Copyright © 2010 Pearson Education, Inc.. Muscle Functions 1.Movement of bones or fluids (e.g.,...


Muscle Functions

1. Movement of bones or fluids (e.g., blood)

2. Maintaining posture and body position

3. Stabilizing joints

4. Heat generation (esp. skeletal muscle)


Skeletal Muscle

• Connective tissue sheaths of skeletal muscle:

• Epimysium: fibrous CT surrounding entire muscle

• Perimysium: fibrous CT surrounding fascicles (groups of muscle fibers)

• Endomysium: delicate CT surrounding each muscle fiber


Perimysium

Endomysium

Muscle fiber

Fascicle(wrapped by perimysium)

Epimysium

Tendon

Epimysium

Muscle fiberin middle ofa fascicle

PerimysiumEndomysium

Fascicle


Skeletal Muscle: Attachments

• Muscles attach to bone by an origin and insertion

• Origin —is fixed and on the immovable bone

• Insertion—is on the movable bone.

• As a contraction occurs the insertion moves towards the origin


NucleusLight I bandDark A band

Sarcolemma

Mitochondrion

Myofibril

• Multiple peripheral nuclei, many mitochondria

• Also contain sarcolemma, myofibrils, sarcoplasmic reticulum, T tubules

Microscopic Anatomy of a Skeletal Muscle Fiber


Myofibril

Myofibrils

Triad:

Tubules of the SR

Sarcolemma

Mitochondria

I band I bandA bandH zone Z discZ disc

• T tubule• Terminal

cisternaeof the SR (2)

M line


Myofibrils• Densely packed, rodlike organelles

• ~80% of cell volume

• Composed of sarcomeres

• Exhibit striations:perfectly aligned repeating series of dark A bands and light I bands

Myofibril


Sarcomere

• Smallest contractile unit (functional unit) of a muscle fiber

• The region of a myofibril between two successive Z discs

• Composed of myofilaments: Thick (myosin) and thin (actin)


Regions of a Sarcomere

• A band (Dark Band)- Contains thin & thick filaments

• H zone: lighter midregion where filaments do not overlap

• M line: proteins that hold adjacent thick filaments together; center of sarcomere

• I band (Light Band)- Contains only thin filaments

• Z disc: proteins that anchor thin filaments; mark start and end of one sarcomere


I band I bandA bandSarcomere

H zoneThin (actin)filament

Thick (myosin)filament

Z disc Z disc

M line

(c) Part of one myofibril

Z disc Z discM line

Sarcomere

Thin (actin)filament

Thick(myosin)filament

Elastic (titin)filaments

(d)


Myofilaments of a Sarcomere

• Thick Filament

• Composed of many myosin proteins

• A single myosin protein has a “tail” and a “head” that can:

• Bind actin and pull it during a contraction

• Hydrolyze ATP to release energy

• Thin Filament

• Twisted double strand of actin protein

• Has active sites for the myosin head


Flexible hinge region

Tail

Tropomyosin Troponin Actin

Myosin head

ATP-bindingsite

Heads Active sitesfor myosinattachment

Actinsubunits

Actin-binding sites

Thick filament Thin filament

Thin filamentThick filament

Longitudinal section of filamentswithin one sarcomere of a myofibril

Portion of a thick filamentPortion of a thin filament

Myosin molecule Actin subunits


I

Fully relaxed

Fully contracted

IA

Z ZH

I IA

Z Z

1

2

•In the relaxed state, thin and thick filaments slightly overlap•During contraction, myosin heads bind to actin, detach, and bind again, propelling thin filaments toward M line•As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and whole muscle shortens


The Neuromuscular Junction

• Defined as:

• Axons of motor neurons: travel from the brain/spinal cord via nerves to skeletal muscles

• Each axon: branches into a number of axon terminals as it enters a muscle

• Each axon ending forms: a neuromuscular junction with a single muscle fiber


Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal ofneuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+

Axon terminalof motor neuron

Synaptic vesiclecontaining ACh

MitochondrionSynapticcleft

Fusing synaptic vesicles

1 Action potential arrives ataxon terminal of motor neuron.

2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.


