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Types of Muscle Tissue

skeletal, cardiac, smooth; each served by one nerve, an artery, and one or more veins

Muscle Fibers

skeletal and smooth muscles; are elongated; diameter ranges from 10 to 100 um (10 times that of an avergage body cell); multiple nuclei

Myo or Mys

root words meaning "muscle"

Sarco

flesh

Skeletal Muscle Tissue

smallest to largest: myofilaments (actin and myosin) bundled to make myofibrils -> bundles of myofibrils wrapped in sarcolemma and wrapped in endomysium make up muscle fibers -> bundles of muscle fibers wrapped in perimysium make up fascicles -> bundles of fascicles wrapped in epimysium make up skeletal muscles (organs).

Cardiac Muscle Tissue

occurs only in the heart, where it constitutes the bulk of the heart walls; striated; not voluntary

Smooth Muscle Tissue

found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages; forces fluids and other substances through internal body channels; elongated "fibers"; not striated; not voluntary

Excitabilty

(responsiveness, irritability); the ability of muscle tissue to recieve and respond to a stimulus, that is any change in the environment either inside or outside the body

Contractility

ability of muscle tissue to shorten forcibly when adequetly stimulated

Extensibilty

the ability of muscle tissue to be stretched or extended

Elasticity

ability of muscle cell to recoil and resume its resting length after being stretched

Muscle Functions

Produce Movement, Maintain Posture and Body Position, Stabalize Joints, Generate Heat

Skeletal Muscle

each is a discrete organ, made up of several kinds of tissues; skeletal muscle fibers predominate, but blood vessels, nerve fibers, and substantial amounts of connective tissue are also present

Connective Tissue Sheaths

hold together and wrap around individual muscle fibers; support each cell and reinforce the muscle as a whole

Epimysium

(outside muscle) an overcoat of dense irregular connective tissue that surrounds the whole muscle

Perimysium

fibrous connective tissue; surrounds each fasicle (grouped muscle fibers that resemble bundles of sticks)

Endomysium

(within muscle) a whispy sheath of connective tissue that surrounds each individual muscle fiber

Insertion

movable bone, moves towards the immovable or less movable bone

Origin

less movable bone; typically lies proximal to the insertion

Direct/ Fleshy Attachments

the epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage

Indirect Attachments

the muscles connective tissue wrapping extend beyond the muscle either as a ropelike tendon or a a sheet like aponeurosis; much more common

Sarcolemma

muscles fibers plasma membrane

Glycosomes

granules of stored glycogen that provide glucose during periods of muscle cell activity

Myoglobin

a red pigment that stores oxygen; similar to hemoglobin (transports oxygen in blood)

Myofibrils

rodlike and run parallel to length of muscle fibers; 1-2 um in daimeter; densely packed in muscle fiber that mitochondria and other organelles appear to be squeezed between them

Striations

a repeating series of dark and light bands; evident along the length of each myofibril

A Bands

dark and perfectly aligned lines in muscle fiber

I Bands

light and perfectly aligned lines in muscle fiber

H Zone

lighter region in each dark A Bands midsection; each is bisected vertically by dark line called M Line

M Line

bisect each H Zone; formed by molecules of the protein muomesin

Z Disc

dark midline interruption in light I Bands; coin shaped sheet composed largley of the protien alpha-actinin, anchors the thin filaments

Sacromere

the smallest contractile unit of a muscle fiber - the functional unit of skeletal muscle; average 2um long; the region of a myofibril between 2 successive Z discs

Myofilaments

smaller structure within sacromeres; the muscle equivalents of the actin or myosin containing microfilaments

Thick Filaments

contain myosin, and extend the entire length of the A band; composed primarily of protien myosin; each myosin molecule consists of two heavy and 4 light polypeptide chains, and has rodlike tail attchaed by flexible hinge to two gobular heads; each contains about 300 myosin molecules

Thin Filaments

contain actin, and extend across the I band and partway into the A band

Cross Bridges

are formed when tail of myosin molecule, which consists of 2 intertwined helical polypeptide heavy chains, contract and link the thick and thin filaments together; the gobular heads, are business end of myosin;

