SOMA

The plasma membrane of the soma is unmyelinated and contacted by inhibitory and excitatory axosomatic synapses: very occasionally, somasomatic and dendrosomatic contacts may be made. The non-synaptic surface is covered by either astrocytic or satellite oligodendrocyte processes.

The cytoplasm of a typical soma (Fig. 3.2) is rich in rough and smooth endoplasmic reticulum and free polyribosomes, indicating a high level of protein synthetic activity. Free polyribosomes often congregate in large groups associated with the rough endoplasmic reticulum. These aggregates of RNA-rich structures are visible by light microscopy as basophilic Nissl bodies or granules. They are more obvious in large, highly active cells, such as spinal motor neurones (Fig. 3.4), which contain large stacks of rough endoplasmic reticulum and polyribosome aggregates. Maintenance and turnover of cytoplasmic and membranous components are necessary in all cells: the huge total volume of cytoplasm within the soma and processes of many neurones requires a considerable commitment of protein synthetic machinery. Neurones also synthesize other proteins (enzyme systems, etc.) involved in the production of neurotransmitters and in the reception and transduction of incoming stimuli. Various transmembrane channel proteins and enzymes are located at the surfaces of neurones where they are associated with movements of ions. The apparatus for protein synthesis (including RNA and ribosomes) occupies the soma and dendrites, but is usually absent from axons.

Fig. 3.4 Spinal motor neurones (toluidine blue stained resin section, rat tissue) showing a group of cell bodies (somata, S), some with the proximal parts of axonal and dendritic processes (P) visible. Their nuclei (N) typically have prominent, deeply staining nucleoli, indicative of metabolically highly active cells; two are visible in the plane of section. Nissl granules (G) are seen in the cytoplasm. Surrounding the neuronal somata is the neuropil, consisting of the interwoven processes of these and other neurones and of glial cells.

The nucleus is characteristically large, round and euchromatic and contains at least one prominent nucleolus; these are features typical of all cells engaged in substantial levels of protein synthesis. The cytoplasm contains many mitochondria and moderate numbers of lysosomes. Golgi complexes are usually close to the nucleus, near the bases of the main dendrites and opposite the axon hillock.

The neuronal cytoskeleton is a prominent feature of its cytoplasm and gives shape, strength and support to the dendrites and axons. A number of neurodegenerative diseases are characterized by abnormal aggregates of cytoskeletal proteins (reviewed in Cairns et al 2004). Neurofilaments (the intermediate filaments of neurones) and microtubules are abundant in the soma and along dendrites and axons: the proportions vary with the type of neurone and cell process. Bundles of neurofilaments constitute neurofibrils which can be seen by light microscopy in silver stained or immunolabelled sections. Neurofilaments are heteropolymers of proteins assembled from three polypeptide subunits, NF-L (68 kilodaltons [kDa]), NF-M (160 kDa) and NF-H (200 kDa). NF-M and NF-H have long C-terminal domains which project as side arms from the assembled neurofilament and bind to neighbouring filaments. They can be heavily phosphorylated, particularly in the highly stable neurofilaments of mature axons, and are thought to give axons their tensile strength. Some axons are almost filled by neurofilaments.

Microtubules are important in axonal transport, although dendrites usually have more microtubules than axons. Centrioles persist in mature postmitotic neurones, where they are concerned with the generation of microtubules rather than cell division. Centrioles are associated with cilia on the surfaces of developing neuroblasts. Their significance, other than at some sensory endings (e.g. the olfactory mucosa, p. 553), is not known.

Pigment granules (Fig. 3.5) appear in certain regions, e.g. neurones of the substantia nigra contain neuromelanin, probably a waste product of catecholamine synthesis. In the locus coeruleus a similar pigment, rich in copper, gives a bluish colour to the neurones. Some neurones are unusually rich in certain metals which may form a component of enzyme systems, e.g. zinc in the hippocampus and iron in the oculomotor nucleus. Ageing neurones, especially in spinal ganglia, accumulate granules of lipofuscin (senility pigment) in residual bodies, which are lysosomes packed with partially degraded lipoprotein material (corpora amylaceae).

