2999867 muscular system

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Muscular System



Histological Features of Skeletal Muscle

Entire muscle composed of numerousfasiculi, or bundles of muscle fibers

Fasicles are from 50 to 200 microns indiameter

Connective Tissue - entire musclesurrounded by the epimysium

– perimysium surrounds each fasicle

– endomysium surrounds the fibers – connective tissues join at the terminal ends to

form the tendons



Muscle Anatomy



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are needed to see this picture.



Fiber Structure

Fibers - each fiber is a multinucleate syncytiummade up of myofibrils

myofibrils 1-3 microns segregated into dark(anisotropic-A) and light (isotropic-I) bands

A and I bands of adjacent myofibrils are lined up

each I band is bisected by a z line

from Z line to Z line is a sarcomere which is thefunctional unit of a muscle



Muscle Fiber Structure



Myofibril Structure

Myofibrils made up of thick (myosin 120 A diameter,

1.6 microns long) and thin (Actin 40 A diameter)filaments

In the middle of each A band is an H zone bisectedby an M line

Each myosin is surrounded by six actin filaments Each actin receives cross bridges from three myosin

Myosin cross bridges - spaced at 143 A, with a 120degree angle of displacement



Sarcomere Structure



Actin-Myosin Configuration



Actin and Myosin



Myosin Subunits



Myosin Polymerization



Sliding Filament Concept

This change in size of the sarcomere is dueto a sliding of the actin filaments across themyosin when actin binds to myosin crossbridges which subsequently move

The length of the A band remains constant,with the Z lines moving closer together

Both the I band and the H band become

narrower. Extreme contraction can lead to deformation

of the myosin filaments.



Sliding

FilamentChanges



Regulatory Molecules

Troponin-tropomyosin is found on the actinfilaments and can inhibit the binding of actinto the myosin cross bridges.

The tropomyosin molecule partially coversthe myosin binding site on the actin and isheld in place by the troponin molecules.

When calcium binds to the troponin, it pulls

tropomyosin to the side and uncovers thecross bridge binding sites.

Removal of the calcium will reverse thisprocess.



Excitation-Contraction Coupling

The presence or absence of calcium isdetermined by the electrical activity of themuscle membrane (action potential).

The sequence of events by which an actionpotential leads to cross bridge activity throughincreased calcium is termed excitation-contraction coupling.



Contraction Cycle

1) Stimulation causes release of Ca2+ from Sarcoplasmic

Reticulum (muscle ER) 2) Ca2+ binds to troponin (C subunit)

3) Structural change in troponin removes tropomyosin from

actin-myosin binding site

4) Myosin head contacts actin molecule

5) Myosin ATPase splits ATP providing energy for "rowing

backward" motion of myosin head

6) Another ATP molecule becomes bound to myosin causing

release, return of head to normal state, and reattachment to actin(if Ca2+ present)

7) Repeat process until contraction attained



Cross Bridge Cycle



Rigor Mortis

Rigor mortis is a rigidity of the body caused by musclescontracting after death from chemical changes within

muscle tissue. It starts in all muscles at the same time.

But it is first noticed in the small muscles of the face,

neck, lower jaw, hands, and feet.

Rigor mortis results from a lack of ATP to break the

binding of actin and myosin, thus leaving the muscle in

the contracted state



Events Leading to Muscle Contraction

– 1. activation of a motor neuron– 2. release of acetylcholine at the myoneural

junction with a resultant action potential

– 3. AP going down T-tubule leading to the release

of calcium by the SR– 4. removal of the troponin-tropomyosin inhibitory

effect and a resultant cross bridge binding andmovement resulting in contraction

– 5. Removal of calcium from the sarcoplasmrestores troponin-tropomyosin inhibitory effectand prevents further cross bridge movement



Muscle Contraction Cycle



Histology of Neuromuscular Junction

As neuron approaches the fiber it looses itsmyelin sheath and divides into a terminalarborization in a muscle groove.

Muscle surface is called the motor end plate.

Synaptic terminals contain acetylcholinevesicles which are released by exocytosis asin any synaptic terminal due to the influx of

calcium.



Motor End Plate Function

EPP (end plate potential) results from thistransmitter release.

It is similar to an EPSP only much larger inamplitude.

Normally able to depolarize to thresholdthose fibers which are innervated (1:1relationship of EPP:AP)

In most skeletal muscle there is no inhibitorypotentials.



