Tonic and phasic spinal cord mechanisms in man

Skilled movements of any sort require the assump-tion of an appropriate posture. Most skilled move-ments comprise rapid, repetitive, alternating and coordinated activity, chiefly of distal muscles. Most postures demand constant sustained activity chiefly of proximal muscles and flexor groups in the upper limb and extensor groups in the lower limb. These functions of voluntary movement, designated respectively as phasic and tonic, have their counter-parts in reflex action. The tendon jerk (or its reflex equivalent elicited by electrical stimulation of afferent fibres) results from a neural discharge so brief as to be classified as a phasic response, while the maintained reflex contraction elicited by more prolonged muscle stretch in a suitably tense subject or a spastic patient has the duration of a tonic response. It is of considerable interest to know whether there are two different populations of alpha motor neurones in the anterior horns of the spinal cord, each with separate characteristics, and each in-nervating a particular type of muscle fibre, or whether a particular cell might alter its properties depending upon the manner of its activation and the prevailing influence of other cells upon it. By recording from single efferent fibres of the ventral nerve roots of decerebrate cats, Granit, Henatsch, and Steg (1956) were able to divide motor neurone responses fairly clearly into two categories. Motor neurones, which discharged once or twice on the rising phase of the stretch stimulus, did not continue to discharge during sustained stretch, and were not affected by a preceding afferent tetanus, were classified as phasic (53 out of 100 motor fibres studied). Motor neurones whose activity continued during the plateau of stretch and was greatly increased by preliminary tetanization were classified as tonic (found in 44 out of 100 fibres). Three fibres fired continuously during sustained stretch with or without a conditioning afferent tetanus. It was noted that small spike potentials were usually those of tonic cells and the larger potentials were as-sociated with phasic responses. Granit, Phillips,