Fig. 3.5 Neurones in the substantia nigra of the human midbrain, showing cytoplasmic granules of neuromelanin pigment.

DENDRITES

Dendrites are highly branched, usually short processes which project from the soma (Fig. 3.2). The branching patterns of many dendritic arrays are probably established by random adhesive interactions between dendritic growth cones and afferent axons that occur during development. There is an overproduction of dendrites in early development, and this is pruned in response to functional demand as the nervous system matures and information is processed through the dendritic tree. There is evidence that dendritic trees may be plastic structures throughout adult life, expanding and contracting as the traffic of synaptic activity varies through afferent axodendritic contacts (for review see Wong & Ghosh 2002). Groups of neurones with similar functions have a similar stereotypic tree structure (Fig. 3.6), suggesting that the branching patterns of dendrites are important determinants of the integration of the afferent inputs that converge on the tree.

Fig. 3.6 Purkinje neurone from the cerebellum of a rat stained by the Golgi–Cox method, showing the extensive two-dimensional array of dendrites.

(By courtesy of Martin Sadler and M Berry, Division of Anatomy and Cell Biology, GKT School of Medicine, London.)

Dendrites differ from axons in many respects. They represent the afferent rather than the efferent system of the neurone, and receive both excitatory and inhibitory axodendritic contacts. They may also make dendrodendritic and dendrosomatic connections (see Fig. 3.9), some of which are reciprocal. Synapses occur either on small projections called dendritic spines or on the smooth dendritic surface. Dendrites contain ribosomes, smooth endoplasmic reticulum, microtubules, neurofilaments, actin filaments and Golgi complexes. Their neurofilament proteins are poorly phosphorylated and their microtubules express the microtubule-associated protein (MAP)-2 almost exclusively in comparison with axons.

Fig. 3.9 The structural arrangements of different types of synaptic contact.

The shapes of dendritic spines range from simple protrusions to structures with a slender stalk and expanded distal end. Most spines are not more than 2 μm long, and have one or more terminal expansions; they can also be short and stubby, branched or bulbous. Free ribosomes and polyribosomes are concentrated at the base of the spine. Ribosomal accumulations near synaptic sites provide a mechanism for activity- dependent synaptic plasticity through the local regulation of protein synthesis.

AXONS

The axon originates either from the soma or from the proximal segment of a dendrite, at a specialized region free of Nissl granules, the axon hillock (Fig. 3.2). Action potentials are initiated here. The axonal plasma membrane (axolemma) is undercoated at the hillock by a concentration of cytoskeletal molecules, including spectrin and actin fibrils, which are important in anchoring numerous voltage-sensitive channels to the membrane. The axon hillock is unmyelinated and often participates in inhibitory axo-axonal synapses. It is unique because it contains ribosomal aggregates immediately below the postsynaptic membrane.

In the CNS, small, unmyelinated axons lie free in the neuropil, whereas in the PNS they are embedded in Schwann cell cytoplasm. Myelin, which is formed around almost all axons >2 μm diameter by oligodendrocytes in the CNS and Schwann cells in the PNS, begins at the distal end of the axon hillock. Nodes of Ranvier are specialized constricted regions of myelin-free axolemma where action potentials are generated and where an axon may branch. In both CNS and PNS, the territory of a myelinated axon between adjacent nodes is called an internode: the region close to a node, where the myelin sheath terminates, is called the paranode. Myelin thickness and internodal lengths are, in general, positively correlated with axon diameter. The density of sodium channels in the axolemma is highest at nodes of Ranvier, and very low along internodal membranes: sodium channels are spread more evenly within the axolemma of unmyelinated axons. Fast potassium channels are present in the paranodal regions of myelinated axons. Fine processes of glial cytoplasm (astrocytic in the CNS, Schwann cell in the PNS) surround the nodal axolemma.

