the spinal cord and, together with sensory nerves from the same level, become part of one spinal nerve. Therefore each spinal nerve carries somatic motor fibers to muscles that originally developed from the related somite. A myotome is that portion of a skeletal muscle innervated by a single spinal cord level or, on one side, by a single spinal nerve.

Myotomes are generally more difficult to test than dermatomes because each skeletal muscle in the body is usually innervated by nerves derived from more than one spinal cord level ( Fig. 6.5 ).

Fig. 6.5, Myotomes.

Testing movements at successive joints can help in localizing lesions to specific nerves or to a specific spinal cord level. For example:

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    Muscles that move the shoulder joint are innervated mainly by spinal nerves from spinal cord levels C5 and C6.

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    Muscles that move the elbow are innervated mainly by spinal nerves from spinal cord levels C6 and C7.

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    Muscles in the hand are innervated mainly by spinal nerves from spinal cord levels C8 and T1.

Visceral part of the nervous system

The visceral part of the nervous system, as in the somatic part, consists of motor and sensory components:

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    Sensory nerves monitor changes in the viscera.

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    Motor nerves mainly innervate smooth muscle, cardiac muscle, and glands.

The visceral motor component is commonly referred to as the autonomic division of the PNS and is subdivided into sympathetic and parasympathetic parts.

Like the somatic part of the nervous system, the visceral part is segmentally arranged and develops in a parallel fashion ( Fig. 6.6 ).

Fig. 6.6, Development of the visceral part of the nervous system.

Visceral sensory neurons that arise from neural crest cells send processes medially into the adjacent neural tube and laterally into regions associated with the developing body. These sensory neurons and their processes, referred to as general visceral afferent fibers (GVAs), are associated primarily with chemoreception, mechanoreception, and stretch reception.

Visceral motor neurons that arise from cells in lateral regions of the neural tube send processes out of the anterior aspect of the tube. Unlike in the somatic part, these processes, containing general visceral efferent fibers (GVEs), synapse with other cells, usually other visceral motor neurons, that develop outside the CNS from neural crest cells that migrate away from their original positions close to the developing neural tube.

The visceral motor neurons located in the spinal cord are referred to as preganglionic motor neurons and their axons are called preganglionic fibers; the visceral motor neurons located outside the CNS are referred to as postganglionic motor neurons and their axons are called postganglionic fibers.

The cell bodies of the visceral motor neurons outside the CNS often associate with each other in a discrete mass called a ganglion.

Visceral sensory and motor fibers enter and leave the CNS with their somatic equivalents ( Fig. 6.7 ). Visceral sensory fibers enter the spinal cord together with somatic sensory fibers through posterior roots of spinal nerves. Preganglionic fibers of visceral motor neurons exit the spinal cord in the anterior roots of spinal nerves, along with fibers from somatic motor neurons.

Fig. 6.7, Basic anatomy of a thoracic spinal nerve.

Postganglionic fibers traveling to visceral elements in the periphery are found in the posterior and anterior rami (branches) of spinal nerves.

Visceral motor and sensory fibers that travel to and from viscera form named visceral branches that are separate from the somatic branches. These nerves generally form plexuses from which arise branches to the viscera.

Visceral motor and sensory fibers do not enter and leave the CNS at all levels ( Fig. 6.8 ):

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    In the cranial region, visceral components are associated with four of the twelve cranial nerves (CN III, VII, IX, and X).

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    In the spinal cord, visceral components are associated mainly with spinal cord levels T1 to L2 and S2 to S4.

Fig. 6.8, Parts of the CNS associated with visceral motor components.

Visceral motor components associated with spinal levels T1 to L2 are termed sympathetic. Those visceral motor components in cranial and sacral regions, on either side of the sympathetic region, are termed parasympathetic:

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    The sympathetic system innervates structures in peripheral regions of the body and viscera.

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    The parasympathetic system is more restricted to innervation of the viscera only.

Sympathetic system

The sympathetic part of the autonomic division of the PNS leaves thoracolumbar regions of the spinal cord with the somatic components of spinal nerves T1 to L2 ( Fig. 6.9 ). On each side, a paravertebral sympathetic trunk extends from the base of the skull to the inferior end of the vertebral column where the two trunks converge anteriorly to the coccyx at the ganglion impar. Each trunk is attached to the anterior rami of spinal nerves and becomes the route by which sympathetics are distributed to the periphery and all viscera.

Fig. 6.9, Sympathetic part of the autonomic division of the PNS.

Visceral motor preganglionic fibers leave the T1 to L2 part of the spinal cord in anterior roots. The fibers then enter the spinal nerves, pass through the anterior rami and into the sympathetic trunks. One trunk is located on each side of the vertebral column (paravertebral) and positioned anterior to the anterior rami. Along the trunk is a series of segmentally arranged ganglia formed from collections of postganglionic neuronal cell bodies where the preganglionic neurons synapse with postganglionic neurons. Anterior rami of T1 to L2 are connected to the sympathetic trunk or to a ganglion by a white ramus communicans , which carries preganglionic sympathetic fibers and appears white because the fibers it contains are myelinated.

Preganglionic sympathetic fibers that enter a paravertebral ganglion or the sympathetic trunk through a white ramus communicans may take the following four pathways to target tissues:

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    Peripheral sympathetic innervation at the level of origin of the preganglionic fiber

Preganglionic sympathetic fibers may synapse with postganglionic motor neurons in ganglia associated with the sympathetic trunk, after which postganglionic fibers enter the same anterior ramus and are distributed with peripheral branches of the posterior and anterior rami of that spinal nerve ( Fig. 6.10 ). The fibers innervate structures at the periphery of the body in regions supplied by the spinal nerve. The gray ramus communicans connects the sympathetic trunk or a ganglion to the anterior ramus and contains the postganglionic sympathetic fibers. It appears gray because postganglionic fibers are nonmyelinated. The gray ramus communicans is positioned medial to the white ramus communicans.

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    Peripheral sympathetic innervation above or below the level of origin of the preganglionic fiber

Fig. 6.10, Course of sympathetic fibers that travel to the periphery in the same spinal nerves in which they travel out of the spinal cord.

Preganglionic sympathetic fibers may ascend or descend to other vertebral levels where they synapse in ganglia associated with spinal nerves that may or may not have visceral motor input directly from the spinal cord (i.e., those nerves other than T1 to L2) ( Fig. 6.11 ).

