Clinical Neuroanatomy

How is the neural tube formed?

Beginning around the 18th gestational day, a midline notochordal thickening anterior to the blastopore forms the neural plate. A midsagittal groove called the neural groove appears in the plate, and the sides elevate to form the neural folds. As the folds fuse, the neural tube is formed. Some cells at the edges of the fold do not fuse into the tube and become neural crest cells.

What types of cells are derived from the neural crest cells?

Neural crest cells give rise to (1) unipolar sensory cells, (2) postganglionic cells of sympathetic and parasympathetic ganglia, (3) chromaffin cells of the adrenal medulla, (4) some microglial cells, (5) pia mater, (6) some arachnoid cells, (7) melanocytes, and (8) Schwann cells.

What are the alar plate and the basal plate?

As the neural tube is formed, a longitudinal groove appears on each side and divides the neural tube into a dorsal half, or alar plate, and a ventral half, or basal plate. The alar plate gives rise to the prosencephalon; the sensory and coordinating nuclei of the thalamus; the sensory neurons of the cranial nerves; the coordinating nuclei including cerebellum, inferior olives, red nucleus, and quadrigeminal plate; and the posterior horn area (sensory) of the spinal cord. The basal plate stops at the level of the diencephalon and gives rise to the motor neurons of the cranial nerves and anterior horn (motor) area of the spinal cord.

What is the process of formation of the ventricles, prosencephalon, mesencephalon, and rhombencephalon?

Around the end of the first gestational month, a series of bulges anterior to the first cervical somites appears. The first bulge is the prosencephalon, or forebrain. The cavity of this bulge forms the lateral ventricles and third ventricle. Secondary outpouchings from the forebrain are called optic vesicles and eventually form the retina, pigment epithelium, and optic nerve. The second bulge is the mesencephalon, or midbrain. The cavity of this bulge forms the cerebral aqueduct. The third bulge is the rhombencephalon, or hindbrain. This cavity gives rise to the fourth ventricle.

Which structures arise from the prosencephalon, mesencephalon, and rhombencephalon?

The prosencephalon develops into the telencephalon, which includes the cerebral cortex and basal ganglia, and the diencephalon, which includes the thalamus and the hypothalamus. The mesencephalon gives rise to the midbrain. The rhombencephalon gives rise to the metencephalon (pons plus cerebellum) and myelencephalon (medulla) ( Table 2-1 ).

What is the histologic organization of skeletal muscle?

Skeletal muscle is composed of long, thin, cylindrical, multinucleated cells called muscle fibers (or myofibrils). Each fiber has a motor endplate at its neuromuscular junction and is surrounded by connective tissue called endomysium . Groups of fibers, or a fascicle, are surrounded by a connective tissue layer called the perimysium . Fascicles are grouped together and surrounded by epimysium.

What is found at the A band, H band, I band, and Z line?

The A band contains the thin filaments (actin) and the thick filaments (myosin). The H band is the portion of the A band that contains only myosin, and the I band is the portion that contains only actin. The actin is anchored at the Z line ( Fig. 2-1 ).

How does the muscle contract?

When the sarcoplasmic reticulum is depolarized, calcium ions enter the cell and bind to troponin. This causes a conformational change that allows exposure of the actin-binding site to myosin. The myosin attaches to the actin-binding site and flexes, causing the actin filament to slide by the myosin filament. Adenosine triphosphate is required to allow the myosin-actin crossbridge to release and the muscle to relax.

What is meant by the term motor unit?

The motor unit is one motor nerve (lower motor neuron) and all muscle fibers that it innervates.

What is the muscle stretch reflex?

The muscle stretch reflex is a reflex arc that responds to stretching of muscle fibers to keep the muscle in an appropriate state of tension and tone, ready to contract or relax as needed. The sensory input (afferent) of the reflex is from two structures in the muscle called spindles and Golgi tendon organs . The output (efferent) is the alpha motor neuron that contracts (tightens) the muscle. (The muscle fibers are sometimes referred to as extrafusal fibers .)

What type of nerve fiber innervates the muscle?

An anterior horn motor neuron, called an alpha motor neuron, innervates the muscle. It is the final common pathway for muscle contraction.

What is the function of the Ia nerve fiber?

