Module 10.1 Overview of Muscle Tissue

Types of Muscle Tissue

Muscle tissue is essential for movement, stability, and heat production in the human body. It consists of specialized cells called myocytes or muscle cells. There are three main types of muscle tissue:

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

• Cardiac muscle: Involuntary, striated, found only in the heart.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

Muscle tissue generates muscle tension, a force fundamental to its function.

• Other functions: movement, posture, joint stabilization, heat production, and moving substances through organs.

Striated Muscle Tissue

• Skeletal muscle cells have striations and are voluntary. They are multinucleated and contract consciously.

• Cardiac muscle cells are striated, branched, and involuntary. They are found only in the heart and connected by intercalated discs for synchronized contraction.

Smooth Muscle Tissue

• Smooth muscle cells lack striations and are involuntary.

• They are spindle-shaped, have a single nucleus, and are found in organs such as the digestive tract, blood vessels, and glands.

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.

• Extensibility: Ability to stretch without damage.

• Elasticity: Ability to return to original shape after stretching.

Structure of Muscle Cells

• Sarcoplasm: Cytoplasm of muscle cells.

• Sarcolemma: Plasma membrane of muscle cells.

• Myofibrils: Cylindrical organelles responsible for contraction, composed of bundles of specialized proteins.

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

Module 10.2 Structure and Function of Skeletal Muscle Fibers

Structure of the Skeletal Muscle Fiber

• Skeletal muscle fibers are long, cylindrical cells with multiple nuclei.

• Myofibrils are the most abundant organelle, containing contractile proteins.

• The SR surrounds myofibrils and regulates calcium release.

• Terminal cisternae: Enlarged SR sections flanking T-tubules, forming a triad.

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.

• Thick filaments have myosin heads that bind to actin during contraction.

• Thin filaments regulate contraction via troponin and tropomyosin.

Myofilament Arrangement and the Sarcomere

• Striations are alternating light (I band) and dark (A band) regions.

• Specific regions include:

  • I band

  • A band

  • H zone

  • M line

Module 10.4 The Process of Skeletal Muscle Contraction and Relaxation

The Neuromuscular Junction (NMJ)

• The NMJ is where a motor neuron communicates with muscle fibers.

• Key components:

  • Axon terminal

  • Synaptic cleft

  • Motor end plate

• Neurotransmitters (e.g., acetylcholine) trigger muscle contraction.

Steps of Skeletal Muscle Contraction

1. Action potential causes release of acetylcholine (ACh).

2. ACh binds to muscle cell receptors, opening sodium channels.

3. Action potential travels down T-tubules, triggering calcium release from SR.

4. Calcium binds to troponin, exposing actin binding sites.

5. Myosin heads bind to actin, forming cross-bridges and performing a power stroke.

6. ATP binds to myosin, causing detachment from actin.

7. Cycle repeats as long as ACh and ATP are available.

8. Acetylcholinesterase breaks down ACh, ending contraction.

9. Calcium is pumped back into SR, muscle relaxes.

Muscle Relaxation

• Occurs when ACh release stops and calcium concentration returns to resting levels.

Module 10.5 Energy Sources for Skeletal Muscle

Immediate Sources of Energy

• ATP stored in muscle fibers is the immediate energy source.

• Creatine phosphate rapidly regenerates ATP.

Glycolytic Energy Sources

• Glycolysis breaks down glucose to pyruvate, providing energy for short bursts of activity.

• If oxygen is present, pyruvate enters mitochondria for oxidative catabolism.

• If oxygen is absent, pyruvate is converted to lactic acid.

Oxidative Energy Sources

• Oxidative catabolism uses oxygen to produce ATP for sustained activity.

• Myoglobin stores oxygen in muscle cells.

• When glucose is low, amino acids and fatty acids can be used for ATP production.

Module 10.6 Muscle Tension at the Fiber Level

Twitch Contraction

• A muscle twitch is the smallest unit of contraction.

• Phases of a twitch (as seen on a myogram):

  • Latent period

  • Contraction period

  • Relaxation period

• Refractory period: Time when muscle cannot respond to new stimulus.

Classes of Skeletal Muscle Fibers

• Type I fibers (slow-twitch): Small diameter, high myoglobin, many mitochondria, fatigue-resistant, red color.

• Type II fibers (fast-twitch): Larger diameter, low myoglobin, fewer mitochondria, fatigue quickly, white color.

• Type II subtypes: IIa (fast oxidative-glycolytic), IIb (fast glycolytic).

• Most muscles contain a mix of fiber types.

Module 10.7 Muscle Tension at the Organ Level

Motor Units

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

• Motor units can be slow or fast, depending on fiber type.

• 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 for posture and readiness.

• Isometric contractions: Muscle length remains unchanged, force equals resistance.

Muscular Fatigue

• Fatigue is the inability to maintain intensity of activity.

• Caused by:

  • Depletion of ATP-producing metabolites

  • Decreased oxygen availability

  • Accumulation of chemicals (calcium, ADP, Pi)

  • Environmental conditions

Key Table: Comparison of Muscle Fiber Types

Key Equations

• ATP hydrolysis:

• Creatine phosphate reaction:

• Glycolysis summary:

Examples and Applications

• Postural muscles (e.g., back muscles) have more slow-twitch fibers for endurance.

• Eye muscles have more fast-twitch fibers for rapid movement.

Additional info: The notes have been expanded for clarity and completeness, with definitions, examples, and academic context added for self-contained study.