Neuromuscular Junction & Impulse Transmission
If you are here from the Overview of Muscle Contraction or Neuron Structure & Excitation Impulse, then you are familiar with where we are located in the body and likely want a detailed understanding of how the innervations of the muscle work in respect to the neuromuscular junction. If you have not read the articles mentioned above, I encourage you to do so.
We are now at the end of a neuron axon, called the axon terminal. At the axon terminal are a series of different processes that enable the neuron to communicate with the muscle, allowing information to pass in such a manner that is efficient and strong enough to stimulate the beginning of the actual muscular contraction itself. As you read, you will be introduced to a breakdown of these processes for a worthy examination of this physiological event.
So, again, the nerve impulse has traveled the length of the neuron and has now reached the axon terminal. It is at this point that the neuron reaches its end and sends the impulse to the muscle. This is best explained if we understand the structure of the neuromuscular junction.
Post Synaptic Membrane*
As the impulse travels to the axon terminal, it encounters the end of the axon, and as it encounters the end of the axon, it reaches a gap between the end of the axon and the muscle; this is called the synaptic cleft.
The synaptic cleft is made up of two parts:
1. Pre-Synaptic Membrane: This membrane is the last possible point of the axon. It is the barrier between the extracellular area between membranes and the intracellular area. Imbedded in this membrane are several protein channels that carry out various tasks, depending on the protein channel (discussed later).
2. Post Synaptic Membrane: Similar to the presynaptic membrane, the post synaptic membrane is the part, as with most cells, which keep extracellular ions out and the intracellular ions in. It, too, is studded with channels of a particular type.
Now, if we focus on intracellular area of the presynaptic membrane (the inside portion of the neuron passing on the neural impulse), we will find that the inside membrane is filled with vesicles. Vesicles are essentially casings that carry a substance called a neurotransmitter.
As I’ve mentioned before, the body runs on a complex electrochemical system of communication (as was first seen within the neuron in Neuron Structure & Excitation Impulse). This system is most evident when discussing neurotransmitters. As the neural impulse travels to the end of the axon, the electrical impulse cannot travel across the synaptic cleft, so it relies on a chemical system (neurotransmitters) to carry that message to the next neuron (or, in our case, the muscle).
As the neuron’s electrical impulse reaches the end of the axon the impulse changes the structure of the presynaptic membrane to allow the movement of chemicals and ions – let’s examine that further.
Extra and Intracellular Changes
The impulse reaches the axon terminal and at this point, the membrane changes to allow extracellular calcium (Ca2+) ions to flood through via calcium channels imbedded in the membrane. This action immediately pulls the intracellular vesicles toward a different series of channels called membrane fusion proteins. Now, the vesicles anchor to these proteins releasing the neurotransmitters inside each vesicle into the synaptic cleft, the extracellular area between the two membranes.
The more calcium is released inside the presynaptic membrane, the more neurotransmitters are then released in the synaptic cleft. Now, the neurotransmitter acetylcholine (ACh) is now saturated in the synaptic cleft, so the process is completely chemical at this point, and as that is the case, it is up to acetylcholine to transfer the message to the post synaptic membrane.
Membrane Fusion Proteins
Motor End Plate* and End Plate Potential
Specific to our scenario is the motor end plate. The motor end plate is, essentially, the post synaptic membrane of the muscle fiber, hence the term. However, specific to the motor end plate are a few key pieces.
Like other post synaptic membranes, the end plate is covered in acetylcholine receptors; however, the motor end plates are not relatively smooth as they have grooves to allow a high amount of acetylcholine receptors to be imbedded into the membrane.
As acetylcholine attaches to these receptors, this forces open the receptor channels and an exchange of sodium (Na+) and potassium (K+) occurs (similar to the action potential discussed in Neuron Structure and Excitation Impulse). This exchange of ions leads to depolarization of the end plate to a 0mV charge called an end plate potential (EPP). The neuron has now successfully carried the impulse across itself, over the synaptic cleft, and started it back up on the muscle fiber; it is at this point the muscle fiber goes through a series of reactions to contract.
Finally, a cleanup process is necessary for the next impulse to do its duty. This process begins in the synaptic cleft. Of course, the channels on the presynaptic membrane close, stopping the release of acetylcholine, but the previous release of acetylcholine from the impulse we just discussed left a significant amount of acetylcholine attached to the acetylcholine receptors of the motor end plate and as such, the body releases another enzyme to do the janitorial duties – (if you couldn’t guess by the title of this section..) this enzyme is called acetylcholinesterase.
Acetylcholinesterase is an interesting little enzyme, because not only does it break up acetylcholine into substrates acetate and choline for resubmission into the neural network to be resynthesized into actetylcholine, but without it, the body’s channels stay open and the body cannot repolarize effectively making another impulse transmission impossible.
Writer: Nicolas Verhoeven
Information gleaned from the Physiology Department at East Carolina University via lecture by Dr. Ronald Cortright.
So, there we have it. The neuron impulse reaches the axon terminal, turns purely chemical as it passes through the neuron membrane (pre synaptic membrane) with the neurotransmitter acetylcholine, at which point the acetylcholine attaches to receptors on the motor end plate (post synaptic membrane) which depolarizes the motor end plate leading to a new impulse that stimulates the muscle fiber.