Cytology of the normal central nervous system

 

 

 

INTRODUCTION

 

The central nervous system (CNS) is composed of various types of cell including neurons, glial cells (astrocytes, oligodendrocytes, ependyma and choroid plexus epithelium), blood vessel elements and microglia. Leptomeninges (pia mater and arachnoid mater) surround the CNS, and the pachymeninges (dura mater) form an outer coating which separates the brain from 'the skull and the spinal cord from the spine.

There are illustrations throughout subsequent chapters which depict many of the histological and macroscopic features of the nervous system. This chapter, however, is more concerned with tlte basic structure and function of the different cell types in the brain and spinal cord, an under-standing of which is a prerequisite for an appreciation of the pathology of the nervous system.

 

 

NEURONS

 

Nerve cells vary enormously in size and con-figuration. The cerebellum presents the best example of morphological extremes: the Purkinje cells and their impressive dendritic trees dwarf the neighbouring granule cells and their short dendrites (Figs 1.1 and 1.2).

The shape of the nerve cell body, the perikaryon, also varies throughout the central nervous system: pyramidal, polygonal, round, oval and fusiform forms can be distinguished. Neuronal cell processes fall into two categories: dendrites and axons. Whilst a neuron usualJy has only a single axon, the number of dendrites ranges from one to nearly one hundred. The length and arborisation of processes also display considerable individual variations. Silver impregnation techniques of classical neurohistology, electron microscopy, enzyme histochemistry and more recently immunocyto­chemistry have all contributed.. to the under-standing of neuronal morphology and function. The previous morphological classifications of nerve cells are now gradually being replaced by classifications based on functions: neurons can be distinguished according to the neurotransmitters they use.

 

 

The nucleus

 

Nerve cell nuclei are round or oval, although the

nuclear profile is occasionally indented. The nucico- or karyoplasm is separated frorn the cytoplasm by the nuclear membrane which is com­posed of two laminae enclosing the perinuclear cisterna: the inner lamina is smooth, whilst the outer one, studded with ribosomes, is frequently continuous with the rough endoplasmic reticulum. Finely granular chromatin is evenly dispersed with some clumping at the nuclear membrane.

The nucleolus is prominent and this helps to distinguish neurons from astrocytes which contain only a small inconspicuous nucleolus. Electron microscopy reveals the nucleolus to be composed of granular and fibrillar constituents. Attached to the nucleolus is the nucleolar satellite or sex chromatin: a dense, coarsely fibrillated body which occurs only in females.

Nuclear inclusions of various types, filamen­tous, granulofibrillar, tubular and vesicular,' often occur in normal neurons and can be occasion­ally discerned in the light microscope. Variations in nuclear morphology exist according to neuronal type, topography and functional activity. The large vesicular nuclei of the Purkin)e cells or the pyramidal cells of the cerebral cortex are in sharp contrast to the small, dense nuclei of the cerebellar granule cells .

 

Cytoplasmic organelles

 

Nissi substance

 

This cytoplasmic component is intensely baso­philic when stained with cresyl violet or methyl­ene blue. Electron microscopy shows that Nissl substance is composed of stacks of the rough-surfaced endoplasmic reticulum (RER) (Fig. 1.4). The cisternae are usually arranged in parallel arrays and their outer surface is covered by ribo­somes. The amount of RER varies from neuron to neuron and appears to be related to the length of the axon: it is particularly abundant in the anterior horn cells of the spinal cord and the pyramidal neurons of the cerebral cortex. It is present not only in the perikaryon but also in the dendrites, but it is absent from the axon hillock and from the axon itself. Since the RER is the site of protein synthesis, its amount is related to the metabolic

activity of the cell. The Nissl substance forms an integral component of the neuronal membrane system which is associated with the transfer of membrane molecules amongst organdies, the packaging and transport of exportable protein and energy metabolism.2 In addition to the membrane-bound particles of the RER, ribosomes are scattered throughout the cytoplasm either singly or in groups as polyribosomes or ribosomal rosettes.

 

 

the axon and dendrites. Although the precise function of the agranular reticulum is not known, its widespread distribution within the cytoplasm and its close spatial relationship to the cell membrane suggest a role in intracellular transport.

