Muscle Tissue: Structure, Function, and Physiology

Module 10.1 Overview of Muscle Tissue

A. Types of Muscle Tissue

Muscle tissue is a specialized tissue composed of muscle cells (myocytes) that generate force and movement. There are three main types of muscle tissue, each with distinct structural and functional characteristics.

• Skeletal muscle: Voluntary, striated muscle attached to bones, responsible for body movement.

• Cardiac muscle: Involuntary, striated muscle found only in the heart, responsible for pumping blood.

• Smooth muscle: Involuntary, non-striated muscle found in walls of hollow organs (e.g., intestines, blood vessels).

Muscle tissue generates a force called muscle tension, which is essential for movement, posture, joint stabilization, and heat production.

Striated Muscle Tissue

• Skeletal muscle cells are long, multinucleated, and have visible striations due to the arrangement of contractile proteins. Their contractions are voluntary.

• Cardiac muscle cells are branched, usually have one or two nuclei, and are connected by intercalated discs. Their contractions are involuntary.

Smooth Muscle Tissue

• Smooth muscle cells are spindle-shaped, have a single nucleus, and lack striations. Their contractions are involuntary.

• Found in the walls of hollow organs, eyes, skin, and some glands.

B. Properties of Muscle Cells

Muscle cells possess unique properties that enable their function:

• Contractility: Ability to contract and generate force.

• Excitability: Ability to respond to stimuli by generating electrical changes across the plasma membrane.

• Extensibility: Ability to be stretched without being damaged.

• Elasticity: Ability to return to original length after being stretched.

C. Structure of Muscle Cells

• Sarcoplasm: The cytoplasm of a muscle cell.

• Sarcolemma: The plasma membrane of a muscle cell.

• Myofibrils: Cylindrical organelles composed of bundles of specialized proteins for contraction. They are more numerous in cardiac and skeletal muscle than in smooth muscle.

• Sarcoplasmic reticulum (SR): Modified endoplasmic reticulum that surrounds myofibrils and stores calcium ions.

Module 10.2 Structure and Function of Skeletal Muscle Fibers

A. Structure of the Skeletal Muscle Fiber

• Skeletal muscle fibers are long, cylindrical cells containing many myofibrils and multiple nuclei.

• Myofibrils are the most abundant organelle and are responsible for muscle contraction.

• The sarcoplasmic reticulum (SR) surrounds myofibrils and stores/releases calcium ions.

• Transverse (T) tubules: Invaginations of the sarcolemma filled with extracellular fluid, conducting action potentials into the cell.

• Terminal cisternae: Enlarged SR sections flanking each T-tubule; together with a T-tubule, they form a triad.

B. Structure of the Myofibril

• Myofibrils contain hundreds to thousands of myofilaments:

  • Thick filaments: Composed of myosin.

  • Thin filaments: Composed of actin, tropomyosin, and troponin.

  • Elastic filaments: Composed of titin, providing elasticity and stability.

• Thick filaments have myosin heads and tails; heads bind to actin during contraction.

• Thin filaments have binding sites for myosin and regulatory proteins (tropomyosin and troponin).

C. Myofilament Arrangement and the Sarcomere

• Striations are due to the arrangement of thick and thin filaments in repeating units called sarcomeres.

• Regions of the sarcomere:

  • I band: Light band, contains only thin filaments.

  • A band: Dark band, contains thick filaments (with overlapping thin filaments).

  • H zone: Central region of A band with only thick filaments.

  • M line: Middle of the sarcomere, supporting proteins.

Module 10.4 The Process of Skeletal Muscle Contraction and Relaxation

A. The Neuromuscular Junction (NMJ)

The NMJ is the synapse where a motor neuron communicates with a muscle fiber to initiate contraction.

• Neurotransmitters (e.g., acetylcholine, ACh) are released from the neuron and bind to receptors on the muscle cell.