Fig. 6.11, Course of sympathetic nerves that travel to the periphery in spinal nerves that are not the ones through which they left the spinal cord.

The postganglionic fibers leave the distant ganglia via gray rami communicantes and are distributed along the posterior and anterior rami of the spinal nerves.

The ascending and descending fibers, together with all the ganglia, form the paravertebral sympathetic trunk, which extends the entire length of the vertebral column. The formation of this trunk, on each side, enables visceral motor fibers of the sympathetic part of the autonomic division of the PNS, which ultimately emerge from only a small region of the spinal cord (T1 to L2), to be distributed to peripheral regions innervated by all spinal nerves.

White rami communicantes only occur in association with spinal nerves T1 to L2, whereas gray rami communicantes are associated with all spinal nerves.

Fibers from spinal cord levels T1 to T5 pass predominantly superiorly, whereas fibers from T5 to L2 pass inferiorly. All sympathetics passing into the head have preganglionic fibers that emerge from spinal cord level T1 and ascend in the sympathetic trunks to the highest ganglion in the neck (the superior cervical ganglion), where they synapse. Postganglionic fibers then travel along blood vessels to target tissues in the head, including blood vessels, sweat glands, small smooth muscles associated with the upper eyelids, and the dilator of the pupil.

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    Sympathetic innervation of thoracic and cervical viscera

Preganglionic sympathetic fibers may synapse with postganglionic motor neurons in ganglia and then leave the ganglia medially to innervate thoracic or cervical viscera ( Fig. 6.12 ). They may ascend in the trunk before synapsing, and after synapsing the postganglionic fibers may combine with those from other levels to form named visceral nerves, such as cardiac nerves. Often, these nerves join branches from the parasympathetic system to form plexuses on or near the surface of the target organ, for example, the cardiac and pulmonary plexuses. Branches of the plexus innervate the organ. Spinal cord levels T1 to T5 mainly innervate cranial, cervical, and thoracic viscera.

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    Sympathetic innervation of the abdomen and pelvic regions and the adrenals

Fig. 6.12, Course of sympathetic nerves traveling to the heart.

Preganglionic sympathetic fibers may pass through the sympathetic trunk and paravertebral ganglia without synapsing and, together with similar fibers from other levels, form splanchnic nerves (greater , lesser , least , lumbar, and sacral), which pass into the abdomen and pelvic regions ( Fig. 6.13 ). The preganglionic fibers in these nerves are derived from spinal cord levels T5 to L2.

Fig. 6.13, Course of sympathetic nerves traveling to abdominal and pelvic viscera.

The splanchnic nerves generally connect with sympathetic ganglia around the roots of major arteries that branch from the abdominal aorta. These ganglia are part of a large prevertebral plexus that also has input from the parasympathetic part of the autonomic division of the PNS. Postganglionic sympathetic fibers are distributed in extensions of this plexus, predominantly along arteries, to viscera in the abdomen and pelvis.

Some of the preganglionic fibers in the prevertebral plexus do not synapse in the sympathetic ganglia of the plexus but pass through the system to the adrenal gland, where they synapse directly with cells of the adrenal medulla. These cells are homologues of sympathetic postganglionic neurons and secrete adrenaline and noradrenaline into the vascular system.

Parasympathetic system

The parasympathetic part of the autonomic division of the PNS ( Fig. 6.14 ) leaves cranial and sacral regions of the CNS in association with:

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    cranial nerves III, VII, IX, and X: III, VII, and IX carry parasympathetic fibers to structures within the head and neck only, whereas X (the vagus nerve) also innervates thoracic and most abdominal viscera; and

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    spinal nerves S2 to S4: sacral parasympathetic fibers innervate inferior abdominal viscera, pelvic viscera, and the arteries associated with erectile tissues of the perineum.

Fig. 6.14, Parasympathetic part of the autonomic division of the PNS.

Like the visceral motor nerves of the sympathetic part, the visceral motor nerves of the parasympathetic part generally have two neurons in the pathway. The preganglionic neurons are in the CNS, and fibers leave in the cranial nerves.

Sacral preganglionic parasympathetic fibers

In the sacral region, the preganglionic parasympathetic fibers form special visceral nerves (the pelvic splanchnic nerves), which originate from the anterior rami of S2 to S4 and enter pelvic extensions of the large prevertebral plexus formed around the abdominal aorta ( Fig. 6.15 ). These fibers are distributed to pelvic and abdominal viscera mainly along blood vessels. The postganglionic motor neurons are in the walls of the viscera. In organs of the gastrointestinal system, preganglionic fibers do not have a postganglionic parasympathetic motor neuron in the pathway; instead, preganglionic fibers synapse directly on neurons in the ganglia of the enteric system.

Fig. 6.15, Pelvic extensions of the prevertebral plexus. A. Anterior view. B. Anteromedial view of right side of plexus.

Cranial nerve preganglionic parasympathetic fibers

The preganglionic parasympathetic motor fibers in CN III, VII, and IX separate from the nerves and connect with one of four distinct ganglia, which house postganglionic motor neurons ( Fig. 6.16 ). These four ganglia are near major branches of CN V. Postganglionic fibers leave the ganglia, join the branches of CN V, and are carried to target tissues (salivary, mucous, and lacrimal glands; constrictor muscle of the pupil; and ciliary muscle in the eye) with these branches.

Fig. 6.16, Secretomotor (parasympathetic) innervation of the salivary glands and lacrimal gland.

The vagus nerve [X] gives rise to visceral branches along its course. These branches contribute to plexuses associated with thoracic viscera or to the large prevertebral plexus in the abdomen and pelvis. Many of these plexuses also contain sympathetic fibers.

When present, postganglionic parasympathetic neurons are in the walls of the target viscera.

Visceral sensory innervation (visceral afferents)

Visceral sensory fibers generally accompany visceral motor fibers.

Visceral sensory fibers accompany sympathetic fibers

Visceral sensory fibers follow the course of sympathetic fibers entering the spinal cord at similar spinal cord levels. However, visceral sensory fibers may also enter the spinal cord at levels other than those associated with motor output. For example, visceral sensory fibers from the heart may enter at levels higher than spinal cord level T1. Visceral sensory fibers that accompany sympathetic fibers are mainly concerned with detecting pain.

Visceral sensory fibers accompany parasympathetic fibers

Visceral sensory fibers accompanying parasympathetic fibers are carried mainly in IX and X and in spinal nerves S2 to S4.

