6: The Nervous System

Part 6
The Nervous System


For descriptive purposes, the nervous system can be divided, topographically, into two parts: the central nervous system and the peripheral nervous system.

The central nervous system comprises the brain and spinal cord, which are located, respectively, in the cranial cavity and the vertebral (spinal) canal, and are continuous with each other at the foramen magnum (where the medulla oblongata of the brain stem adjoins the spinal cord).

The peripheral nervous system comprises the twelve pairs of cranial nerves, thirty‐one pairs of spinal nerves, the ganglia associated with the cranial and spinal nerves, and the right and left ganglionated sympathetic chains.

This division into central and peripheral systems is somewhat arbitrary, as the two ‘systems’ are physically connected and normally function in an integrated and co‐ordinated manner.

The brain

On a functional basis, the brain may be pictured as being made up of four major divisions:

  1. brainstem (comprising, from below upwards, the medulla oblongata, pons and midbrain);
  2. cerebellum;
  3. diencephalon (comprising, mainly, the thalamus and hypothalamus);
  4. cerebral hemispheres.

By another convention, based on embryological development, the brain may be divided into the forebrain (comprising the cerebral hemispheres and diencephalon), midbrain and hindbrain (made up of the pons, medulla oblongata and cerebellum).

The brainstem

Extending from just above the tentorial hiatus to just below the foramen magnum, the brainstem is a stalk‐like structure that is continuous superiorly with the diencephalon, and inferiorly with the spinal cord. As has been stated previously, the brainstem and the cerebellum are situated in the posterior cranial fossa.

The brainstem serves three major functions by:

  1. housing the nuclei of all but two of the twelve pairs of cranial nerves (the exceptions being the cranial nerve pairs I and II which may both be regarded as peripheral extensions of the forebrain);
  2. acting as a ‘thoroughfare’ for the various ascending and descending nerve tracts running to and from the cerebral cortex, and for other tracts that project to the cerebellum;
  3. containing the reticular formation (a fine and diffuse network of nerve cells and nerve fibres) and the reticular activating system. The reticular formation spans the entire length of the brainstem and harbours the ‘vital centres’ – important reflex centres that regulate respiratory and cardiovascular function. The reticular formation and reticular activating system regulate the individual’s level of awareness and wakefulness. Damage to the reticular formation in the upper part of the brainstem may cause the patient to be in a state of prolonged coma.

The medulla oblongata

The medulla oblongata is 2.5 cm (1 in) in length and about 1.8 cm (0.75 in) in diameter. It is continuous below, through the foramen magnum, with the spinal cord and above with the pons; posteriorly, it is connected with the cerebellum by way of the right and left inferior cerebellar peduncles.

External features (Fig. 234)

The base of the brain displaying the cranial nerve roots and their relationships to the circle of Willis, with lines marking the cranial nerves (I–XII) at the left side and arteries at the right side.
Anterior aspect of the brainstem with lines marking the pons, oculomotor nerve III, trochlear nerve IV, hypoglossal nerve, pyramid, accessory nerve XI, facial nerve VII, glossopharyngeal nerve IX, etc.
Posterior aspect of the brainstem with lines marking the midbrain, pineal body, medulla oblongata, floor of IVth ventricle, cerebellar peduncles, superior colliculus, and inferior colliculus.

Fig. 234 (a) The base of the brain showing the cranial nerve roots and their relationships to the circle of Willis. (b) Anterior aspect of the brainstem. (c) Posterior aspect of the brainstem.

The anterior surface of the medulla is grooved by an anteromedian fissure, on either side of which are longitudinal prominences termed pyramids which contain the pyramidal tracts. These pyramids, in turn, are separated from the olivary eminences by the anterolateral sulcus along which the rootlets of the XIIth cranial nerve emerge. Emerging from the posterolateral aspect of the olivary eminences are the rootlets of cranial nerves IX, X and XI. The posterior median sulcus of the cord is continued halfway up the medulla, where it widens out to form the inferior limits of the diamond‐shaped outline of the 4th ventricle. On either side of the sulcus the posterior columns of the spinal cord expand to form two distinct tubercles, corresponding to the gracile and cuneate nuclei.

The blood supply of the medulla is derived from the vertebral arteries directly and from their posterior inferior cerebellar branches.

