Ch. 11 anatomy

what is excitability?

when stimulated by chemical signals, stretch, and electrical changes across the plasma membrane

What is conductivity?

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

What is contractility?

shortens when stimulated

What is extensibility?

capable of being stretched between contractions

What is elasticity?

returns to its original rest length after being stretched

What are skeletal muscles?

voluntary, striated muscle usually attached to bones

what is voluntary?

usually subject to conscious control

How long are myofibers?

as long as 30 cm

what are the connective tissue wrappings?

endomysium, perimysium, and epimysium

What is the endomysium?

connective tissues around muscle cell

what is the perimysium?

connective tissue around the muscle fascicle

what is the epimysium?

connective tissue around the entire muscle

what are tendons?

* attachments between muscle and bone
* continuous with collagen fibers of tendons

what is collagen?

* somewhat extensible and elastic
* stretches slightly under tension and recoils when released

what does collagen do?

* resists excessive stretching and protects muscle from injury
* returns muscle to its resting length
* contributes to power output and muscle efficiency

what is the sarcolemma?

plasma membrane of a muscle fiber

what is the sarcoplasm?

cytoplasm of a muscle fiber

what are myofibrils?

long protein cords occupying most of sarcoplasm

what is glycogen?

carbohydrate stored to provide energy for exercise

what is myoglobin?

red pigment; provides some oxygen needed for muscle activity

what are multiple nuclei?

flattened muscles pressed against the inside of the sarcolemma

what are myoblasts?

Stem cells that fused to form each muscle fiber early in development; muscle building cells

what are satellite cells?

* unspecialized myoblasts remaining between the muscle fiber and endomysium
* play a role in regeneration of damaged skeletal muscle tissue

where are the mitochondria?

packed into spaces between myofibrils

what is the sarcoplasmic reticulum?

* smooth ER that forms a network around each myofibril
* acts as a calcium reservoir; releases calcium through channels to activate contraction

what is the terminal cisterns?

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

what are t tubules?

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

what is a triad?

a t tubule and two terminal cisterns associated with it

what are thick filaments?

* made up of several hundred myosin molecules
* each molecule is shaped like gold club
* two chains intertwined to form a shaft-like tail
* double globular head

what are thin filaments?

* 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
* when relaxed, each blocking six or seven active sites on G actin subunits
* Troponin molecule: small, calcium-binding protein on each troponin molecule

what are elastic filaments?

* titin: huge, springy protein that makes elastic filament
* run through cord of thick filament and anchor it to z disc and M line
* help stabilize and position the thick filament
* prevent overstretching and provide recoil

what are contractile proteins?

myosin and actin do the work of contraction

what are regulatory proteins?

* tropomyosin abd troponin
* act like a switch that determines when fiber can (and cannot) contract
* contraction activated by release of calcium into scar plasm and its binding to troponin
* troponin changes shape and move tropomyosin off the active sites on actin

what is dystrophin?

* clinically important protein
* links actin in outermost myofilamnets 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

what are striations?

* result from the precise organization of myosin and actin in cardiac and skeletal muscle cells
* alternate A-bands (dark) and I-bands (light)

what are A bands?

* darkest parts is where thick filaments overlap a hexagonal array of think filaments

where is the H band?

* middle of A band
* thick filaments only
* not as dark

where is the M line?

middle of H band

what is the I band?

* the way bands reflect polarized light
* Z disc: provides anchorage for thin filaments and elastic filaments
* z disc bisects I band and “zig-zags”

where is the sarcomere?

* segments from z disc to z disc
* functional contractile unit of muscle fiber

why do muscle cells shorten?

their individual sarcomeres shorten

what changes during muscle shortening?

only the amount of overlap changes

what pulls on extrcellu.ar proteins during shortening?

* dystrophin and linking proteins
* transfers pull to extracellular tissue

what is the order of skeletal muscles form biggest to smallest?

muscle, fascicle, muscle fiber, myofibril, sarcomere, myofilament

when do skeletal muscle contract?

when stimulated by a nerve

what happens when nerve connections or severed or poisoned?

muscle is paralyzed

what is denervation atrophy?

shrinkage of paralyzed muscle when nerve remains disconnected

what are somatic motor neurons?

nerve cells whose cell bodies are in the brainstem and spinal cord

what are somatic motor fibers?

* axons lead to the skeletal muscle
* each nerve fiber branches out to a number of muscle fibers
* each muscle fiber is supplied by only one motor neuron

what is a motor unit?

one nerve fiber and all the muscle fibers innervated by it

what does a motor unit do?

* dispersed throughout muscle
* contact in unison
* produce was contraction over wide area
* provide ability to sustain long-term contraction as motor units take turns contracting
* effective contraction usually requires contraction of several motor units at once

what are small motor units?

* control degree of control
* three to six muscle fibers per neuron
* eye and hand muscles

what are large motor units?

* control more strength then control
* powerful contractions supplied by large motor units with hundreds of fibers

what is a synapse?

point where a nerve fiber meets its target cell

what is the neuromuscular junction?

* when target cell is a muscle fiber
* each terminal branch of the nerve fiber within the NMJ forms separate synapse with the muscle fiber

what is a axon terminal?