Actin

has kidney shaped polypeptide subunits, called globular actin, or G actin, which bear the active sites to which the myosin heads attach during contraction

Tropomyosin

polypeptide strands; a rod-shaped protien, spiral about the actin core and help stiffen and stabalize it; arrranged end to end along the actin filaments and in relaxed muscle fibe

Troponin

another major protien in thinn filaments; globular three polypeptide complex; one of its polypeptides is an inhinitory subunit that binds to actin; the third binds calcium ions

Elastic Filament

composed of the giant protien titin; extends from the Z disc to the thick filament, and then runs within the thick filament (forming its core) to attach to M line; holds thick filaments in place, and maintains organization of A band, and helps muscle cell to spring back into shape after being streched

Dystrophin

structural protien; links thin filaments to integral protiens of the sarcolemma

Sarcoplasmic Reticulum

major role is ti regulate intercellular levels of ionic calcium; an elaborate smooth endoplasmic reticulum; interconnecting tubules surround each myofibril; most tubules run longitudially along the myofibril communicating at H Zone; mitochondrai and glycogen granules are closely associated with SR and both involved in producing energy used during contraction

Terminal Cisternae

other tubules that form larger, perpendicular cross channels at the A band - I band junction and they always occur in pairs

T Tubules

elongated tube; formed by the cell interior in sarcolemma of cell muscle that protrudes deep into cell interior; increase muscle fibers surface area

Triads

formed by t tubules that run between the paired teminal cisternae of SR; organelles come into closest contact here; encircle each sarcomere

Triad Relationships

T tubules conduct impulses deep into muscle fiber ; Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes; T tubule proteins: voltage sensors; SR foot proteins: gated channels that regulate Ca2+ release from the SR cisternae

Contraction

refers to activiation of myosin's cross bridges, which are force generating sites; Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening

Sliding Filament Model of Contraction

states that during contraction the thin actin and myosin filaments slide past the thick ones so that actin and myosin filaments overlap to a greater degree; During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line; As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

Requirements for Skeletal Muscle Contraction

1. Activation: stimulation by nerve ending so that change in membrane potential occurs
2. generate and propegate an electrical current, called an action potential, along its sarcolemma
3. a short lived rise in intercellular calcium ion levels that is the final trigger for contraction must occur

Somatic Motor Neurons

nerve cells that activate skeletal muscle fibers; reside in the brain of spinal cord

Axons

long threadlike extensions of motor neurons, which travel bundled within nerves , to muscle cells they serve; ends are called axon terminals

Neuromuscular Junction

formed by curling branches of axons; each muscle fiber has only one; Site where motor neuron excites a skeletal muscle fiber; Chemical synapse consisting of points of contact between axon terminals of motor neuron and motor end plate od skeletal muscle fiber

Synaptic Cleft

space between axon terminal and muscle fiber, which is filled with a gel like extracellular substance rich in glycoprotiens and collagen fibers

Synaptic Vesicles

small membranous sacs containing the neurotransmitter acetylocholine (ACh); inside the axon terminal

Junctional Folds

trough like part of muscle fibers sarcolemma; provide a large surface area for millions of ACh receptors located here

Acetylcholinesterase

enzyme located in synaptic cleft; acetic acid and choline which break down ACh

Sarcolemma

like plasma membrane polarized; their is a potential voltage difference across the membrane and the inside is negative relative to the outer membrane face

Action Potential

electrical charge which occurs along the entire surface of the sarcolemma; 3 steps involved for this to occur

Generation of an Action Potential Across the Sarcolemma (1)

(local depolarization and generation of an end plate potential); Binding of ACh molecules to ACh receptors at neuromuscular junction opens (ligand) gated ion channels that allow Na+ and K+ to pass; More Na+ diffuses in then K+ diffuses out and interior of sarcolemma becomes less negative (depolarization [local electrical event called, end plate potential)

Polarized

-70 (resting plasma potential)

Depolarized

30 (for action potential to occur)

Generation of an Action Potential Across the Sarcolemma (2)

