Skeletal Muscle Tissue

Functions of Skeletal Muscle Tissue

Skeletal muscle tissue is essential for various physiological and mechanical functions in the human body.

• Movement: Skeletal muscles contract to produce voluntary movements of the body.

• Posture Maintenance: Muscles help maintain body posture and stabilize joints.

• Support of Soft Tissues: Muscles support and protect internal organs.

• Guarding Entrances and Exits: Sphincter muscles control the opening and closing of body orifices.

• Heat Production: Muscle contractions generate heat, helping to maintain body temperature.

• Storage of Nutrients: Muscles store glycogen and proteins that can be broken down for energy.

Types of Muscle Tissue

Three Types of Muscle Tissue

Muscle tissue in the human body is classified into three main types, each with distinct characteristics and functions.

• Skeletal Muscle

  • Voluntary or Involuntary: Voluntary

  • Function: Movement of the skeleton, posture, heat production

• Cardiac Muscle

  • Voluntary or Involuntary: Involuntary

  • Function: Pumping blood through the heart and circulatory system

• Smooth Muscle

  • Voluntary or Involuntary: Involuntary

  • Function: Movement of substances through hollow organs (e.g., intestines, blood vessels)

Structure of a Muscle

Connective Tissue Components

Muscles are organized into bundles surrounded by connective tissue layers:

• Epimysium: Dense connective tissue that surrounds the entire muscle.

• Perimysium: Connective tissue that surrounds bundles of muscle fibers called fascicles.

• Endomysium: Thin connective tissue that surrounds each individual muscle fiber.

Tendon vs. Aponeurosis:

• Tendon: Cord-like structure that connects muscle to bone.

• Aponeurosis: Broad, flat sheet of connective tissue that attaches muscles to bones or other muscles.

Skeletal Muscle Cell

Cellular Components

• Nuclei: Skeletal muscle fibers are multinucleated to support high metabolic activity and protein synthesis.

• Myofibrils: Cylindrical structures within muscle fibers responsible for contraction.

• Myofilaments: Protein filaments (actin and myosin) that slide past each other to produce contraction.

• Sarcomere: The functional contractile unit of a muscle fiber, defined by Z-lines.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions (), essential for muscle contraction.

Sliding Filament Theory: Explains how muscles contract by the sliding of actin (thin) filaments over myosin (thick) filaments, shortening the sarcomere.

Membrane Potentials

Cell Polarization

All body cells are polarized, meaning there is a difference in electrical charge across the cell membrane.

• Resting Membrane Potential: The inside of the cell is negatively charged relative to the outside, typically around -70 mV in neurons.

• Significance: This polarization is essential for the generation and propagation of action potentials, which are necessary for muscle contraction and nerve impulse transmission.

Action Potentials

Phases of an Action Potential

• Stimulus (Start): A triggering event causes the membrane potential to become less negative.

• Depolarization: Sodium channels open, allowing to enter the cell, making the inside more positive.

• Repolarization: Potassium channels open, allowing to exit the cell, restoring the negative charge inside.

Propagation: Action potentials spread along the membrane, allowing rapid communication and muscle contraction.

Neuromuscular Junction

Structure and Function

• Control: Each muscle fiber is controlled by a single motor neuron, but one neuron can control multiple fibers (motor unit).

• Motor Neuron: A nerve cell that transmits signals from the central nervous system to muscle fibers, causing contraction.

• Parts of the Neuromuscular Junction:

  1. Axon terminal of the motor neuron

  2. Synaptic cleft

  3. Motor end plate of the muscle fiber

• Steps of Activity:

  1. Action potential arrives at the axon terminal.

  2. Release of acetylcholine (ACh) into the synaptic cleft.

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

  4. Depolarization of the muscle fiber membrane.

  5. Initiation of muscle action potential.

  6. Breakdown of ACh by acetylcholinesterase.

Excitation-Contraction Coupling

Steps of Excitation-Contraction Coupling

This process links the electrical signal (action potential) to muscle contraction.

1. Action potential travels along the sarcolemma and down T-tubules.

2. Triggers release of from the sarcoplasmic reticulum.

3. binds to troponin, causing a conformational change.

4. Tropomyosin shifts, exposing binding sites on actin.

5. Myosin heads bind to actin, forming cross-bridges.

6. Contraction cycle begins.

Muscle Fiber Contraction Cycle

Steps of the Contraction Cycle

1. ATP binds to myosin head, causing it to detach from actin.

2. ATP is hydrolyzed, energizing the myosin head.

3. Myosin head binds to a new position on actin.

4. Power stroke: Myosin head pivots, pulling actin filament.

5. ADP and phosphate are released.

6. Cycle repeats as long as and ATP are present.

7. Contraction ends when is pumped back into the SR.

Muscle Twitch and Contraction

Muscle Twitch

• Definition: A single, brief contraction and relaxation cycle in a muscle fiber.

• Phases: Latent period, contraction phase, relaxation phase.

Motor Units and Muscle Control

Motor Units

• Motor units consist of a single motor neuron and all the muscle fibers it controls.

• Different motor units allow for graded control of muscle force.

Types of Muscle Contractions

Classification and Description

ATP Sources in Muscle

Energy for Contraction

• Creatine Phosphate: Provides rapid ATP regeneration for short bursts.

• Anaerobic Glycolysis: Produces ATP without oxygen, yielding lactic acid.

• Aerobic Respiration: Most efficient, produces the most ATP using oxygen.

Muscle Fatigue and Recovery

Muscle Fatigue

• Definition: The decline in ability of a muscle to generate force.

• Recovery Period: Time required for muscles to return to pre-exertion conditions.

• Oxygen Debt: Extra oxygen required after exercise to restore metabolic conditions.

• Lactic Acid: Produced during anaerobic metabolism; converted back to pyruvate when oxygen is available.

Types of Skeletal Muscle Fibers

Classification and Comparison

• Determination: Genetics and training influence fiber type distribution.

• Hypertrophy: Increase in muscle size due to increased fiber size.

• Atrophy: Decrease in muscle size due to disuse or disease.