Visceral sensory fibers in IX carry information from chemoreceptors and baroreceptors associated with the walls of major arteries in the neck, and from receptors in the pharynx.

Visceral sensory fibers in X include those from cervical viscera, and major vessels and viscera in the thorax and abdomen.

Visceral sensory fibers from pelvic viscera and the distal parts of the colon are carried in S2 to S4.

Visceral sensory fibers associated with parasympathetic fibers primarily relay information to the CNS about the status of normal physiological processes and reflex activities.

The enteric system

The enteric nervous system consists of motor and sensory neurons and their support cells, which form two interconnected plexuses, the myenteric and submucous nerve plexuses, within the walls of the gastrointestinal tract ( Fig. 6.17 ). Each of these plexuses is formed by:

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    ganglia, which house the nerve cell bodies and associated cells, and

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    bundles of nerve fibers, which pass between ganglia and from the ganglia into surrounding tissues.

Fig. 6.17, Enteric part of the nervous system.

Neurons in the enteric system are derived from neural crest cells originally associated with occipitocervical and sacral regions. Interestingly, more neurons are reported to be in the enteric system than in the spinal cord itself.

Sensory and motor neurons within the enteric system control reflex activity within and between parts of the gastrointestinal system. These reflexes regulate peristalsis, secretomotor activity, and vascular tone. These activities can occur independently of the brain and spinal cord but can also be modified by input from preganglionic parasympathetic and postganglionic sympathetic fibers.

Sensory information from the enteric system is carried back to the CNS by visceral sensory fibers.

Nerve plexuses

Nerve plexuses are either somatic or visceral and combine fibers from different sources or levels to form new nerves with specific targets or destinations ( Fig. 6.18 ). Plexuses of the enteric system also generate reflex activity independent of the CNS.

Fig. 6.18, Nerve plexuses.

Somatic plexuses

Major somatic plexuses formed from the anterior rami of spinal nerves are the cervical (C1 to C4), brachial (C5 to T1), lumbar (L1 to L4), sacral (L4 to S4), and coccygeal (S5 to Co) plexuses. Except for spinal nerve T1, the anterior rami of thoracic spinal nerves remain independent and do not participate in plexuses ( Fig. 6.18 ).

Visceral plexuses

Visceral nerve plexuses are formed in association with viscera and generally contain efferent (sympathetic and parasympathetic) and afferent components ( Fig. 6.18 ). These plexuses include cardiac and pulmonary plexuses in the thorax and a large prevertebral plexus in the abdomen anterior to the aorta, which extends inferiorly onto the lateral walls of the pelvis. The massive prevertebral plexus supplies input to and receives output from all abdominal and pelvic viscera.

Structural subdivisions of the nervous system

The CNS (central nervous system) is composed of the brain and spinal cord, both of which develop from the neural tube in the embryo ( Fig. 6.1 ).

The PNS (peripheral nervous system) is composed of all nervous structures outside the CNS that connect the CNS to the body ( Fig. 6.1 ). Elements of this system develop from neural crest cells and as outgrowths of the CNS. The PNS consists of the cranial and spinal nerves, visceral nerves and plexuses, and the enteric system.

Central nervous system

Brain

The parts of the brain are the cerebral hemispheres, the cerebellum, and the brainstem ( Fig. 6.19 ). The cerebral hemispheres consist of an outer portion, or the gray matter, containing cell bodies; an inner portion, or the white matter, made up of axons forming tracts or pathways; and the ventricles, which are spaces filled with cerebrospinal fluid (CSF) ( Fig. 6.20 ).

Fig. 6.19, Lateral view of the brain.

Fig. 6.20, Coronal section through brain.

The cerebellum has two lateral lobes and a midline portion. The components of the brainstem are classically defined as the diencephalon, midbrain, pons, and medulla. However, in common usage today, the term “brainstem” usually refers to the midbrain, pons, and medulla ( Fig. 6.21 ).

Fig. 6.21, Sagittal section of the brain.

Development

During development the brain can be divided into five continuous parts ( Figs. 6.19 and 6.21 ). From rostral (or cranial) to caudal they are:

The telencephalon (cerebrum) becomes the large cerebral hemispheres. The surface of these hemispheres consists of elevations (gyri) and depressions (sulci), and the hemispheres are partially separated by a deep longitudinal fissure. The cerebrum fills the area of the skull above the tentorium cerebelli and is subdivided into lobes based on position.

The diencephalon, which is hidden from view in the adult brain by the cerebral hemispheres, consists of the thalamus, hypothalamus, and other related structures, and classically is considered to be the most rostral part of the brainstem. (However, in common usage today, the term brainstem usually refers to the midbrain, pons, and medulla.)

The mesencephalon (midbrain), which is the first part of the brainstem seen when an intact adult brain is examined, located at the junction between and in both the middle and posterior cranial fossae.

The metencephalon, which gives rise to the cerebellum (consisting of two lateral hemispheres and a midline part in the posterior cranial fossa below the tentorium cerebelli) and the pons (anterior to the cerebellum and is a bulging part of the brainstem in the most anterior part of the posterior cranial fossa against the clivus and dorsum sellae).

The myelencephalon (medulla oblongata), the caudalmost part of the brainstem, ends at the foramen magnum or the uppermost rootlets of the first cervical nerve and to which cranial nerves VI to XII are attached.

Blood supply

The brain receives its arterial supply from two pairs of vessels, the vertebral and internal carotid arteries ( Fig. 6.22 ), which are interconnected in the cranial cavity to produce a cerebral arterial circle (of Willis).

Fig. 6.22, Arterial supply to the brain. A. Diagram. B. Magnetic resonance angiogram showing normal carotid and vertebral arteries. C. Enhanced CT scan of carotid vessels.

The two vertebral arteries enter the cranial cavity through the foramen magnum and just inferior to the pons fuse to form the basilar artery.

The two internal carotid arteries enter the cranial cavity through the carotid canals on either side.

Vertebral arteries

Each vertebral artery arises from the first part of each subclavian artery ( Fig. 6.22 ) in the lower part of the neck and passes superiorly through the transverse foramina of the upper six cervical vertebrae. On entering the cranial cavity through the foramen magnum each vertebral artery gives off a small meningeal branch.

Continuing forward, the vertebral artery gives rise to three additional branches before joining with its companion vessel to form the basilar artery ( Figs. 6.22 and 6.23 ):

Fig. 6.23, Arteries on the base of the brain.

