Motor Neurons and Spinal Control of Movement

Motor neurons are among the largest neurons in the central nervous system and have long axons that travel along peripheral nerves to innervate skeletal muscles. They are the final common pathway through which the brain controls bodily movement. Motor neurons receive excitatory and inhibitory synaptic inputs from sensory afferents and from pathways of supraspinal origin either directly or via interneurons. The intrinsic properties of motor neurons determine how these inputs are transformed into a sequence of action potentials that elicit muscle contraction. Motor neuron intrinsic properties are affected by neuromodulatory inputs from supraspinal and spinal pathways. Motor neurons are recruited in ways that make effective use of the muscle fibres they innervate. All motor actions – whether stereotyped behaviours (such as locomotion or withdrawal reflexes), or uniquely specified sequences of muscle contraction – reflect the interaction of supraspinal commands, sensory inputs and spinal cord interneurons. Key Concepts: Motor neurons are large neurons in the brainstem and spinal cord that innervate skeletal muscle; those innervating the same muscle are grouped in proximity to form a motor neuron pool. The electrophysiological properties of an individual motor neuron are appropriately matched to the contractile properties of the muscle fibres it innervates; together, a motor neuron and its muscle fibres form a motor unit. Motor neurons receive two types of synaptic inputs from descending pathways, spinal interneurons and segmental afferents: (1) excitatory and inhibitory inputs that directly affect membrane potential and (2) neuromodulatory inputs that alter the effects of the excitatory and inhibitory inputs on membrane potential. The intrinsic properties of a motor neuron and the neuromodulatory influences it receives determine how its excitatory and inhibitory synaptic inputs are translated into a firing pattern that elicits muscle contraction. Motor neurons are recruited in a stereotyped order according to the ‘size principle’, in which low-force motor units are activated first, and sequentially higher force motor units are subsequently added to produce the total muscle force appropriate for the current action. Networks (often called central pattern generators) composed of spinal interneurons support locomotion and other cyclical movement patterns by distributing rhythmic synaptic activity among different motor neuron pools in appropriate temporal sequences. Keywords: spinal cord; motoneuron; motor neuron; reflex; movement; motor control; neuromodulation