The terminals of an axon are unmyelinated and most expand into presynaptic boutons which may form connections with axons, dendrites, neuronal somata or, in the periphery, muscle fibres, glands and lymphoid tissue. Exceptions include the free afferent sensory endings in e.g. the epidermis, which are unspecialized structurally, and the peripheral terminals of afferent sensory fibres with encapsulated endings (see Fig. 3.30). Axon terminals may themselves be contacted by other axons, forming axoaxonal presynaptic inhibitory circuits. Further details of neuronal microcircuitry are given in Kandel et al (2000).

Fig. 3.30 Some major types of sensory ending of general afferent fibres (omitting neuromuscular, neurotendinous and hair-related types). The traces below each type of ending indicate (top) their response (firing rate [vertical lines] and adaption with time) to an appropriate stimulus (below) of the duration indicated. The Pacinian corpuscle response to vibration (rapid sequence of on-off stimuli) is also shown.

Axons contain microtubules, neurofilaments, mitochondria, membrane vesicles, cisternae, and lysosomes. Axons do not usually contain ribosomes or Golgi complexes, except at the axon hillock: exceptionally, the neurosecretory fibres of hypothalamo-hypophysial neurones contain the mRNA of neuropeptides. Organelles are differentially distributed along axons, e.g. there is a greater density of mitochondria and membrane vesicles in the axon hillock, at nodes, and in presynaptic endings. Axonal microtubules are interconnected by cross-linking MAPs, of which tau is the most abundant. Microtubules have an intrinsic polarity, and in axons all microtubules are uniformly orientated with their rapidly growing ends directed away from the soma towards the axon terminal. Neurofilament proteins ranging from high to low molecular weights are highly phosphorylated in mature axons, whereas growing and regenerating axons express a calmodulin-binding membrane-associated phosphoprotein, growth-associated protein-43 (GAP-43), as well as poorly phosphorylated neurofilaments.

Neurones respond differently to injury depending on whether the damage occurs in the CNS or the PNS. The glial microenvironment of a damaged central axon does not facilitate axonal regrowth, consequently reconnection with original synaptic targets does not normally occur. In marked contrast, the glial microenvironment in the PNS is capable of facilitating axonal regrowth. However, functional outcome of clinical repair of a large mixed peripheral nerve, especially if the injury occurs some distance from the target organ, or produces a long defect in the damaged nerve, is frequently unsatisfactory.

SYNAPSES

Transmission of impulses across specialized junctions (synapses) between two neurones is largely chemical and depends on the release of neurotransmitters from the presynaptic side. This causes a change in the electrical state of the postsynaptic neuronal membrane, resulting in either its depolarization or hyperpolarization.

The patterns of axonal termination vary considerably. A single axon may synapse with one neurone (e.g. climbing fibres ending on cerebellar Purkinje neurones), or more often with many neurones (e.g. cerebellar parallel fibres, which provide an extreme example of this phenomenon (p. 302). In synaptic glomeruli, e.g. in the olfactory bulb and cerebellum, and synaptic cartridges, groups of synapses between two or many neurones form interactive units encapsulated by neuroglia (Fig. 3.7).

Fig. 3.7 The arrangement of a complex synaptic unit. A cerebellar synaptic glomerulus with excitatory (‘+’) and inhibitory (‘−’) synapses grouped around a central axonal bouton. The directions of transmission are shown by the arrows.

Electrical synapses (direct communication via gap junctions) are rare in the human CNS and are confined largely to groups of neurones with tightly coupled activity, e.g. the inspiratory centre in the medulla. They will not be discussed further here.

Classification of chemical synapses

Chemical synapses have an asymmetric structural organization (Fig. 3.8 and Fig. 3.9) in keeping with the unidirectional nature of their transmission. Typical chemical synapses share a number of important features. They all display an area of presynaptic membrane apposed to a corresponding postsynaptic membrane: the two are separated by a narrow (20–30 nm) gap, the synaptic cleft. Synaptic vesicles containing neurotransmitter lie on the presynaptic side, clustered near a dense plaque on the cytoplasmic aspect of the presynaptic membrane. A corresponding region of submembrane density is present on the postsynaptic side. Together these define the active zone, the area of the synapse where neurotransmission takes place.