The Neuromuscular Junction

• Axon terminal and muscle fiber are: separated by space called the synaptic cleft

• Synaptic vesicles within axon terminal contain: the neurotransmitter acetylcholine (ACh)

• Junctional folds of the sarcolemma contain: ACh receptors (chemically-gated channels)


Events at the Neuromuscular Junction1) A nerve impulse: arrives at the axon terminal

2) Ca2+ floods into axon terminal

3) Ca2+ entry causes synaptic vesicles to release Ach

4) ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma

5) Ach binding opens channels

6) Na+ floods into muscle fiber and K+ floods out making the interior of cell less negative

7) Once threshold is reached an AP is generated


• The AP is an unstoppable, electrical event that travels along the entire sarcolemma conducting the electrical impulse from one end of cell to the other

• Repolarization :The muscle cell returns to its resting state mainly by the exit of K+

The Action Potential

Copyright © 2010 Pearson Education, Inc. Figure 9.8

Nucleus

Actionpotential (AP)

Myelinated axonof motor neuron

Axon terminal ofneuromuscular junction

Sarcolemma ofthe muscle fiber

Ca2+Ca2+


Synaptic vesiclecontaining AChMitochondrionSynapticcleft

Junctionalfolds ofsarcolemma

Fusing synaptic vesicles

ACh

Sarcoplasm ofmuscle fiber

Postsynaptic membraneion channel opens;ions pass.

Na+ K+

Ach–

Na+

K+

Degraded ACh

Acetyl-cholinesterase

Postsynaptic membraneion channel closed;ions cannot pass.

1 Action potential arrives ataxon terminal of motor neuron.

2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.

3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.

4 Acetylcholine, aneurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.

5 ACh binding opens ionchannels that allow simultaneous passage of Na+ into the musclefiber and K+ out of the muscle fiber.

6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.


Destruction of Acetylcholine

• ACh effects are quickly terminated by the enzyme acetylcholinesterase

• Prevents continued muscle fiber contraction in the absence of additional stimulation


Na+

Na+

Open Na+

Channel

Closed Na+

Channel

Closed K+

Channel

Open K+

Channel

Action potential++++++

+++++

+

Axon terminal

Synapticcleft

ACh

ACh

Sarcoplasm of muscle fiber

K+

2 Generation and propagation ofthe action potential (AP)

3 Repolarization

1 Local depolarization:

K+

K+Na+

K+Na+

Wave ofde

po

lari

zatio

n



Muscle fiberTriad

One sarcomere

Synaptic cleft

Setting the stage

Sarcolemma

Action potentialis generated

Terminal cisterna of SR ACh

Ca2+


Steps inE-C Coupling:

Terminal cisterna of SR

Voltage-sensitivetubule protein

T tubule

Ca2+

releasechannel

Ca2+

Sarcolemma

Action potential ispropagated along thesarcolemma and downthe T tubules.

1


Steps inE-C Coupling:

Terminal cisterna of SR

Voltage-sensitivetubule protein

T tubule

Ca2+

releasechannel

Ca2+

Sarcolemma

Action potential ispropagated along thesarcolemma and downthe T tubules.

Calciumions arereleased.

1

2


Role of Calcium (Ca2+) in Contraction

• At low intracellular Ca2+ concentration:

• Active sites on actin are blocked

• Myosin heads cannot attach to actin

• Muscle fiber relaxes


Troponin Tropomyosinblocking active sitesMyosin

Actin

Active sites exposed and ready for myosin binding

Ca2+

Calcium binds totroponin and removesthe blocking action oftropomyosin.

The aftermath

3


Role of Calcium (Ca2+) in Contraction

• At higher intracellular Ca2+ concentrations:

• Ca2+ causes binding sites on actin to be exposed

• Events of the cross bridge cycle occur

• When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends


Troponin Tropomyosinblocking active sitesMyosin

Actin

Active sites exposed and ready for myosin binding

Ca2+

Myosincross bridge

Calcium binds totroponin and removesthe blocking action oftropomyosin.