The pons

External features (Fig. 234)

The pons lies between the medulla and the midbrain and is connected to the right and left cerebellar hemispheres by the right and left middle cerebellar peduncles, respectively. It is 2.5 cm (1 in) in length and 3.8 cm (1.5 in) in width. Its ventral surface presents a shallow, vertical median groove and numerous transverse ridges, which are continuous laterally on either side with the corresponding middle cerebellar peduncle. The dorsal surface of the pons forms part of the floor of the 4th ventricle. Ventrally, its junction with the medulla is marked close to the midline by the emergence of the VIth cranial nerves and, in the angle between the pons and the cerebellum, by the VIIth and VIIIth cranial nerves. Both the motor and sensory roots of the Vth cranial nerve leave the lateral part of the pons near its upper border.

The blood supply of the pons is derived from multiple small pontine branches of the basilar artery (Fig. 234), which in turn is formed by the confluence of the two vertebral arteries.

The midbrain

The midbrain is the shortest part of the brainstem; it is just under 2.5 cm (1 in) long and connects the pons and cerebellum to the diencephalon. It lies within the tentorial hiatus (the gap in the tentorium cerebelli) and is largely hidden by the surrounding structures.

External features (Fig. 234)

The only parts of the midbrain visible from the ventral aspect of the brain are the two cerebral peduncles, which emerge from the substance of the cerebral hemisphere and pass downwards and medially, connecting the internal capsule to the pons. The right and left oculomotor nerves (cranial nerve III) emerge between the two cerebral peduncles in the interpeduncular fossa. Viewed from the lateral aspect, the midbrain is seen to consist of three distinct portions: the basis pedunculi ventrally, the midbrain tegmentum centrally and the tectum dorsally. The trochlear nerve (IV), the optic tract and the posterior cerebral artery wind around this aspect of the midbrain. The dorsal aspect of the midbrain (Fig. 234c) presents the four colliculi (or corpora quadrigemini) and the superior medullary velum between the two superior cerebellar peduncles. The pineal body is a midline structure. It rests between the two superior colliculi and is attached by a stalk to the posterior part of the thalamus. The pineal secretes melatonin and has an important role in setting the circadian rhythm. When calcified, the pineal gland is easily identified on skull radiographs. It may then give the important radiological sign of lateral displacement by a space‐occupying lesion of the cerebral hemisphere.

The cerebellum

External features (Fig. 234; see Fig. 236)

The cerebellum is the largest part of the hindbrain and occupies most of the posterior cranial fossa. It is made up of the right and left cerebellar hemispheres and a median vermis. Inferiorly, the vermis is clearly separated from the two hemispheres and lies at the bottom of a deep cleft, the vallecula; superiorly, it is marked off from the hemispheres only as a low median elevation. A small ventral portion of each cerebellar hemisphere lies on the middle cerebellar peduncle of its side, and is almost completely separated from the rest of the cerebellum. It is termed the flocculus. The surface of the cerebellum is divided into numerous narrow folia and, by a few deep fissures, into a number of lobules. The effect of this fissuring is to give the cerebellum in section the appearance of a many‐branched tree (the arbor vitae) (see Fig. 243).

Internal structure

The structure of the cerebellum is remarkably uniform. It consists of a cortex of grey matter (in which all the afferent fibres terminate) covering a mass of white matter, in which deep nuclei of grey matter are buried.

The cerebellum is connected to the brainstem by way of three pairs of cerebellar peduncles. The inferior peduncles connect it to the dorsolateral aspect of the medulla; the middle cerebellar peduncles to the pons; and the superior peduncles to the caudal midbrain. Ventrally, the cerebellum is related to the 4th ventricle and to the medulla and pons; laterally, to the sigmoid venous sinus and the mastoid antrum and air cells; and posterosuperiorly, it is separated from the cerebral hemispheres by the tentorium cerebelli.

The blood supply of the cerebellum is derived from three pairs of arteries (Fig. 212); the posterior inferior cerebellar branches of the vertebral arteries supply the posterior aspect of the vermis and hemispheres, and the anterior inferior and superior cerebellar branches of the basilar artery supply the anterolateral part of the undersurface and the superior aspect of the cerebellum, respectively.

Functions of the cerebellum

The principal function of the cerebellum is to regulate and maintain balance, and to co‐ordinate timing and precision of body movements. The cerebellum has multiple connections with the cerebral cortex, reticular formation in the brainstem, thalamus and vestibular nuclei. Through these intricate connections, the cerebellum constantly monitors proprioceptive sensory input from joints, muscles and tendons, and accordingly refines and co‐ordinates the contractions of skeletal muscles.

However, unlike the cerebral cortex of the primary motor area, the cerebellum is incapable of initiating movement, nor is the cerebellum involved in the conscious perception of somatic or visceral sensations.

