Chapter 9: Muscle Tissue

Terminology

Muscle-related prefixes are commonly used in anatomy:

• myo-, mys-, sarco-: These prefixes refer to muscle or flesh. For example, myocyte means muscle cell, and sarcoplasm refers to the cytoplasm of a muscle cell.

Muscle Fibers

Muscle fibers are the basic cellular units of muscle tissue.

• Muscle fibers: Elongated, multinucleated cells specialized for contraction.

Types of Muscle

There are three main types of muscle tissue, each with distinct characteristics.

• Skeletal Muscle

  • Location: Attached to bones throughout the body.

  • Appearance: Striated (striped) due to organized sarcomeres.

  • Control: Voluntary, controlled by conscious effort.

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

• Cardiac Muscle

  • Location: Walls of the heart.

  • Appearance: Striated, branched fibers with intercalated discs.

  • Control: Involuntary, regulated by the autonomic nervous system.

  • Special feature: Intercalated discs allow rapid transmission of electrical impulses.

• Smooth Muscle

  • Location: Walls of hollow organs (e.g., intestines, blood vessels).

  • Appearance: Non-striated, spindle-shaped cells.

  • Control: Involuntary, regulated by the autonomic nervous system.

  • Function: Movement of substances through organs (e.g., peristalsis).

Comparison of Muscle Types

• Striated types: Skeletal and cardiac muscle.

• Involuntary types: Cardiac and smooth muscle.

• Voluntary type: Skeletal muscle.

Muscle Characteristics

Muscle tissue possesses four key properties:

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

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

Connective Tissue Sheaths

Muscles are wrapped in layers of connective tissue:

• Epimysium: Surrounds the entire muscle.

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

• Endomysium: Surrounds individual muscle fibers.

Levels of Organization

Muscle structure is organized from outer to inner:

• Muscle → Fascicle → Muscle fiber → Myofibril → Myofilament

Muscle Fiber Structure

• Sarcolemma: Plasma membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber, containing organelles and nutrients.

• Myoglobin: Oxygen-binding protein in muscle cells.

• Glycosomes: Storage granules for glycogen.

Myofilaments

• Thin filaments: Composed primarily of actin.

• Thick filaments: Composed primarily of myosin.

Regulatory Proteins

• Tropomyosin: Covers actin binding sites, preventing contraction.

• Troponin: Binds calcium, causing tropomyosin to move and expose actin sites.

Sarcomere Structure

The sarcomere is the functional unit of muscle contraction.

• A band: Dark region containing thick filaments.

• I band: Light region containing thin filaments only.

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

• M line: Center of H zone; anchors thick filaments.

• Z disc: Boundary of sarcomere; anchors thin filaments.

Sliding Filament Theory

Muscle contraction occurs as myosin heads pull actin filaments toward the center of the sarcomere.

• Requires ATP and calcium ions.

• Results in shortening of the sarcomere and muscle fiber.

Cross Bridge Cycle

The cross bridge cycle describes the steps of muscle contraction:

1. Calcium released from the sarcoplasmic reticulum (SR).

2. Calcium binds to troponin, shifting tropomyosin.

3. Myosin heads bind to actin, forming cross bridges.

4. Myosin head pivots, pulling actin (power stroke).

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

6. ATP is hydrolyzed, re-cocking the myosin head.

7. Cycle repeats as long as calcium and ATP are present.

Neuromuscular Junction

• Neurotransmitter: Acetylcholine (ACh).

• Function: ACh transmits the nerve impulse to the muscle fiber, triggering contraction.

Steps starting from action potential (AP) down the axon:

1. AP travels down motor neuron axon.

2. ACh released into synaptic cleft.

3. ACh binds to receptors on sarcolemma.

4. AP generated in muscle fiber.

5. AP travels along sarcolemma and T-tubules.

6. Calcium released from SR.

Action Potential

• Depolarization: Membrane potential becomes less negative as sodium enters.

• Repolarization: Membrane potential returns to resting as potassium exits.

• Threshold: Minimum stimulus required to trigger AP.

• Refractory period: Time during which the cell cannot respond to another stimulus.