One branch joins with its companion from the other side to form the single anterior spinal artery, which then descends in the anterior median fissure of the spinal cord.

A second branch is the posterior spinal artery, which passes posteriorly around the medulla and then descends on the posterior surface of the spinal cord in the area of the attachment of the posterior roots—there are two posterior spinal arteries, one on each side (although the posterior spinal arteries can originate directly from the vertebral arteries, they more commonly branch from the posterior inferior cerebellar arteries).

Just before the two vertebral arteries join, each gives off a posterior inferior cerebellar artery.

The basilar artery travels in a rostral direction along the anterior aspect of the pons ( Fig. 6.23 ). Its branches in a caudal to rostral direction include the anterior inferior cerebellar arteries, several small pontine arteries, and the superior cerebellar arteries. The basilar artery ends as a bifurcation, giving rise to two posterior cerebral arteries.

Internal carotid arteries

The two internal carotid arteries arise as one of the two terminal branches of the common carotid arteries ( Fig. 6.22 ). They proceed superiorly to the base of the skull where they enter the carotid canal.

Entering the cranial cavity each internal carotid artery gives off the ophthalmic artery, the posterior communicating artery, the middle cerebral artery, and the anterior cerebral artery ( Fig. 6.23 ).

Cerebral arterial circle

The cerebral arterial circle (of Willis) is formed at the base of the brain by the interconnecting vertebrobasilar and internal carotid systems of vessels ( Fig. 6.22 ). This anastomotic interconnection is accomplished by:

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    an anterior communicating artery connecting the left and right anterior cerebral arteries to each other, and

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    two posterior communicating arteries, one on each side, connecting the internal carotid artery with the posterior cerebral artery ( Figs. 6.22 and 6.23 ).

Spinal cord

The spinal cord is the part of the CNS in the superior two- thirds of the vertebral canal. It is roughly cylindrical in shape and is circular to oval in cross section with a central canal.

The spinal cord extends from the foramen magnum to approximately the level of the disc between vertebrae LI and LII in adults, although it can end as high as vertebra TXII or as low as the disc between vertebrae LII and LIII ( Fig. 6.24 ). In neonates, the spinal cord extends approximately to vertebra LIII but can reach as low as vertebra LIV. The distal end of the cord (the conus medullaris) is cone shaped. A fine filament of connective tissue (the pial part of the filum terminale) continues inferiorly from the apex of the conus medullaris.

Fig. 6.24, Spinal cord.

The spinal cord is not uniform in diameter along its length. It has two major swellings or enlargements in regions associated with the origin of spinal nerves that innervate the upper and lower limbs. A cervical enlargement occurs in the region associated with the origins of spinal nerves C5 to T1, which innervate the upper limbs. A lumbosacral enlargement occurs in the region associated with the origins of spinal nerves L1 to S3, which innervate the lower limbs.

The external surface of the spinal cord is marked by a number of fissures and sulci ( Fig. 6.25 ):

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    The anterior median fissure extends the length of the anterior surface.

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    The posterior median sulcus extends along the posterior surface.

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    The posterolateral sulcus on each side of the posterior surface marks where the posterior rootlets of spinal nerves enter the cord.

Fig. 6.25, Features of the spinal cord.

Internally, the cord has a small central canal surrounded by gray and white matter:

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    The gray matter is rich in nerve cell bodies, which form longitudinal columns along the cord, and in cross section these columns form a characteristic H-shaped appearance in the central regions of the cord ( Fig. 6.25 ).

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    The white matter surrounds the gray matter and is rich in nerve cell processes, which form large bundles or tracts that ascend and descend in the cord to other spinal cord levels or carry information to and from the brain.

Vasculature

Arteries

The arterial supply to the spinal cord comes from two sources ( Fig. 6.26 ). It consists of:

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    longitudinally oriented vessels, arising superior to the cervical portion of the cord, which descend on the surface of the cord; and

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    feeder arteries that enter the vertebral canal through the intervertebral foramina at every level; these feeder vessels, or segmental spinal arteries, arise predominantly from the vertebral and deep cervical arteries in the neck, the posterior intercostal arteries in the thorax, and the lumbar arteries in the abdomen.

Fig. 6.26, Arteries that supply the spinal cord. A. Anterior view of spinal cord (not all segmental spinal arteries are shown). B. Segmental supply of spinal cord.

After entering an intervertebral foramen, the segmental spinal arteries give rise to anterior and posterior radicular arteries ( Fig. 6.26 ). This occurs at every vertebral level. The radicular arteries follow, and supply, the anterior and posterior roots. At various vertebral levels, the segmental spinal arteries also give off segmental medullary arteries ( Fig. 6.26 ). These vessels pass directly to the longitudinally oriented vessels, reinforcing these.

The longitudinal vessels consist of:

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    a single anterior spinal artery, which originates within the cranial cavity as the union of two vessels that arise from the vertebral arteries—the resulting single anterior spinal artery passes inferiorly, approximately parallel to the anterior median fissure, along the surface of the spinal cord; and

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    two posterior spinal arteries, which also originate in the cranial cavity, usually arising directly from a terminal branch of each vertebral artery (the posterior inferior cerebellar artery)—the right and left posterior spinal arteries descend along the spinal cord, each as two branches that bracket the posterolateral sulcus and the connection of posterior roots with the spinal cord.

The anterior and posterior spinal arteries are reinforced along their length by eight to ten segmental medullary arteries ( Fig. 6.26 ). The largest of these is the arteria radicularis magna or the artery of Adamkiewicz ( Fig. 6.26 ). This vessel arises in the lower thoracic or upper lumbar region, usually on the left side, and reinforces the arterial supply to the lower portion of the spinal cord, including the lumbar enlargement.

Veins

Veins that drain the spinal cord form a number of longitudinal channels ( Fig. 6.27 ):

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    Two pairs of veins on each side bracket the connections of the posterior and anterior roots to the cord.

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    One midline channel parallels the anterior median fissure.

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    One midline channel passes along the posterior median sulcus.

Fig. 6.27, Veins that drain the spinal cord.

These longitudinal channels drain into an extensive internal vertebral plexus in the extradural (epidural) space of the vertebral canal, which then drains into segmentally arranged vessels that connect with major systemic veins, such as the azygos system in the thorax. The internal vertebral plexus also communicates with intracranial veins.

