Muscle Tissue

General Functions of Muscle Tissue

Muscle tissue is essential for various physiological processes in the human body. It is responsible for movement, posture, and many vital functions.

• Produce skeletal movement: Muscles contract to move bones and body parts.

• Maintain posture and body position: Continuous muscle contractions stabilize joints and maintain posture.

• Support soft tissues: Muscles protect and support internal organs.

• Guard entrances and exits: Sphincter muscles control openings of digestive and urinary tracts.

• Maintain body temperature: Muscle contractions generate heat as a byproduct.

• Store nutrient reserves: Some muscles store glycogen and other nutrients for energy.

Identification, General Location, and Comparative Characteristics of Skeletal, Cardiac, and Smooth Muscle Tissue

Muscle tissue is classified into three main types, each with distinct structure, location, and function.

• Skeletal muscle:

  • Structure: Long, cylindrical, striated, and multinucleated fibers.

  • Location: Attached to bones via tendons; combined with connective tissues.

  • Function: Moves or stabilizes the position of the skeleton; guards entrances/exits; generates heat; protects internal organs.

• Cardiac muscle:

  • Structure: Short, branched, striated fibers; usually single nucleus; intercalated discs present.

  • Location: Heart wall.

  • Function: Circulates blood; maintains blood pressure.

• Smooth muscle:

  • Structure: Short, spindle-shaped, nonstriated fibers; single, central nucleus.

  • Location: Walls of blood vessels and hollow organs (digestive, respiratory, urinary, reproductive tracts).

  • Function: Moves food, urine, and reproductive tract secretions; controls diameter of passageways; regulates diameter of blood vessels.

Comparative Table: Characteristics of Muscle Tissue

Detailed Gross and Microscopic Anatomy of Skeletal Muscle

Organization of Skeletal Muscles

Skeletal muscles are organized from the cellular level (muscle fiber) to the whole muscle organ.

• Skeletal muscle (organ): Composed of bundles of muscle fibers.

• Muscle fascicle: Bundle of muscle fibers.

• Muscle fiber (cell): Individual muscle cell.

Connective Tissue Layers

Three connective tissue layers surround skeletal muscle components:

• Epimysium: Dense connective tissue surrounding the entire muscle.

• Perimysium: Dense, irregular connective tissue surrounding muscle fascicles.

• Endomysium: Elastic connective tissue surrounding individual muscle fibers.

Components within Skeletal Muscle Fiber

• Sarcolemma: Plasma membrane of a muscle cell.

• Transverse tubules (T-tubules): Narrow tubes continuous with sarcolemma, extending deep into the cell.

• Sarcoplasmic reticulum: Network of tubules around myofibrils; stores calcium ions.

• Myofibril: Organized collections of myofilaments in skeletal and cardiac muscle cells.

• Thick myofilament: Cytoskeletal filament composed of myosin.

• Thin myofilament: Cytoskeletal filament composed of actin, troponin, and tropomyosin.

• Troponin: Globular protein that binds to tropomyosin.

• Tropomyosin: Fibrous protein that covers actin active sites.

Sarcomere Structure

• Sarcomere: Smallest contractile unit of striated muscle cell.

• Z-disc (line): Proteins called actinins; bisect I band and mark boundary between sarcomeres.

• M-line: Center of A band; stabilizes thick filaments.

Neuromuscular Junction

The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber to initiate contraction.

• Consists of an axon terminal, motor end plate, and synaptic cleft.

• Neurotransmitter (acetylcholine) is released to stimulate muscle contraction.

Physiology of Skeletal Muscle Contraction and Relaxation

Sliding Filament Theory

The sliding filament theory explains how muscle fibers contract at the molecular level.

• Thin filaments slide toward the M line, alongside thick filaments.

• The width of the A zone remains constant.

• Z lines move closer together.

Sequence of Events in Muscle Contraction

1. Action potential arrives at neuromuscular junction.

2. Release of acetylcholine stimulates muscle fiber.

3. Calcium ions released from sarcoplasmic reticulum.

4. Cross-bridge formation and contraction cycle.

5. Relaxation occurs when calcium is reabsorbed and acetylcholine is broken down.

Nomenclature of Skeletal Muscles

Muscle names often reflect their location, attachments, shape, or function.

• Examples: biceps brachii (two heads, arm), rectus femoris (straight, thigh).

Location, General Attachments, and Actions of Major Skeletal Muscles

Muscles are grouped by location and function. Attachments are typically at origin (fixed point) and insertion (moving point).

• Origin: Stationary attachment site.

• Insertion: Movable attachment site.

• Action: Movement produced by muscle contraction.

Group Actions of Skeletal Muscles

Muscles work together to produce coordinated movements. They are classified by their roles in movement.

• Prime mover (agonist): Main muscle responsible for movement.

• Antagonist: Muscle that opposes the action of the agonist.

• Synergist: Assists the prime mover.

• Fixator: Stabilizes the origin of the prime mover.

Example Table: Muscle Group Actions

Key Terms and Definitions

• Myofibril: Contractile organelle within muscle fiber.

• Sarcomere: Functional unit of muscle contraction.

• Actin: Protein forming thin filaments.

• Myosin: Protein forming thick filaments.

• Acetylcholine: Neurotransmitter at neuromuscular junction.

Important Equations

• Muscle force equation:

• Sliding filament theory (distance):

Additional info: Equations above are general physics equations relevant to muscle contraction force and movement. For more advanced study, consider biochemical equations for ATP hydrolysis in muscle contraction.