The Ia nerve arises from annulospiral endings within the muscle spindle. When the muscle spindle is stretched (i.e., when the muscle is relaxed), the Ia sensory nerve, through the dorsal root, monosynaptically stimulates the alpha motor neuron, which fires and contracts (shortens) the muscle. Thus, the muscle stretch reflex maintains tone and tension in the muscle, by contracting it when it becomes too relaxed.

Is the Ia reflex monosynaptic or polysynaptic?

The Ia reflex is monosynaptic, but it initiates a polysynaptic inhibition of the antagonist muscle group.

In the spinal cord, which nerve fibers synapse on the alpha motor neuron?

Both the corticospinal tract and afferent Ia sensory nerves regulate the alpha motor neuron by snapping on it in the anterior horn of the spinal cord. Renshaw cells are interneurons that are stimulated by the alpha motor neuron and then, by a feedback mechanism, inhibit the alpha motor neuron, causing autoinhibition.

What is the role of the gamma efferent nerve?

The muscle spindle is kept tense and responsive by tiny muscle fibers inside it, called intrafusal fibers . The gamma efferent nerve fibers keep the muscle spindles “tight” by innervating and contracting the intrafusal fibers in the muscle spindle. This process ensures that the spindle remains sensitive to any stretch.

Where does the Ib fiber originate?

The Ib fiber originates from the Golgi tendon organ, another structure that monitors muscle stretch and acts to inhibit muscle contraction (not shown in the diagram).

Where does the Ib neuron synapse?

At the spinal cord level, the Ib sensory nerve polysynaptically inhibits the alpha motor neuron to prevent muscle contraction and also stimulates the gamma efferent nerve to the intrafusal fiber to reset the muscle spindle.

Which roots make up the lumbar plexus?

Roots of L1, L2, L3, L4, and sometimes T12 make up the lumbar plexus.

What are the two largest branches of the lumbar plexus?

• Obturator nerve (L2, L3, L4) . It leaves the pelvis through the obturator foramen and supplies the adductors of the thigh.

• Femoral nerve (L2, L3, L4) . It exits the pelvis with the femoral artery and supplies the hip flexors and knee extensors. Distally, it continues as the saphenous nerve to supply sensation to the medial anterior knee and medial distal leg, including the medial malleolus ( Fig. 2-2 ).

  

What are the other branches of the lumbar plexus?

• Iliohypogastric nerve (L1) —sensation to skin over hypogastric and gluteal areas; to abdominal muscles

• Ilioinguinal nerve (L1) —sensation to skin over groin and scrotum or labia

• Genitofemoral nerve (L1, L2) —enters the internal inguinal ring and runs in the inguinal canal

• Lateral femoral cutaneous nerve (L2, L3) —sensation to skin over anterior and lateral parts of the thigh

Which nerve is at risk during appendectomy (McBurney’s incision)?

The iliohypogastric nerve may be inadvertently cut as it passes between the external and internal oblique muscles. This results in weakness in the inguinal canal area, putting the patient at risk for direct inguinal hernia.

What is meralgia paresthetica?

Meralgia paresthetica is numbness and tingling in the lateral thigh secondary to compression of the lateral femoral cutaneous nerve as it runs over the inguinal ligament. It commonly occurs in obese or pregnant patients. It can also be caused by placing hard objects in the pockets of pants with a low waistline such as so-called low rider jeans.

Which nerve supplies the gluteus maximus?

The inferior gluteal nerve (L5, S1, S2) supplies the gluteus maximus muscle.

What is the largest nerve in the body?

The sciatic nerve (L4, L5, S1, S2, S3), the largest nerve in the body, is composed of the common peroneal nerve (L4, L5, S1, S2) in its dorsal division and the tibial nerve (L4, L5, S1, S2, S3) in its ventral division ( Fig. 2-3 ).

What is the only nerve in the sacral plexus that emerges through the greater sciatic foramen, superior to the piriformis muscle?

The superior gluteal nerve (L4, L5, S1) supplies the gluteus medius and minimus and tensor fascia lata (abduction and medial rotation of the thigh).

Which nerve supplies the inferior buttock and posterior thigh?

The posterior femoral cutaneous nerve (S1, S2, S3), which runs with the inferior gluteal nerve, supplies the inferior buttock and posterior thigh.

Which nerve supplies the structures in the perineum?

The pudendal nerve (S2, S3, S4) supplies the perineum.

What is the only muscle supplied by the sciatic nerve that receives innervation exclusively from the dorsal division (i.e., peroneal component) of the sciatic nerve?