Meninges

The meninges ( Fig. 6.28 ) are three connective tissue coverings that surround, protect, and suspend the brain and spinal cord within the cranial cavity and vertebral canal, respectively:

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    The dura mater is the thickest and most external of the coverings.

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    The arachnoid mater is against the internal surface of the dura mater.

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    The pia mater is adherent to the brain and spinal cord.

Fig. 6.28, Arrangement of meninges in the cranial cavity.

Between the arachnoid and pia mater is the subarachnoid space, which contains CSF.

Cranial meninges.

The cranial meninges are continuous with, and similar to, the spinal meninges through the foramen magnum, with one important distinction—the cranial dura mater consists of two layers, and only one of these is continuous through the foramen magnum ( Fig. 6.29B ).

Fig. 6.29, Cranial meninges. A . Superior coronal view. B. Continuity with the spinal meninges.

Cranial dura mater

The cranial dura mater is a thick, tough, outer covering of the brain. It consists of an outer periosteal layer and an inner meningeal layer ( Fig. 6.29A ):

The outer periosteal layer is firmly attached to the skull, is the periosteum of the cranial cavity, contains the meningeal arteries, and is continuous with the periosteum on the outer surface of the skull at the foramen magnum and other intracranial foramina ( Fig. 6.29B ).

The inner meningeal layer is in close contact with the arachnoid mater and is continuous with the spinal dura mater through the foramen magnum.

The two layers of dura separate from each other at numerous locations to form two unique types of structures ( Fig. 6.29A ):

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    dural partitions, which project inward and incompletely separate parts of the brain, and

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    intracranial venous structures.

Dural partitions

The dural partitions project into the cranial cavity and partially subdivide the cranial cavity. They include the falx cerebri, tentorium cerebelli, falx cerebelli, and diaphragma sellae.

Falx cerebri.

The falx cerebri ( Fig. 6.30 ) is a crescent-shaped downward projection of meningeal dura mater from the dura lining the calva that passes between the two cerebral hemispheres. It is attached anteriorly to the crista galli of the ethmoid bone and frontal crest of the frontal bone. Posteriorly it is attached to and blends with the tentorium cerebelli.

Fig. 6.30, Dural partitions. A. Diagram. B. Dissection.

Tentorium cerebelli.

The tentorium cerebelli ( Fig. 6.30 ) is a horizontal projection of the meningeal dura mater that covers and separates the cerebellum in the posterior cranial fossa from the posterior parts of the cerebral hemispheres. It is attached posteriorly to the occipital bone along the grooves for the transverse sinuses. Laterally, it is attached to the superior border of the petrous part of the temporal bone, ending anteriorly at the anterior and posterior clinoid processes.

The anterior and medial borders of the tentorium cerebelli are free, forming an oval opening in the midline (the tentorial notch), through which the midbrain passes.

Falx cerebelli.

The falx cerebelli ( Fig. 6.30 ) is a small midline projection of meningeal dura mater in the posterior cranial fossa. It is attached posteriorly to the internal occipital crest of the occipital bone and superiorly to the tentorium cerebelli. Its anterior edge is free and is between the two cerebellar hemispheres.

Diaphragma sellae.

The final dural projection is the diaphragma sellae ( Fig. 6.30 ). This small horizontal shelf of meningeal dura mater covers the hypophyseal fossa in the sella turcica of the sphenoid bone. There is an opening in the center of the diaphragma sellae through which passes the infundibulum, connecting the pituitary gland with the base of the brain, and any accompanying blood vessels.

Arterial supply

The arterial supply to the dura mater ( Fig. 6.31 ) travels in the outer periosteal layer of the dura and consists of:

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    anterior meningeal arteries in the anterior cranial fossa,

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    the middle and accessory meningeal arteries in the middle cranial fossa, and

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    the posterior meningeal artery and other meningeal branches in the posterior cranial fossa.

Fig. 6.31, Dural arterial supply.

All are small arteries except for the middle meningeal artery, which is much larger and supplies the greatest part of the dura.

The anterior meningeal arteries are branches of the ethmoidal arteries.

The middle meningeal artery is a branch of the maxillary artery. It enters the middle cranial fossa through the foramen spinosum and divides into anterior and posterior branches:

The anterior branch passes in an almost vertical direction to reach the vertex of the skull, crossing the pterion during its course.

The posterior branch passes in a posterosuperior direction, supplying this region of the middle cranial fossa.

The accessory meningeal artery is usually a small branch of the maxillary artery that enters the middle cranial fossa through the foramen ovale and supplies areas medial to this foramen.

The posterior meningeal artery and other meningeal branches supplying the dura mater in the posterior cranial fossa come from several sources ( Fig. 6.31 ):

The posterior meningeal artery, the terminal branch of the ascending pharyngeal artery, enters the posterior cranial fossa through the jugular foramen.

A meningeal branch from the ascending pharyngeal artery enters the posterior cranial fossa through the hypoglossal canal.

Meningeal branches from the occipital artery enter the posterior cranial fossa through the jugular foramen and the mastoid foramen.

A meningeal branch from the vertebral artery arises as the vertebral artery enters the posterior cranial fossa through the foramen magnum.

Innervation

Innervation of the dura mater ( Fig. 6.32 ) is by small meningeal branches of all three divisions of the trigeminal nerve [V 1 , V 2 , and V 3 ], the vagus nerve [X], and the first, second, and, sometimes, third cervical nerves. (Possible involvement of the glossopharyngeal [IX] and hypoglossal nerves [XII] in the posterior cranial fossa has also been reported.)

Fig. 6.32, Dural innervation.

In the anterior cranial fossa meningeal branches from the ethmoidal nerves, which are branches of the ophthalmic nerve [V 1 ], supply the floor and the anterior part of the falx cerebri.

Additionally, a meningeal branch of the ophthalmic nerve [V 1 ] turns and runs posteriorly, supplying the tentorium cerebelli and the posterior part of the falx cerebri.

The middle cranial fossa is supplied medially by meningeal branches from the maxillary nerve [V 2 ] and laterally, along the distribution of the middle meningeal artery, by meningeal branches from the mandibular nerve [V 3 ].

The posterior cranial fossa is supplied by meningeal branches from the first, second, and, sometimes, third cervical nerves, which enter the fossa through the foramen magnum, the hypoglossal canal, and the jugular foramen. Meningeal branches of the vagus nerve [X] have also been described. (Possible contributions from the glossopharyngeal [IX] and hypoglossal [XII] nerves have also been reported.)

