Lab 7 – Muscle Physiology

Skeletal Muscle Physiology

Skeletal muscles, attached to bones by tendons, are essential for movement and support.  They are composed of numerous muscle fibers (muscle cells) running the length of the muscle body.  These fibers are rich in proteins, especially the major contractile proteins, actin and myosin, which are responsible for muscle contraction.  Skeletal muscles are under voluntary control and are stimulated to contract by somatic motor neurons.  A somatic motor neuron and the muscle fibers it innervates form a motor unit.  When a single action potential (AP) from the motor neuron reaches the muscle fiber, it triggers a muscle twitch (a single contraction). The twitch’s magnitude is influence by various factors, including the load on the muscle.  The overall force generated by the muscle depends on the number of activated motor units (recruitment) and the force produced by individual muscle fibers (e.g., summation of muscle twitches with increased stimulation frequently).

Skeletal muscles can also be stimulated to contract through the skin using an electrical stimulus.  Certain sensitive spots, known as motor points, elicit a strong response and are usually located on the muscle belly where the motor neuron enters.

In this experiment, you will stimulate the abductor digiti minimi (innervated by the ulnar nerve) and the flexor digitorum superficialis (innervated by the median nerve). The abductor digiti minimi, located on the medial surface of the palm, functions to abduct the little finger.  The flexor digitorum superficialis, located on the anterior surface of the forearm, flexes the middle phalanges first, followed by the proximal phalanges and wrist with continued action.  This muscle is primarily involved in rapid, forceful flexion of the digits during grasping movements.  The motor point (median nerve) for the flexor digitorum superficialis is found on the lateral side of the anterior forearm, while the motor point for the abductor digiti minimi is on the medial side of the hand, just distal to the wrist. Begin by locating the abductor digiti minimi motor point; it difficulty arises, try finding the motor point for the flexor digitorum superficialis.

Single Muscle Twitch

A skeletal muscle comprises many muscle fibers (muscle cells) that extend the entire length of the muscle.  Each muscle fiber is packed with myofibrils, which are bundles of contractile proteins organized into functional units called sarcomeres.  The key contractile proteins in these myofibrils include actin (a primary component of the thin filaments) and myosin (thick filaments).

When a muscle fiber is stimulated to contract by a somatic motor neuron, an action potential is generated in the sarcolemma (the muscle fiber’s plasma membrane).  This action potential is propagated across the sarcolemma and deep into the muscle fiber through transverse tubules, or T tubules, which are extensions of the sarcolemma.  As the action potential travels down the T tubules, it triggers the release of calcium ions from the sarcoplasmic reticulum (smooth ER) into the sarcoplasm (cytoplasm).

The calcium ions bind to the troponin complex, causing a conformational change that moves tropomyosin away form the myosin-binding sites on actin.  This exposure allows the myosin heads (crossbridges) to attach to the actin filaments.  Using the energy from ATP, the myosin crossbridges pivot (power stroke) and pull the actin filaments toward the center of the sarcomere, causing the sarcomere to shorten and generate muscle contraction.

The muscle fiber relaxes when calcium ions are actively transported back into the sarcoplasmic reticulum.  This reduces the calcium ion concentration in the sarcoplasm, causing the myosin crossbridges to detach from the actin filaments, allowing the actin filaments to slide back to their original positions, and the sarcomere returns to its resting length.

Muscle Twitch

A muscle twitch is the mechanical response of a muscle cell or whole muscle to a single stimulus.  Upon close observation, it becomes evident that the contraction phase is quicker than the relaxation phase. The latent period, which precedes contraction, represents the time required for the muscle to begin generating tension after receiving the stimulus.  This period is necessary because several sequential events must occur before contraction ensues: 1) depolarization of the sarcolemma and T tubules, 2) release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm, 3) attachment of myosin crossbridges to actin, and 4) tensioning of elastic elements within the muscle cell.

There are two main types of muscle twitches: isometric and isotonic. The distinction lies in whether the muscle changes length during contraction.  In an isometric contraction, tension builds within the muscle but it does not overcome the external load, thus the muscle does not visibly change in length.  The term “isometric” stems from “iso-“ meaning “same” and “-metric” referring to “length”, emphasizing that the muscle length remains constant while tension develops. An example of an isometric contraction is maintaining a plank position.

Conversely, in an isotonic contraction, the muscle generates sufficient force to move the load, causing the muscle to shorten as it pulls on the tendon.  This result in a visible movement of the load.  The term “isotonic” combines “iso-“ meaning “same” and “-tonic” referring to “force/load”, indicating a constant force exerted throughout the movement.  An example of an isotonic contraction is performing bicep curls.  Notably, every isotonic twitch begins with an isometric phase where the muscle initially contracts without shortening until enough force is generated to move the load.

Threshold Stimulus and Motor Unit Recruitment: The Effect of Stimulus Strength on Twitch Amplitude

Skeletal muscles are composed of functional units known as motor units.  Each motor unit compromises a single somatic motor neuron that branches to innervate multiple muscle fibers.  When stimulated, all muscle fibers within a motor unit contract simultaneously, exerting the maximum force they can generate.  To ensure smooth muscle function and reduce fatigue, motor units are activated asynchronously- not all units contract at the same time; some relax while others contract.

In muscles like the abductor digiti minimi, motor units vary in size and ease of activation.  Small motor units, with thin motor neurons innervating fewer muscle fibers, respond to weaker stimuli and contract first. In contrast, large motor units, with thicker motor neurons innervating more muscle fibers, require stronger stimuli to activate.  The overall muscle contraction strength results from the combined force of all concurrently active motor units.