The diencephalon

The diencephalon comprises principally the hypothalamus and thalamus, which are continuous with each other. A vertically disposed, median, cleft‐like space is present between the right and left halves of the diencephalon, and is called the 3rd ventricle (Fig. 235). In addition to the thalamus and hypothalamus, the diencephalon includes two small but functionally important regions: the epithalamus and ventral thalamus. The epithalamus is the dorsal portion of the diencephalon and contains the pineal body. The ventral thalamus (also known as the subthalamus) contains the subthalamic nucleus, which is one of the basal ganglia. It is believed to be the main regulator and modulator of the other basal ganglia, and thus is a significant influence on motor activity.

The thalamus and 3rd ventricle in coronal section of the brain with lines marking the corpus callosum, lateral ventricle, septum pellucidum, choroid plexus, optic tract, infundibulum, hypothalamus, etc.

Fig. 235 The thalamus and 3rd ventricle in coronal section.

The hypothalamus (Figs 234a, 235)

The hypothalamus forms the floor of the 3rd ventricle, and also the lower part of its lateral wall. Viewed from below, the hypothalamus is seen to include, from before backwards, the optic chiasma, the tuber cinereum, the infundibular stalk (leading down to the posterior lobe of the pituitary), the mamillary bodies and the posterior perforated substance. In each of these there is a number of cell masses or nuclei and a fibre pathway – the medial forebrain bundle – which runs throughout the length of the hypothalamus and serves to link it with the midbrain postero‐inferiorly and the basal forebrain areas anterosuperiorly.

Sherrington described the hypothalamus as the head ganglion of the autonomic system. It is largely concerned with autonomic activity and can be divided into a posteromedial sympathetic area and an anterolateral area concerned with parasympathetic activity.

The hypothalamus plays an important role in endocrine control by the formation of releasing factors or release‐inhibiting factors. These substances, following their secretion into the hypophyseal portal vessels, influence the production by the cells of the anterior pituitary of adrenocorticotrophin, follicle‐stimulating hormone, luteinizing hormone, prolactin, somatotrophin, thyrotrophin and melanocyte‐stimulating hormone.

The hormones oxytocin and vasopressin (antidiuretic hormone) are produced by two distinct aggregations of neurones (the paraventricular and supra‐optic nuclei, respectively) in the hypothalamus and released at their axon terminals in the posterior pituitary. Oxytocin and antidiuretic hormone are thus neurosecretions.

In addition to its major influence on the autonomic nervous system and on pituitary function, the hypothalamus also plays a significant role in temperature regulation, water and electrolyte balance, regulation of appetite and sleep–wake patterns.

The thalamus (Fig. 235; see Fig. 237)

The thalamus is an oval mass of grey matter that forms the upper part of the lateral wall of the 3rd ventricle; it extends from the interventricular foramen rostrally to the midbrain caudally. Laterally, it is related to the internal capsule (and through it to the basal ganglia), and dorsally to the floor of the lateral ventricle. Medially, it is frequently connected with its fellow of the opposite side through the massa intermedia (interthalamic connexus). Posteriorly, it presents three distinct eminences, the pulvinar, and the medial and lateral geniculate bodies, the last two are the thalamic relay nuclei of hearing and vision, respectively.

The thalamus is the principal sensory relay nucleus that projects impulses from the main sensory pathways onto the cerebral cortex. It does this via a number of thalamic radiations in the internal capsule.

The blood supply of the thalamus is derived principally from the posterior cerebral artery through its thalamostriate branches, which pierce the posterior perforated substance to supply also the posterior part of the internal capsule. Thalamic damage, by occlusion of this blood supply, results in contralateral sensory loss of the face and body.

The pituitary gland (hypophysis cerebri)

This is an example of a ‘two‐in‐one’ organ, of which nature appears so keen; compare the two glandular components of the suprarenal cortex and medulla, and the exocrine and endocrine parts of the pancreas, testis and ovary. The pituitary comprises a larger anterior and smaller posterior lobe, the latter connected by the hollow infundibulum (pituitary stalk) to the tuber cinereum in the floor of the 3rd ventricle. The two lobes of the pituitary are connected by a narrow zone termed the pars intermedia.

The pituitary lies in the cavity of the pituitary fossa covered over by the diaphragma sellae, which is a fold of dura mater. This fold has a central aperture through which passes the infundibulum. Below the pituitary is the body of the sphenoid, laterally on either side lies the corresponding cavernous sinus and its contents separated by dura mater (Fig. 215), with intercavernous sinuses running in front, behind and below the pituitary. The optic chiasma lies above, immediately in front of the infundibulum.


