Excitation-Contraction Coupling

• Academic year: 2024-2025
• Year: 1
• Semester: 2
• Module: locomotor system (LCS) 105
• Presentation date: 16/2/2025
• Title: Excitation Contraction Coupling
• Presenter: Prof. Dr. Mohamed Hassan Abdelsattar, Professor - Faculty of Medicine - Al-Azhar University
• Department: Medical Physiology

Objectives

• Explain excitation contraction coupling.
• Define elements of the sarcomere that underlie striated muscle contraction.
• Describe the role for Ca++ in excitation-contraction coupling.
• Describe the role of sarcoplasmic reticulum in excitation contraction coupling.
• Explain how tropomyosin and troponin control muscle contraction and relaxation.
• List the events that occur during cross-bridge cycles and describe the role of ATP in muscle contraction.
• Describe morphological changes after muscle contraction.
• List the events that occur during muscle relaxation.
• Define rigor mortis and explain its cause and significance.

Introduction

• The myofibrils contain alternating dark (A) & light (I) bands
• The I band has a dark (Z) line in the middle
• The A band has a lighter H band in the middle
• Sarcomere: The contractile unit of the skeletal muscle fiber and the area between two adjacent Z lines.

The Contractile Proteins

• Thick myosin filaments extend along the length of the dark (A) band only.
• Thin actin filaments extend from the Z line to the H band.
• The A band contains both actin & myosin.
• The I band only contains actin.
• The region in the A band where actin filaments do not overlap myosin filaments is the H band.

Myosin Filament

• Complex actin-binding protein with 2 heads and a long tail.
• Myosin filaments show small projections called cross bridges whose interaction with actin causes muscle contraction.
• Each head has 2 active sites: an actin-binding site and an ATP-binding site with ATPase activity.

Actin Filaments (F-actin)

• Formed of G-actin, tropomyosin, and troponin.
• Actin (G-actin) is formed of double strands of actin molecules, forming active sites that interact with myosin cross bridges to cause muscle contraction.

Tropomyosin

• Formed of 2 protein strands in between actin strands and loosely attached to them.
• In a resting condition, tropomyosin strands cover active sites on actin strands, preventing their interaction with myosin (Relaxing protein).

Troponin

• Formed of 3 globular proteins.
• Troponin T binds other troponin to tropomyosin.
• Troponin I covers the myosin binding site on actin, inhibiting interaction of myosin with actin.
• Troponin C binds Ca++ that initiates contraction.

Sarcotubular System

• Formed of T-tubular system and sarcoplasmic reticulum (SR).

T-tubular system

• It is an invagination on the surface of the muscle membrane.
• Consists of transverse tubules that terminate in the sarcolemma by minute pores.
• Contains extracellular fluid.
• T tubule membrane contains the dihydropyridine receptor.
• Function: conduct the action potential from the surface of the muscle to the sarcoplasmic reticulum.

Sarcoplasmic Reticulum (SR)

• Forms longitudinal tubules parallel to muscle fibrils.
• They are dilated by large chambers called terminal cisterns.
• SR membrane contains the ryanodine receptor that keeps Ca++ from flowing out.
• T-tubule with the terminal cisterns is called the triad.
• Function: concerned with Ca++ storage and release.

Channels

• The channels in the T-tubules are mechanically coupled to Ca++ release channels in the SR.
• SR opens when Ca++ diffuses out.

Mechanical Changes

• These follow skeletal muscles stimulation and are known as Excitation-contraction coupling:
• It’s the process by which the action potential that reaches the muscle fiber initiates contraction.
• These events start after the arrival of the action potential to the sarcoplasm.

Neuromuscular transmission

• A motor neuron discharges.
• Depolarization of the nerve terminal causes Ca++ entry.
• Voltage-gated Ca++ channels rupture vesicles, which releasing acetylcholine (Ach).
• Ach binds to nicotinic Ach receptors in the sarcolemma → depolarization.
• Voltage-gated channels stimulate production of EPP (end plate potential). When the EPP reaches the firing level, an action potential then occurs.

Steps of Excitation-contraction Coupling

• Release of Ca++ from the sarcoplasmic reticulum (SR) occurs first via a propagation of AP (action potential) on either side of the muscle surface and to the inside of the muscle fiber along the T-tubules.
• Depolarization of T Tubule activates voltage-gated dihydropyridine receptors (DHP), opening ryanodine receptors (RyR, Ca++ release channels) in the terminal cisternae and permitting Ca++ to diffuse rapidly into the sarcoplasm and initiate muscle contraction.
• This Ca++ then binds with troponin C.
• Binding of troponin I to actin is weakened, and tropomyosin moves laterally from actin → exposing its active sites.
• Also, Ca++ then increases ATPase activity in the myosin heads → breakdown of ATP.
• Active sites of actin are exposed, and myosin heads (cross bridge) become attached to them → sliding of actin between myosin → muscle shortening.
• This allows sliding of actin on the myosin with the actin filaments moving towards the center of the myosin filaments by bending of myosin head → power stroke for pulling the actin toward the center of myosin & shortening occurs.
• Heads of myosin come into contact with the active sites of actin and act as an enzyme (ATPase) that catalyzes the splitting of ATP → ADP + Pi + E. The energy liberated is an active process consumed in contraction.

