Muscular System: Histology and Physiology Study Notes

Chapter 10: Muscular System: Histology and Physiology

Functions and Properties of Muscle Tissue

• Functions: Muscle tissue serves multiple vital roles in the body, including:

  • Movement of the body
  • Maintenance of posture
  • Respiration
  • Production of body heat
  • Communication (e.g., facial expressions)
  • Constriction of organs and vessels (e.g., stomach, blood vessels)
  • Contraction of the heart (myocardium)
  • Stabilizing joints

• Properties: Muscle tissue exhibits several key properties:

  • Contractility: Ability to shorten forcibly when stimulated.
  • Excitability (responsiveness): Ability to receive and respond to stimuli.
  • Conductivity: Ability to conduct electrical impulses.
  • Extensibility: Ability to be stretched.
  • Elasticity: Ability to return to original shape after being stretched.

Skeletal Muscle Tissue

Type of Muscle Tissue: Skeletal

• Structure:

  • Long, cylindrical, striated muscle fibers.
  • Cells are multinucleated, containing multiple nuclei per fiber.

• Location: Primarily attached to the skeleton.

• Voluntary/Involuntary: Voluntary control.

• Function: Produces movement of the body.

Muscle Cells/Fibers Structure

• Components:

  • Plasma Membrane: Known as Sarcolemma.
  • Cytoplasm: Referred to as Sarcoplasm.
  • Myofibril: Formed of filaments, arranged in a specific pattern.
  • Sarcoplasmic Reticulum (SR): A specialized endoplasmic reticulum for calcium storage.
  • Myofilaments: Components of myofibrils which include:
  • Thick filaments: Myosin.
  • Thin filaments: Actin, Tropomyosin, Troponin.
  • Elastic filaments: Titin.

• Endomysium: A connective tissue layer surrounding individual muscle fibers.

Structure of Myofibrils

• Each myofibril is composed of numerous myofilaments of different types:
  • Contractile proteins: Myosin (thick) and Actin (thin).
  • Regulatory proteins: Troponin and Tropomyosin regulate contractile processes.
  • Structural proteins: Such as Titin which anchors thick filaments and maintains sarcomere structure.

Myofilament Arrangement and the Sarcomere

• I band: Region containing only thin filaments.
• Z disc: Boundary of each sarcomere, marking where thin filaments attach.
• A band: Full length of thick filaments, including overlap with thin filaments.
• H zone: Region of the A band that contains only thick filaments.
• M line: Midline of the sarcomere that anchors thick filaments.

The Sliding-Filament Mechanism of Contraction

• Relaxed Sarcomere: When muscle is not contracted - regions of I band and H zone are present.
• Contracted Sarcomere: When muscle fibbers shorten; I bands shorten, H zone disappears, A band remains constant.

Levels of Organization within a Skeletal Muscle

• Muscle fibers grouped into fascicles, surrounded by perimysium.
• Each fascicle then surrounded by epimysium.
• Epimysium converges at the end of a muscle to form a tendon.
• Fascia: A connective tissue layer encasing the entire muscle.

Membrane Potentials in Muscle Cells

• Definition: Membrane potentials arise from the unequal distribution of ions, generating a polarized resting state.

  • Negative charge inside the cell, positive outside.
  • Creates an electrical gradient, representing potential energy.

• When barriers (membranes) break down, ion flow results in action potential.

• Voltage: The difference in charge between two points.

Ion Channels and Gradients

• Types of Ion Channels:
  • Leak Channels: Always open to maintain resting potential.
  • Gated Channels: Controlled openings, including ligand-gated, voltage-gated, and mechanically-gated channels.
• Na+/K+ Pump: Actively moves three sodium ions out and two potassium ions into the cell per ATP consumed, maintaining gradients critical for muscle contraction and resting potential.

Electrochemical Gradients

• Definitions:
  • Concentration gradient affects the movement of uncharged solutes.
  • Movement of ions is determined by both concentration and electrical gradients, forming an electrochemical gradient.

Action Potentials

• Stages of Action Potential:
  • Depolarization: Triggered by opening voltage-gated Na+ channels, causing Na+ influx.
  • Repolarization: After depolarization, Na+ channels close while K+ channels open for K+ efflux.

The Neuromuscular Junction (NMJ)

• Definition: A synapse where a motor neuron communicates with multiple muscle fibers.
• Components: Includes the axon terminal/synaptic knob, synaptic cleft, and motor end plate.

Behavior of Skeletal Muscle Fibers

Phases of Contraction and Relaxation

1. Excitation: Nerve action potentials lead to muscle action potentials.
2. Excitation-contraction coupling: Links action potentials from sarcolemma to activation of myofilaments.
3. Contraction: Muscle fibers develop tension and may shorten.
4. Relaxation: Muscle fibers return to resting length after contraction.

