Muscular System Notes

Cardiac Muscle

Found in the heart, involuntary.

Skeletal Muscle

Voluntary, striated, primarily responsible for movement.

Smooth Muscle

Involuntary, found in walls of hollow organs.

Movement

Involves both voluntary (e.g., lifting an arm) and involuntary actions (e.g., digestion, blood pumping).

Stability

Maintains posture and prevents undesired movements.

Control of Body Openings and Passages

Sphincter muscles regulate movement (e.g., in the digestive tract).

Heat Generation

Skeletal muscles produce significant body heat (up to 30% at rest, 40x during exercise).

Glycemic Control

Regulates blood glucose levels by absorbing sugar.

Skeletal Muscle Voluntariness

Skeletal muscle operates under conscious control.

Striations

Visible light and dark bands due to internal proteins that allow contraction.

Muscle Structure

Skeletal muscle cells are known as muscle fibers, characterized by long slender shapes, multiple nuclei, and bundled contractile proteins (myofibrils).

Components of Muscle Fibers

Includes membrane (sarcolemma) with T tubules for electrical conduction and sarcoplasmic reticulum for calcium storage.

Myofibrils

Packed with contractile proteins: Thick filaments (myosin) and thin filaments (actin strands intertwined with tropomyosin and troponin).

A Bands

Dark bands where both thick and thin filaments overlap.

I Bands

Light bands consisting of only thin filaments.

Z Disks

Proteins anchoring thin filaments, marking the borders of myofibrils (sarcomeres).

Neural Stimulation

Skeletal muscles contract only when stimulated by motor neurons in the brain/spinal cord.

Neuromuscular Junction

Site of axon terminal and muscle fiber connection, separated by synaptic cleft.

Acetylcholine (ACh)

Released neurotransmitter that binds to receptors on the muscle fiber, triggering contraction.

AChE

Acetylcholinesterase enzyme that breaks down ACh to halt stimulation and allow muscle relaxation.

Excitation

Electrical nerve signal triggers muscle fiber activation via ACh release.

Steps of Excitation

1. Nerve signal leads to ACh release. 2. ACh binds to receptors. 3. Ion influx initiates action potential.

Contraction

Involves sliding filament model leading to muscle shortening through cross-bridge formation.

Steps of Contraction

1. Myosin binds to ATP, hydrolyzing it. 2. Myosin head binds to actin. 3. Power stroke pulling actin filaments. 4. New ATP binding resets myosin.

Relaxation Process

1. End of ACh release. 2. Breakdown of ACh. 3. Reabsorption of calcium. 4. Troponin-tropomyosin blocks myosin attachment sites.

Isometric Contraction

Muscle tension without shortening (e.g., holding a weight).

Isotonic Contraction

Muscle tension changes while maintaining contraction.

Concentric Contraction

Muscle shortens while maintaining tension (e.g., lifting).

Eccentric Contraction

Muscle lengthens while maintaining tension (e.g., lowering weight).

ATP Generation

Essential for muscle contraction via anaerobic fermentation (2 ATP) and aerobic respiration (30 ATP).

Fatigue and Endurance Factors

Includes glycogen depletion, calcium leakage, K+ buildup; endurance influenced by mitochondrial density.

Slow-twitch Fibers

Aerobic, resistant to fatigue, contain many mitochondria.

Fast-twitch Fibers

Anaerobic, quick to contract, fatigue rapidly, rich in anaerobic enzymes.

Resistance Training

Increases muscle fiber size, improves strength but not endurance.

Endurance Exercises

Improve fatigue resistance, enhance efficiency in ATP production.

Cross-training

Combines resistance and endurance training for maximized performance.

What are thick filaments composed of?

Thick filaments are primarily composed of myosin.

What are thin filaments comprised of?

Thin filaments are composed of actin strands, which are intertwined with tropomyosin and troponin.