7.1 PreLab Reading – Muscle Physiology

Skeletal Muscle Physiology

Skeletal muscles, which attach to bones via tendons, are essential for movement and structural support.  They consist of numerous muscle fibers containing contractile proteins, actin, and myosin, responsible for muscle contraction.  Skeletal muscles are under voluntary control and are stimulated by somatic motor neurons.  A motor unit consists of a motor neuron and the muscle fiber it controls.  When an action potential (AP) from the motor neuron reaches a muscle fiber, it triggers a muscle twitch. The force of a muscle contraction depends on the number of motor units activated (recruitment) and the force generated by individual muscle fibers, which can increase through the summation.

Sarcomere Anatomy: Actin and myosin filament. (Figure by https://pressbooks.ccconline.org/bio106/chapter/muscular-levels-of-organization/ 

During muscle contraction, multiple motor units fire repetitively. The electromyogram (EMG) records the electrical activity, which corresponds to the muscle contraction duration. The strength of the contraction is proportional to the electrical activity, which can be quantified by integrating the EMG spikes, creating an area under the curve that is linearly proportional to contraction strength.

Muscles can be stimulated through electrical impulses applied to motor points, where motor neurons enter the muscle. In this experiment, electrical stimulation will target the abductor digiti minimi (innervated by the ulnar nerve) and the flexor digitorum superficialis (innervated by the median nerve). These muscles play key roles in finger and wrist flexion. The motor points for these muscles are located on the medial surface of the palm, and lateral forearm (flexor digitorum superficialis). Your instructor will help locate the motor points during the experiment.

The nerves of the forearm: A. The motor point for the flexor digitorium superficialis (median nerve). B. The motor point for the abductor digiti minimi (ulnar nerve). (Figure by McKinley, 1st edition).

In this week’s experiment, students will measure grip strength using a hand dynamometer and record EMG activity from forearm muscles.  The relationship between EMG activity and grip strength will be analyzed by plotting maximum grip strength against the area under the EMG curve.  Data will be collected from both forearms to compare muscle strength and electrical activity relative to forearm circumference.  The rate of fatigue will also be assessed by recording prolonged grip strength and EMG activity.

Excitation-Contraction Coupling

Skeletal muscle fibers contain myofibrils, which are made up of sarcomeres, the functional units of contraction.  These sarcomeres are composed of actin (thin filaments) and myosin (thick filaments).  When a motor neuron stimulates a muscle fiber, an action potential travels across the sarcolemma (muscle fiber’s plasma membrane) and into the fiber via transverse tubules (T tubules). This action potential triggers the release of calcium ions from the sarcoplasmic reticulum (a type of smooth endoplasmic reticulum) which bind to troponin complex on actin filaments, exposing myosin-binding sites. Myosin binds to actin and, using ATP, pull the filaments inward, causing the sarcomere to shorten and the muscle to contract.

Structure of a skeletal muscle fiber:  Neuromuscular junction. (Figure by Standfield, 4th edition).

The strength of the contraction depends on the amount of calcium released and the number of crossbridge formed.  When muscles are stimulated repeatedly, twitches can combine into a sustained contraction, or tetanus, if the stimulation frequency is high enough that the muscle fibers don’t fully relax.

Muscle Twitch

A muscle twitch is the response to a single stimulus. The contraction phase is faster than the relaxation phase, and there is a latent period before contraction begins.  This period includes the depolarization of the sarcolemma and T-tubules, calcium release, myosin-actin binding, and tension development.

Muscle Twitch: Phases of a muscle twitch. (Figure by OpenStax is used under a Creative Commons Attribution license.)

There are two types of muscles twitches:

• Isometric: The muscle generates tension but does not no change length (e.g., holding a plank).
• Isotonic: The muscle shortens as it lifts a load (e.g., bicep curls).

Isotonic contractions begin with an isometric phase where the muscle contracts without shortening until enough force is generated.

Muscle Twitch: Isotonic contraction and isometric contraction. (Figure by OpenStax is used under a Creative Commons Attribution license.)

Excitation-Contraction Coupling and Muscle Fatigue

Excitation-contraction coupling converts neural signals into muscle contraction. When a motor neuron releases acetylcholine, it triggers an action potential in the muscle fiber, leading to the processes outlined above.