Fig. 3.8 Electron micrographs demonstrating various types of synapse. A, Cross-section of a dendrite (D) upon which two synaptic boutons (B) end. The upper bouton contains round vesicles, and the lower bouton contains flattened vesicles of the small type. A number of pre- and postsynaptic (P) thickenings mark the specialized zones of contact. B, A type I synapse (S, postsynaptic site) containing both small, round, clear vesicles and also large dense-cored vesicles of the neurosecretory type. C, A large terminal bouton (B) of an optic nerve afferent fibre, which is making contact with a number of postsynaptic processes, in the dorsal lateral geniculate nucleus of the rat. One of the postsynaptic processes (^*) also receives a synaptic contact from a bouton (Bf) containing flattened vesicles. D, Reciprocal synapses (S) between two neuronal processes in the olfactory bulb.

(By courtesy of Professor AR Lieberman, Department of Anatomy, University College, London.)

Chemical synapses can be classified according to a number of different parameters including the neuronal regions forming the synapse, their ultrastructural characteristics, the chemical nature of their neurotransmitter(s) and their effects on the electrical state of the postsynaptic neurone. The following classification is limited to associations between neurones. Neuromuscular junctions share many (though not all) of these parameters, and are often referred to as peripheral synapses. They are described separately on page 63.

Synapses can occur between almost any surface regions of the participating neurones. The most common type occurs between an axon and either a dendrite or a soma, when the axon is expanded as a small bulb or bouton (Fig. 3.8, Fig. 3.9). This may be a terminal of an axonal branch (terminal bouton) or one of a row of bead-like endings, when the axon makes contact at several points, often with more than one neurone (bouton de passage). Boutons may synapse with dendrites, including dendritic spines or the flat surface of a dendritic shaft, a soma (usually on its flat surface, but occasionally on spines), the axon hillock and the terminal boutons of other axons.

The connection is classified according to the direction of transmission, and the incoming terminal region is named first. Most common are axodendritic synapses, although axosomatic connections are frequent. All other possible combinations are found but are less common, i.e. axoaxonic, dendroaxonic, dendrodendritic, somatodendritic or somatosomatic. Axodendritic and axosomatic synapses occur in all regions of the CNS and in autonomic ganglia, including those of the ENS. The other types appear restricted to regions of complex interaction between larger sensory neurones and microneurones, e.g. in the thalamus.

Ultrastructurally, synaptic vesicles may be internally clear or dense, and of different size (loosely categorized as small or large) and shape (round, flat or pleomorphic, i.e. irregularly shaped). The submembranous densities may be thicker on the postsynaptic than on the presynaptic side (asymmetric synapses), or equivalent in thickness (symmetrical synapses). Synaptic ribbons are found at sites of neurotransmission in the retina and inner ear. They have a distinctive morphology, in that the synaptic vesicles are grouped around a ribbon- or rod-like density orientated perpendicular to the cell membrane (Fig. 3.9).

Synaptic boutons make obvious close contacts with postsynaptic structures, but many other terminals lack specialized contact zones. Areas of transmitter release occur in the varicosities of unmyelinated axons, where effects are sometimes diffuse, e.g. the aminergic pathways of the basal ganglia and in autonomic fibres in the periphery. In some instances, such axons may ramify widely throughout extensive areas of the brain and affect the behaviour of very large populations of neurones, e.g. the diffuse cholinergic innervation of the cerebral cortices. Pathological degeneration of these pathways can therefore cause widespread disturbances in neural function.

