Motor neuron

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In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons.

Contents

Anatomy and physiology

According to their targets, motor neurons are classified into three broad categories:

Somatic motor neurons, which directly innervate skeletal muscles, involved in locomotion (such as the muscles of the limbs, abdominal, and intercostal muscles).

Special visceral motor neurons — also called “branchial motor neurons”— which directly innervate branchial muscles (that motorize the gills in fish and the face and neck in land vertebrates).

General visceral motor neurons — "visceral motor neurons" for short — which indirectly innervate cardiac muscle and smooth muscles of the viscera ( the muscles of the arteries): they synapse onto neurons located in ganglia of the autonomic nervous system (sympathetic and parasympathetic), located in the peripheral nervous system (PNS), which themselves directly innervate visceral muscles (and also some gland cells).

In other words:

  • the motor command of skeletal and branchial muscles is monosynaptic (involving only one motor neuron, respectively, somatic and branchial, which synapses onto the muscle).
  • the command of visceral muscles is disynaptic (involving two neurons: the “general visceral motor neuron” located in the CNS, which synapses onto a ganglionic neuron, located in the PNS, which synapses onto the muscle).

It could be argued that, in the command of visceral muscles, the ganglionic neuron — parasympathetic or sympathetic — is the real “motor neuron”, being the one that directly innervates the muscle (whereas the “general visceral motor neuron” is, strictly speaking, a “preganglionic” neuron). But, for historical reasons, the term motor neuron is reserved for the CNS neuron.

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