Chap 11 Muscular Tissue

Excitabilty (responsiveness)

ability to respond to change in enviornment. typically a chemical signal (pH change or neurotrasmitter)

conductivity

movement of an impulse from 1 part of the cell to another

contractility

ability to forcibly shorten w/o damage

extensibility

ability to stretch w/o damage

elasticity

recoil to shorten length when relaxed

sarcolemma

muscle cell membrane

sarcoplasm

muscle cell cytoplasm. contains typical organelles but also myofibrils, glycogen, myoglobin, and many many mitochondria

myofibrils

functional unit of muscle cell. long cord of protein. 1 muscle fiber contains 100s to 1000s of myofibrils. tightly packed together and make up 80% of muscle volume. organelles are fit between myofibrils. myofibrils contain myofilaments

glycogen

storage form of glucose held within the sarcoplasm in glycosomes. used during intense activity

myoglobin

oxygen storage molecule. has pigment for red color of muscle

sarcoplasmic reticulum (SR)

smooth endoplasmic reticulum that forms network around each myofibril. this network consists of terminal cisternae and t tubles which create a triad.

terminal cisternae

dialated end-sacs of SR which cross the muscle fiber from 1 side to the other. these store calcium to be released in order to activate contractions.

t tubules

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

triad

1 T-tubule and 2 terminal cisternae

myofilaments

The contractile proteins, actin and myosin, of muscle cells. there are thick and thin types. also regulatory proteins

regulatory proteins

tropomyosin and troponin. these dont contract but instead regulate contractions to allow or not allow them to happen. exclusive to thin myofilaments

thick filaments

made of myosin. has 2 heads attached to hinge which is connected to a tail. heads contain actin binding sites that form cross bridges with actin of thin filaments. tails make it thick.

tropomyosin

regulatory protein that stabilizes actin filaments. in a relaxed muscle, tropomyosin covers binding site for actin and myosin

tropoin

sits on tropomyosin. bonds actin, tropomyosin, and calcium. when calcium binds to troponin, tropomyosin in released and it moves, exposing the actin binding site.

excitation

nerve action potential gets converted to muscle action potential.

nerve action potential causes calcium voltage gated channels to open and calcium flows in. ACh releases into synaptic cleft. Ach receptors in the sarcolemma open and let sodium and potassium move down their electrochemical gradients. NA enters the muscle cell and K leaves. this creates a voltage change that becomes a chain reaction of conductivity, making all Na / K channels on sarcoplasm open. eventually the reaction will go down a T-Tubule and lead into the next step, excitation-contraction coupling.

thin filament

composed of actin which will bind to myosin. made of g actin, which is a singular protein. f actin is a string of g actin. 1 thin filament is 2 f actins wrapped around each other. also has regulatory filaments tropomyosin and troponin

Titin (elastic filaments)

large, spring-like protein runs through thick filament and anchors it to the z disk and m line

Sarcomere

Contractile unit of muscle. distance from 1 z disk to another

what are striations caused by in skeletal muscle

actin and myosin (thick and thin filaments)

A band

dark area where thick and thin filaments overlap

H band

thick filaments only

I band

light area where no thick filaments are

M line

found in the middle of the H band. where protein links thick filaments

Z disk

dense region that separates one sarcomere from another. in the middle of the I band

sliding filament theory

muscle will contract when thick pulls thin filaments, shortening the sarcomere. thick filament myosin heads grab onto thin filament actin binding sites and pull the thin filament toward the M line. this shortens the sarcomere and creates overlap between thick and thin filaments

Structual hierarchy of muscle

Largest to smallest. Muscle, fascicle, fiber, myofibril, sarcomere, myofilaments

neuromuscular junction (NMJ)

type of synapse. connection points between the nervous and muscular systems. one nerve fiber articulates with muscle fiber at multiple points to ensure entire muscle is stimulated. this increases the speed of the reaction

contents of NMJ

synaptic knob, synaptic cleft, basal lamina

synapse

point where a nerve fiber meets its target cell

synaptic knob

Swollen end of a nerve fiber. Contains synaptic vesicles filled with acetylcholine (ACh)

Acetylcholine (ACh)

neurotransmitter contained within synaptic knob

synaptic cleft

gap between the synaptic knob and sarcolemma.

basal lamina

mat of collagen and glycoprotein that isolates the NMJ from surrounding tissue. contains acetylcholinesterase enzyme.

acetylcholinesterase

enzyme that breaks down acetylcholine to allow for fiber relaxation. stored in basal lamina

active site

The part of an enzyme or antibody where the chemical reaction occurs. on the filaments, this occurs at the actin binding site found on the heads of thick filament.

actin binding site

place on the myosin head that physically makes contact with actin to create cross bridges

steps of NMJ communication

1. A nerve impulse will cause the release of ACh via exocytosis

2. ACh will diffuse across the synaptic cleft and bind to receptos on surface of sarcolemma.

3. Binding of neurotransmitter ACh initializes an electical signal on the sarcolemma which causes contraction of the muscle fiber

