Module 7 Lecture 2

Length-tension relationship

During an isometric contraction, at the level of the sarcomere the maximum active force (tension developed) is dependent on the degree of actin and myosin overlap

Understretched

Sarcomere < 2.0 µm, actin and myosin overlap excessively, actin filaments from opposite ends of the sarcomere may even interfere with each other or with the M-line, this limits the effective number of cross-bridges that can form between actin and myosin. Resulting in decreased force production due to inefficient contraction.

Optimal length

Sarcomere is 2.0 - 2.2 µm, actin and myosin filaments have maximum overlap without interference, this allows the maximum number of cross-bridges to form. Resulting in maximal force production — this is the most effective length for contraction.

Overstretched

Sarcomere > 2.2 µm, very little or no overlap between actin and myosin filaments, few or no cross-bridges can form. Resulting in greatly reduced or no force production, because contraction can't effectively occur.

Total tension

Active force + passive force

Muscle has elastic components, active tension is dependent on sarcomere length

Passive force

Increases as the muscle is stretched due to the passive elements

Active force

Developed via cross-bridge cycling and dependent on actin-myosin overlap

Motor-unit

Consists of a motor neuron and all the muscle fibres it innervates

Excitation-contraction coupling

Excitation (7 steps) and contraction (7 steps)

Excitation Step 1

Depolarisation of axon terminal on motor neuron

Excitation Step 2

Causes voltage-gated calcium channels to open, flooding the axon terminal

Excitation Step 3

Synaptic vesicles of acetylcholine are released via exocytosis

Excitation Step 4

Acetylcholine diffuses across synaptic cleft to bind to ACh-gated cation channels which cause a conformational change

Excitation Step 5

ACh-gated cation channels open

Excitation Step 6

A large influx of Na²⁺ ions and a small efflux of K⁺ ions into and out of the muscle fibre making muscle cell less negative (End-plate potential/EPP)

Excitation Step 7

Once membrane potential reaches threshold, voltage-gated Na⁺ channels open and AP is generated along sarcolemma and T-tubule. Acetylcholine unbinds

Clearing of acetylcholine from synaptic cleft

Diffuses out or broken down by acetylcholinesterase into acetic acid and choline, choline is retaken back into the axon terminal for resynthesis of acetylcholine

AP in skeletal muscle

Similar to nerve cell and AP is ~2 ms

Contraction Step 1

AP conducted down T-tubule is in close contact with sarcoplasmic reticulum (SR), forming a triad

Contraction Step 2

Dihydrogen pyridine receptors (DHPRs) sense depolarisation and physically interact with ryanodine receptors (RyR1) on SR, causing them to open

Contraction Step 3

Ca²⁺ released into the cytosol

Contraction Step 4

Ca²⁺ binds with troponin when concentrations reach a critical threshold, the myosin binding sites on the actin filaments are exposed

Contraction Step 5

Cross-bridge cycle occurs

Contraction Step 6

Contraction ends with concentration of Ca²⁺ ions decrease as they are actively pumped back into the SR via Ca²⁺-ATPase pumps

Contraction Step 7

Tropomyosin moves back covering there myosin binding site

Muscle metabolism

Sources of ATP are creatine phosphate, anaerobic glycolysis, aerobic metabolism

Creatine phosphate

Used at the very start of contractions, creating phosphates acts as ATP store but duration of energy is ~15 seconds, anaerobic

Anaerobic glycolysis

Fast but inefficient, dominant system from about 10 - 30 s of maximal effort, build of metabolites (H⁺ limits duration to 120s), anaerobic

Aerobic metabolism

Efficient but slow, max 300 W, important for postural muscles and endurance exercise, aerobic

Muscle fibres

Slow twitch fibres (Type 1) and Fast twitch fibres (Type 2)

Slow twitch fibres (Type 1)

Smaller, darker due to more myoglobin, slow oxidative, maintaining posture and walking

Fast twitch fibres (Type 2)

Bigger, fast glycolytic, fatigue rapidly but develop large forces, jumping and weight lifting

Regulation of force

Rate of stimulation of individual motor units and the number of motor units recruited

Unfused (incomplete) tetanus

Low stimulation frequency, temporal summation

Fused (complete) tetanus

High stimulation frequency, temporal summation

Tetanus

Infection causes tetanus of muscle which suppresses inhibition of motor neuron activity = lots of AP