Controlling Muscle Controlling Muscle Contraction Contraction &&Muscle ResponseMuscle Response

Chapter 8: Muscular SystemChapter 8: Muscular System

Unit 2: Support and Unit 2: Support and MovementMovement

Types of MuscleTypes of Muscle

Recall there are three types of Recall there are three types of musclemuscle– Cardiac and Smooth muscle are Cardiac and Smooth muscle are

considered considered involuntaryinvoluntary You cannot consciously control these You cannot consciously control these

musclesmuscles

– Skeletal muscle is the only muscle Skeletal muscle is the only muscle tissue that is under tissue that is under voluntary voluntary controlcontrol

You think to move your skeletal muscleYou think to move your skeletal muscle You control your movements using your You control your movements using your

nervous system.nervous system.

Nervous SignalNervous Signal

When you want to move you big toe…When you want to move you big toe…– You think about moving your big toeYou think about moving your big toe– Your brain generates an electrical signalYour brain generates an electrical signal– The signal is sent down your spineThe signal is sent down your spine– The signal is sent down a nerve in your legThe signal is sent down a nerve in your leg– The signal reaches the end of the nerve’s The signal reaches the end of the nerve’s

axon at the axon at the motor end platemotor end plate

Neuromuscular Neuromuscular JunctionJunction

AcetylcholineAcetylcholine

A nerve impulse will be sent to A nerve impulse will be sent to the end of a motor neuron axon.the end of a motor neuron axon.

– This impulse will stimulate the This impulse will stimulate the release of acetylcholine into the release of acetylcholine into the synaptic cleft.synaptic cleft.

Acetylcholine diffuses across synaptic Acetylcholine diffuses across synaptic cleft and react with receptors in motor cleft and react with receptors in motor plate of muscle.plate of muscle.

– Once acetylcholine in the muscle fiber Once acetylcholine in the muscle fiber membrane associates with a receptor, a membrane associates with a receptor, a Muscle ImpulseMuscle Impulse is stimulated. is stimulated.

Muscle ImpulseMuscle Impulse

Muscle Impulse AnimationMuscle Impulse Animation

Muscle ImpulseMuscle Impulse

The muscle impulse travels through The muscle impulse travels through the transverse tubules, deep into the transverse tubules, deep into the muscle fibers, and reaches the the muscle fibers, and reaches the sarcoplasmic reticulum.sarcoplasmic reticulum.

– The sarcoplasmic reticulum contains a The sarcoplasmic reticulum contains a high concentration of calcium ions.high concentration of calcium ions.

– Like a reservoir of calcium ready to Like a reservoir of calcium ready to burstburst

Role of Calcium IonsRole of Calcium Ions

In response to a muscle impulse, In response to a muscle impulse, the SR opens gates that allows the SR opens gates that allows Ca2+ to move out.Ca2+ to move out.

– Ca2+ ions bind with troponin, which Ca2+ ions bind with troponin, which changes shape and moves changes shape and moves tropomyosin off the actin binding tropomyosin off the actin binding sites.sites.

As a result, myosin is able to bind to As a result, myosin is able to bind to actin and begin the contraction cycle actin and begin the contraction cycle described earlierdescribed earlier

Muscle RelaxationMuscle Relaxation

When nerve impulses cease, two When nerve impulses cease, two events lead to muscle relaxation:events lead to muscle relaxation:

1.1. Acetylcholine is decomposed by Acetylcholine is decomposed by

AcetylcholinesteraseAcetylcholinesterase..

2. 2. Calcium ions are actively pumped Calcium ions are actively pumped back into the sarcoplasmic reticulum.back into the sarcoplasmic reticulum.

Threshold StimulusThreshold Stimulus

When a muscle fiber is exposed to a When a muscle fiber is exposed to a series of nervous stimuli of increasing series of nervous stimuli of increasing strength, the fiber remains strength, the fiber remains unresponsive until certain strength of unresponsive until certain strength of stimulation is applied.stimulation is applied.

– The minimum strength required to cause The minimum strength required to cause a contraction is called the a contraction is called the Threshold Threshold StimulusStimulus..

All-or-None ResponseAll-or-None Response

Muscle fiber exposed to a stimulus of Muscle fiber exposed to a stimulus of threshold strength respond to its threshold strength respond to its fullest extent.fullest extent.– In other words, all the myosin heads react In other words, all the myosin heads react

by binding and pulling on the actinby binding and pulling on the actin

– The extent of shortening depends on the The extent of shortening depends on the resistance & the number of motor units resistance & the number of motor units that are recruited for the liftthat are recruited for the lift

This phenomenon in which muscles always This phenomenon in which muscles always contracting fully is called the contracting fully is called the All-or None All-or None ResponseResponse..

