The Sliding Filament Theory “How Do Muscles Contract?”

The Sliding Filament Theory This theory states that during contraction, the thin filaments slide pass the thick filaments so that they over lap by a greater degree.

Sliding Filament Model The result is that the I bands shorten and the distance between the Z discs decrease. The H band disappears and the A bands remain the same length.

1 2 Z H I A Fully relaxed sarcomere of a muscle fiber Fully contracted sarcomere of a muscle fiber A Z H 1 2

The Sliding Filament Theory For a muscle to contract, three events need to occur: a) The muscle needs to be stimulated by a nerve ending. This leads to a change in the membrane potential. The site of this is called the neuromuscular junction.

The Sliding Filament Theory For a muscle to contract, three events need to occur: b) An electrical current (action potential) then needs to be generated along the sarcolemma.

The Sliding Filament Theory For a muscle to contract, three events need to occur: c) The electrical current results in the final trigger which is a short lived rise in intracellular calcium ions which results in the in the contraction.

The Neuromuscular Junction The nerve cells that activate skeletal muscle fibers at the neuromuscular junction called somatic (body) motor (think muscles) neurons.

The Neuromuscular Junction The motor neurons reside in the spinal column and brain, they have long cyctoplasmic extensions called axons.

The Neuromuscular Junction The motor neurons reside in the spinal column and brain, they have long cytoplasmic extensions called axons. These enter the muscle and divide extensively so that each muscle fiber (cell) has its own axon terminal which forms a neuromuscular junction.

Figure 9.13 A motor unit consists of a motor neuron and all the muscle fibers it innervates. Spinal cord Motor neuron cell body Muscle Branching axon to motor unit Nerve Motor unit 1 unit 2 fibers axon Axon terminals at neuromuscular junctions Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle. terminals form neuromuscular junctions, one per muscle fiber (photo- micrograph 330x). (b) (a)

The Neuromuscular Junction The axon does NOT come into direct contact with the sacrolemma of the muscle fiber. T There is a 1 to 2 nm cleft between them called the synaptic cleft. This cleft is not empty but is filled with a gel like extracellular matrix.

The Neuromuscular Junction The nerve impulse is transmitted across this cleft by the release of a neurotransmitter. This crosses the space and attaches to specific membrane receptors on the sacrolemma.

The Neuromuscular Junction The typical neurotransmitter found at these synaptic junctions is acetylcholine.

The Neuromuscular Junction This molecule resides is vesicles in the axon and is released upon depolarization of the axon terminal.

The Neuromuscular Junction These diffuse across the cleft and attach to receptors which then stimulate the depolarization of the muscle fiber.

The Neuromuscular Junction Recall that all cells are polar, they are positively charged on the outside and negatively charged on the inside. Sodium ions, Na+ are in high concentration on the outside and potassium ions, K+, are in high concentration on the inside.

The Neuromuscular Junction

The Neuromuscular Junction Acetylcholine binds to its receptor on the sarcolemma and a gated ion channel is opened. This causes sodium ions to diffuse into the muscle fiber and potassium ions to diffuse out.

Myasthenia Gravis This is an autoimmune disease that specifically attacks the acetylcholine receptor. Symptoms include: Weakness starting with the eye lids (ptosis) Progressing to a general weakness Ends with difficulty swallowing and SOB

Paralytic Drugs Curare competitively binds to the acetyl choline receptor but does not lead to depolarization. Death from asphyxiation quickly follows

Figure 9.8 Events at the Neuromuscular Junction (2 of 4) 1 Action potential arrives at axon terminal of motor neuron. Ca2+ Ca2+ 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Synaptic vesicle containing ACh Mitochondrion Axon terminal of motor neuron Synaptic cleft 3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. Fusing synaptic vesicles ACh 4 Junctional folds of sarcolemma Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. Sarcoplasm of muscle fiber

Figure 9.8 Events at the Neuromuscular Junction (3 of 4) Postsynaptic mem- brane ion channel opens; ions pass. Na+ K+ 5 ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber.

Generation and Propagation of the Action Potential The initial depolarization at the neuromuscular junction ignites an action potential that spreads out in all directions across the sarcolemma. The depolarization opens voltage- gated sodium channels.

Generation and Propagation of the Action Potential As the polarization moves down the sarcolemma, other voltage gated channels are opened and the process continues.

Local depolarization: generation of the end plate potential on the Figure 9.9 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber. Na+ K+ Axon terminal Synaptic cleft ACh– ACh 1 Local depolarization: generation of the end plate potential on the sarcolemma Open Na+ Channel Closed Na+ Closed K+ Open K+ 2 Generation and propagation of the action potential (AP) 3 Repolarization Sarcoplasm of muscle fiber

Repolarization This process restores the resting potential. The sodium channels initially opened by the depolarization close and at the same time a potassium channel opens, letting potassium to diffuse out of the cell, restoring the negative voltage inside the muscle fiber.

Na+ channels close, K+ channels open Depolarization due to Na+ entry Figure 9.10 Action potential scan showing changes in Na+ and K+ ion channels. Na+ channels close, K+ channels open K+ channels close Repolarization due to K+ exit Threshold Na+ channels Depolarization due to Na+ entry

Muscle Fiber Contraction: Cross Bridge Activity Before the muscle fiber contracts, there has to be an excitation coupling. This is the sequence of steps where the action potential along the sarcolemma leads to changes in the levels of calcium ions which results in the mechanical contraction.

Sarcoplasmic Reticulum and T Tubules These are two sets of intracellular tubules that participate in the regulation of muscle contraction and excitation coupling.

T Tubules These are found at each A and I band junction. A T-tubule (or transverse tubule), is a deep invagination of the plasma membrane (sarcolemma). These invaginations allow depolarization of the membrane quickly to the interior of the cell.

