Cells of the nervous system

Neuronal structure

The main structural features common to all neurones have been described briefly in Chapter 1 . There are, however, numerous variations on the basic plan. The size of the cell body (soma) varies considerably, depending upon its location and function. For example, some interneurones in the CNS have cell bodies as small as 5 µm in diameter, while the cell bodies of motor neurones innervating striated muscle may exceed 100 µm. The size of the cell body is usually correlated with the length of the axon. Therefore small interneurones usually have short axons, perhaps only a fraction of a millimetre in length. At the other extreme, large motor neurones possess long axons (e.g. those passing from the spinal cord to the muscles of the foot have axons about 1 m in length).

The dendritic arborisation of neurones also shows great variation in the number, size and density of branches, which reflects the complexity of afferent inputs to the cell. For example, pyramidal cells in the cerebral cortex have one or two apical dendrites which course towards the pial surface ( Fig. 2.1A ), while Purkinje cells in the cerebellar cortex have highly complex, tree-like dendritic arborisations ( Fig. 2.1B ).

The configuration of the cell body in relation to the dendrites and axon follows one of three basic patterns ( Fig. 2.2 ). Multipolar neurones are by far the most common. Typically, they possess an axon and a number of dendrites that arise directly from the cell body. Motor neurones are a good example. Bipolar neurones have a centrally placed cell body, from which extend a single dendrite and a single axon. Bipolar neurones occur in the afferent pathways of the visual, auditory and vestibular systems. Unipolar neurones possess a single process emerging from the cell body. This divides into dendritic and axonal branches. Neurones of this type constitute the primary afferents of spinal and some cranial nerves, having their cell bodies in the dorsal root ganglia and sensory ganglia of cranial nerves.

Like most other cells, neurones possess a nucleus. This is usually located in the centre of the cell body and contains the chromosomal DNA. The rest of the intracellular space is occupied by cytoplasm, which contains numerous organelles and inclusions ( Fig. 2.3 ). Many of these are common to cells other than neurones, but some have particular prominence or significance in neurones. Numerous microscopic clumps of Nissl granules (Nissl bodies, Nissl substance) can usually be seen in nerve cell bodies stained with basophilic dyes. Nissl granules consist of rough endoplasmic reticulum and associated ribosomes. The ribosomes contain RNA (which accounts for the basophilic staining properties) and are the sites of protein synthesis. Nerve cells are highly metabolically active and, therefore, the Nissl granules are often very prominent ( Fig. 2.4 ).

Neurones contain a complex meshwork of structural protein strands called neurofilaments , which are assembled into larger neurofibrils ( Fig. 2.3 ). They also possess a system of neurotubules that are involved in the transport of materials throughout the cell. Transport of materials occurs both away from and towards the cell body ( anterograde and retrograde transport , respectively). This phenomenon is exploited in experimental neuroanatomical tracing techniques used to elucidate neuronal connectivity.

Some neurones contain pigment granules. Neuromelanin is a brown-black pigment produced as a by-product of the synthesis of catecholamines. Neuromelanin thus occurs most abundantly in cell groups that utilise catecholamines as their neurotransmitter, notably the pars compacta of the substantia nigra in the midbrain and the locus coeruleus in the pons. Lipofuscin is a yellow-brown pigment that accumulates in some neurones with age.

Each nerve cell is a separate physical entity with a limiting cell membrane. In order for information processing to occur in networks of neurones, therefore, information has to pass between neurones. This occurs at synapses. The basic structure of the synapse has been outlined in Chapter 1 . The most common location for synapse formation is between the terminal axonal arborisation of one neurone and the dendrite of another ( axodendritic synapse). Other locations are also possible and they give axosomatic , axoaxonal and dendrodendritic synapses. Neurotransmission between neurones occurs by release of specific chemical agents from the presynaptic ending that act upon receptors in the postsynaptic membrane.