The anterior lobe is extremely cellular and consists of chromophobe, eosinophilic and basophilic cells. The pars intermedia contains large colloid vesicles reminiscent of the thyroid. The posterior lobe is made up of nerve fibres whose cell bodies lie in the hypothalamus.


The posterior lobe is a diverticulum of the diencephalon. The anterior lobe and the pars intermedia develop from Rathke’s pouch in the roof of the embryonic buccal cavity. Occasionally, a tumour grows from remnants of the epithelium of this pouch (craniopharyngioma). These tumours are often cystic and calcified.

Owing to their strikingly different histological appearances and different embryological origins, the anterior and posterior lobes of the pituitary are also referred to as the adenohypophysis and neurohypophysis, respectively.

The cerebral hemispheres

The cerebral hemispheres which, in man, have developed out of all proportion to the rest of the brain, comprise the cerebral cortex, the basal ganglia, and their afferent and efferent connections. The lateral ventricles, containing cerebrospinal fluid (CSF), are at their centre.

The cerebral cortex

The cortex of the cerebral hemispheres is divided on topographical and functional grounds into four lobes – frontal, parietal, temporal and occipital (Fig. 236).

Lateral aspect of the brain depicting the localization of function in the cerebral cortex, with lines marking the motor area, central sulcus, sensory area, visual area, cerebellum, auditory area, etc.
Lateral aspect of the brain depicting the localization of function in the cerebral cortex, with lines marking the sensory, motor, and visual areas; and cingulate, calcarine, and parietooccipital sulci.

Fig. 236 Localization of function in the cerebral cortex. (a) Lateral aspect. (b) Medial aspect.

Frontal lobe

This includes all the cortex anterior to the central sulcus of Rolando. Its important cortical areas are as follows:

  1. The motor cortex. The primary motor area occupies a large part of the precentral gyrus. It receives afferents from the premotor cortex, thalamus and cerebellum and is concerned with voluntary movements. Stimulation of this area results in discrete muscle movements. Details of localization of function in the motor cortex are considered on page 372.
  2. The premotor cortex. This lies anterior to the precentral gyrus and the adjoining lower part of the frontal gyri. It, too, is concerned with voluntary movement, but its stimulation results in movements of groups of muscles with a common function, and thus lacks the precise definition of muscle movement that is a feature of stimulation of the precentral gyrus.
  3. Eye motor field. This lies in the middle frontal area anterior to the premotor cortex. Lesions of this area result in impaired eye movement with deviation of gaze to the side of the lesion.
  4. Broca’s speech area. Lesions of the area around the posterior part of the inferior frontal gyrus of the dominant hemisphere (the left hemisphere in 98% of individuals) were shown by Broca to affect the motor element in speech.
  5. Frontal association cortex (clinically called the prefrontal cortex). This comprises a considerable part of the frontal lobe and is one of the remarkable, evolutionary advances seen in the human brain. Its afferents are derived from the thalamus, limbic area and also from other cortical areas; it probably sends efferents to the thalamus and hypothalamus. From a functional point of view the lateral aspect of the frontal lobe appears to be related to ‘intellectual activity’ (i.e. cognitive functions – analysis, judgement and planning), the medial and orbital surfaces to affective (or emotional) behaviour and the control of autonomic activity.

Parietal lobe

The parietal lobe is bounded anteriorly by the central sulcus and behind by a line drawn from the parieto‐occipital sulcus to the posterior end of the lateral (Sylvian) sulcus. The important cortical areas of the parietal lobe are as follows.

  1. The primary somatosensory cortex. The postcentral gyrus receives afferent fibres from the thalamus and is concerned with all forms of somatic sensation. Details of localization along the sensory cortex are considered on pages 370 and 371.
  2. The parietal association cortex, comprising the remainder of the parietal lobe, is concerned largely with the recognition of somatic sensory stimulation and its integration with other forms of sensory information. It also receives afferents from the thalamus and, when damaged, gives rise to more complex defects than simple loss of sensation. An example of this is the inability to recognize somatic stimuli, which is called astereognosis; put a pen or a coin in the patient’s hand – he is aware of the object but is unable to recognize what it is. The lower part of the parietal lobe in the subject’s dominant hemisphere interacts with the somatosensory visual and auditory associations and has a key role in language.

The temporal lobe

This is arbitrarily separated from the occipital lobe by a line drawn vertically downwards from the upper end of the lateral sulcus.

