Overview of the nervous system

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Overview of the nervous system

This chapter provides an overview of the main structural and functional components of the nervous system and introduces the topographical anatomy of the brain and its protective coverings. Many of the concepts introduced here will be discussed in more detail in subsequent chapters.

Parts of the nervous system

Central and peripheral nervous systems

The nervous system is divided into central and peripheral parts (Fig. 1.1). The central nervous system (CNS) is made up of the brain and spinal cord, encased within the bones of the skull and vertebral column. The CNS represents the main integrating and decision-making centre of the nervous system. The peripheral nervous system (PNS) includes 31 pairs of spinal nerves which emerge between the vertebrae (Fig. 1.2) and 12 pairs of cranial nerves which arise from the base of the brain (Fig. 1.3). At the roots of the upper and lower limbs, sensory and motor fibres are redistributed in the brachial and lumbosacral plexuses to enter a number of named peripheral nerves (Figs 1.4 and 1.5). The function of the PNS is to transmit nerve impulses to and from the brain and spinal cord.

Fig. 1.1 Central and peripheral components of the nervous system.
The central nervous system (CNS) includes the brain and spinal cord. The peripheral nervous system (PNS) is made up of sensory and motor fibres passing to and from the CNS.

Fig. 1.2 Lateral view of the vertebral column in the lumbosacral region.
The mixed spinal nerves can be seen emerging from the intervertebral foramina.

Fig. 1.3 Ventral aspect of the brain illustrating the cranial nerves, which are conventionally labelled using Roman numerals.
The olfactory nerve (cranial nerve I) is not seen here; it consists of millions of filaments that arise from the nasal cavity and synapse in the olfactory bulb.

Fig. 1.4 The brachial plexus giving rise to the main named peripheral nerves of the upper limb.

Fig. 1.5 The lumbosacral plexus giving rise to the main named peripheral nerves of the lower limb.

The motor component of the peripheral nervous system is further subdivided into somatic and autonomic parts. The somatic nervous system innervates the skeletal musculature and is responsible for ‘voluntary’ (consciously initiated) actions. The autonomic nervous system supplies the internal organs and other visceral structures and operates at an automatic (mainly unconscious) level.

Autonomic nervous system

The autonomic nervous system (ANS) or visceromotor system regulates the activities of the cardiovascular, respiratory, digestive and urogenital systems. It operates autonomously (Greek: autos, self; nomos, governed by law). The ANS depends upon a constant stream of sensory information from the internal organs and blood vessels which it uses to regulate the contraction of cardiac and smooth muscle and the secretory activity of glands. Ultimate control of the autonomic nervous system resides in the hypothalamus, a tiny region at the centre of the brain that is sometimes referred to as the ‘head ganglion’ of the ANS (described later; see also Ch. 3).

Divisions of the ANS

The autonomic nervous system has two divisions that widely innervate the body and tend to have opposing effects on their target structures (Fig. 1.6):

Fig. 1.6 Comparison of the sympathetic and parasympathetic divisions of the autonomic nervous system.
The two limbs of the ANS tend to have opposing or complimentary effects on their target structures.

The sympathetic nervous system is most active during highly stressful situations that provoke intense fear or rage, typified by the ‘fight or flight’ response: e.g. dilated pupils, dry mouth, racing pulse. It is said to act ‘in sympathy’ with the emotions.

The parasympathetic nervous system generally has an antagonistic or complimentary effect on a given organ or tissue and is more associated with vegetative activities (‘resting and digesting’).

It is important to note that most structures are innervated by both divisions of the ANS and can be controlled independently (e.g. it is possible to dilate the pupils without increasing the heart rate); but the ‘fight or flight’ response is a useful way to remember the effects of the sympathetic division.

The two divisions of the ANS have distinct anatomical origins. Sympathetic fibres originate in the thoracic and upper lumbar segments of the spinal cord (T1–L2/3) as the thoracolumbar outflow (Fig. 1.7). This feeds into the sympathetic chain, on either side of the vertebral column, which provides sympathetic innervation to the entire body.

Fig. 1.7 The thoracolumbar outflow gives rise to the sympathetic division of the autonomic nervous system.
Sympathetic fibres from the thoracic and upper lumbar spinal cord (T1–L2/3) innervate the sympathetic chain, which is located on either side of the vertebral column. Superior and inferior extensions of the sympathetic chain provide sympathetic innervation to the entire body.

