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Physiology and Pathophysiology I

Lectures 9/10: Skeletal Muscle: Excitation–Contraction Coupling and Smooth Muscle Contraction

Instructor: Yue-Qiao (George) Huang

Dept of Pharmaceutical Sciences

School of Pharmacy

[email protected]

678 407 7371

Office 3033

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Learning Objectives

  • Outline the composition of skeletal muscle, including levels-of-organization and the arrangement of contractile proteins in the sarcomere.

  • Describe the structures of myosin and actin.

  • List the steps for the molecular basis of muscle contraction.

  • Explain the role of calcium in the excitation and contraction of a skeletal muscle.

Learning Objectives

  • List the steps for the relaxation of a skeletal muscle.

  • Outline the makeup of a skeletal muscle, including the arrangement of connective and nerve tissue with skeletal muscle tissue.

  • Define a motor unit and explain how the nervous system controls motor movement.

  • Compare the characteristics of smooth muscle tissue to skeletal muscle tissue.

Thought Questions to guide your study

  • 1. What are the levels of organization in a skeletal muscle?
  • 2. What produces the striated appearance of skeletal muscles?
  • 3. What is a functional unit of skeletal muscle?
  • 4. What are the composition of thick and thin filaments?
  • 5. What is a motor unit?
  • 6. What is the sliding filament mechanism of muscle contraction? How do cross-bridge power strokes bring about shortening of the muscle fiber?
  • 7. What is excitation-contraction coupling? Compare the excitation–contraction coupling process in skeletal muscle with that in smooth muscle.

Table 9.3

Bone

Perimysium

Endomysium

(between individual

muscle fibers)

Muscle fiber

Fascicle

(wrapped by perimysium)

Epimysium

Tendon

Epimysium

Muscle fiber

in middle of

a fascicle

Blood vessel

Perimysium

Endomysium

Fascicle

(a)

(b)

Skeletal Muscle

Epi=on, at, above, out

Peri=around

Endo=inside, inner

Nucleus

Sarcolemma

Sarco-=flash,

muscle

Mitochondrion

(b) Diagram of part of a muscle fiber showing the myofibrils. One
myofibril is extended afrom the cut end of the fiber.

Myofibril

Myofibrils

  • Densely packed, rod-like elements
  • Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands

Z line

A band

I band

Portion

of myofibril

M line

Sarcomere

H zone

Thick filament

Thin filament

Cross

bridges

M line

H zone

Z line

A band

I band

Fig. 8-2, p. 255

DArk band is the A band. LIght band is the I band.

HAZI (HAZY): H zone is in A-band. Z disc is in the I band.

H zone, German helle, meaning bright; M, German mittel, middle

I=Isotropic; A=Anisotropic. "Isotropic“ means that the I band does not alter polarized light, while the "anisotropic" A band is birefringent in polarized light.

Sarcomere

  • Sarcomere
  • Functional unit of skeletal (and cardiac) muscle

  • Found between two Z lines (connects thin filaments of two adjoining sarcomeres)

  • Regions of sarcomere
  • A band
  • Made up of thick filaments along with portions of thin filaments that overlap on both ends of thick filaments
  • H zone
  • Lighter area within middle of A band where thin filaments do not reach
  • M line
  • Extends vertically down middle of A band within center of H zone
  • I band
  • Consists of remaining portion of thin filaments that do not project into A band

Myosin (Thick Filament)

  • Protein molecule consisting of two identical subunits shaped like a golf club
  • Tail ends are intertwined around each other
  • Globular heads project out at one end
  • Tails oriented toward center of filament and globular heads protrude outward at regular intervals
  • Heads form cross bridges between thick and thin filaments

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

Thin Myofilament

Figure 12.4

  • Primary structural component of thin filaments
  • Subunit spherical in shape (globular=G actin)
  • Thin filament also has two other proteins
  • Tropomyosin and troponin
  • Each actin molecule has special binding site for attachment with myosin cross bridge
  • Binding results in contraction of muscle fiber

Tropomyosin and Troponin

Tropomyosin

Thread-like molecules that

lie end to end alongside

groove of actin spiral

  • covers actin sites blocking interaction that leads to muscle contraction

Troponin

Three subunits

TnT binds to tropomyosin

TnI to actin

TnC with Ca2+

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Role of Calcium in Cross-Bridge Formation

  • Troponin
  • When not bound to Ca2+, troponin stabilizes tropomyosin in blocking position over actin’s cross-bridge binding sites
  • When Ca2+ binds to troponin, tropomyosin moves away from blocking position
  • With tropomyosin out of way, actin and myosin bind, interact at cross-bridges
  • Muscle contraction results

Role of Calcium in Cross-Bridge Formation

Changes in Banding Pattern During Shortening

Contraction is

accomplished by thin

filaments from opposite

sides of each

sarcomere sliding

closer together

between thick filaments

Neither thick nor thin filament shortens.

