Unit IIIMuscular System Structure

and Physiology

Chapter 10

Muscle functionsProducing body motions

Walking, running, nodding, grasping, etc.

Stabilizing body positionsSustained contractions of neck hold head up

Storing & moving substances within body

GI tract, cardiac muscle

Generating heat (_______________)Exercise, shivering

4 Properties of muscle tissue

_________________ – respond to stimuli by producing electrical signals (AP)___________ – ability of muscle tissue to contract forcefully when stimulated by AP_____________ – stretch w/out being damaged ____________ – return to original length & shape after contraction or extension

Structure of skeletal muscle, fig 10.1

Each _______________________________composed of 100-1000s cells* muscle ______= muscle _____ = myofibers

Endomysium = CT surrounds each fiberB.V. & nerves penetrate into muscle

Perimysium = CT surrounds the fascicle_______= 10s-100s of cells (fibers) bundled together

Epimysium = CT surrounds many fascicles to bundle a whole muscle together as an organ

Structure of skeletal muscle (2)

All 3 layers of CT (endo, peri, epimysium)

protect and strengthenextend from deep fascia - dense irregular CT bind muscles w/ similar function

________ = fibrous membrane covering

Supporting & separating muscles

Submicroscopic skeletal muscle

_____________= muscle fiber’s plasma mem.Skeletal muscle fibers can have >100 nuclei just beneath the sarcolemma____________ = invaginations in sarcolemma

tunnel into center of fiber, filled w/ ECFassists in exciting entire muscle fiber during AP

_____________-cytoplasm within muscle fibercontains ___________________ ATPstuffed w/ myofibrils

Myofibrils – threadlike structurescontractile elements, extend entire length of muscleArranged in ___________- basic functional units

Striated appearance

Myofilaments - composed of contractile proteins

DO NOT extend whole length of fiber2 types:

______________________________________________

_____________- red, oxygen binding proteinonly in muscle fibersContributes oxygen for ATP synthesis

Mitochondria- many in skeletal muscle Myoglobin & sarcoplasm have ingredients for ATP production:

O2

Glucose

_____________________ (SR)- fluid-filled system of membranous sacs, similar to sER

in relaxed muscle stores Ca 2+ release of Ca 2+ causes muscle contraction

____________- appearance due to light I bands and dark A bands____________- small mesodermal cells

Embryonic: skeletal muscle fibers arise from fusion of 100 or more100’s of nuclei

Satellite cells- myoblasts persisting in mature skeletal muscle

capacity to fuse w/ one another or w/ damaged muscle fibers Regenerate functional muscle fibers

Motor neuron & its muscle fibers

Figure 10.11- neuromuscular junction (NMJ)__________________- motor neuron & all the muscle fibers it stimulates__________________- region of sarcolemma that includes Ach receptors

Near synaptic end bulbs

___________________ (NMJ)- a synapse between axon terminals of a motor neuron & sarcolemma of a muscle fiber

Graded potential________________ from membrane potential that makes the membrane more or less polarized (Na+ & Ca 2+ in, K+ out)occur in dendrites & cell body of the motor neuronif graded potential reaches the axon:voltage-gated ion channels openAP

All-or-none principleWhen ______________ voltage is reached voltage gated channels will open and an action potential occursDifferent neurons may have different thresholds BUT the point is:________________________________

Push over first domino, the rest fall

__________________- molecules within axon terminals

Released into synaptic cleft in response toNerve impulseChange in membrane potential

___________________- NT released at NMJ1000s of molecules in each synaptic vesicle

Acetylcholine receptor (AChR)- at each motor end plate 30-40 million

Transmembrane protein binds AChBinding opens ligand-gated ion channels

Acetylcholinesterase (AChE)- enzyme breaks down ACh, attached to ECM in synaptic cleft

ACh binding lasts only brieflyBreaks down excess not bound to receptor

Nerve elicits a muscle action potential:1.Release of ACh- diffuses across synaptic

cleft.2.Activation of AChR: binding opens gated

ion channels allowing flow of small cations (most importantly Na+)

3.Production of muscle AP: inflow of Na+ inside fiber + charged, changing membrane potential, trigger AP

4.Termination of ACh activity: effect of ACh binding lasts briefly (ACh rapidly broken down by AChE)

*NMJ usually located at midpoint of muscle fibers & propagate toward both ends

Excitation - depolarizationIncreased Ca 2+ concentration in cytosol initiates muscle contraction[Ca 2+] due to depolarization of muscle cell membrane = sarcolemma

AP from neuron ACh receptors opening Na+ channels on the sarcolemma which depolarizes the muscle- muscle AP travels along T tubules causing SR to release Ca 2+