The parasympathetic fibres originate in cranial nerves III, VII, IX and X and the sacral spinal cord (S2–4). This constitutes the craniosacral outflow (Fig. 1.8). The vagus nerve (cranial nerve X) has a wide territory of distribution that includes most of the thoracic and abdominal viscera (Latin: vagus, wandering).

Fig. 1.8 The craniosacral outflow corresponds to the parasympathetic division of the autonomic nervous system.
The craniosacral outflow includes contributions from cranial nerves III, VII, IX and X and the pelvic splanchnic nerves which arise from segments S2–S4 of the sacral spinal cord. Note that the vagus nerve arises in the brain stem but descends in the neck (dashed blue line) to supply most of the thoracic and abdominal organs.

The term enteric nervous system refers to an extensive network of neurons within the wall of the gastrointestinal tract (Greek: enteron, intestine). This regulates the activity of digestive glands and coordinates the waves of peristalsis that propel food through the bowel lumen. Although it is a component of the ANS, it is sometimes regarded as a separate system since it is capable of coordinated activity, independent of the brain and spinal cord.

Key Points

The central nervous system (CNS) is composed of the brain and spinal cord, encased within the bones of the skull and vertebral column.

The peripheral nervous system (PNS) includes 12 pairs of cranial nerves and 31 pairs of spinal nerves which carry sensory and motor fibres to and from the CNS.

At the roots of the upper and lower limbs, sensory and motor fibres are redistributed in the brachial and lumbosacral plexuses to form a large number of mixed peripheral nerves.

The motor component of the peripheral nervous system can be separated into somatic (somatomotor) and autonomic (visceromotor) divisions.

The somatic nervous system innervates the voluntary (skeletal) musculature.

The autonomic nervous system (ANS) innervates smooth muscle, cardiac muscle and glands within the internal organs (viscera) and blood vessels. It is controlled by the hypothalamus.

The physiological actions of the sympathetic division of the ANS (originating from the thoracolumbar outflow) are typified by the ‘fight or flight’ response.

The parasympathetic division of the ANS (represented by the craniosacral outflow) generally has an opposing action on a given organ or tissue.

Cells of the nervous system

Neural tissue contains two specialized cell types: neurons and glia. These will be introduced here and discussed further in Chapter 5.

Nerve cells (neurons)

The basic structural and functional unit of the nervous system is the neuron or nerve cell. Neurons are electrically excitable cells that are highly specialized for the receipt, integration and transmission of information via rapid electrochemical impulses. A typical neuron is illustrated in Fig. 1.9.

The cell body ranges from 5–120 µm in diameter and contains the nuclear DNA and the biological machinery for protein synthesis and other housekeeping functions. Two types of process (or neurite) arise from the cell body. A profusely branching ‘tree’ of dendrites (Greek: dendron, tree) is specialized to receive and integrate information and may receive projections from many thousands of other neurons.

Nerve impulses are triggered in the cell body and transmitted away from the neuron along the slender nerve fibre or axon. A typical nerve cell has a single axon which may be up to two metres long in humans. Axons make contact with their target cells (and may branch to influence more than one target cell) at swellings called axon terminals. Axons often give rise to branches (or collaterals) which typically arise at right angles to the long axis of the nerve fibre.

The point of contact between two neurons is called a synapse (Greek: sunapsis, point of contact). Neurons influence effector structures such as muscle fibres and glandular tissue at neuroeffector junctions. The specific junction between a somatic motor neuron and a skeletal muscle fibre is the neuromuscular junction (NMJ) (see Ch. 4). The electrical activity of the target cell is influenced by neurotransmitters, chemical mediators that are released at the axon terminal in response to the arrival of a nerve impulse. Neurotransmitters are stored in membrane-bound organelles within the axon terminal called synaptic vesicles (see Ch. 7).

Neurons can be classified into three functional types (Fig. 1.10):

Fig. 1.10 Afferent neurons, interneurons and efferent neurons.
The vast majority of nerve cells in the central nervous system are interneurons (association neurons).

Afferent neurons carry nerve impulses towards the central nervous system. The term afferent fibre (or simply ‘afferent’) is also used to refer to any nerve process carrying impulses towards a particular structure. For example, ‘cortical afferents’ are fibres carrying impulses to the cerebral cortex. The term sensory neuron is usually reserved for afferent neurons conveying information that is to be consciously perceived.

