Neuromuscular junctions and neurotransmitters

Neuromuscular junction

Whereas neurons communicate with other neurons at synapses, an alpha-motor neuron communicates with muscle fibers at a site known as a neuromuscular junction. The function of the neuromuscular junction is essentially the same as that of a synapse. In fact, the proximal part of the neuromuscular junction is the same: It starts with the axon terminals of the motor neuron, which release neurotransmitters into the space between the motor nerve and the muscle fiber in response to an action potential. However, in the neuromuscular junction, the axon terminals protrude into motor end plates, which are troughlike segments on the plasmalemma. Picture below shows.

The motor end plate is invaginated(folded to form cavities). The cavity thus formed is called the synaptic gutter. As with synapses, the space between the neuron and the muscle fiber is the synaptic cleft.

Neurotransmitters released from the alpha-motor neuron axon terminals diffuse across the synaptic cleft and bind to receptors on the muscle fiber’s plasmalemma. This binding typically causes depolarization by opening sodium ion channels, allowing more sodium to enter the muscle fiber. As always, if the depolarization reaches the threshold, an action potential is formed. It spreads across the plasmalemma into the T-tubules, inititating muscle fiber contraction. As in the neuron, the plasmalemma, once depolarized, must undergo repolarization. During the period of repolarization, the sodium gates are closed and the potassium gates are open; thus, like the neuron, the muscle fiber is unable to respond to any further stimulation. This period is reffered to as the refractory period. Once the electrical conditions of the muscle fiber are restored to resting levels, the fiber can respond to another stimulus. Thus, the refractory period limits the motor unit’s firing frequency.

Now we know how the impulse is transmitted between two cells. But to understand what happens once the impulse is transmitted, we must first examine the chemical signals that accomplish transmission.

Neurotransmitters

More than 50 neurotransmitters have been positively identified or are suspected as potential candidates. These can be cathegorized as either (a) small-molecule, rapid-acting neurotransmitters or (b) neuropeptide, slow-acting neurotransmitters. The small-molecule, rapid-acting transmitters, which are responsible for most neural transmissions, are our main concern.

Acetylholine and norepinephrine are the two major neurotransmitters involved in regulating our physiological responses to exercise. Acetylholine is the primary neurotransmitter for the motor neurons that innervate skeletal muscle and for most parasympathetic neurons. It is generally an excitatory neurotransmitter, but it can have inhibitory effects at some parasympathetic nerve endings, such as in the heart. Norepinephrine is the neurotransmitter for the most sympathetic neurons, and it too can be either excitatory or inhibitory, depending on the receptors involved.

Once the neurotransmitters binds to the post-synaptic receptor, the nerve impulse has been successfully transmitted. The neurotransmitter is then either degraded by enzymes, actively transported back into the presynaptic terminals for reuse, or diffused away from the synapse.