The internal reticular apparatus of Golgi appears as tortuous anastomosing strands in the light microscope. Located between the nucleus and the cell membrane or at the base of a dendrite, the Golgi apparatus is composed of arrays of closely packed, smooth-surfaced cisternae and numerous associated vacuoles and vesicles (Fig.

1.4). Its curved configuration allows a convex, external forming face and a concave, internal mature face to be distinguished. The internal face of the Golgi apparatus forms an integral part of the GERL system (Golgi apparatus, smooth endo­plasmic reticulum and lysosomes): a spatially closely related group of organelles which is devoted to the synthesising and condensing

activity of the Golgi apparatus.3 This polarisation of the Golgi apparatus, although not evident in all neurons, is thought to serve the efficient transport of materials. The demonstration of concanavalin A binding sites on Golgi membranes strongly suggests that this organelle is concerned with glycoprotein metabolism.1 The formation of new plasma membranes during mitosis and the maintenance of normal membrane structure in resting cells are important functions of the Golgi apparatus.4

The Golgi apparatus itself promotes the move­ment of membranes: vacuoles fuse to form a cisterna at the forming face, while vesicles are budding off at the mature face. The diameter of these vesicles ranges from 20-60 ~m and they are usually smooth-surfaced, round or elliptical. Coated or alveolate vesicles of 50-60 ~m diameter are distinguished by their regu]arly spaced arms or striac. These vesicles are concerned with the trans-

port of hydrolytic enzymes to the lysosomal system. Larger alveolate vesicles, lOOnm in diameter, derived from the cell surface by invagin­ation or pinocytosis, ingest proteins which then are transferred to the lysosomes to be digested. In

 

-1addition, many neurons, particularly those of the supraoptic and paraventricular nuclei, contain dense-cored, neurosecretory granules up to -150 nm in diameter.

Multivesicular bodies are spherical structures of 0~5 ~m in diameter which contain vesicles and an assortment of inclusions. Although frequently associated with the Golgi apparatus, they are not part of these organelles and occur throughout the perikaryon and cell processes. Multivesicular bodies sequester material transported to them by coated vesicles, and hydrolytic enzymes acquired from smaller coated vesicles convert them into lysosomes.

 

 

 

Lysosomes

 

Primary lysosomes are round or oval, uniformly dense bodies up to 1 pm in diameter, bound by a unit membrane. They contain various hydrolytic enzymes, including acid phosphatase which serves as a marker enzyme for their identification.3 Secondary lysosomes are larger, more irregular and usually contain various ingested materials.

 

 

Lipofuscin granules

 

Although visible by light microscopy, lipofuscin can be better appreciated in the electron micro­scope: a single membrane encloses a dense granule and one or two peripheral vacuoles (Fig. 1.3). Lipofuscin or lipochrome is frequently referred to as 'wear-and-tear' pigment, since the number of granules within the neurons increases with age. These granules are end products of intracellular peroxidation and polymerisation of unsaturated fatty acids. Their origin is controversial, but according to the most widely accepted concept they are denv~d from lysosomes. The precise function of lipofuscin is unknown. Degenerated cytoplasmic  constituents  contribute  to  its formation, which requires intracellular oxidants and antioxidants. Lipofuscin is thus the by­product of metabolic activity; it is stored in lyso­

somes and is only disposed of by expulsion during mitosis or by cell death.5 Since mature nerve cells do not divide, lipofuscin granules accumulate during life. Appearing as light brown granules in haematoxylin and eosin stained sections, lipo­fuscin can be demonstrated by a variety of fat stains including Sudan black B, Sudan III and Nile blue sulphate. It is acid fast and PAS positive and reduces ferric ferricyanide to Prussian blue (Schmorl's reaction).

 

 

Melanin

 

This pigment is present in the leptomeningeal cells and in the pigmented nuclei of the brainstem. Melanin-containing cells in the leptomeninges are most plentiful over the ventral aspect of the brain-stem; this is the area in which the rare primary malignant melanomas most often develop. Two major accumulations of pigmented neurons occur in the brainstem: in the substantia nigra of the midbrain and in the much smaller locus ceruleus of the pontine tegmentum. The cells of the substantia nigra and locus ceruleus are dopaminergic and noradrenergic  respectively.  Other  brainstem nuclei, including the motor nucleus of the vagus, also contain melanin in small amounts.