As stimulus strength increases, more motor units are recruited to contract, enhancing the force of muscle contraction.  This recruitment process increases the number of active motor units.  Addiionally, muscle contraction strength can be increased through summation, where successive stimuli are applied before the muscle fiber completely relaxes from the preceding twitch.  Summation causes twitches to combine, generating greater force.

Most mammalian skeletal muscles contain two main types of muscle fibers: fast-twitch and slow-twitch fibers, each characterized by unique myosin forms influencing contraction speed.  Fast-twitch fibers include fast glycolytic fibers, which primarily use glycolysis for ATP production, and fast oxidative fibers, which rely on aerobic respiration.  Fast glycolytic fibers, with he largest diameter, produce the strongest contractions but fatigue quickly due to inefficient glucose breakdown and lactic acid accumulation.  Fast oxidative fibers are intermediate in size and strength.  Slow oxidative fibers, with the smallest diameter, primarily rely on aerobic respiration and are highly resistant to fatigue.

Skeletal muscles typically contain a mix of these fiber types.  However, individual motor units generally consist of only fiber type.  During muscle activation, the brain first recruits motor units containing slow oxidative fibers for low-intensity tasks.  As demands increase, signals stimulate motor units with fast oxidative fibers for additional force.  Motor units with fast glycolytic fibers are recruited last, activated during high-intensity activities like weightlifting.

In summary, as the need for muscle tension increases, recruitment of muscle fibers occurs in the following order: slow oxidative fibers first, followed by fast oxidative fibers, and finally, fast glycolytic fibers.

Effect on Load on Isotonic Contractions

The effect of load on isotonic contractions is crucial to understanding muscle performance.  When a muscle is stimulated to lift a weight, it initially develops tension without shortening (isometric contraction). If the muscle generates enough tension to overcome the load, it shortens, and the load is lifted (isotonic contraction). The time between stimulation and the start of shortening is known as the latent period.

The load significantly impacts isotonic contractions in several ways:

1. Latent Period: As the load increases, the latent period lengthens, requiring more time to initiate lifting.
2. Duration of Shortening: Heavier loads shorten the duration of muscle shortening, reducing how long the load can be lifted.
3. Velocity of Shortening: With heavier loads, the velocity of muscle shortening decreases, resulting in slower lifting.
4. Distance Shortened: The amount of shortening distance decreases as the load becomes heavier, limiting how far the load is lifted.

The size of the load directly affects the amount of work a muscle can perform.  Work is defined as the product of the force applied and the distance over which is applied (WORK = Force x Distance). During purely isometric contractions, where no movement occurs, the muscle performs no work.  In this lab, work will be measured as the mass of weight moved by your finger multiplied by the amplitude of the muscle twitch observed in your finger.

Summation and Tetanus

Two primary factors determine the force generated by a muscle: 1) the recruitment of muscle fibers and 2) the force produced by each individual muscle fiber through summation.

Summation occurs when a muscle fiber is stimulated before it completes its current contraction, resulting in subsequent twitches adding to the previous ones.  This process, enhances muscle contraction strength.  When stimuli are closely spaced, each muscle fiber contracts more forcefully due to increased calcium ions in the sarcoplasm.  The initial action potential triggers the release of calcium ions from the sarcoplasmic reticulum, and not all are pumped back before the next action potential arrives.  This higher concentration of calcium ions exposes more myosin-binding sites on actin, facilitating more myosin crossbridge bindings and greater tension production.

To visualize this, consider a tug of war with a rope: Adding more people to your end of the rope allows your team to pull more strongly.

As the frequently of stimulation increases, the muscle fiber has less time to relax, leading to summation and eventually incomplete tetanus.  In incomplete tetanus, although the individual twitches are still discernible, there is a higher average tension compared to a single muscle twitch.  Further increasing the stimulus frequently results in complete tetanus, where no relaxation occurs between stimulations, leading to a sustained maximum tension output.

During complete tetanus, the muscle maintains a plateau of tension.  Once the frequency required for tetanic contraction is achieved, increasing the stimulus rate further does not enhance the force of contraction.  The rate of muscle relaxation after tetanus is slower than after a single muscle twitch because it takes more time to reuptake the excess calcium back into the sarcoplasmic reticulum.

Under normal conditions, motor units are activated by many stimuli in rapid succession rather than a single stimulus.  This asynchronous firing of motor unit allows the muscle to produce a smooth a nd sustained contraction.

Muscle Fatigue

When a muscle fiber is stimulated frequently, it eventually experiences a decrease in tension production known as muscle fatigue.  Several factors contribute to muscle fatigue, including the type of exercise involving both the rate and force of contraction, and the adequacy of blood supply to the muscle.  During high-intensity exercises like weightlifting or sprinting, fast glycolytic fibers are recruited.  These fibers rely on fermentation (glycolysis) to generate ATP, leading to the accumulation of lactic acid.  The resulting change in pH affects the shape and function of contractile proteins and calcium pumps within the sarcoplasmic reticulum.  Additionally, very intense exercise can cause neuromuscular fatigue, where somatic motor neurons exhaust their neurotransmitter supply due to frequent stimulation.

Conversely, during low-intensity, long-duration exercises that primarily engage slow oxidative and fast oxidative fibers, muscle fatigue stems largely from the depletion of fuel sources like glycogen needed for cellular respiration.

While it’s evident that energy is essential for muscle contraction, it’s equally crucial for muscle relaxation. Relaxation requires the active transport of calcium ions back into the sarcoplasmic reticulum from the cytoplasm, a process that takes approximately 0.033 seconds.  Inadequately ATP prolongs the presence of calcium ions in the cytoplasm, sustaining interaction between actin and myosin. Moreover, without ATP, the myosin head remains attached to the actin active site, preventing muscle relaxation from occurring efficiently.