Cells Of The Nervous System I: Neurones
There are two major classes of cells in the nervous system: the neuroglial cells and neurones, with the latter making up only 10– 20% of the whole population. The neurones are specialized for excitation and nerve impulse conduction (see Chapters 14, 15 and 17), and communicate with each other by means of the synapse (see Chapter 16) and so act as the structural and functional unit of the nervous system.
The cell body (soma) is that part of the neurone containing the nucleus and surrounding cytoplasm. It is the focus of cellular metabolism, and houses most of the neurone’s intracellular organelles (mitochondria, Golgi apparatus and peroxisomes). It is typically associated with two types of neuronal processes: the axon and dendrites. Most neurones also contain basophilic staining, termed Nissl substance, which is composed of granular endoplasmic reticulum and ribosomes and is responsible for protein synthesis. This is located within the cell body and dendritic processes but is absent from the axon hillock and axon itself, for reasons that are not clear. In addition, throughout the cell body and processes are neurofilaments which are important in maintaining the architecture or cytoskeleton of the neurone. Furthermore, two other fibrillary structures within the neurone are important in this respect: microtubules and microfilaments, structures that are also important for axoplasmic flow (see below) and axonal growth.
The dendrites are neuronal cell processes that taper from the soma outwards, branch profusely and are responsible for convey- ing information towards the soma from synapses on the dendritic tree (axodendritic synapses; see also Chapter 17). Most neurones have many dendrites (multipolar neurones) and while some inputs synapse directly onto the dendrite, some do so via small dendritic spines or gemmules. Thus, the primary role of dendrites is to increase the surface area for synapse formation allowing integration of a large number of inputs that are relayed to the cell body. In contrast, the axon, of which there is only one per neurone, conducts information away from the soma towards the nerve ter- minal and synapses (see Chapter 15). Although there is only one axon per neurone, it can branch to give several processes. This branching occurs close to the soma in the case of sensory neurones (pseudo-unipolar neurones; see Chapter 31), but more typically occurs close to the synaptic target of the axon. The axon originates from the soma at the axon hillock where the initial segment of the axon emerges. This is the most excitable part of a neurone because of its high density of sodium channels, and so is the site of initiation of the action potential (see Chapter 15). All neurones are bounded by a lipid bilayer (cell membrane) within which proteins are located, some of which form ion channels (see Chapter 14); others form receptors to specific chemicals that are released by neurones (see Chapters 18 and 19) and others act as ion pumps moving ions across the membrane against their electrochemical gradient, e.g. Na+–K+ exchange pump (see Chapter 15).
The axonal surface membrane is known as the axolemma and the cytoplasm contained within it, the axoplasm. The ion channels within the axolemma imbue the axon with its ability to conduct action potentials while the axoplasm contains neurofilaments, microtubules and mitochondria. These latter organelles are not only responsible for maintaining the ionic gradients necessary for action potential production, but also allow for the transport and recycling of proteins away from (and to a lesser extent towards) the soma to the nerve terminal. This axoplasmic flow or axonal transport is either slow (∼1 mm/day) or fast (∼100–400 mm/day) and is not only important in permitting normal neuronal/synaptic activity but may also be important for neuronal survival and development and as such may be abnormal in some neurodegenerative disorders such as motor neurone disease as well as disorders associated with abnormalities of certain proteins such as tau (see Chapter 60). Many axons are surrounded by a layer of lipid, or myelin sheath, which acts as an electrical insulator. This myelin sheath alters the conducting properties of the axon, and allows for rapid action potential propagation without a loss of signal integrity (see Chapter 15). This is achieved by means of gaps, or nodes (of Ranvier), in the myelin sheath where the axolemma contains many ion channels (typically Na+ channels) which are directly exposed to the tissue fluid. The nodes of Ranvier are also those sites from which axonal branches originate, and these branches are termed axon collaterals. The myelin sheath encompasses the axon just beyond the initial segment and finishes just prior to its terminal arborization. The myelin sheath is formed by Schwann cells in the PNS and by oligodendrocytes in the central nervous system (CNS) (see Chapter 13), with many CNS axons being ensheathed by a single oligodendrocyte while in the PNS, one Schwann cell provides myelin for one internode.
The synapse is the junction where a neurone meets another cell, which in the case of the CNS is another neurone. In the PNS the target can be muscle, glandular cells or other organs. The typical synapse in the nervous system is a chemical one, which is composed of a presynaptic nerve terminal (bouton or end-bulb), and a synaptic cleft, which physically separates the nerve terminal from the postsynaptic membrane and across which the chemical or neurotrans-mitter from the presynaptic terminal must diffuse (see Chapter 16). This synapse is typically between an axon of one neurone and the dendrite of another (axodendritic synapse) although synapses are found where the point of contact between the axon and the postsynaptic cell is either at the level of the cell body (axosomatic synapses) or, less frequently, the presynaptic nerve terminal (axoaxonic synapse; see Chapter 17). A few synapses within the CNS do not possess these features but are low-resistance junctions (gap junctions) and are termed electrical synapses. These synapses allow for rapid conduction of action potentials without any integration and as such tend to enable populations of cells to fire together or in synchrony (see Chapters 16 and 61). They may also be important in the coupling of activity across cortical areas which may be important in some of the synchronized responses seen in the brain in sleep-wakefulness (see Chapters 43 and 44).
The specific loss of neurones is seen in a number of neurological disorders, and those diseases in which this is the primary event are discussed in Chapter 60.