Overview of the nervous system

Spinal cord

The spinal cord lies within the vertebral column, in the upper two-thirds of the vertebral canal, and is continuous rostrally with the medulla oblongata of the brainstem. For the most part, the spinal cord receives afferent input from, and controls the functions of, the trunk and limbs. Afferent and efferent connections between the periphery and the spinal cord travel in 31 pairs of segmentally arranged spinal nerves that are attached to the cord as a linear series of dorsal and ventral rootlets. Adjacent groups of rootlets unite to form dorsal and ventral roots that combine to form the spinal nerves proper ( Fig. 24.2 ). Dorsal and ventral roots are functionally distinct: dorsal roots carry primary afferent nerve fibres from neuronal cell bodies located in dorsal root ganglia, whereas ventral roots carry efferent fibres from neuronal cell bodies located in the spinal grey matter.

Internally, the spinal cord consists of a central core of grey matter surrounded by white matter. The grey matter is configured in a characteristic H, or butterfly, shape that has projections known as dorsal (posterior) and ventral (anterior) horns ( Fig. 24.3 ). In general, neurones situated in the dorsal horn are primarily concerned with sensory functions whilst those in the ventral horn are mostly associated with motor activities. At thoracic and upper lumbar levels of the spinal cord a small lateral horn is additionally present, marking the location of the cell bodies of preganglionic sympathetic neurones. The central canal, a vestigial component of the ventricular system, lies at the centre of the spinal grey matter and runs the length of the cord. The white matter of the spinal cord consists of ascending and descending tracts that link spinal cord segments to one another and the spinal cord to the brain.

Brain

The brain receives information from, and controls the activities of, the trunk and limbs, mainly via connections with the spinal cord. It also possesses 12 pairs of cranial nerves through which it communicates with structures that are mostly in the head and neck.

The brain is divided into major regions on the basis of ontogenetic growth and phylogenetic principles ( Figs 24.4 – 24.6 ). Ascending in sequence from the spinal cord, the principal divisions are the rhombencephalon (hindbrain), mesencephalon (midbrain) and prosencephalon (forebrain).

The rhombencephalon is subdivided into the myelencephalon or medulla oblongata, metencephalon or pons, and the cerebellum. The medulla oblongata, pons and midbrain are collectively referred to as the brainstem: it lies upon the basal portions of the occipital and sphenoid bones (clivus). The medulla oblongata is the most caudal part of the brainstem and is continuous with the spinal cord below the level of the foramen magnum. The pons lies rostral to the medulla and is distinguished by a mass of transverse nerve fibres that connect it to the cerebellum. The midbrain is a short segment of brainstem, rostral to the pons. The cerebellum consists of paired hemispheres united by a median vermis: it lies within the posterior cranial fossa, dorsal to the pons, medulla and caudal midbrain, with all of which it has numerous fibre connections.

The prosencephalon is subdivided into the diencephalon and the telencephalon. The diencephalon equates mostly to the thalamus and hypothalamus, but also includes the smaller epithalamus and subthalamus. The telencephalon is composed mainly of the cerebral hemisphere or cerebrum. The diencephalon is almost completely embedded in the cerebrum and is, therefore, largely hidden from the exterior. The human cerebrum constitutes the major part of the brain. It occupies the anterior and middle cranial fossae and is directly related to the calvaria. The surface of the cerebral hemisphere is convoluted into a complex pattern of ridges (gyri) and furrows (sulci). Internally, the hemisphere has an outer layer of grey matter, the cerebral cortex, beneath which lies a thick mass of white matter ( Fig. 24.7 ). One of the most important components of the cerebral white matter, the internal capsule (see Fig. 24.9 ), contains nerve fibres that pass to and from the cerebral cortex and lower levels of the neuraxis. Several large nuclei of grey matter, the basal ganglia or basal nuclei, are partly embedded in the subcortical white matter. Nerve fibre connections between corresponding areas on either side of the brain cross the midline within commissures, the largest being the corpus callosum.

