Muscle Contraction & Motor Unit Firing Rates (Temporal Summation)

Announcements

  • Tuesday is Truth and Reconciliation Day; there are no classes.

  • Next Thursday, Professor Ali Hall will cover the lecture.

Historical Context: Luigi Galvani and Muscle Electrostimulation

  • Clue: Images of frog legs, known for their ability to jump 1010 times their body length, requiring strong muscles.

  • Luigi Galvani's Discovery: Experimenting with frog legs, Galvani discovered that muscles could be artificially stimulated to contract using electricity.

  • Modern Relevance: This principle forms the basis for studying muscle responses to electrical stimulation, particularly how they contract and how motor neurons naturally stimulate contractions.

Muscle Contraction Control

  • Previous Topic Recap (Recruitment): The central nervous system controls force output by recruiting and derecruiting motor units, effectively changing the active muscle mass.

  • Temporal Summation (Rate Coding): The other primary factor influencing force output is temporal summation, which involves changing the efficiency of contractile qualities by varying the frequency of excitation to the muscle.

Experimental Study of Temporal Summation

  • Methods to Study Muscle Response:

    • Electromyography (EMG): Measures the neural drive to the muscle. By recording the firing rate (rate coding) of motor unit action potentials during voluntary contractions and correlating it with force output.

    • Artificial Electrical Stimulation: Directly applying electricity to the muscle, similar to Galvani's approach, offers the advantage of precise control over stimulation parameters (e.g., current, voltage, frequency), making it easier to interpret muscle force responses compared to EMG, which can be complex.

  • Experimental Setup: Stimulate muscles (e.g., 100%100\% slow and 100%100\% fast twitch muscles) at varying frequencies: 11 pulse/second (Hz), 55 Hz, 1010 Hz, 2020 Hz, 5050 Hz, 100100 Hz.

Muscle Responses to Different Frequencies

  • Observations (Force Output vs. Frequency):

    • Slow-Twitch Muscles: Exhibit a summation effect even at lower frequencies (10−2010-20 Hz), not fully returning to baseline between pulses. They take longer to achieve full tension and relax, allowing subsequent pulses to build upon existing force.

    • Fast-Twitch Muscles: Reach a peak quickly and return to baseline rapidly. Higher frequencies are needed for significant summation.

    • General Principle: Regardless of muscle type, higher activation rates generally lead to greater force output.

Definitions of Contractile Responses

  • Twitch Response: The muscle's response to a single electrical shock or one pulse.

  • Unfused Tetanus (Subtetanic): Occurs at frequencies where individual twitches begin to summate, increasing force, but there is still some perceptible relaxation between pulses.

  • Fused Tetanus: Occurs at high stimulation frequencies where pulses are delivered so rapidly that there is no opportunity for relaxation between them. The force output plateaus at a maximum level (maximal contractile summation).

Force-Frequency Curves

  • Description: A graphical representation plotting force output (up to 100%100\% of maximal force) against input stimulation frequency (e.g., 1−2001-200 Hz).

  • Muscle Type Differences: The curves for fast and slow muscles are distinct, showing different frequency requirements to achieve a given percentage of maximal force.

  • Plateau: Both fast and slow muscles eventually plateau at higher frequencies (e.g., 500500 pulses/sec), indicating that maximal force has been reached and further increases in stimulation frequency will not generate more force. The slow muscle typically plateaus sooner.

  • Sigmoid (S-shaped) Curve: The classic shape of a force-frequency curve.

  • Sensitive Region: The steep part of the curve indicates high sensitivity, where small changes in stimulation rate lead to relatively large changes in force.

Why Does Tetanus Produce More Force Than a Maximal Twitch?

  • Question: Even with maximal current/voltage, why do higher frequencies (tetanus) yield more force than a single twitch?

  • Underlying Mechanism: Excitation-Contraction Coupling

    • Three Curves:

      1. Membrane Potential (Electrical Event): Rapid depolarization and repolarization (e.g., 10−2010-20 ms), ending quickly after the stimulus.

      2. Calcium Concentration (Chemical Event): Calcium release from the sarcoplasmic reticulum peaks shortly after the electrical event, influencing the 'active state' of the muscle.

      3. Tension/Force (Mechanical Response): The force peak occurs after both the electrical and calcium events have returned to baseline. This is a relatively delayed and smoothed response.

    • Practical Value of Delayed/Smoothed Response: The muscle transforms fast, high-frequency input signals into a blunted, delayed, and smoothed output. This prevents jerky movements and allows for controlled contractions. If twitches were as fast as electrical signals, force summation would require extremely high and energy-expensive motor neuron firing rates.

  • Key Factors:

    1. Calcium Accumulation: Repetitive electrical shocks (or higher firing rates from motor neurons) lead to the accumulation of calcium in the sarcoplasm. Calcium is not cleared entirely between pulses, enhancing the 'active state' of the muscle and sustaining contraction.

    2. Muscle Compliance (Hill's Model): Muscles possess parallel and series elastic components that absorb and modify tension before it reaches the bone. A single twitch only takes up a small amount of this internal tension. Repeated pulses in tetanus effectively