 

 

 

 

Mitochondria

 

These organelles vary considerably in shape and size, depending upon their location within the neurons. They are apparently distributed at random in the perikaryon and also occur in den­drites and axons, including the presynaptic terminals. Mitochondria have a smooth outer membrane and an irregular inner membrane. The inner membrane forms complex folds (cristac); the inner compartment of the mitochondrion is filled with a moderately dense matrix (Fig. 1.3). The inner surfaces of the cristae are rendered uneven by many minute projections, each ending in a knob. The inner and outer membranes differ in composition, structure and function; the electron transport mechanism and many hydrogenases are located in the inner membrane, whereas the monoamine oxidase system is found in the outer membrane.1

 

The cytoskeIeton:-4

 

The neuronal cytoskeleton is composed of three elements:  neurofilaments,  microtubules  and microfilaments.

 

Neurofilaments.  These filaments are 10 nm in diameter and of indeterminate length. They belong to the class of intermediate filaments, their diameter being in between that of the micro-filaments (4-6nm) and that of microtubules (24 nm). Of the five types of intermediate filaments found in animal cells, two occur in the central nervous system: neurofilaments in nerve cells and glial filaments in astrocytes. Although the various types of intermediate filaments appear to be similar morphologically, they are composed of different proteins.

In cross-section, neurofilaments display a central hollow core) surrounded by a wall of 3 nm in thickness. The basic unit of the wall is composed of four globular subunits, each 3.5 nm in diameter, which are linked together by connecting arms of 2~5 nm in thickness. These units are stacked one upon another and rotated in the transverse plane by 45~ to each other.6 In longitudinal section, neurofilaments appear as two dense, parallel lines which enclose the central hollow core. They are dispersed throughout the perikaryon, but tend to accumulate at the base of large processes, into which they extend, forming parallel arrays.

 

Neurofilaments isolated from the nervous system are formed by three polypeptides with the approximate molecular weights of 70, 150 and 200 kilodaltons.7 Recent investigation has revealed that immunohistochemical differences may exist between neurofilaments in the perikarya, den­drites and axons.

 

Although the morphology and biochemistry of neurofilaments have been established,9 very little is known about their functions. They are intimately related to microtubules, the other component of the neuronal cytoskeleton, and with them they could play a role not only in stabilising nerve cells, but also in axonal transport.10 Neurofilaments are also major intrinsic deter­minants of axonal diameter in large myclinated nerve fibres: the expression of a single set of neuron-specific genes encoding neurofilaments directly determines axonal calibre."

Alicrotubules.  Microtubules  are  of indeter­minate length and 24nm in diameter. Their walls are 6nm thick and composed of 13 globular subunits, each representing a constituent proto­filament.'0 Microtubules are intermingled with neurofilaments both in the penkaryon and in cell processes: the ratio of neurofilaments to micro-tubules decreases as the axon becomes smaller, and in thin unmyelinated axons usually only micro-tubules are present. Tubulin, the major protein component of microtubules.

 

In an adult brain this protein comprises 10-30" of the soluble protein'0 and more recently a group of microtubule-associated proteins has also been recognised. Many functions have been attributed to microtubules: skeletal support, maintenance of axonal flow, transport of various substances and of cytoplasmic vesicles, cellular contraction and a r6le in mitosis.' Microtubules also play a r6le in the maintenance and function of the Golgi apparatus."

 

Microfilaments.  These are not obvious in nerve cells: they have a diameter of 4-6nm and are composed of actin, a globular protein with a mol­ecular weight of 42000." Microfilaments, in addition to their stabilising function, also play a r6le in axonal transport.10

 

 

Synapses

 

Synapses are specialised interneuronal contacts which can be electrical or chemical. Electrical synapses function by the propagation of electrical impulses and do not require elaborate structural organisation of the plasma membrane and cyto­plasmic organelles. These synapses, which are rare in matnmals but common in lower vertebrates, are formed by the close apposition of the cell membranes at a gap junction. Chemical synapses, in contrast, utilise various substances-neuro­transmitters and neuropeptides-for intercellular communication:  this  chemical  transmission requires a sophisticated subcellular mechanism well demonstrated by electron microscopy. By light microscopy, using silver impregnation tech­niques, only the profile of the end-bulb or bouton terminal of the axon abutting onto the surface of the perikaryon or dendrite can be seen. A synapse is composed of three constituents: the presynaptic element, the synaptic cleft and the postsynaptic component (Fig. 1.5). Both the pre- and the postsynaptic membranes display densities along the speci~ised stretch of cell membrane and these, together with the intervening cleft, are referred to as the synaptic junction.

