BIO 210: Major Muscles of the Body

Major Muscles of the Body: Anatomical Overview

Anterior View - Superficial and Deep Structures: * Head and Neck: * Occipitofrontalis (Frontal belly): Located on the forehead. * Epicranial aponeurosis: A connective tissue sheet covering the superior aspect of the skull. * Sternocleidomastoid: Major muscle for head flexion and rotation. * Thorax and Abdomen: * Pectoralis major: Large superficial chest muscle. * Pectoralis minor: Deep chest muscle. * Serratus anterior: Found on the lateral rib cage. * Deltoid: Triangular shoulder muscle. * Rectus abdominis: The vertical midline abdominal muscle. * Abdominal external oblique: The outermost slanted muscle layer of the abdominal wall. * Upper Limb: * Biceps brachii: Anterior upper arm muscle. * Brachialis: Deep to the biceps, primary forearm flexor. * Brachioradialis: Found in the forearm. * Pronator teres: Forearm muscle involved in pronation. * Flexor carpi radialis: Forearm muscle involved in wrist flexion. * Thigh and Lower Limb: * Iliopsoas: Deep muscle group for hip flexion. * Pectineus: Medial thigh muscle. * Adductor longus: Large medial thigh muscle. * Gracilis: Most medial muscle of the thigh. * Sartorius: Long, narrow muscle crossing the thigh diagonally. * Tensor fasciae latae: Located on the lateral aspect of the hip/thigh. * Rectus femoris: Central quadriceps muscle. * Vastus lateralis: Lateral quadriceps muscle. * Vastus medialis: Medial quadriceps muscle. * Fibularis longus: Lateral lower leg muscle. * Tibialis anterior: Primary muscle on the front of the shin. * Soleus and Gastrocnemius: Muscles of the posterior lower leg visible from the anterior view.
Posterior View - Superficial and Deep Structures: * Head and Neck: * Occipitofrontalis (Occipital belly): Posterior portion of the scalp muscle. * Splenius capitis: Muscle of the upper neck. * Shoulder and Back: * Levator scapulae: Elevates the scapula. * Trapezius: Large superficial muscle of the upper back and neck. * Supraspinatus: Deep shoulder muscle (rotator cuff). * Teres minor and Teres major: Muscles of the scapula. * Infraspinatus: Muscle located on the posterior surface of the scapula. * Rhomboids: Deep muscles connecting the scapula to the spine. * Latissimus dorsi: Large muscle of the lower back. * Serratus posterior inferior: Deep muscle of the trunk. * Upper Limb (Posterior): * Triceps brachii: Muscles for forearm extension. * Extensor carpi radialis, Extensor digitorum, Extensor carpi ulnaris: Muscles involved in wrist and finger extension. * Flexor carpi ulnaris: Forearm muscle. * Hip and Thigh (Posterior): * Gluteus maximus: Largest superficial buttock muscle (often dissected to reveal deeper layers). * Gluteus medius: Intermediate gluteal muscle. * Gluteus minimus: Deepest gluteal muscle. * Gemellus muscles: Small muscles of the posterior hip. * Biceps femoris, Semitendinosus, Semimembranosus: The three muscles comprising the hamstrings. * Lower Leg (Posterior): * Peroneus longus: Lateral leg muscle. * Tibialis posterior: Deep muscle of the posterior lower leg. * Gastrocnemius: The bulky calf muscle (often dissected). * Soleus: Deep to the gastrocnemius.

Learning Outcomes for Muscle Anatomy

Outcome 19: Describe the actions and roles of agonists, also known as prime movers.
Outcome 20: Describe the actions and roles of antagonists.
Outcome 21: Describe the actions and roles of fixators.
Outcome 22: Describe the actions and roles of synergists.
Outcome 23: Describe the layers of connective tissue packaging skeletal muscle, including specifically: * Epimysium * Perimysium * Fascicles * Endomysium
Outcome 24: Explain the criteria used for naming skeletal muscles and provide examples for each criterion.
Outcome 25: Define the anatomical origin and insertion of a skeletal muscle.
Outcome 26: List all muscles and their specific actions as presented in the curriculum.

Classification and Characterization of Muscle Types

Skeletal Muscle: * Classification: Voluntary and Striated. * Description: These are the muscles under conscious control; they are the muscles typically targeted during exercise.
Smooth Muscle: * Classification: Involuntary and Smooth (not striated). * Description: Muscles that cannot be consciously controlled. They function in the internal parts of the body, specifically the viscera (internal organs).
Cardiac Muscle: * Classification: Involuntary and Striated. * Description: Exclusive to the heart. Key characteristics include resistance to fatigue. * Myogenic property: Contraction originates from within the muscle cell itself rather than needing external nerve stimulation for each beat.

Functional Roles of Skeletal Muscles

Primary Function: Production of movement.
Heat Production: Maintenance of body temperature through metabolic activity.
Posture: Sustaining an upright or stable position.
Protection and Support: Guarding internal organs and contributing to structural integrity.
Storage and Movement of Materials: Regulation of internal flow through sphincters and pumps.

