Overview

Sensory systems are the nexus between the external world and the central nervous system (CNS). Afferent neurones of the somatosensory system continuously ‘taste their environment’ (Koltzenburg, 1999). They respond in a co-ordinated fashion, in order to instruct an integrated efferent response, which will retain the homeostatic integrity of the organism and curtail any tissue-damaging stimuli. This chapter will consider the peripheral apparatus that responds (and in some cases adapts) to a potentially injurious or noxious stimuli. Nociception forms an integral part of the somatosensory nervous system, whose main purpose can be described by exteroceptive, proprioceptive and interoceptive functions.

Exteroceptive functions include mechanoreception, thermoception and nociception. Proprioceptive functions provide information on the relative position of the body and limbs that arise from input from joints, muscles and tendons. Interoceptive information details the status and well-being of the viscera. These broad sensory modalities can be further subdivided in order to integrate more subtle stimuli (e.g. difference between flutter and vibration). In order to cope with the immense variety and magnitude of stimuli that impinge upon the CNS; sensory neurones are vastly heterogeneous and exquisitely specialized.

Heterogeneity of sensory neurones

Primary sensory neurones, whose cell bodies reside in the dorsal root ganglia (DRG), can be classified according to their cell body size, axon diameters, conduction velocity, neurochemistry, degree of myelination and ability to respond to neurotrophic factors (NTFs) (see Figure 2.1 and Table 2.1 for overview of classification).