 

Originally two types of synapse were distin­guished on ultrastructural examination of the pyramidal cells of the cerebral cortex. The type I junction is usually extensive, the postsynaptic density prominent, rendering the synapse asym­metrical, and the wide synaptic cleft contains a dense plaque. In the type Ii synapse, in contrast, the postsynaptic density is less readily identifiable and the narrower cleft does not contain a dense plaque; the overall configuration is symmetrical.15 The presynaptic element in most cases is an axon, either at its end (bouton terminal) or along its course (bouton en passant): the axonal terminal can form a synapse with any part of another neuron and thus axo-dendritic, axo-somatic and axo­axonal synapses are distinguished. The presyn­aptic element, however, does not need to be an axonal terminal and synapses can be formed between any part of two neurons, providing a wide variety of synapses.

 

The presynaptic element contains the synaptic vesicles which, in turn, are filled with neuro­transmitters (Fig. 1.5). The size, shape and content of the vesicles vary, depending upon the type of synapse. The most frequently Occurring vesicles are apparently clear, spherical and 40-50 nm in diameter. Elongated or flattened vesicles with clear centres are 20 nm wide and 50 nm long. There have been many attempts to correlate vesicular shape with functional activity: spherical vesicles occur in excitatory, type I synapses, whilst flattened vesicles are associated with inhibitory, type II synapses. Larger vesicles of up to 60nm in diameter with a dense core are encountered in areas of catecholamine activity and contain noradrenaline, dopamine or 5-hydroxy-tryptamine. Another, larger dense-cored vesicle, up to lSOnm with a spherical dense core of 50-70 nm in diameter, is most frequently seen in the presynaptic terminals of autonomic ganglia, but also occurs in various parts of the central nervous system intermingled with clear vesicles. The presynaptic element, particularly the axonal terminal, also contains other organelles, including mitochondria, neurofilaments,  microtubules, smooth endoplasmic reticulum and glycogen.

 

The synaptic cleft, separating the pre- and postsynaptic membranes, is 20-3Onm wide and usually contains a dense plaque of intercellular material. In addition, filaments appear to traverse the cleft and contribute to the formation of the plaque.

 

The  postsynaptic  membrane  is  rendered conspicuous by an accumulation of dense material on its cytoplasmic surface (Fig. 1.5). This post-synaptic density, composed of granular material and an occasional filamentous structure, is more pronounced in asymmetrical, type I, synapses. The existence of postsynaptic density is dependent upon the presence of the presynaptic element: if the  presynaptic  structures  degenerate,  the postsynaptic density will eventually disappear. The postsynaptic element ~so contains cisternac of the smooth endoplasmic reticulum and the spine apparatus, which is composed of 2-3 flattened cisternac, separated by plaques of dense material.

 

The morphological appearances of synapses reflect  the  functional  dynamics  of neuro­transmission. Accordingly, in chemical synapses the vesicles contain quanta of the neurotransmitter which is discharged into the synaptic cleft after the vesicle has fused with the presynaptic membrane. Thus, the uptake of calcium into the nerve terminal triggers off a chain of events leading to exocytosis: vesicular apposition, membrane fusion and fission.16 Although this vesicular or exocytotic hypothesis of neurotransmission'7 provides a plausible explanation for the quantal nature of  transmitter release, an increasing body of evidence now suggests that this mav not be the only mechanism by which chemical signals in the central nervous system are propagated. Chemical substances may be released from non-synaptic axon terminals and even from regions of nerve cells other than presynaptic axon.

 

A parasynaptic system is envisaged in which so-called neuroactive or informational substances reach specific target cell receptors by diffusion from release points through the extracellular space. The mechanism of exocytosis itself remains poorly understood not only morpho­logically, but also neurochemically, and various alternative biochemical mechanisms have been considered to explain the release of neuro­transmitters.