Recording a Muscle Recording a Muscle ContractionContraction

A muscle can be stimulated A muscle can be stimulated electrically, and when it electrically, and when it contracts, its movement can be contracts, its movement can be recorded.recorded.

– The resulting muscle contraction The resulting muscle contraction pattern is called a pattern is called a MyogramMyogram..

Recording Recording ContractionsContractions If a muscle is exposed to a single If a muscle is exposed to a single

stimulus to activate some of its stimulus to activate some of its motor units, the muscle will motor units, the muscle will contract and then relax.contract and then relax.

– A single contraction that lasts only a A single contraction that lasts only a fraction of a second is called a fraction of a second is called a TwitchTwitch..

The delay between the time the stimulus The delay between the time the stimulus was applied and the time the muscle was applied and the time the muscle responded is the responded is the Latent PeriodLatent Period..

Recording Recording Contractions Contractions ContinuedContinued The latent period lasts less than The latent period lasts less than

0.001 seconds.0.001 seconds.

– This period is followed by a This period is followed by a Period Period of Contractionof Contraction when the muscle when the muscle pulls at its attachments.pulls at its attachments.

A A Period of RelaxationPeriod of Relaxation when it when it retunes to its former length.retunes to its former length.

SummationSummation

A muscle fiber exposed to a series A muscle fiber exposed to a series of stimuli of increasing frequency of stimuli of increasing frequency reaches a point when it is unable reaches a point when it is unable to completely relax before the to completely relax before the next stimulus in the series next stimulus in the series arrives.arrives.

– The force of individual twitches The force of individual twitches combines by the process of combines by the process of SummationSummation..

Series of TwitchesSeries of Twitches

SummationSummation

Tetanic ContractionTetanic Contraction

When the resulting forceful, sustained When the resulting forceful, sustained contraction lacks even partial relaxation, it is contraction lacks even partial relaxation, it is called a called a Tetanic ContractionTetanic Contraction..

Use and Disuse of Use and Disuse of Skeletal MuscleSkeletal Muscle

Skeletal muscles are very Skeletal muscles are very responsive to use and disuse.responsive to use and disuse.

– Forcefully exercising muscles causes Forcefully exercising muscles causes them to enlarge (them to enlarge (Muscular Muscular HypertrophyHypertrophy))

– Unused muscle undergoes Unused muscle undergoes AtrophyAtrophy decreasing their size and strength.decreasing their size and strength.

Muscle FibersMuscle Fibers

There are two types of muscle There are two types of muscle fibers:fibers:

Slow Twitch FibersSlow Twitch Fibers

Fast Twitch FibersFast Twitch Fibers These fiber types respond These fiber types respond

differently to exercise/stress.differently to exercise/stress.

Slow FibersSlow Fibers

A muscle contracting weakly A muscle contracting weakly activates a specialized group of activates a specialized group of muscle fibers called muscle fibers called Slow FibersSlow Fibers..

– When stimulated, slow fibers When stimulated, slow fibers produce more mitochondria, and a produce more mitochondria, and a more extensive capillary networkmore extensive capillary network Mitochondria for extra ATP productionMitochondria for extra ATP production Capillaries to deliver more oxygenCapillaries to deliver more oxygen *Their overall size does not change*Their overall size does not change

Fast FibersFast Fibers Forceful exercise, in which a Forceful exercise, in which a

muscle exerts more than 75% of its muscle exerts more than 75% of its maximum force, utilizes specialized maximum force, utilizes specialized muscle fibers called muscle fibers called Fast FibersFast Fibers..

– In response to strenuous exercise, In response to strenuous exercise, these fibers produce new filaments of these fibers produce new filaments of actin and myosin.actin and myosin.

The diameter of the muscle fibers The diameter of the muscle fibers increase, and the entire muscle enlarges.increase, and the entire muscle enlarges.

The extra myosin and actin fibers give the The extra myosin and actin fibers give the muscle more strengthmuscle more strength

DisuseDisuse

If regular exercise stops:If regular exercise stops:

– Capillary network shrinkCapillary network shrink

– Number of mitochondria within the Number of mitochondria within the muscle fibers dropsmuscle fibers drops

– Number of actin and myosin Number of actin and myosin filaments decreasefilaments decrease

Unused muscle may decrease to less Unused muscle may decrease to less than half its usual size within a few than half its usual size within a few months.months.