Efferent neurons carry nerve impulses away from the central nervous system. Again, the term efferent fibre (or ‘efferent’) can be used to refer to any process carrying impulses away from a particular structure. The term motor neuron is best reserved for efferent neurons that are involved in pathways concerned with voluntary movement.

Association neurons (or interneurons) make up the vast network of interconnections within the brain and spinal cord. They have an integrative function, transforming sensory inputs into appropriate motor responses. The majority of neurons therefore fall into this category.

Neurons can also be classified by the number of processes that they have. The example shown in Fig. 1.9 is a multipolar neuron because it has several processes. Neurons with one or two processes are termed unipolar and bipolar respectively. The vast majority of neurons in the CNS are multipolar.

Grey and white matter

CNS tissue can be divided into grey and white matter (Fig. 1.11). Grey matter is composed mainly of neuronal cell bodies, dendrites and synapses. It is sharply demarcated from the adjacent white matter, which is made up of nerve fibres travelling to other parts of the nervous system. The pale colour of white matter is due to the lipid-rich myelin sheath that surrounds axons and enhances their conduction velocity (see Fig. 1.9; see also Chs 5 & 6). Discrete groups of neurons within the central nervous system are referred to as nuclei (singular: nucleus). In the peripheral nervous system, collections of neuronal cell bodies form aggregates that resemble knots on a piece of string. These are referred to as ganglia (singular: ganglion).

Neuroglial cells

In addition to neurons, the nervous system contains a variety of support cells that are known collectively as glia. These cells were originally thought to offer mainly physical support, literally holding nervous tissue together (Greek: glia, glue) but they are now known to carry out a wide range of important functions. The four main types of glial cell (discussed further in Chapter 5) are:

Astrocytes, which provide structural and metabolic support to neurons and help to regulate the exchange of molecules between the bloodstream and the brain.

Oligodendrocytes, which invest axons with a myelin sheath in the brain and spinal cord.

Schwann cells, the peripheral counterparts of oligodendrocytes.

Microglia, the resident phagocytic and immunocompetent cells of the central nervous system.

Although glial cells are relatively small, with an average diameter of around 4–8 µm, they outnumber neurons in a ratio of 10 : 1 and make up approximately 50% of the total volume of the CNS. Glial cells are able to undergo cell division (unlike neurons, which are post-mitotic cells).

Key Points

The neuron is the basic structural and functional unit of the nervous system. The main types are afferent (sensory), efferent (motor) and association (interneurons).

The dendritic tree is the afferent portion of the nerve cell, which may receive synaptic contacts from thousands of other neurons. Nerve impulses are generated in the cell body and transmitted to other nerve cells via the axon.

Neurons communicate with other nerve cells, muscle fibres and glands at synapses. The arrival of a nerve impulse at the axon terminal triggers the release of a chemical neurotransmitter which influences the target cell.

Grey matter is composed mainly of neuronal cell bodies, whereas white matter is made up of myelinated axons. Collections of nerve cells form nuclei in the CNS and ganglia in the PNS.

Astrocytes are the main type of support (glial) cell. They perform important metabolic functions and help to regulate the passage of molecules between the CNS and the bloodstream.

Axons are wrapped in a lipid-rich myelin sheath by central oligodendrocytes and peripheral Schwann cells. Myelin increases axonal conduction velocity.

Basic cerebral topography

The human brain is a pale pink organ with a mean mass of around 1.3 kg and a very soft, gelatinous consistency. It can be separated into three major parts – the cerebrum, cerebellum and brain stem (Fig. 1.12). The brain stem is further subdivided into the midbrain, pons and medulla oblongata.

Fig. 1.12 The human brain, illustrating the major subdivisions and some of the key anatomical landmarks.
**(A)** Lateral surface; **(B)** Median sagittal section. From Crossman: Neuroanatomy ICT 4e (Churchill Livingstone 2010) with permission.

The corpus striatum is the largest component of the basal ganglia. These are a collection of nuclei (the term ‘ganglia’ is a misnomer) that are involved in movement control and are affected in Parkinson’s disease (Ch. 13). They also contribute to cognition, emotion and behaviour. The amygdala is concerned with emotional responses (especially anxiety and fear). It has close links to the limbic lobe, part of the brain that is particularly concerned with emotion and memory.