The biceps femoris has only dorsal innervation. This is important clinically to differentiate lesions caused by damage to the common peroneal nerve versus the sciatic nerve itself.

Which muscles are supplied by the tibial nerve?

The tibial nerve supplies plantar flexors and invertors of the foot.

What are the two divisions of the common peroneal nerve?

• Deep peroneal nerve —dorsiflexion of the foot and toes and sensation to a small area of skin between the first and second toes.

• Superficial peroneal nerve —evertors of the foot and sensation to the skin of the dorsal and lateral foot.

The brachial plexus comprises which roots?

The brachial plexus comprises the ventral rami of C5, C6, C7, C8, and T1 ( Fig. 2-4 ).

Which nerves arise from the ventral rami of the roots before formation of the brachial plexus?

• Dorsal scapular nerve , from C5 to rhomboid and levator scapula muscles; responsible for elevation and stabilization of the scapula.

• Long thoracic nerve , from C5, C6, and C7 to serratus anterior; responsible for abduction of the scapula.

Testing these nerves is useful in differentiating between root and plexus lesions. If there is a deficit in one of these nerves (clinically or electrically), the lesion is proximal to the plexus.

Which roots form the three trunks of the brachial plexus?

(1) Superior trunk, formed by C5 and C6; (2) middle trunk, formed by C7; and (3) lower trunk, formed by C8 and T1.

What is the only nerve from the trunks of the brachial plexus?

The suprascapular nerve (C5) comes off the upper trunk and supplies the supraspinatus (abduction) and infraspinatus (external rotation) of the shoulder.

Which vascular structure is associated with the three cords of the brachial plexus?

The lateral cord (C5, C6, C7), medial cord (C8, T1), and posterior cord (C5, C6, C7, C8) are named in relationship to the axillary artery.

What are the nerves off the cords of the brachial plexus?

Lateral cord:

• Lateral pectoral nerve (C5, C6, C7)—to pectoralis minor

• Musculocutaneous nerve (C5, C6)—to brachialis and coracobrachialis (elbow flexion)

• Median nerve (partial; C6, C7)—to pronator teres, flexor carpi radialis, part of flexor digitorum superficialis, part of palmaris longus

Medial cord:

• Medial pectoral nerve (C8, T1)—to pectoralis major (shoulder adduction)

• Ulnar nerve (C8, T1)—ulnar wrist and long finger flexors

• Median nerve (partial; C8, T1)—long finger flexors and small hand muscles

• Medial brachial cutaneous nerve—skin over medial surface of arm and proximal forearm

• Medial antebrachial cutaneous nerve—skin over medial surface of forearm

Posterior cord:

• Upper subscapular nerve (C5, C6)—to subscapularis (medial rotation of the humerus)

• Thoracodorsal nerve (C6, C7, C8)—to latissimus dorsi (shoulder adduction)

• Lower subscapular nerve (C5, C6)—to teres major (adducts the humerus)

• Axillary nerve (C5, C6)—to deltoid (abduction of the humerus) and teres minor (lateral rotation of humerus)

• Radial nerve (C5, C6, C7, C8, T1)—to extensor muscles of upper limb ( Figs 2-5 and 2-6 )

  
  

What is Erb’s palsy?

Erb’s palsy is an injury to the upper brachial plexus (C5, C6) resulting from excessive separation or stretch of the neck and shoulder (such as from a sliding injury or from pulling on an infant’s neck during delivery). The result is decreased sensation in the C5 and C6 dermatomes and paralysis of scapular muscles. The arm may be held in adduction, with the fingers pointing backward, so-called waiter’s tip position. Distal strength in the upper extremity remains intact.

What is Klumpke’s palsy?

Klumpke’s palsy is an injury to the lower brachial plexus (C8, T1) leading to weakness and anesthesia in a primarily ulnar distribution, which may be the result of maximal abduction of the shoulder.

What is Parsonage–Turner syndrome?

Parsonage–Turner syndrome is an acute brachial plexus neuritis, commonly also affecting the long thoracic, musculocutaneous, and axillary nerves. It causes patchy upper extremity weakness and numbness, usually accompanied by pain. Symptoms are bilateral in 20% of patients. This condition is associated with diabetes, systemic lupus erythematosus, and polyarteritis nodosa and may follow immunizations or viral infections. One-third of patients recover within 1 year and 90% within 3 years.

What deficit results from poorly fitting crutches?