Dural venous sinuses

The dural venous sinuses include the superior sagittal, inferior sagittal, straight, transverse, sigmoid, and occipital sinuses, the confluence of sinuses, and the cavernous, sphenoparietal, superior petrosal, inferior petrosal, and basilar sinuses ( Fig. 6.33 , Table 6.1 ).

Fig. 6.33, Veins and dural venous sinuses.

Table 6.1
Dural venous sinuses {GAFS5 Table 8.3 }
Dural sinus Location Receives
Superior sagittal Superior border of falx cerebri Superior cerebral, diploic, and emissary veins and CSF
Inferior sagittal Inferior margin of falx cerebri A few cerebral veins and veins from the falx cerebri
Straight Junction of falx cerebri and tentorium cerebelli Inferior sagittal sinus, great cerebral vein, posterior cerebral veins, superior cerebellar veins, and veins from the falx cerebri
Occipital In falx cerebelli against occipital bone Communicates inferiorly with vertebral plexus of veins
Confluence of sinuses Dilated space at the internal occipital protuberance Superior sagittal, straight, and occipital sinuses
Transverse (right and left) Horizontal extensions from the confluence of sinuses along the posterior and lateral attachments of the tentorium cerebelli Drainage from confluence of sinuses (right—transverse and usually superior sagittal sinuses; left—transverse and usually straight sinuses); also superior petrosal sinus, and inferior cerebral, cerebellar, diploic, and emissary veins
Sigmoid (right and left) Continuation of transverse sinuses to internal jugular vein; groove of parietal, temporal, and occipital bones Transverse sinuses, and cerebral, cerebellar, diploic, and emissary veins
Cavernous (paired) Lateral aspect of body of sphenoid Cerebral and ophthalmic veins, sphenoparietal sinuses, and emissary veins from pterygoid plexus of veins
Intercavernous Crossing sella turcica Interconnect cavernous sinuses
Sphenoparietal (paired) Inferior surface of lesser wings of sphenoid Diploic and meningeal veins
Superior petrosal (paired) Superior margin of petrous part of temporal bone Cavernous sinus, and cerebral and cerebellar veins
Inferior petrosal (paired) Groove between petrous part of temporal bone and occipital bone ending in internal jugular vein Cavernous sinus, cerebellar veins, and veins from the internal ear and brainstem
Basilar Clivus, just posterior to sella turcica of sphenoid Connect bilateral inferior petrosal sinuses and communicate with vertebral plexus of veins

Superior sagittal sinus

The superior sagittal sinus is in the superior border of the falx cerebri ( Fig. 6.33 ). It begins anteriorly at the foramen cecum, where it may receive a small emissary vein from the nasal cavity, and ends posteriorly in the confluence of sinuses, usually bending to the right to empty into the right transverse sinus. The superior sagittal sinus communicates with lateral extensions (lateral lacunae) of the sinus containing numerous arachnoid granulations.

The superior sagittal sinus usually receives cerebral veins from the superior surface of the cerebral hemispheres, diploic and emissary veins, and veins from the falx cerebri.

Inferior sagittal and straight sinuses

The inferior sagittal sinus is in the inferior margin of the falx cerebri ( Fig. 6.33 ). It receives a few cerebral veins and veins from the falx cerebri and ends posteriorly at the anterior edge of the tentorium cerebelli, where it is joined by the great cerebral vein and together with the great cerebral vein forms the straight sinus ( Fig. 6.33 ).

The straight sinus continues posteriorly along the junction of the falx cerebri and the tentorium cerebelli and ends in the confluence of sinuses, usually bending to the left to empty into the left transverse sinus.

The straight sinus usually receives blood from the inferior sagittal sinus, cerebral veins (from the posterior part of the cerebral hemispheres), the great cerebral vein (draining deep areas of the cerebral hemispheres), superior cerebellar veins, and veins from the falx cerebri.

Confluence of sinuses, transverse and sigmoid sinuses

The superior sagittal and straight sinuses, and the occipital sinus (in the falx cerebelli) empty into the confluence of sinuses, which is a dilated space at the internal occipital protuberance ( Fig. 6.33 ) and is drained by the right and left transverse sinuses.

The paired transverse sinuses extend in horizontal directions from the confluence of sinuses where the tentorium cerebelli joins the lateral and posterior walls of the cranial cavity.

The right transverse sinus usually receives blood from the superior sagittal sinus and the left transverse sinus usually receives blood from the straight sinus.

The transverse sinuses also receive blood from the superior petrosal sinus, veins from the inferior parts of the cerebral hemispheres and the cerebellum, and diploic and emissary veins.

As the transverse sinuses leave the surface of the occipital bone, they become the sigmoid sinuses ( Fig. 6.33 ), which turn inferiorly, grooving the parietal, temporal, and occipital bones, before ending at the beginning of the internal jugular veins. The sigmoid sinuses also receive blood from cerebral, cerebellar, diploic, and emissary veins.

Cavernous sinuses

The paired cavernous sinuses are against the lateral aspect of the body of the sphenoid bone on either side of the sella turcica ( Fig. 6.34 ). They are of great clinical importance because of their connections and the structures that pass through them.

Fig. 6.34, Cavernous sinuses.

The cavernous sinuses receive blood not only from cerebral veins but also from the ophthalmic veins (from the orbit) and emissary veins (from the pterygoid plexus of veins in the infratemporal fossa). These connections provide pathways for infections to pass from extracranial sites into intracranial locations. In addition, because structures pass through the cavernous sinuses and are located in the walls of these sinuses they are vulnerable to injury due to inflammation.

Structures passing through each cavernous sinus are:

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    the internal carotid artery, and

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    the abducent nerve [VI].

Structures in the lateral wall of each cavernous sinus are, from superior to inferior:

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    the oculomotor nerve [III],

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    the trochlear nerve [IV],

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    the ophthalmic nerve [V 1 ], and

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    the maxillary nerve [V 2 ].

Connecting the right and left cavernous sinuses are the intercavernous sinuses on the anterior and posterior sides of the pituitary stalk ( Fig. 6.33 ).

Sphenoparietal sinuses drain into the anterior ends of each cavernous sinus. These small sinuses are along the inferior surface of the lesser wings of the sphenoid and receive blood from the diploic and meningeal veins.