The thalamus and hypothalamus belong to the diencephalon. This region lies at the centre of the brain, surrounding the cavity of the third ventricle and is normally hidden from view between the cerebral hemispheres (Greek: dia-, between; enkephalos, brain) (Fig. 1.16). The thalamus is known as the ‘gateway’ to the cerebral cortex, since most ascending sensory pathways relay in one of its nuclei in order to reach their cortical targets. The hypothalamus controls the ANS and endocrine system and is involved in the maintenance of homeostasis.

Pituitary gland

The pituitary gland is just beneath the hypothalamus, connected to it by the pituitary stalk. It is separated into anterior (glandular) and posterior (neural) portions. The pituitary gland is an endocrine structure which releases hormones that control growth, metabolism and sexual function. It mainly regulates other glands (rather than having a direct physiological effect) and is therefore referred to as the ‘master gland’ of the endocrine system. Its activity is controlled in turn by soluble mediators released by the hypothalamus. These reach the anterior lobe of the pituitary gland via a capillary network called the hypothalamo-pituitary portal system. The hypothalamus communicates with the posterior lobe more directly via a bundle of nerve fibres termed the hypothalamo-hypophyseal tract.

Hemispheric white matter

The subcortical white matter is composed of numerous interlacing tracts, which are groups of axons with a common origin, destination and function. Two or more tracts running in company make up a bundle or fasciculus (plural: fasciculi). There are three main types of white matter bundle in the cerebrum, illustrated in Figs 1.17–1.19:

Fig. 1.17 Coronal section of the cerebral hemispheres showing the main association, commissural and projection fibres in the subcortical white matter.
The corpus callosum is the main white matter connection between the cerebral hemispheres; the internal capsule is a massive white matter bundle consisting of fibres passing to and from the cerebral cortex.

Fig. 1.18 Lateral view of the cerebral hemisphere, illustrating white matter pathways.
Some of the major long association fibres are shown projected on to the surface of the cerebral hemisphere. The approximate position of the corpus callosum appears in transverse section; notice the genu and splenium, which give rise to fibres interconnecting the frontal lobes and the posterior parts of the cerebral hemispheres respectively.

Fig. 1.19 Axial (horizontal) section of the cerebral hemispheres showing the anterior and posterior limbs of the internal capsule passing through the deep grey matter.
The internal capsule (blue) is seen in transverse section and consists of ascending and descending axons passing to and from the cerebral cortex [to understand the orientation of the fibres, compare with Fig. 1.17]. The minor and major forceps are also seen in this view, which represent commissural fibres crossing the midline in the genu and splenium of the corpus callosum [compare with Fig. 1.18].

Association fibres interconnect parts of the same hemisphere. Short association fibres (subcortical ‘U-fibres’) loop between neighbouring gyri. Long association fibres link more distant areas within a particular hemisphere.

Commissural fibres cross the midline to connect with matching areas of the opposite hemisphere (Latin: commissūra, a joining together). The largest commissure is the corpus callosum, composed of some 200–300 million myelinated axons. The anterior and medial temporal lobes are linked by the much smaller anterior commissure (see Ch. 3, Fig. 3.25).

Projection fibres pass to and from the cerebral cortex. The internal capsule is a massive white matter system composed of 20 million projection fibres that pass through the corpus striatum.

The internal capsule contains the pyramidal tract (primary voluntary motor pathway). This originates in the motor areas of the frontal lobe and projects to the brain stem and spinal cord.

Key Points

The brain consists of the cerebrum, cerebellum and brain stem.

The cerebrum is dominated by the paired cerebral hemispheres which are responsible for personality, behaviour, language, intellect and emotion.

The cerebral cortex is folded to form convolutions (gyri) separated by furrows (sulci). The central and lateral sulci help to demarcate the frontal, parietal, temporal and occipital lobes.

The nuclei in the base of the cerebral hemisphere (basal nuclei) include the corpus striatum and amygdala. The corpus striatum is part of the basal ganglia and is involved in voluntary movement, cognition and emotion. The amygdala is concerned with emotional responses.

The diencephalon is hidden between the cerebral hemispheres, surrounding the third ventricle. It includes the thalamus (‘gateway to the cerebral cortex’) and hypothalamus.

