Gross Anatomy of the Muscular System

GROSS ANATOMY OF THE MUSCULAR SYSTEM

STRUCTURE AND ORGANIZATION OF HUMAN MUSCLES

Date: 11/15/2025

AGENDA ITEMS

  • Overview of the Muscular System

  • Structure of Skeletal Muscles

  • Major Muscle Groups in the Human Body

  • Connective Tissue Structures Associated with Muscles

  • Functional Organization and Movement

MUSCULAR SYSTEM – AN INTRODUCTION

  • The muscular system primarily consists of skeletal muscles.

  • Muscle organization:

    • Affects power, range, and speed of movement.

  • Essential functions of the muscular system:

    • Body movement

    • Posture

    • Heat production.

  • Types of muscle tissues:

    • Comprised of three distinct types, each with unique characteristics and roles.

  • Approximately 700 skeletal muscles in the human body.

  • Naming convention of muscles incorporates:

    • Region of the body (e.g., temporalis)

    • Position, direction, or fascicle arrangement

    • Structural characteristics

    • Action of the muscle.

Types of Muscle Tissue

  1. Skeletal Muscle

    • Characteristics:

      • Voluntary and striated.

      • Enables controlled body movements through contraction of fibers.

  2. Cardiac Muscle

    • Features:

      • Involuntary.

      • Found only in the heart.

      • Responsible for rhythmic contractions of the heart.

  3. Smooth Muscle

    • Function:

      • Involuntary.

      • Located in internal organs and blood vessels.

      • Controls slow, sustained movements.

GROSS ANATOMICAL FEATURES OF SKELETAL MUSCLES

  • Muscle Fiber Bundles:

    • Muscle fibers grouped into bundles facilitating force generation for movement.

  • Connective Tissue Layers:

    • Surround muscle fibers and bundles, providing structural support.

  • Hierarchical Muscle Structure:

    • Supports efficient force generation and movement control.

  • Muscle Belly:

    • The thick central part of a skeletal muscle, generating force during contraction.

  • Tendons:

    • Connect muscles to bones, transmitting force generated during muscle contractions, enabling movement.

  • Muscle Fascicles:

    • Bundles of muscle fibers within the muscle belly, facilitating precise control and coordination of contractions.

  • Role of Aponeuroses:

    • Broad, flat tendons that anchor muscles and enhance range of motion and stability.

SKELETAL MUSCULAR SYSTEM - CLASSIFICATION

Fascicle Arrangement

  • Classification of muscles based on fascicle arrangement patterns:

    1. Parallel Muscles:

    • Fascicles run parallel to the long axis of the muscle.

    • Some are flat, while cylindrical muscles have a central body (belly).

    • Example: biceps brachii.

    • Tension developed during contractions depends on total number of myofibrils.

    1. Convergent Muscles:

    • Muscle fibers spread like a fan and converge on an attachment site.

    • Example: pectoralis muscles.

    • Works on tendon, aponeurosis, or raphe (slender band of collagen fibers).

    • Muscle fibers can pull in different directions depending on activity.

    1. Pennate Muscles:

    • Muscle fibers pull at an angle relative to the tendon.

    • Compared to parallel muscles:

      • They do not move their tendons as far.

      • Contain more myofibrils.

      • Develop more tension.

      • Types:

        • Unipennate: All fascicles on one side of the tendon (e.g., extensor digitorum).

        • Bipennate: Fascicles on both sides of a central tendon (e.g., rectus femoris).

        • Multipennate: Tendon branches within the muscle (e.g., deltoid).

    1. Circular Muscles (Sphincters):

    • Act as valves in digestive and urinary tracts.

    • Surround body openings and hollow organs, reducing diameter with contraction.

    • Example: orbicularis oris of the mouth.

MUSCULAR SYSTEM - LEVERS

  • Role of Levers:

    • Almost all skeletal muscles are attached to bones.

    • The connection site affects force, speed, and range of movement.

    • Each bone acts as a lever (rigid moving structure) moving on a fixed point (fulcrum) when muscles apply force to overcome the load.

    • Joints act as fulcrums.

    • Levers can change:

    • Direction of applied force (AF).

    • Distance and speed produced by AF.

    • Effective strength of AF.

  • Classes of Levers (based on relative positions of applied force, fulcrum, and load):

    1. First-class lever:

    • Fulcrum is between applied force and load.

    • Example: extension of the neck/lifting the head (like a pry bar or crowbar).

    1. Second-class lever:

    • Load is between applied force and fulcrum.

    • Example: ankle extension (plantar flexion) by calf muscles (like a wheelbarrow).

    1. Third-class lever:

    • Applied force is between load and fulcrum.

    • Most common lever in the body, minimizes effective force to maximize distance and speed.

FUNCTIONAL GROUP - SKELETAL MUSCULAR SYSTEM

Origins and Insertions

  • Origin:

    • Fixed point of muscle attachment to the bone, usually proximal and provides a stable base during contraction.

  • Insertion:

    • The movable point of attachment that moves during muscle contraction.

