The term primary afferents refers to the cells whose axons serve somatosensory receptors and that proceed to the central nervous system (CNS) through the dorsal roots of sensory nerve input to the spinal cord. The cell bodies of primary afferents are dorsal root ganglion cells located in the peripheral nervous system. The central axon synapses to postsynaptic cells at the segment level, at close segments, or at some distance from the entry point, even to the first synapse being in the brain stem at the dorsal column nuclei.
Primary afferents serve receptors for touch, thermal sensations, proprioceptive sensations from displacement of muscles and joints, and pain. Their sensory nerve classification is into groups I, II, III, and IV, on the basis of decreasing axonal diameter and decreasing conduction velocity. Most human physiology texts have a table showing these parameters, and most are currently incorrect (Peters & Brooke, 1998). This is because the data were obtained using feline nerves, which conduct much faster than human ones. Avery rough rule of thumb is to halve the velocity and diameter to translate from cat to human. Human Ia fibers conduct in the approximate range of 40 to 60 meters per second.
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