The subcortical white matter contains: (i) short and long association fibres, linking cortical areas within a hemisphere; (ii) commissural fibres, which cross the midline to enter the opposite hemisphere; and (iii) projection fibres, passing to and from the cerebral cortex.

Cerebellum

The cerebellum (Latin: diminutive of cerebrum; cf. pig and piglet) is disproportionately large and well-developed in humans compared to other mammals. It clasps the brain stem from behind, forming the roof of the fourth ventricle. The cerebellum is involved in balance, muscle tone and coordination.

The cerebellum is composed of two large cerebellar hemispheres connected in the midline by the narrow vermis (which is said to resemble a segmented garden worm; Latin: vermis, worm) (Fig. 1.20A). The hemispheres and vermis are further subdivided into anterior and posterior lobes by the primary fissure. The cerebellum also contains a much smaller flocculonodular lobe that is composed of the paired flocculi (Latin: flocculus, tuft of wool) and the nodule of the vermis (see Chs 2 and 3).

Fig. 1.20 Gross anatomy of the cerebellum.
**(A)** Oblique view of the cerebellum from the posterolateral aspect, showing its intimate relationship to the brain stem. The division of the cerebellar hemispheres and vermis into anterior and posterior lobes is illustrated. The small flocculonodular lobe is not visible in this view (it can only be seen from the anterior aspect); **(B)** An oblique slice through the cerebellar hemisphere showing the cortex, white matter and dentate nucleus.

Like the cerebrum, the cerebellum also has a folded outer layer of grey matter overlying a central core of white matter. However, the cortex of the cerebellum is arranged in parallel ridges called folia (separated by creases called fissures) and has a comparatively simple three-layered structure. Slicing the cerebellar hemisphere reveals the dentate nucleus embedded in the subcortical white matter (Fig. 1.20B). This is the principal efferent (outflow) nucleus of the cerebellum.

Cerebellar peduncles

The cerebellum is attached to the brain stem (on each side) via three white matter bundles (Fig. 1.21):

Fig. 1.21 Posterior view of the brain stem with the left half of the cerebellum removed, exposing the diamond-shaped floor of the fourth ventricle.
The three cerebellar peduncles have been cut (on the left-hand side) and are therefore seen in transverse section. [NB: The inferior cerebellar peduncle ascends vertically through the medulla and then passes directly backwards into the cerebellum, making a 90-degree bend.]

The superior cerebellar peduncles are attached to the midbrain and contain a major projection from the dentate nucleus to the opposite thalamus and frontal lobe.

The middle cerebellar peduncles arise from the pons and represent the principal afferent supply to the cerebellar hemispheres, which mainly originates from the opposite frontal lobe (see below).

The inferior cerebellar peduncles are connected to the medulla and transmit afferent sensory information from the spinal cord.

The motor and premotor areas of the frontal lobe project to the basal pons (on the same side) via the frontopontine pathway. The pontine nuclei then give rise to transverse pontine fibres which project to the contralateral cerebellar hemisphere (via the middle cerebellar peduncle). Each frontal lobe thus projects to the contralateral cerebellar hemisphere. The cerebellum is therefore ‘uncrossed’ (with respect to the cerebrum) so that unilateral cerebellar damage affects coordination on the same (ipsilateral) side of the body.

Key Points

The cerebellum is composed of the cerebellar hemispheres and vermis. It is divided into a small anterior lobe and a large posterior lobe by the primary fissure.

Like the cerebrum, the cerebellum has an outer cortical layer (thrown into parallel ridges called folia) and a central core of white matter. The main efferent structure is the dentate nucleus.

The cerebellum is connected to the brain stem by three pairs of white matter bundles: the superior, middle and inferior cerebellar peduncles. These fibre systems connect to the midbrain, pons and medulla respectively.

The main functions of the cerebellum include the maintenance of balance, posture, muscle tone and coordination. Lesions cause ipsilateral incoordination.

Brain stem

The cerebrum and cerebellum are both attached to the brain stem, which is composed of the midbrain, pons and medulla oblongata (discussed separately in Ch. 3).

Taken as a whole, the brain stem can be divided into two longitudinal regions called the base and tegmentum (Fig. 1.22). In other animals, such as rodents, the base of the brain stem lies inferiorly and the tegmentum is above it (Latin: tegmentum, a covering). Since humans are bipedal (and stand upright) the ‘base’ is anterior to the tegmentum.

