Motor neurons

In vertebrates, motor neurons (also called motoneurons) are efferent neurons that originate in the spinal cord or brain stem and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity.

Anatomy and physiology
Motoneurons of both the somatic and autonomic nervous system (ANS) originate in the ventral gray column of the spinal cord. Somatic fibers innervate skeletal muscle while autonomic fibers innervate cardiac muscle of the heart and smooth muscle of the visceral organs and glands.

In the somatic nervous system, the pathway of a motoneuron from the spinal cord to the skeletal muscle fiber is composed of a single motoneuron. By contrast, the analogous pathway in the ANS is composed of two motoneurons that synapse in an autonomic ganglion. Motoneurons of the ANS are thus called preganglionic and postganglionic depending on their position relative to their ganglion.

Motoneurons are further classified depending on the neurotransmitter they release. Those that release noradrenaline (norepinephrine) are called adrenergic, while those that release acetylcholine are dubbed cholinergic. All motoneurons are cholinergic except for most postganglionic fibers of the sympathetic nervous system, which are adrenergic. An exception is made for sympathetic postganglionic fibers that innervate sweat glands and certain blood vessels; these fibers are cholinergic.

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. This is why muscle relaxants work by acting on the nerves that innervate muscles (by decreasing their electrophysiological activity) or on cholinergic neuromuscular junctions, rather than on the muscles themselves.

Somatic motoneurons
There are two types of somatic motoneurons: 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 ol a motor unit.