Contraction begins

The aftermath

3

4


Cross Bridge Cycle

• Continues as long as the Ca2+ signal and adequate ATP are present

• Cross bridge formation: high-energy myosin head attaches to thin filament

• Power stroke: myosin head pivots and pulls thin filament toward M line


Cross Bridge Cycle

• Cross bridge detachment: ATP attaches to myosin head and the cross bridge detaches

• “Cocking” of the myosin head: energy from hydrolysis of ATP cocks the myosin head into the high-energy state


1

Actin

Cross bridge formation.

Cocking of myosin head. The power (working) stroke.

Cross bridge detachment.

Ca2+

Myosincross bridge

Thick filament

Thin filament

ADP

Myosin

Pi

ATPhydrolysis

ATP

ATP

24

3

ADP

Pi

ADPPi


Spinal cord

Motor neuroncell body

Muscle

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motorneuronaxon

Axon terminals atneuromuscular junctions

• Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies


Graded Muscle Responses

• Defined: Variations in the degree of muscle contraction

Responses are graded by:

1. Changing the frequency of stimulation

2. Changing the number of muscle cells being stimulated at one time (by changing strength of stimulus)

Copyright © 2010 Pearson Education, Inc. Figure 9.15a

Contraction

Relaxation

Stimulus

Single stimulus single twitch

A single stimulus results in a single contractile response called a muscle twitch

Response to Change in Stimulus Frequency


Response to Change in Stimulus Frequency

• Increase frequency of stimulus muscle doesn’t have time to completely relax (btwn. stimuli)

• Ca2+ release stimulates further contraction temporal (wave) summation

• Further increase in stimulus frequency unfused (incomplete) tetanus

• If stimuli are given quickly enough, fused (complete) tetanus results

Copyright © 2010 Pearson Education, Inc. Figure 9.15b

Stimuli

Partial relaxation

Low stimulation frequencyunfused (incomplete) tetanus

(b) If another stimulus is applied before the muscle relaxes completely, then more tension results.

Copyright © 2010 Pearson Education, Inc. Figure 9.15c

Stimuli

High stimulation frequencyfused (complete) tetanus

(c) At higher stimulus frequencies, there is no relaxation at all between stimuli. This is fused (complete) tetanus.


Muscle Metabolism: Energy for Contraction

• ATP is the only source used directly for contractile activities

• Available stores of ATP are depleted in 4–6 seconds



• ATP is regenerated by:

• Direct phosphorylation of ADP by creatine phosphate (CP)

• Anaerobic pathway

• Aerobic pathway


• Direct phosphorylation of ADP by creatine phosphate (CP)

• CP is more concentrated in muscle fibers than ATP (~4 X more)

• When ATP stores are depleted: muscle fibers use CP to regenerate ATP

• Products are: 1 ATP/ CP

• Provides energy for: ~ 15 seconds of activity


Copyright © 2010 Pearson Education, Inc. Figure 9.19a

Coupled reaction of creatinephosphate (CP) and ADP

Energy source: CP

(a) Direct phosphorylation

Oxygen use: NoneProducts: 1 ATP per CP, creatineDuration of energy provision:15 seconds

Creatinekinase

ADPCP

Creatine ATP


Anaerobic Pathway

• Under intense muscle activity or when oxygen delivery is impaired: the body switches to the anaerobic pathway

• Begins just like aerobic pathway (Glucose breakdown) but pyruvic acid is converted into lactic acid

• Products are: 2ATP/glucose

• Provides energy for : 60 seconds of activity

Copyright © 2010 Pearson Education, Inc. Figure 9.19b

Energy source: glucose

Glycolysis and lactic acid formation

(b) Anaerobic pathway

Oxygen use: NoneProducts: 2 ATP per glucose, lactic acidDuration of energy provision:60 seconds, or slightly more

Glucose (fromglycogen breakdown ordelivered from blood)

Glycolysisin cytosol

Pyruvic acid

Releasedto blood

net gain

2

Lactic acid

O2

O2ATP


Aerobic Pathway

• Produces 95% of ATP during rest and light to moderate exercise

• Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids

• Products are: 32 ATP/glucose, CO2 and H2O

• Provides energy for: hours (endurance activities)

