Study Guide: Introduction to Muscles and Skeletal Muscle Anatomy

Gross Structure of Skeletal Muscle

Connective Tissue Layers

Skeletal muscles are composed of muscle fibers and several layers of connective tissue that provide structure, protection, and attachment points. These layers are essential for muscle function and organization.

• Endomysium: Connective tissue that wraps around individual muscle fibers (cells), providing support and insulation.

• Perimysium: Surrounds groups of muscle fibers, forming bundles called fascicles.

• Epimysium: Encloses the entire muscle, providing an outer protective layer.

• Fascia: The most superficial connective tissue sheath, separating individual muscles from one another. The epimysium is continuous with the fascia.

• Tendon: Dense connective tissue that attaches muscle to bone, transmitting force for movement.

• Aponeurosis: A flat, sheet-like tendon that connects muscles to bones or other muscles.

Arrangement of Fascicles: Fascicles are groups of muscle fibers that often run parallel to each other, contributing to the muscle's overall shape and function.

Fascicle (in nerves): A bundle of axons surrounded by the perineurium (note: in muscle, fascicle refers to a bundle of muscle fibers).

Microscopic Structure of Skeletal Muscle Cells

Muscle Fiber Anatomy

A muscle fiber is a single muscle cell, specialized for contraction. Its structure includes several unique features:

• Sarcolemma: The plasma membrane of the muscle cell, analogous to the skin of a sausage.

• Transverse Tubules (T-tubules): In-foldings of the sarcolemma that penetrate deep into the cell, allowing rapid transmission of electrical signals for synchronized contraction.

• Sarcoplasmic Reticulum: A specialized, net-like organelle that stores and releases calcium ions, essential for muscle contraction.

• Myofibrils: Cylindrical structures within the muscle fiber responsible for contraction.

Filaments and Bands

• Thin Filament (Actin): Protein strands that serve as scaffolds for muscle contraction. Actin filaments are pulled by myosin during contraction.

• Thick Filament (Myosin): Composed of myosin proteins, which bind to actin and perform the power stroke to generate force.

• A-band: Dark region where actin and myosin overlap, visible under a microscope.

• I-band: Light region containing only actin filaments.

• Z-disc (Z-line): Dark, zig-zag protein plates that anchor actin filaments and mark the boundaries of a sarcomere.

• Sarcomere: The fundamental contractile unit of muscle, spanning from one Z-disc to the next. Muscle contraction occurs via the sliding filament mechanism.

Sliding Filament Mechanism: Muscle contraction is achieved when myosin heads bind to actin, pull the filaments toward the center of the sarcomere, and shorten the muscle fiber.

General Considerations Regarding Skeletal Muscles

Muscle Attachments and Actions

Skeletal muscles produce movement by pulling on bones. The terminology for muscle attachments and actions is as follows:

• Origin: The attachment site on a more stable bone.

• Insertion: The attachment site on a more movable bone.

• Action: The movement produced when the insertion is pulled toward the origin. Muscles can only pull, not push.

• Antagonists: Muscles that perform opposite actions (e.g., flexion vs. extension).

• Synergists: Muscles that perform the same action.

• Prime Mover: The main muscle responsible for a specific movement among a group of synergists.

Major Skeletal Muscles and Their Actions

Muscles of Facial Expression

• Frontalis: Located on the forehead; elevates the eyebrows.

• Occipitalis: Located at the back of the head; wrinkles the forehead. Combines with frontalis to form the epicranius.

• Orbicularis oculi: Encircles the eye; closes the eyelids (winking, blinking).

• Orbicularis oris: Encircles the mouth; puckers the lips.

• Zygomaticus: Pulls the corners of the mouth upward (smiling).

Muscles of Mastication

• Masseter: Powerful muscle for chewing.

• Temporalis: Synergist in mastication; originates from the temporal bone and attaches to the mandible.

Muscles That Move the Head

• Sternocleidomastoid: Flexes and rotates the head; named for its attachments (sternum, clavicle, mastoid process).

• Trapezius: Moves and stabilizes the scapula; assists in head movement.

Muscles That Move the Trunk

• External Intercostals: Elevate the ribs during inspiration.

• Internal Intercostals: Depress the ribs during expiration.

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

• Abdominal Wall Muscles: Include rectus abdominis, external oblique, internal oblique, and transverse abdominis; provide support and movement for the trunk.

Postural Muscles of the Back

• Erector Spinae: Group of muscles running alongside the vertebral column; maintain upright posture.

Muscles That Move the Shoulder

• Deltoid: Covers the shoulder; responsible for abduction of the arm.

• Pectoralis Major: Adducts and flexes the arm.

• Latissimus Dorsi: Extends, adducts, and medially rotates the arm.

