The motor unit is the only connection between the central nervous system (= the brain) and the skeletal muscles
A. Definitions and Structural components required:
A motor unit consists of:
a motor nerve cell which is located in the spinal cord.
an axon that goes from the nerve cell to a skeletal muscle.
several skeletal muscle fibers that are innervated by this particular motor axon.
The axon can be very short or very long, depending upon the distance from the spinal cord to the muscle. For example, in the case of the muscles in your big toe, the axon has to go through the length of your leg; about one meter!
In this figure, the motor nerve is connected to 5 muscle fibers. This is actually a small motor unit. Usually, a motor unit will connect (=innervate) many more muscle fibers. Typically a few hundred to a few thousand muscle fibers are connected to one single motor nerve.
B. A bit of information about the spinal cord:
The spinal cord is a part of the central nervous system. It is located in the vertebral column and runs from the base of the head down to the sacral region.
In this cross section (figure), the spinal cord consists of two parts; the greymatter and the white matter.
Make sure you understand the orientation of the spinal cord in this cross section; the upper part is dorsal and the lower part is ventral.
Inside the spinal cord, there are two types of tissues; the white matter and the grey matter (matter = tissue). The white matter consists of myelinated nerve fibers. They form tracks that go up (= to the brain) and down (=from the brain). It is white because of the myelination.
The grey matter consists mainly of nerve cell bodies (=nerve,soma), hence the grayish color.
The spinal cord is connected to the body by the spinal roots.
Between every pair of vertebra there are two roots coming into the spinal cord (at the dorsal side) and two roots going out of the spinal cord (at the ventral side).
The dorsal roots (incoming) contain the axons from sensors in the body. The ventral roots (outgoing) contain the axons that are connected to the effectors in the body, mainly muscles and glands.
Therefore the incoming nerves are called the sensory nerves while the outgoing nerves are called the motor nerves.
Those parts of the grey zones in the spine that are close to the in- or outgoing roots are called the horns.
Because of the Ventral-Dorsal orientation, these horns are therefore called the ventral horns and the dorsal horns.
The ventral hornis the home of the motor nerve cells (that is their soma).The axons of these cells go out of the spinal cord through the ventral roots and to their respective muscle cells that they innervate.