Excitation-Contraction Coupling

Process linking muscle fiber excitation to contraction:

1. AP arrives at muscle fiber.

2. Calcium released from SR.

3. Calcium binds to troponin.

4. Tropomyosin shifts, exposing actin sites.

5. Cross bridge cycle begins.

Calcium Role

• Stored in: Sarcoplasmic reticulum (SR).

• Binds to: Troponin.

• Result: Initiates muscle contraction.

Clinical Applications

• Botox: Blocks ACh release, causing muscle paralysis.

• Black Widow venom: Causes excessive ACh release, leading to muscle spasms.

• Rigor Mortis: Postmortem muscle stiffness due to lack of ATP and calcium accumulation.

Chapter 10: Muscular System

Muscle Actions & Roles

Muscles generate force by contracting; they cannot push, only pull.

• Muscles can only pull, never push.

Roles of Muscles

• Agonist (Prime Mover): Main muscle responsible for a specific movement.

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

• Synergist: Assists the agonist by adding force or reducing unnecessary movement.

• Fixator: Stabilizes the origin of the agonist.

Origin vs Insertion

• Origin: Attachment site that remains stationary during movement.

• Insertion: Attachment site that moves during contraction.

Muscle Naming

Muscles are named based on several criteria:

• Location: Where the muscle is found (e.g., brachialis in the arm).

• Shape: Describes the muscle's form (e.g., deltoid is triangular).

• Size: Relative size (e.g., maximus for largest).

• Fiber direction: Orientation of fibers (e.g., rectus means straight).

• Number of origins: Number of tendons (e.g., biceps has two origins).

• Action: Movement produced (e.g., flexor bends a joint).

Muscle Attachments

• Tendon: Cord-like connective tissue attaching muscle to bone.

• Aponeurosis: Sheet-like connective tissue attaching muscle to bone or other muscles.

Facial Muscles

• Function: Control facial expressions.

• Orbicularis oculi: Closes the eyelids.

• Orbicularis oris: Closes and protrudes the lips.

• Zygomaticus: Raises the corners of the mouth (smiling).

Muscles of Mastication

• Masseter: Elevates the mandible (chewing).

• Temporalis: Elevates and retracts the mandible.

• Buccinator: Compresses the cheek (whistling, chewing).

Breathing Muscles

• Diaphragm: Main muscle for inspiration; contracts to increase thoracic volume.

• External intercostals: Elevate ribs for inspiration.

• Internal intercostals: Depress ribs for forced expiration.

Forced Inspiration Muscles

• Scalenes: Elevate first two ribs.

• Sternocleidomastoid: Elevates sternum.

Abdominal Muscles

• Rectus abdominis: Flexes vertebral column.

• External oblique: Compresses abdomen, rotates trunk.

• Internal oblique: Compresses abdomen, rotates trunk.

• Transversus abdominis: Compresses abdomen.

Upper Limb Muscles

Arm

• Biceps brachii: Flexes elbow, supinates forearm.

• Triceps brachii: Extends elbow.

• Deltoid: Abducts arm.

Forearm

• Flexors: Flex wrist and fingers.

• Extensors: Extend wrist and fingers.

Lower Limb Muscles

Hip & Thigh

• Adductors: Adduct thigh.

• Iliopsoas: Flexes hip.

• Sartorius: Flexes, abducts, and laterally rotates thigh.

• Quadriceps: Extends knee.

• Hamstrings: Flex knee, extend hip.

Lower Leg

• Tibialis anterior: Dorsiflexes foot.

• Gastrocnemius: Plantar flexes foot.

• Soleus: Plantar flexes foot.

Concept Check

• Why do muscles need antagonists? To allow controlled movement and return to original position; prevents injury and allows fine motor control.

• Difference between origin and insertion: Origin is stationary, insertion moves during contraction.

• How does structure relate to function? Muscle fiber arrangement, attachment points, and connective tissue sheaths determine the strength, direction, and type of movement produced.

Example: The biceps brachii (agonist) flexes the elbow, while the triceps brachii (antagonist) extends it. Their coordinated action allows smooth movement.

Additional info: Muscle contraction is governed by the interaction of actin and myosin, regulated by calcium and ATP. The organization of muscle fibers and connective tissue sheaths ensures efficient force transmission and movement.