• Rotator Cuff: Group of four muscles (supraspinatus, infraspinatus, teres minor, subscapularis) that stabilize the shoulder joint.

Muscles That Move the Forearm at the Elbow

• Biceps Brachii: Flexes the forearm.

• Triceps Brachii: Extends the forearm.

Muscles That Move the Hip

• Iliopsoas: Flexes the hip.

• Iliotibial Tract: Band of connective tissue aiding in hip stability.

• Gluteus Maximus: Extends and laterally rotates the hip.

• Adductor Group: Includes adductor magnus, adductor longus, adductor brevis; adduct the thigh.

Muscles That Move the Lower Leg at the Knee

• Sartorius: Flexes and laterally rotates the thigh.

• Hamstrings: Includes biceps femoris, semitendinosus, semimembranosus; flex the knee.

• Quadriceps: Includes rectus femoris, vastus lateralis, vastus medialis, vastus intermedius; extend the knee.

• Patellar Tendon: Connects quadriceps to the tibia.

Muscles That Move the Foot at the Ankle

• Tibialis Anterior: Dorsiflexes the foot.

• Gastrocnemius: Plantar flexes the foot.

• Calcaneal (Achilles) Tendon: Attaches gastrocnemius to the heel bone.

Surface Landmarks and Clinical Considerations

Key Anatomical Landmarks

• Triangles of the Neck: Formed by specific muscles; the anterior and posterior triangles are important for clinical assessment.

• Inguinal Ligament: Runs from the anterior superior iliac spine to the pubic tubercle.

• Inferior Angle of the Scapula: Located at the level of the T7 vertebra.

• Iliac Crests: A horizontal line through the most superior points intersects the L4 vertebra.

• Axilla (Armpit): The base is the deep valley; the anterior axillary fold is formed by the pectoralis major, and the posterior axillary fold by the latissimus dorsi.

• Muscles Used for Intramuscular Injections: Deltoid, gluteus medius, vastus lateralis.

• Cubital Fossa: The triangular area on the anterior aspect of the elbow.

• Popliteal Fossa: The diamond-shaped area behind the knee; bordered by the biceps femoris (laterally) and semimembranosus, semitendinosus (medially).

• Femoral Triangle: Bordered by the inguinal ligament (superior), sartorius (lateral), and adductor longus (medial).

Key Terms and Definitions

• Endomysium: Connective tissue covering an individual muscle fiber.

• Sarcomere: Structural and functional unit of skeletal muscle.

• Perimysium: Connective tissue covering around a fascicle.

• Myofibril: The part of the muscle fiber that contracts.

• Epimysium: Connective tissue covering on the outside of a whole muscle.

Muscle Actions

• Flexion: Decreasing the joint angle.

• Extension: Increasing the joint angle.

Summary Table: Major Muscle Groups and Actions

Muscle Group	Example Muscles	Primary Action
Facial Expression	Frontalis, Occipitalis, Orbicularis oculi, Orbicularis oris, Zygomaticus	Movement of facial features (e.g., smiling, blinking)
Mastication	Masseter, Temporalis	Chewing
Head Movement	Sternocleidomastoid, Trapezius	Flexion, rotation, stabilization
Trunk Movement	External/Internal Intercostals, Diaphragm, Abdominal muscles	Breathing, trunk flexion/rotation
Back/Posture	Erector spinae	Upright posture
Shoulder Movement	Deltoid, Pectoralis major, Latissimus dorsi, Rotator cuff	Abduction, adduction, rotation
Forearm Movement	Biceps brachii, Triceps brachii	Flexion, extension
Hip Movement	Iliopsoas, Gluteus maximus, Adductor group	Flexion, extension, adduction
Knee Movement	Sartorius, Hamstrings, Quadriceps	Flexion, extension
Ankle/Foot Movement	Tibialis anterior, Gastrocnemius	Dorsiflexion, plantar flexion

Muscle Contraction: Sliding Filament Theory

Mechanism of Contraction

Muscle contraction is based on the interaction between actin and myosin filaments within the sarcomere. The process is described by the sliding filament theory:

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

• ATP is hydrolyzed, providing energy for the myosin head to perform a power stroke, pulling actin toward the center of the sarcomere.

• This shortens the sarcomere and generates force.

Key Equation:

$\text{Force} = \text{Number of Cross-Bridges} \times \text{Force per Cross-Bridge}$

Additional info: The sliding filament mechanism is fundamental to all striated muscle contraction, including skeletal and cardiac muscle.

Summary

This guide covers the gross and microscopic anatomy of skeletal muscle, major muscle groups and their actions, key anatomical landmarks, and the mechanism of muscle contraction. Understanding these concepts is essential for studying the muscular system in anatomy and physiology.