8 Skeletal Muscle II & SM

Skeletal Muscle II & Smooth Muscle Overview

Presented by: Gianfranco Calafiore, M.S., D.C.

Page 1: Introduction

This section provides a comprehensive overview of skeletal and smooth muscle types, highlighting the key differences in structure, function, and innervation of these muscle types.

Page 2: Neuromuscular Junction

Initiation of Muscle Action Potential:

• Somatic motor neuron releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft at the neuromuscular junction.

• Muscle fiber membrane depolarization occurs due to the net entry of Na+ ions through the ACh receptor-channel, leading to the initiation of a muscle action potential.

Components:

• Key structural components at the neuromuscular junction include:

  • Z disk: Anchoring point for thin filaments (actin).

  • T-tubule: Invagination of the muscle fiber membrane that propagates action potentials.

  • Thick filaments (myosin): Motor proteins that interact with actin for contraction.

  • DHP (Dihydropyridine) calcium channel and RyR (Ryanodine receptor-channel): Critical for calcium release from the sarcoplasmic reticulum during excitation-contraction coupling.

Page 3: Excitation-Contraction Coupling

Contraction Mechanism:

1. An action potential traveling down the T-tubule alters the conformation of the DHP receptor.

2. The DHP receptor's alteration opens the RyR channels in the sarcoplasmic reticulum (SR), causing Ca2+ to be released into the cytoplasm.

3. Ca2+ binds to troponin, which causes a conformational change that moves tropomyosin, exposing actin binding sites for myosin.

4. Myosin heads, now bound to actin, execute the power stroke, sliding the actin filaments toward the center of the sarcomere, resulting in muscle contraction.

Page 4: Relaxation Phase

Mechanism of Muscle Relaxation:

• The sarcoplasmic Ca2+-ATPase actively pumps Ca2+ back into the SR, decreasing cytosolic [Ca2+].

• Reduced calcium levels lead to Ca2+ unbinding from troponin, allowing tropomyosin to cover the binding sites on actin, resulting in relaxation of the muscle.

• Elastic elements within the muscle pull the filaments back to a relaxed position.

Page 5: E-C Coupling Timing

Graph Representation:

• Graphical representation illustrating the timing of action potentials in the axon terminal that leads to a muscle twitch. Phases include:

  • Latent period: Initial delay before muscle tension begins to increase.

  • Contraction phase: The muscle actively contracts and generates force.

  • Relaxation phase: Muscle tension decreases as muscle relaxes.

• Duration of each phase typically ranges from 10-100 msec.

Page 6: Muscle Energy Sources

Aerobic Metabolism:

• Requires oxygen and generates 36 ATP per glucose molecule, with water (H2O) and carbon dioxide (CO2) as byproducts.

Anaerobic Metabolism:

• Occurs in the absence of oxygen, producing only 2 ATP per glucose molecule along with lactic acid as a byproduct.

Phosphocreatine:

• A rapid energy source, producing 1 ATP per molecule of phosphocreatine without oxygen, utilized for extremely short bursts of activity (up to 15 seconds).

Page 7: Detailed Pathways for Energy Production

1. Direct phosphorylation:

   • Energy source: Phosphocreatine, yielding 1 ATP per creatine; duration of energy provision is approximately 15 seconds.

2. Anaerobic mechanism:

   • Energy source: Glucose leading to production of 2 ATP and lactic acid; duration is approximately 30-60 seconds.

3. Aerobic mechanism:

   • Energy source includes glucose, pyruvic acid, fatty acids, and amino acids; producing 36 ATP with a duration of hours.

Page 8: Phosphocreatine Overview

Resting Muscle:

• Stores energy in phosphocreatine for rapid ATP production during muscle activity.

Working Muscle:

• Utilizes phosphocreatine to regenerate ATP vital for muscle contractions and recovery during relaxation periods.

Page 9: Creatine Supplementation

• Discusses how creatine supplementation may enhance muscle performance, increase the ability to generate force and provide a temporary "pump" effect during high-intensity workouts through increased intracellular energy availability.

Page 10: Types of Muscle Contraction

Concentric contraction:

• Muscle shortens as it generates force while opposing resistance (gravity).

Eccentric contraction:

• Muscle lengthens while still producing force, often used for controlled movements against gravity.

Isometric contraction:

• Produces tension without any visible movement, maintaining posture or resisting gravity.

Page 11: Length-Tension Relationship

Optimal Resting Length:

• Importance of maintaining an optimal degree of overlap between thick (myosin) and thin (actin) filaments, as too much or too little overlap adversely affects the muscle's tension-generating capacity.

Page 12: Summation of Contractions

• Single Twitches:

  • Occur when muscle fully relaxes between stimuli, resulting in low tension.

• Summation:

  • When stimuli arrive closely together, muscle does not completely relax, leading to increased tension.

  • Important in differentiating between unfused (incomplete) and complete tetanus contractions.

Page 13: Smooth Muscle Contractions

Phasic Smooth Muscle:

• Alternates between contraction and relaxation; typically found in organs like the intestines where rhythmic contraction is essential for peristalsis.

Tonic Smooth Muscle:

• Typically maintains a state of partial contraction and adjusts tension based on physiological needs, e.g., sphincters that regulate the passage of materials.

Page 14: Smooth Muscle Coordination

Single-unit Smooth Muscle:

• Comprises interconnected cells through gap junctions, resulting in synchronized contraction as a single unit.

Multi-unit Smooth Muscle:

• Consists of individual muscle fibers that are not electrically coupled; requires independent stimulation by nerves to contract.

Page 15: Smooth Muscle Structure

• Lacks organized sarcomeres, instead relying on a network of intermediate fibers anchored to dense bodies.

• Smooth muscle myosin has hinged heads distributed along its length, allowing for more versatile contraction patterns compared to striated muscle.

Page 16: Smooth Muscle Contraction & Relaxation

• Cytosolic Calcium:

  • Functions as the primary signaling molecule for initiating contraction in smooth muscle tissues.

• Relaxation Mechanisms:

  • Calcium removal is achieved via the activity of myosin light-chain kinase (MLCK) and myosin phosphatase, which are critical for muscle relaxation and returning to the resting state.

Page 17: Clinical Applications

Calcium-Channel Blockers (e.g., Amlodipine):

• These pharmacological agents are employed in treating conditions like hypertension and vasospasm; they facilitate the relaxation and vasodilation of smooth muscles, thereby lowering blood pressure and improving blood flow.