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.