The Muscular System

The Muscular System

Introduction to the Muscular System

• The muscular system consists only of skeletal muscles.
• Muscle organization dramatically affects the power, range, and speed of movement.

Fascicle Arrangement

• Skeletal muscle fibers form bundles called fascicles.
• Muscles are classified based on patterns of fascicle arrangement:
  • Parallel muscles
  • Convergent muscles
  • Pennate muscles
  • Circular muscles

Parallel Muscles

• Fascicles are parallel to the long axis of the muscle.
• Some are flat.
• Cylindrical muscles have a central body (belly).
  • Example: biceps brachii
• Tension developed during a contraction depends on the total number of myofibrils.
• Examples include:
  • Biceps brachii
  • Rectus abdominis (parallel with tendinous bands)
  • Supinator (wrapping muscle)

Convergent Muscles

• Muscle fibers spread out like a fan and converge on an attachment site.
  • Example: Pectoralis muscles
• Muscle may pull on:
  • Tendon
  • Aponeurosis
  • Raphe (slender band of collagen fibers)
• Fibers pull in different directions, depending on the activity.
• Example: Pectoralis.

Pennate Muscles

• Muscle fibers pull at an angle relative to the tendon.
• Compared to parallel muscles, pennate muscles:
  • Do not move their tendons as far.
  • Contain more myofibrils.
  • Develop more tension.

Types of Pennate Muscles

• Unipennate:
  • All fascicles are on the same side of the tendon.
  • Example: Extensor digitorum
• Bipennate:
  • Fascicles are on both sides of a central tendon.
  • Example: Rectus femoris
• Multipennate:
  • Tendon branches within the muscle.
  • Example: Deltoid

Circular Muscles (Sphincters)

• Act as valves in digestive and urinary tracts.
• Surround body openings and hollow organs.
  • Contraction makes the diameter of opening smaller.
  • Example: Orbicularis oris of the mouth

Levers

• Almost all skeletal muscles attach to bones.
  • The site of connection to a bone affects force, speed, and range of movement.
• Each bone acts as a lever (a rigid, moving structure).
  • Moves on a fixed point (fulcrum) when muscles provide applied force to overcome the load.
  • Each joint is a fulcrum.
• Levers can change:
  • Direction of applied force (AF)
  • Distance and speed produced by AF
  • Effective strength of AF
• Three classes of levers are based on the relative positions of applied force, fulcrum, and load:
  • First-class lever
  • Second-class lever
  • Third-class lever

First-Class Lever

• Fulcrum lies between applied force and load.
• Like a pry bar or crowbar.
• Example: Extension of the neck and lifting the head.

Second-Class Lever

• Load lies between applied force and fulcrum.
• Like a wheelbarrow.
• A small force moves a large weight.
• Example: Ankle extension (plantar flexion) by calf muscles.

Third-Class Lever

• Applied force is between load and fulcrum.
• Like a pair of tongs.
• Most common lever in the body.
• Maximizes speed and distance traveled at the expense of effective force.

Origins and Insertions

• Fixed point of attachment of a muscle to bone is the origin.
• Movable point of attachment is the insertion.
• The origin is usually proximal to the insertion.

Actions

• Movements produced by muscle contraction.
• Example: adduction, elevation, pronation, etc.
• Described in terms of effect on bone or joint.
  • Example: Flexion of the forearm or flexion at the elbow.

Muscle Interactions

• Muscles work in groups to maximize efficiency.
• Smaller muscles reach maximum tension first, followed by larger, primary muscles.
• Four terms refer to how muscles work together:
  • Agonist
  • Antagonist
  • Synergist
  • Fixator

Agonist (Prime Mover)

• Mostly responsible for producing a particular movement.

Antagonist

• Opposes the movement of a particular agonist.

Synergist

• A smaller muscle that assists a larger agonist.

Fixator

• A synergist that assists an agonist by preventing movement at another joint.

Muscle Opposition

• Agonists and antagonists work in pairs.
  • When one contracts, the other stretches.
  • Such as flexors–extensors and abductors–adductors.
• Flexion and Extension
  • Muscles whose lines of action cross the anterior side of a joint are flexors of that joint.
  • Muscles whose lines of action cross the posterior side of a joint are extensors of that joint.
• Abduction and Adduction
  • Muscles whose lines of action cross the medial side of a joint are adductors of that joint.
  • Muscles whose lines of action cross the lateral side of a joint are abductors of that joint.
• Medial and Lateral Rotation
  • Movement or turning of the body part occurs around its axis.
  • Muscles that cross the anterior aspect of the shoulder joint produce medial rotation at the joint.
  • Muscles that cross the posterior aspect of the shoulder joint produce lateral rotation at the shoulder.

