Skeletal Muscle Anatomy and Organization

Microscopic Anatomy and Organization of Skeletal Muscle

Overview

• Skeletal muscle is the predominant muscle tissue in the body, attaching to the skeleton or associated connective tissue.
• It enables movement (walking, running, etc.) and other actions (drawing, playing instruments, facial expressions).
• Smooth muscle forms the walls of hollow organs.
• Cardiac muscle forms the walls of the heart.
• Smooth and cardiac muscles facilitate the movement of materials within the body (e.g., digestion, blood flow).

Learning Outcomes

• Define muscle fiber, myofibril, and myofilament and describe their structural relationships.
• Describe thick (myosin) and thin (actin) filaments and their relationship to the sarcomere.
• Discuss the structure and location of T tubules and terminal cisterns.
• Define endomysium, perimysium, and epimysium and relate them to muscle fibers, fascicles, and entire muscles.
• Define tendon and aponeurosis and describe the difference between them.
• Describe the structure of skeletal muscle from gross to microscopic levels.
• Explain the connection between motor neurons and skeletal muscle, and discuss the structure and function of the neuromuscular junction.

Pre-Lab Quiz

1. Skeletal muscle cells are also known as fibers (not tubules) due to their size and cylindrical shape.
2. Each muscle fiber is surrounded by a thin connective tissue called the endomysium.
3. A cordlike structure that connects a muscle to another muscle or bone is a tendon.
4. The junction between an axon and a muscle fiber is called a neuromuscular junction.
5. The contractile unit of muscle is the sarcomere (not sarolemma).

Microscopic Anatomy of Skeletal Muscle

Skeletal Muscle Cells (Muscle Fibers)

• Skeletal muscles consist of long, cylindrical cells called muscle fibers.
• These cells range from 10 to 100 μm in diameter and can be up to 30 cm long.
• Muscle fibers are multinucleate because they are formed from the fusion of hundreds of embryonic cells.
• Multiple oval nuclei are located just beneath the plasma membrane (sarcolemma).
• The nuclei are pushed to the periphery by myofibrils.

Myofibrils

• Myofibrils are long, rod-shaped organelles that nearly fill the sarcoplasm (muscle cell cytoplasm).
• They exhibit alternating light (I) and dark (A) bands, giving the muscle fiber a striped appearance (striations).

Myofilaments

• Myofibrils are composed of smaller threadlike structures called myofilaments.
• Myofilaments consist primarily of actin and myosin, which are contractile proteins.
• These filaments slide past each other during muscle activity, leading to muscle cell contraction.

Sarcomeres

• Sarcomeres are the contractile units of muscle.
• They extend from one Z disc (middle of an I band) to the next Z disc along the myofibril's length.
• Cross sections show that each thick filament is surrounded by six thin filaments, and each thin filament is surrounded by three thick filaments.

T Tubules and Sarcoplasmic Reticulum (SR)

• At the junction of the A and I bands, the sarcolemma indents into the muscle cell, forming a transverse tubule (T tubule).
• T tubules run deep into the muscle fiber between terminal cisterns of the sarcoplasmic reticulum (SR).
• The sarcoplasmic reticulum is a smooth endoplasmic reticulum.
• Regions where SR terminal cisterns border a T tubule on each side are called triads.

Voluntary and Striated Muscle

• Skeletal muscle is also known as voluntary muscle because it can be consciously controlled.
• It is also called striated muscle because of its striped appearance.

Organization of Skeletal Muscle

Connective Tissue Wrappings

• Muscle fibers are bundled together with connective tissue to form skeletal muscles.
• Each muscle fiber is enclosed by the endomysium (areolar connective tissue sheath).
• Several sheathed muscle fibers are wrapped by the perimysium (collagenic membrane), forming a fascicle.
• Numerous fascicles are bound together by the epimysium (dense irregular connective tissue), which sheathes the entire muscle.
  Epimysium: An overcoat of dense irregular connective tissue that surrounds the entire muscle.
  Perimysium: A collagenic membrane that wraps several sheathed muscle fibers, forming a fascicle.

Tendons and Aponeuroses

• The endomysium, perimysium, and epimysium converge to form tendons (cordlike) or aponeuroses (sheetlike), which attach muscles to each other or to bones.
• The muscle's more movable attachment is the insertion, while its fixed attachment is the origin.
• Tendons provide durability and conserve space.
• They can span rough bony projections and allow more tendons than fleshy muscles to pass over a joint.

Changes with Age

• As we age, muscle fiber mass decreases, and connective tissue increases, making muscles more sinewy.

The Neuromuscular Junction

Stimulation by Motor Neurons

• Voluntary skeletal muscle cells are stimulated by motor neurons via nerve impulses.
• The neuromuscular junction is the junction between an axon of a motor neuron and a muscle fiber.

Structure of the Neuromuscular Junction

• Each axon of a motor neuron divides into terminal branches as it approaches the muscle.
• Each branch ends in an axon terminal, forming a neuromuscular junction with a single muscle fiber.
• A single neuron and all the muscle fibers it stimulates form a motor unit.
• The neuron and muscle fiber membranes do not touch but are separated by the synaptic cleft.
• Axon terminals contain mitochondria and vesicles filled with acetylcholine (ACh).

Function of the Neuromuscular Junction

1. An action potential reaches the axon terminal.
2. Voltage-gated Ca^{2+} channels open, and Ca^{2+} enters the axon terminal.
3. Ca^{2+} influx causes ACh release via exocytosis.
4. ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma.
5. Receptor binding causes ion channels to open, depolarizing the sarcolemma.
6. Subsequent contraction of the muscle fiber occurs.