FHA - Muscular System

Basic Properties of Muscle

Excitability, contractility, extensibility, elasticity

Excitability

The ability to respond to a stimulus

Contractility

• The ability to shorten and exert tension

• Length of actin or myosin filaments do not change

Extensibility

The ability to contract over a range of resting lengths

Elasticity

The ability to rebound to original length after contraction

Major Functions of Skeletal Muscle

Produce movement, maintain posture, support soft tissues, regulate entry/exit of materials, maintain body temperature, pulling on tendons attached to bones, and maintain posture from constantly adjusting and stabilizing joints, heat generated through muscle contractions

Skeletal Muscles Supporting Soft Tissues

Forming muscular walls and pelvic floor support

Skeletal Muscles Regulating Entry and Exit of Materials

Through voluntary control of mouth, urethra, and anus

Epimysium

Dense connective tissue surrounding the entire muscle

Fascicle

A bundle of muscle fibers

Perimysium

Dense connective tissue surrounding each fascicle.

Endomysium

Dense connective tissue surrounding each muscle fiber

Tendon Formation

The merging of epimysium, perimysium, and endomysium

Muscle Fiber

A single muscle cell

Sarcolemma

The muscle cell membrane

Sarcoplasm

The cytoplasm of a muscle fiber

Myofibrils

Long protein bundles inside muscle fibers containing thick and thin filaments

Thick Filaments

Myosin

Thin Filaments

Actin

Sarcoplasmic Reticulum

A specialized ER that stores calcium

T-tubulues

Transverse tubules that conduct electrical impulses into the muscle fiber

Triad

A T‑tubule with two terminal cisternae

Sarcomere

The functional contractile unit of a muscle fiber

Boundaries of a Sarcomere

Z‑lines

M-line

The center of the sarcomere where myosin anchors

I-band

Region containing only actin

A-band

Region containing both actin and myosin

H-band

Region containing only myosin

Zone of Overlap

Area where actin and myosin overlap

Sliding Filament Theory

Actin slides over myosin, shortening the sarcomere

Z-lines during Contraction

They move closer together

I-band & H-band during contraction

They get smaller

Triggers of Contraction

Calcium binding to troponin

Exposes Active Sites on Actin

Troponin–tropomyosin shifting after calcium binds

Forms a Cross-bridge

Myosin heads binding to actin

Causes the Power Stroke

Myosin pulling actin toward the M‑line

Neuromuscular Junction (NMJ)

The synapse between a motor neuron and a muscle fiber

Neurotransmitter Released at the NMJ

Acetylcholine (ACh)

Enzyme Breaks Down ACh

Acetylcholinesterase (AChE)

ACh When Broken Down

The action potential stops

Calcium During Relaxation

It is actively pumped back into the SR

Calcium once removed from Troponin

Tropomyosin covers actin’s active sites again

Motor Unit

A motor neuron and all the muscle fibers it innervates, all fibers in a motor unit contract together

Size Principle

Small motor units activate first; larger ones activate as more force is needed

Muscle Tone

Tension in a muscle at rest

Muscle Spindles

Sensory receptors that detect muscle stretch and regulate tone

Muscle Atrophy

Loss of muscle size, tone, and strength due to disuse

Three Types of Skeletal Muscle Fibers

Fast (FG), slow (SO), and intermediate (FOG)

Fast Fibers

Fast glycolytic (FG), Type IIb/IIx, Low myoglobin (appear pale), Quick, powerful contractions, and fatigue quickly

Slow Fibers

Slow oxidative (SO), Type I, High myoglobin and oxygen supply (appear red), Endurance and sustained contractions, Postural muscles and lower extremities (abundance)

Intermediate Fibers

Fast oxidative glycolytic (FOG), Type IIa, Moderate endurance and moderate power

Brachii

The arm

Tibialis Anterior

A muscle on the anterior tibia

Trapezius

A trapezoid-shaped muscle

Deltoid

Triangular

Rectus

Straight

Oblique

Angled

Biceps

Two heads

Sternocleiomastoid

Origin on sternum and clavicle; insertion on mastoid

Genioglossus

From chin to tongue

Flexor Carpi Radialis

A wrist flexor on the radial side

Adductor Longus

A long muscle that adducts the thigh

Buccinator

Trumpet player, Compresses the cheek for blowing and controlling airflow

Sartorius

Tailor, Tailors traditionally sat cross‑legged, a movement produced by the sartorius

Muscle Fibers with Age

They shrink in diameter due to loss of myofibrils

Glycogen Reserves with Age

Reserves decrease

Fibrosis in Aging Muscle

Increased connective tissue replacing muscle fibers