Muscular System


The Muscular System

Introduction to the Muscular System

  • The muscular system consists only of skeletal muscles.

  • Muscle organization affects power, range, and speed of muscle movement.

  • Fascicles are bundles of muscle cells (fibers).

Fascicle Arrangement

Classification of Skeletal Muscles

  • Skeletal muscles are classified by:

    • How fascicles are organized.

    • Relationships of fascicles to tendons.

Organization of Skeletal Muscle Fibers

  • Four patterns of fascicle organization:

    • Parallel

    • Convergent

    • Pennate

    • Circular

Parallel Muscles

  • Fibers run parallel to the long axis of the muscle (e.g., biceps brachii).

  • The tension developed depends on the total number of myofibrils, which directly relates to the cross-section of the muscle.

  • 1 square inch (1 in21 \, \text{in}^2 or 6.45 cm26.45 \, \text{cm}^2) of cross-section develops 50 lb (23 kg) of tension.

Convergent Muscles

  • A broad area converges on an attachment site (tendon, aponeurosis, or raphe).

  • Muscle fibers pull in different directions, depending on the stimulation (e.g., pectoralis muscles).

Pennate Muscles

  • Fascicles form an angle with the tendon.

  • Do not move as far as parallel muscles.

  • Contain more myofibrils than parallel muscles.

  • Develop more tension than parallel muscles.

  • Types of pennate muscles:

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

    • Bipennate: Fibers on both sides of the tendon (e.g., rectus femoris).

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

Circular Muscles

  • Also called sphincters.

  • Open and close to guard entrances of the body (e.g., orbicularis oris muscle of the mouth).

Levers

Skeletal Motion

  • Skeletal muscles attach to the skeleton to produce motion.

  • The type of muscle attachment affects power, range, and speed of muscle movement.

Levers

  • Mechanically, each bone is a lever (a rigid, moving structure) and each joint is a fulcrum (a fixed point).

  • Muscles provide the applied force (AF).

  • The applied force is required to overcome the load (L).

Function of a Lever

  • To change:

    • Direction of an AF.

    • Distance and speed of movement produced by an AF.

    • Effective strength of an AF.

The Three Classes of Levers

  • Depend on the relationship between applied force, fulcrum, and resistance.

    • First-class lever

    • Second-class lever

    • Third-class lever

First-Class Lever

  • The center fulcrum is between the applied force and the load.

  • Force and load are balanced.

Second-Class Lever

  • The center resistance is between the applied force and the fulcrum (e.g., wheelbarrow).

  • A small force moves a large weight.

Third-Class Lever

  • The center applied force is between the load and the fulcrum.

  • Greater force moves a smaller load.

  • Maximizes speed and distance traveled.

  • Most common levers in the body (e.g., biceps brachii muscle).

Muscle Attachments to Other Tissues

Origins and Insertions

  • Muscles have one fixed point of attachment (origin) and one moving point of attachment (insertion).

  • Most muscles originate or insert on the skeleton.

  • The origin is usually proximal to the insertion.

Actions

  • Movements produced by muscle contraction.

  • Body movements (e.g., flexion, extension, adduction, etc.).

  • Described in terms of bone, joint, or region.

Muscle Interactions

  • Muscles work in groups to maximize efficiency.

  • Smaller muscles reach maximum tension first, followed by larger, primary muscles.

Muscle Terminology Based on Function

  • Agonist (or prime mover): Produces a particular movement.

  • Antagonist: Opposes movement of a particular agonist.

  • Synergist: A smaller muscle that assists a larger agonist; helps start motion or stabilize the origin of the agonist (fixator).

Muscle Opposition

  • Agonists and antagonists work in pairs.

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

Naming Skeletal Muscles

  • Correct names of muscles include the term "muscle."

  • Exceptions: Platysma, Diaphragm.