(generation and propagation of the action potential); [neuron send AP through axon] end plate potential ignites AP that spreads in all directions from neuromuscular junction across sarcolemma; this depolarization (end plate potential) spreads to adjacent membrane areas and opens voltage gated sodium channels; Na+ enters and reaches voltage threshold (+30), and an action potential is generated. AP is propegated along length of sarcolemma as depolarization wave spreads to adjacent areas of sarcolemma and opens voltage gated sodium channels there; again, Na+ difusses into cell

Generation of an Action Potential Across the Sarcolemma (3)

(repolarization); sarcolemma is restored to intitial polarized state; Na+ channels close and voltage-gated K+ channels open; K+ efflux rapidly restores the resting polarity; Fiber cannot be stimulated and is in a refractory period until repolarization is complete; Ionic conditions of the resting state are restored by the Na+-K+ pump

Excitation-Contraction Coupling

Sequence of events that convert s action potentials in a muscle fiber to a contraction; Action potential travels across entire sarcolemma; occur during hidden (laten) period, between AP initiation and the beginning of mechanical activity (contraction); electrical signal does not act directly on myofilaments (it causes rise in intracellular calcium ion concentration that allows filaments to slide)

Excitation-Contraction Coupling: Step 1

action potential travels across entire sarcolemma and are rapidly conducted to interior of muscle fibers by transverse tubules

T Tubules

Regularly spaced infoldings of sarcolemma that branch extensively throughout the muscle fiber; At numerous junctions, make contact with calcium storing membranous network known as sarcoplasmic reticulum

Terminal Cisternae

formed by SR (On portion of t tubule and adjacent terminal cisternae); saclike bulges where it abuts t tubules

Excitation-Contraction Coupling (2)

traveling down of action potential causes t tubule voltage sensitive protein to change shape; opens a calcium release channel in SR allowing calcium ions to flee sarcplasm; this rapid influx of calcium triggers contraction of muscle fibers

Excitation-Contraction Coupling (3)

calcium binds to tropinin and removes the blocking action of tropomyosin; when Ca2+ binds, troponin changes shape, exposing binding sites for myosin on the thin filaments

Excitation-Contraction Coupling (4)

contraction begins; myosin binding to actin forms cross bridges and contraction begins; at this point E-C coupling is over

Role of Calcium (Ca2+) in Contraction

At low intracellular Ca2+ concentration:
(Tropomyosin blocks the active sites on actin) (Myosin heads cannot attach to actin) (Muscle fiber relaxes)
At higher intracellular Ca2+ concentrations:
[Ca2+ binds to troponin ] [Troponin changes shape and moves tropomyosin away from active sites] [Events of the cross bridge cycle occur] [When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends]

Sacromere

Functional unit of contraction in skeletal muscle fiber; Shorten when myosin heads in thick myofilaments form cross bridges with actin molecules in thin myofilaments

Formation of Cross Bridge

Initaited when calcium ions released from SR bind to troponin (This causes tropinin to change shape); Tropomyosin moves away from myosin binding sites on actin allowing myosin head to bind actin and form a cross bridge; Myosin head has to be activated before a cross bridge cycle can begin; ATP combines with myosin head and is hydrolized to ADP and inorganic phosphate (Energy from hydrolozied Atp activates myosin head forcing It to be in cocked position)

Cross Bridge Cycle: Step 1

cross bridge formation:Activated myosin head binds to actin forming a cross bridge; Inorganic phospahte released; Bond between myosin and actin becomes stronger

Cross Bridge Cycle: Step 2

the power stroke: ADP released and activated myosin head pivotes; Slides thin myofilament toward center of sarcomere

Cross Bridge Cycle: Step 3

cross bridge detachment: Link between mysoin head and actin weakens when another ATP ataches to myosin head; Myosin head detaches

Cross Bridge Cycle: Step 4

reactivation of myosin head: ATP hydrolized to ADP and inorganic phospahte; Energy released during hydrolizes reactivates myosin head returning it to cocked postion

Muscle Tension

the force exerted by a containing muscle on an object

Load

the opposing force exerted on the muscle by the weight of the object to be moved

Isometric

if muscle tension develops but the load is not moved; increasing muscle tension is measured