The important cortical areas of the temporal lobe are the following.

  1. The auditory cortex. This lies in the superior temporal gyrus on the lateral and superior surfaces of the hemisphere. Its afferent fibres are from the medial geniculate body and it is concerned with the perception of auditory stimuli.
  2. The temporal association cortex. The area surrounding the auditory cortex is responsible for the recognition of auditory stimuli and for their integration with other sensory modalities. Lesions of this area result in auditory agnosia, i.e. the inability to recognize or to understand the significance of meaningful sounds. The cortical region just above and behind this area on the dominant hemisphere (Wernicke’s area) is of considerable importance in the sensory aspects of language comprehension. This visual area of the occipital lobe connects with the temporal lobe and is concerned with visual recognition. The antero‐inferior aspect of the frontal lobe connects with the medial aspect of the temporal lobe and is concerned with behaviour.

The parahippocampal gyrus  The cortex of the most medial part of the undersurface of the temporal lobe is known as the parahippocampal gyrus, much of which is referred to as the entorhinal cortex. It receives widespread association cortical afferents and is a significant source of inputs to the hippocampus. Anteriorly, it is related to the olfactory cortex of the uncus. Medially, it is in direct continuity with the layer of in‐rolled cortex which is the hippocampus and which is one of the most important sources of afferents to this structure. The hippocampus occupies the whole length of the floor of the inferior horn of the lateral ventricle and extends to the amygdala. It sends its efferents into the overlying layer of white matter known as the alveus. The fibres of the alveus collect on the medial margin of the hippocampus to form a compact bundle, the fimbria, which, as it arches under the corpus callosum, becomes known as the fornix. The fornix passes forwards and then downwards in front of the interventricular foramen and finally backwards into the hypothalamus to terminate in the mamillary body. It also gives fibres to the thalamus and the hypothalamus.

Projection of the hippocampus to the hypothalamus is part of the limbic system. This is an important substrate for emotions, behaviour and memory. The circuit is completed by projections of the hypothalamus to the thalamus, from the thalamus to the cingulate gyrus and from thence back to the hippocampus. Bilateral hippocampal damage results in inability to form new long‐term memories.

The amygdaloid nuclear complex  The amygdaloid nuclear complex is also a prominent temporal lobe structure, situated immediately rostral to the hippocampus. It is conveniently divided into three groups of nuclei: corticomedial, central and basolateral, which receive largely olfactory, gustatory and association cortical afferents, respectively. These divisions also have more or less separable projections to the hypothalamus and septum, brainstem autonomic centres and ventral striatum. The amygdala is involved in the control of emotional behaviour and conditioned reflexes. Its neuroanatomical connections are clearly appropriate for such a role, since it is in a position to affect emotional responses in endocrine, autonomic and motor domains. Destruction of the amygdala is particularly associated with reduced aggressive behaviour, whereas the very high density of benzodiazepine receptors here has suggested amygdaloid involvement in anxiety and stress and their treatments.

Occipital lobe

The occipital lobe lies behind the parietal and temporal lobes. On its medial aspect it presents the Y‐shaped calcarine and postcalcarine sulci (Fig. 236). The following cortical areas are noteworthy.

  1. The visual cortex surrounds the calcarine and postcalcarine sulci and receives its afferent fibres from the lateral geniculate body of the thalamus of the same side; it is concerned with vision of the opposite (contralateral) half field of sight (Fig. 237).
  2. The occipital association cortex lies anterior to the visual cortex. This area is particularly concerned with the recognition and integration of visual stimuli.
The basal ganglia and internal capsule in horizontal section through the cerebrum, with lines marking the lateral ventricle, corpus callosum, caudate nucleus, fornix, pyramidal tract, thalamus, etc.

Fig. 237 The basal ganglia and internal capsule shown in horizontal section through the cerebrum.

The insula (Figs 235, 237)

If the lips of the lateral sulcus are separated, it is seen that there is a considerable area of cortex buried in the floor of this sulcus. This area is known as the insula of Reil. It is divided into a number of small gyri and is crossed by the middle cerebral artery. Apart from its upper part, which abuts on the sensory cortex and probably represents the taste area of the cerebral cortex, the function of the insula is unknown. Its stimulation excites visceral effects such as belching, increased salivation, gastric movements and vomiting.