Descriptive Names for Skeletal Muscles

  • Location in the body

  • Origin and insertion

  • Fascicle organization

  • Relative position

  • Structural characteristics

  • Action

Location in the Body

  • Identifies body regions (e.g., temporalis muscle).

Origin and Insertion

  • The first part of the name indicates the origin, and the second part indicates the insertion (e.g., genioglossus muscle).

Fascicle Organization

  • Describes fascicle orientation within the muscle (e.g., rectus (straight), transversus, oblique).

Position

  • Externus (superficialis): Visible at the body surface.

  • Internus (profundus): Deep muscles.

  • Extrinsic: Muscles outside an organ.

  • Intrinsic: Muscles inside an organ.

Structural Characteristics

  • Number of tendons: bi = 2, tri = 3

  • Shape: Trapezius, deltoid, rhomboid

  • Size: Many terms refer to muscle size

Action

  • Movements (e.g., flexor, extensor, retractor)

  • Occupations or habits (e.g., risor = laughter)

Terms Indicating Specific Regions of the Body

  • Abdominis (abdomen)

  • Anconeus (elbow)

  • Auricularis (auricle of ear)

  • Brachialis (brachium)

  • Capitis (head)

  • Carpi (wrist)

  • Cervicis (neck)

  • Cleido-/-clavius

  • Coccygeus (coccyx)

  • Costalis (ribs)

  • Cutaneous (skin)

  • Femoris (femur)

  • Genio- (chin)

  • Glosso-/-glossal (tongue)

  • Hallucis (great toe)

  • Ilio- (ilium)

  • Inguinal (groin)

  • Lumborum (lumbar region)

  • Nasalis (nose)

  • Nuchal (back of neck)

  • Oculo- (eye)

  • Oris (mouth)

  • Palpebrae (eyelid)

  • Pollicis (thumb)

  • Popliteus (posterior to knee)

  • Psoas (loin)

  • Radialis (radius)

  • Scapularis (scapula)

  • Temporalis (temples)

  • Thoracis (thoracic region)

  • Tibialis (tibia)

  • Ulnaris (ulna)

  • Uro- (urinary)

Terms Indicating Position, Direction, or Fascicle Organization

  • Anterior (front)

  • Externus (superficial)

  • Extrinsic (outside)

  • Inferioris (inferior)

  • Internus (deep, internal)

  • Intrinsic (inside)

  • Lateralis (lateral)

  • Medialis/medius (medial, middle)

  • Oblique

  • Posterior

  • Profundus (deep)

  • Rectus (straight, parallel)

  • Superficialis

  • Superioris (superior)

  • Transversus (transverse)

Terms Indicating Structural Characteristics of the Muscle

  • Nature of Origin

    • Biceps (two heads)

    • Triceps (three heads)

    • Quadriceps (four heads)

  • Shape

    • Deltoid (triangle)

    • Orbicularis (circle)

    • Pectinate (comblike)

    • Piriformis (pear-shaped)

    • Platy- (flat)

    • Pyramidal (pyramid)

    • Rhomboid

    • Serratus (serrated)

    • Splenius (bandage)

    • Teres (long and round)

    • Trapezius (trapezoid)

  • Other Striking Features

    • Alba (white)

    • Brevis (short)

    • Gracilis (slender)

    • Lata (wide)

    • Latissimus (widest)

    • Longissimus (longest)

    • Longus (long)

    • Magnus (large)

    • Major (larger)

    • Maximus (largest)

    • Minimus (smallest)

    • Minor (smaller)

    • Vastus (great)

Terms Indicating Actions

  • General

    • Abductor

    • Adductor

    • Depressor

    • Extensor

    • Flexor

    • Levator

    • Pronator

    • Rotator

    • Supinator

    • Tensor

  • Specific

    • Buccinator (trumpeter)

    • Risorius (a laughter)

    • Sartorius (like a tailor)

Divisions of the Muscular System

  • Axial muscles:

    • Position head and spinal column.

    • Move rib cage.