Isotonic

if the muscle tension developed overcomes the load and muscle shortening occurs; amount of muscle shortening is measured

Motor Unit

the nerve-muscle functional unit; consists of a motor neuron and all the muscle fibers it supplies; small motor units (more precise movement) [ex: fingers], larger motor units (less precise movement) [ex: hip muscles, bone]

Motor Nerve

served each muscle; each motor nerve contains axons of up to hundreds of motor neurons

Myogram

a graphic recording of contractile activity; line recording activity is called tracing

Muscle Twitch

response of a motor neuron to a single action potential of its motor neuron

Latent Period

the first few milliseconds following stimulation when excitation-contraction coupling is occuring; during this period, muscle tension is beggining to increase

Period of Contraction

cross bridges are active, from the onset to the peak of tension development, and the myogram tracing rises to a peak

Period of Relaxation

final phase, lasting 10-100ms, is initiated by reentry of Ca2+ into the SR; muscle tension decreases to zero and tracing returns to baseline

Graded Muscle Responses

muscle contraction can be graded in two ways: (1) by changing the frequency of stimulation and (2) by changing the strength of stimulation

Temporal/Wave Summation

if two identical stimuli (electrical shocks or nerve impulses) are delivered to muscle in rapid succession, the second twitch will be stronger then the first; temoral or wave summation is this second twitch; this occurs because second contraction occurs before the muscle has completely relaxed; primary function is to produce smooth continous muscle cells

Unfused or Incomplete Tetanus

if the stimulus strength is held canstant and the muscle is stimulated at an increasingly faster rate, the relaxation time between the twitches become shorter and shorter, the concentration of Ca2+ in the cytosol higher and higher, and the degree of wave sumation greater and greater, progressing to a sustained but quivering contraction

Fused or Complete Tetanus

as the stimulation frequency continues to increase, muscle tension increases until a maximal tension is reached; at this point all evidence of muscle relaxation dissapears and the contractions fuse into a smooth, sustained contraction plateau

Recruitment (Multiple Motor Unit Summation)

controls force of contraction; achieved by delivering shocks of increasing voltage to the muscle, calling more and more muscle fibers to play

Subthreshold Stimulus

stimuli that produce no observable contractions

Threshold Stimulus

the stimulus at which the first observable contraction occurs

Maximal Stimulus

the strongest stimulus that produces increased contractile force; represent the point at which all the muscles motor units are recruited

Muscle Tone

relaxed muscles that are almost slightly contracted; its due to spinal reflexes that activate first one group of motor units then another in response to activation of stretch receptors in muscles

Isotonic Contractions

muscle length changes and moves the load, the tension remains relatively constant through the rest of the contractile period; come in two flavors concentric and eccentric

Concentric Contractions

those in which the muscle shortens and does work, such as picking up a book or kicking a ball;

Eccentric Contractions

the muscle generates force as it lengthens; are important for coordination and purposeful movements; occur in calf muscle for example; 50% more forceful then concentric

Isometric Contractions

tension may build to the muscles peak tension producing capacity, but the muscle neither shortens nor lengthens; occur when a muscle attempts to move a load that is greater then the force (tension) the muscle is able to develop

Direct Phosphorylation

ATP producing way; Metabolic Pathway; chemical reaction where CP (creatine phosphate) and ADP are used. Phosphate from CP is taken and put into ADP and turns to ATP; 1 to 1 ration; energy last for less then 10 seconds; does not require Oxygen;first stage body goes to to make energy; occurs in cytoplasm of cell

Anearobic Pathway (Glycolosis)

metabolic pathway without oxygen; produced 4 ATP; 10 step process; every step is a chemical reaction and changes everytime (10 times); 1st 5 steps are known as energy investment phase (they require energy) [2 ATPS to produce these 5 steps]; 2nd Stages known as Reinvestment stage; makes only 2 ATPs; this process occurs in cytoplasm of cell; 2 energy sources glucose and glycogen; makes 2 or 3 ATP + 2 Pyruvic acids