Tissue Organization and Nervous System Interaction

Skeletal Muscle Tissue: Organized into units called fascicles, which are groups of muscle fibers.
Connective Tissue Components: Various layers provide structure, support, and integration with the skeletal system.
Nervous Tissue: * Nervous tissue attaches directly to skeletal muscle fibers. * Function: To stimulate muscle contraction via electrical impulses.

Hierarchical Connective Tissue Structures

Endomysium: A delicate membrane of connective tissue that covers each individual skeletal muscle fiber.
Perimysium: The tough connective tissue that binds multiple skeletal muscle fibers together into a fascicle.
Fascicles: Functional groups or bundles of muscle fibers wrapped in perimysium.
Epimysium: A coarse sheet of connective tissue that covers the entire muscle.
Tendon: * Formed when the epimysium, perimysium, and endomysium merge at the end of the muscle. * Functionally, it is an extension of the periosteum of the bone. * Action: Connects muscle to bone. * Clinical Note: The Achilles Tendon is a specific example prone to tearing.

Advanced Connective Tissue Components

Aponeurosis: * A broad, flat sheet of connective tissue extending from the muscle's fibrous coverings. * Function: Connects muscle to muscle.
Tendon Sheath: * A fibrous connective tissue enclosure lined with a synovial membrane. * Function: Encloses certain tendons (like those in the wrist and ankle) to allow for smooth, frictionless movement.
Deep Fascia: * Dense fibrous connective tissue. * Extensions of deep fascia form the epimysium, perimysium, and endomysium. * Function: Separates individual muscles from one another.

Muscle Morphology: Size, Shape, and Arrangement

General Relationship: The strength and type of movement produced is directly related to muscle fiber orientation, overall shape, bone attachments, and joint involvement.
Diverse Muscle Shapes: * Latissimus dorsi: Characterized as both BROAD and FLAT. * Sartorius: Characterized as both NARROW and LONG. * Gastrocnemius: Characterized as BULKY. * Orbicularis oris: Characterized as CIRCULAR. * Deltoid: Characterized as TRIANGULAR.

Patterns of Fascicle Arrangement

Parallel Muscles: * Long strap-like muscles where fascicles run parallel to the long axis. * Orientation can be horizontal (side to side) or vertical (up and down). * Example: Biceps brachii.
Convergent Muscles: * Fascicles converge from a broad area to a single, narrow attachment point. * Example: Pectoralis muscles.
Oblique Muscles: * Muscles where fibers are slanted relative to the midline. * Examples: External oblique, Internal oblique.
Pennate Muscles ("Feather-like"): * Unipennate: Fascicles anchor to only one side of the central connective tissue shaft (e.g., Extensor digitorum). * Bipennate: Double feather arrangement; fascicles attach to both sides of the central shaft (e.g., Rectus femoris). * Multipennate: Numerous quill-like fascicles converge on a single common attachment point (e.g., Deltoid).
Circular Muscles (Sphincters): * Fascicles encircle body tubes or openings. * Examples: Orbicularis oris, Orbicularis oculi.

Mechanisms of Muscle Attachment and Movement

Articulation: Most muscles span across a joint by attaching to articulating bones.
Muscle Contraction Mechanics: * One bone remains in a fixed position while the other bone moves. * Origin: The point of attachment that remains stationary during contraction. * Insertion: The point(s) of attachment that move when the muscle contracts. * Movement Direction: Typically, contraction moves the insertion closer to the origin. * Stabilization: The bone of origin is usually stabilized by other muscles to prevent movement. * Complexity: Many muscles possess multiple points of both origin and insertion.

Functional Categories of Muscle Actions

Group Coordination: Muscles almost always act in groups; while some contract, others must relax.
Prime Mover (Agonist): * Definition: The muscle or group directly performing a specific movement. * Example: In Plantarflexion, the Gastrocnemius and Soleus are prime movers.
Antagonists: * Definition: Muscles that oppose the movement of the prime mover. This means they have the opposite action; they do not "fight" the movement. * State: They are relaxed when the prime mover is active. * Example: In Plantarflexion, the Tibialis Anterior is the antagonist.
Synergists: * Definition: Muscles that contract simultaneously with prime movers. * Function: They facilitate or complement the prime mover making the action more efficient. * Example: In Hip Flexion, the four quadriceps muscles act as synergists.
Fixator Muscles: * Definition: Muscles that stabilize joints. * Function: They maintain balance and posture during the activity of prime movers.

The Mechanism of Movement

Skeletal muscle attaches to bones via tendons, spanning a joint.
The skeletal muscle is stimulated by a nerve impulse.
The muscle contracts (shortens).
The contraction pulls on the bone(s), producing movement at the joint.

