Synaptic organization of sensory and motor neurones innervating triceps brachii muscles in the bullfrog

Abstract

The anatomy and physiology of sensory-motor pathways were studied in the brachial spinal cord of adult bullfrogs to characterize the properties and specificity of these connections. Motoneurons innervating a given forelimb muscle are located in discrete and reproducible regions of the lateral motor column. Only a fraction of the motoneurons in a particular region innervates any 1 muscle. The central projections of sensory afferent axons from the triceps muscles extend throughout the rostro-caudal length of the brachial spinal cord. Within this region, these projections terminate in an area containing many motoneuronal dendrites. Within the triceps motor pool, sensory neurons from the triceps muscles produce monosynaptic potentials only in triceps motoneurons even though these motoneurons are mingled with motoneurons innervating other muscles. Motoneurons innervating each of the 3 heads of the triceps muscles, medial, internal and external, receive monosynaptic input from their own, homonymous muscle head. Sensory fibers from the medial head also innervate 98% of the heteronymous motoneurons projecting to the internal or external heads, and nearly 90% of the medial triceps motoneurons are innervated by sensory axons from the other 2 heads. Other brachial motoneurons receive monosynaptic input from sensory axons in their own muscle nerves. Most of the synaptic potentials evoked in triceps motoneurons by stimulation of muscle nerves other than triceps are of longer latency and probably involve polysynaptic pathways. The pattern of synaptic connections between muscle sensory afferents and motoneurons in the frog''s spinal cord is specific. Comparison with homologous pathways in the cat''s spinal cord suggests that the strength and pattern of these connections are similar.