Glucose Anearobic Pathway

yields 2 ATPs; does not require oxygen; also yeilds 2 pyruvic acids

Glycogen Anearobic Pathway

used when their is not enought glucose; stored fat; Yields 3 ATPs because it has allready been partially hydrated; also yields 2 pyruvic acids

Lactic Acid

made if their is no oxygen present or work is more then you can breathe; pyruvic acid is transformed to lactic acid; made either way by glucose or glycogen; liver can use or send back to be formed to pyruvic

Aerobic Pathway

yields the most ATP with Oxygen; needs pyruvic acid to be shuttled from mitochondria (it is shuttled from Anearobic Pathway when 2 ATP and 2 Pyruvic acids); Pyruvic acid is transformed to Acetylcoa (happens in cytoplasm of cell); Krebs cycle produces 32 ATP

Aerobic Endurance

the length of time a muscle can continue to contract using aerobic pathways

Anaerobic Threshold

the point at which muscle metabolism converts to anaerobic glycolysis

Muscle Fatigue

the state of physiological inability to contract even though the muscle still may be receiving stimuli; due to a problem in excitation-contraction coupling or, in rare cases, problems at the neuromuscular junction

Contractures

lack in ATP; states of continuous contraction because the cross bridges are unable to detach

Oxygen Deficit

the extra amount of oxygen that the body must take in for these restorative processes

Force of Muscle Contraction

affected by, (1) the number of muscle fibers stimulated [more motor units the greater the muscle force], (2) the relative size of the fibers [the bulkier the muscle the more tension it can develop and the greater its strength, (3) the frequency of stimulation [more frequency allows time for more effective transfer of tension to noncontractile components]
, and (4) the degree of muscle stretch [Length-tension relationship—muscles contract most strongly when muscle fibers are 80-120% of their normal resting length]

Velocity and Duration of Contraction

influenced by muscle fiber type, load, and recriutment

2 Major Muscle Type Characteristics

speed of contraction: there are slow and fast fibers; difference in speed reflects how fast their myosin ATPases split ATP, and on the pattern of electrical activity of their motor neurons

the major pathways for forming ATP: oxidative fibers rely mostly on the oxygen using aerobic pathways for ATP gerneration; glycolytic fibers rely more on anaerobic glycolisis

Classification of Skeletal Muscle Cells

slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers

Aerobic Exercise

such as swimming, jogging, fast walk, and biking; Leads to increased Muscle capillaries, Number of mitochondria, Myoglobin synthesis; Results in greater endurance, strength, and resistance to fatigue; May convert fast glycolytic fibers into fast oxidative fibers

Resistance Exercise

(typically anaerobic) results in Muscle hypertrophy (due to increase in fiber size), Increased mitochondria, myofilaments, glycogen stores, and connective tissue

The Overload Principle

Forcing a muscle to work hard promotes increased muscle strength and endurance; Muscles adapt to increased demands; Muscles must be overloaded to produce further gains

Smooth Muscle

Found in walls of most hollow organs (except heart); Usually in two layers (longitudinal and circular); lack coarse connective tissue sheath; smooth muscle fibers SR is less developed then that of skeletal muscle and lacks specific pattern relative to myofilaments (no sacromeres, myofibrils or t tubules)

Peristalsis

Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs; (Longitudinal layer) contracts; organ dilates and shortens ; (Circular layer) contracts organ constricts and elongates

Varicosities

numerous bulbous swellings in smooth muscle; release neurotransmitter into a wide synaptic cleft in the general area of the smooth muscle cells. Such junctions are called diffuse junctions

Caveolae

pouchlike infoldings that sequester bits of extracellular fluid containing a high concentration of Ca2+ close to the membrane; when calcium channels open here Ca2+ influx occurs rapidly

Myofilaments in Smooth Muscle

Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2); Thick filaments have heads along their entire length; No troponin complex; protein calmodulin binds Ca2+; Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner; Dense bodies: proteins that anchor noncontractile intermediate filaments to sarcolemma at regular intervals

