Action Potentials and Muscle Contraction

Introduction

The action potential is the brief (about a thousandth of a second) reversal of the electrical polarization of the membrane of a nerve cell (neuron) or muscle cell. In the neuron, an action potential produces the nerve impulse, and in the muscle cell, it produces the contraction required for all movement. Sometimes called a propagated potential because a wave of excitation is actively transmitted along with the nerve or muscle fibre, an Action Potentials and Muscle Contraction is conducted at speeds that vary from 1 to 100 meters (3 to 300 feet) per second, depending on the properties of the fibre and its environment.

Before stimulation, a neuron or muscle cell has a slightly negative electrical polarization; that is, its interior is negatively charged compared to the extracellular fluid. This polarized state is created by a high concentration of positively charged sodium ions outside the cell and a high concentration of negatively charged chloride ions (as well as a lower concentration of positively charged potassium) inside. The resulting resting potential typically measures around -75 millivolts (mV), or -0.075 volts, with the minus sign indicating a negative charge inside.

In action potential generation, stimulation of the cell by neurotransmitters or sensory receptor cells partially opens channel-shaped protein molecules in the membrane. Sodium diffuses into the cell, shifting that part of the membrane toward a less negative polarization. If this local potential reaches a critical state called the threshold potential (measuring about -60 mV), the sodium channels open fully. Sodium floods that part of the cell, which instantly depolarizes to an action potential of about +55 mV. The depolarization activates sodium channels in adjacent parts of the membrane so that the impulse moves along the fibre.

If the entry of sodium into the fibre were not balanced by the exit of another positively charged ion, an action potential could not drop from its maximum value and return to the resting potential. The decaying phase of the action potential is caused by the closing of sodium channels and the opening of potassium channels, allowing a charge approximately equal to that entering the cell to leave as potassium ions.

Subsequently, protein transport molecules pump sodium ions out of the cell and potassium ions in. This restores the original ion concentrations and prepares the cell for a new action potential. The Nobel Prize in Physiology or Medicine was awarded in 1963 to Sir A.L. Hodgkin, Sir A.F. Huxley, and Sir John Eccles for formulating these ionic mechanisms involved in nerve cell activity.

How do the bones of the human skeleton move? Skeletal muscles contract and relax to mechanically move the body. Messages from the nervous system trigger these muscle contractions. The entire process is called the mechanism of muscle contraction and can be summarized in three steps:

(1) A message travels from the nervous system to the muscular system, causing chemical reactions.

(2) Chemical reactions cause muscle fibres to rearrange in a way that shortens the muscle: that’s contraction.

(3) When the signal from the nervous system is no longer present, the chemical process reverses and the muscle fibres reorganize again and the muscle relaxes.

1. A muscle contraction is triggered when an action potential travels along the nerves to the muscles

Muscle contraction begins when the nervous system generates a signal. The signal, an impulse called an action potential, travels through a type of nerve cell called a motor neuron. The neuromuscular junction is the name for the place where the motor neuron reaches a muscle cell. Skeletal muscle tissue is made up of cells called muscle fibres. When the signal from the nervous system reaches the neuromuscular junction, the motor neuron releases a chemical message. The chemical message, a neurotransmitter called acetylcholine, binds to receptors on the outside of the muscle fibre. That starts a chemical reaction within the muscle.

2. Acetylcholine is released and binds to receptors on the muscle membrane

A multi-step molecular process within the muscle fibre begins when acetylcholine binds to receptors on the muscle fibre membrane. Proteins within muscle fibres are organized into long chains that can interact with each other, rearranging themselves to shorten and relax. When acetylcholine reaches receptors on the membranes of muscle fibres, the channels in the membrane open, and the process that contracts relaxed muscle fibres begins:

  • The open channels allow the entry of sodium ions into the cytoplasm of the muscle fibre.
  • Sodium influx also sends a message within the muscle fibre to trigger the release of stored calcium ions.
  • Calcium ions diffuse into the muscle fibre.
  • The relationship between protein chains within muscle cells changes, leading to contraction.

3. Muscle fibres relax when the signal from the nervous system is no longer present

When the stimulation of the motor neuron that provides the impulse to the muscle fibres stops, the chemical reaction that causes the rearrangement of the proteins of the muscle fibres stops. This reverses the chemical processes in the muscle fibres and the muscle relaxes.

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