| Motor nerve | |
|---|---|
 Diagram of afferent and efferent innervation | |
| Details | |
| Identifiers | |
| Latin | nervus motorius |
| FMA | 5867 |
| Anatomical terms of neuroanatomy | |
Amotor nerve, orefferent nerve, is anerve that contains exclusivelyefferent nerve fibers and transmits motor signals from thecentral nervous system (CNS) to the effector organs (muscles andglands), as opposed tosensory nerves, which transfer signals from sensory receptors in the periphery to the CNS.[1] This is different from themotor neuron, which includes a cell body and branching of dendrites, while the nerve is made up of a bundle ofaxons. In the strict sense, a "motor nerve" can refer exclusively to the connection to muscles, excluding other organs.[2][3] The vast majority of nerves contain both sensory and motor fibers and are therefore calledmixed nerves.[4]
Motor nerve fiberstransduce signals from the CNS to peripheral neurons of proximal muscle tissue. Motor nerve axon terminals innervateskeletal andsmooth muscle, as they are heavily involved inmuscle control. Motor nerves tend to be rich inacetylcholine vesicles because the motor nerve, a bundle of motor nerve axons that deliver motor signals and signal for movement and motor control.[5] Calciumvesicles reside in theaxon terminals of the motor nerve bundles. The high calcium concentration outside of presynaptic motor nerves increases the size ofend-plate potentials (EPPs).[6]
Within motor nerves, each axon is wrapped by theendoneurium, which is a layer of connective tissue that surrounds themyelin sheath. Bundles of axons are calledfascicles, which are wrapped inperineurium. All of the fascicles wrapped in theperineurium are wound together and wrapped by a final layer of connective tissue known as theepineurium. These protective tissues defend nerves from injury, pathogens and help to maintain nerve function. Layers of connective tissue maintain the rate at which nerves conductaction potentials.[7]
_(14742157516).jpg)
Most motor pathways originate in themotor 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 throughmotor neurons once they exit the spinal cord.[1][7]
Motor nerves can vary based on the subtype ofmotor neuron they are associate with.[8]
Alpha motor neurons targetextrafusal 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.[8]
Beta motor neurons innervateintrafusal fibers ofmuscle spindles. These nerves are responsible for signaling slow twitch muscle fibers.[8]
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.[8]
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.[9]
There are problems withneuroregeneration 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 intoastrocytes which assists the surrounding neurons.Schwann cells 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.[10][11][12][13]
{{cite book}}: CS1 maint: multiple names: authors list (link)