• Components of the NMJ:

  • Axon terminal

  • Synaptic cleft

  • Motor end plate

B. Steps of Skeletal Muscle Contraction

1. An action potential arrives at the axon terminal, causing release of ACh.

2. ACh diffuses across the synaptic cleft and binds to receptors on the motor end plate.

3. Binding opens Na+ channels, causing depolarization of the muscle cell.

4. The action potential travels along the sarcolemma and down T-tubules.

5. Calcium ions are released from the SR into the cytosol.

6. Calcium binds to troponin, causing tropomyosin to move and expose active sites on actin.

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

8. Myosin heads perform a power stroke, pulling actin filaments toward the center of the sarcomere.

9. ATP binds to myosin, causing it to detach from actin and reset for another cycle.

10. Contraction continues as long as calcium and ATP are available.

11. Relaxation occurs when ACh is broken down, calcium is pumped back into the SR, and the muscle returns to its resting length.

C. Muscle Relaxation

• Occurs when ACh release stops and calcium is re-sequestered in the SR.

• The muscle fiber returns to its resting state.

Module 10.5 Energy Sources for Skeletal Muscle

A. Immediate Sources of Energy for Muscle Contraction

• ATP stored in the muscle fiber is the immediate energy source, but is rapidly depleted.

• Creatine phosphate in the cytosol can quickly regenerate ATP for short bursts of activity (about 10 seconds).

B. Glycolytic Energy Sources

• Glycolysis: Anaerobic process in the cytosol that breaks down glucose to pyruvate, producing ATP.

• If oxygen is present, pyruvate enters mitochondria for oxidative metabolism; if not, it is converted to lactic acid.

C. Oxidative Energy Sources

• Oxidative phosphorylation in mitochondria uses oxygen to produce large amounts of ATP for sustained activity.

• Myoglobin stores oxygen in muscle cells for use during contraction.

• When glucose is low, amino acids and fatty acids can be used to generate ATP.

Module 10.6 Muscle Tension at the Fiber Level

A. Twitch Contraction

A muscle twitch is the response of a muscle fiber to a single action potential. It consists of three phases:

• Latent period: Time between stimulus and contraction.

• Contraction period: Muscle fiber shortens and tension increases.

• Relaxation period: Muscle fiber returns to resting length.

The refractory period is the time during which the muscle cannot respond to another stimulus.

B. Classes of Skeletal Muscle Fibers

• Type I fibers (slow-twitch): Small diameter, contract slowly, high endurance, rely on oxidative metabolism, abundant mitochondria and myoglobin (dark red color).

• Type II fibers (fast-twitch): Larger diameter, contract quickly, fatigue rapidly, rely more on glycolytic metabolism, less myoglobin (pale color).

• Type II fibers are further subdivided into:

  • IIa: Fast oxidative-glycolytic

  • IIx: Fast glycolytic

• Most muscles contain a mix of fiber types, with proportions depending on function.

Module 10.7 Muscle Tension at the Organ Level

A. Motor Units

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

• Muscles requiring fine control have small motor units; those requiring strength have large motor units.

• Recruitment: Increasing the number of active motor units to generate more force.

• Muscle tone: Baseline level of involuntary activation, important for posture and readiness for movement.

• Isometric contraction: Muscle length remains unchanged while tension increases.

B. Muscular Fatigue

• Muscular fatigue is the inability to maintain a given level of intensity during activity.

• Causes include:

  • Depletion of ATP and other key metabolites

  • Decreased oxygen availability

  • Accumulation of chemicals (e.g., calcium ions, ADP, Pi)

  • Environmental conditions

Key Table: Comparison of Muscle Tissue Types

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Location	Attached to bones	Heart	Walls of hollow organs
Control	Voluntary	Involuntary	Involuntary
Striations	Present	Present	Absent
Cell Shape	Long, cylindrical	Branched	Spindle-shaped
Nuclei	Multiple, peripheral	One or two, central	Single, central
Special Features	Fast contractions, fatigue easily	Intercalated discs, rhythmic contractions	Slow, sustained contractions

Key Equations

• ATP hydrolysis (energy for contraction): $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Creatine phosphate reaction: $\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

• Glycolysis (simplified): $\text{Glucose} \rightarrow 2~\text{Pyruvate} + 2~\text{ATP}$

Example: During a sprint, skeletal muscle first uses stored ATP, then creatine phosphate, and finally switches to glycolysis and oxidative phosphorylation for sustained energy.