Note: Sensory fibres carried by the olfactory, optic and vestibulocochlear nerves are not dealt with in this chapter. Consult Chapters 18, 20 and 23.

The basic plan of sensory systems

The basic sensory system consists of three neuronal groups:

• primary sensory neurons from receptor to central nucleus, with its cell body in a peripheral sensory ganglion;

• secondary sensory neurons from nucleus to diencephalon (usually the thalamus);

• tertiary sensory neurons from thalamus to cortex.

There are no synapses outside the brain and spinal cord: the first synapse is in the central nervous system (CNS) between primary and secondary sensory neurons.

Primary sensory neuron: receptor to sensory nucleus

This extends from peripheral receptor to CNS. The cell body is situated in a peripheral ganglion (dorsal root ganglion for spinal nerves) and the neuron is usually pseudounipolar, that is to say, it gives rise to a single axon which bifurcates into a peripheral process passing to the receptor, and a central process passing into the CNS. The cell body is thus both structurally and electrically out on a limb. The central process of the primary sensory neuron terminates by synapsing in a central nucleus which consists of cell bodies of the neuron next in the pathway …

Secondary sensory neurons: sensory nucleus to thalamus

The axons of these neurons ascend from the nucleus, which contains their cell bodies, to the contralateral thalamus (in the diencephalon), decussating soon after leaving the nucleus.