Unit 3: Muscle

what is a mucle

tissue specialized in converting biochemical reactions into mechanical work

2 main functions of mucles

generate motion and force

muscles can only __ and cannot _

contract, expand (except when pulled by an antagonistic muscle group)

3 types of muscle in the human body

skeletal, cardiac, smooth

function of skeletal muscle

attached to bone of the skeleton. responsible for positioning and movement of the body

how does contraction of skeletal muscle get triggered

by signal from somatic motor neuron; can’t initiate contraction on its own or be influenced by hormones

how does skeletal muscle look visually

has striations (stripes)

function of smooth muscle

found on stomach, blood vessels and bladder to move material throughout the body

how does smooth muscle look

no striations

function of cardiac muscle

found only on the heart → pumps blood around the body

how does cardiac muscle look visually

has striations

how does skeletal muscle attach to bones

tendons

what are tendons made of

dense connective tissue composed of collagen (cable-like fiber protein)

what is the outer layer of skeletal muscle

epimysium (connective tissue)

what is contained within the outer epimysium layer of skeletal muscle

bundles of fascicles

what is the outer layer of a fascicle

perimysium (connective tissue sheath)

what is contained in a fascicle

muscle fibers (cells)

what covers each muscle fiber within a fascicle

endomysium (connective tissue sheath)

what is found within a muscle fiber

• myofibrils (functional units of skeletal muscle)→ so many there is little room for other organelles

• glycogen granules (energy storage)

• mitochondria (ATP synthesis)

• SR

general structure of a muscle fiber

• long, cylindrical cell

• 100s of nuclei on the surface

sarcolemma

cell membrane of a muscle fiber

myofibril

highly organized bundles of contractile elastic protein found within muscle fibers

sarcoplasmic reticulum

specialized endoplasmic reticulum found within muscle cells

T tubules

transverse tubules—associate with the sarcoplasmic reticulum. they are tubes of lumen continuous with the ECF

what do T tubules do

sequester (store) Ca2+. allow for rapid action potential diffusion into muscle fiber

triad

2 terminal cisternae sandwiching a T tubule

terminal cisternae

enlarged regions of the sarcoplasmic reticulum (SR)

sarcoplasm

cytoplasm of a muscle cell

what 3 category of proteins are contained within myofibrils

contractile proteins, regulatory proteins and accessory proteins

what contractile proteins are found in myofibrils

actin and myosin

what regulatory proteins are found in myofibrils

troponin (bead-like) and tropomyosin (rope-like). they regulate muscle contraction

what accessory proteins are found in myofibrils

nebulin (aligns thin filament) and titin (elastic—returns stretched muscle to relaxed state)

contractile proteins: myosin

motor protein consisting 2 thick coiled protein molecules (make a head and tail region) joined by a flexible hinge. makes up thick filament

how is the coil of the myosin arranged

heads are at the ends (z disc), tails are together (m line)

contractile proteins: actin

forms the basis of thin filament with 2 F-actin chains twisted together

• subunit: G actin (globular)

• polymer: F-actin (filamentous)

thin filament

made of coiled F-actin (2 twisted strands) associated with regulatory proteins troponin and tropomyosin

sarcomere

repeating pattern units of striations. made of 2 Z discs and filament between them

Z-line (disks)

site of thin filament attachment

I band

isotropic (light). sarcomere region with only thin filament, has a Z disc running through it thus ½ of the I band is part of a different sarcomere

A band

anisotropic (dark). sarcomere region containing thick and thin filament overlapping at the outer edges and only thick filament in the center

H zone

part of the A band containing only thick filament (center region; lighter than outer edges)

M line

site of thick filament attachment—center of the sarcomere

what is muscle tension

force created by a contracting muscle

what is a load

weight/force that opposes contraction

do muscles elongate or shorten when they contract

shorten

who discovered sliding filament theory of contraction

huxley and rolf in the 50s → observed the a band length doesn’t shorten during contraction

sliding filament theory

during contraction, thick and thin filaments slide past each other with no change in the length of the filament itself → brings Z disks together

sliding filament theory: what changes during a contraction

sarcomere shortens, I band decreases, H zone decrease

sliding filament theory: what doesn’t change during a contraction

A band remains constant

what triggers contraction in skeletal muscle

only signals from the nervous system (somatic motor neurons)

excitation-contraction coupling

electrical and mechanical events that lead to muscle contraction

EPP

end plate potential → depolarization at the motor end plate (synapse of neuron and skeletal muscle) leading to action potential

what is the first step in the excitation contraction coupling series for skeletal muscle contraction

ACh released by neuron → ACh binds to nicotinic cholinergic receptors on motor end plate → Na and K channels open

what happens once the Na and K channels open during excitation contraction coupling of skeletal muscle

Na and K move across the membrane → net Na influx (bcz it exceeds K efflux) leading to EPP (depolarization at the synapse)

what happens once the EPP occurs during excitation contraction coupling of skeletal muscle

epp (action potential) moves down the T-tubule system → depolarization changes the conformation of DHP receptors → RyR conformation changes and opens

what happens once RyR receptors open during excitation contraction coupling of skeletal muscle