The connections of the cerebral cortex

As has been indicated, most areas of the cerebral cortex receive their main afferent input from the thalamus, but, in addition to this, there are well‐established commissural connections with the corresponding area of the opposite hemisphere by way of the corpus callosum. Intracortical association fibres also link neighbouring cortical areas on the same side and, in some cases, connect distant cortical areas; thus, the frontal, occipital and temporal lobes within the same hemisphere are directly connected by long association pathways.

The basal ganglia (Figs 235, 237)

These compact masses of grey matter are situated deep in the substance of the cerebral hemisphere and comprise the corpus striatum (composed of the caudate nucleus, the putamen and the globus pallidus) and the claustrum. Together with the cerebellum, they are involved in co‐ordination and control of movement.

The corpus striatum

The caudate nucleus is a large homogeneous mass of grey matter consisting of a head, anterior to the interventricular foramen and forming the lateral wall of the anterior horn of the lateral ventricle; a body, forming the lateral wall of the body of the ventricle; and an elongated tail, which forms the roof of the inferior (temporal) horn of the ventricle. It is largely separated from the putamen by the internal capsule, but the two structures are connected anteriorly. The putamen is a roughly ovoid mass closely applied to the lateral aspect of the globus pallidus; together, they are called the lentiform nucleus. The corpus striatum receives afferent connections from the cerebral cortex and sends efferents to the globus pallidus. From there, fibres project to the thalamus and, thence, back to the premotor cortex. Dopaminergic fibres project from the substantia nigra to the corpus striatum and efferent fibres also pass to the thalamus, hypothalamus, red nucleus, substantia nigra and the inferior olivary nucleus.

The long ascending and descending pathways

The somatic afferent pathways (Fig. 238)

  1. Proprioceptive and tactile impulses pass uninterruptedly through the posterior root ganglia, through the ipsilateral posterior columns of the spinal cord to the gracile and cuneate nuclei in the lower part of the medulla. In the posterior columns there is a fairly precise organization of the afferent fibres; those from sacral and lumbar segments are situated medially in the tracts whereas fibres from thoracic and cervical levels are successively added to their lateral aspect. This arrangement according to body segments is maintained in the gracile and cuneate nuclei and in the efferents from these nuclei to the contralateral thalamus. The fibres arising from the gracile and cuneate nuclei immediately cross over to the opposite side in the sensory decussation of the medulla and continue up to the thalamus as a compact contralateral bundle – the medial lemniscus.
  2. Dorsal root fibres subserving pain and temperature, together with some tactile afferents, end ipsilaterally in the substantia gelatinosa of the posterior horn. They then synapse and cross to the contralateral anterior lateral columns of the cord and are relayed to the contralateral thalamus. The fibre crossing occurs in the anterior white commissure of the spinal cord. In the brainstem these fibres come to lie immediately lateral to the medial lemniscus and are sometimes known as the spinal lemniscus (Fig. 238). They terminate in the thalamus.
The long ascending pathways of the dorsal columns and spinothalamic tracts, with 2 discretely shaded lines from the cord passing through medulla, to pons, to midbrain, and to cerebrum.

Fig. 238 The long ascending pathways of the dorsal columns (yellow lines) and spinothalamic tracts (red lines).

These somatic afferents are relayed from the thalamus, through the posterior limb of the internal capsule (Fig. 237) to the somatic sensory cortex of the postcentral gyrus. In the internal capsule the fibres are arranged in the sequence ‘face, arm, trunk and leg’ from before backwards, and this segregation persists in the sensory cortex, where the leg is represented on the dorsal and medial part of the cortex, the trunk and arm in its middle portion and the face most inferiorly. Since the size of the area of cortical representation reflects the density of the peripheral innervation and hence complexity of the function being performed rather than the area of the receptive field, there is a good deal of distortion of the body image in the cortex, the cortical representation of the face and hand being much greater than that of the limbs and trunk.

The motor pathways (Fig. 239)

The long descending pathway of the pyramidal tract starting from the cerebrum to midbrain, to pons, to medulla, and to cord.

Fig. 239 The long descending pathway of the pyramidal tract.

It is customary to divide the motor pathways of the brain and spinal cord into pyramidal and extrapyramidal systems. Although the latter is an imprecise term, it nevertheless provides a useful collective term for the many motor structures not confined to the pyramidal tracts in the medulla.

