Muscular System: Functions and Tissues

Muscular System Overview

• Refers to the skeletal system, with most muscles attached to bones
• Constitutes 40-50% of total body weight
• Converts chemical energy into mechanical work, enabling action via microscopic striations
• Contractions are usually voluntary and result to movement

Myology

• The scientific study of muscles

Unique Muscle Tissues

• Composed of muscle cells that undergo modification to become muscle fibers

Muscle Functions

• Movement results from brain-prompted contractions
• Posture is maintained through partial contraction
• Respiration relies on the diaphragm, the chief muscle undergoing continuous contraction and communicating with the heart
• Body heat is generated by muscle contraction
• Communication involves skeletal muscles for speaking, writing, gesturing, and facial expressions
• Constriction, mainly by smooth muscle, constricts internal organs and vessels.
• Contraction of cardiac muscle enables the heart to beat, propelling blood throughout the body

Characteristics and Types of Muscles

• Includes skeletal, smooth, and cardiac muscle
• All are made of muscular tissue made up of muscle cells or muscle fibers

Key Muscles

• Trapezius extends from front to back
• Pectoralis major lies above the pectoralis minor
• Biceps brachii are arm muscles at the front
• Triceps brachii are arm muscles at the back
• Latissimus dorsi is the widest muscle of the body
• Abdominal muscles are paired (10 packs)
• Gluteus maximus is the largest muscle, with gluteus minimus underneath
• Sartorius, the longest muscle, attaches to the femur
• Biceps femoris are hamstring muscles
• Rectus femoris is located at the front of the thigh
• Gastrocnemius is located at the calf
• Achilles tendon was discovered by Achilles

Characteristics of Muscles

• Enables muscles to carry out functions and contributes to homeostasis
• Excitability/Irritability: ability to respond to stimuli
• Conductivity: ability to transmit impulses
• Contractility: ability to shorten and thicken to generate work
• Extensibility: ability to be stretched without tissue damage
• Elasticity: ability to return to original form after contraction or lengthening

Skeletal Muscle Tissue

• Muscle fibers are skeletal muscle cells

Stimulus Classification

• Minimal/Liminal (Threshold): smallest stimulus amount to produce a muscle twitch
• Sub-Minimal/Subliminal (Subthreshold): any stimulus of lesser strength that cannot produce a twitch

Connective Tissue Components

• Surround and protect muscle tissue

Fascia

• A large sheet-like connective tissue covering the entire muscle

Tendons

• Cords of dense connective tissue that attach muscle to bone

Aponeurosis

• Broad, flattened connective tissue that attaches muscle to another muscle

Connective Tissue Coverings

• Epimysium: Outermost layer that surrounds muscles and binds muscle fibers together

Perimysium

• Surrounds bundles of muscle fibers (fascicles)

Endomysium

• Innermost layer that penetrates each muscle fiber

Muscle Fiber Structure

• Myofibrils are thread-like proteins that make up muscle fibers
• Myofilaments are proteins that make up myofibrils
• Sarcolemma is the cell membrane containing T-tubules
• T-tubules wrap around sarcomeres at the A band and are associated with the sarcoplasmic reticulum
• Sarcoplasmic reticulum surrounds myosin and stores/releases Ca2+
• Sarcoplasm: cytoplasm of muscle fiber (cell)

Major Muscle Proteins

• Actin is the protein composing thin myofilaments
• Myosin is the protein composing thick myofilaments

Actin and Myosin Myofilaments

• Actin are thin myofilaments resembling two strands of pearls
• Myosin are thick myofilaments resembling golf clubs
• Troponin: attachment site on actin for Ca2+
• Tropomyosin a filament on grooves of actin that serves as an attachment site on actin for myosin

Sarcomeres

• Basic contractile units that contain actin and myosin
• Z disk: protein fibers forming attachment site for actin
• M line: where myosin is anchored
• H zone: center of sarcomere with only myosin
• I band: contains only actin
• A band: where actin and myosin overlap

Sliding Filament Theory

• Explains how muscles contract through the sliding of actin myofilaments past myosin myofilaments during contraction
• Cross-bridge: binding of myosin to actin

Tonic Contraction

• Produces muscle tone, but the muscles do not shorten
• Only a few fibers contract at a time
• Example: standing position