Pressure from crutches in the axilla results in a lesion of the posterior cord or the radial nerve, leading to weakness of the elbow, wrist, and digits.

Which nerve is commonly affected in shoulder dislocation or fracture of the humerus?

The axillary nerve is affected, resulting in a lesion that causes decreased abduction of the shoulder and anesthesia over the lateral part of the proximal arm.

What is thoracic outlet syndrome (TOS)?

Classically, TOS consists of decreased upper extremity pulses, with tingling and numbness in the medial aspect of the arm secondary to compression of the medial cord of the brachial plexus and the axillary artery by a cervical rib or other structures.

What is found in the ventral nerve root?

The ventral nerve root contains principally motor axons.

What is found in the dorsal nerve root?

The dorsal nerve root contains principally sensory axons.

What synapse is found in the dorsal root ganglia?

There is no synapse in the dorsal root ganglia. The dorsal root ganglia are made up of unipolar cell bodies for the sensory system.

What are the dermatomes of the following landmarks: thumb, middle finger, little finger, breast nipple, umbilicus, medial knee, big toe, and little toe?

Thumb—C6, middle finger—C7, little finger—C8, breast nipple—T4, umbilicus—T10, medial knee—L4, big toe—L5, little toe—S1.

What are the common signs and symptoms of lumbar radiculopathies?

Lumbar radiculopathies cause back pain with radiation below the knee. The pain increases with a Valsalva maneuver or leg stretch (such as the straight leg raising test). Weakness or numbness may develop in the distribution of the involved root. An S1 radiculopathy diminishes ankle reflexes, whereas an L4 radiculopathy decreases knee reflexes. Statistically, an L5 radiculopathy is more common than S1, followed by L4. This is because the intervertebral discs at these levels are under greatest pressure from the curvature of normal lumbar lordosis and thus are most vulnerable to herniation and compression of the spinal roots.

What are the common signs and symptoms of cervical radiculopathies?

Cervical radiculopathies usually involve the lower cervical roots (C6, C7, C8). Patients typically complain of pain in the back of the neck, frequently with radiation to the arm in a dermatomal distribution. Paresthesias are often present in one or two digits. Absent biceps, brachioradialis, or triceps reflexes suggest lesions of C5, C6, and C7, respectively, and these muscles also may lose strength.

How is the spinal cord organized?

Sections of the spinal cord cut perpendicular to the length of the cord reveal a butterfly-shaped area of gray matter with surrounding white matter. The white matter consists mainly of longitudinal nerve fibers, carrying the ascending and descending tracts up and down the cord. Midline grooves are present on the dorsal and ventral surfaces (the dorsal median sulcus and ventral median fissure). The gray matter of the cord contains dorsal and ventral enlargements known as dorsal horns and ventral horns .

In a given transverse section of the spinal cord, how is the gray matter subdivided?

The gray matter can be subdivided into groups of nuclei. When the spinal cord is cut along its length, these nuclei appear to be arranged in cell columns or laminae. Rexed divides the cord into 10 laminae. Each lamina extends the length of the cord, with lamina I at the most dorsal aspect of the dorsal horn, lamina IX at the most ventral aspect of the ventral horn, and lamina X surrounding the central canal. Lamina II is also called the substantia gelatinosa and is the area of synapse for the spinothalamic tract. Lamina IX is the site of the cell bodies for the anterior horn motor cells.

What are the major ascending tracts in the spinal cord?

(1) Dorsal columns; (2) spinothalamic tract; (3) dorsal spinocerebellar tract ; and (4) ventral spinocerebellar tract.

What are the major descending tracts in the spinal cord?

(1) Intermediolateral columns; (2) lateral corticospinal tract; (3) lateral reticulospinal tract; (4) lateral vestibulospinal tract; (5) medial longitudinal fasciculus (MLF); and (6) ventral corticospinal tract.

Going from rostral to caudal, what are the five divisions of the spinal cord?

Cervical, thoracic, lumbar, sacral, and coccygeal are the five divisions of the spinal cord.

In the adult, at what vertebral level does the spinal cord end?

The spinal cord ends at vertebral level L1 to L2.

How many spinal nerves exit from each region of the spinal cord?

Spinal nerves exit the spinal cord in pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each spinal nerve is composed of the union of the dorsal sensory root and the ventral motor root.

What is the filum terminale?