  • H zone shortens
  • Sarcomere shortens
  • Myofibrils shorten
  • Myofibers shorten
  • Fascicles shorten

Fig. 8-9, p. 260

Basic 4 steps

Sliding Filament Mechanism

  • Increase in Ca2+ starts filament sliding
  • Decrease in Ca2+ turns off sliding process
  • Thin filaments on each side of sarcomere slide inward over stationary thick filaments toward center of A band during contraction
  • As thin filaments slide inward, they pull Z lines closer together
  • Hydrolysis of ATP provides energy for muscle contraction

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Figure 8.9: Cross-bridge activity.

(a) During each cross-bridge cycle, the cross bridge binds with an actin molecule, bends to pull the thin filament inward during the power stroke, then detaches and returns to its resting conformation, ready to repeat the cycle. (b) The power strokes of all cross bridges extending from a thick filament are directed toward the center of the thick filament. (c) Each thick filament is surrounded on each end by six thin filaments, all of which are pulled inward simultaneously through cross-bridge cycling during muscle contraction.

Neuromuscular Junction

Excitation-Contraction Coupling

Transverse Tubules and SR

  • Most Ca++ in SR is in terminal cisternae

  • Running along terminal cisternae are T tubules

12-30

Terminal button

Acetylcholine-

gated cation

channel

Acetylcholine

T tubule

Surface membrane of muscle cell

Lateral

sacs of

sarcoplasmic

reticulum

Tropomyosin

Troponin

Cross-bridge binding

Myosin cross bridge

Actin

Fig. 8-12, p. 262

Transverse Tubules

  • T tubules are extensions of sarcolemma

  • Run perpendicularly from surface of muscle cell membrane into central portions of the muscle fiber

  • Since membrane is continuous with surface membrane – action potential on surface membrane also spreads down into T-tubule

  • Spread of action potential down a T tubule triggers release of Ca2+ from sarcoplasmic reticulum into cytosol

Sarcoplasmic Reticulum

  • Modified endoplasmic reticulum

  • Consists of fine network of interconnected compartments that surround each myofibril

  • Not continuous but encircles myofibril throughout its length

  • Segments are wrapped around each A band and each I band
  • Ends of segments expand to form saclike regions – lateral sacs (terminal cisternae)

A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates.

 

motor unit 

Neuromuscular Junction

NMJ is the synapse between the motor neuron and the muscle fiber it innervates.

End plate is the convoluted postsynaptic membrane region where muscle ACh receptors are located.  

Neuromuscular Junction (cont)

Acetylcholine receptors are ligand-gated cation channels

  • Acetylcholine binding to its receptor allows Na flux into the muscle cell, depolarizing the cell membrane
  • End-plate potentials (EPPs): graded potentials
  • Graded potentialsReaches thresholdAction potentials

ENa (Equilibrium Potential for Na+)

EK (Equilibrium Potential for K+)

Ionic Movement towards the Equilibrium Potential

The driving force for K is small compared to Na

VDF = Vm − Ex

Action Potential is generated

when the muscle cell membrane is depolarized to the threshold level, APs are generated

Action Potentials travel to the T Tubules

SERCA

Ca2+ under resting condition, is sequestered by calsequestrin near the SR Terminal cisternae.

DHP Receptor = dihydropyridine receptors (DHPRs) =L type (Long-Lasting) voltage gated Ca channel =voltage sensor for EC Coupling

RyR =Ryanodine receptor =foot proteins =Ca Release Channels

DHP Receptors and Ryanodine Receptors

Summary of Excitation-Contraction Coupling

E-C
Coupling
Summary

12-33

DHP Receptor

Ryanodine receptor, Maligant Hyperthermia,

Dantrolene

Neostigmine

Myasthenia Gravis,

Anti-malarial

Artimisinin,

Qinghaosu SERCA

Steps leading to muscle contraction

■ Depolarization of the motor neuron terminal results in Na+/Ca2+ influx.

■ Vesicles of the axon terminal release acetylcholine.

■ Binding of acetylcholine by nicotinic receptors results in an endplate potential.

