Muscle Physiology and Intro to Muscular system

skeletal, cardiac, and smooth

Three types of muscle tissue

Generating force (muscle tension)

basic function of all muscle tissue

Myocyte

muscle cell

Sarcoplasm

myocyte’s cytoplasm

Sarcolemma

myocyte’s plasma membrane

Sarcoplasmic reticulum (SR)

modified endoplasmic reticulum; surrounds myofibrils; stores and releases calcium ions

Myofibrrils

cylindrical organelles; bundles of specialized proteins; allow for contraction; other organelles (such as mitochondria) are packed between myofibrils; made up of mostly contractile proteins

structure of the skeletal muscle fiber

skeletal muscle tissue consists of many fibers and surrounding endomysium (extracellular matrix); Fibers are thin cylinders

Transverse tubules (T-tubules)

extensions of sarcolemma; surround each myofibril; form tunnel-like network within muscle fibers

Terminal cisternae

enlarged sections of SR; flank each T-tubule

structure of the myofibril

each myofibril contains hundreds to thousands of myofilaments

Thick filaments

bundles of contractile protein myosin; globular heads at each end linked by intertwining tails; each head has active site that binds with actin

Thin filaments

proteins actin, tropomyosin, and troponin; Two intertwined strings of actin in thin filament; each bead-shaped actin has active site- binds with myosin heads

Elastic filaments

spring-like structural protein (titin); stabilizes myofibril structure; resists excessive stretching

Tropomyosin

rope-like protein that twists around each actin and covers active sites

Troponin

small protein that holds tropomyosin in place; assists with turning contractions on and off

I band

only thin filaments

Z disc

in middle of I band; anchor thin filaments in place to one another and serve as attachment points for elastic filaments

A band

contains overlap of thick and thin filaments

H zone

middle of A band where only thick filaments exist

M line

dark line in middle of A band; proteins hold thick filaments in place

form a fascicle

multiple muscle fibers (each surrounded by endomysium)

perimysium

each fascicle is surrounded by perimysium

a skeletal muscle

bundles of fascicles make up this (surrounded by epimysium)

Perimysium and epimysium

what comes together to form a tendon

Fascia

skeletal muscles are surrounded by (anchors them to surrounding tissues; holds groups of muscles together)

Sliding filament mechanism

Myosin heads attach to actin; pull thin filaments toward M line; brings Z-discs closer together (shorten sarcomere); both I band and H zone narrow; A band unchanged; Sarcomeres are arranged end to end within each myofibril; when simultaneously contracted, shorten whole muscle fiber

-85mV

Resting membrane potential of a muscle fiber

Na+/ K+ pump

3 Sodium (Na) ions for every two Potassium (K+), per ATP (active transport)

Action potentials

A brief change in the membrane potential; generated by opening and closing of Na+ and K+ channels in the plasma membrane in response to stimuli

K+ and Na+ movement

Na+ into the cell while K+ moves out of the cell

Depolarization

Na+ channels open and Na+ flows into the cell; membrane potential becomes less negative

Repolarization

Na+ channels close, K+ channels open and K+ flows out of the cell; membrane potential becomes more negative and K+ channels close once cell reaches resting membrane potential

innervated (connected to a neuron)

All skeletal muscles are… (each connection is a synapse)

Neuromuscular Junction (NMJ)

Synapse where a single motor neuron communicates with many muscle fibers; transmits the action potential from neuron to sarcolemma

Axon terminal, Synaptic cleft, and motor end plate

Three components of neuromuscular junction

Axon terminal

synaptic vesicles filled with acetylcholine (ACh)

Synaptic cleft

space between axon terminal and muscle fiber

Motor end plate

specialized region of muscle fiber plasma membrane; folded surface has ligand gated Na+ channels; ACh is a ligand that opens gates, allowing Na+ to diffuse into muscle cell

Step 1: Excitation

Action potential from brain or spinal cord arrives at synaptic terminal of motor neuron; ACh is released into synaptic cleft and binds to ligand-gated ion channels of motor end plate; Ligand gated channels open and Na+ ions flow into the muscle fiber, generating end-plate potential (depolarization of motor end plate)

Step 2: Excitation-contraction coupling

End-plate potential (depolarization of motor end plate) opens voltage gated Na+ channels in sarcolemma triggers an action potential; Action potentials propagate- depolarization of one area of membrane triggers next few channels to open (like a chain reaction); Action potential signals terminal cisternae to open voltage-gated Ca2+ channels, releasing Ca2+ into the cytosol of the muscle fiber.

Step 2a: In preparation for muscle contraction

Calcium released from terminal cisternae binds to troponin; When calcium binds to troponin it triggers tropomyosin to move off actin and expose the active sites

Step 3: Contraction

Actin’s active site is exposed, initiating crossbridge cycle: Myosin head cocked once ATP is bound; Myosin head is now able to bind to active site of actin (crossbridge); ATP’s energy is harnessed causing myosin heads to pull actin toward M line (power stroke) then it pivots to a relaxed position; Myosin can bind to another ATP and break the link with the actin active site; Detachment of myosin head from actin does not allow thin filament to slide backward because some myosin heads are still attached to actin

Contraction cycle

May repeat as long as stimulus to contract continues and ATP is available; Myosin head recocked, binds to first actin molecule, and power stroke repeats, Myosin binds to second actin, and so on over, and over

Isotonic contractions

tensions generated is constant, but muscle length changes

Isotonic eccentric contractions

constant tension but muscle lengthens

Isotonic concentric contractions

constant tension while muscle shortens

Isometric contractions

Muscle length remains unchanged because external force applied equals that generated by muscle

Skeletal Muscle Relaxation

Acetylcholinesterase degrades

remaining Ach; ligand-gated Na+

channels close; end plate potential

ends; final repolarization begins

2. Sarcolemma returns to resting

membrane potential

3. Ca2+ ions pumped back into

sarcoplasmic reticulum ; returns

cytosol concentration to resting level

4. In absence of calcium, troponin

and tropomyosin block active sites

of actin, and muscle relaxes;

myofilaments slide back into original

positions

• Motor neuron action potentials stop signaling for release of acetylcholine from

axon terminals

Endomysium

thin connective tissue layer surrounding each individual muscle cell (fiber)

Perimysium

connective tissue layer surrounding each fascicle

Fascicle

several (between 10-100) muscle cells bundled together

Epimysium

fibrous connective tissue wrapping around all fascicles

Fascia

most superficial connective tissue sheath; continuous with epimysium

Parallel

strap-like muscle; evenly spaced fascicles; muscle and tendon are same width

Convergent

broad triangular-shaped muscle that tapers down into single tendon

Pennate

fascicles attach to tendon at angle; feather-like appearance

Unipennate

fascicles only attached to one side of associated tendon

Bipennate

fascicles attached to both sides of associated tendon

Multipennate

several regions of fascicles joined by connective tissue; each section contributes to form single tendon

Sphincters

circular fascicle arrangements; surround body openings; provide voluntary control over defecation and urination

Spiral

muscles that wrap around another structure (bone)

Fusiform

muscle midsection (belly) is thicker than each tapered end

Muscle origin and insertion

skeletal muscles begin and end at distinct anatomical locations

origin

anchoring point on bone, where skeletal muscle “originates from”; typically not involved directly with movement of joint

Insertion

moving end of muscle whose tendon attaches to bone or other structures; usually on far side of joint