The Synaptic Link Between the Sensory and Motoneurones in the Eye-Withdrawal Reflex of the Crab

Abstract

The axon of the larger of the two motoneurones in the optic tract of Carcinus which bring about the rapid eye withdrawal is 30–50 µ in diameter and has an unusually thin sheath compared with crab leg neurones of similar diameter. Within the brain the axon of the large eye-withdrawal motoneurone tapers fairly rapidly to 5–10µ. It extends diagonally across one side of the brain and branches in two near the mid line but does not pass the contralateral side of the brain. The axon terminals and the cell body have not been seen. A single stimulus to the afferent inputs of the motoneurone produces a characteristic burst of impulses followed by an irregular train of impulses. The excitatory effects of all tested afferent inputs to the motoneurone are summed before affecting a common spike-initiating locus on the motoneurone. Intracellular recordings from the motoneurone within the brain show two types of subthreshold activity to follow a single pre-synaptic volley : a smooth graded depolarization and small superimposed depolarizations of constant amplitude and duration. The synaptic link between afferents and the eye-withdrawal motor neurone is thought to be via a direct, possibly monosynaptic pathway and also by way of interneurone collaterals.