a&p ch 11

SKELETAL MUSCLE

Fiber-shaped, voluntary, striated, multinucleated cells that usually attach to bone(s)

Three types of myofilaments:

Thick filament composed of bundled myosin contractile protein Elastic filament composed of titan protein forms the Z-line Thin filaments composed of actin, troponin, and tropomyosin

myofilaments

Thick filament composed of bundled myosin contractile protein Elastic filament composed of titan protein forms the Z-line Thin filaments composed of actin, troponin, and tropomyosin

Myofilaments:

Three types of long protein cords that fill most of the muscle cell cytoplasm and create contractions do the work of contraction & called contractile proteins

Synaptic knob

bulb on end of motor nerve axon nestled into depression on sarcolemma; knob contains synaptic vesicles filled with a neurotransmitter called acetylcholine (Ach)

Junctional folds

folded area below knob on sarcolemma to increase surface are on muscle cell and synthesizes the Ach receptors and other sarcolemma proteins. Loss of these receptors leads to muscle paralysis seen in myasthenia gravis

Synaptic cleft

gap or space between knob and muscle cell, ~ 10 nm

Somatic motor neurons

have axon connected to a skeletal muscle. Nerve fibers (axons) branch at distal end of axon Axon penetrate endomysium to innervate one muscle fiber at neuromuscular junction

cardiac & smooth muscles have in common

involuntary

end plate potential

ion channels opne sodium and potassium diffuses across plasma membrane

contact proteins found in myofibril

myosin actin

action potential

polarization change generated by the process of muscle excitation

sacromere

portion of a myofibril from one Z disc to another

creatine phosphate and ATP

powers the energy of short bursts

aerobic fermentation

produces lactic acid and small amounts of ATP

Cardiocytes

propel blood with each contraction; involuntary Sustained contractions would be disastrous Coupled end to end by gap junctions; cells joined together by intercalated discs Has own pacemaker to produce contraction = autorhythmic

Contraction stage

sliding filament theory Myosin ATPase binds myosin head power stroke = myosin heads bend ATP releases myosin head to its original position = recovery stroke

Contraction

the production of internal muscle tension that may or may not shorten the muscle.

Satellite cells

between muscle cell and endomysium; used to repair damaged muscle Most repair = fibrosis replacement of muscle cells by scar tissue (collagen) and normal function is not restored; minimal new muscle cells can form

Membrane potential

both neurons and muscle are excitable mVolt charge created by charge in ions (K, Na, Cl) difference across PM Membrane is polarized (charged). Resting membrane potential = ~ -75 -90 mV Inside of membrane is more negative than outside When stimulated, ion gates open. Na rushes in, K rushes out Membrane potential changes = AP (action potential) Thus the plasma membrane is electrically polarized (charged like a little battery) due to ion distribution between the ICF and the ECF causing a RMP (resting membrane potential) =charge difference between inside and outside of cell

Striated

inner contractile proteins create alternating light & dark transverse bands

dystrophin

located between sarcolemma and the outmost filaments and functions to link actin to a sarcolemma protein that enables the endomysium to bind to the surface of a muscle cell; movement of actin then pulls on the dystrophin ultimately linking the endomysium to all of the other series elastic CT and bone. Genetic defects for this protein lead to the disabling effects of muscular dystrophy (MS)

Rigor mortis

temporary rigidity post mortem, due to ATP depletion following the cessation of cellular respiration after death formed cross-bridges are not released. Myosin heads are released from actin after tissue begins to degrade 12+ hours after death

Myoblasts

the embryonic cells with a single nucleus that fuse to form a skeletal muscle cell (fiber) making skeletal muscle cells multinucleated

Recruitment Effects of varying stimulus intensity and frequency

the process of stimulating more motor units Multilple motor unit (MMU) summation ↑ # motor units that contract ↑strength of contraction of whole muscle

Relaxation phase

time to return to resting length (muscle loses tension)

Voluntary

under conscious control

One skeletal muscle cell =

"muscle fiber"

White fibers

(fast twitch; fast glycosidic (FG), Type II fibers) muscles that are used for short, quick work Have higher ATPase activity (ATP ADP + P), contract / relax quickly Most energy from glycolysis, few mitochondria, fatigue rapidly (white meat) Leg muscles high in white fibers makes a good sprinter (e.g. 100-yd dash) In arms, this makes good power lifter, for strength

Thin

2 intertwined strands of contractile protein called fibrous (F) actin; Each F actin consists of a string of beads called globular (G) actin in twisted shape with active site for myosin heads.