Although the spinal cord ends at the lower border of vertebral level L1, the pia mater continues caudally as a connective tissue filament, the filum terminale, which passes through the subarachnoid space to the end of the dural sac, where it receives a covering of dura and continues to its attachment to the coccyx.

What is the cauda equina?

The lumbar and sacral spinal nerves have very long roots, descending from their respective points in the spinal cord to their exit points in the intervertebral foramina. These roots descend in a bundle from the conus, termed the cauda equina for its resemblance to a horse’s tail.

Describe the blood supply of the spinal cord.

The one anterior spinal artery and the two posterior spinal arteries travel along the length of the cord to supply blood to the cord. These arteries originate from the vertebral arteries. Other arteries replenish the anterior and posterior spinal arteries and enter the spinal canal through the intervertebral foramina in association with the spinal nerves. They are called radicular arteries if they supply only the nerve roots, and radiculospinal arteries if they supply blood to both the roots and the cord. Each radiculospinal artery supplies blood to approximately six spinal cord segments, with the exception of the great radicular artery of Adamkiewicz, which usually enters with the left second lumbar ventral root (range T10 to L4) and supplies most of the caudal third of the cord.

What type of information is carried in the dorsal columns?

The dorsal columns convey tactile discrimination, vibration, and joint position sense.

What types of receptors are stimulated to sense this information?

Muscle spindles and Golgi tendon organs perceive position sense, Pacinian corpuscles perceive vibration, and Meissner corpuscles perceive superficial touch sensation needed for tactile discrimination. Pacinian and Meissner corpuscles are examples of mechanoreceptors.

What type of peripheral nerve fiber is involved with transmission of dorsal column information?

Large, myelinated, fast-conducting nerve fibers carry dorsal column-type information.

What is the pathway by which this information reaches the cerebral cortex?

Sensation on skin → afferent sensory nerve → dorsal column on ipsilateral side (fasciculus gracilis and cuneatus) → lower medulla → synapse in nucleus gracilis and cuneatus → arcuate fibers → cross to the contralateral side into the medial lemniscus → ascend to the ventral posterolateral (VPL) nucleus of the thalamus → synapse → through the posterior limb of the internal capsule → postcentral gyrus of the cortex.

Where do dorsal column fibers decussate? At what locations do they synapse?

The dorsal columns decussate in the lower medulla, after synapsing in the nucleus gracilis and cuneatus. They also synapse in the VPL of the thalamus before going to the cortex.

What type of information is carried in the spinothalamic tract?

The spinothalamic tract conveys pain, temperature, and crude touch.

What type of peripheral nerve fiber is involved with transmission of spinothalamic information?

Small, myelinated, and unmyelinated fibers carry spinothalamic-type information.

What is the pathway by which this information reaches the cerebral cortex?

Sensation on skin → afferent sensory nerve → substantia gelatinosa of the ipsilateral dorsal horn → synapse → cross via the anterior white commissure → contralateral spinothalamic tract → ascend to the VPL nucleus of the thalamus → synapse → through the posterior limb of the internal capsule → postcentral gyrus of the cortex.

Where do the spinothalamic fibers decussate? At what locations do they synapse?

These fibers decussate at the level they enter the spinal cord, after synapsing in Rexed’s lamina II (substantia gelatinosa). They also synapse in the VPL of the thalamus before going to the cortex.

What types of receptors are stimulated to sense this information?

Pain and temperature are perceived by naked terminals of A delta and C fibers and by many specialized chemoreceptors that are excited by tissue substances released in response to noxious and inflammatory stimuli. Substance P is thought to be the neurotransmitter released by A delta and C fibers at their connections with the interneurons in the spinal cord.

Where in the internal capsule do the afferents travel from the VPL thalamic nucleus?

The sensory tracts from the VPL travel in the posterior aspect of the posterior limb of the internal capsule.

To which anatomic locations do the afferents from the VPL project?

They project to the postcentral gyrus (Brodmann’s area 3, 1, 2; also called somatosensory I ) and to somatosensory II (the posterior aspect of the superior lip of the lateral fissure).

Which pathway carries proprioception from the lower limbs to the cerebellum?

Proprioception travels from the legs to the cerebellum in the dorsal columns.

Where does cerebellar proprioception for the lower limb synapse?

These fibers synapse in the midthoracic level of the spinal cord in the nucleus dorsalis of Clarke.

Where is the spinocerebellar tract located?

The spinocerebellar tract lies lateral to the corticospinal tract in the cord.