 

Neuronal processes

 

AXONS

 

The transitional zone between the neuronal perikaryon and the axon is the cone-shaped axon hillock. In larger, multipolar neurons the axon hillock lacks Nissl substance, and it is in this region that cytoskeletal elements, the microtubules and neurofilaments, converge to become parallel as they enter the axon. The microtubules then form a fascicle in the initial segment of the axon, a feature 'vhich enables axons to be distinguished from dendrites, in which microtubules are more evenly distributed. The more distal portion of the axon may contain various organelles, but granular endoplasmic reticulum and polyribosomes are usually absent. The internal structure of the axon has recently been visualised as a three-dimensional lattice: the longitudinally orientated neurofila­ments and microtubules are extensively cross-linked to each other and to the plasma membrane by thin filaments. Similar bridges also connect membrane-bound organelles with the components of the cytoskeleton and with each other .There is axonal transport of material from the perikaryon towards the periphery (anterograde transport) and to a lesser extent in the opposite direction (retrograde transport). Since the axon terminal, the most active site of neurotrans­mission, can be far removed from the perikaryon in which proteins are synthesised, this axonal transport is vital for the proper functioning of the nerve cell. Axonal flow has a slow and a fast component, 0~2-8~0mm and 50-500 mm per day respectively. Fast axoplasmic flow carries organ­elles, including vesicles and mitochondria, and membrane-bound substances like proteins and neurotransmitters  materials which are essential for synaptic activity. The fast axonal flow is effected by microtubules, but the possibility that some membrane-bound proteins are transported by the smooth endoplasmic reticulum Or another tubulo-vesicular system cannot be excluded. Slow flow, in contrast, transports high molecular weight and soluble materials which are involved in the growth and maintenance of the axon. The structural basis of slow transport is controversial, but the movement of the cytoplasmic matrix itself may represent the prime force.23 Mechanisms of anterograde and retrograde transport are similar: both need metabolic energy, have similar ionic requirements and are sensitive to the same drugs.23 Both anterograde and retrograde transport can be blocked by low temperatures and suspended by colchicine and vinblastine, agents which disrupt microtubules. The factors which determine the direction of axonal transport remain conjectural: alternatives envisage either two, oppositely polar­ised transport systems or a single mechanism in which the direction of movement is determined by the nature of the material or organelle to be trans­ported. Investigation of the molecular mechan­isms of axonal transport should contribute to our further understanding of this vital neuronal function.

 

Dendrites

 

There is wide variation in the number and organisation of these cell processes in the central nervous system. The character of the dendritic tree reflects the afferent neuronal connections and consequently indicates the functional activity of the cell. The complex configuration of dendrites is best appreciated in Golgi preparations; these have allowed the classification of neurons into three groups based on their dendritic arborisations (Fig. 1.1). Isodendritic neurons issue straight dendrites which run in all directions, whilst allodendritic cells are distinguished by shorter, branched den­drites which are restricted in their wavy course. Idiodendritic neurons have a unique dendritic tree characteristic  of and  determined  by  their location.27 A further, more detailed classification of neurons has been achieved by the dendritic branching pattern.

Although clear differences exist, it may not be always easy to distinguish a dendrite from an axon. Dendrites  have  irregular  contours,  taper gradually, branch at relatively acute angles, are unmyelinated and contain Nissi substance. Axons, in contrast, display a smoother contour, have a relatively even diameter along their course, branch at obtuse or at right angles, can be myelinated or unmyclinated and lack Nissl substance. The smaller axons and dendrites become, the more difficult is their positive identification; the presence of clustered ribosomes, particularly in association with cisternac of rough-surfaced endoplasmic reticulum, remains the most reliable identifying feature of small dendritic processes.