Copyright © 2010 Pearson Education, Inc. Figure 9.19c

Energy source: glucose; pyruvic acid;free fatty acids from adipose tissue;amino acids from protein catabolism

(c) Aerobic pathway

Aerobic cellular respiration

Oxygen use: RequiredProducts: 32 ATP per glucose, CO2, H2ODuration of energy provision: Hours

Glucose (fromglycogen breakdown ordelivered from blood)

32

O2

O2

H2O

CO2

Pyruvic acidFattyacids

Aminoacids

Aerobic respirationin mitochondriaAerobic respirationin mitochondria

ATP

net gain perglucose


Short-duration exerciseProlonged-durationexercise

ATP stored inmuscles isused first.

ATP is formedfrom creatinePhosphateand ADP.

Glycogen stored in muscles is brokendown to glucose, which is oxidized togenerate ATP.

ATP is generated bybreakdown of severalnutrient energy fuels byaerobic pathway. Thispathway uses oxygenreleased from myoglobinor delivered in the bloodby hemoglobin. When itends, the oxygen deficit ispaid back.


MUSCLE IDENTIFICATION AND NAMING


Naming Skeletal Muscles

• Location—bone or body region associated with the muscle

• Shape—e.g., deltoid muscle (deltoid = triangle)

• Relative size—e.g., maximus (largest), minimus (smallest), longus (long)

• Direction of fibers or fascicles—e.g., rectus (fibers run straight), transversus, and oblique (fibers run at angles to an imaginary defined axis)


Naming Skeletal Muscles

• Number of origins—e.g., biceps (2 origins) and triceps (3 origins)

• Location of attachments—named according to point of origin or insertion

• Action—e.g., flexor or extensor, muscles that flex or extend, respectively


Shoulder

Arm

Forearm

Pelvis/thigh

Thigh

Leg

Head Facial

Neck

Thorax

Abdomen

Thigh

Leg

TrapeziusDeltoid

Triceps brachiiBiceps brachiiBrachialis

Hand, wrist and finger flexors

IliopsoasPectineus

Rectus femorisVastus lateralisVastus medialis

Fibularis longusExtensor digitorum longusTibialis anterior

Temporalis Epicranius, frontal bellyOrbicularis oculiZygomaticusOrbicularis oris

Sternocleidomastoid

Pectoralis major

External oblique

Rectus abdominisInternal obliqueTransversus abdominis

SartoriusAdductorsGracilis

GastrocnemiusSoleus

Masseter

Platysma


Muscles of Facial Expression• Epicranius (Frontal belly and Occipital belly)

• Raises eyebrows, wrinkles forehead (frontal belly)

• Pulls scalp posteriorly (occipital belly)

• Orbicularis Oculi

• Closes eyes, squinting, blinking

• Orbicularis Oris

• Closes mouth, protrudes lips

• Buccinator

• Flattens cheek (as in whistling)

• Zygomaticus

• Pulls corners of mouth superiorly (as in smiling)

• Platysma

• Pulls corners of mouth inferiorly


Orbicularis oculi

Zygomaticus

Buccinator

Orbicularis oris

Platysma

Temporalis

MasseterSternocleidomastoidTrapezius


Muscles of Mastication

• Temporalis and Masseter

• Elevate the mandible (closing jaw)


Orbicularisoris

Temporalis

MasseterBuccinator

(a)


Muscles of the Neck

• Sternocleidomastoid—major head flexor (also rotates the head)


1st cervicalvertebra

Sternocleido-mastoid

(a) Anterior

Base ofoccipital boneMastoidprocess


AnteriorTrunk Muscles• Pectoralis Major

• Adducts and flexes the arm

• Rectus Abdominis

• Flexes vertebral column

• External and Internal obliques

• Flex vertebral column

• Transversus Abdominis

• Compresses abdomen


Pectoralis minor

Serratus anteriorSternum

(a)

Subscapularis


External oblique

(a)

Pectoralis major

Linea alba

Tendinousintersection

Rectusabdominis


• Trapezius

• Extends head; elevates, depresses scapula

• Latissimus Dorsi

• Adducts and extends humerus

• Erector Spinae

• Extends vertebral column

Posterior Trunk Muscles


Trapezius

(c)