Excitation Phase

• An action potential arriving at the axon terminal causes:
  • Voltage-gated Ca2+ channels to open,
  • Influx of Ca2+ triggers release of acetylcholine (ACh),
  • ACh binds to receptors on the motor end plate, generating an end-plate potential.

Excitation-Contraction Coupling

• The end-plate potential stimulates an action potential which propagates down T-tubules, leading to Ca2+ release from the sarcoplasmic reticulum.

Inside: Preparing for Muscle Contraction

• Calcium binds to troponin, moving tropomyosin and exposing actin's active sites.

Crossbridge Cycle

• The cycle of myosin head binding to actin, pulling it toward the center of the sarcomere through ATP hydrolysis. The sequence includes:
  • Myosin head in a cocked position.
  • Binding of myosin to actin.
  • Power stroke as phosphate detaches from myosin head.

Skeletal Muscle Relaxation

• Ca2+ is pumped back into the SR, tropomyosin blocks actin binding sites again leading to muscle relaxation.

Muscle Contraction Overview

• Steps include action potential traveling to the NMJ, ACh release, Na+ influx, and Ca2+ release from the SR causing contraction.

Toxins and the NMJ

• Toxins interfering with synaptic function can induce paralysis:
  • Spastic Paralysis: Continuous contraction due to toxins like tetanus.
  • Flaccid Paralysis: Muscles limp due to toxins such as curare and botulism.

Rigor Mortis

• A state of muscle stiffening after death, due to calcium leak, beginning within 3-4 hours due to a lack of ATP.

Sources of Energy for Muscle Contraction

• ATP is Required For:
  • Powering Na+/K+ pumps;
  • Releasing myosin heads from actin;
  • Pumping calcium back into the SR during relaxation.
• ATP Production: Via immediate reactions (creatine phosphate), glycolytic and oxidative processes, which may occur simultaneously during contraction.

Twitch Contraction

• Phases of twitch (on a myogram):

  • Latent period
  • Contraction period
  • Relaxation period

• Factors affecting tension: Timing/frequency of stimulation, fiber length, and type of muscle fiber.

The Length-Tension Relationship

• Optimal length of sarcomere: 100-120% of its natural length for maximum tension.

Types of Skeletal Muscle Fibers

Motor Units and Recruitment

• Motor Unit: Consists of a single motor neuron and all muscle fibers it innervates.
• Recruitment occurs to increase muscle force.
• Muscle Tone: Baseline level of involuntary activation of motor units.

Types of Muscle Contractions

• Isotonic: Muscle tension remains constant as length changes.
  • Isotonic Concentric: Muscle shortens against resistance.
  • Isotonic Eccentric: Muscle lengthens while contracting.
• Isometric: Muscle length remains unchanged despite tension.

Changes Caused By Physical Training

• Myoplasticity: Structural changes linked to functional adaptations from physical training.
• Satellite cells can repair injured muscle; atrophy vs hypertrophy depend on training type.

Muscular Fatigue and EPOC

• Fatigue: Inability to maintain intensity; caused by metabolic depletion, oxygen demands, chemical accumulation, and environmental stress.
• Excess Postexercise Oxygen Consumption (EPOC): Increased breathing rate post-exercise restores oxygen levels.

Smooth Muscle

Structure

• Single nucleus, non-striated, connected by gap junctions.

Locational Context

• Found in walls of hollow organs, skin, and eyes.

Voluntary/Involuntary

• Functions involuntarily to regulate organ dimensions.

Types

• Single Unit: Acts as a syncytium.
• Multi-Unit: Functions independently.

Smooth Muscle Contraction and Relaxation

• Contracting through a process where extracellular Ca2+ binds to calmodulin, activating myosin light chain kinase.
• Key events include activation of myosin ATPase and crossbridge cycling.

Steps of Smooth Muscle Contraction

1. Hormonal binding or depolarization activates G protein mechanism.
2. Ca2+ influx through channels.
3. Binding of Ca2+ with calmodulin activates myosin kinase.
4. Myosin heads phosphorylated and initiate contraction.
5. Crossbridge cycle initiates resulting in muscle contraction.
6. Relaxation facilitated by myosin phosphatase.

Cardiac Muscle

Structure

• Short, branched, striated cells with intercalated discs and typically one or two nuclei.

Location

• Exclusively in the heart.

Voluntary/Involuntary

• Functions involuntarily to maintain rhythmic heart contractions.

Functional Characteristics

• Coordinated by pacemaker cells; has abundant mitochondria for energy.