Muscle Physiology (235 #10, 230 #8)

Question 1

Functions of Muscle

Answer 1

1) Producing Body Movements
2) Stabilizing Body Positions
3) Storing and Moving Substances within the body
4) Generating heat - thermogenesis (i.e. shivering, etc)

Question 2

Properties of Muscle

Answer 2

1) Electrical Excitability
2) Contractility - the ability to contract forcefully and generate tension when stimulated
3) extensibility - the ability to stretch without being damaged
4) Elasticity - the ability to return to original length and shape after contraction/extension

Question 3

Connective Tissue Components

Answer 3

1) sub-Q or hypodermis separates muscle from skin, composed of aerolar connective tissue and adipose.
2) fascia - sheet or broad band of irregular connective tissue - holds muscles with similar functions together but allows freedom of mvmt, carries nerves, blood vessels, lymphatics, etc.
3) protection and strengthening of muscles = extends from fascia:

epimysium (dense irregular) encircles muscle
perimysium (dense, irregular) surrounds groups of 10-100 muscle fibres, or fascicles
endomysium (reticular) penetrates the interior of fascicle and separates individual muscle fibres

Question 4

tendon

Answer 4

attaches a muscle to the periosteum of a bone

Question 5

aponeurosis

Answer 5

connective tissue extends as a broad flat sheet

Question 6

sarcolemma/sarcoplasm

Answer 6

plasma membrane of a muscle cell and cytoplasm of a muscle cell, containing substantial amounts of glycogen, used for synth of ATP. Also contains myoglobin to bind O2 and release it to mitochondria for ATP synth. Filled also with little threads of myofibrils!

Question 7

transverse (T) tubules

Answer 7

invaginations of the sarcolemma, filled with interstitial fluid. APs travel along the sarcolemma and through the T tubules so that all parts of the fiber are excited simultaneously.

Question 8

myofibrils

Answer 8

contractile organelles of skeletal muscle. 2um in diameter and extend the length of the fibre. Contain STRIATIONS

Question 9

sarcoplasmic reticulum

Answer 9

fluid-filled system of membranes similar to smooth ER in nonmuscular cells. Dialated end sacs - terminal cisterns - butt against the T tubules from both sides, making a TRIAD. SR wraps around each myofibril and the terminal cisterns (or latent sacs) act as a resevoir for Ca2+ - the release of ions from here triggers muscle contraction. To relax muscle, contain active transport pumps for Ca2+ to pull cytosolic Ca2+ back into SR to stop contractions. Inside SR, calsequestrin binds with Ca2+, which allows even more Ca2+ to be stored within the SR.

Question 10

thin & thick filaments

Answer 10

within myofibrils, directly involved in contractile process. Two thin for every thick filament:

1) thin = mostly actin filaments, 8nm in diam, 1-2um long
2) thick = mostly myosin, 16nm in diam, 1-2um long

Arranged into sarcomeres

Question 11

sarcomere

Answer 11

Basic functional unit of a myofibril:

1) Z discs - narrow plate-shaped region of dense protein to separate sarcomeres
2) A band - darker middle part, expands entire length of thick filaments, includes zone of overlap where there are both thin and thick filaments
3) I band - lighter less dense area, contains thin but no thick filaments, with Z disc through centre
4) H zone contains thick but no thin
5) M line - supporting proteins that hold the thick filaments in the centre of the H zone.

The letter ‘I’ is THIN, while the letter ‘H’ is THICK.

Question 12

Contractile Proteins

Answer 12

1) myosin - thick filaments. molecule consists of tail and two heads (twisted golf club handles) which bind to myosin binding sites on actin molecules of thin filaments. Pulls cellular structures to achieve movement by converting ATP chemical energy to mechanical energy.
2) actin - thin filaments, spherical molecule has a myosin binding site where myosin heads bind during contraction. Filaments are joined molecules twisted into a helix.

Question 13

Regulatory Proteins

Answer 13

1) troponin - thin filaments, when Ca2+ ions bind to it, conformational changes moves tropomyosin away from myosin-binding sites on actin, so myosin can bind
2) tropomyosin - in relaxed muscle, these threadlike proteins bind to actin’s myosin-binding sites and block myosin.

Question 14

Structural Proteins

Answer 14

keep alignment of thick/thin, give elasticity and extensibility, link myofibrils to sarcolemma and ECM.