"Muscle fiber" or "myofibers"

= an individual skeletal muscle cell: multinucleated, extra-ordinarily long, some thick, muscle cells are usually attached to tendon, bone, or skin; ~100um in diameter and 3 cm long with a fiber shaped

Excitation

AP reaches knob vesicles release Ach into cleft AP starts on muscle

Excitation-contraction coupling

AP travels to T-tubules enters SER Ca+ released Ca+ binds troponin tropomyosin bends exposing active sites for myosin heads to bind to

Phosphagen system

ATP & CP provides nearly all the energy used for short bursts of intense activity. E.g. 100 yd dash, baseball, weight lifting, 1st 30 seconds of intense work ATP made by cells, gives a few seconds of energy via glycolysis, ADP + P ATP Aerobic respiration uses oxygen from O2 from muscle myoglobin for a short time Two enzymes important in the P transfers to ATP: Myokinase Creatine kinase

Muscle cell fatigue at low intensity, long-duration exercise thought to be due to:

ATP depletion as glycogen stores are exhausted Electrolyte loss through sweating Central fatigue little understood fatigue of the central nervous system where the CNS produces less signaling to the muscle cells. Psychological factors may play a part in overcoming this hurtle during competitive sports

sarcomeres

Actin and myosin are organized into repeating units called give the cell its striated appearance transverse bands are designated as A,H, and I

Long-term energy:

After 40 seconds respiratory and CV system "catch up" deliver new oxygen to muscles Oxygen consumption levels off after 3-4 minutes If exercise last for >10 minutes., 90% of ATP is produced aerobically After 20 minutes of mid range exercise (50-60%) body breaks down fat for energy If high intensity exercise (>60%) body uses carbohydrate sources for energy

Myofibril (NOT myofibers)

Bundles of long, parallel protein cords that occupy most of the cytoplasm; arranged end to end into sarcomeres made up of microfilaments called myofilaments;

perimysium

CT; bundles muscle fibers together into fascicles

Cross-training:

Combines both resistance and endurance training If muscles are inactive, they become deconditioned (weaker and fatigue easily) very quickly

Skeletal muscle

Contracts with single quick contraction when stimulated = twitch Series of separate stimuli are received to produce a sustained contraction = tetany Strength and duration of tetanic contraction depends on number of fibers involved and number of stimuli received Movements may require quick or sustained responses (red vs. white fibers)

Other organelles are packed between the adjacent myofibrils and include:

Cytosol Mitochondria Sarcoplasmic reticulum (SR) SER that forms a network around each myofibril and along T-tubules forming sacs called terminal cisternae sequesters calcium SR is reservoir for Ca ++ and has Ca ++ gated channels to release Ca ++ into cytosol Current through T-tubules causes release of Ca ++ from terminal cisternae

Skeletal muscles attached to bone via this series of CT:

Endomysium, perimysium, epimysium, fascia, and tendon

Cardiac muscle

Limited to the heart; contract and relax in alternating rhythm Rich in myoglobin, large stores of glycogen, exclusively uses aerobic respiration Striated, unicleate, and branched

Neuromuscular junction (NMJ,

Motor end plate) synapse between a nerve & muscle cell and is often the site of attack by toxins and other paralyzing agents in the environment

Two functional categories of smooth muscle:

Multiunit Single-unit

CONTRACTION:

Myosin heads bind to ATP using myosin kinase to release a P from ATP releasing energy that activates the myosin heads which extend (cock) Linkages called cross-bridges form between actin binding site and myosin cocked heads as ATP binds myosin. Myosin release the energy and flexes into a bent position tugging on the actin. This is a power stroke (myosin heads cock) remaining bound to the actin A second ATP binds the myosin head destabilizing it and breaking the cross-bridges. Myosin is now ready to bind another ATP and recock (called recovery stroke) to produce another power stroke About half of the myosin heads are always bound to, in this way, actin filaments slide inward along the myosin filaments shortening the distance between two Z-lines and the length of each sarcomere (hundreds of sarcomeres are end to end in a single muscle cell and each muscle cell (fiber) shortens over its length as sarcomeres contract (shorten)

EXCITATION:

Nerve impulse called "action potential" (AP) reaches the motor end plate synaptic knob and opens voltage gated Ca+ channels → Ca+ ions diffuse into synaptic knob Ca+ stimulates knob to release neurotransmitter acetylcholine (ACh) into synaptic cleft Synaptic vesicles move to edge of knob and release Ach into synaptic cleft (gap) → ACh binds to muscle cell sarcolemma ligand-gated receptors on surface of the muscle cell These Na/K channels pump Na+ out and K+ into muscle cell cytoplasm changing membrane potential of muscle from -90 mV to -70 mV to create an end plate potential (EPP) that depolarizes the muscle cell (excites it) The EPP excites other Na channels next to the end late to open and move Na into the cell. Other K channels also open to move K out of the cell

RELAXATION: Events of muscle cell relaxation

Nerve signal stops and muscle loses tension. calcium levels in the sarcoplasm fall. myosin releases thin filaments

Events leading to the conduction of a nerve impulse:

Neuron fiber cell membrane has a resting membrane potential (charge) and is polarized. Threshold stimulus is received by neuron and depolarizes nerve fiber. Sodium channels in a local region of the neuron cell membrane open Sodium ions diffuse into neuron cytoplasm, causing the cell membrane to depolarize Potassium channels in cell membrane open Potassium ions diffuse out of cell causing the membrane to repolarize The resulting action potential (AP) causes an ionic current that stimulates adjacent portions of the cell membrane in axon Wave of APs (action potentials) travels the length of the nerve axon as a nerve impulse

Sarcolemma

PM or CM

Muscle cell fatigue at high intensity, short-duration exercise thought to be due to:

Potassium accumulation in ECF lowering membrane potential making it harder to stimulate muscle ADP/P accumulation which slows further hydrolysis of ATP Lactic acid accumulation in ICF lowers pH which interferes with Ca+ use in the cell

Universal characteristics of all muscle cells

Responsive/ Excitable (irritable) Conductivity Contractility Extensibility Elasticity

Sarcoplasmic reticulum (SR)

SER that forms a network around each myofibril and along T-tubules forming sacs called terminal cisternae sequesters calcium SR is reservoir for Ca ++ and has Ca ++ gated channels to release Ca ++ into cytosol Current through T-tubules causes release of Ca ++ from terminal cisternae

Membrane charge (resting membrane potential RMP) due to:

Selective permeability of membrane to certain ions Electrostatic attraction between + and - ions Diffusion of ions down their concentration gradients through PM depolarization Ion concentrations are different inside and outside the PM Na-K pump maintains this concentration difference (more Na outside, more K inside) which causes repolarization of the membrane returns it to RMP

Resistance Training;

Short term (<2 minutes) or high intensity weight bearing or tension generating workouts (> 65%) are anaerobic do not burn fat but builds muscles and strength [(220 - age -RHR) X %] + RHR = target HR during your workout RHR = pulse/ min taken in morning before you get out of bed

Striations

Striations due to organization of actin and myosin into dark (A) bands and light (I) bands: Appears very dark where both actin and myosin overlap Dark (A) banding thick filaments laying side by side Light (I) banding a thin band in the middle with no thin filaments Actin and myosin are organized into repeating units called sarcomeres that give the cell its striated appearance transverse bands are designated as A,H, and I.

Contraction strength of twitches:

Subthreshold stimulus produces no twitch. Twitches are produced only at threshold when enough stimulus is received to initiate a twitch. Increasing the stimulus voltage above threshold will still produce twitches of the same strength. Therefore, muscle excitation does follow an all-or-none response like neurons (once they reach threshold, they will respond), but muscle fibers do NOT exhibit an all-or-none twitch in response to that excitation twitches vary in strength Why? Twitch strength varies with stimulation frequency faster arriving stimuli produce stronger twitches (treppe and tetany) Twitches vary in the amount of Ca+ in the sarcoplasm which varies signal frequency Twitch strength is dependent of the stretch of muscle before stimulus Warmer muscles cotract more strongly because enzymes work faster Twitches are weaker if pH falls in the sarcoplasm producing muscle fatigue Twitches vary with the state of hydration of the muscle. Dehydration impacts cross-bridge formation and slows muscle responses A single twitch is not strong enough to do any useful work. Muscles must be able to contract at varying strength to do different types of work.