Naming Skeletal Muscles

• The body has approximately 700 skeletal muscles.
• Names of muscles include descriptive information about:
  • Region of the body (e.g., temporalis)
  • Position, direction, or fascicle arrangement
  • Structural characteristics
  • Action

Terms indicating specific regions of the body

• Abdominal (abdomen)
• Ancon (elbow)
• Auricular (ear)
• Brachial (arm)
• Capitis (head)
• Carpi (wrist)
• Cervicis (neck)
• Coccygeal (coccyx)
• Costal (rib)
• Cutaneous (skin)
• Femoris (thigh)
• Glossal (tongue)
• Hallux (great toe)
• Ilium (groin)
• Inguinal (groin)
• Lumbar (lumbar region)
• Nasalis (nose)
• Nuchal (back of neck)
• Ocular (eye)
• Oris (mouth)
• Palpebra (eyelid)
• Pollex (thumb)
• Popliteal (posterior to knee)
• Psoas (loin)
• Radial (forearm)
• Scapular (scapula)
• Temporal (temple)
• Thoracic (thorax)
• Tibial (tibia; shin)
• Ulnar (ulna)

Position, direction, or fascicle arrangement

• Externus (superficialis)
  • Muscles visible at body surface
• Internus (profundus)
  • Deeper muscles
• Extrinsic muscles
  • Position or 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
    • Example: Rectus abdominis
• Anterior (front)
• External (on the outside)
• Extrinsic (outside the structure)
• Inferior (below)
• Internal (away from the surface)
• Intrinsic (within the structure)
• Lateral (on the side)
• Medial (middle)
• Oblique (slanting)
• Posterior (back)
• Profundus (deep)
• Rectus (straight)
• Superficial (toward the surface)
• Superior (toward the head)
• Transverse (crosswise)

Structural characteristics

• Origin and Insertion
  • First part of name indicates origin
  • Second part indicates insertion
    • Example: genioglossus
• Number of Tendons
  • Example: biceps brachii
• Shape and Size
  • Example: trapezius, deltoid, rhomboid
  • Many terms refer to muscle size
• Terms indicating structural characteristics
  • Nature of origin
    • Biceps (two heads)
    • Triceps (three heads)
    • Quadriceps (four heads)
  • Shape
    • Deltoid (triangle)
    • Orbicularis (circle)
    • Pectinate (comblike)
    • Piriformis (pear shaped)
    • Platysma (flat plate)
    • Pyramidal (pyramid)
    • Rhomboid (parallelogram)
    • Serratus (serrated)
    • Splenius (bandage)
    • Teres (round and long)
    • Trapezius (trapezoid)
  • Other striking features
    • Alba (white)
    • Brevis (short)
    • Gracilis (slender)
    • Latae (wide)
    • Latissimus (widest)
    • Longissimus (longest)
    • Longus (long)
    • Magnus (large)
    • Major (larger)
    • Maximus (largest)
    • Minimus (smallest)
    • Minor (smaller)
    • Vastus (great)

Action

• Movements
  • Example: flexor, extensor, pronator, etc.
• Occupations or Habits
  • Example: buccinator means “trumpeter”
• Terms indicating general actions
  • Abductor (movement away)
  • Adductor (movement toward)
  • Depressor (lowering movement)
  • Extensor (straightening movement)
  • Flexor (bending movement)
  • Levator (raising movement)
  • Pronator (turning into prone position)
  • Supinator (turning into supine position)
  • Tensor (tensing movement)
• Terms indicating specific actions
  • Buccinator (trumpeter)
  • Risorius (laugher)
  • Sartorius (like a tailor)

Axial and Appendicular Muscles

• Divisions of the muscular system:
  • Axial muscles
    • 60 percent of skeletal muscles
    • Position head and vertebral column
    • Move rib cage
    • Form pelvic floor
  • Appendicular muscles
    • Move and support pectoral and pelvic girdles and limbs

Axial Muscles

• Grouped 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 the Head and Neck

• Muscles of facial expression
  • Originate on skull
• Muscles of mastication
  • Move the mandible
• Muscles of the tongue
  • Names end in glossus
• Muscles of the pharynx
  • Begin swallowing process
• Extrinsic eye muscles
  • Originate on the surface of the orbit
  • Control the position of eyes
• Muscles of the anterior neck
  • Control position of larynx
  • Depress the mandible and tense floor of mouth
  • Support muscles of tongue and pharynx

Muscles of Facial Expression

• Orbicularis oris
  • Constricts the mouth opening
• Buccinator
  • Moves food across the teeth
  • In infants, provides suction for nursing
• Muscles of the epicranium (scalp)
  • Temporoparietalis
  • Occipitofrontalis
    • Frontal belly and occipital belly are separated by epicranial aponeurosis
• Platysma
  • Covers anterior surface of the neck

