Afferent System Overview

The afferent, or sensory, systems include visual, auditory, somatosensory, and interoceptive (ie, pain, temperature, and visceral sensation) inputs to the central nervous system. This chapter briefly reviews principles of transduction, relay, and processing of sensory information. The dorsal column–medial lemniscal system is reviewed in more detail. However, pain, vision, olfaction, and hearing are reviewed in subsequent chapters. Sensory transduction refers to the transformation of a stimulus into an electric signal. This process involves several distinct families of cation channels and associated receptor types.

Evoked potentials

Evoked potentials (EPs) occur in the peripheral and the central nervous system. The low amplitude signals are extracted from noise by averaging multiple time epochs time-locked to a sensory stimulus. The mechanisms of generation, the techniques for stimulation and recording are established. Clinical applications provide robust information to various questions. The importance of EPs is to measure precisely the conduction times within the stimulated sensory system. Visual evoked potentials to a pattern reversal checker board stimulus are commonly used to evaluate the optic nerve. Auditory evoked potentials following ‘click’ stimuli delivered by a headset are most often used to test the auditory nerve and for prognostication in comatose patients. Somatosensory evoked potentials to electrical stimulation of distal nerves evaluate the peripheral nerve and the lemniscal system, and have various indications from demyelinating diseases to the monitoring of operations and prognosis of comatose patients.

Response Properties of Whisker-Related Neurons in Rat Second Somatosensory Cortex

In addition to a primary somatosensory cortex (SI), the cerebral cortex of all mammals contains a second somatosensory area (SII); however, the functions of SII are largely unknown. Our aim was to explore the functions of SII by comparing response properties of whisker-related neurons in this area with their counterparts in the SI. We obtained extracellular unit recordings from narcotized rats, in response to whisker deflections evoked by a piezoelectric device, and compared response properties of SI barrel (layer IV) neurons with those of SII (layers II to VI) neurons. Neurons in both cortical areas have similar response latencies and spontaneous activity levels. However, SI and SII neurons differ in several significant properties. The receptive fields of SII neurons are at least five times as large as those of barrel neurons, and they respond equally strongly to several principal whiskers. The response magnitude of SII neurons is significantly smaller than that of neurons in SI, and SII neurons are more selective for the angle of whisker deflection. Furthermore, whereas in SI fast-spiking (inhibitory) and regular-spiking (excitatory) units have different spontaneous and evoked activity levels and differ in their responses to stimulus onset and offset, SII neurons do not show significant differences in these properties. The response properties of SII neurons suggest that they are driven by thalamic inputs that are part of the paralemniscal system. Thus whisker-related inputs are processed in parallel by a lemniscal system involving SI and a paralemniscal system that processes complimentary aspects of somatosensation.