 

The dendritic tree represents the most sub­stantial receptive area of the nerve cells Synapses are formed either on the dendritic trunks them­selves or on the dendritic spines, the specialised structures projecting from them. The spines are composed of a stalk or neck which connects the dendrite with the ovoid bulb or head. There is a considerable variation in their configuration, but their overall length appears to be relatively con­stant at 2 um. Spines can also be present on the perikaryon and they are responsible for 43()() of the total surface area of the dendrites and the cell body: there are 4000 spines on a single pyramidal neuron.~ The function of the dendritic spines is still largely unknown. Although they synapse with axon terminals, their role is not restricted to increasing receptive surfaces; they may regulate the excitatory input to a neuron.1

 

 

The neuropil and the extracellular space

 

The neuropil is composed of the complex and intricate network of neuronal cell processes. This intermingling and interconnection of myelinated and unmyelinated axons and dendrites may appear random, but these processes form the neuronal circuitry of a particular area. Cell processes of astrocytes, oligodendrocytes and microglial cells further add to the variety of structures in the neuropil (Fig. 1.3).

Cell processes and perikarya, bounded by typical unit membranes, are separated by an extra-cellular space with an average width of 15-20 run. Electron-dense markers, electrical impedance and the distribution of radiotracers have conclusivelv demonstrated the existence of this extracellular space, which constitutes 18~25o() of the total gre~' and white matter.29

 

Myelin

 

Myelin in the central nervous system is produced by oligodendrocytes (see p. 22-24). The myclin sheath is formed by the spiral wrapping of the cytoplasmic processes of the myelinating cells around axons, and the subsequent extrusion of the cytoplasm leads to a compact, tightly spiralled, multi-layered envelope.  Electron microscopy reveals the characteristic structure of the myelin sheath: major dense lines alternate with thinner intraperiod lines to form the repeating units. The major dense line results from the fusion of the thicker, inner leaflet of the oligodendrocytic plasma membrane, whereas the intraperiod line is formed by the apposition of the thinner, outer leaflet of this membrane. Unlike Schwann cells (the myelinating cells of the peripheral nervous system which are concerned with the myelination of a single axon), each oligodendrocyte may provide myelin for many axons. From this it follows that the motion by which the sheath is produced by the myelinating cell may be different in the central and peripheral nervous Systems. Whilst Schwann cells can, in principle, lay down myelin by rotating around a single axon, the oligodendrocyte, being in connection with many axons, cannot perform a revolving motion. Con­sequently, ensheathment takes place by the progressive lengthening of the oligodendrocytic process which encircles the axon completely and an internal mesaxon is formed by apposition of the free edges of the myclinating process. The outer­most lamella of the sheath encloses the outer tongue and this, together with the internal mesaxon, represents the tenuous connections which remain between the fully formed myclin and the myelinating cell.

 

Nodes of Ranvier occur regularly along the course of central myclinated fibres; the segment between two nodes is referred to as internodal myclin, and the region where the lamellae terminate is the paranode. The thickeness of the myelin sheath is related to axonal diameter: larger axons are surrounded by thicker myelin sheaths. Evidence from experimental animals has con­vincingly shown that the formation of myelin is preceded by the proliferation of glial precursors which develop into young oligodendrocytes. The processes of these active oligodendrocytes are frequently wrapped around axons, but this direct connection is difficult to demonstrate in later life.' Biologically, the formation of myclin is a two-step process: first, the oligodendrocyte relates to the axon in response to an axonal myelinogenic stimulus and, second, it produces myelin, the volume of which is determined by the internodal axonal surface area. 30

Mvelination in man progresses slowly and generally proceeds centripetally: it commences peripherally and whilst many axons, particularly in the spinal cord) are myclinated at birth, the more central fibres in the frontal and parietal lobes remain unmyclinated well into posmatal life. Completion of myelin formation is achieved largely during the first two years after birth.31 Immunohistochemistry for myelin basic protein confirms earlier findings of the chronological sequence of myelination: phylogenetically older regions acquire myelin first.32 The sequential nature of myelination reflects physiological demand: the time and rapidity of myelination are related to the relative significance of a fibre system at various periods of cerebral development.

 

Biochemically, myelin is composed of altern­ating layers of proteins and lipids. Of the myelin proteins, a proteolipid protein constitutes approximately 50 mu, basic protein (the antigenic agent capable of inducing experimental allergic encephalomyclitis) 30%, and an acidic proteolipid protein 200/0 .3' Myelin-associated glycoprotein is an acidic, concanavalin A binding minor com­ponent which is also present in the cytoplasm of oligodendrocytes before and during myelination. The various lipids to be found in the myelin include cholesterol, phosphatidylethanolamine, phosphatidylserine  and  phosphatidylcholine, sphingomyelin   and   glycolipids.   Galacto­cerebroside is the main glycolipid component and the cell membrane of the myelinating cell contains a high percentage of this compound. This suggests that galactocerebroside may play an important role in the successive layering of cell membranes, a

process unique to myelination.

