Recurrent inhibitory connexions among neck motoneurones in the cat.

1. Intracellular recordings were made from motoneurones innervating neck muscles in the cat. Dorsal roots were cut and muscle nerves electrically stimulated to activate alpha motor axons. 2. Recurrent inhibitory post-synaptic potentials (i.p.s.p.s) evoked by antidromic volleys in homonymous or heteronymous nerves were found in the majority of motoneurones studied, including those to dorsal neck muscles (biventer cervicis, splenius and complexus) as well as to occipitoscapularis and levator scapulae ventralis. 3. Central latencies of the recurrent i.p.s.p.s indicate disynaptic transmission. Amplitudes ranged from 100 microV (criterion level) to 2.2 mV. Average amplitudes were less than 0.6 mV. 4. The recurrent i.p.s.p.s were distributed to non-synergistic as well as to synergistic motoneurones. Analysis of relative strength of recurrent inhibition indicates influence of proximity of motoneurone pools, functional relatedness of muscles, as well as other factors. Variation in intrinsic motoneuronal properties probably underlies positive correlations (independent of variation in resting potential) between recurrent i.p.s.p.s evoked from different sources in motoneurones of a single pool. 5. Recordings (mainly extracellular) were also made from interneurones (Renshaw cells), located in the C3 and C4 segments of the spinal cord, that were excited by antidromic volleys in muscle nerves. The response varied from a single action potential to a burst of up to nineteen action potentials. Central latencies to the first response indicate monosynaptic transmission. Many Renshaw cells were excited by antidromic volleys in several muscle nerves, though this was restricted to nerves of the same segmental level as the Renshaw cell. All the muscle nerves studied were effective in activating Renshaw cells. 6. The results indicate that in many ways the recurrent i.p.s.p.s and the responses of Renshaw cells recorded in the neck segments resemble those in the hind-limb segments. Thus, the basic organization of recurrent inhibition in the neck segments resembles that occurring elsewhere in the spinal cord. A difference is the tendency for recurrent i.p.s.p.s in neck motoneurones to be relatively small in amplitude and Renshaw cell responses to be less strong than those recorded in the hind-limb segments. It is suggested that this is related to the segmentation of neck muscles and their motoneurone pools. 7. It is concluded that recurrent inhibition is a prominent feature of spinal organization governing neck muscles. It can therefore be expected to participate in control of head movements.