Muscle Tissue

General Functions and Properties of Muscle Tissue

Muscle tissue is specialized for contraction, enabling movement, posture, and heat production. There are three main types of muscle tissue: skeletal, cardiac, and smooth.

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

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

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs, responsible for regulating internal movements.

General Properties: Excitability, contractility, extensibility, elasticity.

Structure: Muscle fibers (cells), connective tissue coverings, blood vessels, and nerves.

Function: Movement, posture maintenance, joint stabilization, heat generation.

Connective Tissue Structures Associated with Muscles

Muscles are organized and supported by connective tissue layers:

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Endomysium: Surrounds individual muscle fibers.

These layers provide structural support, route for nerves and blood vessels, and help transmit force generated by muscle contraction.

Histology of Skeletal Muscle

Skeletal muscle is composed of long, cylindrical, multinucleated fibers with striations due to the arrangement of contractile proteins.

• Muscle Fiber: The basic cell, containing myofibrils.

• Cross Striation: Alternating light and dark bands due to sarcomere structure.

• Sarcomere: The functional contractile unit, defined by Z-lines.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum storing calcium ions.

• Transverse Tubules (T-tubules): Invaginations of the sarcolemma that transmit action potentials.

• Myofibril: Composed of repeating sarcomeres.

• Thick Filament (Myosin): Motor protein responsible for contraction.

• Thin Filament (Actin): Protein that interacts with myosin for contraction.

Neuromuscular Junction: The synapse between a motor neuron and a muscle fiber, where acetylcholine is released to initiate contraction.

Sliding Filament Theory and Muscle Contraction

The sliding filament theory explains how muscles contract:

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

• ATP hydrolysis powers the movement of myosin heads, pulling actin filaments toward the center of the sarcomere.

• Muscle shortens as sarcomeres contract.

• Relaxation occurs when calcium is re-sequestered and cross-bridges detach.

Role of Acetylcholine and Acetylcholinesterase: Acetylcholine initiates contraction; acetylcholinesterase breaks it down to end the signal.

Immediate Source of Energy for Muscle Contraction

ATP is the immediate energy source for muscle contraction. Pathways for regenerating ATP include:

• Phosphagen System: Creatine phosphate donates phosphate to ADP.

• Anaerobic Glycolysis: Glucose is broken down without oxygen.

• Aerobic Respiration: Glucose and fatty acids are metabolized with oxygen.

Stimulus Types: All-or-none principle, threshold stimulus, subthreshold stimulus.

Muscle Fiber Contraction Terms

Muscle contraction involves several phases and terms:

• Myogram: Graph of muscle contraction.

• Latent Period: Time between stimulus and contraction.

• Contraction Period: Muscle shortens.

• Relaxation Period: Muscle returns to resting length.

• Refractory Period: Time when muscle cannot respond to another stimulus.

• Graded Muscle Contraction: Variation in strength due to frequency and number of fibers stimulated.

• Summation: Increased contraction due to rapid stimuli.

• Tetanus: Sustained contraction from high-frequency stimulation.

• Recruitment: Activation of multiple motor units.

• Incomplete Tetanus: Partial relaxation between stimuli.

• Complete Tetanus: No relaxation between stimuli.

• Fused Tetanus: Maximum sustained contraction.

• Temporal (Wave) Summation: Increased force due to rapid stimuli.

Muscle Tone

Muscle tone is the continuous and passive partial contraction of muscles, important for posture and readiness to respond to stimuli.

Myoglobin and Muscle Fiber Types

Myoglobin: Oxygen-binding protein in muscle, especially abundant in oxidative fibers (red fibers).

• Slow Twitch (Type I) Fibers: High myoglobin, fatigue-resistant, aerobic metabolism.

• Fast Twitch (Type II) Fibers: Low myoglobin, rapid contraction, anaerobic metabolism.

Characteristics: Slow twitch fibers are suited for endurance; fast twitch for rapid, powerful movements.

Histology and Physiology of Smooth Muscle

Smooth muscle is non-striated, involuntary, and found in walls of hollow organs.

• Similarities to Skeletal Muscle: Both contract via actin-myosin interaction.

• Differences: Smooth muscle lacks striations, contracts more slowly, and can maintain contraction longer.

• Contraction Physiology: Calcium binds to calmodulin, activating myosin light chain kinase.

• Intermediate Filaments and Dense Bodies: Provide structural support and anchor contractile proteins.

• Intrinsic Rhythmicity: Some smooth muscle can contract spontaneously.

• Single Unit vs. Multiunit Smooth Muscle: Single unit contracts as a group; multiunit contracts independently.

Histology and Physiology of Cardiac Muscle

Cardiac muscle is striated, involuntary, and found only in the heart.

• Myocardium: Heart muscle tissue.

• Diad: Two components at the Z-line: T-tubule and sarcoplasmic reticulum.

• Functional Syncytium: Cells act as a unit due to gap junctions.

• Intercalated Disc: Specialized junctions for cell-to-cell communication and mechanical strength.

• Functional Significance: Allows rapid, coordinated contraction.

• Similarities to Skeletal Muscle: Both are striated and use actin-myosin contraction.

• Differences: Cardiac muscle has intercalated discs, is involuntary, and has intrinsic rhythmicity.

• Similarities to Smooth Muscle: Both are involuntary and can contract without neural input.

• Differences: Cardiac muscle is striated; smooth muscle is not.

Summary Table: Muscle Tissue Types