Cross bridge cycle

• The sliding of the thin filaments between the thick filaments is produced by repeated forming then breaking of cross linkages between actin and myosin. This occurs in 4 steps:
  • 1st step: Binding: The heads of the myosin molecules bind to specific sites at the actin molecules.
  • 2nd step: Bending: Cross bridges slide the actin over myosin. The energy needed for bending is obtained from hydrolysis of ATP.
  • 3rd step: Detachment: (separated) of the myosin heads from actin. ATP is needed to decrease the affinity of cross bridges to active sites.
    • If ATP is not available, the thick and thin filaments can not be separated → muscle contracture (sustained contraction).
  • 4th step: Return: Cross bridges return to their original site to re-connect at the next sites of actin, and a new cycle is initiated.

Repeating the Cycle

• Process of binding, binding, detachment and return of the myosin heads is repeated → sliding of actin filaments → muscle shortening.
• Cycling continues as long as Ca++ is attached to troponin & energy is available.

Morphological Changes Post-Contraction

• Thin filaments slide between the thick filaments:
  • The Z lines move closer, and the sarcomeres becoming smaller.
  • I bands widths decreases
  • A bands remain constant.
  • H bands become narrower. These filaments overlap when muscle shortening is marked, H bands then disappear
• These changes reduce the distance between Z-lines
• Muscle shortening occurs due to actin movement over the myosin.

Relaxation

• Conformation of the DHP receptor closes the Ca++ release channels.
• Ca++ is transported from the sarcoplasm into the sarcoplasmic reticulum. This is achieved by an ATР—dependent Ca++ pump called SERCA (sarcoplasmic reticulum Ca++-ATPase).
• This pump concentrates the Ca++ ions about 10,000-fold inside the tubules and contains a special binding protein called calsequestrin.
• Each molecule of calsequestrin is able to bind up to 40 Ca++ ions.
• The Ca++ pump causes troponin I to bind with actin and tropomyosin covering the active sites of actin, which stops the interaction between actin & myosin, resulting in relaxation.
• The breakdown of ATP necessary for active Ca++ pump
• Both contraction & relaxation are active & require ATP.

Steps

• Contraction -Discharge of motor neuron -Release of acetylcholine at motor end-plate -Binding of acetylcholine to nicotinic acetylcholine receptors -Increased Na+ and K+ conductance in end-plate membrane -Generation of end-plate potential -Generation of action potential in muscle fibers -Inward spread of depolarization along T tubules -Release of Ca2+ from terminal cisterns of sarcoplasmic reticulum and diffusion to thick and thin filaments -Binding of Ca2+ to troponin C, uncovering myosin-binding sites on actin -Formation of cross-linkages between actin and myosin and sliding of thin on thick filaments, producing shortening.
• Relxation -Ca2+ pumped back into sarcoplasmic reticulum -Release of Ca2+ from troponin -Cessation of interaction between actin and myosin.

Summary of excitation-contraction coupling

• Action potential generated in the sarcolemma, propagated down in T tubules
• Action potential triggers Ca release from terminal cisternae SR
• Calcium ions bind to troponin, Troponin changes shape as a result of the blocking of tropomyosin and active action sites are exposed.
• myosin heads connect over over with actin filaments toward the center of the sarcomere release of ADP energy by atp occurs.
• Blockage restores, blockage of block action sites, contraction ends in the muscular fibre relaxes.
• remove cause active transport.

Rigor Mortis

• Stiffening of skeletal muscles within 4-5 hours post mortem (after death.)
• Starts to disappear 11-15 hours later, and complete muscle relaxation occurs after 25 hours.
• Time relations are important for determining the time of death in forensic medicine.
• It is a type of muscle contracture (shortening) due to depletion of ATP. ATP is needed to separate the bonds after muscle contraction. When a body dies, there is no atp so depletion occurs, and muscles are stiff.
• Thus, the myosin heads remain attached to actin in an abnormal fixed and resistant way leading to muscle rigor.
• Relaxation of muscles that follows is due to autolysis of the muscle proteins by proteolytic enzymes released from the lysosomes of dead muscle fibres, and followed by putrefaction due to bacterial action.
• Cold storage of dead bodies delays the onset and higher temperatures hasten rigor mortis.

Interactive Question

• Propagation of action potential along the membrane of the T-tubule causes the release of Ca++ from the terminal cisternae.

Summary

• The myosin filaments show small projections from the whole side called cross bridges.
• The sarcoplasmic reticulum is concerned with Ca++ storage & release.
• Heads of myosin come into contact with the active sites of actin during action and acts as an enzyme (ATPase).
• Both contraction & relaxation are active & need ATP.
• The cross bridge cycle consist of binding, Detachment and Return.
• Rigor mortis is stiffening of skeletal muscles that occurs within 4-5 hours after death.