Extrinsic Eye Muscles (Oculomotor Muscles)

• Inferior rectus
• Medial rectus
• Superior rectus
• Lateral rectus
• Inferior oblique
• Superior oblique

Muscles of Mastication

• Masseter
  • Strongest jaw muscle
• Temporalis
  • Helps elevate the mandible
• Pterygoid muscles
  • Elevate, depress, and protract the mandible
  • Slide the mandible from side to side (lateral excursion)

Muscles of the Tongue

• All named for origin and insertion
  • Palatoglossus
  • Styloglossus
  • Genioglossus
  • Hyoglossus

Muscles of the Pharynx

• Pharyngeal constrictor muscles
  • Move food into the esophagus
• Palatal muscles
  • Elevate the soft palate and adjacent portions
  • Pull open entrance to auditory tube
• Laryngeal elevators
  • Raise the larynx

Muscles of the Anterior Neck

• Digastric
  • Controls position of the larynx
  • Extends from chin to hyoid bone and from the hyoid to mastoid portion of the temporal bone.
• Mylohyoid
  • Elevates the floor of the mouth
  • Depresses the jaw
• Geniohyoid
  • Extends between the hyoid bone and chin
• Stylohyoid
  • Between the hyoid bone and styloid process of the skull
• Sternocleidomastoid
  • Extends from the clavicle and sternum to the mastoid
  • Turns the head obliquely to the opposite side
• Omohyoid
  • Attaches the scapula, clavicle, first rib, and hyoid

Muscles of the Vertebral Column

• Erector spinae muscles
  • Superficial and deep layers
• Spinal flexors

Erector Spinae (Superficial Layer)

• Spinalis group
• Longissimus group
• Iliocostalis group

Erector Spinae (Deep Layer)

• Semispinalis group
• Multifidus
• Interspinales
• Intertransversarii
• Rotatores

Spinal Flexors

• Neck
  • Longus capitis and longus colli
    • Rotate and flex the neck
• Lumbar region
  • Quadratus lumborum
    • Flexes the vertebral column and depresses the ribs

Oblique and Rectus Muscles

• Lie within the body wall
  • Oblique muscles
    • Compress underlying structures
    • Rotate vertebral column
  • Rectus muscles
    • Flex vertebral column
    • Oppose erector spinae

Oblique Muscles

• Cervical region
  • Scalene muscles
    • Flex the neck and elevate the ribs
• Thoracic region
  • External and internal intercostal muscles
    • Aid in breathing movements of ribs
  • Transversus thoracis
    • Crosses posterior surface of the sternum
• Abdominopelvic region (same pattern as thoracic)
  • External oblique
  • Internal oblique
  • Transversus abdominis

Rectus Muscles

• Rectus abdominis
  • Between the xiphoid process and pubic symphysis
  • Divided longitudinally by the linea alba
  • Divided transversely by tendinous inscriptions

The Diaphragm

• Divides thoracic and abdominopelvic cavities
• The major muscle used in breathing

Muscles of the Pelvic Floor

• Function to:
  • Support organs of pelvic cavity
  • Flex sacrum and coccyx
  • Control movement of materials through urethra and anus

Perineum

• Region bounded by the inferior margins of the pelvis
• Divided by ischial tuberosities into:
  • Anterior urogenital triangle
  • Posterior anal triangle
• Pelvic diaphragm
  • Forms the muscular foundation of the anal triangle
  • Extends to pubic symphysis
• Urogenital and pelvic diaphragms
  • Do not completely close the pelvic outlet
  • Urethra, anus, vagina (in females), muscles, nerves, and blood vessels pass through
  • Sphincters permit voluntary control of urination and defecation

Appendicular Muscles

• Position and stabilize pectoral and pelvic girdles.
• Move upper and lower limbs
• Two groups 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 fingers

Muscles that Position the Pectoral Girdle

• Trapezius
  • Large and superficial
  • Covers the back and portions of the neck
  • Extends to base of skull
  • Originates on midline of neck and back
  • Inserts on clavicles and scapular spines
• Serratus anterior
  • Fan-shaped muscle on chest
  • Originates along ribs
  • Inserts on anterior margin of scapula
• Subclavius
  • Originates on ribs
  • Inserts on clavicle
• Pectoralis minor
  • Originates on ribs
  • Attaches to coracoid process of the scapula
• Rhomboid major, rhomboid minor, and levator scapulae
  • Deep to trapezius
  • Attach to cervical and thoracic vertebrae
  • Insert on vertebral border of each 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
  • Produces flexion and adduction at the shoulder
• Pectoralis major
  • Between the anterior chest and greater tubercle of the humerus
  • Produces flexion at the shoulder joint
• Latissimus dorsi
  • Between the thoracic vertebrae and humerus
  • Produces extension at the shoulder joint