Fig. 1.22 Midsagittal view of the brain stem with the cerebellum still attached, illustrating the basal and tegmental regions.
Note that the midbrain also has a ‘roof plate’ (the tectum) that lies dorsal to the cerebral aqueduct, but there is no corresponding region in the pons or medulla.

The basilar region is composed mainly of descending axons, such as the pyramids of the medulla which contain the primary motor pathway or pyramidal tract. The tegmentum is the central core of the brain stem. It contains the nuclei of the lower ten cranial nerves (III–XII) and a diffuse network of neurons referred to as the reticular formation (described below). The brain stem transmits several long tracts, including ascending (sensory) and descending (motor) pathways.

Reticular formation

The reticular formation forms a polysynaptic network in the tegmentum of the brain stem. It contains the so-called vital centres (respiratory and cardiovascular) and mediates the airway-protective brain stem reflexes (e.g. cough, sneeze, gag). It also coordinates several stereotyped actions concerned with feeding, via connections with the cranial nerve nuclei. These include salivating, chewing, swallowing and vomiting. Other activities include control of: (i) bladder emptying (the micturition reflex); (ii) conjugate gaze (via the vertical and horizontal gaze centres of the midbrain and pons); and (iii) posture, muscle tone and gait.

The ascending reticular activating system (ARAS) is a diffuse projection that arises from the rostral brain stem. It receives afferents from each of the sensory systems and influences cortical excitability by release of excitatory neurotransmitters including acetylcholine and noradrenaline (Fig. 1.23). Activity in this system is influenced by general and special sensory afferents and is vital for maintaining wakefulness. For this reason, brain stem damage may result in coma (Clinical Box 1.1).

Clinical Box 1.1: Coma and brain stem death

Interference with the ascending reticular activating system (e.g. following brain stem injury) may cause coma. This is defined as a profound state of unconsciousness characterized by lack of responsiveness to external stimuli and absence of sleep–wake cycles. More severe damage can result in loss of spontaneous respiration and cardiovascular drive which is not compatible with survival. This is termed brain stem death. Artificial ventilation can prolong life, but if the brain stem is irreversibly damaged, recovery will not be possible.

Fig 1.23 The afferent and efferent projections of the ascending reticular activating system (ARAS).
These connections contribute to wakefulness and coma.

Diffuse neurochemical systems

A number of small brain stem nuclei give rise to extremely diffuse neurochemical projections that influence the entire CNS (Fig. 1.24). These diffuse modulatory systems release the neurotransmitters serotonin, noradrenaline, dopamine and acetylcholine via synaptic terminals distributed throughout the brain and spinal cord. The main pathways travel together between the brain stem and the cerebral hemisphere within the medial forebrain bundle.

The suffix ‘-ergic’ is used to refer to fibres releasing a common neurotransmitter such that those releasing serotonin are said to be ‘serotonergic’ and those releasing noradrenaline are referred to as ‘noradrenergic’. Coordinated activity in these diffuse projections influences neural functions on a global scale, including arousal, vigilance, sleep–wake cycles and mood (Clinical Box 1.2).

Clinical Box 1.2: Amines and depression

During the 1960s a theory emerged which aimed to explain depression as a disorder of neurotransmitter chemistry. It had been noted that some patients receiving the drug reserpine for the treatment of high blood pressure became depressed or even suicidal. It was also known that the mechanism of action of reserpine was to deplete neurons of amine neurotransmitters. Later, attempts to produce new agents for the treatment of tuberculosis lead to the development of the drug iproniazid which turned out to have mood-elevating properties. The mechanism appeared to be potentiation of endogenous amines via inhibition of a key enzyme responsible for their degradation (monoamine oxidase, MAO). Thus it was postulated that depression might represent a deficiency of CNS amines such as serotonin and noradrenaline, which came to be known as the biogenic amine hypothesis. Indeed, all modern anti-depressant agents appear to potentiate these neurotransmitter systems. However, despite rapid changes at central synapses, there is a delay of around 2–4 weeks before symptoms start to improve. This suggests that ‘knock-on’ effects in neuronal biochemistry may be responsible for the symptomatic benefit. Nevertheless, it is important to exercise caution when attempting to extrapolate a theory of disease causation from the mechanism of a drug used to treat it: analogous to repairing a pair of spectacles with a dab of glue and then concluding that they must have been ‘glue-deficient’.