Superior and inferior petrosal sinuses

The superior petrosal sinuses drain the cavernous sinuses into the transverse sinuses. Each superior petrosal sinus begins at the posterior end of the cavernous sinus, passes posterolaterally along the superior margin of the petrous part of each temporal bone, and connects to the transverse sinus ( Fig. 6.33 ). The superior petrosal sinuses also receive cerebral and cerebellar veins.

The inferior petrosal sinuses also begin at the posterior ends of the cavernous sinuses. These bilateral sinuses pass posteroinferiorly in a groove between the petrous part of the temporal bone and the basal part of the occipital bone, ending in the internal jugular veins. They assist in draining the cavernous sinuses and also receive blood from cerebellar veins and veins from the internal ear and brainstem.

Basilar sinuses connect the inferior petrosal sinuses to each other and to the vertebral plexus of veins. They are on the clivus, just posterior to the sella turcica of the sphenoid bone ( Fig. 6.33 ).

Arachnoid mater

The arachnoid mater is a thin, avascular membrane that lines, but is not adherent to, the inner surface of the dura mater ( Fig. 6.35 ). From its inner surface thin processes or trabeculae extend downward, cross the subarachnoid space, and become continuous with the pia mater.

Fig. 6.35, Arrangement of the meninges and spaces.

Unlike the pia, the arachnoid does not enter the grooves or fissures of the brain, except for the longitudinal fissure between the two cerebral hemispheres.

Pia mater

The pia mater is a thin, delicate membrane that closely invests the surface of the brain ( Fig. 6.35 ). It follows the contours of the brain, entering the grooves and fissures on its surface, and is closely applied to the roots of the cranial nerves at their origins.

Arrangement of meninges and spaces

There is a unique arrangement of meninges coupled with real and potential spaces within the cranial cavity ( Fig. 6.35 ).

A potential space is related to the dura mater, while a real space exists between the arachnoid mater and the pia mater.

Extradural space

The potential space between dura mater and bone is the extradural space ( Fig. 6.35 ). Normally, the outer or periosteal layer of dura mater is firmly attached to the bones surrounding the cranial cavity.

This potential space between dura and bone can become a fluid-filled actual space when a traumatic event results in a vascular hemorrhage. Bleeding into the extradural space due to rupture of a meningeal artery or a torn dural venous sinus results in an extradural hematoma.

Subdural space

Anatomically, a true subdural space does not exist. Blood collecting in this region (subdural hematoma) due to injury represents a dissection of the dural border cell layer, which is the innermost lining of the meningeal dura. Dural border cells are flattened cells surrounded by extracellular spaces filled with amorphous material. While very infrequent, an occasional cell junction may be seen between these cells and the underlying arachnoid layer. Bleeding due to the tearing of a cerebral vein as it crosses through the dura to enter a dural venous sinus can result in a subdural hematoma.

Subarachnoid space

Deep to the arachnoid mater is the only normally occurring fluid-filled space associated with the meninges, the subarachnoid space ( Fig. 6.35 ). It occurs because the arachnoid mater clings to the inner surface of the dura mater and does not follow the contour of the brain, while the pia mater, being against the surface of the brain, closely follows the grooves and fissures on the surface of the brain. The narrow subarachnoid space is therefore created between these two membranes ( Fig. 6.35 ).

The subarachnoid space surrounds the brain and spinal cord and in certain locations it enlarges into expanded areas (subarachnoid cisterns). It contains cerebrospinal fluid (CSF) and blood vessels.

Cerebrospinal fluid is produced by the choroid plexus, primarily in the ventricles of the brain. It is a clear, colorless, cell-free fluid that circulates through the subarachnoid space surrounding the brain and spinal cord.

The CSF returns to the venous system through arachnoid villi. These project as clumps (arachnoid granulations) into the superior sagittal sinus, which is a dural venous sinus, and its lateral extensions, the lateral lacunae ( Fig. 6.35 ).

Spinal meninges

Spinal dura mater

The spinal dura mater is the outermost meningeal membrane and is separated from the bones forming the vertebral canal by an extradural space ( Fig. 6.36 ). Superiorly, it is continuous with the inner meningeal layer of cranial dura mater at the foramen magnum of the skull. Inferiorly, the dural sac dramatically narrows at the level of the lower border of vertebra SII and forms an investing sheath for the pial part of the filum terminale of the spinal cord. This terminal cord-like extension of dura mater (the dural part of the filum terminale) attaches to the posterior surface of the vertebral bodies of the coccyx.

Fig. 6.36, Meninges.

As spinal nerves and their roots pass laterally, they are surrounded by tubular sleeves of dura mater, which merge with and become part of the outer covering (epineurium) of the nerves.

Arachnoid mater

The arachnoid mater is a thin delicate membrane against, but not adherent to, the deep surface of the dura mater ( Fig. 6.36 ). It is separated from the pia mater by the subarachnoid space. The arachnoid mater ends at the level of vertebra SII (see Fig. 6.24 ).

Subarachnoid space

The subarachnoid space between the arachnoid and pia mater contains CSF ( Fig. 6.36 ). The subarachnoid space around the spinal cord is continuous at the foramen magnum with the subarachnoid space surrounding the brain. Inferiorly, the subarachnoid space terminates at approximately the level of the lower border of vertebra SII (see Fig. 6.24 ).

Delicate strands of tissue (arachnoid trabeculae) are continuous with the arachnoid mater on one side and the pia mater on the other; they span the subarachnoid space and interconnect the two adjacent membranes. Large blood vessels are suspended in the subarachnoid space by similar strands of material, which expand over the vessels to form a continuous external coat.

The subarachnoid space extends further inferiorly than the spinal cord. The spinal cord ends at approximately the disc between vertebrae LI and LII, whereas the subarachnoid space extends to approximately the lower border of vertebra SII (see Fig. 6.24 ). The subarachnoid space is largest in the region inferior to the terminal end of the spinal cord, where it surrounds the cauda equina. As a consequence, CSF can be withdrawn from the subarachnoid space in the lower lumbar region without endangering the spinal cord.

Pia mater

The spinal pia mater is a vascular membrane that firmly adheres to the surface of the spinal cord ( Fig. 6.36 ). It extends into the anterior median fissure and reflects as sleeve-like coatings onto posterior and anterior rootlets and roots as they cross the subarachnoid space. As the roots exit the space, the sleeve-like coatings reflect onto the arachnoid mater.