The pyramidal tract  The pyramidal system is the main ‘voluntary’ motor pathway and derives its name from the fact that projections to the motor neurones in the spinal cord are grouped together in the medullary pyramids. The fibres in this pathway arise from a wide area of the cerebral cortex. About two‐thirds derive from the motor and premotor cortex of the frontal lobes; however, about one‐third arise from the primary somatosensory cortex. In both the motor and premotor cortex there is an organization comparable to that seen in the sensory area. Again, the body is inverted so that the ‘leg area’ is situated in the dorsomedial part of the precentral gyrus encroaching on the medial surface of the hemisphere, supplied by the anterior cerebral artery. The ‘face area’ is near the lateral sulcus, while the ‘arm area’ occupies a central position, both supplied by the middle cerebral artery. Again, the body image is greatly distorted; the areas representing the hand, lips, eyes and foot are exaggerated out of proportion to the rest of the body and in accordance with the complexity of the tasks they perform.

From the cortex, the motor fibres pass through the posterior limb of the internal capsule (Fig. 237) where they are again organized in the sequence of ‘face, arm, leg’, anteroposteriorly. From the internal capsule the fibres form a compact bundle that occupies the central third of the cerebral peduncle. From there they pass through the ventral pons, where they are broken up into a number of small bundles between the cells of the pontine nuclei and the transversely disposed pontocerebellar fibres. Near the lower end of the pons they again collect to form a single bundle, which comes to lie on the ventral surface of the medulla and forms the elevation known as the ‘pyramid’. As it passes through the brainstem, the pyramidal system gives off, at regular intervals, contributions to the somatic and branchial arch efferent nuclei of the cranial nerves. Most of these corticobulbar fibres cross over in the brainstem, but many of the cranial nerve nuclei are bilaterally innervated.

Near the lower end of the medulla the great majority of the pyramidal tract fibres cross over to the opposite side and come to occupy a central position in the lateral white column of the spinal cord. This is the so‐called ‘crossed pyramidal tract’ shown in Fig. 245. A small proportion of the fibres of the medullary pyramid, however, remain uncrossed until they reach the segmental level at which they finally terminate. This is the direct or uncrossed pyramidal tract, which runs downwards close to the anteromedian fissure of the cord, with fibres passing from it at each segment to the opposite side.

In view of the frequent involvement of the pyramidal tract in cerebrovascular accidents, its blood supply is listed here in some detail:

  • motor cortex – leg area: anterior cerebral artery; face and arm areas: middle cerebral artery;
  • internal capsule – branches of the middle cerebral artery;
  • cerebral peduncle – posterior cerebral artery;
  • pons – pontine branches of basilar artery;
  • medulla – anterior spinal branches of vertebral artery;
  • spinal cord – segmental branches of anterior and posterior spinal arteries.

The extrapyramidal motor system  This should, by definition, include all those motor projections that do not pass physically through the medullary pyramids. It was once thought to control movement in parallel with and, to a large extent, independently of the pyramidal motor system and the pyramidal/extrapyramidal division was used clinically to distinguish between two motor syndromes: one characterized by spasticity and paralysis whereas the other involved involuntary movements, or immobility without paralysis. It is now clear that many ‘extrapyramidal’ structures, particularly the basal ganglia, actually control movement by altering activity in the premotor cortex and, thus, the pyramidal motor projections. This clearly emphasizes the blur between the two systems.

Components of the extrapyramidal system include the red nuclei, vestibular nuclei, superior colliculus and reticular formation in the brainstem, all of which project via discrete pathways to influence spinal cord motor neurones. Cerebellar projections (see page 361) are also included since they influence not only these brainstem motor pathways but also the motor cortex itself via the dentatothalamic projection.

Perhaps the most important structures to retain an extrapyramidal definition are the basal ganglia (see page 370). The neostriatum (caudate and putamen) receives widespread cortical afferents, including those from high‐order sensory association and motor areas, and projects mainly to the globus pallidus. The latter nucleus is the major outflow for the basal ganglia and, via the ventral anterior thalamus, exerts its major influence on premotor and hence the motor cortices. This pattern of connections suggests that the basal ganglia are involved in complex aspects of motor control, including motor planning and the initiation of movement.

A variety of motor disorders are associated with basal ganglia pathology and, in some instances, neuroanatomically discrete deficits in specific neurotransmitters. For example, Parkinson’s disease involves the degeneration of dopaminergic neurones in the substantia nigra in the midbrain. This pigmented nucleus provides the neostriatum with a dense dopaminergic innervation which may be completely lost in severe cases of Parkinsonism. Knowledge of this selective chemical neuropathology has resulted in the development of a treatment of the disease which involves the oral administration of the dopamine precursor L‐dopa.

The membranes of the brain and spinal cord (the meninges)

Three concentrically arranged membranes, known as meningeal layers or meninges, surround the brain and spinal cord. From outside in, these three layers are the dura mater, arachnoid mater and pia mater.

