Muscular System Overview

Introduction

The muscular system is essential for movement, posture, and various bodily functions. This study guide focuses on the identification and understanding of skeletal muscle cells and the major muscles of the human body, as required for Anatomy & Physiology I lab practicum.

Skeletal Muscle Cell Structure

Key Structures and Functions

• Endomysium: A thin layer of connective tissue that surrounds each individual muscle fiber, providing structural support and insulation.

• Muscle Fiber (Muscle Cell): The basic unit of skeletal muscle; a single, elongated, multinucleated cell responsible for contraction.

• Sarcolemma: The plasma membrane of the muscle fiber, involved in conducting electrical impulses.

• Sarcoplasm: The cytoplasm of the muscle cell, containing organelles and contractile proteins.

• Multiple Peripheral Nuclei: Skeletal muscle fibers contain many nuclei located at the cell's periphery, supporting high protein synthesis.

• Myofibrils: Cylindrical structures within the muscle fiber, composed of contractile proteins (actin and myosin).

• Striations: Alternating light and dark bands seen in skeletal muscle, resulting from the arrangement of myofibrils.

• Z Line (Z Disc): The boundary of a sarcomere; anchors actin filaments.

• Sarcomere: The smallest functional (contractile) unit of muscle, defined as the region between two Z lines.

• Motor Neuron: A nerve cell that transmits signals to muscle fibers, initiating contraction.

• Axon: The long, slender projection of a neuron that conducts electrical impulses to the neuromuscular junction.

• Neuromuscular Junction (NMJ): The synapse between a motor neuron and a muscle fiber, where neurotransmitters stimulate muscle contraction.

Key Facts:

• A muscle fiber is a single cell.

• Skeletal muscle cells are multinucleated, with nuclei at the periphery.

• Sarcolemma is the plasma membrane; sarcoplasm is the cytoplasm.

• Sarcomere is the smallest contractile unit.

• Myofibrils contain contractile proteins.

• Endomysium surrounds each muscle fiber.

Example: The contraction of skeletal muscle begins when a motor neuron releases acetylcholine at the NMJ, triggering an action potential in the sarcolemma and leading to sarcomere shortening.

Types of Muscle Tissue

Anatomical Differences

• Skeletal Muscle: Voluntary, striated, multinucleated, long cylindrical fibers; attached to bones for movement.

• Cardiac Muscle: Involuntary, striated, single nucleus per cell, branched fibers; found only in the heart.

• Smooth Muscle: Involuntary, non-striated, single nucleus per cell, spindle-shaped fibers; found in walls of hollow organs (e.g., intestines, blood vessels).

Example: Skeletal muscle contracts to move limbs, cardiac muscle contracts to pump blood, and smooth muscle contracts to move food through the digestive tract.

Major Muscles by Region

Facial Muscles

• Frontalis: Raises eyebrows, wrinkles forehead.

• Occipitalis: Moves scalp backward.

• Orbicularis oculi: Closes eyelids.

• Orbicularis oris: Closes and protrudes lips.

• Buccinator: Compresses cheek.

• Zygomaticus major/minor: Elevates corners of mouth (smiling).

• Risorius: Draws corner of mouth laterally.

• Mentalis: Protrudes lower lip.

• Levator labii superioris: Elevates upper lip.

• Depressor labii inferioris: Depresses lower lip.

• Depressor anguli oris: Depresses corner of mouth.

• Masseter: Elevates mandible (chewing).

• Temporalis: Elevates and retracts mandible.

Neck Muscles

• Sternocleidomastoid: Flexes and rotates head.

• Splenius capitis: Extends and rotates head.

• Sternohyoid: Depresses hyoid bone.

• Omohyoid: Depresses hyoid bone.

• Levator scapulae: Elevates scapula.

Respiratory Muscles

• Diaphragm: Main muscle of respiration; contracts to increase thoracic volume.

• External intercostals: Elevate ribs during inhalation.

• Internal intercostals: Depress ribs during exhalation.

• Transversus thoracis: Depresses ribs.

Abdominal Muscles

• Rectus abdominis: Flexes vertebral column; forms "six-pack".

• External oblique: Compresses abdomen, flexes and rotates trunk.

• Internal oblique: Compresses abdomen, flexes and rotates trunk.

• Transversus abdominis: Compresses abdomen.

Back Muscles

• Trapezius: Moves scapula, extends neck.

