10 Muscle System

Skeletal

Bones, voluntary, striped, many nuclei

Innervated = neuron

vascular = blood vessels

Cardiac

heart, involuntary, striped, branching

intercalated discs connects neighboring cells

Smooth

organs/blood vessels, involuntary, no stripes, spindle-shaped

Epimysium

wraps entire muscle

Perimysium

wraps bundle of fibers

houses many blood vessels and nerves

Endomysium

wraps individual muscle fiber

support capillaries, helps electrically insulate fibers, binds neighboring cells

Tendon

cord connecting muscle to another structure

dense regular CT

Aponeurosis

flat sheet connecting muscle to another structure

dense irregular CT

Sarcolemma

cell membrane

Sarcoplasm

cytoplasm

Sarcoplasmic reticulum (SR)

stores calcium

T-tubules

carry electrical signals inward

Myoglobin

stores oxygen

Glycogen

stored fuel

Actin

thin filament

binding sites for myosin

contains troponin and tropomyosin

Myosin

thick filament

Heads = actin, ATPase site

Troponin

binds calcium

tropomyosin

blocks actin’s binding sites when relaxed

Sarcomere

basic unit of muscle contraction

Muscle Contraction

More calcium available to bind troponin

Nerve signal arrives > ACh is release and binds to receptors > electrical signal travels along sarcolemma and T-tubules > SR releases Ca2+ > calcium binds to troponin, moving tropomyosin > myosin binds actin and pulls thin filament > sarcomere shorten, producing contraction

Muscle Functions

Movement, Posture, Protection and support, control of elimination, heat production

Excitability

responds to a stimulus

Ex: muscle receives signal from nerve

Conductivity

Carries an electrical signal along muscle cell

Contractility

Can shorten and produce force

Extensibility

Can be stretched

Elasticity

Returns toward its original length

Skeletal Muscle Organization

Whole muscle > fascicle > muscle fiber > myofibril > myofilaments

Fascicle

bundle of muscle fibers

Muscle fiber

one muscle cell

Myofibril

long contractile structures inside a muscle fiber

contain thick and thin filaments

Myofilaments

protein strands that slide to produce contraction

Deep Fascia

dense irregular CT

outside of epimysium

nerves, blood vessels, lymph vessels

Superficial Fascia

areolar/adipose

above deep fascia; separates muscle from skin

Neuromuscular junction

neuron communicates with muscle fiber

Terminal cisternae

enlarged SR sacs

store calcium

Triad

2 terminal cisternae + 1 T-tubule

Embryonic development

myoblasts (satellite cells) fuse to form muscle fibers

Key of Muscle System

Calcium!

Tropomyosin

covers myosin-binding sites on actin when muscle relaxed

Troponin

binds calcium

Muscles Shorten

filaments do not get shorter; they slide past each other

Synaptic knob

end of motor neuron

contains vesicles filled with ACh and voltage-gated calcium channels

components of skeletal muscle fibers

Synaptic cleft

Small fluid-filled gap btwn nerve and muscle

contains acetylcholinesterase

Motor and plate

region of sarcolemma containing many ACh receptors

Acetylcholine (ACh)

neurotransmitter that communicates the nerve signal to muscle

Acetylcholinesterase (AChE)

breaks down ACh, helping terminate the signal

Resting Membrane Potential

-90mV (resting)

inside cell is electrically (-) compared with outside

leak channels/sodium-potassium pumps maintain resting state

voltage-gated channels are closed

Muscle Relaxes

Calcium removed from sarcoplasm

nerve signal/ACh stop > AChE breaks down remaining ACh > ACh receptors close > Calcium-release channels in the SR close > Calcium pumps move calcium back into SR > Troponin returns to original shape >Tropomyosin covers binding sites again > muscle returns to original

Phosphate Transfer

very short, intense activity

fastest ATP replacement

Ex: 50 m sprint (less than 10 secs)

Glycolysis

short, intense activity

anaerobic (no O2); net 2 ATP per glucose

Ex: 400 m sprint (less than a minute)

Aerobic respiration

longer activity

uses O2 and mitochondria; 30 ATP

Ex: 1500 m run (more than a minute)

Type 1 - slow oxidative

slow, endurance, many mitochondria, lots of myoglobin

Red = endurance

Type 2a - fast oxidative

fast and powerful, primarily aerobic

Type 2b - fast glycolytic

very fast and powerful, fatigues quickly, less myoglobin

white = quick power

Motor unit

1 motor neuron and all muscle fibers it controls

Small motor unit

precise movements

Ex: moving fingers

Large motor unit

powerful movements, less precision

Ex: lifting a heavy weight

Cross-bridge

a connection formed when myosin head binds to actin

ATP

detaches myosin from actin and energizes myosin head

powers calcium pumps to move back to SR

Hypertrophy

muscle gets bigger

Atrophy

muscle shrinks

Mitochondria

determines dark/white muscle

Main Proteins of Muscle Contraction

Myosin and Actin

Sliding Filaments Theory

release calcium when binding tg

Z discs

boundaries of each sarcomere

thin filaments attached

A band

entire length of thick filaments - actin and myosin overlap

stay same during contraction

I band

only thin filaments

get shorter during contraction

H zone

central region containing only thick filaments

get smaller/ disappear during contraction

M line

middle line of sarcomere

hold thick filaments in place

Action potential

electrical shot across membrane

Creatine Kinase

transfer phosphate from creatine phosphate to ADP to make ATP

Myokinase

transfer phosphate group btwn 2 ADP molecules, producing ATP and AMP

Muscle Fatigues

Decreased glycogen - muscles run low on stored fuel

Reduced stimulation - muscle don’t receive signals as effectively

Reduced contraction ability - muscle have trouble contracting with same force

Isometric Contraction

tension develops while overall muscle length stays approximately constant.

Isotonic Contraction

changes muscle length while moving a load

Tetanus

repeating stimulation at a high frequency can produce a sustained contraction