On each side of the spinal cord, a longitudinally oriented sheet of pia mater (the denticulate ligament) extends laterally from the cord toward the arachnoid and dura mater ( Fig. 6.36 ).

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    Medially, each denticulate ligament is attached to the spinal cord in a plane that lies between the origins of the posterior and anterior rootlets.

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    Laterally, each denticulate ligament forms a series of triangular extensions along its free border, with the apex of each extension being anchored through the arachnoid mater to the dura mater.

The lateral attachments of the denticulate ligaments generally occur between the exit points of adjacent posterior and anterior rootlets. The ligaments function to position the spinal cord in the center of the subarachnoid space.

Peripheral nervous system

Cranial nerves

The 12 pairs of cranial nerves are part of the peripheral nervous system (PNS) and pass through foramina or fissures in the cranial cavity. All nerves except one, the accessory nerve [XI], originate from the brain.

In addition to having somatic and visceral components similar to those of spinal nerves, some cranial nerves also contain special sensory and motor components ( Tables 8.4 and 8.5 ).

The special sensory components are associated with hearing, seeing, smelling, balancing, and tasting.

Special motor components include those that innervate skeletal muscles derived embryologically from the pharyngeal arches and not from somites.

In human embryology, six pharyngeal arches are designated, but the fifth pharyngeal arch never develops. Each of the pharyngeal arches that does develop is associated with a developing cranial nerve or one of its branches. These cranial nerves carry efferent fibers that innervate the musculature derived from the pharyngeal arch.

Innervation of the musculature derived from the five pharyngeal arches that do develop is as follows:

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    first arch—trigeminal nerve [V 3 ],

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    second arch—facial nerve [VII],

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    third arch—glossopharyngeal nerve [IX],

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    fourth arch—superior laryngeal branch of the vagus nerve [X],

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    sixth arch—recurrent laryngeal branch of the vagus nerve [X].

Olfactory nerve [I]

The olfactory nerve [I] carries special afferent (SA) fibers for the sense of smell. Its sensory neurons have:

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    peripheral processes that act as receptors in the nasal mucosa, and

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    central processes that return information to the brain.

The receptors are in the roof and upper parts of the nasal cavity, and the central processes, after joining into small bundles, enter the cranial cavity by passing through the cribriform plate of the ethmoid bone ( Fig. 6.37 ). They terminate by synapsing with secondary neurons in the olfactory bulbs ( Fig. 6.38 ).

Fig. 6.37, Cranial nerves exiting the cranial cavity.

Fig. 6.38, Cranial nerves on the base of the brain.

Optic nerve [II]

The optic nerve [II] carries SA fibers for vision. These fibers return information to the brain from photoreceptors in the retina. Neuronal processes leave the retinal receptors, join into small bundles, and are carried by the optic nerves to other components of the visual system in the brain.

The optic nerve [II] is not a true cranial nerve, but rather an extension of the brain carrying afferent fibers from the retina of the eyeball to the visual centers of the brain. The optic nerve is surrounded by the cranial meninges, including the subarachnoid space, which extend as far forward as the eyeball.

Any increase in intracranial pressure therefore results in increased pressure in the subarachnoid space surrounding the optic nerve. This may impede venous return along the retinal veins, causing edema of the optic disc (papilledema), which can be seen when the retina is examined using an ophthalmoscope.

The optic nerve leaves the orbit through the optic canal ( Fig. 6.39 ). It is accompanied in the optic canal by the ophthalmic artery.

Fig. 6.39, Innervation of the orbit and eyeball.

Oculomotor nerve [III]

The oculomotor nerve [III] carries two types of fibers:

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    General somatic efferent (GSE) fibers innervate most of the extra-ocular muscles.

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    General visceral efferent (GVE) fibers are part of the parasympathetic part of the autonomic division of the PNS.

The oculomotor nerve [III] leaves the anterior surface of the brainstem between the midbrain and the pons ( Fig. 6.38 ). It enters the anterior edge of the tentorium cerebelli, continues in an anterior direction in the lateral wall of the cavernous sinus ( Figs. 6.37 and 6.38 ; see Fig. 6.34 ), and leaves the cranial cavity through the superior orbital fissure.

Just before entering the orbit the oculomotor nerve [III] divides into superior and inferior branches ( Fig. 6.40 ). These branches enter the orbit through the superior orbital fissure, lying within the common tendinous ring ( Fig. 6.39 ).

Fig. 6.40, Oculomotor nerve [III] and its divisions.

Inside the orbit the small superior branch passes upward over the lateral side of the optic nerve to innervate the superior rectus and levator palpebrae superioris muscles ( Fig. 6.40 ).

The large inferior branch divides into three branches:

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    one passing below the optic nerve as it passes to the medial side of the orbit to innervate the medial rectus muscle,

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    a second descending to innervate the inferior rectus muscle, and

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    the third descending as it runs forward along the floor of the orbit to innervate the inferior oblique muscle ( Fig. 6.40 ).

As the third branch descends, it gives off the branch to the ciliary ganglion. This is the parasympathetic root to the ciliary ganglion and carries preganglionic parasympathetic fibers that will synapse in the ciliary ganglion with postganglionic parasympathetic fibers. The postganglionic fibers are distributed to the eyeball through short ciliary nerves and innervate the sphincter pupillae and ciliary muscles.

GSE fibers in the oculomotor nerve innervate levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, and inferior oblique muscles.

The GVE fibers are preganglionic parasympathetic fibers that synapse in the ciliary ganglion and ultimately innervate the sphincter pupillae muscle, responsible for pupillary constriction, and the ciliary muscles, responsible for accommodation of the lens for near vision.

Ciliary ganglion

The ciliary ganglion is a parasympathetic ganglion of the oculomotor nerve [III]. It is associated with the nasociliary branch of the ophthalmic nerve [V 1 ] and is the site where preganglionic and postganglionic parasympathetic neurons synapse as fibers from this part of the autonomic division of the PNS make their way to the eyeball. The ciliary ganglion is also traversed by postganglionic sympathetic fibers and sensory fibers as they travel to the eyeball.

The ciliary ganglion is a very small ganglion, in the posterior part of the orbit immediately lateral to the optic nerve and between the optic nerve and the lateral rectus muscle ( Fig. 6.41 ). It is usually described as receiving at least two, and possibly three, branches or roots from other nerves in the orbit.

Fig. 6.41, Course of the nasociliary nerve (from [V 1 ]) in the orbit.

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