• Latissimus dorsi: Extends, adducts, and medially rotates arm.

• Rhomboid major/minor: Retract scapula.

• Serratus anterior/posterior: Moves scapula, assists in respiration.

• Longissimus thoracis: Extends vertebral column.

• Quadratus lumborum: Flexes vertebral column laterally.

Shoulder Muscles

• Deltoid: Abducts arm.

• Pectoralis major: Flexes, adducts, and medially rotates arm.

• Pectoralis minor: Depresses and protracts scapula.

• Supraspinatus: Abducts arm (rotator cuff).

• Infraspinatus: Laterally rotates arm (rotator cuff).

• Teres major: Extends and medially rotates arm.

• Teres minor: Laterally rotates arm (rotator cuff).

• Subscapularis: Medially rotates arm (rotator cuff).

Rotator Cuff Muscles (SITS): Supraspinatus, Infraspinatus, Teres minor, Subscapularis.

Arm and Forearm Muscles

• Biceps brachii: Flexes elbow, supinates forearm.

• Brachialis: Flexes elbow.

• Coracobrachialis: Flexes and adducts arm.

• Triceps brachii: Extends elbow.

Anterior Forearm:

• Pronator teres: Pronates forearm.

• Flexor carpi radialis: Flexes and abducts wrist.

• Palmaris longus: Flexes wrist.

• Flexor carpi ulnaris: Flexes and adducts wrist.

• Flexor digitorum superficialis: Flexes fingers.

Posterior Forearm:

• Brachioradialis: Flexes elbow.

• Extensor carpi radialis longus/brevis: Extends and abducts wrist.

• Extensor digitorum: Extends fingers.

• Extensor carpi ulnaris: Extends and adducts wrist.

• Extensor digiti minimi: Extends little finger.

• Supinator: Supinates forearm.

• Anconeus: Extends elbow.

• Abductor pollicis longus: Abducts thumb.

• Extensor pollicis brevis: Extends thumb.

Hip and Thigh Muscles

• Iliacus: Flexes thigh.

• Psoas major: Flexes thigh.

• Iliopsoas: Combination of iliacus and psoas major; primary hip flexor.

• Tensor fasciae latae: Abducts and medially rotates thigh.

• Gluteus maximus: Extends and laterally rotates thigh.

• Gluteus medius: Abducts and medially rotates thigh.

• Sartorius: Flexes, abducts, and laterally rotates thigh; helps cross legs.

• Gracilis: Adducts thigh.

• Adductor longus/magnus: Adducts thigh.

Quadriceps: Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius (all extend knee).

Hamstrings: Biceps femoris, Semitendinosus, Semimembranosus (all flex knee and extend hip).

Leg Muscles

• Tibialis anterior: Dorsiflexes and inverts foot.

• Extensor digitorum longus: Extends toes.

• Gastrocnemius: Plantar flexes foot; forms calf.

• Soleus: Plantar flexes foot.

• Fibularis (Peroneus) longus/brevis: Everts and plantar flexes foot.

• Tibialis posterior: Plantar flexes and inverts foot.

• Flexor hallucis longus: Flexes big toe.

• Flexor digitorum longus: Flexes toes.

• Popliteus: Unlocks knee.

High-Yield Facts

• Front of the forearm = Flexor muscle group.

• Front of the leg = Extensor muscle group.

• Sartorius helps cross the legs.

• Rectus abdominis forms the "six-pack".

• Gastrocnemius forms the calf.

• Iliacus + Psoas major = Iliopsoas.

Study Tips

• Spend most study time with laboratory models for muscle identification.

• Practice identifying every muscle without looking at labels.

• Review the skeletal muscle cell model until you can identify every structure from memory.

• Learn muscle names by region (face, neck, shoulder, arm, abdomen, thigh, leg) for better retention.

• Pay attention to superficial versus deep muscles, especially in the shoulder, forearm, and leg.

Summary Table: Types of Muscle Tissue

Summary Table: Rotator Cuff Muscles

Key Formula: Muscle Contraction (Sliding Filament Theory)

The force generated by muscle contraction is proportional to the overlap of actin and myosin filaments within the sarcomere.

Where: F = total force n = number of cross-bridges formed f = force per cross-bridge

Additional info: This formula is a simplified representation; actual muscle force depends on many factors including muscle length, velocity, and neural activation.