Microscopic and Functional Anatomy of Skeletal Muscle Fibers

Microscopic Anatomy of Skeletal Muscle Fiber

Structure of Skeletal Muscle Fibers

• Skeletal muscle fibers are multinucleate, containing many nuclei and abundant mitochondria beneath the sarcolemma (the specialized plasma membrane of muscle cells).

• The cytoplasm of the muscle cell is called sarcoplasm.

• Myofibrils are long, cylindrical organelles that fill up the sarcoplasm and are responsible for muscle contraction.

Specialized Structures

• Sarcoplasmic reticulum (SR): A network that surrounds each myofibril, storing and releasing calcium ions necessary for muscle contraction.

• Transverse (T) tubules: Extensions of the sarcolemma that penetrate into the cell, allowing electrical impulses to travel deep into the muscle fiber.

Myofibril Organization

• Myofibrils are composed of even finer fibers called myofilaments.

• There are two types of myofilaments:

  • Thick myofilaments: Made of the protein myosin.

  • Thin myofilaments: Made of the protein actin.

• The arrangement of actin and myosin gives skeletal muscle its striated appearance.

• Striations form repeating units called sarcomeres, which are the functional units of muscle contraction.

Sarcomeres

Structure and Function

• A sarcomere is the segment between two Z discs and is the basic contractile unit of muscle fiber.

• Each sarcomere contains overlapping thick (myosin) and thin (actin) filaments, which slide past each other during contraction.

Stimulation and Contraction of Single Skeletal Muscle Cells

Functional Properties of Skeletal Muscle

• Irritability (Responsiveness): Ability to receive and respond to a stimulus.

• Contractility: Ability to forcibly shorten when an adequate stimulus is received.

• Extensibility: Ability of muscle cells to be stretched.

• Elasticity: Ability to recoil and resume resting length after stretching.

Motor Unit and Neuromuscular Junction

• Skeletal muscles must be stimulated by a motor neuron to contract.

• A motor unit consists of one motor neuron and all the skeletal muscle fibers it stimulates.

• The neuromuscular junction is the site where the axon terminal of a motor neuron meets the sarcolemma of a muscle cell.

• The neurotransmitter released is acetylcholine (ACh), which stimulates the muscle fiber.

• The synaptic cleft is the gap between the nerve ending and the muscle fiber, filled with interstitial fluid.

Events at the Neuromuscular Junction

1. Nerve impulse reaches the axon terminal of the motor neuron.

2. Calcium channels open, and calcium ions enter the axon terminal.

3. Calcium ion entry causes synaptic vesicles to release acetylcholine (ACh).

4. ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma.

5. Attachment of ACh triggers an action potential over the sarcolemma and into the T tubules, causing the sarcoplasmic reticulum to release calcium.

6. Calcium binds to regulatory proteins on actin, exposing attachment points. Myosin heads attach to actin, forming cross-bridges, and the filaments slide past each other, shortening the sarcomere and causing muscle contraction.

7. When nerve impulses stop, ACh is no longer released. Acetylcholinesterase breaks down remaining ACh, and calcium is pumped back into the SR. The muscle fiber relaxes.

Sliding Filament Theory

• Muscle contraction occurs as myosin heads bind to actin, pivot, and pull the thin filaments toward the center of the sarcomere, powered by ATP.

• ATP is required for both the power stroke and the detachment/re-cocking of myosin heads.

Types of Body Movements

Muscle Attachments

• Origin: Attachment to an immovable or less movable bone.

• Insertion: Attachment to a movable bone.

• When a muscle contracts, the insertion moves toward the origin.

• Skeletal muscles can only pull; they never push.

Common Body Movements

• Flexion: Decreases the angle of a joint (e.g., bending the elbow).

• Extension: Increases the angle of a joint (e.g., straightening the knee).

• Hyperextension: Extension beyond the anatomical position.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Circumduction: Circular movement of a limb.

• Rotation: Movement around a longitudinal axis.

• Dorsiflexion/Plantar flexion: Movements at the ankle (upward/downward).

• Inversion/Eversion: Movements of the sole of the foot inward/outward.

• Supination/Pronation: Rotational movements of the forearm.

• Opposition: Movement of the thumb to touch the fingertips.

Interactions of Skeletal Muscles in the Body

• Muscles that produce opposite actions are generally located on opposite sides of a joint.

• Prime mover: Main muscle responsible for a movement.

• Antagonist: Muscle that opposes or reverses a prime mover.

• Synergist: Muscle that aids a prime mover or reduces unwanted movements.

• Fixator: Specialized synergist that stabilizes the origin of a prime mover.

Actions of Major Skeletal Muscles

Muscle	Action
Sternocleidomastoid	Flexes the neck and laterally rotates the head
Trapezius	Raises, retracts (pulls), and rotates the scapula
Pectoralis Major	Adducts and flexes the humerus
Deltoid	Abducts the arm
Biceps brachii	Flexes the elbow; supinates the forearm
Triceps brachii	Extends the elbow
External oblique	Flexes and rotates the vertebral column

Example: The biceps brachii acts as a prime mover for elbow flexion, while the triceps brachii acts as its antagonist during this movement.

Additional info: ATP is essential for both contraction and relaxation phases of muscle activity. Calcium ions play a central role in regulating the interaction between actin and myosin.