Moter Nerves

Moter Nerves In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (CNS) which project their axons outside the CNS and directly or indirectly control muscles. The term is synonymous with efferent neurons.

Moto ne urons ar e classi fied into thr ee br oad ca te go ries "Somatic motoneurons", which directly innervate skeletal muscles, involved in locomotion (such as muscles of the limbs, abdominal and intercostal muscles ).

“Special visceral motoneurons” ►

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

“ General visceral motoneurons ”  “General visceral motoneurons” — "visceral motoneurons" for short— which indirectly innervate smooth muscles of the viscera (like the heart, or 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).

Neurotransmitters Branch of NS

Position

Neurotransmitter

Somatic

n/a

Acetylcholine

Parasympathetic

Preganglionic

Acetylcholine

Parasympathetic

Ganglionic

Acetylcholine

Sympathetic

Preganglionic

Acetylcholine

Sympathetic

Ganglionic

Norepinephrine

Function • The interface between a motoneuron and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motoneuron releases a flood of neurotransmitters that bind to postsynaptic receptors and triggers a response in the muscle fiber.

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In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory

• For vertebrates, however, the response of a muscle fiber to a neurotransmitter can only be excitatory, in other words, contractile. Muscle relaxation and inhibition of muscle contraction in verterbrates is obtained only by inhibition of the motoneuron itself. This is why muscle relaxants work by acting on the motoneurons that innervate

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muscles (by decreasing their electrophysiological activity) or on cholinergic neuromuscular junctions, rather than on the muscles themselves.

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