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Nervous System and Contractions

Nervous System and Contractions

Description:

This lesson will investigate the role of the nervous system in muscle contractions.

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Tutorial

What's Covered

This lesson will focus on the relationship of the nervous system and muscle contractions by examining:

  1. The Steps of Muscle Contraction
  2. The Role of Neurotransmitters

1. Steps of Muscle Contraction

Muscle contractions are basically controlled by signals from the nervous system. In order for a skeletal muscle contractions to happen it needs a signal from the nervous system to occur. Motor neurons are what signal the start and stop of the contraction of sarcomeres, the basic units of contraction found in muscle fibers.

Terms to Know

    • Motor Neuron
    • A neuron that delivers signals to muscles or glands.
    • Sarcomere
    • The basic units of muscle contractions.

Before discussing the steps that occur for muscle contraction to happen, let's review the structure of a sarcomere. Myosin is referred to as the thick filament in a sarcomere, and myosin heads will attach to actin ,the thin filaments, pulling the z bands of a sarcomere closer together. This allows for the shortening of that sarcomere, which in turn causes the shortening of the muscle fiber, and therefore a muscle contraction. Troponin and tropomyosin are proteins that are found on actin filaments.

NervousMuscleContraction.png

Terms to Know

    • Actin
    • A protein referred to as the "thin filaments" involved in contractions.
    • Myosin
    • A protein referred to as the "thick filaments" involved in contractions.
    • Troponin
    • A protein found on actin filaments that plays a role in muscle contractions.
    • Tropomyosin
    • A protein found on actin filaments that plays a role in muscle contractions.

The first step is a signal coming from the nervous system that triggers the release of calcium from the sarcoplasmic reticulum, the endoplasmic reticulum in muscle cells that stores and releases calcium.

IN CONTEXT
Think back to the lesson on homeostasis and bone remodeling. Remember how bone remodeling allows for our blood calcium levels to stay within a certain range? 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.

Term to Know

    • Calcium
    • An ion necessary for muscle contractions to occur.

Next, calcium that's been released from the sarcoplasmic reticulum will bind to troponin on actin filaments. The binding of the calcium will cause troponin to move, which in turn, twists tropomyosin away from the binding site. In a resting filament, the tropomyosin is covering the binding site, so myosin is not allowed to bind to actin. Once myosin can bind to actin, a contraction can occur.

Lastly, when a muscle contraction is done, the end of the nerve impulse signals calcium to move back to the sarcoplasmic reticulum, and when calcium moves back, the binding site will be covered again. Myosin will no longer be attached to actin, and that filament will be at rest.


2. Role of Neurotransmitters

Nerve impulses will arrive near the nerve cell at something called a neuromuscular junction. If we look at this word, neuro means nerve and muscular means muscle. In other words, this is a junction between a nerve and a muscle. A gap, called a synapse, will separate the ends of the neuron from the muscle cell. If it's two neurons communicating, a synapse will separate the two as well, but for the purpose of this lesson, you will focus on a synapse that separates the muscle cell from the neuron. Neurotransmitters are the chemical messengers that carry signals across the synapse.

Terms to Know

    • Neuromuscular Junction
    • The area where the end of a motor neuron comes close to a muscle fiber.
    • Synapse
    • The gap between the end of a motor neuron and a muscle fiber.
    • Neurotransmitter
    • A chemical messenger that carries signals across the synapse between cells.

nervous muscle contraction2.png

This diagram illustrates the synapse between a neuron and muscle fiber. When a signal is sent from a neuron to a muscle cell, calcium is going to flow into the neuron. ACH, or acetylcholine, is a neurotransmitter that will, in the vesicles, bind with the neuron plasma membrane. It will be released into the synapse. Once it's in the synapse, it's going to travel towards the muscle fiber, and then will bind with receptors on the muscle fiber. After this occurs, it's going to generate an electrical impulse. That electrical impulse is going to travel through the muscle fiber into the sarcoplasmic reticulum, causing calcium ions to be released, and then a contraction can proceed

Summary

There are several steps of muscle contraction. First a signal comes from the nervous system that triggers the release of calcium from the sarcoplasmic reticulum. Calcium will then bind to troponin on actin filaments, which will twist tropomyosin away from the binding site. This allows myosin to bind to actin, causing a contraction. When contraction is done, calcium is signaled to move back to the sarcoplasmic reticulum, the binding site is covered by tropomyosin again, myosin will no longer be bound to actin, and the filament will be at rest. The role of neurotransmitters in the process is very important. The Neurotransmitter is what crosses the synapsis to bring signals from the neuron to the muscle fiber. ACH is the neurotransmitter used to signal the release of calcium in the muscle fiber, causing contraction.

Keep up the learning and have a great day!

Source: This work is adapted from Sophia Author Amanda Soderlind

TERMS TO KNOW
  • Motor Neuron

    A neuron that delivers signals to muscles or glands.

  • Sarcomere

    The basic units of muscle contractions.

  • Calcium

    An ion necessary for muscle contractions to occur.

  • Actin

    A protein referred to as the "thin filaments" involved in contractions.

  • Myosin

    A protein referred to as the "thick filaments" involved in contractions.

  • Neuromuscular Junction

    The area where the end of a motor neuron comes close to a muscle fiber.

  • Synapse

    The gap between the end of a motor neuron and a muscle fiber.

  • Neurotransmitter

    A chemical messenger that carries signals across the synapse between cells.

  • ​Troponin

    A protein found on actin filaments that plays a role in muscle contractions.

  • ​Tropomyosin

    A protein found on actin filaments that plays a role in muscle contractions.