■ An action potential is initiated and is propagated along the sarcolemma and down the T-tubules.

■ A conformational change in the dihydropyridine receptor of the T-tubule is transduced to a conformational change in the ryanodine receptor of the sarcoplasmic reticulum.

■ Ca2+ is released from the sarcoplasmic reticulum, initiating contraction.

Relaxation

  • Depends on reuptake of Ca2+ into sarcoplasmic reticulum

  • Acetylcholinesterase breaks down ACh at neuromuscular junction

  • Muscle fiber action potential stops

  • When local action potential is no longer present, Ca2+ moves back into sarcoplasmic reticulum by SERCA

  • No more Ca, tropomyosin slips back to its blocking position, contraction ends, actin filament slides back to its original position

Smooth Muscle

Found in walls of hollow organs and tubes

Smooth Muscle

  • Found in walls of hollow organs and tubes
  • No striations
  • Filaments do not form myofibrils
  • Not arranged in sarcomere pattern found in skeletal muscle
  • Spindle-shaped cells with single nucleus
  • Cells usually arranged in sheets within muscle
  • Have dense bodies containing same protein found in Z lines

Smooth muscle has three types of filaments

Intermediate

filament

Thick filament

Thin filament

Dense body

Smooth Muscle

  • Cell has three types of filaments
  • Thick myosin filaments
  • Longer than those in skeletal muscle
  • Thin actin filaments
  • Contain tropomyosin but lack troponin
  • Filaments of intermediate size
  • Do not directly participate in contraction
  • Form part of cytoskeletal framework that supports cell shape

Marieb, Figure 9.27

Varicosities release

their neurotransmitters

into a wide synaptic

cleft (a diffuse junction).

Innervation of Smooth Muscle

Smooth

muscle

cell

Synaptic

vesicles

Mitochondrion

Autonomic

nerve fibers

innervate

most smooth

muscle fibers.

Varicosities

Single-unit and Multiunit Smooth Muscles

Multiunit Smooth Muscle

  • Neurogenic
  • Consists of discrete units that function independently of one another
  • Units must be separately stimulated by nerves to contract
  • Found
  • In walls of large blood vessels
  • In large airways to lungs
  • In ciliary muscle of lens
  • In iris of eye
  • At base of hair follicles
  • Self-excitable or myogenic (does not require nervous stimulation for contraction)
  • Also called visceral smooth muscle, and found in GI tract, uterus and small-diameter blood vessels
  • Fibers become excited and contract as single unit
  • Cells electrically linked by gap junctions
  • Can also be described as a functional syncytium
  • Contraction is slow and energy-efficient
  • Well suited for forming walls of hollow organs

Single-unit Smooth Muscle

Calcium Activation of Myosin Cross Bridge

in Smooth Muscle

Comparison of Role of
Calcium In Contraction of Smooth and Skeletal Muscle

X

Not entirely true!

Summary

  • Structure of muscle, fasicle, myofiber, myofibrils, sarcomere
  • Thick filament, myosin, ATPase
  • Thin filament, actin, tropomyosin, troponin
  • Sliding model, Cross bridge, power stroke
  • Excitation-contraction coupling: pivotal role by Ca
  • Motor Unit, NMJ, End Plate, ACh, AChR, AChE
  • End Plate Potential, Action Potential, Sarcolemma, T Tubules (DHPR, Voltage sensor, L type Ca Chanel), SR (RyR, Ca release channel); calsequestrin, SERCA
  • Relaxation
  • Malignant hyperthermia (Dantrolene, RyR), Myasthenia Gravis (Neostigmine, AChE),
  • Smooth muscle, Multiple unit and single unit, MLCK, Compare and contrast with skeletal muscle

Take Home Messages

  • Skeletal muscle fibers are striated by a highly organized internal arrangement. Myosin forms the thick filaments. Actin is the main structural component of the thin filaments.
  • A motor unit is made up of a motor neuron and all the skeletal muscle fibers innervated by that motor neuron's axonal terminals.
  • Sliding model: During contraction, cycles of cross-bridge binding and bending pull the thin filaments inward.
  • Calcium is the link between excitation and contraction. Calcium is released from the Sarcoplasmic Reticulum (SR).
  • Relaxation: the contractile process is turned off and relaxation occurs when Ca2 is returned to the lateral sacs
  • Smooth muscle cells are small and un-striated.
  • Smooth muscle cells are turned on by Ca2-dependent phosphorylation of myosin.