A motor nerve, or efferent nerve, is a nerve that contains exclusively efferent nerve fibers and transmits motor signals from the central nervous system (CNS) to the effector organs ( and ), as opposed to , which transfer signals from sensory receptors in the periphery to the CNS.
Structure and function
Motor nerve fibers transduce signals from the CNS to peripheral neurons of proximal muscle tissue. Motor nerve axon terminals innervate
Skeletal muscle and smooth muscle, as they are heavily involved in
muscle control. Motor nerves tend to be rich in
acetylcholine vesicles because the motor nerve, a bundle of motor nerve axons that deliver motor signals and signal for movement and motor control.
Calcium vesicles reside in the
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. Bundles of axons are called
Nerve fascicle, 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. 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.
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
once they exit the spinal cord.
Motor nerve types
Motor nerves can vary based on the subtype of
motor neuron they are associate with.
Alpha
Alpha motor neurons target extrafusal muscle fibers. 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 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
which assists the surrounding neurons.
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.
See also
