MECHANISMS OF ACTIVATION AND MOTOR CONTROL OF STRETCH RECEPTORS IN LOBSTER AND CRAYFISH

Abstract

Stretch receptors recently described by Alexandrowicz situated in the tails of lobsters and cray-fish were investigated. In general design, physiological organization and performance they resemble the vertebrate muscle spindles. The receptor organs are made up of 2 functionally and anatomically distinct units. They consist of 2 fine muscle strands, each with one sensory neuron which sends its terminal branches into a noncontractile portion dividing the muscular elements. The latter are innervated by motor nerves which cause contraction and thereby afferent discharges in the sensory neuron. The receptors can be excited by external (passive) stretch or by direct neural control activating a mechanical trigger system within the sense organ itself. The mechanisms of this action were explored in situ and in complete isolation. One receptor muscle, with fine striation and connected with a fast-adapting sensory neuron gives twitch-like contractions and has a high fusion frequency (about 50/second) when excited through its motor nerve. The 2d receptor muscle strand, with coarse striation and linked to a slowly adapting neuron produces relatively slow contractile changes and has a low fusion frequency (about 5/second). These 2 muscular elements were named the "fast" (twitch) and "slow" bundle. The tension changes in the fast and slow bundles following motor nerve stimulation were recorded and correlated with afferent discharges. In a "quiescent" receptor, efferent impulses can initiate sensory discharges. These tend to be single or grouped, when arising in the neuron of the fast receptor bundle, reflecting the individual twitch-like tension increments which are set up by each motor stimulus. The afferent discharge frequency from the slow receptor strand increases relatively smoothly until a constant level is attained, reflecting the slowly rising and then steadily maintained tension changes. In both receptor systems the effectiveness of motor nerve stimulation is increased by higher initial stretch and decreased by lower stretch of the receptor organ. The results of alterations of external stretch on the sensory discharge can be largely compensated for by changing the frequency of motor stimulation. The receptor muscle bundles consist of small individual fibers with a resting potential of 60-75 mV measured with intracellular leads. Motor nerve stimuli set up relatively slow potential changes which were identified as endplate potentials (epps). The potentials rise to 5-25 mV in several mseconds and decay along an approximately exponential time course in a further 30-50 mseconds in the fast fibers and in 100-150 mseconds in the slow fibers. With repetitive stimulation successive epps increase in size (facilitation), their potentials sum and reach a depolarization plateau. Using intracellular recording, epp can be found at all places in the receptor muscles and from the spatial distribution it is concluded that individual muscle elements are densely innervated along their whole course. Several profusely branching motor nerves innervate each muscle bundle. In the slow receptor bundle no spikes with membrane reversal (overshoot) were seen and it is concluded that this bundle is principally activated through local graded contractions which occur at multiple points. In the fast bundle spikes accompany the contractions which, however, may be confined to a portion of the muscle strand. Stretch greatly facilitated the processes which lead to activation of the contractile system.