Motor End Plate Function Acetylcholinesterase breaks down Ach to terminate

transmission Curare blocks the binding sites for Ach without

inducing a response

Organophosphate pesticides and nerve gases block

the action of Achesterase which leads to prolongeddepolarization

Botulinin toxin blocks Ach release 0.0001mg can killa man (500mg entire pop)

myasthemia gravis - decreased Ach receptor sitesdue to a self destructive auto-immune response



Myoneural Junction




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Role of Sarcoplasmic Reticulum

Lateral sacs of Sarcoplasmic Reticulum store

calcium.

Lateral sacs are connected to each other by way of avesicular network.

Lateral sacs are adjacent to T-tubules which are

invaginations of the sarcolemma down which AP’stravel.

Each T-tubule is in close association with two lateralsacs and forms a triad.

In the frog the triads are at the Z line, while inmammalian muscle they are at the junction of the Aand I bands.



Roles of Calcium in Muscle Function

Leads to the removal of troponin’s inhibitoryeffect on actin and myosin binding

Activates a calcium ATPase pump which overa longer period of time returns the calcium tothe SR.



Temporal Events of Contraction

Action potential reaches its peak about 1.5

msec after stimulation

Latent period before contraction of 3-5 msectension development takes from 50-120 msec

Decay of contraction takes from 100-500msec

A second stimulus can add to the tension of afirst stimulus (summation)

15-120 stimuli/second produces tetany whichis a strong maintained contraction



Cycle of Contraction



Latent Period

Due to several factors – Time for AP propagation along the t-tubules

into the fiber

– Release of calcium from the sarcoplasmicreticulum

– Diffusion of calcium to the troponin site

– Binding of calcium

– Activation of myosin cross bridges



Coordination of Contractions

Most useful movements of body due to repeated

individual contractions These must be coordinated to produce useful work

Muscles are a contractile element arranged in

parallel with one elastic component and in serieswith another

– Parallel components are the membrane and connective

tissues in parallel with them

– Series components are tendons, CT linking musclefibers to tendons and perhaps Z disks

– The myosin cross bridges themselves are also part of

the series elastic components



Elastic Components Function

Isometric

phase

Isotonic

phase

i



Active State

This is the period of time in which internal tension

is building within a muscle

Under resting conditions muscles show little

resistance to stretch except from the resistance due

to CT During this phase resistance to stretch builds up

quickly

Brief increase in tension due to cross-bridge

movement is called a twitch

Under prolonged stimulation external tension can

equal internal tension resulting in tetany

i h d



Twitch and Tetany

M h f T i h d T



Myographs of Twitch and Tetany

M l M h i



Muscle Mechanics

Many properties of muscular mechanics were worked out

prior to elucidation of the contraction mechanism Contractions are classified based upon what happens to the

length of the active muscle

– Isotonic - the muscle shortens as force is generated (in

the strictest sense the tension remains constant) – Isometric - the muscle length is fixed and internal

tension builds up ( a small degree of internal shortening

is possible)

– Previous material on mechanics of the twitch basedupon an isometric contraction

I i V I i



Isometric Vs Isotonic



F V l it R l ti hi



Force-Velocity Relationship

Originally worked out by attaching a muscle tovarious weights and determining the time frame of

the contraction

As load increases, the shortening velocity

decreases (assuming the optimal overlap of actin

and myosin)

Two reasons for this

– Average force generated by cross bridges decrease asspeed increases

– Total number of cross bridges activated at any one time

drops as speed increases



The relationship between force (tension) development and velocity (rate) of muscle contraction is said to be inverse.

In this sense, eccentrics are considered "slower" than isometric contractions,

and isometrics are considered "slower" than concentric contractions.

("eccentric" implies that the working muscle is being overloaded to the point

where it cannot hold the external weight.) Eccentric contractions can develop more tension than isometric contractions,

and isometric contractions can develop more tension than concentric

contractions.

Faster concentric contractions develop less tension than slower concentric

contractions.

Faster eccentric contractions develop more tension than slower eccentric

contractions.

Isometric contractions develop more tension than concentrics, but less than

eccentrics.

