Effects of muscle and cutaneous hindlimb afferents on L4 neurons whose activity is modulated by neck rotation.

We recorded extracellularly, in decerebrate, labyrinthectomized cats, from spontaneously active L4 neurons whose activity was modulated by head rotation, and studied the effects of stimulation of ipsilateral hindlimb nerves. Rotation of the head about the longitudinal (roll) axis was more effective than rotation about the transverse (pitch) axis or vertical (yaw) axis for this group of neurons. Most units received convergent excitatory or inhibitory inputs from several nerves, with excitation being more prominent. The most effective muscle nerves were quadriceps (37/43 neurons), sartorius (19/21) and tibialis anterior (17/35); stimulation of biceps posterior-semitendinosus, biceps anterior-semimembranosus, or gastrocnemius rarely influenced the firing of the neurons. Group I effects were present in only a small fraction of neurons; however, short latency (central latency less than or equal to 5 ms) group II effects were observed in almost one-third. Longer latency group II as well as group III inputs were also common. All neurons received inputs from mixed and cutaneous nerves which usually had low thresholds and central latencies greater than 5 ms. Most recording sites were in medial lamina VII or lamina VIII; some of the units were identified by antidromic stimulation as propriospinal neurons which projected to the lumbar enlargement.