Motor neuron

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. Motor neuron is often synonymous with efferent neuron.

Anatomy and physiology
According to their targets, motoneurons are classified into three broad categories:

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

“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” — "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).

In other words:
 * the motor command of skeletal and branchial muscles is monosynaptic (involving only one motoneuron —respectively “somatic “ and “branchial”— which synapses onto the muscle).
 * the command of visceral muscles is disynaptic (involving two neurons: the “general visceral motoneuron” 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 “motoneuron”, being the one that directly innervates the muscle (while the “general visceral motoneuron” is, strictly speaking, a “preganglionic” neuron). But, for historical reasons, the term motoneuron is reserved for the CNS neuron.

All motoneurons are cholinergic (that is, release the neurotransmitter acetylcholine). Parasympathetic ganglionic neurons are also cholinergic, while most sympathetic ganglionic neurons are noradrenergic (that is, release the neurotransmitter noradrenaline). (see Table)

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.


 * 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.  Although muscle innervation may eventually play a role in the maturation of motor activity. This is why muscle relaxants work by acting on the motoneurons that innervate muscles (by decreasing their electrophysiological activity) or on cholinergic neuromuscular junctions, rather than on the muscles themselves.

Somatic motoneurons
Somatic motoneurons are further subdivided into two types: alpha efferent neurons and gamma efferent neurons. (Both types are called efferent to indicate the flow of information from the central nervous system (CNS) to the periphery.)
 * Alpha motoneurons innervate extrafusal muscle fibers (typically referred to simply as muscle fibers) located throughout the muscle.
 * Gamma motoneurons innervate intrafusal muscle fibers found within the muscle spindle.

In addition to voluntary skeletal muscle contraction, alpha motoneurons also contribute to muscle tone, the continuous force generated by noncontracting muscle to oppose stretching. When a muscle is stretched, sensory neurons within the muscle spindle detect the degree of stretch and send a signal to the CNS. The CNS activates alpha motoneurons in the spinal cord which cause extrafusal muscle fibers to contract and thereby resist further stretching. This process is also called the stretch reflex.

Gamma motoneurons regulate the sensitivity of the spindle to muscle stretching. With activation of gamma neurons, intrafusal muscle fibers contract so that only a small stretch is required to activate spindle sensory neurons and the stretch reflex.

Motor units
A single motoneuron may synapse with one or more muscle fibers. The motoneuron and all of the muscle fibers to which it connects is a motor unit.