lecture 16 pt2

Cholinesterase and Acetylcholine

• Cholinesterase: An enzyme that breaks down acetylcholine; often referred in textbooks as acetylcholinesterase.

  • Simplified to just cholinesterase to reduce confusion.

Repolarization of Muscles

• After acetylcholine is broken down by cholinesterase, muscles must repolarize.

• Active Transport: The mechanism of moving molecules against their concentration gradient using a carrier.

  • Sodium (Na+): Predominantly found outside cells, pushed out by active transport.

  • Potassium (K+): Located within the muscle; positive charges repel each other, enabling repolarization.

  • This process is managed by the Sodium-Potassium Pump.

Sodium-Potassium Pump

• An active transport mechanism crucial for repolarization.

  • Function: Restores the original charge across the muscle cell membrane after contraction.

  • Key point: Sodium and Potassium are elements and distinct from sodium chloride (table salt).

All or None Law

• Definition: Once the threshold is reached, all muscle cells that have the threshold will contract fully.

  • Mechanism: If threshold (e.g., 25 mV) is not reached, no contraction occurs. Once it is reached (e.g., 30 mV), contraction is full, not partial.

• Entire muscles are composed of thousands of muscle cells, each with varying threshold points.

Impulse Transmission in Muscles

• A single nerve impulse branches into various muscle bundles, stimulating multiple muscle cells.

  • Impulses are uniform; if one target requires 20 volts, all receive 20 volts equally across connected cells.

Summation in Muscle Contraction

• Definition: The cumulative effort of each muscle cell or motor unit contributing to total muscle force.

• Example: If a threshold of 25 is reached, all corresponding muscle cells contract as a collective unit.

Muscle Structure and Membranes

• Muscle Composition:

  • Made of muscle bundles, each containing numerous muscle cells.

• Membranes Surrounding Muscles:

  • Epimysium: Encases the entire muscle; translates to tendons, attaching to bones.

  • Perimysium: Surrounds each muscle bundle.

  • Endomysium: Surrounds individual muscle cells.

Motor Units

• Definition: Groups of muscle cells that share the same threshold for activation.

  • Each unit consists of muscle cells that respond collectively to a frequency signal.

• Implications: Lower thresholds contract with lower frequencies, while higher thresholds require stronger signals.

Muscle Strength and Exercise

• Muscle strength correlation with exercise; larger muscle cells require lower firing frequency to achieve the same workload.

• For example, females generally possess smaller muscle cells compared to males and require higher frequencies to exert similar strength.

Myasthenia Gravis

• Disease Mechanism: An autoimmune condition where the body attacks acetylcholine receptors, leading to weakness.

  • Some thresholds become unreachable, requiring greater frequencies to achieve muscle contractions.

  • Over time, muscle functionality declines, affecting basic movements including breathing.

Curare and Historical Context

• Curare: A toxin that blocks acetylcholine, used in historical hunting tools.

  • Also used during surgeries to manage muscle response to prevent involuntary contractions.

Maximum Muscle Contraction

• The maximum contraction of any muscle is limited to approximately 50% of available motor units at one time to ensure muscle safety and efficiency.

  • Exceeding this can cause muscle cell death, impacting long-term muscle health.

Adrenaline and Muscle Function

• Adrenaline can override normal muscle contraction limits temporarily, explaining exceptional strength in emergencies (fight or flight responses).

Overview of Muscle Physiology

• Muscle contractions and nerve functions operate on principles of electricity and chemical interactions, highlighting the intricate balance and regulation needed for muscle control and movement.