F V l it



Force-Velocity

L th T i R l ti hi



Length-Tension Relationship each muscle has an optimal length to give maximal

tension development this is based on the sliding filament theory of contraction

175% of optimal length-no overlap of actin and myosin

optimal length - maximum number of cross bridges can

be activated below optimal length - thin filaments can overlap during

contraction and thus decrease the number of active crossbridges

at less than 80% of optimal length there is also reducedcalcium release

most muscles in the body when relaxed are at theoptimal length

L th T i R l ti hi



Length-Tension Relationship



M l E ti



Muscle Energetics

Two major processes require ATP Hydrolysis of ATP by myosin cross-bridges

Pumping of Calcium back into the

sarcoplasmic reticulum (2ATP/calcium) During tetany ATP is being hydrolyzed by

both myosin ATPase and calcium pumps

Research indicates that calcium pumps useabout 25-30% of total ATP consumption

E ti



Energetics

Phosphagen S stem



Phosphagen System

Muscle Types



Muscle Types

Several properties determine muscle fiber type

– Electrical properties - if membrane produces

AP’s twitch is all or nothing

– Rate of cross-bridge detachment - based on

chemical nature of myosin heavy chains

– Density of calcium pumps

– Number of mitochondria and density of blood

supply

Four Types



Four Types

Tonic muscle fibers - contract slowly and do not produce twitches - postural muscles of

amphibians, reptiles and birds - stretch receptors

Slow twitch fibers-contract slowly and fatigue

slowly-mammalian postural

Fast twitch oxidative - activate quickly but

relatively resistant to fatigue - flight muscles

Fast twitch glycolytic - contract very rapidly andfatigue quickly - breast of domestic fowl

Muscle Types



Muscle Types

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Photo - JPEG decompressor are needed to see this picture.

Features of Working Muscles



Features of Working Muscles

Three factors determine the efficiency of muscles in completion of their work

– Length-tension relationship - degree of overlap

between thick and thin filaments

– Relative velocity of shortening during work -

determines the power and efficiency of muscle

– The timing and duration of the muscle’s active

state

Metabolic Types



Metabolic Types

The different types of muscles discussed areeach best adapted to particular types of

activities as described in your text

– Power - frog jumping – Contrasting actions - steady vs escape

swimming in fish

– Speed - sound production in toadfish (short

bursts) vs rattttlesnakes (prolonged)

Frog Jump



Frog-Jump

Steady Swimming



Steady Swimming

Escape Swimming



Escape Swimming

Toadfish Muscles



Toadfish Muscles

Time for transport of

Calcium back into

sarcoplasmic reticulum

Red vs Sonic Fibers



Red vs Sonic Fibers

High Calcium Threshold

and lack of tetany

at higher frequency

Low Calcium Thresholdand tetany at lower frequency

Sonic vs Shaker Muscle



Sonic vs Shaker Muscle

Calcium time

frame

Twitch

Time

Course

Vertebrate Motor Control



Vertebrate Motor Control

Muscles typically arranged in antagonistic pairs Each muscle has several nerves that can innervate

it (motor pool)

One neuron and the fibers it innervates make up a

motor unit (typically 100 fibers)

Size of motor unit determines precision of

movement

All vertebrate motor neurons are excitatory

Motor Unit



Motor Unit

Vertebrate Control



Vertebrate Control

Twitch vs Tonic Fibers



Twitch vs Tonic Fibers

In twitch fibers there is one a one:one relationship betweenneuronal AP and muscle AP

Graded contraction of a muscle depends upon activation of

multiple motor units

Tonic fibers receive multiterminal innervation There are no all or nothing AP’s therefore graded

responses are possible due to the number of terminals

activated

Frequency of stimulation is a major factor in determiningmuscle tension

Neuron type is often matched to the type of fibers that they

innervate (ie. Fast fibers - high frequency AP neurons)

Arthropod Muscle



Arthropod Muscle

Few neurons - one may innervate an entire muscle no action potentials in many

often show graded responses which allows a wide

range of tension not possible with the typical

vertebrate system

Summation of both inhibitory and excitatory

synapses possible in many cases

Multiple muscle types present, from rapid all or nothing fibers to slow fibers and all ranges in

between

Arthropod Control



Arthropod Control

Types of Arthropod Fibers



Types of Arthropod Fibers

Asynchronous Flight Muscle



Asynchronous Flight Muscle

In most muscle high activity would require many SR calcium channels and calcium pumps which is the case in

some vertebrate muscle that contracts very rapidly

This would require massive cellular space to be occupied

by SR and mitochondria at the expense of myofilamaents

for force

In these flight muscles a single AP can produce many

contractions, thus limiting the need for calcium sources

These muscles can contract at rates much higher than the

potential rate of AP production despite slow changes in

calcium levels and limited stores of ATP

They thus can contract at rates higher than synchronous

flight muscle found in some insects can.

