A&P: Chapter 11: Muscle

excitability (responsiveness)

respond to stimulus

• to chemical signals, stretch, and electrical changes across the plasma membrane

conductivity

stimulus transmitted from cell to cell to cell …

• local electrical excitation sets off a wave of excitation that travels along the muscle fiber

• NOT IN SKELETAL MUSCLE (must be connected to a nerve cell); only cardiac & smooth (gap junctions)

contractility

shortens when stimulated

extensibility

capable of being stretched (w/out damage) between contractions

elasticity

returns to its original rest length after being stretched 

Endomysium

connective tissue around muscle cell 

• inner most layer

• on top of sarcolemma 

Perimysium

connective tissue around muscle fascicle

Epimysium

connective tissue surrounding entire muscle 

myoglobin

• red pigment- gives red color

• provides some oxygen needed for muscle activity

Aponeurosis

flat tendon, connected to fat muscles

myofibril

• organelle present within a muscle cell

• long protein cords occupying most of sarcoplasm

myofilament

contractile protein of organelle

sarcolemma

plasma membrane of a muscle fiber

sarcoplasm

• cytoplasm of a muscle fiber

• flooded with calcium

• attached to TnC

glycogen

carbohydrate stored to provide energy for exercise 

Multiple nuclei

flattened nuclei pressed against the inside of the sarcolemma

Mitochondria

packed into spaces between myofibrils 

Sarcoplasmic reticulum (SR)

smooth ER that forms a network around each myofibril

• functions: storage, conc. of calcium

  • releases calcium through channels to activate contraction 

• Terminal cisterns

Terminal cisterns

dilated end-sacs of SR which cross the muscle fiber from one side to the other

T tubules

tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side 

• continuous with sarcolemma 

• transfers action potentials deep into the cell 

Triad

a T tubule and two terminal cisterns associated with it 

needed for muscle contraction

calcium 

resting cell membrane potential

intracellular memb. is slightly negative

hyperpolarization

polarity increases (more negative)

depolarization

polarity decreases (more positive)

thick filaments

made of several hundred myosin molecules

• shaped like a golf club 

  • two chains intertwined to form a shaft-like tail

  • double globular head

• heads directed outward in a helical array around the bundle

  • heads on one half of the thick filament angle to the left, while heads on the other half angle to the right

  • bare zone with no heads in the middle 

head of myosin

1. active binding site for actin (high affinity site) 

2. binding sites for ATP 

3. are an ATPase (ATP is broken down to ADP + energy)  

extends

high energy state of head (movement)

bends

low energy state of head (movement)

thin filaments

structural & regulatory proteins

Fibrous (F) actin

two intertwined strands

• string of globular (G) actin subunits  each with an active site that can bind to head of myosin molecule 

Tropomyosin molecules

each blocking six or seven active sites on G actin subunits

• fibrous protein, parallel to F actin 

• coveres myosin binding site → prevents myosin + actin from attaching 

Troponin molecule

small, calcium-binding protein on each tropomyosin molecule 

• has 3 subunits (TnT, TnC, TnI)

cross bridge formation

1. calcium binds

2. troponin changes shape 

3. pull tropomyosin off actin’s binding site

4. myosin can attack 

TnT

tropomyosin 

TnC

binds to calcium, comes from terminal cisterae

TnI

binds to G-actin, inhibitory

dystrophin

• links actin in outermost myofilaments to membrane proteins that link to endomysium 

• transfers forces of muscle contraction to connective tissue ultimately leading to tendon 

• genetic defects in dystrophin produce disabling disease muscular dystrophy (MD) 

  • deficient dystrophin 

striations

• result from the precise organization of myosin and actin in cardiac and skeletal muscle cells 

• altering A-bands (dark) and I-bands (light) 

A band

dark, anisotropic

• darkest part where thick filaments overlap a hexagonal array of thin filaments 

• H band: not as dark; middle of A band; thick filaments only 

• M line: middle of H band 

I band

light, isotropic

• the way the bands reflect polarized light 

• Z disc: provides anchorage for thin filaments and elastic filaments 

  • Bisects I band

sarcomere

segment from Z disc to Z disc

• functional contractile unit of muscle fiber

synapse

point where a nerve fiber meets its target cell

• connection of neuron to another cell

Neuromuscular junction (NMJ)

when target cell is a muscle fiber 

Axon Terminal

swollen end of nerve fiber

• contains synaptic vesicles with acetylcholine (ACh)

synaptic cleft

gap between axon terminal and sarcolemma

motor endplate

area of sarcolemma where folds are in close proximity

excitation

process in which nerve action potentials lead to muscle action potentials 

excitation-contraction coupling

events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract

contraction

step in which the muscle fiber develops tension and may shorten

relaxation

when stimulation ends, a muscle fiber relaxes and returns to its resting length, stopping interaction 