Fibrous actin (F actin)

entire string of g actin. 2 wrapped around each other creates a thin filament

Globular actin (G-actin)

individual proteins. many g actins create 1 f actin string

synaptic vesicles*

Membrane-bounded compartments in which synthesized neurotransmitters are kept, such as ACh

Depolarization

When stimulated, sodium channels open and let Na+ flow down its gradient into the cell. this turns the normal 90 mV to +75 MV

Repolarization

K+ channels open letting K+ out of the cell. returns the cell back to resting membrane potential at -90 mV

resting membrane potential*

-90 mV, muscle relaxed, ICF negative, ECF positive

electrochemical gradient*

The combination of forces that acts on membrane potential. Electro comes from elements moving to opposite charges, chemical comes from elements moving down their gradient.

electrical potential*

potential energy due differences in positive and negative charges between the ECF and ICF respectively

polarized*

membranes of muscles and nerves are polarized because one side of the cell is positive (ECF) and the other is negative (ICF)

action potential*

a neural impulse; a brief electrical charge that travels down an axon. relationship between depolarization and repolarization

Electrophysiology*

the study of the electrical activity of cells

excitation-contraction coupling

events that link the excitation of muscle are translated to myofilaments, preparing them for contraction.

action potential from the excitation stage travels down the T-Tubule and into the cell, causing calcium channels from the SR to open. Ca releases and binds to troponin on thin filaments, moving the troponin-tropomyosin complex. this exposes the active sites on actin and allows myosin heads to grab the actin.

end plate potential (EPP)

the rapid depolarization and repolarization as Na enters the cell, depolarizing it at +75 mV, and K leaves the cell, repolarizing it at -90 mV.

ACh receptors

proteins on the sarcolemma that bind 2 ACh molecules and allows Na / K to flow in and out respectively, leading to depolarization + repolarization which is end plate potential. also creates conductivity of voltage along the sarcoplasm which opens Na / K channels.

power stroke

action of myosin pulling actin inward toward the M line. myosin releases ADP and Pi, flexing its head back, thereby pulling the thin filament with it. 5 per second. 1 power stroke is 1 ATP molecule used

recovery stroke

Return of the myosin head to its original position after cross-bridge release and ATP is rebound to myosin head

contraction cycling

ATPase enzyme in the myosin head hydrolyzes (breaks down) ATP molecule into ADP and inorganic phosphate (Pi). This activates the head and cocks it, expanding it further out. head binds to actin active site, forming a myosin-actin cross-bridge. once cross bridge is formed, ADP and Pi come off of myosin head, causing it to reset and thereby pull the actin, contracting the muscle toward the m line. this can be repeated when more ATP is bound to myosin head, its hydrolyzed, etc.

cross-bridge

The connection of a myosin head group to an actin filament during muscle contraction (the sliding filament theory).

relaxation

nerve stimulation and Ach release stops, AChE breaks down ACh left and fragments are reabsorbed into knob. Stimulation by ACh stops. Calcium is pumped back into SR via active transport, binding to calsequestrin while in storage in the SR. Tropomyosin blocks active sites, and finally the muscle fiber returns to rest length due to recoil of elastic components and contraction of antagonistic muscles.

Basic steps of contraction

1. Nerve signal causes ACh to release into synaptic cleft

2. ACh receptors in sarcolemma open and let Na and K move down their electrochemical gradients

3. Chain reaction of depolarization / repolarization down the sarcolemma

4. Reaction travels down T-Tubule and releases Ca from SR

5. Ca binds to troponin once in cytosol of cell

6. Troponin-Tropmyosin complex moves and reveals the active site for myosin to bind to on actin

7. Using ATP, myosin pulls on actin, shortening the sarcomere

8. When nerve signal stops, all of the above actions are reversed

Calsequestrin

calcium-binding protein within the sarcoplasmic reticulum which aids in storage of intracellular Ca2+

motor units

a single nerve cell and all of the muscles it services. 1 motor unit supplies about 200 muscles fibers, however there are small and large types that differ. muscle fibers are dispersed throughout the cell, contract in unison, and produce a weak contraction over wide area. effective contractions require many motor units to contract simultaneously.

small motor units

3-6 muscle fibers per neuron. precise movements. hand and eye muscles

large motor units

1000 muscle fibers per neuron. fast and forceful movement that lacks precision in exchange for strength. leg muscles

twitch

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

threshold

minimum amount of a stimulus that is required for any muscle contraction to occur. too little stimulation does not meet threshold and thus the muscle cannot contract.

how do muscles contract with variable strength?

