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CHAPTER 3 – Nervous system

The nervous system has two major divisions, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord and contains the majority of neuronal cell bodies. The PNS includes all nervous tissue outside the central nervous system and is subdivided into the cranial and spinal nerves, peripheral autonomic nervous system (ANS) (including the enteric nervous system of the gut wall, ENS) and special senses (taste, olfaction, vision, hearing and balance). It is composed mainly of the axons of sensory and motor neurones that pass between the CNS and the body. However, the ENS contains as many intrinsic neurones in its ganglia as the entire spinal cord, is not connected directly to the CNS, and may be considered separately as a third division of the nervous system (Gershon 1998).

The CNS is derived from the neural tube (Ch. 24). The cell bodies of neurones are often grouped together in discrete areas termed nuclei, or they may form more extensive layers or masses of cells; collectively they constitute grey matter. Neuronal dendrites and synaptic contacts are mostly confined to areas of grey matter, and they form part of its meshwork of neuronal and glial processes termed the neuropil. Their axons join bundles of nerve fibres that tend to be grouped separately to form tracts. In the spinal cord, cerebellum, cerebral cortices (Chs 20, 22) and some other areas, concentrations of tracts constitute the white matter, so called because the axons are often ensheathed in lipid-rich sheaths of myelin which is white when fresh (Fig. 3.1).

The PNS is composed of the efferent axons (fibres) of motor neurones situated inside the CNS, and the cell bodies of sensory neurones (grouped together as ganglia) and their afferent processes. Sensory cells in dorsal root ganglia give off both centrally and peripherally directed processes: there are no synapses on their cell bodies. Also included are ganglionic neurones of the ANS, which receive synaptic contacts from the peripheral fibres of preganglionic autonomic neurones whose cell bodies lie within the CNS. Neuronal cell bodies in peripheral ganglia are all derived embryologically from cells which migrate from the neural crest (p. 201).

When the neural tube is formed during prenatal development its walls thicken greatly but do not completely obliterate the cavity within. The latter remains in the spinal cord as the narrow central canal, and in the brain it becomes greatly expanded to form a series of interconnected cavities called the ventricular system. In the fore- and hindbrains, parts of the neural tube roof do not generate nerve cells but become thin folded sheets of secretory tissue which are invaded by blood vessels and are called the choroid plexuses. The plexuses secrete cerebrospinal fluid (CSF) which fills the ventricles and subarachnoid spaces and penetrates the intercellular spaces of the brain and spinal cord to create their interstitial fluid. The CNS has a rich blood supply, which is essential to sustain its high metabolic rate. The blood–brain barrier places considerable restrictions on the substances which can diffuse from the bloodstream into the nervous tissue.

Neurones encode information, conduct it over considerable distances, and then transmit it to other neurones or to various non-neural cells. The movement of this information within the nervous system depends on the rapid conduction of transient electrical impulses along neuronal plasma membranes. Transmission to other cells is mediated by secretion of neurotransmitters at special junctions either with other neurones (synapses), or with cells outside the nervous system, e.g. muscle cells (neuromuscular junctions), gland cells, adipose tissue, etc. and this causes changes in their behaviour.

The nervous system contains large populations of non-neuronal cells, neuroglia or glia, which, whilst not electrically active in the same way, are responsible for creating and maintaining an appropriate environment in which neurones can operate efficiently. Indeed, it is now known that two-way communication between neurones and glial cells is essential for normal neural activity (reviewed in Fields & Stevens-Graham 2002). In the CNS, glia outnumber neurones by 10–50 times and consist of microglia and macroglia. Macroglia are further subdivided into three main types, oligodendrocytes, astrocytes and ependymal cells. The principal glial cell of the PNS is the Schwann cell. Satellite cells surround each neuronal soma in ganglia.


Fig. 3.1  Section through the human cerebellum stained to show the arrangement in the brain of the central white matter (WM, deep pink), and the highly folded outer grey matter (GM). In the cerebellum, GM consists of an inner granular layer of tightly packed small neurones (blue) and an outermost molecular layer (pale pink) where neuronal processes make synaptic contacts.


Most of the neurones in the CNS are either clustered into nuclei, columns or layers, or dispersed within grey matter. Neurones of the PNS are confined to ganglia. Irrespective of location, neurones share many general features, which are discussed here in the context of central neurones. Special characteristics of ganglionic neurones and their adjacent tissues are discussed on page 55–6.

Neurones exhibit great variability in their size (cell bodies range from 5 to 100 μm diameter) and shapes. Their surface areas are extensive because most neurones display numerous narrow branched cell processes. They usually have a rounded or polygonal cell body (perikaryon or soma). This is a central mass of cytoplasm which encloses a nucleus and gives off long, branched extensions, with which most intercellular contacts are made. Typically, one of these processes, the axon, is much longer than the others, the dendrites (Fig. 3.2). Dendrites conduct electrical signals towards a soma whereas axons conduct impulses away from it.


Fig. 3.2  Schematic view of a typical neurone showing the soma (cut away to show the nucleus and cytoplasmic organelles), the dendritic tree with synaptic contacts (also shown contacting the soma), the axon hillock and the proximal part of the axon.

Neurones can be classified according to the number and arrangement of their processes. Multipolar neurones (Fig. 3.3) are common: they have an extensive dendritic tree which arises from either a single primary dendrite or directly from the soma, and a single axon. Bipolar neurones, which typify neurones of the special sensory systems, e.g. the retina (p. 697), have only a single dendrite which emerges from the soma opposite the axonal pole. Unipolar neurones which transmit general sensation, e.g. dorsal root ganglion neurones, have a single short process which bifurcates into peripheral and central processes (p. 55). This arrangement arises by the fusion of the proximal axonal and dendritic processes of a bipolar neurone during development: they may also, therefore, be termed pseudounipolar. Neurones are also classified according to whether their axons terminate locally on other neurones (interneurones), or transmit impulses over long distances, often to distinct territories via defined tracts (projection neurones).


Fig. 3.3  The variety of shapes of neurones and their processes. The inset shows a human multipolar retinal ganglion cell, filled with fluorescent dye by microinjection.
(Inset by courtesy of Drs Richard Wingate, James Morgan and Ian Thompson, King’s College, London.)

Neurones are postmitotic cells and, with few exceptions, they are not replaced when lost.


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.

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