    • 60% of skeletal muscles.

  • Appendicular muscles:

    • Support pectoral and pelvic girdles.

    • Support limbs.

    • 40% of skeletal muscles.

Axial Musculature

Divisions Based on Location and Function

  • Muscles of the head and neck

  • Muscles of the vertebral column

  • Oblique and rectus muscles

  • Muscles of the pelvic floor

Muscles of Facial Expression

  • Originate on the skull.

Extrinsic Eye Muscles

  • Originate on the surface of the orbit.

  • Control the position of the eye.

Muscles of Mastication

  • Move the mandible.

Muscles of the Tongue

  • Names end in "-glossus."

Muscles of the Pharynx

  • Begin the swallowing process.

Anterior Muscles of the Neck

  • Control the position of the larynx.

  • Depress the mandible.

  • Support the tongue and pharynx.

Muscles of Facial Expression Details

  • Orbicularis oris constricts the mouth opening.

  • Buccinator moves food around the cheeks.

  • Muscles of the epicranium (scalp)

    • Temporoparietalis

    • Occipitofrontalis

      • Frontal and occipital bellies

      • Separated by epicranial aponeurosis

  • Platysma: Covers the anterior surface of the neck.

Six Extrinsic Eye Muscles (Oculomotor Muscles)

  • Inferior rectus

  • Medial rectus

  • Superior rectus

  • Lateral rectus

  • Inferior oblique

  • Superior oblique

Muscles of Mastication Details

  • Masseter: The strongest jaw muscle.

  • Temporalis: Helps lift the mandible.

  • Pterygoid muscles: Position the mandible for chewing.

Muscles of the Tongue Details

  • All named for origin and insertion.

    • Palatoglossus

    • Styloglossus

    • Genioglossus

    • Hyoglossus

Muscles of the Pharynx Details

  • Pharyngeal constrictor muscles: Move food into the esophagus.

  • Laryngeal elevator muscles: Elevate the larynx.

  • Palatal muscles: Lift the soft palate.

Anterior Muscles of the Neck Details

  • Digastric: From chin to hyoid, and hyoid to mastoid.

  • Mylohyoid: Floor of the mouth.

  • Geniohyoid: Between the hyoid and chin.

  • Stylohyoid: Between the hyoid and styloid.

  • Sternocleidomastoid: From clavicle and sternum to mastoid.

  • Omohyoid: Attaches scapula, clavicle, first rib, and hyoid.

Muscles of the Vertebral Column

  • Spinal extensors or erector spinae muscles (superficial and deep).

  • Spinal flexors (transversospinalis).

Superficial Spinal Extensors

  • Spinalis group

  • Longissimus group

  • Iliocostalis group

Deep Spinal Extensors

  • Semispinalis group

  • Multifidus muscle

  • Interspinalis muscles

  • Intertransversarii muscles

  • Rotatores muscles

Spinal Flexors

  • Neck

    • Longus capitis and longus colli; rotate and flex the neck.

  • Lumbar

    • Quadratus lumborum muscles; flex the spine and depress ribs.

Oblique and Rectus Muscles

  • Lie within the body wall.

  • Oblique muscles

    • Compress underlying structures.

    • Rotate the vertebral column.

  • Rectus muscles

    • Flex the vertebral column.

    • Oppose erector spinae.

Oblique Muscles

  • Cervical region

    • Scalene muscles; flex the neck.

  • Thoracic region

    • Intercostal muscles (external and internal); respiratory movements of ribs.

    • Transversus thoracis; cross the inner surface of ribs.

  • Abdominopelvic region

    • External oblique muscles

    • Internal oblique muscles

    • Transversus abdominis

Rectus Muscles

  • Rectus abdominis: Between xiphoid process and pubic symphysis; divided longitudinally by linea alba; divided transversely by tendinous inscriptions.

  • Diaphragmatic muscle or diaphragm: Divides thoracic and abdominal cavities; performs respiration.

