Skeletal Muscle Physiology: Structure, Function, and Clinical Applications

Skeletal Muscle Physiology

Introduction

Skeletal muscle physiology is fundamental to understanding movement, posture, and homeostasis in the human body. Skeletal muscles are responsible for voluntary actions such as walking, breathing, and chewing, and play a critical role in regulating blood sugar, maintaining body temperature, and supporting circulation. At the cellular level, muscle function relies on excitable membranes, ion channels, and ATP-dependent processes, which enable precise neural control and contraction. Studying skeletal muscle physiology is essential for comprehending athletic performance, neuromuscular function, and medical conditions like muscular dystrophy and myasthenia gravis.

Muscle Characteristics and Functions

Muscle Characteristics

• Excitable: Responds to stimuli and produces action potentials.

• Contractile: Can shorten and thicken.

• Extensible: Can stretch when pulled.

• Elastic: Returns to original shape after contraction or extension.

Muscle Functions

• Movement: Enables actions such as walking and breathing.

• Posture and Facial Expression: Maintains body position and allows communication.

• Heat Production: Generates heat to maintain body temperature (~37°C).

• Protection of Viscera: Supports and shields internal organs.

Neuromuscular Junction

Structure and Function

The neuromuscular junction is the site where a motor neuron communicates with a skeletal muscle fiber. Each muscle fiber is innervated by only one neuron, but a single motor neuron can branch to innervate multiple muscle fibers, forming a motor unit.

• Presynaptic Cell: The neuron’s axon terminal contains vesicles filled with acetylcholine (ACh).

• Postsynaptic Cell: The motor end plate of the muscle fiber’s sarcolemma has many ACh receptors.

• Synaptic Cleft: The space between the pre- and postsynaptic membranes.

Mechanism of Muscle Fiber Stimulation

1. Action potential reaches the axon terminal.

2. Voltage-gated calcium channels open.

3. Ca2+ enters the neuron, causing exocytosis of ACh.

4. ACh binds to receptors on the motor end plate.

5. Chemically-gated channels open; Na+ enters, creating an End Plate Potential (EPP).

6. EPP opens voltage-gated Na+ channels, generating an action potential on the sarcolemma.

7. Action potential propagates along the sarcolemma and down T-tubules.

Note: One neuron action potential always creates one EPP, which always creates one muscle action potential. Inhibition of contraction requires inhibition of the motor neuron.

Molecular Basis of Skeletal Muscle Contraction

Sliding Filament Theory

Skeletal muscle contraction is governed by the interaction of actin and myosin within sarcomeres. The process is powered by ATP and regulated by Ca2+, which binds to troponin, causing tropomyosin to shift and expose myosin binding sites on actin.

• In a relaxed muscle, tropomyosin covers myosin binding sites on actin; myosin head is activated.

• Upon Ca2+ binding to troponin, tropomyosin moves, allowing myosin to bind to actin.

Steps of Contraction

1. Excitation: Sarcolemma depolarizes; action potential propagates down T-tubules.

2. Excitation-Contraction Coupling: Action potential triggers Ca2+ release from sarcoplasmic reticulum; Ca2+ binds to troponin; tropomyosin shifts.

3. Contraction: Myosin heads attach to actin (cross bridge formation); power stroke occurs; ATP binds to myosin, causing release and recovery stroke; cycle repeats if Ca2+ remains high.

Sliding Filament Mechanism: Sarcomeres shorten, H zone and I band shorten, A band remains the same length. Myofibrils and muscle shorten, but actin and myosin filaments retain their length.

Muscle Fiber Relaxation

Steps of Relaxation

1. ACh is broken down by acetylcholinesterase (AChE) on the motor end plate.

2. Sarcoplasmic reticulum actively takes up Ca2+ (Ca2+-ATPase pump).

3. ATP binds to and releases myosin heads.

4. Tropomyosin moves back to cover myosin binding sites on actin.

ATP is necessary for cross bridge release, myosin activation, Ca2+ pumping, and Na+/K+-ATPase activity.

Example: The breakdown of acetylcholine (ACh) by acetylcholinesterase (AChE) produces acetic acid and choline. Acetic acid enters the Krebs cycle as Acetyl CoA, while choline is recycled for further neurotransmitter synthesis.

Clinical Applications

Botulism

• Caused by Clostridium botulinum (improper canning).

• Prevents exocytosis of ACh, resulting in flaccid paralysis.

• Medical uses: Treats uncontrolled blinking, crossed eyes, cosmetic applications (Botox).

Rigor Mortis

• Postmortem muscle stiffness due to increased Ca2+ and lack of ATP.

• Crossbridges form but cannot release; starts ~3 hours after death, peaks at 12 hours, subsides over days.

Myasthenia Gravis

• Autoimmune condition reducing ACh receptors.

• Results in flaccid paralysis; treated with AChE inhibitors.

Curare Poisoning

• Prevents ACh from binding to receptors, causing flaccid paralysis.

• Historically used in surgery to prevent movement.

Nicotine

• Mimics ACh effect, causing muscle spasms.

Black Widow Spider Venom

• Triggers massive ACh release, causing continuous contraction and respiratory failure.

Muscle Tension

Factors Affecting Muscle Tension

• Frequency of Stimulation: Single stimulus produces a twitch; repeated stimuli cause wave summation, incomplete tetanus, or complete tetanus.

• Fiber Length: Optimal resting length allows maximum cross bridge formation; tension decreases if fibers are too short or too long.

• Size of Fiber: Thicker fibers (more myofibrils) generate more tension; increased by exercise and testosterone.

• Fatigue: Reduces maximum tension.

Fiber Types

• Fast Fibers: Contract and relax rapidly; white (low myoglobin).

• Slow Fibers: Contract and relax slowly; red (high myoglobin); e.g., postural muscles.

Whole Muscle Tension

• Number of fibers contracting (motor unit recruitment).

• Number of fibers per motor unit.

• Muscle size.

• Fatigue.

Muscle Tone

Muscle tone is a low level of tension maintained by alternating stimulation of motor units. It provides firmness to muscles and is essential for posture and readiness for action.

Whole Muscle Contraction

Types of Contraction

• Isotonic: Muscle changes length; tension exceeds load resistance; e.g., flexion at the elbow.

• Isometric: Muscle length remains constant; tension increases but does not exceed load; e.g., holding a book.

Both types use ATP for cross bridge cycling and tension generation.

Muscle Metabolism

Energy for Contraction

• Resting Conditions: Fatty acids used for ATP (aerobic); storage of glycogen and creatine phosphate.

• Short-Term Exercise (<1 min): Anaerobic; uses available ATP, creatine phosphate, and glycogen (produces lactic acid).

• Long-Term Exercise (1 min to hours): ATP from aerobic pathways; glucose and fatty acids used; O2 from hemoglobin and myoglobin.

Equation for Creatine Phosphate:

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

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

Muscle Fatigue

Types of Fatigue

• Physiological Fatigue: Inability to maintain tension; protective mechanism; due to depletion of energy supplies, build-up of end products (H+, Pi), and failure of action potentials.

• Psychological Fatigue: CNS fails to send commands; often related to lactic acid accumulation.

Muscles and EPOC

Excess Post-exercise Oxygen Consumption (EPOC)

EPOC refers to increased oxygen consumption after exercise, necessary for recovery. Oxygen is used to replenish glycogen, creatine phosphate, and O2 stores, and to convert lactic acid to pyruvic acid or glucose. Elevated body temperature increases O2 demand due to faster chemical reactions.

Additional info: Academic context and expanded explanations were added to ensure completeness and clarity for college-level study.