Skeletal contraction & proteins

Myofibrils built from 3 types protein;________________ proteins- generate force

Actin and myosin

________________ proteins- switch on and offTroponin and tropomyosin

Both are part of the thin filament

________________ proteins- proper alignment, elasticity, extensibility, link myofibrils to sarcolemma and ECM

12: titan, myomesin, dystrophin

Sliding filament theory fig 10.7,8

Muscle contraction:______ heads attach & “walk” along _____Walking towards both ends of sarcomere

Thin filaments M lineThick filaments Z disc

Length of thick and thin NOT changing

Sarcomere shortening whole muscle fiber shortens shorten entire muscle

_______- motor protein (push and pull)found in all 3 types muscle300 molecules/thick filament______________- convert chemical energy in ATP to mechanical energy of motion & force productionShaped like 2 golf clubs twisted together

______- molecules join to form filament in form of helix

On each actin molecule is a myosin binding site for myosin head to attach

The contraction cycle, fig 10.8

Ca 2+ released from SRBinds troponin-tropomyosin complex & move it

In relaxed muscle, myosin binding sites are blocked by:

______________- Are held in place by: _______________

Contraction cycle can begin:ATP hydrolysisMyosin attaches actin, form crossbridgesPower stroke Detachment of myosin from actin

Contraction of skeletal muscle

Treadmill analogyMyosin moving draws Z discs together

neighboring sarcomeres pulled together

Skeletal muscle shortens, pulls CT & tendonsTension passes thru tendon, move boneFig. 10.11 to summarize contraction

Calcium’s role [Ca2+] in cytosol starts contraction

(decrease stops contraction)

Muscle fiber relaxed: [Ca2+] low, BUT huge amt of Ca2+ stored in SR.AP propogates along sarcolemma T tubules, ________________________ in SR membrane openCa2+ flows out into cytosol, combines with troponin to change its shape

Myosin binding sites are now free

Sources of energy Figure 10.12______________: powering contraction cycle, pumping Ca2+ to SR for relaxation (& other metabolic rxns)

Relaxed state- modest amount usedContracting- using at rapid paceAmt present only enough for a few seconds of contraction

If strenous activity more ATP made…

3 ways to produce ATP_____________________________

Unique to muscle fibersWhile relaxed, muscle making more ATP than neededExcess creatine phosphate – an energy rich molecule

Enzyme: creatine kinase (CK) catalyzes transfer of one phosphate of ATP to creatine creatine phosphate & ADPWhen contraction begins, ADP levels so CK transfers phosphate group from creatine phosphate back to ADP creating ATP (enough energy to last 15 sec)

______________________________________________________________

Cellular respiration & muscle

Anaerobic- ATP-producing rxns, without O2

Muscle activity but no creatine phosphate glucose is catabolized to generate ATPGlucose: blood muscle fibers, & glycogen breakdown within muscle

Glycolysis: 1 glucose (10 rxns) 2 pyruvate yields 2 ATP

Pyruvic acid enters mitochondria & enters series of O2 requiring rxns to produce large amt of ATP

If no O2, pyruvic acid lactic acid in cytosolLiver cells take lactic acid glucose

Aerobic cellular respiration- series of O2 requiring rxns in the mitochondria produce ATP

muscle activity longer than ½ minutePyruvic acid ATP, CO2, H2O, and heat

Slower than glycolysis BUT yields more: 1 glucose 36-38 ATP molecules.

F.a. molecule over 100 ATP moleculesOxygen comes from:

Diffuses into muscle from bloodReleased from myoglobin within muscle fibers

Motor unit recruitmentProcess of ____________________________

Typically different motor units in a whole muscle are NOT stimulated to contract in unison

Alternation delays muscle fatigueContraction of whole muscle can be sustained for long pds

One factor responsible for producing smooth movements rather than series of jerks

Recruitment causes small changes in muscle tension

Comparing isotonic & isometric

___________ contraction- iso= same, tonic= tension

Contraction where tension remains sameOccurs when constant load is moved thru the range of motions possible at a jointLifting a book off a table

_______________ contraction- iso = same, metric = measure

Contraction in which tension of the muscle increases but there is only minimal shortening so that no visible movement is produced Holding a book in an outstretched hand

Simple twitch figure 10.15__________________- brief contraction of all muscle fibers in motor unit due to a single AP in motor neuron

MyogramSkeletal muscle twitch= 20-200msec

_____________ period- brief delay between application of stimulus & beginning of contraction (@2msec)

Ca2+ being released from SR & filaments exert tension, elastic components stretch, shortening begins

_______________ period= 10-100msec_______________ period= 10-100msec

Active transport of Ca2+ back into SR

Duration of all periods depends on type of muscle fiber (see table 10.1)

Fast twitch (as in eye) – 10 msec for each contraction and relaxationSlow twitch (as in legs) – 100 msec for each contraction and relaxation

Repolarization happens…During the relaxation period Ca2+ is actively transported back to the SR