Ca2+ leaves the SR, increasing cytosolic Ca2+

what is DHP

dihydropyridine receptors, found in the T-tubule membrane. L-type calcium channels

what is RyR

stands for ryanodine receptors, they are Ca2+ channels in the sarcoplasmic reticulum. linked to DHP receptors

what happens after Ca leaves the SR during excitation contraction coupling of skeletal muscle

calcium binds to troponin on thin filament → moves tropomyosin and reveals actin binding sites → crossbridge cycle

what 2 components interact during the crossbridge cycle

myosin head and actin

cross bridge cycling: what is the first step

active sites on actin are exposed (Ca binds to troponin) → myosin head binds to actin binding site to form a weak crossbridge

cross bridge cycling: what happens once myosin and actin form a weak crossbridge

phosphate releases from myosin (from atp) → myosin head pivots toward the M line (power stroke) → thin filament moves inward

cross bridge cycling: what happens once the power stroke occurs

ADP gets released from myosin → myosin gets strongly bound to actin (rigor state)

what characterizes rigor state during the crossbridge cycle

no ATP/ADP bound to myosin, ONLY tightly bound to actin

cross bridge cycling: what happens once the rigor state occurs

new molecule of ATP attaches to the myosin head, allowing actin to detach

cross bridge cycling: what happens once a new atp attatches to the myosin and actin detaches

myosin head’s ATPase hydrolyzes ATP to ADP + P, returning myosin head to cocked position. if Ca is still bound to troponin, cycle starts again

do all cross bridges move simaultaneously

no, only about 50% of crossbridge attached at any given moment in contraction

how does skeletal muscle relaxation occur

Ca2+ gets pumped back into SR via Ca²⁺-ATPase (goes from low [ ] in cytoplasm to high [ ] in sr) → Ca2+ unbinds from troponin

what happens when Ca2+ unbinds from troponin

tropomyosin shifts back to ‘off’ and covers actin binding sites → elastic muscle elements pull filaments back to the relax position

what kind of molecule is ATP

nucleotide triphosphate

aerobic process

occurs in the presence of oxygen

anaerobical process

occurs in the absence of oxygen

glycolysis and ATP production

anaerobic and in cytoplasm, makes limited ATP (2), generates unwanted lactic acid

oxidative metabolism and ATP production

aerobic and in mitochondria, makes lots of ATP (15x more than glycolysis), doesn’t generate toxic products

what molecules can undergo glycolysis

blood glucose and muscle glycogen

what happens to the pyruvate produced from glycolysis

• anaerobically → becomes lactic acid in the blood

• aerobically → oxidative metabolism

what molecules can undergo oxidative metabolism

pyruvic acid (from glycolysis), fatty acids and blood glucose

creatine phosphate

• high energy phosphate molecule → quickly donates p to adp to make atp

• found in high [ ] in the muscles

• provides limited atp

what reaction does creatine phosphate undergo with ADP, which enzyme catalyzes is

creatine phosphate + ADP → ATP + creatine, catalyzed by creatine kinase (CK)

creatine kinase

enzyme → large amounts of it are in the muscles; resting muscles store energy as creatine phosphate

2 important terms of muscles contraction

twitch and latent period

twitch

single contraction-relaxation cycle

laten period

short delay between action potential stimulus and the beginning of the muscle tension → this is the time it takes for excitation-contraction coupling to occur

tension over time during a twitch

after stimulus (action potential), there is a latent period of no tension, then contraction period of growing tension, then relaxation period of falling tension

3 general types of muscle fibers

slow twitch oxidative fibers (type 1), fast twitch oxidative glycolytic fibers (type IIA) and fast twitch glycolytic fibers (IIX)

3 general types of muscle fibers: what does oxidative or glycolytic indicate in the name

refer to the primary source of ATP

3 general types of muscle fibers: what does fast or slow indicate in the name

refers to rate of myosin ATPase activity → fast fibers split ATP faster and develop tension faster

what causes some muscle fibers to be fast twitch and other to be slow

different isoforms of myosin lead to different rates of ATP hydrolysis

physical characteristics of oxidative fibers vs glycolytic fibers

oxidative fibers are smaller, have lots of mitochondria and have more blood vessels

myoglobin

oxygen-carrying haeme protein in oxidative fiber; causes these fibers to look red

length of twitch: fast vs short fibres

fast has shorter twitch → more twitches per unit time

what dictates whether fibers to have a short or long twitch

determined by the rate of removal of Ca2+ from the cytosol; fast twitch fibers have the highest rate of Ca2+ removal

what is short twitch duration good for

rapid small muscle contractions → typing

what is long twitch duration good for

long sustained movements → lifting heavy loads

what 2 factors influence the tension exerted by a single twitch

muscle type and sarcomere length before contraction

what 2 factors influence the tension exerted by a single twitch: muscle type

fast twitch generates more tension because of its structure

what 2 factors influence the tension exerted by a single twitch: sarcomere length at the start of contraction

refers to degree of filament overlap

• too little overlap → few cross bridges, little force

• too much overlap → actin filaments interfere with each other, less force

• way too much overlap → thick filaments collide with Z disks, force decreases

important notes about single twitch force and maximum force

single twitch force DOES NOT represent the max force the fibre can develop → force can be increased by increases ap rate

summation

increase in force generated by a muscle from repeated ap stimulation before muscle can fully relax

tetanus

state of a muscle when it reaches max force of contraction. can be unfused or fused