Levatorscapulae

RhomboidminorRhomboidmajor


Arm Muscles

• Anterior Flexor Muscles

• Brachialis, Biceps brachii, Brachioradialis

• Forearm flexors

• Posterior extensor muscles

• Triceps brachii

• Extend forearm


Biceps brachiiBrachialisBrachioradialis

(a) Anterior view


• Deltoid

• Abducts arm, but can do all angular movements

• Rotator Cuff Muscles

• Supraspinatus

• Infraspinatus

• Subscapularis

• Teres Minor

Shoulder Muscles


(b) Posterior view

Triceps brachii: Lateral head Long head

Teres minor

Supraspinatus

Infraspinatus


Muscles of the Forearm

• Actions: movements of the wrist, hand, and fingers

• Most anterior muscles are flexors and insert via the flexor retinaculum

• Most posterior muscles are extensors and insert via the extensor retinaculum


Brachioradialis

Flexor retinaculum

Medial head oftriceps brachii

(a)


Hip/Thigh Muscles

• Iliopsoas and Sartorius

• Hip/thigh flexion


Psoas majorIliopsoas Iliacus

Sartorius

(a)

5th lumbar vertebra


• Gluteus Maximus

• Lateral thigh

• Extends leg at hip

• Gluteus Medius

• Abducts thigh

• Adductor Muscles

• Medial thigh

• Adductor thigh

Hip/Thigh Muscles


Gluteus medius (cut)

Gluteus minimus

Gluteusmaximus(cut)

(c)


(b)

O = origin I = insertion

Adductormagnus

Pectineus(cut)

Adductorbrevis

Adductorlongus

Femur


• Hamstring muscles (Biceps femoris, Semitendinosus, Semimembranosus )

• Posterior thigh

• All three flex leg at knee and extend hip

Thigh Muscles


Long head

SemitendinosusSemimembranosus

Short headBicepsfemoris

Hamstrings

(a)


Muscles of the Thigh that Move the Knee Joint

• Quadriceps femoris (Vastus Medialis, Lateralis, intermedius and rectus femoris)

• Anterior Thigh

• All extend the knee


Quadriceps femoris• Rectus femoris (superficial

to vastus intermedius)

• Vastus lateralis

• Vastus medialis

(a)

Patella

Tendon of quadriceps femoris

Copyright © 2010 Pearson Education, Inc. Figure 10.5

ArmTriceps brachiiBrachialisForearmBrachioradialis

Extensor carpiulnaris Extensor digitorum

Iliotibial tract

LegGastrocnemiusSoleusFibularis longus

NeckEpicranius, occipital bellySternocleidomastoidTrapezius

Shoulder

HipGluteus mediusGluteus maximus

Thigh

Biceps femoris

Adductor magnus


Latissimus dorsiRhomboid major

InfraspinatusDeltoid

Teres majorExtensors of forearm

Calcaneal(Achilles) tendon

Hamstrings:


Gluteus medius

Gluteus maximus

Adductor magnusGracilisIliotibial tract


Bicepsfemoris

Hamstrings

(a)


Muscles of the Anterior Compartment of the Leg

• Tibialis anterior & Extensor digitorum longus

• Primary toe extensors and ankle dorsiflexors


Tibialis anteriorExtensor digitorum longus

(a)


Muscles of the Lateral Compartment ofthe Leg

• Fibularis longus

• Plantar flexion and eversion of the foot


Head of fibula

Fibularis longus

Lateral malleolus

(a)


Muscles of the Posterior Compartment of the Leg

• Gastrocnemius and Soleus

• Both do plantar flexion of foot

• Gastrocnemius also does flexion at knee


Gastrocnemius Medial headLateral head

Tendon ofgastrocnemius

Calcaneal tendon

Medial malleolus Lateral malleolus

Calcaneus

(a) Superficial view of the posterior leg.

Copyright © 2010 Pearson Education, Inc.. Muscle Functions 1.Movement of bones or fluids (e.g.,...

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