1) stimulating the nerves of motor units with higher voltage. higher voltage excites more nerve cells which stimulate motor units to contract and vice versa. multiple motor unit summation

2) higher frequency of stimulations increases contraction strength. low stimuli produce identical twitches that have little power. high-frequency stimuli produce temporal / wave summation

strongest contractions will combine high voltage and high frequency. (MMU + Temporal summation)

temporal/ wave summation

high frequency twitching that allows for each twitch to ride off of the power of the last, only allowing for partial relaxation. this generates higher tension

Recruitment (multiple motor unit summation)

process of utilizing more motor units to strengthen a contraction. occurs due to higher voltage. follows size principle:

weak, low voltage stimuli recruit small motor units

strong, high voltage stimuli recruit small AND large motor units for powerful movements

what causes stronger twitches

warmer temps and proper hydration

modes of ATP synthesis required for muscle contractions

anaerobic fermentation and aerobic respiration

anaerobic fermentation

used during intense activity. short-term energy. enables cells to produce ATP in absence of oxygen through glycolysis. this yields little ATP in contrast to aerobic respiration and builds up lactic acid to be eliminated by the liver. 2 ATP per glucose

aerobic respiration

used during regular to moderate activity. long-term energy. requires a constant supply of oxygen to generate lots of ATP without lactic acid generation. 30-38 ATP per glucose

phosphagen system

myokinase and creatine kinase control phosphate transfer in muscle cells. combined efforts create enough ATP to power 6 seconds of high activity such as a sprint before muscles switch to aerobic fermentation to get their ATP

myokinase

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

creatine kinase

obtains Pi from creatine phosphate molecules (CP) and gives it to ADP, making it ATP. also creates creatine

Muscle Fatiuge

loss of ability to contract due to prolonged use. cause of fatigue depends on excercise type

cause of muscle fatigue in high intensity results from

- potassium accumulation in the ECF which reduces excitability of fibers

- excess ADP and Pi slow cross bridge movements (myosin/ actin interaction)

cause of muscle fatigue in low intensity results from

- fuel depletion as glycogen and glucose decline

- electrolyte loss through sweat, decreasing excitability

- central fatigue when less motor signals are issued from the brain

Excessive Postexcercise oxygen consumption (EPOC)

breathing heavily after excercise in order to repay oxygen debt, replenish ATP aerobically, replace oxygen that was used in myoglobin, provide liver with oxygen to dispose of lactic acid, and provide oxygen to cells that have elevated metabolic rates after exercise

oxygen debt

the amount of oxygen required after physical exercise to convert accumulated lactic acid to glucose

axon terminal

The endpoint of a neuron where neurotransmitters are stored

cardiomyocytes

cardiac muscle cells

contractile proteins

actin (thin filament) and myosin (thick filament) myosin heads grab onto actin binding sites once troponin-tropomyosin complex is moved by calcium

Endurance training (aerobic exercise)

builds resistance by improving fatigue resistant muscles. slow twitch fibers will create more mitochondria, glycogen, and acquire a greater density of capillaries. will enhance the cardiovascular, respiratory, skeletal, and nervous systems.

resistance training (weightlifting)

builds strength only. growth is from cellular enlargement (hypertrophy). muscle fibers create more myofilaments and myofibrils.

slow oxidative fibers / red fibers / slow twitch

utilize aerobic respiration. slow and sustained contractions for long term use. good at fatigue resistance. ATP is used more slowly due to slow ATPase and SR that releases calcium slowly. abundant in mitochondria, myoglobin, and capillaries which leads to its red color. small amount of glycogen. grouped in small motor units

PURPOSE IS SLOW, PRECISE, AND SUSTAINED MOVEMENT

fast glycolytic fibers / white fibers / fast twitch

utilize glycolysis and anaerobic fermentation. strong and fast contractions for short term use. bad at fatigue resistance. ATP used fast due to form of myosin with ATPase and large SR that quickly releases calcium. small amount of capillaries, myoglobin, and mitochondria which lead to its white color. many glycogen. grouped in large motor units

PURPOSE IS FAST, STRONG, AND UNSUSTAINABLE MOVEMENT

how are slow and fast twitch fibers distributed in the body and among individuals

every muscle has a combination of both, but one will dominate the other depending on the muscles function. distribution varies across people depending on their life styles. marathon runners will have more slow oxidative / red/ slow twitch fibers to sustain their long runs while sprinters will have more fast glycolosis/ white/ fast twitch fibers to give them the ability to run as fast as they can for a short period

intercalated discs

seen in cardiac muscle and are mechanical junctions that prevent cardiocytes from ripping apart.

involuntary movement

unconscious control over movement as seen in cardiac and smooth muscle

muscle cells aka muscle fiber

made up of myofibrils, sarcomeres, and myofilaments. called fibers due to their long shape.

muscle tone

optimum resting length of skeletal muscles. there's some contraction but very little. thin filaments still overlap thick to allow for cross bridges

myocytes/ myofiber

muscle cells