During prenatal development, the walls of the neural tube thicken greatly but never completely obliterate the central lumen, which remains in the spinal cord as the vestigial central canal and becomes greatly expanded in the brain to form a series of interconnected cavities or ventricles ( Ch. 25 ). In the forebrain and hindbrain, parts of the roof of the neural tube do not generate nerve cells but become thin, folded sheets of highly vascular secretory tissue, the choroid plexuses which secrete cerebrospinal fluid into the ventricles. The cavity of the rhombencephalon becomes expanded to form the fourth ventricle dorsal to the pons and upper half of the medulla. Caudally, the fourth ventricle is continuous with a canal in the caudal medulla and, through this, with the central canal of the spinal cord. It is continuous with the subarachnoid space through three openings (the foramina of Luschka and of Magendie) and with a narrow channel, the cerebral aqueduct, at its rostral extent. The cerebral aqueduct passes through the midbrain and opens out rostrally into the third ventricle, a narrow, slit-like, midline cavity bounded laterally by the diencephalon. At the rostral end of the third ventricle, a small aperture on each side leads into the large lateral ventricle located within each cerebral hemisphere (see Fig. 24.4C ).

Overview of ascending sensory pathways

Sensory modalities are conventionally described as being either special senses or general senses. The special senses are olfaction, vision, taste, hearing and vestibular function: afferent information is encoded by highly specialized sense organs and transmitted to the brain in the olfactory, optic, facial, vestibulocochlear and glossopharyngeal nerves.

The general senses include touch (ranging from light to discriminative), pressure, vibration, pain, thermal sensation and proprioception (perception of posture and movement). Stimuli from the external and internal environments activate a diverse range of receptors in the skin, fascia, viscera, muscles, bones, tendons and joints ( Ch. 3 ). Afferent impulses from the trunk and limbs are conveyed to the spinal cord in spinal nerves, while those from the head are carried to the brain in cranial nerves. The detailed anatomy of the complex pathways by which the various general senses impinge on consciousness levels is better understood by reference to certain common organizational principles. Whilst undoubtedly oversimplified and subject to exceptions, this schema is helpful in emphasizing the essential similarities that exist between the ascending sensory systems.

In essence, ascending sensory projections related to the general senses consist of a sequence of three neurones that extends from peripheral receptor to contralateral cerebral cortex ( Fig. 24.8 ). These are often referred to as primary, secondary and tertiary neurones or first-, second- and third-order neurones. Primary afferents have peripherally located sensory endings and cell bodies that lie in dorsal root ganglia or the sensory ganglia associated with certain cranial nerves. Their axons enter the CNS through spinal or cranial nerves and terminate by synapsing on the cell bodies of ipsilateral second-order neurones, the precise location of which depends upon the modality.

Primary afferent fibres carrying pain, temperature and light touch/pressure information from the trunk and limbs terminate in the dorsal horn of the spinal grey matter, near their point of entry into the spinal cord. Homologous fibres from the head terminate in the spinal nucleus of the trigeminal nerve in the brainstem. The cell bodies of second-order neurones are located in either the dorsal horn or the spinal nucleus of the trigeminal nerve. Their axons decussate and ascend to the ventral posterior nucleus of the contralateral thalamus as the spinothalamic or the trigeminothalamic tract, respectively; they synapse on the cell bodies of third-order neurones in the thalamus. Axons of third-order neurones pass through the internal capsule to reach the cerebral cortex, terminating in the primary somatosensory cortex in the postcentral gyrus of the parietal lobe.

Primary afferent fibres carrying proprioceptive information and fine (discriminative) touch from the trunk and limbs ascend ipsilaterally in the spinal cord as the dorsal columns (fasciculus gracilis and fasciculus cuneatus) and end by synapsing on second-order neurones in the dorsal column nuclei (nucleus gracilis and nucleus cuneatus) of the medulla. Axons of second-order neurones decussate in the medulla and then ascend as the medial lemniscus to the ventral posterior nucleus of the contralateral thalamus, where they synapse on the cell bodies of third-order neurones. Axons of third-order neurones pass through the internal capsule and terminate in the primary somatosensory cortex. A similar homologous projection to the cortex exists for afferents derived from the head, except that primary afferent fibres carrying discriminative touch from the face and oral cavity terminate by synapsing on second-order neurones in the principal sensory nucleus of the trigeminal nerve.

Overview of descending motor pathways

Corticofugal fibres descend through the internal capsule and pass into the brainstem, where many of them terminate in various cranial nerve nuclei and other brainstem nuclei such as the red nucleus, reticular nuclei, and olivary nuclei. The term ‘corticobulbar’ has been used for many years to describe cortical projections to brainstem nuclei but it is a misnomer. The suffix ‘bulbar’ is derived from ‘bulb’, an archaic name for the medulla oblongata. The term ‘corticonuclear’ is, therefore, more accurate to describe these connections although the term ‘bulb’ is still used (e.g. pseudobulbar palsy).