Key Points

The brain stem is composed of the midbrain, pons and medulla.

The tegmentum of the brain stem contains the nuclei of cranial nerves III–XII, the reticular formation and several ascending (sensory) tracts. The base contains descending (motor) tracts.

The reticular formation maintains normal cardiorespiratory function, mediates airway-protective brain stem reflexes and is involved in wakefulness and coma.

The diffuse neurochemical systems arise from small nuclei in the brain stem and base of the cerebral hemisphere. They release the transmitters serotonin, noradrenaline, dopamine and acetylcholine throughout the CNS and contribute to mood, arousal and sleep–wake cycles.

Other brain stem centres mediate conjugate gaze, regulation of bladder emptying and the maintenance of normal muscle tone, posture and gait.

The spinal cord

The spinal cord is a slender continuation of the brain stem which is contained within the bony spinal canal. It is 40–50 cm in length, up to 1.5 cm in width and contains around a billion neurons. A lateral view of the spinal cord shows the cervical and lumbar enlargements which are required for the considerable sensory and motor supply to the upper and lower limbs (Fig. 1.25).

Fig. 1.25 The relationships between the vertebral column, spinal cord and spinal nerves.
The cervical nerves emerge above their corresponding vertebrae, apart from the C8 nerve which emerges below the seventh cervical vertebra. This is because there are eight cervical nerves but only seven vertebrae. All other spinal nerves (thoracic, lumbar and sacral) exit below their corresponding vertebrae. The spinal cord ends at the conus medullaris (at the level of the L1/L2 disc).

The spinal cord is significantly shorter than the vertebral column and terminates at the lower border of the first lumbar vertebra (L1/2) as the tapering conus medullaris. A consequence of this length-discrepancy is that the upper roots leave the cord horizontally, but the lower roots follow a progressively more oblique course. Below the conus medullaris, the roots form an almost-vertical leash called the cauda equina (Latin: horse’s tail).

The 31 pairs of spinal nerves are attached to the cord via the dorsal (sensory) and ventral (motor) roots which arise from a series of rootlets. Each dorsal root bears a dorsal root ganglion which contains the cell bodies of sensory neurons.

Internal anatomy

The spinal cord contains a central, H-shaped core of grey matter (with dorsal and ventral horns) that is surrounded by a thick layer of white matter. The spinal cord white matter is arranged in three longitudinal columns that contain ascending and descending pathways (Fig. 1.26).

Fig. 1.26 A transverse section through the lumbar spinal cord. Modified from Nolte: The Human Brain in Photographs and Diagrams 3e (Mosby 2007) with permission.

The posterior columns are located between the dorsal roots and are separated by the dorsal median sulcus. The anterior columns lie between the ventral roots and are separated by the more substantial ventral median fissure. The lateral columns are situated between the attachments of the dorsal and ventral nerve roots on each side of the cord.

Spinal reflexes contribute to normal muscle tone and mediate a number of simple motor responses (e.g. withdrawal from a painful stimulus). The spinal cord also contains more complex neuronal networks called central pattern generators (CPGs). These coordinate semi-automatic actions such as walking and are recruited and modulated by descending projections from the brain.

Key Points

The spinal cord is a continuation of the brain stem which occupies the bony spinal canal. It terminates at the lower border of the first lumbar vertebra as the conus medullaris.

The cervical and lumbar enlargements of the spinal cord reflect the considerable sensory and motor supply to the upper and lower limbs.

The spinal cord gives rise to the ventral (motor) roots and dorsal (sensory) roots which unite to form the mixed spinal nerves. The dorsal roots bear the dorsal root (sensory) ganglia.

The spinal cord transmits nerve impulses to and from the brain and mediates several important reflexes. It also coordinates more complex motor sequences (e.g. those required for walking).

Protective coverings of the CNS

The central nervous system is protected by the bones of the skull and spinal column and by three layers of investing membranes. In addition, a fluid-filled space around the brain and spinal cord provides a cushioning or ‘shock-absorbing’ effect.

