Chapter 11: The Muscular System – Structure, Function, and Integration

Chapter 11: The Muscular System

Structural Variations Among Skeletal Muscles

The structure of skeletal muscles determines their power, range, and speed of movement. Muscles are classified based on the arrangement of their fascicles, which are bundles of muscle fibers.

• Parallel Muscles: Fascicles run parallel to the long axis of the muscle. Example: biceps brachii.

• Convergent Muscles: Fascicles converge from a broad area to a single tendon. Example: pectoralis major.

• Pennate Muscles: Fascicles are arranged at an angle to the tendon. Types include unipennate, bipennate, and multipennate. Example: rectus femoris (bipennate).

• Circular Muscles (Sphincters): Fascicles are arranged in concentric rings. Example: orbicularis oris.

Example: The arrangement of fascicles in the deltoid (multipennate) allows for greater force production but less range of motion.

Levers and Muscle Efficiency

Bones act as levers to increase muscle efficiency. A lever is a rigid structure that moves around a fixed point called the fulcrum. Levers modify the direction, strength, distance, and speed of movement.

• First-Class Lever: Fulcrum is between the applied force and the load. Example: neck muscles lifting the head.

• Second-Class Lever: Load is between the fulcrum and applied force. Example: calf muscles raising the body on tiptoe.

• Third-Class Lever: Applied force is between the fulcrum and load. Most common in the body. Example: biceps brachii flexing the elbow.

Example: The elbow joint acts as a third-class lever during forearm flexion.

Origins, Insertions, and Actions of Muscles

Each muscle is identified by its origin, insertion, and action. The origin is the fixed attachment, while the insertion is the movable attachment. The action is the movement produced when the muscle contracts.

• Origin: Site of attachment of the fixed end of a muscle.

• Insertion: Site where the movable end attaches to another structure.

• Action: Movement produced by muscle contraction.

Example: The biceps brachii originates at the scapula and inserts on the radius, producing forearm flexion.

Functional Classification of Muscles

Muscles are classified by their function during movement:

• Agonist (Prime Mover): Muscle responsible for a specific movement.

• Antagonist: Muscle that opposes the action of the agonist.

• Synergist: Muscle that assists the agonist.

• Fixator: Muscle that stabilizes the origin of the agonist.

Example: During elbow flexion, the biceps brachii is the agonist, the triceps brachii is the antagonist, and the brachialis acts as a synergist.

Muscle Naming Conventions

The names of muscles often provide clues to their characteristics:

• Body Region: Example: brachialis (arm).

• Origin and Insertion: Example: sternocleidomastoid (originates at sternum and clavicle, inserts on mastoid process).

• Fascicle Arrangement: Example: rectus (straight).

• Position: Example: externus (superficial).

• Structural Characteristics: Example: longus (long).

• Action: Example: flexor (flexes a joint).

Axial and Appendicular Musculature

Muscles are grouped based on their location and function:

• Axial Musculature: Arises on the axial skeleton; positions the head, vertebral column, and moves the rib cage.

• Appendicular Musculature: Stabilizes or moves the limbs of the appendicular skeleton.

Example: The trapezius (axial) positions the head and neck, while the deltoid (appendicular) moves the arm.

Innervation of Muscles

Innervation refers to the distribution of nerves that control a region or organ, including muscles. Proper innervation is essential for muscle contraction and coordination.

Axial Muscles: Head, Neck, Vertebral Column, Trunk, and Pelvic Floor

Axial muscles are organized by location and function:

• Facial Expression: Includes orbicularis oris, buccinator, occipitofrontalis, and platysma.

• Extrinsic Eye Muscles: Six muscles control eye movement: inferior/superior rectus, lateral/medial rectus, inferior/superior oblique.

• Mastication: Masseter, temporalis, and pterygoid muscles.

• Tongue Muscles: Palatoglossus, styloglossus, genioglossus, hyoglossus.

• Pharynx Muscles: Pharyngeal constrictors, laryngeal elevators, palatal muscles.

• Neck Muscles: Digastric, sternocleidomastoid, and seven muscles associated with the hyoid bone.

• Spine Muscles: Superficial groups: spinalis, longissimus, iliocostalis; deep muscles: longus capitis, longus colli, quadratus lumborum.

• Oblique Muscles: Scalene, intercostal, transversus muscles; diaphragm is crucial for respiration.

• Pelvic Floor: Muscles form the pelvic diaphragm; perineum divided into urogenital and anal triangles.

Appendicular Muscles: Shoulders, Upper Limbs, Pelvis, and Lower Limbs

Appendicular muscles support and move the limbs:

• Shoulder Girdle: Trapezius, rhomboid, levator scapulae, serratus anterior, subclavius, pectoralis minor.

• Arm Movers: Deltoid, supraspinatus (abductors); subscapularis, teres major (medial rotation); infraspinatus, teres minor (lateral rotation); coracobrachialis (flexion/adduction).

• Shoulder Flexion/Extension: Pectoralis major (flexes), latissimus dorsi (extends).

• Forearm and Hand Movers: Biceps brachii, triceps brachii (elbow); brachialis, brachioradialis (flexion); anconeus (opposes flexion); flexor carpi ulnaris, flexor carpi radialis, palmaris longus (wrist flexion); extensor carpi radialis, extensor carpi ulnaris (wrist extension); pronator teres, pronator quadratus (pronation); supinator (supination).

• Thigh Movers: Gluteus maximus, tensor fasciae latae (abductors); piriformis, obturator (lateral rotators); adductors (varied movements); psoas major, iliacus (form iliopsoas, hip flexor).

• Leg Movers: Biceps femoris, semimembranosus, semitendinosus (hamstrings, knee flexors); sartorius; popliteus (unlocks knee); quadriceps femoris (knee extensors: vastus intermedius, lateralis, medialis, rectus femoris).

• Foot and Toe Movers: Gastrocnemius, soleus (plantar flexion); fibularis brevis, longus (eversion/extension); fibularis tertius (eversion/flexion); smaller muscles control precise toe movements.

Integration of the Muscular System with Other Body Systems

Exercise demonstrates the muscular system's integration with other systems:

• Cardiovascular System: Increased heart rate and blood flow during muscle activity.

• Respiratory System: Enhanced oxygen intake and carbon dioxide removal.

• Integumentary System: Regulates body temperature through sweat and blood flow.

• Nervous System: Coordinates muscle contraction and movement.

• Endocrine System: Hormones regulate muscle growth and metabolism.

Example: During aerobic exercise, muscles require increased oxygen, leading to coordinated responses from the cardiovascular and respiratory systems.

Table: Muscle Naming Conventions

Characteristic	Example
Body Region	Brachialis (arm)
Origin/Insertion	Sternocleidomastoid (sternum, clavicle, mastoid)
Fascicle Arrangement	Rectus (straight)
Position	Externus (superficial)
Structural Characteristic	Longus (long)
Action	Flexor (flexes a joint)

Table: Lever Classes in the Human Body

Lever Class	Arrangement	Example
First-Class	Fulcrum between AF and L	Neck muscles lifting the head
Second-Class	Load between F and AF	Calf muscles raising body on tiptoe
Third-Class	AF between F and L	Biceps brachii flexing the elbow

Formula: Lever Mechanics

The mechanical advantage (MA) of a lever is given by:

$MA = \frac{\text{Length of effort arm}}{\text{Length of resistance arm}}$

Additional info: This formula helps explain how levers in the body can amplify force or speed depending on their arrangement.