Corticospinal (pyramidal tract) fibres originate from widespread regions of the cerebral cortex, including the primary motor cortex of the frontal lobe where the contralateral half of the body is represented in a detailed somatotopic fashion. The fibres descend throughout the length of the brainstem and the majority then cross to the contralateral side in the decussation of the pyramids in the ventral medulla. Thereafter, the majority of fibres continue caudally as the lateral corticospinal tract of the spinal cord and terminate in association with interneurones and motor neurones of the spinal grey matter ( Fig. 24.9 ). The principal function of the corticonuclear and corticospinal tracts is the control of fine, fractionated movements, particularly of those parts of the body where delicate muscular control is required. These tracts are particularly important in speech (corticonuclear fibres) and movements of the hands (corticospinal fibres).

The concept of ‘upper’ and ‘lower’ motor neurones is fundamental in clinical neurology because the motor signs and symptoms of damage to each category are different and are indicative of the anatomical site of the lesion. Lower motor neurones are the alpha motor neurones located in the brainstem and spinal cord that innervate the extrafusal muscle fibres of skeletal muscle. The term upper motor neurones refers collectively to all the descending pathways that impinge upon the activity of lower motor neurones but, in clinical parlance, the term is often equated with the corticonuclear and corticospinal tracts. The terms upper and lower motor neurone lesion are used clinically to distinguish, for example, between the effects of a stroke in the internal capsule (a typical upper motor neurone lesion) and those of motor neurone disease (a typical lower motor neurone lesion).

Lower motor neurone lesions cause paralysis or paresis of specific muscles because the affected muscles have lost their direct innervation. There is also loss or reduction of tendon reflex activity and reduced muscle tone. Spontaneous muscular contractions (fasciculation) occur and affected muscles atrophy over time. Upper motor neurone lesions cause paralysis or paresis of movements as a result of loss of higher control. There is increased tendon reflex activity and increased muscle tone, but no muscle atrophy. The combination of paralysis, increased tendon reflex activity and hypertonia is referred to as spasticity. A positive plantar (Babinski) reflex is present with corticospinal lesions.

The pathophysiology underlying the symptoms of upper motor neurone lesions is complex. This is because many descending pathways other than the corticonuclear and corticospinal tracts that also influence lower motor neurone activity may be compromised to varying extents, depending on the site of a lesion. These pathways include corticofugal projections to the brainstem that traverse the internal capsule (e.g. corticoreticular and corticopontine fibres), and numerous pathways that originate within the brainstem itself (e.g. reticulospinal and vestibulospinal tracts): their involvement is believed to be important in the pathophysiological mechanisms that underlie the generation of spasticity. Pure corticospinal tract lesions are exceedingly rare in humans because corticospinal tract fibres lie in close relationship to other pathways throughout most of their course: when they occur, they are believed to cause specific deficits in delicate, fractionated movements and to induce a positive plantar reflex.

Two other major systems that contribute to the control of movement are the basal ganglia (basal nuclei) and the cerebellum. The basal ganglia are a group of large subcortical nuclei, the major components being the caudate nucleus, putamen and globus pallidus (see Fig. 24.7 ; Ch. 31 ). These structures have important connections with the cerebral cortex, certain nuclei of the thalamus and subthalamus, and with the brainstem. They appear to be involved in the selection of appropriate behavioural patterns/movements and the suppression of inappropriate ones. Disorders of the basal ganglia cause either too little movement (akinesia) or abnormal involuntary movements (dyskinesias), as well as tremor and abnormalities of muscle tone ( ). The basal ganglia are sometimes described as being part of the so-called ‘extrapyramidal system’. This somewhat archaic term was introduced to distinguish between the effects of basal ganglia disease and those of damage to the ‘pyramidal’ (corticospinal) system. However, the progressive elucidation of the anatomy of the basal ganglia and of the pathophysiology of motor disorders has revealed the close functional interrelationship between the two ‘systems’, and has rendered the terms that distinguish them largely obsolete ( ). The cerebellum ( Ch. 29 ) has rich connections with the brainstem, particularly the reticular and vestibular nuclei, and with the thalamus. It is concerned with the coordination of movement: cerebellar disorders cause ataxia, intention tremor and hypotonia.