Muscle Attachments

• Involves OIA (Origin, Insertion, Action)
• Tendons: white glistening fibrous bands of connective tissue attaching muscle to bone
• Origin: less movable, more stable attachment site, usually proximal
• Insertion: more movable, less stable attachment site, usually distal

Muscle Tones

• Persistent state of slight muscle contraction
• Represents tension in a muscle at rest

Nomenclature of Skeletal Muscles

• Direction relative to the midline of the body
• Rectus: example, rectus abdominis
• Transverses: example, transverse abdominis, deep into the abdomen
• Oblique (External): example, external abdominal oblique
• Oblique (Internal): example, internal abdominal oblique

Muscles According to Location

• Frontalis: specific to frontal
• Tibialis Anterior: front of lower leg, inverts foot
• Tibialis Posterior: back of lower leg

Muscles According to Size

• Maximus: largest
• Minimus: smallest
• Longus: longest
• Brevis: short
• Major: large
• Minor: small

Quadriceps

• Four origins
• Examples: Rectus femoris, Vastus intermedius, Vastus medialis, Vastus lateralis

Muscles According to Origin and Insertion

• Example: Sternocleidomastoid

Muscle Shape

• Based on organization of fascicles
• Types include: Orbicularis oris, Unipennate, Bipennate, Convergent, Parallel, Multipennate

Muscles According to Number of Origins

• Biceps: 2 origins
• Triceps: 3 origins

Muscles of Facial Expressions

• Frontalis: draws scalp anteriorly, raises eyebrows
• Orbicularis oculi: closes the eyelids
• Zygomaticus: smiles or laughs
• Risorius: grins
• Buccinator: assists with blowing or kissing
• Levator labii superioris: elevates the upper lip
• Depressor labii inferioris: depresses the lower lip
• Depressor anguli oris: causes grief
• Mentalis: elevates and protrudes lower lip
• Orbicularis oris: closes and purses lips

Muscles of Mastication

• Masseter: elevates the jaw
• Temporalis: elevates and retracts the jaw
• Lateral pterygoid: protrudes the jaw
• Medial pterygoid: moves the jaw from side to side

Muscles of the Eyeball

• Superior rectus: directs eyeball upwards
• Inferior rectus: directs eyeball downwards
• Medial rectus: directs eyeball medially
• Lateral rectus: directs eyeball laterally
• Superior oblique: rotates eyeball laterally and downward
• Inferior oblique: rotates eyeball laterally and upward

Muscles That Move the Head and Neck

• Sternocleidomastoid flexes the head forward
• Platysma retracts the angle of the mouth and jaw

Muscles That Move The Shoulder

• Rhomboids retract the scapula
• Serratus anterior protracts the scapula
• Pectoralis minor depresses the scapula
• Pectoralis major flexes the shoulder and adducts the arm
• Teres major extends the shoulder and adducts/rotates the arm
• Latissimus dorsi extends shoulder and adducts arm
• Infraspinatus extends and rotates the shoulder

Muscles of the Abdominal Wall

• Rectus abdominis compresses the abdomen which aids in defecation, urination, and forced expiration
• External Oblique compresses abdomen and rotates the vertebral column laterally
• Internal Oblique compresses abdomen and rotates the vertebral column laterally
• Transversus compresses abdomen

Muscles Used in Breathing

• Diaphragm is the key muscle for respiration
• External intercostals compress the thorax
• Internal intercostals expand the thorax

Upper Extremity Muscles

• Triceps brachii extends forearm
• Biceps brachii flexes and supinates forearm
• Brachialis flexes forearm
• Pronators pronate forarm
• Brachioradialis flexes and pronates the forearm
• Supinator supinates the forearm

Lower Extremity Muscles

• Iliopsoas: chief flexor of the hip
• Tensor fasciae latae: flexes hip and abducts thigh
• Gluteus maximus: chief extensor of the hip
• Gluteus medius: abducts the thigh
• Quadriceps femoris group: extensor muscles, extends knee
• Sartorius: extends knee
• Hamstring group: flexor muscles
• Adductor group: adducts thigh
• Tibialis anterior: dorsiflexes ankle
• Gastrocnemius: plantar flexes ankle
• Soleus: plantar flexes ankle
• Peroneus: flexes and everts the ankle