Actions

  • Movements produced by muscle contraction include:

    • Adduction

    • Elevation

    • Pronation

  • Described in terms of effect on bones or joints, such as flexion of the forearm or flexion at the elbow.

  • Muscle contractions lead to actions like flexion, extension, and rotation for body movement.

Muscle Interactions

  • Muscles work in groups to maximize efficiency, with smaller muscles reaching maximum tension before larger primary muscles.

  • Key terms describing muscle cooperation:

    • Agonist (Prime mover):

    • Primarily responsible for producing a specific movement.

    • Antagonist:

    • Opposes the action of an agonist, controlling or reversing movements.

    • Synergist:

    • Smaller muscle aiding a larger agonist, ensuring smooth movement.

    • Fixator:

    • A synergist that stabilizes a joint, providing a solid base for effective muscle action.

Muscle Opposition

  • Agonists and antagonists work in pairs:

    • When one contracts, the other stretches (e.g., flexors/extensors and abductors/adductors).

SKELETAL MUSCLES - NAMING

Regions of the Body

  • Abdominal (abdomen)

  • Ancon (elbow)

  • Auricular (ear)

  • Brachial (arm)

  • Capitis (head)

  • Carpi (wrist)

  • Cervicis (neck)

Naming Based on:

  1. Position, Direction, or Fascicle Arrangement:

    • Externus (superficialis): Nearby surface muscles.

    • Internus (profundus): Deeper muscles.

    • Extrinsic Muscles: Stabilize an organ.

    • Intrinsic Muscles: Located entirely within an organ.

    • Transversus Muscles: Run across the long axis of the body.

    • Oblique Muscles: Run at a slant to the long axis.

    • Rectus (straight) Muscles: Run along the long axis (e.g., rectus abdominis).

  2. Structural Characteristics:

    • Indicates origin and insertion, e.g. genioglossus

    • Number of tendons (e.g., biceps brachii)

    • Shape and size (e.g., trapezius, deltoid, rhomboid)

SKELETAL MUSCULAR SYSTEM - DIVISION

Divisions of the Muscular System

  • Axial Muscles:

    • Comprising about 60% of skeletal muscles, responsible for:

    • Positioning head and vertebral column.

    • Moving rib cage.

    • Forming pelvic floor.

    • Grouped by location and function:

    • Muscles of the head and neck

    • Muscles of the vertebral column

    • Oblique and rectus muscles

    • Muscles of the pelvic floor.

  • Appendicular Muscles:

    • Responsible for moving and supporting pectoral and pelvic girdles and limbs:

    • Muscles of the shoulders and upper limbs.

    • Muscles of the pelvis and lower limbs.

AXIAL AND APPENDICULAR MUSCLES

Axial Muscles:

  • Include:

    • Frontal belly of occipitofrontalis

    • Temporoparietalis (reflected)

    • Temporalis

    • Sternocleidomastoid

    • Rectus abdominis

    • External oblique

    • Linea alba

Appendicular Muscles (Upper Limb):

  • Include:

    • Trapezius

    • Deltoid

    • Pectoralis major

    • Latissimus dorsi

    • Serratus anterior

    • Biceps brachii

    • Triceps brachii

    • Brachialis

    • Brachioradialis

    • Extensor carpi radialis longus

    • Extensor carpi radialis brevis

    • Palmaris longus

    • Flexor carpi radialis

    • Flexor carpi ulnaris

    • Flexor digitorum superficialis

Appendicular Muscles (Lower Limb):

  • Include:

    • Gluteus medius

    • Tensor fasciae latae

    • Iliotibial tract

    • Sartorius

    • Rectus femoris

    • Vastus lateralis

    • Vastus medialis

    • Tibialis anterior

    • Gastrocnemius

    • Soleus

    • Extensor digitorum longus

MAJOR MUSCLES OF THE MUSCULAR SYSTEM

Anterior Muscles

  • Deltoid: Abduction of the arm at the shoulder (Example: Bowling a cricket ball).

  • Biceps: Flexion of the arm at the elbow joint (Example: Pulling the paddle when kayaking).

  • Pectorals: Movement around the shoulder joint (Example: A chest pass in netball).

  • Trapezius: Rotation of the scapula and supporting the arm (Example: The butterfly stroke in swimming).

  • Triceps: Extension of the arm at the elbow joint (Example: Throwing a javelin).

  • Latissimus Dorsi: Movement at the shoulder and lateral flexion of the spine (Example: Pulling the oars when rowing).

  • Gluteal Muscles: Movement of the leg around the hip (Example: Pushing your feet off the ground when sprinting).

  • Hamstring: Flexion of the leg at the knee joint (Example: Lifting your leg to kick a football).

  • Abdominals: Flexion of the trunk and assisting with breathing (Example: A pike hold in gymnastics).

  • Quadriceps: Extension of the leg at the knee joint (Example: Pushing the pedals when cycling).

  • Gastrocnemius: Flexion of the foot at the ankle joint and the leg at the knee joint (Example: Taking off when performing a high jump).

