There are only two basic types of motor endings, those found between somatic motor neurons and skeletal muscle fibers (neuromuscular junctions), and those found between autonomic motor fibers and their effectors. We will be covering the more complex and more numerous somatic neuromuscular junction here.
The Neuromuscular Junction:
Imagine one of your quadricep muscles. Picture the nerve that innervates it. Each somatic motor neuron contained within the nerve branches wildly once it enters the muscle. These branches will ultimately innervate each individual skeletal muscle fiber present. The junction between these axon terminals and the muscle fiber cell membranes is know as the neuromuscular junction.
As each axon branch reaches its target muscle fiber, it splits into several more smaller branches called axon terminals. Each terminal of this group of axon terminals forms a junction, or synapse with a specific region of the muscle cell membrane (sarcolemma). The picture above illustrates the end of a single axon terminal and its junction with a small piece of sarcolemma called a motor end plate.
There are three key structures present in all neuromuscular junctions.
1. The axon terminal
2. The synaptic cleft
Once a motor impulse travels down the motor neuron and reaches the axon terminal, acetylcholine is released into the synaptic cleft. This chemical travels across the cleft where it binds to acetylcholine ligand-gated channels on the sarcolemma. This results in the opening of these chemically gated channels allowing sodium to rush in at a large rate, while allowing potassium to leave the cell at a slower rate. The result is a net depolarization of the sarcolemma’s membrane potential.
This results in the formation of a graded potential known as an end plate potential. This potential spreads to adjacent patches of membrane where voltage gated channels are present. In response to the end plate potential’s spread across the membrane these voltage gated channels reach threshold and open, triggering the formation of the action potential ultimately responsible for muscle fiber contraction.
Motor Integration and Levels of Motor Control:
Like the sensory system, the motor system has three basic levels of control.
1. The segmental level: The lowest level of motor control, the segmental level consists primarily of reflexes and relatively simple circuits of short neuron chains.
2. The Projection Level: Named for the nerve fiber type contained in this pathway (projection fibers) this level consists primarily of upper motor neurons. It is the primary role of this level of motor corneal to initiate discrete voluntary movement by carrying action potentials to the lower motor neurons of the spinal cord.
3. The Precommand level: The highest level of motor control, this motor control center is composed of the cerebellum and basal nuclei. These regions receive sensory information from the proprioceptors of the body, and monitor outgoing motor information. This center allows for complex motor planning, adjustment, and constant fine tuning of motor output.