The skull and vertebral column

The skull is composed of the cranium (which encases the brain), together with the facial skeleton and mandible (lower jaw). The brain lies within the cranial cavity, resting on the skull base. It is covered by the dome-like cranial vault. The bones that make up the skull vault are the calvaria (singular: calvarium) (Fig. 1.27). They include the frontal, parietal, occipital and temporal bones, after which the underlying lobes of the cerebral hemispheres are named. The calvarial bones unite at the cranial sutures, but this process is not complete until about 18 months of age.

Fig. 1.27 The bones of the skull vault (the calvaria) which encase the brain.
**(A)** Lateral view; **(B)** Superior view. The separate bones are united at the cranial sutures.

The skull base accommodates the ventral part of the brain and can be divided into three broad recesses or fossae (singular: fossa). The frontal and temporal lobes lie in the anterior and middle fossae respectively, whereas the brain stem and cerebellum occupy the posterior fossa. The brain stem becomes continuous with the spinal cord via a large opening in the skull base, the foramen magnum. The cranial fossae contain numerous openings by which the cranial nerves reach their target structures in the head and neck.

The spinal cord is protected by the vertebral column, which is composed of multiple separate vertebrae that align to form the hollow vertebral canal. The joints, ligaments and muscles attached to the vertebral column provide the necessary stability and protection for the spinal cord, whilst at the same time permitting a good deal of mobility (Fig. 1.28).

Fig. 1.28 Transverse section through the thoracic spine illustrating the relationships between the spinal cord, meninges and vertebral column.

Cranial and spinal meninges

In addition to its bony coverings, the central nervous system is invested by three layers of protective membranes, the meninges (singular: meninx) (Fig. 1.29).

Fig. 1.29 The relationship of the meninges to the surface of the brain.
Note that the cerebral arteries occupy the fluid-filled subarachnoid space and that penetrating blood vessels are invested by a sheath of pia mater as they plunge into the CNS tissue.

The outermost layer is the dura mater (Latin: dura, tough). This is a tough fibrous membrane that is tightly adherent to the inside of the skull and is fused with the periosteum. The dura extends through the foramen magnum to surround the spinal cord as the dural sac which terminates at the level of the second sacral vertebra (S2). The dural sac is separated from the bones of the vertebral column by an extradural venous plexus, which is embedded in fatty connective tissue.

Closely apposed and loosely attached to the dura is the arachnoid mater (Greek: arachnoid, resembling a cobweb). The dura-arachnoid represents a double layer that lines the skull and forms the dural sac.

The innermost membrane is the pia mater (Latin: pia, delicate). This is a thin layer of vascular connective tissue that is intimately associated with the surface of the brain and spinal cord, diving down into every sulcus and fissure. A sheath of pia mater also invests the arteries as they penetrate the substance of the brain and spinal cord.

Subarachnoid space

The arrangement of the three meningeal layers creates the subarachnoid space between the arachnoid and pia. The subarachnoid space is filled with cerebrospinal fluid (CSF) and is in continuity with the cerebral ventricles via three openings at the base of the brain (see Ch. 2). Thus the surface of the CNS is bathed in cerebrospinal fluid, which contains dissolved oxygen and glucose that helps to nourish neural tissues.

The subarachnoid space also has a protective role since it offers buoyancy and cushions against sudden head movements. Further support is provided by fine fibrous connections between the arachnoid and pia (the arachnoid trabeculae) which effectively suspend the brain like puppet strings. Since the CSF is intimately related to the brain and spinal cord it is sometimes sampled clinically to look for evidence of infection, metabolic derangements or tumours (Clinical Box 1.3).

Clinical Box 1.3: Lumbar puncture

A sample of cerebrospinal fluid can be helpful in the assessment of patients with infectious, inflammatory, metabolic or toxic CNS disorders. Tumours can also be diagnosed (very rarely) if the specimen contains malignant cells. The sample is obtained from the spinal subarachnoid space via a lumbar puncture (Fig. 1.30). A needle is inserted between the third and fourth lumbar vertebrae under local anaesthetic (well below L1/L2, where the spinal cord ends) and advanced through the dural sac into the subarachnoid space, pushing aside the nerve roots of the cauda equina. The CSF pressure is measured and approximately 10–20 mL of clear, colourless fluid is collected for analysis. Damage to the spinal cord or nerve roots is extremely rare and most patients complain only of a mild headache following the procedure (attributed to low CSF pressure).