Neurones express a variety of neurotransmitters, either as one class of neurotransmitter per cell or more often as several. Good correlations exist between some types of transmitter and specialized structural features of synapses. In general, asymmetric synapses with relatively small spherical vesicles are associated with acetylcholine (ACh), glutamate, serotonin (5-hydroxytryptamine, 5-HT), and some amines; those with dense-core vesicles include many peptidergic synapses and other amines (e.g. noradrenaline (norepinephrine), adrenaline (epinephrine), dopamine). Symmetrical synapses with flattened or pleomorphic vesicles have been shown to contain either γ-aminobutyric acid (GABA) or glycine.

Neurosecretory endings found in various parts of the brain and in neuroendocrine glands and cells of the dispersed neuroendocrine system (see below; p. 31) share many features with presynaptic boutons. They all contain peptides or glycoproteins within dense-core vesicles. The latter are of characteristic size and appearance: they are often ellipsoidal or irregular in shape, and relatively large, e.g. oxytocin and vasopressin vesicles in the neurohypophysis may be up to 200 nm across.

Synapses may cause depolarization or hyperpolarization of the postsynaptic membrane, depending on the neurotransmitter released and the classes of receptor molecule in the postsynaptic membrane. Depolarization of the postsynaptic membrane results in excitation of the postsynaptic neurone, whereas hyperpolarization has the effect of transiently inhibiting electrical activity. Subtle variations in these responses may also occur at synapses where mixtures of neuromediators are present and their effects are integrated.

Type I and II synapses

There are two broad categories of synapse: type I synapses, in which the cytoplasmic density is thicker on the postsynaptic side, and type II synapses, in which the pre- and post-synaptic densities are more symmetrical but thinner. Type I boutons contain a predominance of small spherical vesicles approximately 50 nm in diameter, and type II boutons contain a variety of flat forms. Throughout the CNS, type I synapses are generally excitatory and type II, inhibitory. In a few instances types I and II synapses are found in close proximity, orientated in opposite directions across the synaptic cleft (a reciprocal synapse).

Mechanisms of synaptic activity

Synaptic activation begins with arrival of one or more action potentials at the presynaptic bouton, which causes the opening of voltage-sensitive calcium channels in the presynaptic membrane. The response time in typical fast-acting synapses is then very rapid; classic neurotransmitter (e.g. ACh) is released in less than a millisecond, which is faster than the activation time of a classic second messenger system on the presynaptic side. The influx of calcium activates Ca²⁺-dependent protein kinases. This uncouples synaptic vesicles from a spectrin-actin meshwork within the presynaptic ending, to which they are bound via synapsins I and II. The vesicles dock with the presynaptic membrane, through processes not yet fully understood, and their membranes fuse to open a pore through which neurotransmitter diffuses into the synaptic cleft.

Once the vesicle has discharged its contents, its membrane is incorporated into the presynaptic plasma membrane and is then more slowly recycled back into the bouton by endocytosis around the edges of the active site. The time between endocytosis and re-release may be approximately 30 seconds; newly recycled vesicles compete randomly with previously stored vesicles for the next cycle of neurotransmitter release. The fusion of vesicles with the presynaptic membrane is responsible for the observed quantal behaviour of neurotransmitter release, both during neural activation and spontaneously, in the slightly leaky resting condition.

Postsynaptic events vary greatly, depending on the receptor molecules and their related molecular complexes. Receptors are generally classed as either ionotropic or metabotropic. Ionotropic receptors function as ion channels, so that conformational changes induced in the receptor protein when it binds the neurotransmitter cause the opening of an ion channel within the same protein assembly, thus causing a voltage change within the postsynaptic cell. Examples are the nicotinic ACh receptor and the N-methyl-D-aspartate (NMDA) glutamate receptor. Alternatively, the receptor and ion channel may be separate molecules, coupled by G-proteins, some via a complex cascade of chemical interactions (a second messenger system), e.g. the adenylate cyclase pathway. Postsynaptic effects are generally rapid and short-lived, because the transmitter is quickly inactivated either by an extracellular enzyme (e.g. acetylcholinesterase, AChE), or by uptake by neuroglial cells. Examples of such metabotropic receptors are the muscarinic Ach receptor and 5-HT receptor.