Frequently stimulation leads to muscle fatigue, influenced by factors such as exercise intensity, the muscle fiber type, and blood supply.  For example, high-intensity exercise recruits fast glycolytic fibers that generate ATP via glycolysis, leading to lactic acid buildup and fatigue.  Whereas, prolonged low-intensity exercise depletes glycogen stores, leading to fatigue in slow-oxidative fibers.

ATP is required for both muscle contraction and relaxation.  Without ATP, calcium ions remain in the cytoplasm, keeping actin and myosin bound, preventing relaxation.

Motor Unit Recruitment

Skeletal muscles are made of motor units, which consist of a motor neuron and the muscle fibers it innervates.  Small motor units (with smaller somas) are recruited first, responding to weaker stimuli, while larger motor units (with larger somas) are recruited as the stimulus increases.  The force of contraction depends on the number and size of motor units recruited.

Motor units are activated asynchronously to prevent fatigue.  Recruitment is increased by the frequency of stimulation, a process known as summation.  If a second stimulus is applied before the muscle has relaxed from the first, it leads to greater force production.  If the frequency is high enough, the muscle may experience tetanus – either incomplete or complete – where the muscle maintains a steady contraction.

Fiber Types and Muscle Contraction

Muscle fibers are classified into three types: fast glycolytic, fast oxidative, and slow oxidative.  Fast glycolytic fibers generate the most force but fatigue quickly.  Fast oxidative fibers are most resistant to fatigue, and slow oxidative are the most enduring but generate the least force.

Muscles typically contain a mix of these fibers.  When higher force is required, larger motor units with fast glycolytic fibers are recruited.  The recruitment order follows slow oxidative first, fast oxidative second, and fast glycolytic last.

Effect of Load on Isotonic Contractions

In isotonic contractions, the muscle generates enough tension to overcome a load, causing it to shorten.  As the load increases:

• The latent period (time before shortening) becomes longer.
• The duration of shortening decreases.
• The velocity of shortening slows down.

The muscle’s work output is affected by the load, as work is the product of force and distance.  No work is done in an isometric contraction, where no distance is covered.

Summation and Tetanus

Summation occurs when successive stimuli increase muscle tension.  This happens because calcium ions remain in the cytoplasm after a previous stimulus, allowing for more myosin-actin binding.  With frequent stimulation, the muscle fiber reaches incomplete tetanus, where individual twitches are still observable, but tension is higher than a single twitch.  With further stimulation, the muscle reaches complete tetanus, where no relaxation occurs, leading to maximum tension.

Complete tetanus results in sustained contraction, and relaxation occurs more slowly as calcium is pumped back into the sarcoplasmic reticulum.  Under normal conditions, motor units fire asynchronously, allowing for smooth, sustained muscle contractions.

Summation and Tetanus: The effect of increasing tension. (Figure by OpenStax is used under a Creative Commons Attribution license.)

Muscle Fatigue

Muscle fatigue, a decrease in tension production following frequent stimulation, is influenced by exercise type, contraction rate, force, and blood supply.  High-intensity activities like weightlifting recruit fast glycolytic fibers, which depend on fermentation (glycolysis) for ATP.  This process generates lactic acid, altering pH and disrupting the function of contractile proteins and calcium pumps in the sarcoplasmic reticulum.  Intense exercise may also lead to neuromuscular fatigue, where somatic motor neurons deplete their neurotransmitter supply.  In contrast, low-intensity, long duration exercises primarily fatigue slow oxidative and fast oxidative fibers through fuel source depletion, such as glycogen.

Energy is crucial not only for contraction but also for relaxation. Calcium ions must actively return to the sarcoplasmic reticulum, a process requiring ATP. Without adequate ATP, calcium remains in the cytoplasm, prolonging actin-myosin interaction and preventing efficient relaxation.

In this lab, students will explore excitation-contraction coupling by stimulating the motor points of the abductor digiti minimi and flexor digitorium superficialis to observe muscle contraction via EMG.  Analyzing grip strength and EMG activity reveals the connection between electrical impulses and muscle force.  Comparisons between dominant and non-dominant forearms provide insights into muscle size, electrical activity, and fatigue, highlighting the physiological basis of muscle performance.

Adapted from Human Physiology Lab Manual by Jim Blevins, Melaney Farr, and Arleen Sawitzke, Salt Lake Community College.