Muscle System Overview

Biology 1108 Lecture Notes: Muscle System

Overview of the Muscular System

  • The muscular system consists of over 600 different muscles.

  • Components include:

    • Nervous tissue

    • Blood vessels

    • Two types of tissue:

    • Connective tissue

    • Muscle tissue

Functions of the Muscular System

  • Movement: Muscles create movement by working with:

    • Bones

    • Ligaments

    • Tendons

  • Digestive Movement: Assists in moving food through the digestive system.

  • Circulation: Moves blood through the body by pumping the heart.

  • Heat Production: Generates heat as a byproduct of muscular activity.

  • Protection: Provides protection to inner organs.

Connective Tissue Coverings

  • There are four types of connective tissue coverings:

    • Deep Fascia: Surrounds the entire skeletal muscle and extends past muscle length to form a tendon.

    • Epimysium: Envelops the entire muscle organ.

    • Perimysium: Covers each fascicle, defined as a bundle of muscle fiber cells.

    • Endomysium: Encloses each individual muscle cell (fiber).

Tendons

  • Definition: Tendons are structures that attach muscles to bones.

  • Composition: Made up of flexible connective tissue strands.

  • Function: When a skeletal muscle contracts (shortens), the tendon pulls on the bone.

Antagonistic Muscle Pairs

  • Muscles work in pairs known as antagonistic pairs to achieve smooth and controlled movements.

  • Basic movement mechanics involve:

    • Flexor: A muscle that contracts to bend a part of the body.

    • Extensor: A muscle that contracts to straighten a part of the body.

Types of Muscle Activation

  • Muscle action can be categorized into two types based on activity:

    • Voluntary Activation:

    • Under conscious control.

    • Controlled by the Somatic Nervous System.

    • Examples: Walking, talking, running, writing, jumping, stretching.

    • Involuntary Activation:

    • Not under conscious control.

    • Controlled by the Autonomic Nervous System.

    • Examples: Heart beating, food movement through the digestive tract, reflexes.

    • Note: Skeletal muscles can function both voluntarily and involuntarily (in reflexes).

Types of Muscle Tissue

  1. Skeletal Muscle:

    • Attached to bones, facilitating movement.

    • Provides protection to inner organs.

  2. Cardiac Muscle:

    • Exclusive to the heart.

    • Responsible for heart contractions.

  3. Smooth Muscle:

    • Lines the digestive tract and blood vessels.

    • Facilitates the movement of food and blood through the body.

Comparison of Muscle Tissue Types

Type

Location

Function

Appearance

Control

Skeletal

Skeleton

Movement, heat, posture

Striated, multi-nucleated, parallel cells

Voluntary

Cardiac

Heart

Pumps blood continuously

Striated, one central nucleus, branching cells

Involuntary

Smooth (Visceral)

G.I. tract, uterus, eye, blood vessels

Peristalsis, blood pressure, pupil size

Non-striated, one central nucleus, spindle-shaped

Involuntary

Organizational Levels of Skeletal Muscle

  • Muscles organized into hierarchical levels:

    • Fascia: Covering muscle

    • Muscle fibers (cells):

    • Organize into Fascicles

    • Composed of Myofibrils

    • Surrounded by Epimysium (entire muscle) and Perimysium (fascicles)

    • Myofibrils contain:

    • Thick and Thin Filaments

    • Myofilaments grouped within Sarcomeres,

    • Contractile unit of muscle fibers.

Muscle Cell Ultrastructure

  • Detailed structure of muscle cell components:

    • Sarcoplasm: Equivalent to cytoplasm in other cells.

    • Sarcoplasmic Reticulum: Stores calcium, critical for muscle contraction.

    • T Tubules: Facilitate the transmission of nervous impulses and ensure simultaneous contraction of muscle fibers.

    • Mitochondria: Provide energy required for contraction.

Sliding Filament Theory of Muscle Contraction

  • Unit of contraction: Sarcomere.

  • Process Overview of Muscle Contraction:

    1. Neural impulse (action potential) travels across the neuromuscular junction with acetylcholine.

    2. Electrical disturbance travels down sarcolemma and into T-tubules.

    3. This disturbance prompts the release of calcium ions from the sarcoplasmic reticulum.

    4. Calcium ions bind to troponin, causing a shape change that opens actin active sites.

    5. Myosin crossbridges attach to these active sites.

    6. ATP is hydrolyzed, allowing myosin heads to flex (at a 45-degree angle) and pull actin filaments inward, contracting the sarcomere.

    7. Result: Reduction of sarcomere length and muscle contraction.

Excitation-Contraction Coupling

  • Series of biochemical and electrical events leading to muscle contraction:

    1. Sodium (Na+) entry generates an action potential.

    2. This potential propagates along the sarcolemma and into T-tubules.

    3. Activation of voltage-sensitive receptors triggers Ca2+ release from the sarcoplasmic reticulum into cytosol.

    4. Calcium binding to troponin removes tropomyosin blockade from actin active sites.

    5. Contraction ensues as myosin heads alternately bind to actin, with ATP hydrolysis powering this process.

    6. After contraction, Ca2+ is actively transported back into the SR, tropomyosin blockade is restored, and muscle fiber relaxes.

Energy Requirements for Muscle Contraction

  • Oxygen and Muscle Contraction:

    • Myoglobin in muscles binds and stores oxygen, supplying it for ATP synthesis through aerobic cellular respiration.

  • Muscle cells require vast amounts of ATP, available via:

    • Aerobic Cellular Respiration

    • Anaerobic Cellular Respiration

    • Direct phosphorylation of creatine.

Methods of ATP Regeneration During Muscle Activity

  • Direct Phosphorylation:

    • Energy source: Creatine Phosphate (CP).

    • Products: 1 ATP per CP, lasts 15 seconds.

    • Oxygen use: None.

  • Anaerobic Mechanism (Glycolysis and Lactic Acid Formation):

    • Energy source: Glucose.

    • Products: 2 ATP per glucose, lasts 30-60 seconds.

    • Oxygen use: None.

  • Aerobic Mechanism (Aerobic Cellular Respiration):

    • Energy source: Glucose, pyruvic acid, free fatty acids, amino acids.

    • Products: 38 ATP per glucose, CO2, H2O.

    • Oxygen use: Required, lasts hours.

Exercise and Muscle Response

  • Prolonged, Moderate Exercise:

    • ATP synthesized primarily through aerobic cellular respiration.

    • When glycogen is depleted, muscle switches to using glucose and fatty acids from blood.

  • Intense Exercise:

    • Muscle action surpasses capacity of respiratory and cardiovascular systems to deliver oxygen.

    • Results in energy production through anaerobic means (glycolysis) producing lactic acid, leading to muscle fatigue.

Oxygen Debt

  • Occurs when oxygen supply is insufficient during high-intensity exercise:

    • Glycolysis continues, converting pyruvic acid to lactic acid.

    • The liver processes lactic acid back to glucose, and oxygen levels in skeletal muscle must be restored to pre-exercise levels.

Heat Production during Muscle Contraction

  • Efficiency of cellular respiration is about 40%, meaning roughly 60% of energy from glucose is lost as heat.

  • Heat generated from muscle contraction plays a crucial role in maintaining body temperature as muscles metabolize nutrients to produce ATP.