Triad

T-tubule + 2 terminal cisternae

Length-Tension relationship

The amount of tension generated by a muscle and the force of the contraction Is dependent on how stretched or how contracted the muscle sarcomeres are. If too stretched, not enough myosin heads can contact actin binding sites If too contracted, no additional sarcomere shortening is possible Both result in a weak / ineffective contraction

EXCITATION-COUPLING:

The muscle cell sarcolemma is stimulated and the impulse travels in all directions across the sarcolemma to T-(transverse) tubules where they open channels in the T-tubule Ca+ ions diffuses out of SR of the muscle cell into the cell cytoplasm Ca+ binds to troponin molecules on the actin filaments Troponin-tropomyosin complex in the groove of the actin filaments changes shape to expose specific binding sites on the actin for the myosin heads

Endurance exercise:

To burn fat put your HR at 50-60% maximum for >20 minutes, do aerobic exercises; aerobic exercise benefits the cardiovascular system, increasing endurance; jogging, biking, running, swimming, walking, etc

Two regulatory proteins troponin and tropomyosin that act as a switch to start or turn off contractions:

Tropomyosin fills spiral groove created by the twisted actin and blocks the active sites on actin where the myosin heads bind Troponin bound to the tropomyosin, binds to Ca ++

Muscle Strength and Conditioning: Science of exercise:

We usually have more muscular strength than we actually need to use, even more than tendon and bones could tolerate. Muscle strength depends on: Muscle size -> thicker muscles are stronger Fascicle arrangement -> pennate are stronger than parallel which are stronger than circular Size of active motor units -> stronger movements require more motor units Temporal summation -> greater frequency of AP at NMJ means stronger contractions Length-tension relationship -> an ideal overlap of actin and myosin is required Fatigue -> rested muscle contracts more forcefully than fatigued ones Resistance exercise -> weight bearing exercise increases size of each muscle cell

Sliding Filament Theory Overview Review:

When an action potential (AP) arrives at end of axon (synaptic knob), Ca+ enters knob, vesicles migrate to axon PM and release acetylcholine into cleft 2) Acetylcholine binds to receptors on the muscle surface. Sends impulse into T-tubules Causes Ca+ release from SER which binds to a molecule on the actin called troponin Troponin molecules are bound to tropomyosin and block the binding sites for myosin 4) Ca+ causes tropomyosin to bend moving troponin off of binding sites on actin. Myosin heads (cross bridges) bind these sites cock back & forth moving on surface. 5) The actin and myosin in the muscle cell slide in on one another to shorten the muscle. This occurs in thousands of sarcomeres along the muscle cell at the same time. Sarcomeres come closer the overall effect is to shorten the entire muscle. ATP is required to achieve a contraction and to release the myosin from the actin to relax the muscle which then returns to its elongated state.

Thick

contractile protein consisting of 200-500 molecules of myosin shaped like gulf clubs with two heads. These are bundled together with the heads directed outward in a helical array around bundles (half face one end of the bundle and half face the other end. There is a bare region in the mid-region of the bundle where there are no heads; These heads pull along surface of actin ↓ length of sarcomere.

Sarcomere

contractile unit joined end to end at Z line, run along the length of muscle cell. The different widths of microfilaments give appearance of striations in muscle.

Twitch

contraction + relaxation; has three phases: Latent period Contraction

Isotonic

contraction with a change in length but no change in tension (bend arm at elbow to raise book in the air) Requires two forms of isotonic movement: a. Concentric muscles shorten as it maintains tension (lifting book) b. Eccentric muscle lengthens as it maintains tension (setting book down)

Oxygen debt

created during a burst of strenuous exercise when the circulatory system cannot deliver sufficient oxygen to keep up with demand Periods of rapid breathing after strenuous workout restores oxygen levels = amount of extra O2 needed to ↓ metabolic rate to resting level Serves to: replace lost oxygen store, replenish the phosphagen system, oxidize lactic acid and elevate metabolic rate to keep body temperature high

Sarcoplasm

cytoplasm

Titan(Elastic)

elastic proteins that flank each myosin filament to connect them to 2 Z lines and stabilize them, center myosin in the sarcomere between the thin filaments, prevents myosin overstretching, and contributes to their elastic recoil

Nerve impulse

electrical signal (due to ion movement across PM of nerve) Travels down nerve and stops at knob creates a chemical signal that crosses gap Chemical = acetylcholine (Ach) (neurotransmitter) stored in synaptic vesicles of knob Released into cleft and travels to receptors on folded sarcolemma surface

Hypercalcemia -

elevated calcium levels reduce Na permeability of plasma membrane and inhibit depolarization of the nerve causing muscle fatigue and weakness.