* the swollen end of nerve fiber
* contains synaptic vesicles with acetylcholine (ACh)

what is a synaptic cleft?

gap between axon terminal and sarcolemma

what does a Schwann cell do?

envelop and isolated NMJ

what do nerve impulse cause?

synaptic vesicles to undergo exocytosis releasing ACh into synaptic cleft

what happens during an unstimulated (resting) cell?

* the more anions on the inside of the membrane than on the outside
* anions make the inside of the plasma membrane negatively charged then to its outer surface
* the plasma membrane is electrically polarized with a negative resting membrane potential
* there are excess sodium ions in the extracellular fluid
* there are excess potassium ions in the intracellular fluid

what happens during stimulated (active) muscle?

* sodium gates open in the plasma membrane
* sodium flows into the cell downs its electrochemical gradient
* these cations override the negative charges in the ICF
* depolarization: inside of the plasma membrane becomes positive
* immediately potassium gates open and potassium rushes out of the cell partly repelled by positive sodium charge and partly because of its concentration
* loss of positive potassium ions turns the membrane negative again (repolarization)

what does an action potential do?

* one point causes another one to happen immediately in front of it, which triggers another one a little further along and so forth
* this wave of excitation is called an impulse

what causes muscles to become paralyzed?

* toxins interfering with synaptic function
* cholinesterase inhibitors bind to acetylcholinesterase and prevent it form degrading ACh

what is spastic paralysis?

state of continual contraction of the muscles; possible suffocation

what is tetanus?

* form of spastic paralysis caused by toxin clostridium tetani
* glycine in the spinal cord normally stops motor neurons form producing unwanted muscle contractions
* tents toxin blocks glycine release in the spinal cord and causes overstimulation and spastic paralysis of the muscles

what is flaccid paralysis?

state in which the muscles are limp and cannot contract

what is curare?

competes with ACh for receptor sites, but does not stimulate the muscles

what is botulism?

* type of food poisoning caused by a neuromuscular toxin
* blocks release of ACh causing flaccid paralysis
* botox cosmetic injections used for wrinkle removal

what are the four major phases of contraction and relaxation?

excitation, excitation-contraction coupling, contraction, relaxation

what is excitation?

process un which nerve action potentials lead to muscle action potentials

what is excitation-contraction coupling?

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

what is contraction?

step in which muscle fiber develops tension and may shorten

what is relaxation?

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

what is length-tension relationship?

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

what happens if a muscle is overly shortened before stimulated?

a weak contraction results, as thick filaments just butt against z discs

what happens when a muscle is too stretched before stimulated?

* a weak contractions results, as the minimal overlap between thick and thin filaments, results in minimal cross-bridge formation
* optimum resting length produces greatest force when muscle contracts

what is rigor mortis?

* hardening of muscles and stiffening of the body beginning 3 to 4 hours after death
* muscle relaxation requires ATP, and ATP production is no longer produced after death
* rigor mortis peaks about 12 hours after death, then diminishes over the next 48 to 60 hours

how does rigor mortis occur?

* deteriorating sarcoplasmic reticulum releases calcium
* deteriorating activates sarcolemma allows calcium to enter cytosol
* calcium activates myosin-actin cross-bridging
* muscle contracts, but cannot relax

what is a myogram?

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

what is the muscle threshold?

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

what is a twitch?

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

what is the latent period?

* very brief delay between stimulus and contraction
* time required for excitation, excitation-contraction coupling, and tensing of elastic components of muscle

what is the contraction phase?

* time when muscle generate external tension
* force generated can overcome the load and cause movement

what is the relation phase?

* time when tension declines to baseline
* SR reabsorbs calcium, myosin releases actin and tension decreases
* takes longer than contraction

why do different stimuli cause twitches varying in strength?

* the muscles starting length influences tension generation
* muscles fatigue after continual use
* warmer muscles’ enzymes work more quickly
* muscle cell’s hydration level influences cross-bridge formation

what are the different contraction strength of twitches?

* with subthreshold stimuli- no contraction at all
* at threshold intensity and above- twitch produced

what do higher voltages create?

* cites more nerve fibers which stimulate more motor units to contract
* more motor units come play with stronger stimuli

what is the muscle size principle?

weak stimuli recruit small units, while strong stimuli recruit small and large units for powerful movements

what does a low frequency create?

produce identical twitches

what does a higher frequency create?

produce temporal (wave) summation

what does a unnaturally high stimulus frequencies create?

cause a steady, contraction called complete (fused) tetanus (only created in lab experiments)

what is isometric muscle contraction (“same length”)?

* muscle produces internal tension but external resistance causes it to stay the same length
* important in postural muscle function and antagonistic muscle joint stabilization

what is a isotonic muscle contraction (“same tension”)?

* muscle changes in length with not change in tension
* creates concentric and eccentric contraction

what is concentric contraction?

muscle shortens as it maintains tension

what is eccentric contraction?

muscle lengthens as it maintains tension

what happens at the beginning of a contraction?

* isometric phase
* muscle tension rises but muscle does not shorten

what happens when tension overcomes resistance of the load?

* tension stopes increasing
* muscle begins to shorten and move the load (isotonic phase)

what does ATP supply depend on?

oxygen and organic energy sources (i.e. glucose and fatty acids)

what are the two main pathways of ATP synthesis?

* anaerobic fermentation
* aerobic respiration

what is anaerobic fermentation?

* enables cells to produce ATP in the absence of oxygen
* yields little ATP and lactate, which needs to be disposed of by the liver