Factors Which Produce Rapidill i



Oscillations

Physiology of the muscle itself Anatomic arrangement of the muscle in the thorax

Mechanical properties of the thorax and wing

joints

Contraction begins only after both proper calcium

levels are reached and proper stretch of the muscle

In these muscles the thorax shape changes during

contraction at a high frequency Muscles do not actually connect to the wing itself

Experimental Setup to Study



Experimental Setup to Study

Contraction Cycle



Contraction Cycle

Calcium low Calcium low

Synchronous vs Asynchronous Flight Muscles



Synchronous vs Asynchronous Flight Muscles

Smooth Muscle



Smooth Muscle

Lack sarcomeres Found in walls of hollow organs and supports visceral

function

There are multiple classes making smooth muscle harder to

characterize than either skeletal or cardiac Often function somewhat independently of the nervous

system

All innervated by neurons of the autonomic nervous

system Can produce more force than some skeletal muscle and

more prolonged contractions with less energy

Smooth Muscle



Smooth Muscle

Contract utilizing actin and myosin Each fiber is an individual cell with a single

nucleus

Little or no sarcoplasmic reticulum and lack T

tubules

Actin and myosin grouped into bundles anchored

in dense bodies or connected to attachment plaque

on the plasma membrane

Smooth Muscle



Smooth Muscle contains three filament types

Thin or Actin - anchored to dense bodies similar to Z lines2X that of skeletal

Thick or myosin - much larger than in skeletal 1/3 that of

skeletal

10-15 actin/myosin intermediate - non contractile fibers

These filaments are not arranged in a regular pattern as in

striated muscle, as a result smooth muscle can operate over

a broader range of stretch. No troponin as a regulatory system, calcium regulates

contraction by controlling an enzyme that phosphorylates

myosin

Vertebrate Smooth Muscle




Single Unit - small spindle shaped cellselectrically coupled by gap junctions

Often contract in response to spontaneous

depolarization (myogenic) Entire group of fibers behave as a “single-

unit”

Autonomic neurons can modulate rate,strength and frequency of contraction





Multi-unit smooth muscle - act independently andcontract in response to neuronal input or in some

cases hormonal input

Autonomic synapses release transmitter from

many varicosities along the length of the axonwithin the smooth muscle tissue

This transmitter can influence numerous cells as it

diffuses from the source In many cases this resembles neuromodulatory

synaptic function

Multi-Unit Smooth Muscle



u t U t S oot usc e



The same transmitter or hormone can have differing effects

on different smooth muscle due to different second

messenger systems.

NE - depolarizes vascular smooth muscle leading to

contraction, but hyperpolarizes visceral smooth muscle

thus decreasing its activity.

The local responses are useful in regulating local blood

flow or other local physiological changes.

Actin and Myosin Organization



y g

Contraction Regulation



g

Contraction cycle regulated by calcium levels Excitation contraction coupling is much slower

than in skeletal muscle

Plasma membrane performs most of the regulation

of calcium entry into and exit from the cell

Depolarization opens voltage gated calcium

channels

In some smooth muscle there are actually calcium produced AP’s

Actin Regulation



g

Binds to actin

Myosin Regulation



y g

Myosin Regulation



y g

Vascular Smooth Muscle Regulation



Myosin

Light Chain

Activation

Modulation of Activity



y

1. spontaneous - pacemaker potential due to cyclic

changes in the sodium-potassium pump leading to

alternating depolarization and hyperpolarization

2. neurotransmitters - can de or hyperpolarize

3. hormones - can de or hyperpolarize

4. local changes in extracellular fluids - oxygen,

osmolarity, ion concentrations

5. stretch

Unusual Characteristics



Stretch can produce depolarization

Net result is to maintain constant tension over a broad

range of stretch

This is responsible for many self regulatory activities such

as in blood pressure control

Stretch induced contractions are partly responsible for

peristalsis

Smooth muscle may reduce the rate of contraction

resulting in a condition referred to as latch or catch in

which the muscles maintain a contracted state with little

energy utilization

Summary of Regulatory Mechanisms



Phasic vs Tonic Smooth Musclein Vertebrates



in Vertebrates

Catch Muscle in Mollusks

2999867 muscular system

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