Major Muscles from the Front (Anterior)

  • Head & Neck: Frontalis, Temporalis, Orbicularis oculi, Orbicularis oris, Sternocleidomastoid.

  • Chest: Pectoralis major, Pectoralis minor, Serratus anterior.

  • Abdomen: Rectus abdominis, External oblique, Internal oblique.

  • Arm: Deltoid, Biceps brachii, Brachialis, Brachioradialis.

  • Leg: Iliopsoas, Sartorius, Quadriceps group (Rectus femoris, Vastus lateralis, Vastus medialis), Tibialis anterior, Gastrocnemius.

Major Muscles from the Back (Posterior)

  • Head & Neck: Occipitalis, Sternocleidomastoid (partially visible from the back).

  • Back & Shoulder: Trapezius (large diamond-shaped muscle covering the upper back), Deltoid (posterior fibers), Infraspinatus, Teres major, and Teres minor, Latissimus dorsi, Rhomboid major, Rhomboid minor.

  • Arm: Triceps brachii (long and lateral heads), Brachialis (partially visible), Brachioradialis (partially visible).

  • Gluteal Region: Gluteus maximus, Gluteus medius.

  • Thigh: Hamstring group (Biceps femoris, Semitendinosus, Semimembranosus), Adductor magnus (partially visible).

  • Leg: Gastrocnemius, Soleus, Fibularis longus, Tibialis posterior, Extensor digitorum longus.

MUSCLES OF THE HEAD AND NECK

Functions

  • Facial Expression Muscles: Enable a wide array of facial expressions essential for non-verbal communication.

  • Mastication Muscles: Facilitate chewing by controlling jaw movements, crucial for eating.

  • Head Movement Muscles: Support nodding, rotation, and stabilization of the head.

Major Muscles of the Head

  • Frontalis: Raises the eyebrows and wrinkles the forehead.

  • Temporalis: Assists in closing the jaw (mastication).

  • Orbicularis oculi: Encircles the eye, responsible for closing eyelids.

  • Orbicularis oris: Encircles the mouth, responsible for closing and protruding lips.

  • Zygomaticus major and minor: Elevate corners of mouth (smiling).

  • Masseter: Elevates the mandible (jaw closure).

  • Buccinator: Compresses the cheek (e.g., in blowing or sucking).

  • Nasalis: Compresses the bridge and depresses the tip of the nose.

MAJOR MUSCLES OF NECK

Anterior Neck Muscles

  • Sternocleidomastoid:

    • Origin: Sternum and clavicle to mastoid process of temporal bone and superior nuchal line of occipital bone.

    • Function: Rotates and flexes the neck.

  • Platysma:

    • Function: Tenses skin of the neck, helps depress mandible, covering anterior neck.

  • Suprahyoid Muscles:

    • Elevate hyoid bone and floor of mouth.

  • Infrahyoid Muscles:

    • Depress hyoid bone and larynx.

Posterior Neck Muscles

  • Splenius capitis and splenius cervicis:

    • Functions in extending, rotating, and laterally flexing the head and neck.

APPENDICULAR MUSCLES

Muscles of the Shoulder (Pectoral Girdle)

  • Superficial muscles are closest to the surface, responsible for moving and stabilizing the scapula and clavicle.
    Key superficial muscles include:

  1. Trapezius:

    • Location: Upper back and neck.

    • Function: Elevates, retracts, and rotates the scapula; extends the neck.

    • Attachments: Originates on midline of neck and back, inserts on clavicles and scapular spines.

  2. Deltoid:

    • Covers shoulder joint.

    • Function: Major abductor of arm; assists in shoulder flexion, extension, and rotation.

  3. Serratus Anterior:

    • Fan-shaped muscle on lateral chest.

    • Function: Protracts and stabilizes scapula; assists in upward rotation.

  4. Pectoralis Minor:

    • Lies deep to pectoralis major.

    • Function: Depresses and protracts scapula.

  5. Subclavius:

    • Stability function for the clavicle.

  6. Levator Scapulae, Rhomboid Major, and Minor:

    • Elevate and retract the scapula, found deep to trapezius.

AXIAL MUSCLES

Overview

  • Grouped based on location and function, which include:

    • Muscles of the head and neck.

    • Muscles of the vertebral column.

    • Oblique and rectus muscles.

    • Muscles of the pelvic floor.

Key Roles

  • Posture support, movement of the head, and control during breathing.

  • Arm and shoulder movements through upper limb muscle function.

MUSCLE ACTIONS IN COMMON MOVEMENTS

Movements Defined

  • Flexion and Extension:

    • Flexion decreases the angle between bones; extension increases it, allowing bending and straightening.

  • Abduction:

    • Movement away from the body's midline, critical for sideward motion.

  • Rotation:

    • Turning around an axis, enabling complex movements like head and torso rotations.

CONCLUSION

Complexity of the Muscular System

  • The muscular system's structural complexity supports a variety of essential body functions.

  • Knowledge of muscular anatomy enhances understanding of movement mechanics and coordination.

  • Clinical Relevance:

    • Important for applications in medical treatments and rehabilitation strategies.