Contraction of Smooth Muscle

Slow, synchronized contractions ; Cells are electrically coupled by gap junctions; Some cells are self-excitatory (depolarize without external stimuli); act as pacemakers for sheets of muscle ; Rate and intensity of contraction may be modified by neural and chemical stimuli; Sliding filament mechanism; Final trigger is high intracellular Ca2+; Ca2+ is obtained from the SR and extracellular space; Very energy efficient (slow ATPases); Myofilaments may maintain a latch state for prolonged contractions

Relaxation requires:
Ca2+ detachment from calmodulin
Active transport of Ca2+ into SR and ECF
Dephosphorylation of myosin to reduce myosin ATPase activity

Role of Calcium Ions

Ca2+ binds to and activates calmodulin ; Activated calmodulin activates myosin (light chain) kinase; Activated kinase phosphorylates and activates myosin ; Cross bridges interact with actin

Neural Regulation

Neurotransmitter binding [Ca2+] in sarcoplasm; either graded (local) potential or action potential; Response depends on neurotransmitter released and type of receptor molecules

Hormones and Local Chemical Factors

May bind to G protein-linked receptors; May either enhance or inhibit Ca2+ entry

Special Features of Smooth Muscle Contraction

Response to stretch, length and tension changes, and hyperplasia

Response to Stretch

Responds to stretch only briefly, then adapts to new length; Retains ability to contract on demand;
Enables organs such as the stomach and bladder to temporarily store contents

Length and Tension Changes

Can contract when between half and twice its resting length

Hyperplasia

Smooth muscle cells can divide and increase their numbers

Single Unit Smooth Muscle

Sheets contract rhythmically as a unit (gap junctions); Often exhibit spontaneous action potentials; Arranged in opposing sheets and exhibit stress-relaxation response

Multiunit Smooth Muscle

Located in large airways, large arteries, arrector pili muscles, and iris of eye; Gap junctions are rare; Arranged in motor units; Graded contractions occur in response to neural stimuli

Developmental Aspects of Muscles

Muscular development reflects neuromuscular coordination; Development occurs head to toe, and proximal to distal; Peak natural neural control occurs by midadolescence; Athletics and training can improve neuromuscular control; Female skeletal muscle makes up 36% of body mass; Male skeletal muscle makes up 42% of body mass, primarily due to testosterone; Body strength per unit muscle mass is the same in both sexes; With age, connective tissue increases and muscle fibers decrease; By age 30, loss of muscle mass (sarcopenia) begins; Regular exercise reverses sarcopenia; Atherosclerosis may block distal arteries, leading to intermittent claudication and severe pain in leg muscles

Musclular Dystrophy

Group of inherited muscle-destroying diseases; Muscles enlarge due to fat and connective tissue deposits; Muscle fibers atrophy

Parallel Muscles

fibers parallel to the long axis of muscle; depends on the total number of myofibrils; directly relates to cross section of muscle; ex: biceps brachii

Convergent muscles

broad area converges on attachment site (tendon, aponeurosis, or raphe); muscle fibers pull in different directions, depending on stimulation; ex: pectoralis muscles

Unipennate muscle

forms an angle with a tendon; do not move as far as parallel; contains more myofibrils than parallel muscles; develop more tension than parallel muscles; fibers on one side of tendon ex: extensor digitorum muscle

Bipennate muscle

forms an angle with a tendon; do not move as far as parallel; contains more myofibrils than parallel muscles; develop more tension than parallel muscles; fibers on both sides of tendon; ex: rectus femoris

Multipennate muscle

forms an angle with a tendon; do not move as far as parallel; contains more myofibrils than parallel muscles; develop more tension than parallel muscles; tendon branches within the muscle; ex: deltoid

Circular muscle

also called sphincters; open and close to guard entrances of body; ex: orbicularis oris muscle of the mouth

Lever

a rigid, moving structure; all bones are these mechanically

Fulcrum

each joint is a fixed point mechanically; also called...

Applied force

muscles provide this to overcome resistance

Effort

force applied to a lever

Load

resistance moved by the effort

First class lever

fulcrum between applied force and resistance; if resistance is closer to the applied force there is a mechanical disadvantage; see-saw is an example

Second class lever

resistance between applied force and fulcrum; small force moves a large weight; uncommon in the body; all these levers work at a mechanical advantage; wheelbarrow is an example

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