|Neuron: Motor neuron|
|Nissl substance ("tigroid" cytoplasm). H&E-LFB stain.|
|Location||Ventral horn of the spinal cord, some cranial nerve nuclei|
|Function||Excitatory projection (to NMJ)|
|Neurotransmitter||UMN to LMN: glutamate; LMN to NMJ: ACh|
|Presynaptic connections||Primary motor cortex via the Corticospinal tract|
|Postsynaptic connections||Muscle fibers and other neurons|
In neurology, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (CNS) that project their axons outside the CNS to directly or indirectly control muscles. Motor neurons are efferent nerves also called effector neurons, that carry signals from the spinal cord to the muscles to produce (effect) movement. Examples of motor neurons are primary motor neurons, alpha motor neurons, beta motor neurons and gamma motor neurons.
A single motor neuron may innervate many muscle fibres (muscle cells), and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch (fasciculation). As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through summation or tetanus. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch. The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. On the other hand, tetanus is caused by constant, very high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, and tension rises smoothly eventually reaching a plateau.
Anatomy and physiology
|Branch of NS||Position||Neurotransmitter|
| *Except fibers to sweat glands and certain blood vessels|
According to their targets, motor neurons are classified into three broad categories:
Somatic motor neurons, which originate in the central nervous system, project their axons to the target tissues, which are always skeletal muscles. Skeletal muscles are involved in locomotion (such as the muscles of the limbs, abdominal, and intercostal muscles).
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.
All vertebrate motor neurons are cholinergic, that is, they release the neurotransmitter acetylcholine. Parasympathetic ganglionic neurons are also cholinergic, whereas most sympathetic ganglionic neurons are noradrenergic, that is, they release the neurotransmitter noradrenaline. (see Table)
The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motor neuron releases a flood of neurotransmitters that bind to postsynaptic receptors and triggers a response in the muscle fiber which leads to muscle movement.
- In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either 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 vertebrates is obtained only by inhibition of the motor neuron itself. Although muscle innervation may eventually play a role in the maturation of motor activity. This is how 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 motor neurons
Somatic motoneurons are the alpha efferent neurons, beta efferent neurons, and gamma efferent neurons. (They are called efferent to indicate the flow of information from the central nervous system (CNS) to the periphery.)
- Alpha motor neurons innervate extrafusal muscle fibers (a type of slow switch fiber) located throughout the muscle. Their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells.
- Beta motor neurons innervate intrafusal muscle fibers of muscle spindles, with collaterals to extrafusal fibres.
- Gamma motor neurons innervate intrafusal muscle fibers found within the muscle spindle.
In addition to voluntary skeletal muscle contraction, alpha motor neurons 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.
According to Purves D, et al, a single motor neuron may synapse with one or more muscle fibers. The motor neuron and all of the muscle fibers to which it connects is a motor unit. Motor units are split up into 3 categories: slow motor units, fast fatiguing motor units, and fast fatigue-resistant motor units.
- Slow motor units are used to stimulate small muscle fibres which contract very slowly and provide small amounts of energy but are very resistant to fatigue, so they are used to sustain muscular contraction such as keeping the body upright.
- Fast fatiguing motor units are used to stimulate larger muscle groups which apply large amounts of force but fatigue very quickly. They are used for tasks that requires large brief bursts on energy such as jumping or running.
- Fast fatigue-resistant motor units stimulate moderate-sized muscles groups that don't react as fast as the FF motor units, but can be sustained much longer, as implied by the name, and provide more force than S motor units.