muscle physiology

three types of muscle tissue

skeletal, smooth, cardiac

which muscle is voluntary

skeletal muscle

which muscle is involuntary

cardiac and smooth muscle

contractility

the ability of muscle to shorten and produce force

excitability

the ability to respond to a stimulus

extensibility

the ability to be stretched

elasticity

the ability to return to its original length

order of muscle organization from largest to smallest

Whole muscle → fascicle → muscle fiber → myofibril → myofilament

fascicle

bundle of muscle fibers

muscle fiber

individual muscle cell

two main myofilaments

actin and myosin

thin myofilament

actin

thick myofilament

myosin

epimysium

surrounds each entire skeletal muscle

perimysium

surrounds each bundle of fascicles

endomysium

surrounds and separates muscle fibers in each fascicle

tendons

attach muscle to bone, skin, or another muscle

aponeurosis

thin, flattened sheet of dense irregular connective tissue

sarcolemma

plasma membrane of muscle fiber

sarcoplasm

cytoplasm of muscle fiber

main function of T-tubules

carry action potentials deep into the muscle fiber

main function of sarcoplasmic reticulum

store calcium ions

sarcomere

smallest functional unit of skeletal muscle

z disk

attachment site for actin myofilaments

l band

light staining regions each contain a Z disk, contains acting, extend to end of myosin myofilament

a band

darker staining regions, overlapping acting and myosin myofilaments

H zone

region in A band where actin and myosin do not overlap

M line

center of sarcolemma, delicate filaments holding myosin in place

titin filaments

elastic protein, helps hold myosin in place, helps muscle return to its resting length

tropomyosin

covers myosin binding sites in relaxed muscle

troponin

binding site for calcium

what happens when calcium binds to troponin

tropomyosin moves away from actin’s active sites

myosin heads

bind to active sites on actin molecules to form cross-bridges

what is a cross-bridge

connection formed when a myosin head attaches to actin

what provides energy for muscle contration

ATP; ATPase enzymes break down ATP and release energy

sliding filament theory

actin slides past myosin causing the sarcomere to shorten

neuromuscular junction

motor neuron communicaets with muscle fiber

acetylcholine (ACh)

neurotransmitter released by motor neuron

synaptic vesicles

small sacs in presynaptic terminal that contain acetylcholine

synaptic cleft

space between presynaptic terminal and muscle fiber

action potential

motor neurons carry electrical signals

what happens when ACh binds to receptors on muscle

sodium enters the muscle and action potential begins

cause of calcium to enter neuron

action potential opens voltage-gated calcium channels

causes ACh to be realeased

calcium entering motor neuron

what causes SR to release calcium

action potential traveling through T tubules

what does calcium bind to

troponin

what happens after tropomyosin moves

myosin attaches to actinw

power stroke

when myosin head bends and pulls actin

ligand-gated ion channels

open when specific neurotransmitter binds to a recepter

voltage-gated ion channel

open or close in response to a specific membrane potential in response to small voltage changes across membrane

what is the sequence of skeletal muscle contraction?

Nerve signal → ACh → Na⁺ enters → action potential → T tubules → Ca²⁺ released → Ca²⁺ binds troponin → tropomyosin moves → myosin attaches to actin → ATP provides energy → actin slides → contraction.

acetylcholinesterase

breaks down ACh

what happens to calcium during relaxation

pumped back into SR

what happens to tropomyosin during relaxation

it covers active sites on actin again

ATP needed for relaxation?

yes

resting membrane potential

charge difference in relaxed cell, but ready to respond

resting membrane results from three factors

1. more potassium inside membrane than outside

2. more sodium outside membrane than inside

3. plasma membrane more permeable to potassium than sodium

sodium-potassium pump

maintains arrangement of potassium and sodium— in resting cell, pump transports potassium from outside to inside cell and sodium from inside to outside the cell

action potentials

1. occur when excitable cell is stimulated

2. lasts approximately 1 millisecond

3. has two phases

   1. depolarization

   2. repolarization

depolarization

cell becomes less negatively charged (sodium enters cell)

repolarization

cell returns to resting state (potassium leaves cell)

muscle twitch

response of a muscle fiber to a single action potential along it’s motor neuron

three phases of muscle twitch

1. lag/latent phase: time between stimulation and contraction

2. contraction phase: calcium is released and cross-bridges form

3. relaxation phase: calcium returns to SR and muscle relaxes

isometric contraction

muscle tension increases but muscle does not change length

isotonic

muscle changes length while producing tension

motor unit

single motor neuron and all muscle fibers innervated by it (controlled by it)

large muscles

many muscle fibers— powerful movements

small muscles

few muscle fibers—precise movement

muscle recruitment

brain can increase muscle strength by activating more motor units

subthreshold stimulus

not strong enough to cause contraction

threshold stimulus

strong enough to cause action potential

submaximal stimuli

some motor units contract

maximal stimulus

all motor units contract

size principal

small motor units recruited first, large motor units recruited later as more force is needed

muscle tone

constant slight tension in muscles, some motor units always slightly active (helps maintain posture and stabilize joints)

slow twitch muscle fibers good for?

endurance and long-lasting activities

slow twitch muscle fibers

contract slower, better aerobic respiration, fatigue resistance, slow variant myosin ATPase, also known as red or type I fibers

slow twitch muscle fibers have many of?

mitochondria, blood vessels, myoglobin

myoglobin

stores oxygen in muscle cells

fast twitch muscle fibers good for?

speed and powerful movements

slow twitch muscle fibers

contract faster, more power, less fatigue resistance, fast variant ATPase, quick responses, white fibers

slow twitch muscle fibers have less of

mitochondria, blood supply

hypertrophy

increase in muscle fiber size

atrophy

decrease in muscle fiber size

heat production

three stages

1. exercise: metabolic rate and heat production increase

2. post-exercise: metabolic rate stays high due to oxygen debt, excess heat lost due to vasodilation and sweating

3. shivering: uncoordinated contraction of muscle fibers resulting in shaking

main energy source for muscles

ATP

transfer energy system

transfer of creatine phosphate by enzyme creatine kinase from ADP to form ATP

anareobic respiration

energy system production that does not require oxygen but produces ATP and lactic acid QUICKLY, good for short, intense exercise

aerobic respiration

energy system production that requires oxygen, occurs mainly in mitochondria and produces much more ATP

which produces more ATP?

aerobic respiration

muscle fatique

a decrease in muscles ability to contract effectively

acidosis and ATP depletion

type of muscle fatigue from increased ATP or decrease in ATP production

oxidative stress

type of muscle fatigue from the buildup of excess reactive oxygen species (ROS; free radicals)

physiological contracture

type of muscle fatique where too little ATP to bind to myosin myofilaments, muscle cannot contract or relax

physiological fatique

muscle fatigue, most common, nervous system tells you that you cannot use muscle any longer

rigor mortis

rigid muscles several hours after death, without ATP myosin cannot detach from actin. Similar to physiological contracture

smooth muscle is found in?

walls of hollow organs and blood vessels

smooth muscle

not striated, spindle fiber shaped, one nucleus, involuntary, contracts slower

calmodulin

instead of troponin, calcium binds to calmodulin in smooth muscle

cardiac muscle is found in?

the heart