Gain Control in Human Primary Afferent Transmission Over Spinal Paths

In humans, Ia transmission monosynaptically to leg motoneurons—for example, the Hoffmann (H) reflex—is attenuated by muscle spindle activation of Ia afferents serving uniarticular extensor muscles of the hip or knee or, probably, ankle. The inhibitory pathway is, at least in part, spinal through presynaptic inhibitory interneurons (Brooke et al., 1997). Such centripetal sensorisensory conditioning is complemented by centrifugal conditioning arising from the brain and from central pattern-generating networks of the spinal cord and brain stem (Rudomin et al., 1998). For example, immediately before voluntary plantar flexion movement, H-reflex magnitudes in the plantar flexor muscle soleus increase considerably.

There seems to be clear separation of the control of Ia afferents and cutaneous afferents during locomotion. Locomotor-induced modulation of cutaneous afferent affects on motoneurons appears to arise from centrifugal conditioning but, unlike the Ia pathways previously described, not as attenuation from centripetal conditioning from somatosensory afferents activated as a consequence of the movement per se (Brooke, McIlroy, Staines, Angerilli, & Peritore, 1999).

Primary afferent activation can also reveal novel membrane characteristics of motoneurons. Abrief burst of Ia afferent firing can reset membrane currents so that plateau potentials occur in mammalian motoneurons (e.g., in stance), being a rapid series of action potentials from a depolarized plateau (Kiehn, 1991). Such repetitive motoneu-ronal firing, continuing well beyond the duration of the Ia afferent burst, can then be terminated by a brief burst of firing of high threshold primary afferents.

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