PSM V71 D109 A single isolated motor nerve cell of an ox spinal cord

A motor nerve is a

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the ...

that transmits motor signals from the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

(CNS) to the muscles of the body. This is different from the

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirect ...

, which includes a cell body and branching of dendrites, while the nerve is made up of a bundle of axons. Motor nerves act as efferent nerves which carry information out from the CNS to muscles, as opposed to afferent nerves (also called sensory nerves), which transfer signals from sensory receptors in the periphery to the CNS. Efferent nerves can also connect to

gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

s or other organs/issues instead of muscles (and so motor nerves are not equivalent to efferent nerves). In addition, there are nerves that serve as both sensory and motor nerves called mixed nerves.

Structure and function

Motor nerve fibers transduce signals from the CNS to peripheral neurons of proximal muscle tissue. Motor nerve axon terminals innervate

skeletal A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...

and

smooth muscle Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...

, as they are heavily involved in

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

control. Motor nerves tend to be rich in

acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

vesicles because the motor nerve, a bundle of motor nerve axons that deliver motor signals and signal for movement and motor control. Calcium

vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...

reside in the

axon terminal Axon terminals (also called synaptic boutons, terminal boutons, or end-feet) are distal terminations of the telodendria (branches) of an axon. An axon, also called a nerve fiber, is a long, slender projection of a nerve cell, or neuron, that cond ...

s of the motor nerve bundles. The high calcium concentration outside of presynaptic motor nerves increases the size of end-plate potentials (EPPs).

Protective tissues

Within motor nerves, each axon is wrapped by the endoneurium, which is a layer of connective tissue that surrounds the myelin sheath. Bundles of axons are called fascicles, which are wrapped in perineurium. All of the fascicles wrapped in the perineurium are wound together and wrapped by a final layer of connective tissue known as the

epineurium The epineurium is the outermost layer of dense irregular connective tissue surrounding a peripheral nerve. It usually surrounds multiple nerve fascicles as well as blood vessels which supply the nerve. Smaller branches of these blood vessels pene ...

. These protective tissues defend nerves from injury, pathogens and help to maintain nerve function. Layers of connective tissue maintain the rate at which nerves conduct action potentials. Hand-book of physiology (1892) (14742157516)

Hand-book of physiology (1892) (14742157516)

Spinal cord exit

Most motor pathways originate in the motor cortex of the brain. Signals run down the brainstem and spinal cord ipsilaterally, on the same side, and exit the spinal cord at the ventral horn of the spinal cord on either side. Motor nerves communicate with the muscle cells they innervate through

s once they exit the spinal cord.

Motor nerve types

Motor nerves can vary based on the subtype of

they are associate with.

Alpha

Alpha motor neuron Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their co ...

s target

extrafusal muscle fiber Extrafusal muscle fibers are the standard skeletal muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. They make up the large mass of skeletal striated muscle ti ...

s. The motor nerves associated with these neurons innervate extrafusal fibers and are responsible for muscle contraction. These nerve fibers have the largest diameter of the motor neurons and require the highest conduction velocity of the three types.

Beta

Beta motor neurons innervate intrafusal fibers of muscle spindles. These nerves are responsible for signaling slow twitch muscle fibers.

Gamma

Gamma motor neurons, unlike alpha motor neurons, are not directly involved in muscle contraction. The nerves associated with these neurons do not send signals that directly adjust the shortening or lengthening of muscle fibers. However, these nerves are important in keeping muscle spindles taut.

Neurodegeneration

Motor neural degeneration is the progressive weakening of neural tissues and connections in the nervous system. Muscles begin to weaken as there are no longer any motor nerves or pathways that allows for muscle innervation. Motor neuron diseases can be viral, genetic or be a result of environmental factors. The exact causes remain unclear, however many experts believe that toxic and environmental factors play a large role.

Neuroregeneration

There are problems with

neuroregeneration Neuroregeneration refers to the regrowth or repair of nervous tissues, cells or cell products. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Neuroregeneration differs between the peripheral nervous syst ...

due to many sources, both internal and external. There is a weak regenerative ability of nerves and new nerve cells cannot simply be made. The outside environment can also play a role in nerve regeneration. Neural stem cells (NSCs), however, are able to differentiate into many different types of nerve cells. This is one way that nerves can "repair" themselves. NSC transplant into damaged areas usually leads to the cells differentiating into astrocytes which assists the surrounding neurons.

Schwann cell Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann) are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory en ...

s have the ability to regenerate, but the capacity that these cells can repair nerve cells declines as time goes on as well as distance the Schwann cells are from site of damage.

References

{{Nervous tissue Nervous system