Refractory period= period of lost excitability

Characteristic of all muscle and nerve cellsDuration varies with muscle involved

Skeletal 5msecCardiac 300msec

___________ = minimum stimulus – the least amount of voltage required for contraction_________________ – the voltage at which maximal force is generated (increasing voltage will not increase force of contraction)

All motor units are stimulatedAll muscle fibers are contracting

Graded response (graded potential) – small deviation in the membrane potential that makes the membrane more polarized or less polarized

_______________________________-Maximal voltage applied to muscle in which all fibers in unit are stimulatedseries of shocks at max voltage causes separate twitcheseach twitch will stronger than the previous

Stimuli all at same intensity, cause muscle to contract more efficiently each time

May be warm-up effect, due to intracellular Ca2+ needed for contraction

Terms assoc. w/ frequency

Wave summation- (summation of contraction) strength of muscle contraction that results when muscle APs occur one after another in rapid succession

frequency = strength of contraction

Tetany- fused tetanus -hyperexcitability of neurons & muscle fibers

Sustained or fused contraction Continuous tonic muscular contractions – individual twitches not discernedMay be due to hypoparathyroidism

Muscle fatigueInability to mantain force of contraction after prolonged activityusually results from ∆ w/in muscle fiberMay feel tired, desire to cease activity

Central fatigue (CNS)Mechanism unknown, possibly protective

Suspect contributing factors:Inadequate release of Ca2+ from SRDepletion of creatine phosphate (ATP levels not much change)Insufficent oxygen, depletion of glycogen,build up of lactic acid and ADP, failure of AP to release Ach

Recovery oxygen uptakeFormerly called ___________________ – the added oxygen that is taken into body after exerciseRecovery: few minutes to several hours depending upon intensity of exerciseDepletions during exercise:

Convert lactic acid glycogen in liverResynthesize creatine phosphate & ATPReplace oxygen removed from myoglobin

Post exercise oxygen needs remain high: body temp chemical rxnsHeart & muscles still working hard ATP useTissue repair processes

Definitions:Tonus= Muscle tone - state of partial contraction

characteristic of normal musclemaintained at least in part by a continuous motor impulses originating from reflex, and serves to maintain body posture AKA muscle tone

______________- wasting away or decrease in size of a part due to a failure, abnormality of nutrition, or lack of use_______________- excessive enlargement or overgrowth of tissue without cell division

Cardiac muscle

Figure 20.9, table 10.2Only in heart

forms most of the heart wall

StriatedAction is involuntary

alternating contraction & relaxation cannot be consciously influenced.

beats due to pacemaker- ________________

Cardiac muscle (2)Requires constant O2, many mitochondriaHormones and neurotransmitters adjust heart rate by speeding or slowing the pacemaker_______________- thickening of sarcolemma connecting ends of fibers together

Gap junctions to communicate from cell to cell

Smooth muscle, fig 10.18, 19

In walls of hollow internal structures: b.v., airways, and most organs of abdominopelvic cavity, also in skin and attached to hair follicles.

Looks ____________________________Action is usually ___________, some also has autorhythmicity-built in or intrinsic rhythm.Regulated by neurons of autonomic NS and by endocrine hormonesCompared to other types of muscle cells:

Contraction usually slower, lasts longer Can stretch and shorten to greater extent

Muscle disorders & myopathies

Myopathy- signifies disease or disorder of skeletal muscle tissue.Neuromuscular diesase – problems at all 3 sites:

Somatic motor neuron, NMJ & muscle fibers

Myasthenia gravis – autoimmune disease, chronic, progressive damage to NMJ

AB bind and block AchR at motor end plates75% of patients have hyperplasia or tumors of thymus

Thymic abnormality may be the cause

1st affects eye swallowing, chewing, talkinglimbsDeath may result from paralysis of respirtory musclesAnticholinesterase drugs (inhibits AchE)

Muscular dystrophy- group of inherited muscle destroying diseases

Degeneration of skeletal muscle fibersDMD= Duchenne muscular dystrophy

Mutation on X chromosome strikes males almost exclusivelyDifficultly running, jumping, hoppingUnable to walk @12, resp or cardiac failure usually death between 20-30 yrsGene defect – protein dystrophin

Little or no dystrophin, sarcolemma tears during contractionGene therapy- inject myoblasts w/functional gene

_______________- painful, nonarticular rheumatic disorder

Usually ages 25-50, 15X more in femalesAffects fibrous CT components of muscle, tendons and ligamentsPain at tender points from gentle pressureFatigue, poor sleep, headaches, depression

______- sudden involuntary contraction of single muscle of large muscle group

__________- painful spasmodic contraction

______- spasmodic twitching made involuntarily by muscles that are usually under voluntary control

Eyelid, facial muscles

_____________- rhythmic, involuntary, purposeless contraction that produces quivering or shaking_____________-involuntary brief twitch of an entire motor unit that is visible under skin_________________- spontaneous contraction of since muscle fiber that is not visible but can be recorded

May signal destruction of motor neurons