Muscle Physiology: Chapter 9 Study Guide (Anatomy & Physiology)

Muscle Physiology

Muscle Membranes

Muscle tissue is organized into distinct layers, each surrounded by a specific membrane:

• Epimysium: Surrounds the entire muscle organ.

• Perimysium: Encloses bundles of muscle fibers called fascicles.

• Endomysium: Wraps individual muscle fibers.

Example: In a biceps muscle, the epimysium covers the whole muscle, perimysium groups fibers into fascicles, and endomysium surrounds each fiber.

Muscle Attachment Points

Muscles attach to bones at specific sites:

• Origin: The fixed attachment, usually proximal.

• Insertion: The movable attachment, usually distal.

Example: The biceps brachii originates at the scapula and inserts at the radius.

Muscle Cell Structure

Muscle fibers (cells) have specialized structures for contraction:

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasm: The cytoplasm of a muscle cell.

• Myofibrils: Rod-like units within muscle cells containing contractile proteins.

Thick filament: Composed of myosin molecules.

Thin filament: Composed of actin, with regulatory proteins troponin and tropomyosin.

• Sarcomere: The functional unit of muscle contraction, defined by Z lines.

• Sarcoplasmic reticulum (SR): Stores calcium ions (Ca2+), which are released upon stimulation.

• T tubules: Invaginations of the sarcolemma that help transmit action potentials.

Example: During contraction, Ca2+ is released from the SR, allowing actin and myosin to interact.

Sliding Filament Model of Muscle Contraction

Muscle contraction occurs when actin and myosin filaments slide past each other, shortening the sarcomere.

• ATP is required for myosin heads to detach and reattach to actin.

• Calcium ions bind to troponin, moving tropomyosin and exposing binding sites on actin.

Equation:

$\text{ATP} + \text{CP} \rightarrow \text{ADP} + \text{Creatine}$

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber:

• Motor neuron: Releases the neurotransmitter acetylcholine (ACh).

• ACh receptor: Located on the muscle cell membrane, binds ACh to initiate contraction.

• Acetylcholinesterase (AChE): Enzyme that breaks down ACh, ending the signal.

Example: AChE prevents continuous muscle contraction by degrading ACh in the synaptic cleft.

Myasthenia Gravis

Myasthenia gravis is an autoimmune disease where antibodies block or destroy ACh receptors, leading to muscle weakness.

Action Potential and Muscle Contraction

Action potentials trigger muscle contraction by opening ion channels:

• End-plate potential: Local depolarization at the NMJ.

• Depolarization: Na+ channels open, Na+ enters the cell.

• Repolarization: K+ channels open, K+ exits the cell.

Example: Na+ influx initiates the action potential, while K+ efflux restores resting potential.

Isometric vs. Isotonic Contraction

Muscle contractions can be classified as:

• Isometric: Muscle tension increases, but length does not change.

• Isotonic: Muscle changes length while tension remains constant.

Types of isotonic contraction:

• Concentric: Muscle shortens.

• Eccentric: Muscle lengthens.

Motor Unit

A motor unit consists of a motor neuron and all the muscle fibers it innervates.

• Motor units are recruited from small to large as more force is needed.

• Recruitment increases muscle tension.

Summation and Recruitment

Muscle force can be increased by:

• Summation: Increasing the frequency of stimulation.

• Recruitment: Increasing the number of active motor units.

Incomplete tetanus: Partial relaxation between stimuli.

Complete tetanus: No relaxation; sustained contraction.

Muscle Tone

Muscle tone is the continuous and passive partial contraction of muscles, important for posture and readiness.

Sources of ATP in Muscle

Muscles require ATP for contraction, which can be generated by:

• Creatine phosphate (CP): Rapidly regenerates ATP when needed.

• Anaerobic respiration (glycolysis): Produces ATP without oxygen; yields lactic acid.

• Aerobic respiration: Uses oxygen; occurs in mitochondria; produces more ATP.

ATP yield:

• Anaerobic: 2 ATP per glucose

• Aerobic: 32 ATP per glucose

Muscle Fatigue and Oxygen Debt

Muscle fatigue occurs when ATP production cannot keep up with demand, leading to decreased performance.

Oxygen debt: The extra oxygen required after exercise to restore metabolic conditions.

Hemoglobin vs. Myoglobin

Property	Hemoglobin	Myoglobin
Location	Red blood cells	Muscle cells
Function	Transports oxygen in blood	Stores oxygen in muscle
Structure	4 polypeptide chains	1 polypeptide chain

Muscle Fiber Types

Muscle fibers can be classified as:

• Oxidative fibers: Use aerobic respiration; fatigue-resistant; high myoglobin.

• Glycolytic fibers: Use anaerobic respiration; fatigue quickly; low myoglobin.

Example: Marathon runners have more oxidative fibers; sprinters have more glycolytic fibers.

Adaptation to Exercise

Exercise induces changes in muscle:

• Aerobic exercise: Increases mitochondria, myoglobin, and capillaries.

• Resistance exercise: Increases myofibrils and glycogen stores.

Muscle Atrophy

Disuse atrophy is the decrease in muscle size due to inactivity.

Skeletal Muscle vs. Smooth Muscle

Both types share similarities but have key differences:

• Smooth muscle lacks sarcomeres, myofibrils, and T tubules.

• Smooth muscle is found in hollow organs and has circular and longitudinal layers.

• Smooth muscle contracts slower and can sustain contraction longer.

• Calcium for contraction comes from extracellular sources.

• Uses calmodulin instead of troponin.

• Relaxation requires additional steps.

Peristalsis: Wave-like contractions that move contents through hollow organs.

Varicosities: Swellings in autonomic nerve fibers that release neurotransmitters.

Myosin kinase: Enzyme that phosphorylates myosin in smooth muscle.

Muscular Dystrophy

Muscular dystrophy is a group of genetic diseases causing progressive muscle weakness and degeneration.

• The most severe form is Duchenne muscular dystrophy, caused by a mutation in the dystrophin gene.