Figure 1. Diagrammatic representation of a section through a mammalian spinal cord. The central shaded area represents the grey matter, which contains mainly the cells of the central nervous system, while the surrounding lightly shaded area contains myelinated axons passing up or down the cord. (a) A primary afferent (sensory) neuron in the dorsal root ganglion with one of its central processes entering the dorsal horn of the grey matter and a motor neuron in the ventral horn of the grey matter with its process passing via the ventral root to a somatic muscle where it forms a close and specialized junction (a motor end plate) on the striated muscle cells. (b) A sympathetic preganglionic cell located in the lateral horn (characteristically only found in the thoracic region in mammals) of the grey matter. The peripheral process also passes out via the ventral root but it leaves the ventral root to synapse in sympathetic ganglia; the postsynaptic (postganglionic) nerve fibre then forms en passant synapses with exocrine glands or smooth muscle cells.

Figure 2. Highly diagrammatic view of the ventral aspect of a mammalian nervous system. Nerves labelled as in humans. Although eight cervical nerves is the mammalian norm, the numbers of nerves/segments in the thoracic, lumbar, sacral and coccygeal (tail) regions varies between species.

Figure 3. The formation of the myelin sheath on a peripheral nerve.

Figure 4. Diagrammatic representation of the nerves of the thoracolumbar spinal segments and the connections with the sympathetic ganglia. Not shown are the fibres that ascend or descend between ganglia. Some preganglionic fibres ascend into the neck where they form cervical ganglia. Other fibres pass out into the mesentery of the gut to form coeliac and mesenteric ganglia which then supply the intestine.

Figure 5. Diagrammatic summary of the main types of connections made by the motor components of the peripheral nervous system. The horizontal dotted line represents the anatomical limit of the central nervous system. The motor neurons controlling the striated musculature of the body are shown contacting a crosshatched structure. The parasympathetic and sympathetic systems are shown contacting structures representative of smooth muscle. The last two parts of the diagram on the right are the exceptions to the general arrangement of the sympathetic system. One relies on the transmitter (or hormone) from the adrenal gland being carried via the bloodstream to the target cells and the other is the special vasodilator innervation that relaxes the smooth muscle cells wrapped around the arterial blood vessels within limb muscles. Ach, acetylcholine; Norad, noradenaline; Adr, adrenaline.