anatomy: muscle functions and types; muscular system: overview and contraction

movement, posture and position, stabilize, heat production

primary function of muscles

movement

responsible for locomotion(move the skeletal system), responsible for organ manipulation

posture and position

muscles function nearly continuously, small adjusting movements

stabilize

while moving joints, stabilization occurs to maintain bony articulation

heat production

generated with contration, plays a role in maintaining normal body temperature

skeletal, smooth, cardiac

muscle tissue types

myo, mys

muscle

sarco

flesh

epimysium, perimysium, fasicle, muscle fiber, endomysium, sarcolemma, myofibril

gross anatomy of skeletal muscles

myofibrils

rod-like contractile elements, “banded” appearance, made of sarcomeres

muscle fibers

elongated multinucleate cell, “banded” appearance, made of myofibrils, surrounded by endomysium(wrap sheath of areolar connective tissue)

fasicles

discrete bundle of muscle fibers, surrounded by perimysium(dense irregular connective tissue)

muscle

hundreds of thousands of fasicles, includes: blood vessels and nerves, surrounded by epimysium(dense irregular connective tissue)

direct and indirect

connecting to the skeleton

direct connection

“fleshy” attachment, epimysium is fused to the periosteum/pericondrium

indirect connection

tendon, apneurosis, can anchor to bone, cartilage and other muscles; more common

tendon

rope like anchor

aponeurosis

sheet anchor

sarcomere

contractile unit; composed of myofilaments made up of contractile proteins'; 1=z-disc to z-disc

myosin

splits ATP

titin

forms elastic filament

actin

binding site for myosin

tropomyosin

stabilize actin; block myosin binding sites(relaxed)

troponin

helps position tropomyosin and binds Ca+ ions

thick filament

many myosin molecules; around 300 heads

thin filament

two strands of actin twisted into a helix and 2 types of regulatory proteins

sarcomere zones

I band, H zone, M line, Outer edge of A band

I band

thin filaments only

H zone

thick filaments only

M line

thick filaments linked by accessory proteins

outer edge of A band

thick and thin band filaments

sarcoplasmic reticulum

elaborate smooth endoplasmic reticulum, loosley weaves around each myofibril(elongated sacs are terminated cisterns(always come in pairs), stores calcium

T tubules

formed by the sarcolemma, runs between terminal cisterns creating triads, allow for nerve stimulation to reach even the deepest sarcomeres

excitability, extensibility, elasticity, conductivity, contractility

universal characteristics of muscle

motor unit

one motor neuron and all the muscle fibers it innervates

motor neurons

aka somatic motor neurons and efferent somatic neurons, axon splits as it enters the muscle, split branches form the neuromuscular junction(motor end plate)

neuromuscular junction

only one per muscle fiber( around middle fiber), contain multiple axon terminals, very close to the muscle but technically separate(synaptic cleft)

muscle contration

motor neuron fires, release of ACh, ACh binds, depolarization; action potential triggered down the sarcolemma; AP travels to T tubules, release of Ca2+, Ca2+ binds to troponin; contraction

neuromuscular junction

synaptic vesicles(small membranous sacs), contains neurotransmitter acetylcholine(ACh), very close to the muscle but technically separate(synaptic cleft)

first step of neuromuscular junction

action potential arrives at axon terminal

second step of neuromuscular junction

voltage gated Ca2+ channels open. Ca2+ enters the axon terminal, moving fown the electrochemical gradient

third step of neuromuscular junction

Ca2+ entry causes ACh to be released

fourth step of neuromuscular junction

ACh diffuses across the synaptic cleft and binds to ACh receptors on the sarcolemma

fifth step of neuromuscular junction

ion channels now open to allow passage of Na+ into the muscle fiber and K+ out of the muscle fiber. (Na+>K+) results in depolarization(end plate potential)

sixth step of neuromuscular junction

Ach breaks down and is sweapt away from the junction(ion channels close)

action potential across the sarcolemma

once initiated, sweeps across entire sarcolemma; 3 phases: generation of end plate potential, 2. action potential depolarization, 3. repolarization

action potential

result of a predictable sequence of electrical changes

first step in excitation-contraction coupling

the action potential propagates along the sarcolemma and down the T tubules

second step in excitation-contraction coupling

as the ATP moves down the T-tubules, they change shape (voltage sensitive). change in shape leads to release of Ca2+ from the terminal cisterns, creating an increase of Ca2+ in the cytosol

third step in excitation-contraction coupling

Ca2+ binds to troponin causing it to change shape, and removes the blocking action of tropomyosin. results in the myosin binding sites of the actin to be exposed

fourth step in excitation-contraction coupling

contraction begins as the myosin heads are now able to form cross-bridges

sliding filament model of contraction

1. nervous system stimulates the muscle, 2. a cross bridge is formed, 3. thin filaments propelled towards center, 4. muscle cell shortens

cross bridge cycle

1. myosin head attaches to actin(cross bridge), 2. ADP and Pi are released(results in myosin head movement and flex of the cross bridge), 3. actin slides relative to myosin, 4. ATP attaches to myosin(releases from actin)

energy for muscle contraction

ATP

direct phosphorylation

no oxygen use; 1 ATP per CP, creatine made; 15 seconds of energy

anaerobic pathway

no oxygen use; 2 ATP per glucose and lactic acid made; 30-40 seconds of energy provided

aerobic pathway

oxygen use required; 32 ATP per glucose made, CO2, H2O; energy lasts hours

force

directly correlated to the number of myosin cross bridges that are attached to actin; four factors contribute to the strength of a muscle contraction(frequency, number, size, stretch)

velocity and duration

“how fast” and “how long”; three factors contribute to velocity and duration of muscle contraction(type, load, recruitment)

slow oxidative(type I) and fast glycolytic(type II)

subtypes of skeletal muscle

slow oxidative(type I)

endurance; smaller motor units; weaker motor unit strength; smaller relative diameter; aerobic ATP synthesis; good fatigue resistance; abundant and large mitochondria

fast glycolytic(type II)

quick/fast; larger motor unit size; stronger motor unit strength; larger diameter; anaerobic ATP synthesis; poor fatigue resistance; fewer and smaller mitchondria