The dura is a dense membrane which, within the cranium, is often described as being made up of two layers. This description requires some qualification. The so‐called outer layer of the dura is intimately adherent to the inner surface of the skull; the inner layer, which is the true dural layer, is for the most part fused with the outer layer except where the two layers are separated by the intracranial dural venous sinuses and where the inner layer projects inwards to form four, prominent, reduplicated sheets (Fig. 214):

  • the falx cerebri;
  • the falx cerebelli;
  • the tentorium cerebelli;
  • the diaphragma sellae.

These dural reduplications serve to compartmentalize the cranial cavity and act as partitions between different parts of the brain, thereby performing the hugely important function of minimizing the considerable torsional stresses to which the brain is normally subjected.

The arachnoid is a delicate membrane applied, throughout, to the inner surface of the dura, and separated from the dura by the potential subdural space.

The pia is closely moulded to the surface of the brain and spinal cord. It dips down into all the cerebral sulci. The interval between the pia mater and the overlying arachnoid is termed the subarachnoid space. This space contains cerebrospinal fluid and is traversed by trabeculae of fine fibrous strands that run from the arachnoid to the pia.

The ventricular system and the cerebrospinal fluid circulation

The CSF is formed by the secretory activity of the epithelium covering the choroid plexuses in the lateral, 3rd and 4th ventricles; it circulates through the ventricular system of the brain and drains into the subarachnoid space from the roof of the 4th ventricle before being reabsorbed into the dural venous system.

The general appearance of the ventricular system is indicated in Fig. 240. The two lateral ventricles, which are by far the largest components of the system, occupy a considerable part of the cerebral hemispheres. Each has an anterior horn (in front of the interventricular foramen), a body, above and medial to the body of the caudate nucleus, a posterior horn in the occipital lobe and an inferior horn reaching down into the temporal lobe. The choroid plexuses of the lateral ventricles, which are responsible for the production of most of the CSF, extend from the inferior horn, through the body, to the interventricular foramen where they become continuous with the plexus of the 3rd ventricle (Fig. 235).

The ventricular system with lines marking the body of lateral ventricle, anterior horn, 3rd ventricle, 4th ventricle, interventricular foramen, posterior horn, aqueduct, and inferior horn.

Fig. 240 The ventricular system.

The 3rd ventricle is a narrow midline slit‐like cavity between the two halves of the diencephalon. Thus the upper part of the 3rd ventricle lies between the two halves of the thalamus, while the lower part of the ventricle is between the two halves of the hypothalamus. The floor of the 3rd ventricle is formed, from front to back, by the optic chiasma, tuber cinereum, infundibulum and mamillary bodies. The hypothalamus thus contributes to both the lateral wall and floor of the 3rd ventricle. From the 3rd ventricle the CSF passes through the narrow cerebral aqueduct (of Sylvius) in the midbrain to reach the 4th ventricle.

The 4th ventricle is diamond‐shaped when viewed from above and tentshaped as seen from the side. Its floor is formed caudally by the medulla and rostrally by the pons. Its roof is formed by the cerebellum and the superior and inferior medullary vela. The CSF escapes from the 4th ventricle into the subarachnoid space by way of the median and lateral apertures (of Magendie and Luschka, respectively) and then flows over the surface of the brain and spinal cord.

In certain areas the subarachnoid space is considerably enlarged to form distinct cisterns. The most important of these are the cisterna magna between the cerebellum and the dorsum of the medulla; the cisterna pontis over the ventral surface of the pons; the interpeduncular cistern between the two cerebral peduncles; the cisterna ambiens between the splenium of the corpus callosum and the superior surface of the cerebellum (containing the great cerebral vein and the pineal gland); and the chiasmatic cistern around the optic chiasma. Re‐absorption of CSF is principally by way of the superior sagittal sinus and to a lesser extent by the other dural venous sinuses, the modified arachnoid of the arachnoid granulations piercing the dura and bringing the CSF into direct contact with the sinus mesothelium. Along the superior sagittal sinus these granulations (or arachnoid villi) clump together to form the Pacchionian bodies, which produce the pitted erosions readily seen along the median line of the inner aspect of the skull cap.

About one‐fifth of the CSF is absorbed along similar spinal villi or escapes along the nerve sheaths into the lymphatics. This absorption of CSF is passive, depending on its hydrostatic pressure being higher than that of the venous blood.

Jun 28, 2019 | Posted by in ANATOMY | Comments Off on 6: The Nervous System

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