Glycogen

energy storing polysaccharide abundant in muscle

Calsequestrin

enzyme binds to Ca+ in the SR

actylcholinterase

enzyme that breaks down actylcholine

Hypocalcemia -

excessively low calcium levels increase Na permeability of plasma membrane causing the nervous and muscular systems to be over excitable and send spontaneous signals causing spasms, tremors, or tetany (sustained contractions) of muscles

resting membrane potential

measure of voltage across muscle cell

Single-unit

more widespread, called "visceral muscle" Autonomic fibers pass between several myocytes and stimulate them all at once Bead-like varicosities running along length and contain synaptic vesicles No motor end plate. Have binding site scattered on their surface Electrically coupled to each other by gap junctions Directly stimulate each other in large numbers and contract as a large unit Found in most blood vessels, GI, urinary, reproductive & respiratory tracts Occur in layers (usually circular and longitudinal) in hollow viscera

Isometric

muscle develops tension without a change in length (begin to lift a book) maintain posture

Motor unit (motor end unit) in shifts to ↓ fatigue

muscle fiber innervated by a single nerve fiber.

Tonus

muscle tone maintained by CNS Muscle tone keeps muscle sarcomeres at the optimal distance apart for the fastest, most efficient contraction when needed

Relaxation

nerve signal stops, muscle returns to normal, Ca returned to SR by

actylcholine

neurotransmitter released @ a neuromuscular junction

Smooth muscle

not attached to bone, forms tubes in body walls or layers in organs; no Tubes; little SR

Creatine kinase

obtains P from creatine phosphate and donates it to ADP Creatine phosphate (CP) short lived energy storage molecule in muscle cells

Myoglobin

oxygen storing red pigment of muscle

muscle tone

partial contraction of a resting muscle

Temporal (Wave) Summation

partial contraction of a resting muscle produces incomplete tetanus

myogram

record of timing and strength of a muscle's contraction

H band of sarcomere

region within A band that lacks thin filaments

Peristalsis

rhythmic wave of contractions that propel food through GI tract; When you stretch smooth muscle or stimulate it, it contracts in response

Red fibers (

slow twitch; slow oxidative (SO), Type I fibers) require a steady supply of oxygen, have large quantity of mitochondria and myoglobin (like hemoglobin) enhances rapid diffusion of oxygen during strenuous exertion; More myoglobin and less glycogen than white fibers Used for activities that required sustained endurance. (dark meat) Leg muscles high in red fibers makes a good endurance (marathon) runner

Three types of myocytes

smooth, cardiac, and skeletal

Complete tetanus

smooth, prolonged contraction due to 40-50 stimuli / second Muscle produce 4 times the tension as a single twitch (produced in lab setting only) *Not the same as the disease tetanus caused by tetanus toxin

Schwann cell

surround whole junction to isolate it from surrounding interstitial fluid

Multiunit

terminal branches of nerves synapse with individual myocytes Form motor end units contract independently of others Found in large arteries, arrector pili, iris, and pulmonary air sacs

Threshold

the minimum voltage equired to produce a contraction

Glycogen-lactic acid system

the pathway from glycogen to lactic acid Muscles shift to anaerobic respiration until cardiopulmonary system catches up Cells are required to make ATP without oxygen by lactic acid fermentation Gives 30-40 seconds of activity Muscles obtain glucose from blood & from their own stores of glycogen Muscle contractions are weaker without oxygen, accumulate lactic acid cramps occur Muscles fatigue as waste accumulates

intercalated discs

thickened notch ends of cardiac muscle contain gap junctions

Contraction phase

time while muscle shortens First 5 sec is refractory no response to new stimulus

Myokinase

transfers P from other molecules to add to ADP (ADP + P ATP)

regulatory proteins found in myofibril

tropomyosin troponin

Transverse (T) tubules

tunnel-like extensions that penetrate inside of the fiber to carry ionic current from surface to interior of cell when cell is stimulated Nuclei several just below the sarcolemma. Multiple nuclei because skeletal muscle cell formed from fusion of embryonic myoblasts

aerobic metabolism

utilizes glucose and fatty acids to generate ATP

Synapse

where a nerve fiber meets its target cell such as a muscle cell

Treppe

↑ in # stimuli / sec ↑ tension of each successive twitch Muscles still have time to reset therefore muscles work more effectively Eg. Warmup before a competition