Animation 1. View the YouTube animation Sodium and Potassium Gradients (opens in new window)
Now that we understand what an action potential is in theory, we can continue with the steps that precede muscle contraction. We said that the signal to start a contraction comes from the brain. This is because the brain is made out of nervous tissue which, like muscle cells, is excitable and has a polarized membrane. There are some differences between the action potentials we see in nervous and muscular tissue, but the theory is the same.
The brain starts the electrical signal we now know as an action potential. The action potential travels from the brain down a somatic motor neuron until it reaches the skeletal muscle that we are targeting. Remember, soma = body and motor = producing movement, therefore these neurons produce movement/locomotion of the body. The somatic motor neuron branches to stimulate multiple skeletal muscle cells at one time. The junction of these 2 cells – somatic motor neuron and skeletal muscle cell – is called the neuromuscular junction. This presents no mystery as to how the junction got its name. The neuron terminates at the skeletal muscle cells in a bulbous structure called synaptic end bulbs. These bulbs are not directly connected to the skeletal muscle cell the 2 cells are separated by a small gap that is called the synaptic cleft. The signal that traveled down the somatic motor neuron is electrical in nature and if there is a break in an electrical connection, like a broken wire or a synaptic cleft, it is very unsafe for the electricity to arc over that gap. When electricity arcs we cannot be sure of its target and in the body that can be very detrimental. So to resolve this problem when the action potential reaches the end of the somatic motor neuron in the synaptic end bulbs the electrical signal is converted into a chemical one. This chemical signal is safe in the body and will transmit the message to the skeletal muscle on the other side of the synaptic cleft to contract.
Let's examine how this happens. When the action potential reaches the synaptic end bulbs at the end of the axon, the electricity opens voltage gated calcium channels in the membrane of the synaptic end bulb. Calcium, which is in great abundance outside the cell, readily enters the neuron through these open channels. When the calcium enters the neuron it causes vesicles containing the neurotransmitter acetylcholine (ACh) that were being stored in the synaptic end bulbs to undergo exocytosis. This process is where the vesicles merge with the cell membrane of the synaptic end bulb and dump the contents – ACh – into the synaptic cleft. ACh is our chemical signal! We have just seen how the electrical signal triggered a series of events that caused the release of a chemical signal into the cleft, which as we said before is much safer than the electric signal arcing over the cleft.
