Sharing

local electrical charge triggers a wave of excitation that travels along the muscle fiber

Force

Shortens when stimulated

Relaxation

Capable of being stretched

reoil/resume

returns to its original resting length after being stretched

voluntary, striated muscle attached to bones

muslce fibers (myofibers) as long as 30 cm

alternating light and dark transverse bands/striations

striations reflect overlapping arrangement of internal contractile proteins (myofilaments) actin and myosin

Parallel Elastic Components

Series Elastic Components

• Found between muscle fiber and bone or other attachment

• Not excitable or elastic, but are somewhat extensible and elastic

• stretches slightly under tension and recoils when released

• Not contractile or physically able to change length

surrounds contractile parts

connected to each other in linear series

• Endomysium-> Perimysium->epimysium

Joined at end

• Tendon, periosteum, bone matrix

help return muscles to their resting lengths

adds significantly to power output and efficiency of muscles

Multinucleated-- multiple nuclei against inside of Plasma Membrane

Sarcolemma

Sarcoplasm

Sarcoplasmic Reticulum

lemma = husk

plasma membrane

has tunnel-like infoldings or transverse (T) tubules that penetrate the cell

carry out electric current to cell interior

Cytoplasm

• filled with long protein bundles called myofibrils

• myofibrils = bundles of parallel protein microfilaments called myofilaments

• myofilaments = actin and myosin

• glycogen stored for energy

• myoglobin = unit inside muscle cell that is receptive to oxygen

Smooth Endoplasmic Reticulum

Interconnected network

Ca++ resovoir (collecting and distributing Ca++)

1 functional contractile unit of muscle fiber

sarco = muscle

mere = segment

Contractile Proteins: Actin and Myosin

1 myosin surrounded by 6 actin

Thin Filaments

Contractile Protein

• 2 intertwined strands of fibrous (F) actin
• each subunit is a globular (G) Actin with an active site (most F actin, few G)
• Groove holds Tropomyosin molecules, each blocking the active sites of Actin
• One small, calcium-binding troponin molecule stuck to each tropomyosin

Thick Filaments

Contractile Protein

• made of 200-500 myosin molecules
• 2 entwined polypeptides (Golfclubs)
• Arranged in a bundle of with heads (cross bridges) directed outward in a spiral array around the bundled tails
• strategically placed heads to aid in muscle interaction, heads must be lined up in order
• middle = bare zone (no heads, only tails) where there is no interaction

Regulatory Protein

blocks active sites of Actin so that it cannot bind with myosin

Small, Calcium binding molecule that is stuck to each Tropomyosin molecule

Highly sensitive to calcium

contains a Tropin complex

Huge springy protein called titin 

runs through core of each thick filament

connects thick filament to Z disc structure

Elastic Filament

Functions: keep thick and thin filaments aligned with each other

resist overstretching

help the cell recoil to its resting length (elasticity)

Troponin + Tropomyosin = on/off switch

act like a switch that starts and stops shortening of muscle cell

*Ca++ is key*

the release of calcium into sarcoplasm and its binding to troponin activates contraction

Troponin moves the Tropomyosin off the actin active sites

Myosin and Actin

bond if they crossbridge

crossbridging = binding

myosin head binds with actin binding site

dark band regions alternating with lighter band regions

thick filament region

myosin filament

center of filament

area contains no think filament

no actin, only myosin

thin filament region

bisected by Z disc protein called connectin

one at each end of sarcomere

no myosin, only actin

makes up the actual Z line

anchors elastic and thin filaments

Z line = Z disc

from one Z disc (Z line) to the next

hundreds of myosin and actin make up one sarcomere

Muscle cells shorten because their individual sarcomeres shorten

Z discs are pulled closer together

Thick and Thin filaments don't change length during shortening

They overlap (sliding filament theory)-crossbridging

Z discs approximate one another

only myosin

SHRINKZ gets shorter

only actin

SHRINKZ gets shorter

ends

SHRINKZ gets shorter

anywhere there is myosin

SAME do not change shape

• Skeletal muscle must be stimulated by a nerve or it will not contract
• Cell bodies of somatic motor neurons are in brainstem or spinal cord
• Axons of somatic motor neurons are called somatic motor fibers
• each branches into 200 terminal branches that supply 1 muscle fiber each
• each motor neuron and all the muscle dibers it innervates are called a MOTOR UNIT

a motor neuron and the muscle fibers it innervates

dispersed throughout muscle

works in shifts- 1 motor unit stimulates 1 type of muscle fiber; when fatigued, other parts that have been resting can kick in

small motor unit

large motor unit

contain as few as 20 muscle fibers per nerve fiber

means less fibers per nerve

in fingers and fine detailed movements

more muscle fibers per nerve

able to accomplish larger tasks

gastrochemius muscle has 1,000 fibers per nerve fiber

region where a nerve fiber makes a functional connection with its muscle (NMJ)

Neurotransmitter (achetlycholine/ACh) released from the nerve fiber causes stimulation of the muscle cell

neuro = nerve

transmitter = meaning to propogate/transmit

swollen end of nerve fiber (contains ACh)

has synaptic vesicles with neurotransmitter

specialized region of muscle cell surface

has ACh receptors on membrane which bind ACh released from nerve; receptors are just waiting to receive ACh

AChase is an enzyme that breaks down ACh and causes relaxation

plasma membrane/husk that covers muscle fiber

attaches at all points to increase surface area

• Pesticides contain inhibitors that bind to AChase and prevent it from degrading ACh--> spastic paralysis and possible suffocation
• Tetanus or lockjaw is spastic paralysis caused by toxin of Clostridium bacteria- blocks glycine release in spinal cord and causes over stimulation/constant contraction of muscles
• Flaccid Paralysis with limp muscles unable to contract b/c currare takes up space where ACh should be ACh--> respiratory arrest