Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson on nervous system and muscle contractions.
In this lesson today we will discuss how the nervous system plays a role in muscle contractions.
Muscle contractions are basically controlled by signals from the nervous system. In order for a muscle contraction to happen, for skeletal muscles, it needs a signal from the nervous system to occur. Motor neurons are what signal the start and stop of the contraction of sarcomeres. Sarcomeres are those basic units of contraction found in muscle fibers.
There are several steps that occur in order for a muscle contraction to happen. And we'll go through those in just a moment, but I want to start by labeling this diagram down here so we can refer to it as we discuss the steps in a muscle contraction.
This right here is myosin. Myosin is referred to as the thick filament in a sarcomere. And it plays a role in a muscle contraction because myosin heads will attach to actin, and pull the z bands of a sarcomere closer together, allowing for the shortening of that sarcomere. The shortening of the muscle fiber, and therefore a muscle contraction.
These little strands down here that look like beads are actin. Actin are the thin filaments in a sarcomere. And then here we have our z bands. Z bands mark the beginning and end of a sarcomere.
And then we have troponin and tropomyosin. So these are all the components of a sarcomere, that work together in order to allow for muscle contraction to happen.
Troponin and tropomyosin are proteins that are found on actin filaments. And we'll discuss the role of these proteins when we discuss the different steps of a muscle contraction. So again, actin and myosin are the filaments that play a role in muscle contractions by overlapping and shortening, causing a muscle contraction to occur.
So let's go through the steps of muscle contraction. First, we would have a signal coming in from the nervous system that will trigger the release of calcium from the sarcoplasmic reticulum. Calcium is really key to muscle contractions. In order for muscle contraction to occur, we need to have calcium. So if you are familiar with homeostasis and bone remodeling, you know that bone remodeling allows for our blood calcium levels to stay within a certain range. And that range is necessary in order for these muscle contractions to occur.
If blood calcium levels are too low, calcium will be removed from the bones so that the blood calcium level can increase, and if blood calcium levels are too high, then that extra calcium will be deposited into bone. So we need to maintain this certain range of blood calcium levels in order for our nervous system to properly function, so that we can have muscle contractions.
And the sarcoplasmic reticulum is basically like the endoplasmic reticulum in muscle cells, that stores and releases calcium. So we have the incoming nervous signal triggering calcium to be released. From there, that calcium that's been released from the sarcoplasmic reticulum will bind to troponin on actin filaments. So we have our tropinin labeled right here.
So calcium is going to bind to troponin, and that binding of the calcium will cause tropinin to move, which in turn, twists tropomyosin away from the binding site. So in a resting filament, the tropomyosin is basically covering the binding site, so myosin is not allowed to bind to actin in a resting filament, because these proteins are blocking the binding site. But when that nerve impulse comes, and calcium is released, it causes these proteins to move away from the binding site so that myosin can then bind to actin.
Once myosin can bind to actin, a contraction can occur. So contraction occurs when myosin binds to actin, and it pulls these actin filaments inward, therefore pulling the a bands inward, causing the sarcomere to shorten, causing the filament to shorten, and causing the muscle to contract.
When a muscle contraction is done, the end of the nerve impulse signals calcium to move back to the sarcoplasmic reticulum. So at the end it of this contraction, calcium will move back into the sarcoplasmic reticulum, and then the binding site, therefore, will be covered again. So then, myosin will no longer be attached to actin, and that filament will be at rest.
We're going to take a look at another diagram here, and explain this a little bit further. So we will be talking about how neurotransmitters play a role in delivering information from a neuron to a muscle cell.
Nerve impulses will arrive near the nerve cell at something called a neuromuscular junction. If we look at this word, neuro means nerve, muscular means muscle. And then junction. So we have a junction between a nerve and a muscle, is what we're talking about right here. So the nerve impulses will arrive near the nerve cell at this neuromuscular junction. And then a gap, called a synapse, will separate the ends of the neuron from the muscle cell. And sometimes the ends of one neurone from another neuron, too. If it's two neurons communicating, a synapse will separate the two neurons. If a neuron's communicating with a muscle cell, then a synapse will separate the muscle cell from the neuron.
Let's take a look at the process that happens in order for a signal to be transferred from a neuron to a muscle cell to allow for contraction.
The first thing we're going to do here is just quickly label this diagram. So here we have our muscle fiber. This here is the ending of one of our neurons. So we have a neuron ending right here. And then we have our neurotransmitters, which are in these vesicles. And then I mentioned this is our axon ending, and then we have the synapse, here. So remember, the synapse is the gap between the end of the neuron and our muscle fiber.
So basically, what's going to happen when we're sending a signal from a neuron to a muscle cell, is that first of all, calcium is going to flow into the neuron. From there, ACH, which stands for acetylcholine, and acetylcholine is a neurotransmitter which is a chemical signal. So ACH will, in the vesicles, will bind with the neuron plasma membrane. So we have ACH, or acetylcholine, which is our neurotransmitter, in this vesicle. It's going to find here with the neuron plasma membrane. And then it's going to be released into the synapse. From there, once it's in the synapse, it's going to travel towards the muscle fiber. And then it will bind with receptors on the muscle fiber. So these are going to be our receptors on our muscle fiber.
Once it binds with the receptors on the muscle fiber, it's going to generate an electrical impulse. And then that electrical impulse is going to travel through the muscle fiber into the sarcoplasmic reticulum.
The sarcoplasmic reticulum stores and releases calcium ions. So once that nerve impulses travels to the sarcoplasmic reticulum, it will cause calcium ions to be released, and then a contraction can proceed, like you saw on the previous diagram in this lesson. So those are kind of the steps that need to happen in order for a signal to be transmitted from an accent ending to a muscle fiber, in order for a contraction to occur.
So again, ACH, which stands for acetylcholine, is the neurotransmitter involved in this example.
This lesson has been an overview on how the nervous system controls muscle contractions.
A neuron that delivers signals to muscles or glands.
The basic units of muscle contractions.
An ion necessary for muscle contractions to occur.
A protein referred to as the "thin filaments" involved in contractions.
A protein referred to as the "thick filaments" involved in contractions.
The area where the end of a motor neuron comes close to a muscle fiber.
The gap between the end of a motor neuron and a muscle fiber.
A chemical messenger that carries signals across the synapse between cells.
A protein found on actin filaments that plays a role in muscle contractions.
A protein found on actin filaments that plays a role in muscle contractions.