Muscles of the Pelvic Floor

  • Functions of pelvic floor muscles:

    • Support organs of the pelvic cavity.

    • Flex sacrum and coccyx.

    • Control the movement of materials through the urethra and anus.

Perineum

  • Muscular sheet forming the pelvic floor, divided into:

    • Anterior urogenital triangle

    • Posterior anal triangle

Urogenital Diaphragm

  • Deep muscular layer between pubic bones

  • Supports the pelvic floor and muscles of the urethra

  • Superficial muscles of the urogenital triangle support external genitalia

Pelvic Diaphragm

  • Deep muscular layer extending to the pubis

  • Supports the anal triangle

Appendicular Musculature

  • Position and stabilize pectoral and pelvic girdles.

  • Move upper and lower limbs.

  • Two divisions of appendicular muscles:

    • Muscles of the shoulders and upper limbs

    • Muscles of the pelvis and lower limbs

Muscles of the Shoulders and Upper Limbs

  • Four groups:

    • Muscles that position the pectoral girdle

    • Muscles that move the arm

    • Muscles that move the forearm and hand

    • Muscles that move the hand and fingers

Muscles That Position the Pectoral Girdle

  • Trapezius: Superficial; covers back and neck to base of skull; inserts on clavicles and scapular spines.

  • Rhomboid and levator scapulae: Deep to trapezius; attach to cervical and thoracic vertebrae; insert on the scapular border.

  • Serratus anterior: On the chest; originates along ribs; inserts on anterior scapular margin.

  • Subclavius: Originates on ribs; inserts on clavicle.

  • Pectoralis minor: Attaches to the scapula.

Muscles That Move the Arm

  • Deltoid: The major abductor.

  • Supraspinatus: Assists deltoid.

  • Subscapularis and teres major: Produce medial rotation at the shoulder.

  • Infraspinatus and teres minor: Produce lateral rotation at the shoulder.

  • Coracobrachialis: Attaches to the scapula; produces flexion and adduction at the shoulder.

  • Pectoralis major: Between anterior chest and greater tubercle of humerus; produces flexion at shoulder joint.

  • Latissimus dorsi: Between thoracic vertebrae and humerus; produces extension at shoulder joint.

  • The Rotator Cuff: Muscles involved in shoulder rotation: Supraspinatus, subscapularis, infraspinatus, teres minor and their tendons.

Muscles That Move the Forearm and Hand

  • Originate on the humerus and insert on the forearm.

    • Exceptions: The major flexor (biceps brachii) and the major extensor (triceps brachii).

  • Extensors: Mainly on posterior and lateral surfaces of arm.

  • Flexors: Mainly on anterior and medial surfaces.

Flexors of the Elbow

  • Biceps brachii: Flexes elbow; stabilizes shoulder joint; originates on scapula; inserts on radial tuberosity.

  • Brachialis and brachioradialis

Extensors of the Elbow

  • Triceps brachii: Extends elbow; originates on scapula; inserts on olecranon.

  • Anconeus: Opposes brachialis.

Flexors of the Wrist

  • Palmaris longus: Superficial, flexes wrist.

  • Flexor carpi ulnaris: Superficial, flexes wrist, adducts wrist.

  • Flexor carpi radialis: Superficial, flexes wrist, abducts wrist.

Extensors of the Wrist

  • Extensor carpi radialis: Superficial, extends wrist, abducts wrist.

  • Extensor carpi ulnaris: Superficial, extends wrist, adducts wrist.

Pronation and Supination

  • Pronator teres and supinator: Originate on humerus and ulna; rotate radius.

  • Pronator quadratus: Originates on ulna; assists pronator teres.

Muscles That Move the Hand and Fingers

  • Also called extrinsic muscles of the hand.

  • Lie entirely within the forearm.

  • Only tendons cross the wrist (in synovial tendon sheaths).

