What Is a Muscle Contraction?

A muscle contraction is an increase in the tension or a decrease in the length of a muscle. Muscle tension is the force exerted by the muscle on a bone or other object. A muscle contraction is isometric if muscle tension changes, but muscle length remains the same. An example of isometric muscle contraction is holding a book in the same position. A muscle contraction is isotonic if muscle length changes, but muscle tension remains the same. An example of isotonic muscle contraction is raising a book by bending the arm at the elbow. The termination of a muscle contraction of either type occurs when the muscle relaxes and returns to its non-contracted tension or length.

To use our arm wrestling example, if both arm wrestlers have equal strength and they are pulling with all their might, but there is no movement, that is isometric muscle contraction. However, as soon as one arm wrestler starts to win and is able to start pulling the opponents arm down, that is isotonic muscle contraction.

How a Skeletal Muscle Contraction Begins

Excluding reflexes, all skeletal muscle contractions occur as a result of conscious effort originating in the brain. The brain sends electrochemical signals through the somatic nervous system to motor neurons that innervate muscle fibres (to review how the brain and neurons function, see the chapter Nervous System). A single motor neuron with multiple axon terminals is able to innervate multiple muscle fibres, thereby causing all of them to contract at the same time. The connection between a motor neuron axon terminal and a muscle fibre occurs at a site called a neuromuscular junction. This is a chemical synapse where a motor neuron transmits a signal to a muscle fibre to initiate a muscle contraction. The process by which a signal is transmitted at a neuromuscular junction is illustrated in Figure 12.4.2 below.

12.4 Neuromuscular Junction

The sequence of events begins when an action potential is initiated in the cell body of a motor neuron, and the action potential is propagated along the neuron’s axon to the neuromuscular junction. Once the action potential reaches the end of the axon terminal, it causes the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles in the axon terminal. The ACh molecules diffuse across the synaptic cleft and bind to receptors on the muscle fibre, thereby initiating a muscle contraction.

Sliding Filament Theory of Muscle Contraction

Once the muscle fibre is stimulated by the motor neuron, actin and myosin protein filaments within the skeletal muscle fibre slide past each other to produce a contraction. The sliding filament theory is the most widely accepted explanation for how this occurs. According to this theory, muscle contraction is a cycle of molecular events in which thick myosin filaments repeatedly attach to and pull on thin actin filaments, so the filaments slide over one another, as illustrated in Figure 12.4.3. The actin filaments are attached to Z discs, each of which marks the end of a sarcomere. The sliding of the filaments pulls the Z discs of a sarcomere closer together, thus shortening the sarcomere. As this occurs, the muscle contracts.

12.4.3 Sliding Filament Theory

Crossbridge Cycling

Crossbridge cycling is a sequence of molecular events that underlies the sliding filament theory. There are many projections from the thick myosin filaments, each of which consists of two myosin heads (you can see the projections and heads in Figures 12.4.3 and 12.4.4). Each myosin head has binding sites for ATP (or the products of ATP hydrolysis: ADP and Pi) and for actin. The thin actin filaments also have binding sites for the myosin heads. A crossbridge forms when a myosin head binds with an actin filament.

The process of crossbridge cycling is shown in the video “Muscle Contraction 3D” by 3DBiology (below), and in Figure 12.4.4. A crossbridge cycle begins when the myosin head binds to an actin filament. ADP and Pi are also bound to the myosin head at this stage. Next, a power stroke moves the actin filament inward toward the center of sarcomere, thereby shortening the sarcomere. At the end of the power stroke, ADP and Pi are released from the myosin head, leaving the myosin head attached just to the thin filament until another ATP binds to the myosin head. When ATP binds to the myosin head, it causes the myosin head to detach from the actin. ATP is once again split into ADP and Pi and the energy released is used to move the myosin head into a “cocked” position. Once in this position, the myosin head can bind to the actin filament again, and another crossbridge cycle begins.

Muscle Contraction 3D, 3DBiology, 2017.

12.4.4 Crossbridge Cycling

Energy for Muscle Contraction

According to the sliding filament theory, ATP is needed to provide the energy for a muscle contraction. Where does this ATP come from? Actually, there are multiple potential sources, as illustrated in Figure 12.4.5 below.

12.4 Muscle Metabolism

  1. As you can see from the first diagram, some ATP is already available in a resting muscle. As a muscle contraction starts, this ATP is used up in just a few seconds. More ATP is generated from creatine phosphate, but this ATP is used up rapidly as well. It’s gone in another 15 seconds or so.
  2. Glucose from the blood and glycogen stored in muscle can then be used to make more ATP. Glycogen breaks down to form glucose, and each glucose molecule produces two molecules of ATP and two molecules of pyruvate. Pyruvate (as pyruvic acid) can be used in aerobic respiration if oxygen is available. Alternatively, pyruvate can be used in anaerobic respiration, if oxygen is not available. The latter produces lactic acid, which may contribute to muscle fatigue. Anaerobic respiration typically occurs only during strenuous exercise when so much ATP is needed that sufficient oxygen cannot be delivered to the muscle to keep up.
  3. Resting or moderately active muscles can get most of the ATP they need for contractions by aerobic respiration. This process takes place in the mitochondria of muscle cells. In the process, glucose and oxygen react to produce carbon dioxide, water, and many molecules of ATP.

Feature: Human Biology in the News

Basic research on muscle contraction, especially if it is interesting and hopeful, is often in the news, because muscle contractions are involved in so many different body processes and disorders, including heart failure and stroke.

12.4 Summary

12.4 Review Questions

    What is a skeletal muscle contraction?