Sarcoplasmic Reticulum (SR) This is a modified smooth endoplasmic reticulum. Its tubules run longitudinally surround each myofibril. They communicate with each other in the H zone. They function to store calcium ions.

Figure 9.11 Excitation-Contraction Coupling (1 of 4) Axon terminal of motor neuron Muscle fiber Triad One sarcomere Synaptic cleft Setting the stage Sarcolemma Action potential is generated Terminal cisterna of SR ACh Ca2+

Figure 9.11 Excitation-Contraction Coupling (3 of 4) Calcium ions are released. Steps in E-C Coupling: Terminal cisterna of SR Voltage-sensitive tubule protein T tubule Ca2+ release channel Sarcolemma Action potential is propagated along the sarcolemma and down the T tubules. 1 2

Actin and Myosin & Contraction Myosin makes up the thick filament. This is a complex molecule that consists of two heavy and four light polypeptides. These form a molecule with a rod like tail with two flexible globular “heads”.

Figure 9.3 Composition of thick and thin filaments (2 of 3). Flexible hinge region Tail Myosin head ATP- binding site Heads Actin-binding sites Thick filament Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament. Portion of a thick filament Myosin molecule

Actin and Myosin & Contraction Actin makes up the bulk of the thin filament. This molecule has ”kidney shaped” polypeptide subunits called globular actin or G actin which combine with the myosin head during the contracting process.

Actin and Myosin & Contraction Troponin is a globular three polypeptide complex. It has several regulatory roles with actin.   TnI binds to actin TnT binds to Tropomyosin and helps to position it on actin TnC binds calcium ions.

Other Proteins Tropomyosin a rod shaped protein which helps to stabilize the actin molecule. In a relaxed muscle fiber, they block myosin blinding sites on the actin molecule.

Figure 9.3 Composition of thick and thin filaments (3 of 3). Tropomyosin Troponin Actin Active sites for myosin attachment Actin subunits Thin filament A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins (troponin and tropomyosin). Portion of a thin filament

Figure 9.3 Composition of thick and thin filaments (1 of 3). In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Longitudinal section of filaments within one sarcomere of a myofibril

Thick filament (myosin) Myosin heads Figure 9.4 Transmission electron micrograph of part of a sarcomere clearly showing the myosin heads forming cross bridges that generate the contractile force. Thin filament (actin) Thick filament (myosin) Myosin heads

Other Proteins Titin is the primary protein found in the elastic filament. This protein extends from the Z disc to the thick filament. It helps the muscle spring back to its original shape after stretching.

The Role of Calcium The cross bridge formation is the attachment of the myosin heads to the actin. This process requires calcium ions. Calcium is the key ion in the contraction process.

The Role of Calcium The muscle is relaxes when there are low levels of intracellular calcium ions. The myosin binding sites on the actin molecule are blocked by Tropomyosin proteins.

The Role of Calcium As intracellular calcium levels rise, the ions bind to regulatory sites on the protein troponin. This results in a change in troponin’s shape causing it to move the Tropomyosin off the myosin binding sites.

Process of movement Myosin heads bind to the passive actin filaments at the myosin binding sites.

Process of movement Myosin heads bind to the passive actin filaments at the myosin binding sites. Upon strong binding, myosin and actin undergo an isomerization (myosin rotates at the myosin-actin interface) extending an extensible region in the neck of the myosin head.

Figure 9.12 Cross Bridge Cycle (1 of 4) Actin Cross bridge formation. Ca2+ 1 Myosin head Thick filament Thin filament ADP P i

Figure 9.12 Cross Bridge Cycle (2 of 4) The power (working) stroke. 2 ADP P i

Process of movement 3. Shortening occurs when the extensible region pulls the filaments across each other (like the shortening of a spring). Myosin remains attached to the actin.

Process of movement 3. Shortening occurs when the extensible region pulls the filaments across each other (like the shortening of a spring). Myosin remains attached to the actin. 4. The binding of ATP allows myosin to detach from actin. While detached, ATP hydrolysis occurs "recharging" the myosin head. If the actin binding sites are still available, myosin can bind actin again.

Figure 9.12 Cross Bridge Cycle (3 of 4) Cross bridge detachment. 3 ATP

Figure 9.12 Cross Bridge Cycle (4 of 4) Cocking of myosin head. 4 ATP hydrolysis ADP PI

Figure 9.7 The phases leading to muscle fiber contraction (1 of 2). Muscle fiber is stimulated by motor neuron (see Figure 9.8). Action potential (AP) arrives at axon terminal at neuromuscular junction ACh released; binds to receptors on sarcolemma Ion permeability of sarcolemma changes Local change in membrane voltage (depolarization) occurs Local depolarization (end plate potential) ignites AP in sarcolemma

Figure 9.7 The phases leading to muscle fiber contraction (2 of 2). AP travels across the entire sarcolemma AP travels along T tubules SR releases Ca2+; Ca2+ binds to troponin; myosin-binding sites (active sites) on actin exposed Myosin heads bind to actin; contraction begins Phase 2 Excitation-contraction coupling occurs (see Figures 9.9 and 9.11).

Rigor mortis Illustrates the cross bridging requires ATP.

Rigor mortis Illustrates the cross bridging requires ATP. Most muscles stiffen 3 to 4 hours after death.

Rigor mortis Illustrates the cross bridging requires ATP. Most muscles stiffen 3 to 4 hours after death. Calcium leaks into the cells, causing cross bridging.

Rigor mortis Illustrates the cross bridging requires ATP. Most muscles stiffen 3 to 4 hours after death. Calcium leaks into the cells, causing cross bridging. ATP is no longer being produced, leaving the muscles stiff.

Rigor mortis Rigor mortis disappears after the muscle proteins begin to break down.