Principles of Muscle Nomenclature

Classification Criteria: * Location: e.g., Brachialis (named for the arm/brachium). * Function: e.g., Adductor group (moves the leg toward the midline). * Shape: e.g., Deltoid (triangular shape). * Direction of Fibers: * Transversus: Across. * Rectus: Up and down (e.g., Rectus abdominis). * Oblique: Slanting. * Number of Divisions/Heads: Refers to origin points (e.g., Biceps brachii = 2 heads; Triceps brachii = 3 heads). * Points of Attachment: Named for specific bones/markings (e.g., Sternocleidomastoid: origins on sternum and clavicle, insertion on mastoid process). * Size: Comparative size (e.g., Gluteus maximus is the largest compared to Gluteus medius and Gluteus minimus).

Muscles of Facial Expression

Orbicularis oculi: * Action: Encircles and closes the eyes; facilitates blinking.
Zygomaticus major: * Action: Pulls corners of the mouth upward; allows for laughing. * Insertion: Angle of the mouth.
Orbicularis oris: * Action: Encircles the mouth; allows for puckering and kissing.
Buccinator: * Action: Presses lips and cheeks against teeth; facilitates blowing and whistling. * Insertion: Skin at the sides of the mouth.

Muscles of Mastication and Head Movement

Mastication (Chewing): * Temporalis: Elevates the mandible (closes jaw); inserts on the mandible. * Masseter: Elevates the mandible (closes jaw); inserts on the mandible.
Head Movement: * Sternocleidomastoid (Paired Muscles): * Bilateral contraction: Flexes the head toward the thorax (referred to as the "Prayer Muscle"). * Unilateral contraction: Turns the head to the side. * Insertion: Mastoid process of the temporal bone.

Trunk Muscles: Respiration and Abdominal Wall

Muscles of the Thorax (Respiration): * External Intercostals: Elevate the ribs (Inhalation). * Internal Intercostals: Depress the ribs (Exhalation). The difference in function is due to orientation and attachment points. * Diaphragm: Enlarges the thorax to cause inspiration.
Anterior and Lateral Abdominal Wall: * Arranged in three layers for strength and protection. * External Oblique: Outermost layer. * Internal Oblique: Middle layer. * Transversus abdominis: Innermost layer. * Rectus abdominis: Muscle along the midline.
Abdominal Wall Functions: * Compress/tighten abdomen. * Rotate the trunk laterally. * Posture stabilization: Pulls the pelvis upward to flatten the lumbar curve. * Support for defecation, forced expiration, and childbirth. * Rectus abdominis specific action: Flexes the trunk of the body.

Muscles of the Shoulder and Upper Arm

Trapezius: Raises and lowers shoulders (shrugging). Insertions: Clavicle, Scapula.
Serratus Anterior: Pulls shoulder forward and upward (involved in punching movements). Insertion: Anterior surface of the scapula.
Pectoralis Major: Flexes and adducts the upper arm anteriorly (draws across chest). Insertion: Humerus.
Latissimus Dorsi: Extends and adducts the upper arm posteriorly (draws across back). Insertion: Humerus.
Deltoid: Abducts the upper arm; assists with flexion and extension. Insertion: Lateral side of humerus, halfway down.

Muscles Moving the Forearm

Biceps Brachii: Supinates the forearm and hand; flexes the supinated forearm. Insertion: Radius.
Brachialis: Flexes the pronated forearm. Insertion: Ulna.
Brachioradialis: Supinates the forearm and hand; flexes the semipronated or semisupinated forearm. Insertion: Styloid process of the Radius.
Triceps Brachii: Extends the forearm. Insertion: Olecranon Process of the Ulna.

Muscles of the Lower Limb: Thigh and Leg

Muscles Moving the Thigh: * Iliopsoas: Flexes the thigh. Insertion: Femur. * Rectus femoris: Flexes thigh and extends leg. Insertion: Tibia. * Tensor fasciae latae: Abducts the thigh. * Pectineus: Adducts and flexes the thigh. * Adductor Group (Brevis, Longus, Magnus): All adduct the thigh. Insertion: Femur. * Gracilis: Adducts thigh; flexes leg. Insertion: Tibia. * Gluteal Group: * Maximus: Extends thigh. * Medius: Abducts thigh. * Minimus: Abducts thigh.
Muscles Moving the Lower Leg: * Quadriceps femoris group: 1. Rectus femoris: Extends lower leg; flexes thigh. 2. Vastus lateralis: Extends lower leg. 3. Vastus medialis: Extends lower leg. 4. Vastus intermedius: Deep to rectus femoris; extends lower leg. * Note: All Vastus muscles extend the lower leg and all quadriceps insert on the Tibia. * Sartorius: Flexes lower leg; permits sitting "tailor fashion" (indian style). Insertion: Tibia. * Hamstring Group: 1. Biceps femoris: Flexes lower leg; extends thigh. Insertion: Fibula. 2. Semitendinosus: Flexes lower leg; extends thigh. Insertion: Tibia. 3. Semimembranosus: Flexes lower leg; extends thigh. Insertion: Tibia.

Muscles of the Foot and Ankle

Tibialis Anterior: Facilitates dorsiflexion and inverts the foot (inversion).
Gastrocnemius: Performs plantar flexion.
Soleus: Performs plantar flexion.