1) titin - elastic, largest protein, connects M line to Z disc
2) alpha actinin - in Z disc, attaches to actin and titin
3) myomesin - forms M line, binds thick filaments together adn to titin
4) nebulin - wraps entire length of thin filament, anchors them to Z
5) dystrophin - links thin to integral membrane proteins in sarcolemma, then to ECM - reinforces sarcolemma and Tx’s tension generated

Question 15

Levels of Skeletal Muscle Organization

Answer 15

1) Skeletal Muscle
2) Fascicle
3) Muscle Fiber (cell)
4) Myofibril
5) Filaments

Question 16

Contraction or Cross-Bridge Cycle

Answer 16

1) ATP hydrolysis - myosin head contains ATP binding site and an ATPase - enzyme that hydrolizes ATP, which energizes and reorients myosin head. ADP and phosphate remain attached to myosin head
2) Energized myosin head attaches to myosin binding site on actin and releases phosphate group - CROSS BRIDGE
3) Power-stroke: site on cross-bridge where ADP is opens, cross-bridge rotates and releases ADP, generating force.
4) cross-bridge remains firmly attaches to actin until it binds with new ATP, at which point myosin detaches from actin.

In skeletal muscle, magnesium must be attached to ATP before myosin ATPase can split the ATP.

Question 17

Rigor Mortis

Answer 17

when no fresh ATP is available to attach to myosin to allow cross-bridge to detach, the muscle is held in contraction state = death causes Ca2+ to flood sarcoplasm, so muscle contract and cannot release.

Question 18

Excitation-Contraction Coupling

Answer 18

steps that connect excitation (muscle AP) to contraction:

1) muscle AP travels along sarcolemma and T tubules, causing Ca2+ release channels in the SR to open
2) sarcoplasmic Ca2+ increases and binds to troponin, pulling tropomyosin out of the way of the myosin-binding sites on actin
3) contraction cycle begins when myosin forms cross-bridges with actin.

Question 19

Length-Tension Relationship

Answer 19

indicates how forcefulness of muscle contraction depends on the length of sarcomere before contraction.

1) when stretched, as the zone of overlap shortens, fewer myosin heads can make contact with actin so possible tension decreases.
2) when shortened, thick filaments crumple as they are compressed by Z discs so fewer myosin heads can connect.

When sarcomere length is 2-2.4um (resting length), zone of overlap is optimal.

Question 20

Creatine Phosphate

Answer 20

when relaxed, fibers make more ATP than they need for resting. Most excess is used to synth creatine phosphate - an energy rich molecule. Enzyme creatine kinase (CK) catalyzes tx of one high-energy phosphate groups from ATP to creatine, making CP and ADP. When contraction begins and ADP starts to rise, CK catalyzes tx of high-energy P from CP back to ADP to make ATP - VERY QUICK, so first source of energy. Stores of CP and ATP generally last about 15 seconds.

Question 21

Anaerobic Cellular Respiration

Answer 21

glycolysis uses glucose to form two ATP, with 2 pyurvic acid produced as byproducts. Further anaerobic reactions convert most of the pyurvic acid to lactic acid - about 80% diffuses into bloodstream where liver cells can convert some back to glucose to reduce acidity of the blood. Lasts 30-40 seconds.

Question 22

Aerobic Cellular Respiration

Answer 22

Pyurvic acid enters mitochondria and is oxidized. Slow, produces 36 ATP, uses pyurvic acid from glycolysis, fatty acids from breakdown of triglycerides in adipose and amino acids from protein breakdown. In activities longer than 10 mins, 90% is from ACR. At the end of a marathon, 100%!

Question 23

Muscle Fatigue

Answer 23

inability of muscle to maintain force.

1) inadequate release of Ca2+ from SR
2) depletion of CP
3) insufficient O2, glycogen, nutrients
4) failure to release enough ACh
5) buildup of lactic acid and ADP

Question 24

Central/Psychological Fatigue

Answer 24

even before muscle fatigue sets in, feelings of tiredness and desire to cease activity.

Question 25

Oxygen Debt / Recovery Oxygen Uptake

Answer 25

recovery period after exercise. added oxygen, above and beyond the resting consumption that is taken into the body after exercise to pay back and restore metabolic conditions

1) convert lactic acid in liver
2) synth CP and ATP in fibers
3) replace O2 to myoglobin

After exercise, O2 use is also elevated:

1) body temp is up, so more chemical rxns take place
2) heart & breathing muscles are still working harder
3) tissue repair occur at increased pace

Question 26

motor unit

Answer 26

somatic motor neuron plus all of the muscle fibers it stimulates. Avg 150 skeletal muscle fibers, and all in one unit contract in unison - dispersed throughout a muscle rather than clumped together.

1) whole muscles that control precise movements contain many smaller units.
2) large-scale and powerful movements contain fewer really large motor units.

Total strength of contraction depends on how many motor units and the size of them that are contracted simultaneously

Question 27

twitch contraction

Answer 27

brief contraction of all the muscle fibers in a motor unit in response to a single AP. 20-200 msec long.

1) latent period - 2 msec, lasts while AP sweeps over sarcolemma and SR releases Ca2+.
2) contraction period - 10-100 msec, cross-bridge cycling
3) relaxation period - 10-100 msec, Ca2+ actively tx’d to SR
4) refractory period - period of ‘lost excitablility’ when fiber rx’s enough stimulation to contract and temporarily loses excitability.