Action potential (nerve impulse)

1. depolarization 

2. repolarization 

3. hyperpolarization

spastic paralysis

state of continual contraction (stimulated and stay contracted) of the muscles; possible suffocation

Tetanus (lockjaw)

a form of spastic paralysis caused by toxin Clostridium tetani

• not due to synaptic influence

Flaccid paralysis

a state in which the muscles are limp and cannot contract

• paralyze target’s competitive inhibitor, ACh can’t activate channels & stimulate muscle

botulism (botox)

type of food poisoning caused by neuromuscular toxin secreted by the bacterium Clostridium botulinum

• blocks release of ACh causing flaccid paralysis (doesn’t attach to receptors, muscle ins’t stimulated)

• cosmetic injections used for wrinkle removal 

rigor mortis

hardening of muscles and stiffening of body beginning 3-4 hours after death

• cross bridge forms, can’t separate bc ATP is used up 

• peaks about 12 hours after death, the diminishes over the next 48-60 hours

Length-tension relationship

the amount of tension generated by a muscle depends on how stretched or shortened it was before it was stimulated 

weak contraction

result of an overly shortened or too stretched muscle before it was stimulated 

myogram

a chart of the timing and strength of a muscle’s contraction

• activity of muscle plotted on graph 

Treshold

minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction 

sub-threshold

subminimal, no muscles stimulated= no contraction

twitch

a quick cycle of contraction and relaxation when stimulus is at threshold or higher

• muscle response to single stimulus

• 7-100 ms 

maximal stimulus

all muscle and muscle fibers stimulated, provides the max amount of force it can 

beyond maximal stimulus

doesn’t change force of contraction

latent period

very brief delay between stimulus and contraction

• time required for excitation and tensing of elastic components of muscle

contraction phase

time when muscle generates external tension

• force generated can overcome the load and cause movement

relaxation phase

time when tension declines to baseline

• SR reabsorbs Ca²⁺, myosin releases actin and tension decreases

• takes longer than contraction

• cross bridge cycle diminishes

motor unit

motor neuron and all the muscle fibers it innerrates

identical twitches

low frequency stimuli produce…

temporal (wave) summation

higher frequency stimuli (ex: 20 stimuli/s) produce …

treppe (staircase effect)

each new twitch rides on the previous one generating higher tension

complete (fused) tetanus

unnaturally high stimulus frequencies cause a steady contraction

• no relaxation in between

• smooth contraction

• muscle stimulated at high rate

isometric muscle contraction

length of muscle stays same while muscle contracts, muscles have to contract to maintain posture

• muscle produces internal tension but external resistance causes it to stay the same length

• can be a prelude to movement when tension is absorbed by elastic component of muscle

• important in postural muscle function and antagonistic muscle join stabilitation

isotonic muscle contraction

force stays same as muscle contracts

• muscle changes in length with no change in tension 

concentric contraction

muscle shortens as it maintains tension (ex: lifting weight)

eccentric contraction

muscle lengthens as it maintains tension (ex: slowly lowering weight

myokinase and creatine kinase

enzyme systems that control phosphate transfers

myokinase

transfers phosphate from one ADP to another, converting the latter to ATP

creatine kinase

obtains phosphate from a phosphate-storage molecule creatine phosphate (CP) and gives it to ADP

Phosphagen system

the combination of ATP and CP which provides nearly all energy for short bursts of activity

• after stored ATP is used up 

• enough energy for 6 seconds of sprinting 

anaerobic threshold (lactate threshold)

point at which lactate becomes detectable in the blood

• faster but less efficient

• produces enough ATP for 30-40 seconds of maximum activity

lactic acid

stalls conversion of glucose to ATP

aerobic respiration 

produces more ATP per glucose than glycolysis does (another 30 ATP per glucose) 

• more efficient for prolonged exercise 

excess postexercise oxygen consumption (EPOC)

• aka oxygen debt 

• returns cells back to normal oxygen rate 

  • replenishes ATP sources → convert back to resting state

• needed for: 

  • aerobically replenish ATP 

  • replace oxygen reserves on myoglobin

  • provide oxygen to many cells that have elevated metabolic rates after exercise

• can be 6x basal consumption and last an hour (why we heavy breathe after exercising)