Rotator Cuff

• Muscles involved in shoulder rotation
  • Supraspinatus, infraspinatus, teres minor, subscapularis, and their tendons

Muscles that Move the Forearm and Hand

• Most originate on the humerus and insert on the forearm and wrist
• Exceptions:
  • The major flexor (biceps brachii)
  • The major extensor (triceps brachii)
  • Biceps brachii and long head of triceps brachii originate on scapula
• Extensors
  • Mainly on posterior and lateral surfaces of the arm
• Flexors
  • Mainly on anterior and medial surfaces
• Extensors of the elbow
  • Triceps brachii
    • Long head originates on the scapula
    • Inserts on the olecranon of the ulna
  • Anconeus
• Flexors of the elbow
  • Biceps brachii
    • Flexes elbow and supinates forearm
    • Stabilizes shoulder joint
    • Originates on scapula
    • Inserts on radial tuberosity of radius
  • Brachialis and brachioradialis
    • Flex the elbow
• Muscles involved in supination and pronation
  • Supinator and pronator teres
    • Originate on the humerus and ulna
    • Rotate the radius
  • Pronator quadratus
    • Originates on the ulna
    • Assists the pronator teres in opposing actions of the supinator or biceps brachii
• Flexors of the wrist
  • Flexor carpi ulnaris
    • Flexes and adducts the wrist
  • Flexor carpi radialis
    • Flexes and abducts the wrist
  • Palmaris longus
    • Flexes the wrist
• Extensors of the wrist
  • Extensor carpi radialis
    • Extends and abducts the wrist
  • Extensor carpi ulnaris
    • Extends and adducts the wrist
• Tendons of forearm muscles that cross the wrist pass through synovial tendon sheaths.
  • Extensor retinaculum
    • Wide band of connective tissue
    • Posterior surface of wrist
    • Stabilizes tendons of extensor muscles
  • Flexor retinaculum
    • Anterior surface of wrist
    • Stabilizes tendons of flexor muscles

Muscles that Move the Fingers

• Extrinsic muscles of the hand
  • Lie in the forearm
  • Only tendons cross the wrist
  • Provide strength and gross movement of hand and fingers
• Intrinsic muscles
  • Originate on carpal and metacarpal bones
  • No muscles originate on phalanges
  • Only tendons extend across distal joints of fingers
  • Provide fine motor movement of the hand

Muscles of the Pelvis and Lower Limbs

• The Pelvic girdle is tightly bound to the axial skeleton.
  • Permits little movement.
  • Few axial muscles influence the position of the pelvis.
• A range of movements is possible in the lower limbs.
• Muscles that position the lower limbs:
  • Three functional 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
    • To pull on iliotibial tract of lateral surface of thigh
  • Gluteus medius and gluteus minimus
    • Originate anterior to gluteus maximus
    • Insert on greater trochanter of femur
• Lateral rotators
  • Group of six muscles, including the dominant
    • Piriformis
    • Obturator
• Adductors
  • Pectineus, adductor brevis, adductor longus, and gracilis
    • Produce hip flexion and adduction
  • Adductor magnus
    • Produces adduction and extension or flexion
    • Also, medial or lateral rotation at hip
• Iliopsoas
  • Two hip flexors that insert on the same tendon
    • Psoas major
    • Iliacus

Muscles That Move The Leg

• Flexors of the knee
  • Most originate on edges of pelvis
  • Insert on tibia and fibula
• Knee extensors
  • Most originate on shaft of femur
  • Insert on the patella
• Flexors of the knee
  • Hamstrings
    • Biceps femoris
    • Semitendinosus
    • Semimembranosus
  • Sartorius
  • Popliteus
• Knee extensors
  • Quadriceps femoris consist of:
    • Three vastus muscles
    • Rectus femoris

Muscles That Move The Foot and Toes

• Extrinsic muscles that produce plantar flexion
  • Gastrocnemius
  • Soleus
  • Fibularis muscles
  • Tibialis posterior
• Calcaneal tendon (Achilles tendon)
  • Shared by the gastrocnemius and soleus
• Extrinsic muscles that produce flexion at the ankle
  • Tibialis anterior
    • Opposes the gastrocnemius
• Extrinsic muscles that produce extension at toes
  • Extensor digitorum longus
  • Extensor hallucis longus
    • Extensor retinacula stabilize synovial tendon sheaths of these muscles
• Extrinsic muscles that produce flexion at toes
  • Flexor digitorum longus
  • Flexor hallucis longus
• Intrinsic muscles of the foot
  • Originate on tarsal and metatarsal bones
  • Move toes and maintain longitudinal arch of foot

Effects of Exercise

• The muscular system is supported by other systems:
  • Cardiovascular system
    • Delivers oxygen and nutrients
    • Removes carbon dioxide
  • 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