Basic Types of Muscle Tissue

Skeletal Muscle Tissue

Mostly voluntary

Actin and Myosin

Fibers

Cardiac Muscle Tissue

Involuntary

Actin and Myosin

Intercalated disc

Short and fat

Smooth Muscle Tissue

Fibers

Involuntary

Basic Characteristics of Muscle Cells• Excitability

• Contractility

• Extensibility

• Elasticity

General Functions of Muscle Tissue

Bone

Endomysium(between individualmuscle fibers)

Muscle fiber

Fascicle(wrapped by perimysium)

Epimysium

Tendon

Blood vessel

General Structure of a Skeletal Muscle

Epimysium

Muscle fiberin middle ofa fascicle

Perimysium

Endomysium

Skeletal Muscle Attachments

Embryonicmesoderm cells

Embryonicmesoderm cellsundergo celldivision (toincrease number)and enlarge.

Severalmyoblastsfuse togetherto form amyotube.

Myotubematures intoskeletal musclefiber.

Myoblasts

Myotube(immaturemultinucleatemuscle fiber)

Satellite cell

MatureSkeletalmusclefiber1 2 3

Skeletal Muscle Fiber Formation

Nucleus

Sarcolemma

Mitochondrion

Myofibril

Skeletal Muscle Fiber Microanatomy

Sarcoplasm

Myoglobin Glycogen

Nuclei

Fiber

Dark A band

Light I band

Skeletal Muscle Fiber Microanatomy

Sarcomeres

I band I bandA bandSarcomere

H zone

Thin (actin)filament

Thick (myosin)filament

Z disc Z disc

M line

Skeletal Muscle Fiber Microanatomy

Skeletal Muscle Fiber Microanatomy

Z disc Z discM line

Sarcomere

Thin (actin)filament

Thick(myosin)filament

Sarcomere Structure

I band H zone Outer edgeof A band

Cross-sectional view of a sarcomere cut through in different locations.

I IA

Z ZH

1 Fully relaxed sarcomere of a muscle fiber

Sliding Filament Model of Muscle Contraction

I IA

Z Z

2 Fully contracted sarcomere of a muscle fiber

Sliding Filament Model of Muscle Contraction

Excitation-Contraction Coupling

Nucleus

Action potential(AP)

Myelinated axonof motor neuron

Axon terminal ofneuromuscular junction

Sarcolemmaof the musclefiber

Action potential arrives at axon terminal of motor neuron.

Voltage-gated Ca2+

channels open and Ca2+ enters the axon terminal.

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

Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.

Ca2+

Axon terminalof motor neuron

Synaptic vesiclecontaining ACh

Mitochondrion

Synaptic cleft

Junctionalfolds of

sarcolemma

Fusingsynaptic vesicles

ACh

Sarcoplasm of muscle fiber

Ca2+

1

2

3

4

Excitation-Contraction Coupling

Postsynaptic mem-brane ion channel opens; ions pass.

Na+ K+

5 ACh binding opens ion channels that allowsimultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber.

Excitation-Contraction Coupling

Excitation-Contraction Coupling

ACh

Na+

K+

Degraded ACh

Acetylcholinesterase

Postsynaptic membraneion channel closed;ions cannot pass.

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

Na+ K+

Axon terminal

Synapticcleft

ACh–

ACh

1 Local depolarization: generation of the end plate potential on the sarcolemma

Na+

Open Na+

ChannelClosed K+

Channel

K+

K+

2 Generation and propagation of the action potential (AP)

Sarcoplasm of muscle fiber

Na+

Excitation-Contraction Coupling

Na+ channelsclose, K+ channelsopen

K+ channelsclose

Repolarizationdue to K+ exit

Threshold

Na+

channelsopen

Depolarizationdue to Na+ entry

Calciumions arereleased.

Steps inE-C Coupling:

Terminalcisterna of SR

Voltage-sensitivetubule protein

T tubule

Ca2+

releasechannel

Ca2+

Sarcolemma

Action potential ispropagated along thesarcolemma and downthe T tubules.

1

2

Excitation-Contraction Coupling

Flexible hinge region

Tail

Myosin head

ATP-bindingsite

Actin-binding sites

Myosin molecule

Heads

Structure of Myosin

Tropomyosin Troponin Actin

Active sitesfor myosinattachmentActin subunits

Actin subunits

Structure of Actin

Calcium binds totroponin and removesthe blocking action oftropomyosin.

Contraction begins

Troponin Tropomyosinblocking active sitesMyosin

Actin

Active sites exposed andready for myosin binding

Ca2+

Myosincross bridge

3

4

Excitation-Contraction Coupling

What’s Next?• Sarcoplasmic reticulum Ca++ ATPase

• Troponin and tropomyosin revert

• No more cross bridges

• Recoil of the connective tissue

• Gravity

• Return to resting length

Spinal cord

Motor neuroncell body

Muscle

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motorneuronaxon

Axon terminals atneuromuscular junctions

Motor Units

All or None

# of motor units stimulated ∝ tension developed

Stimulus strength

Proportion of motor units excited

Strength of muscle contraction

Maximal contraction

Maximalstimulus

Thresholdstimulus

Motor Units

Latentperiod

Singlestimulus

Period ofcontraction

Period ofrelaxation

Muscle Twitch

Latent period

Extraocular muscle (lateral rectus)

Gastrocnemius

Soleus

Singlestimulus

Muscle Twitch

Contraction

Relaxation

Stimulus

Single stimulus single twitch

Muscle Twitch

Stimuli

Partial relaxation

Low stimulation frequencyunfused (incomplete) tetanus

Wave Summation – Incomplete Tetanus

Stimuli

High stimulation frequencyfused (complete) tetanus

Complete Tetanus

3 kg

3 kg

Musclecontracts(isotoniccontraction)

Tendon

Tendon

Isotonic Concentric Muscle Contraction

Amount ofresistance

Peak tensiondeveloped

Resting length

Musclestimulus

Musclerelaxes

Isotonic Concentric Muscle Contraction

Isotonic Eccentric Muscle Contraction

6 kg 6 kg

Musclecontracts(isometriccontraction)

Isometric Muscle Contraction

Amount of resistance

Peak tensiondeveloped

Resting length

Musclestimulus

Musclerelaxes

Isometric Muscle Contraction

Short-duration exercise Prolonged-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.

Energy Supply for Muscle Activity

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

Energy Supply for Muscle Activity

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

O2

ATP

Energy Supply for Muscle Activity

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, CO

2, H

2O

Duration 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

Energy Supply for Muscle Activity

Muscle Fatigue

Na+ K+

Axon terminal

ACh–

AChK+Na+

Oxygen Debt

Muscle Fiber Types

Muscle Fiber Types

Capillary densityMitochondriaMyoglobin

Endurance Training

Resistance Training

Nucleus

SarcolemmaMitochondrion

Myofibril