Neurohormones

Neurohormones are included in the class of molecules with neurotransmitter-like activity. They are synthesized in neurones and released into the blood circulation by exocytosis at synaptic terminal-like structures. As with classic endocrine gland hormones, they may act at great distances from their site of secretion. Neurones secrete into the CSF or local interstitial fluid to affect other cells, either diffusely or at a distance. To encompass this wide range of phenomena the general term neuromediation has been used, and the chemicals involved are called neuromediators.

Neuromodulators

Some neuromediators do not appear to affect the postsynaptic membrane directly, but they can affect its responses to other neuromediators, either enhancing their activity (by increasing or prolonging the immediate response), or perhaps limiting or inhibiting their action. These substances are called neuromodulators. A single synaptic terminal may contain one or more neuromodulators in addition to a neurotransmitter, usually (though not always) in separate vesicles. Neuropeptides (see below) are nearly all neuromodulators, at least in some of their actions. They are stored within dense granular synaptic vesicles of various sizes and appearances.

ASTROCYTES

Astrocytes are star-shaped glia (Fig. 3.10). Their processes, which ramify through the entire central neuropil (Fig. 3.11), are coupled functionally at gap junctions, forming an interconnected network which ensheathes all neurones, other than at synapses and along the myelinated segments of axons. Some astrocyte processes terminate as end-feet on the basal lamina of blood vessels, and at the pial surface, where they form the glia limitans (glial limiting membrane).

Fig. 3.10 Confocal micrograph of an astrocyte in an adult rat optic nerve, iontophoretically filled with an immunofluorescent dye by intracellular microinjection. (Prepared by Professor A Butt, Portsmouth, and Kate Colquhoun, formerly of Division of Physiology, GKT School of Medicine, London.)

Fig. 3.11 The different types of non-neuronal cell in the CNS and their structural organization and interrelationships with each other and with neurones.

Astrocytes have been divided into two subtypes, fibrous and protoplasmic, on morphological grounds. Fibrous astrocytes occur predominantly in white matter and protoplasmic astrocytes are found mainly in grey matter. The significance of these subtypes is unclear since there are few known functional differences between fibrous and protoplasmic astrocytes. Astrocytes typically have a pale nucleus with a narrow rim of heterochromatin, although this is variable. Their cytoplasm is pale and contains glycogen, lysosomes, Golgi complexes and bundles of glial intermediate filaments that extend into their processes (these last are found particularly in fibrous astrocytes). Glial intermediate filaments are formed from glial fibrillary acidic protein (GFAP): its presence can be used clinically to identify tumour cells of glial origin.

Astrocytes are now thought to provide a network of communication, providing integration and regulation of function in the brain (reviewed in Fields 2004, Volterra & Meldolesi 2005). One form of communication, with other astrocytes, is via interconnecting low resistance gap junctional complexes. There is recent evidence that astrocytic (and other neural) gap junctions may be formed by a novel family of proteins, the pannexins (see p. 7). They signal to each other using intracellular calcium wave propagation, triggered by synaptically-released glutamate. Signalling presumably co-ordinates astrocyte functions that are essential for efficient neuronal activity, such as ion (particularly potassium) buffering; neurotransmitter uptake and metabolism (e.g. of excess glutamate, which is excitotoxic); membrane transport; the secretion of peptides, amino acids, trophic factors etc. The complexity of astrocytic function and dysfunction in neurological disorders is reviewed in Seifert et al (2006).

Injury to the CNS induces astrogliosis, seen as a local increase in the number and size of cells expressing GFAP, and in the production of an extensive meshwork of processes to form a glial scar. It is thought that the microenvironment of a glial scar, which may also include cells of oligodendrocyte lineage and myelin debris, plays an important role in inhibiting regrowth of damaged CNS axons.

Pituicytes are glial cells found in the neural parts of the pituitary gland, the infundibulum and neurohypophysis. They resemble astrocytes, but their processes end mostly on endothelial cells in the neurohypophysis and tuber cinereum.