Tendon Sheaths

  • Extensor retinaculum: Wide band of connective tissue on the posterior surface of the wrist; stabilizes tendons of extensor muscles.

  • Flexor retinaculum: Anterior surface of wrist; stabilizes tendons of flexor muscles.

The Intrinsic Muscles of the Hand

  • Muscles that move the metacarpals and phalanges and originate and insert only on those bones.

Muscles of the Pelvis and Lower Limbs

  • The pelvic girdle is tightly bound to the axial skeleton, permits little movement, and has few muscles.

Muscles That Position the Lower Limbs

  • Three Groups:

    • Muscles that move the thigh

    • Muscles that move the leg

    • Muscles that move the foot and toes

Muscles That Move the Thigh

  • Gluteal muscles

  • Lateral rotators

  • Adductors

  • Iliopsoas

Gluteal Muscles

  • Gluteus maximus: Largest, most posterior gluteal muscle; produces extension and lateral rotation at hip.

  • Tensor fasciae latae: Works with gluteus maximus; stabilizes iliotibial tract.

  • Gluteus medius and gluteus minimus: Originate anterior to gluteus maximus; insert on trochanter.

Lateral Rotators

  • Group of six muscles, including the dominant piriformis and obturator.

Adductors

  • Adductor magnus: Produces adduction, extension, and flexion.

  • Adductor brevis: Hip flexion and adduction.

  • Adductor longus: Hip flexion and adduction.

  • Pectineus: Hip flexion and adduction.

  • Gracilis: Hip flexion and adduction.

Iliopsoas

  • Two hip flexors insert on the same tendon: psoas major and iliacus.

Muscles That Move the Leg

  • Flexors of the knee: Originate on the pelvic girdle.

  • Extensors of the knee: Originate on the femoral surface and insert on the patella.

Flexors of the Knee

  • Hamstrings: biceps femoris, semimembranosus, semitendinosus

  • Sartorius: Originates superior to the acetabulum.

Extensors of the Knee

  • Four muscles of the quadriceps femoris: three vastus muscles and rectus femoris muscle.

Muscles That Move the Foot and Toes

  • Extrinsic muscles that move the foot and toes include:

    • Muscles that produce extension (plantar flexion) at the ankle

    • Muscles that produce flexion (dorsiflexion) at the ankle

    • Muscles that produce extension at the toes

    • Muscles that produce flexion at the toes

Muscles That Produce Extension (Plantar Flexion) at the Ankle

  • Four Muscles:

    • Gastrocnemius

    • Soleus

    • Fibularis (group)

    • Tibialis posterior

  • The Achilles Tendon (calcaneal tendon) is shared by the gastrocnemius and soleus.

Muscles That Produce Flexion (Dorsiflexion) at the Ankle

  • Tibialis anterior opposes the gastrocnemius.

Muscles That Produce Extension at the Toes

  • Extensor digitorum longus

  • Extensor hallucis longus

  • Extensor retinacula fibrous sheaths hold tendons of toes as they cross the ankle.

Muscles That Produce Flexion at the Toes

  • Flexor digitorum longus

  • Flexor hallucis longus

  • Oppose the extensors.

The Intrinsic Muscles of the Foot

  • Muscles that move the tarsals, metatarsals, and phalanges and originate and insert only on those bones.

Effects of Aging on the Muscular System

  • Skeletal muscle fibers become smaller in diameter.

  • Skeletal muscles become less elastic.

  • Develop increasing amounts of fibrous tissue (fibrosis).

  • Decreased tolerance for exercise.

  • Decreased ability to recover from muscular injuries.

Muscular System Integration

  • Cardiovascular System

    • Delivers oxygen and fuel; removes carbon dioxide and wastes.

  • Respiratory System

    • Responds to the oxygen demand of muscles.

  • Integumentary System

    • Disperses heat from muscle activity.

  • Nervous and Endocrine Systems

    • Direct responses of all systems.