Skeletal muscles are told what to do by the nervous system. They only contract when told to do so.
You already learned that the electrically active, signalling cells of the nervous system are the neurons. Neurons are cells with many processes sticking off of them, and one of these processes provides the output of the electrical information for the neuron. This output process is the thinnest process, and there is only one of them; it is called the axon. Do you see the axon in the diagram of this neuron to the left?
You do not need to learn all the names of the parts of the neuron yet. That will come soon enough. All I want you to see for now is that the neuron sends its information down its axon to its presynaptic terminals ("pre" for before, because the information comes from this end). It's presynaptic terminals in this diagram are all at the far right-hand edge of the drawing.
A neuron that communicates directly with a muscle is called a motor neuron ("motor" for movement). So you may also see the presynaptic terminals called the motor neuronal terminals. With what does the motor neuron communicate? Certainly not with the entire muscle.
Think about it this way... if one motor neuron controlled all the muscle fibers in an entire muscle, every time that motor neuron was active, the entire muscle would contract. Our muscular contractions would then be all-or-none. And we would move about this world in a jerky fashion. Since that is not the case, can you figure out what the motor neuron connects to? It connects to muscle fibers. Somewhere between a few muscle fibers and many-- but that is the topic for the next web page on motor units.
You should expect that if the muscle fiber receives information from a motor neuron, it should have an anatomical specialization at the point of motor neuronal contact. The specialized region on the muscle fiber is called the motor end plate (that's just a fancy way of saying that it is the muscle's postsynaptic terminal, "post" for after, because the information is received at this end).
The neuromuscular junction is the term for the connection between the nervous system and the muscle fiber. It is a type of synapse-- a connection from a neuron to either another neuron or a muscle or another effector organ. Regardless, it is a connection from that sends electrical information to another place. This electrical information has to travel from one cell to another. The problem is that each cell is an independent unit, and there is space between the two cells that must communicate electrically.
You probably know from experience that if you cut the electrical cord from a lamp right in half and then plug in the half that can get plugged in, even if you put the two halves of the cord close together, if the two halves don't touch, your light won't turn on. (don't try this, though-- big fire hazard!) Cells that communicate electrically have to have a gap between them in order to be different cells. So how do they bridge this gap? They transfer the electrical information into chemical information to cross the gap, then the chemical information is transformed back into electrical information.
Follow along through the next description on Figure 9.7 in your textbook...
The electrical information (called an Action Potential, or AP) reaches the tip of the motor neuronal terminal. There, the AP causes the motor neuronal terminal to secrete a chemical. The specific chemical is called acetylcholine (or ACh). The ACh, once secreted, simply diffuses across the gap between the motor neuron and the muscle fiber. This gap is called the synaptic cleft, literally meaning the space within the synapse. When the ACh reaches the motor end plate, the membrane has specific receptors to recognize ACh, called acetylcholine receptors (or AChRs). When ACh comes in contact with the AChRs on the motor end plate, the AChRs are able to trigger the generation of an AP on the sarcolemma. Thus, an electrical signal is restored postsynaptically.
To summarize what happens across the neuromuscular junction,
You will get a lot more detail on synapses when we cover the nervous system. Until then, this is enough terminology.
Meanwhile, you should now be able to see that only those muscle fibers that receive information from a particular motor neuron will be excited to contract when that particular motor neuron is electrically active. To understand this further, go on to the motor unit page.
© 2011 STCC Foundation Press