Introduction

It is well known that much of the sensory input to the central nervous system can be modulated by centrifugally organized control systems that originate in the central nervous system (Head and Holmes, 1911; Hagbarth, 1960). The control mechanisms can be excitatory or inhibitory processes that may occur in the periphery or within the central nervous system. Inhibition can be at pre- and/or postsynaptic sites (Fig. 6.1(I)). Presynaptic inhibition at the first central synapse of a sensory pathway has the potential advantage of being able to reduce sensory input prior to wide dissemination of that sensory input within the central nervous system through the activation of interneuronal networks and multiple ascending pathways, for example, in the spinal cord (Schmidt, 1973; see Chapter 3).

Pre- and postsynaptic inhibition can have somewhat different effects on the stimulus-response curves of second-order sensory neurons, as shown in Fig. 6.1(II). Postsynaptic inhibition involves inhibitory postsynaptic potentials that sum with excitatory postsynaptic potentials (Fig. 6.1(IIA)). If there is a linear summation, the stimulus-response curve will be shifted to the right in a parallel fashion (Carstens et al., 1980). However, if the IPSP is generated in a membrane area near that in which the EPSP is generated, the excitatory current may be shunted and the slope of the stimulus-response curve reduced, causing a reduction in the gain of synaptic transmission (Fig. 6.1(IIB)). A similar reduction in gain can be produced by presynaptic inhibition.