 

 

Neurotransmitters and neuropeptides

 

The number of putative neurotransmitters has been dramatically increased during the last decade. In addition to the classical monoamine neurotransmitters (acetylcholine and catechol­amines) and a few amino acids, at least 30 neuro­peptides have been discovered: all these com­pounds can act as chemical messengers in the mammalian nervous system.  Immunohisto­chemistry has allowed the precise localisation of these neurotrarismitters and neuropeptides and thus contributed to the understanding of how certain regions of the brain work. This expanding knowledge of the relationship between structure and function has been accompanied by the realisation that certain neurological and psychiatric disorders are caused by an imbalance (overproduction or deficit) of these substances. It is for this reason that the distribution of neuro­transmitters and neuropeptides is of considerable importance not only to neurobiologists, but also to histopathologists. Neurochemical analyses of post-mortem brains are affected by various factors including sampling, precision of dissection, age, sex, medication, the agonal state of the patient and post-mortem delay.

 

Of the monoamines, acetylcholine is found in the motor nuclei of the cranial nerves and in the motor neurons of the spinal cord; in these locations it serves as the chemical messenger for neuromuscular transmission. Acetylcholine is also present in the intrinsic pathways within the central nervous system, and cholinergic neurons project in a diffuse ascending system from the medial septal nuclei to the hippocampus and from the nucleus basalis of Meynert to the cerebral cortex.39 The basal ganglia are rich in this monoamine and the enzymes related to its metabolism: choline acetyltransferase and acetylcholinesterase, the synthesising and catabolising enzyme respect­ively. Large cholinergic neurons have been recently demonstrated by histochemistry in the human striatum, but only the isolation, purific­ation and immunohistochemical localisation of  choline acetyltransferase have made a more com­prehensive mapping of cholinergic pathways possible.

 

There are three catecholamines in the central nervous system: noradrenaline, adrenaline (epi­nephrine) and dopamine. The noradrenergic svstem is localised in the brainstem nuclei, the largest of which is the locus ceruleus, the pig­mented column of cells in the rostral part of the pontine tegmentum: axons originating from these cells establish extensive connections with the cerebral cortex and hippocampus. The hypo­thalamus is also rich in noradrenergic fibres.

 

The adrenergic system, in contrast, is more restricted: cells in the pons and medulla project to other brainstem structures or to the hypothala­mus.

 

The major dopaminergic pathway originates in the pars compacta of the substantia nigra and ascends to the striatum: devastation of this system is the underlying cause of Parkinson's disease.'2 In addition to this nigrostriatal pathway, there is also a mesocortical and mesolimbic dopaminergic system: cells in the ventral tegmentum of the mid­brain project to the cerebral cortex and to the limbic areas respectively.

 

The raphe nuclei form a long, ill-defined chain in the midline of the brainstem; their nerve cells give rise to the serotoninergic system which contains 5-hydroxytryptamine (serotonin) and projects to various sites in the forebrain including the hypothalamus, basal ganglia and medial fore­brain bundle, and descends to the anterior and posterior horns of the spinal cord.

 

Gamma-aminobutyric acid (GABA), glutamate and glycine are amino acid neurotransmitters. GABA is the principal inhibitory neurotransmitter in the vertebrate nervous system and one-third of all nerve terminals in the rat brain appears to be GABAergic.36 Moreover, neurophysiology, auto-radiography and irrrrnunohistochemistry have all demonstrated that inhibitory synapses of the cerebellum utilise GABA and that the major efferent pathways of the Purkinje cells are also GABAergic. GABA is also found in the spinal cord, although glycine is the maior inhibitory neurotransmitter at this site. Glycine Occurs in the small inhibitory interneurons of the grey matter and acts upon the large motor neurons of the anterior horn. An increasing body of evidencc suggests that glutamate is the universal putativc excitatory neurotransmitter in the central nervous system. The possibility that some excitatorv synapses use aspartate instead of glutamate ca