Muscle Mechanics

Muscle Mechanics - Part Two: Strength of Neural Stimuli and Motor Unit Recruitment

Overview of Graded Muscle Responses

• Muscle contractions are typically graded responses, meaning they are not just "on" or "off." Instead, the muscular system can adjust the force produced based on the requirements of the task.

• Grading of muscle contraction is achieved through two primary mechanisms:

  • Recruitment: Changing the number of motor units stimulated (summation of motor units).

  • Frequency of Stimulation: Changing the rate of firing (temporal summation/tetanus).

• This section focuses on how the strength of neural stimuli leads to spatial summation via recruitment.

Detailed categories of Neural Stimuli

• To understand how stimuli affect tension, we consider an experimental setting where external electrical shocks are applied to a muscle:

  1. Sub-threshold Stimulus

  • Measured at low voltages (e.g., $10\text{ mV}$).

  • A stimulus that is too weak to reach the threshold of any motor neurons associated with the muscle.

  • No action potential is generated; therefore, there is no observable muscle contraction.

  1. Threshold Stimulus (The First Observable Response)

  • The minimum voltage required to trigger the first observable contraction (e.g., $30\text{ mV}$).

  • At this point, the most excitable motor unit (the one with the lowest threshold) reaches its action potential and triggers its associated muscle fibers.

  1. Maximal Stimulus

  • The strongest stimulus that produces an increase in contractile force (e.g., $1,000\text{ mV}$).

  • This represents the point where all possible motor units in the muscle have been recruited.

  • Increasing the voltage further (e.g., to $2,000\text{ mV}$) will not result in additional tension because there are no more motor units left to activate. This is the physiological limit of the muscle's strength for that specific length and condition.

Physiological Basis of Motor Unit Recruitment

• Motor Unit Definition: A single somatic motor neuron and all the skeletal muscle fibers it innervates.

  • When a motor neuron fires, all fibers in that unit contract simultaneously (the All-or-None Law at the cellular level).

  • The whole muscle, however, acts on a sliding scale because it contains hundreds of different motor units.

• Multiple Motor Unit Summation (Recruitment):

  • As stimulus intensity increases, the number of activated motor units increases, leading to a smooth, continuous increase in muscular tension.

The Size Principle (Henneman's Size Principle)

• Recruitment is not random; it follows the Size Principle, which dictates the specific order of motor unit activation based on the size of the motor neuron.

  1. Smallest Motor Units Activated First:

  • These consist of smaller, highly excitable motor neurons.

  • Typically control Slow Oxidative (Type I) fibers, which are fatigue-resistant and used for low-intensity, precision movements.

  1. Medium Motor Units Activated Second:

  • As higher force is needed, medium-sized neurons are recruited.

  • These often control Fast-Oxidative Glycolytic (Type IIa) fibers.

  1. Largest Motor Units Activated Last:

  • These consist of the largest, least excitable motor neurons with the highest threshold.

  • They control Fast Glycolytic (Type IIb/IIx) fibers, which produce the most powerful contractions but fatigue very quickly.

• Excitability Factors: Smaller neurons have a larger surface area-to-volume ratio, allowing them to reach threshold depolarization faster than larger neurons under the same level of synaptic input.

Clinical and Practical Application

• Precision vs. Power: Muscles used for fine control (like the extrinsic eye muscles) have very small motor units ($3$-$10$ fibers per unit). Muscles used for power (like the gastrocnemius) have large motor units (up to $2,000$ fibers per unit).

• Asynchronous Recruitment: During submaximal contractions, the body often alternates which motor units are active to prevent fatigue, allowing some units to rest while others maintain tension.

Summary of Tension Generation

• Weak Stimulation: Recruits only the small, high-excitability units for delicate tasks.

• Maximum Stimulation: Recruits all units for heavy lifting or explosive movements.

• The Cap: Once maximal stimulus is reached, no further force can be generated by increasing neural input alone; recruitment is at $100\%$.