lecture 15 physiology

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• Lecture Title: Human Physiology Lecture 15: Muscle Function

• Instructor: Dr. Suzanne Gray, UPEI

• Semester: Winter 2025

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Overview of Lecture Topics

1. Types and Structure of Muscle

2. Skeletal Muscle Contraction

3. Stimulation and Strength of Contraction

4. Types of Skeletal Muscle Fibers

5. Cardiac and Smooth Muscle

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Types and Structure of Muscle

• Skeletal Muscle

  • Characteristics:

    • Nucleus: Multiple nuclei

    • Striations: Present

• Cardiac Muscle

  • Characteristics:

    • Striations: Present

    • Intercalated disk: Present

    • Nucleus: Central

• Smooth Muscle

  • Characteristics:

    • Nucleus: Single

    • Striations: Absent

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Muscle Types Continued

• Skeletal Muscle

  • Voluntary control; striated muscle fibers

• Cardiac Muscle

  • Involuntary control; striated muscle fibers

• Smooth Muscle

  • Involuntary control; non-striated

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• Skeletal Muscle:

  • Represents all voluntary muscle; most abundant

  • Only cardiac muscle is found in the heart

  • Smooth muscle located primarily in digestive and circulatory vessels

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Structure of Muscle Fibers

• Individual muscle fibers are long cells, composed of multiple fused cells.

• Each muscle fiber contains many sarcomeres, the basic unit of contraction.

• The boundaries of sarcomeres create striations.

• Multiple nuclei present within each fiber.

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Hierarchical Structure of Muscle

• Whole Muscle Structure includes:

  • Tendon

  • Muscle fiber (muscle cell)

  • Muscle fascicle (cell bundle)

  • Artery, vein, and nerve supply

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Sarcomere Structure

• Myofibrils consist of:

  • Z lines

  • M lines

  • Sarcolemma

  • Sarcoplasmic reticulum (SR)

  • T-tubules

  • Triad structures (T-tubule and terminal cisternae)

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Muscle Appearance

• Striated Appearance:

  • Resulting from the arrangement of actin (thin) and myosin (thick) filaments.

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Sarcomere Bands

• I Band: Region with actin filaments

• A Band: Region with both myosin and actin

• H Zone: Area with only myosin

• Crossbridge formation explained with myosin and actin interaction.

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Skeletal Muscle Contraction

• Initiation of contraction begins with myosin and actin interaction.

• Changes in the bands and lines of the contractile apparatus during contraction.

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Contraction Process Overview

• 1. A nerve impulse travels down an axon.

• 2. Contraction regulated via the neuromuscular junction.

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Neuromuscular Junction (NMJ)

• The NMJ is where the axon of a motor neuron meets the muscle fiber.

• ACh (acetylcholine) is released into the synaptic cleft upon nerve impulse arrival.

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Impulse Transmission

• 4. ACh binds to receptors, activating sodium channels.

• 5. The electrical signal passes to T-tubules of the muscle.

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Role of Calcium in Contraction

• 6. Calcium channels open as an electrical signal triggers them.

• Calcium ions interact with actin and myosin, facilitating contraction.

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Myosin Structure

• Myosin filaments possess a tail and head, with:

  • Actin binding site

  • ATP binding site

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Actin Structure

• Actin has additional proteins bound to it:

  • Tropomyosin

  • Troponin

• Calcium ions allow binding of actin and myosin to occur.

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Power Stroke

• Myosin head attachment leads to a power stroke, bending towards the center of the sarcomere, shortening it.

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Cycle Process

1. Myosin head attaches to actin.

2. ATP hydrolysis occurs when the head is unattached.

3. ADP release triggers position change and actin movement.

4. Binding of ATP causes myosin head to return to resting position.

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Muscle Shortening

• After the power stroke:

  • Muscle shortens by approximately 1% but has potential to shorten up to 60%.

  • Cross-bridge cycle allows the power stroke to begin again after detachment.

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Sarcomere States

• Relaxed vs Contracted states of the sarcomere.

• Thin (actin) and Thick (myosin) filaments in both states.

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Sliding Filament Mechanism

• Review of skeletal muscle contraction through the sliding filament mechanism.

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Step-by-Step Contraction Process

1. Nervous stimulation via ACh.

2. Ca+2 leads to conformational change of actin.

3. Cross-bridge formed.

4. Muscle cell shortens due to simultaneous contraction of all sarcomeres.

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Rigor Mortis

• ATP is needed to detach myosin from actin, which cannot occur after death.

• Results in stiff muscles until breakdown occurs.

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Stimulation and Strength of Contraction

• Varying contraction strength and duration relies on the muscle twitch, which is an all-or-nothing response.

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Motor Unit

• Consists of a neuron and all muscle fibers it stimulates.

• Signals from the neuron activate all respective fibers.

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Muscle Twitch Phases

• Phases:

  • Latent Period: Initial milliseconds; connections between actin and myosin start forming.

  • Contraction: Muscle shortens.

  • Relaxation: Returns to original length.

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Muscle Twitch Graph

• Tension vs Time for a single muscular stimulus.

• Latent period, contraction phase, and relaxation phase shown.

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Variability in Muscle Fibers

• Differences in twitch speed depends on muscle fiber diameter, strength variations, and fiber types (slow vs fast twitch).

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Modifying Muscle Contraction

• Key factors: (a) Isometric vs Isotonic contractions(b) Frequency of stimulation(c) Strength of stimulation

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Isometric vs Isotonic Contractions

• Isometric: Same length, changing tension.

• Isotonic: Same tension, changing length (with examples).

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Frequency of Stimulation

• Strength increases with higher frequency of signals; calcium availability and myosin binding site exposure increase.

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Tetanus

• High-frequency stimulation leads to muscle twitches that can overlap, resulting in tetanus where muscle relaxation diminishes.

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Tetanus Clarification

• Distinction between physiological tetanus and the disease tetanus caused by bacteria.

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Strength of Stimulation

• Muscles contract stronger as more fibers are stimulated; requires recruitment of larger fibers as force increases.

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Types of Skeletal Muscle Fibers

• a) Slow and Fast Twitch Fibers

  • Different muscles exhibit varied fiber densities, such as:

    • Soleus: Mostly slow twitch

    • Gastrocnemius: Equal numbers

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Fast vs Slow Twitch Fiber Differences

• Slow Twitch: Efficient in using oxygen, delayed firing, fatigue-resistant.

• Fast Twitch: Quick to fire, better for explosive movements, fatigue quickly.

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Glycolytic vs Oxidative Fibers

• Classification based on metabolic properties:

  • Glycolytic: High glycolytic enzymes, few mitochondria.

  • Oxidative: Rich in mitochondria, smaller diameter, efficient ATP production.

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Fiber Attributes

• Slow oxidative fibers contract slowly using aerobic respiration.

• Fast oxidative fibers contract quickly using aerobic or anaerobic respiration.

• Fast glycolytic fibers primarily use anaerobic glycolysis and fatigue quickly.

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Types of Muscle Fibers Summary

• Three types based on contraction speed and energy production:

  1. Slow oxidative fibers

  2. Fast oxidative fibers

  3. Fast glycolytic fibers

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Cardiac Muscle Differences

• Striated like skeletal muscle but branched and interconnected.

• Intercalated discs with gap junctions, forming a single functional unit.

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Smooth Muscle Characteristics

• Location: walls of blood vessels, digestive tract, etc.

• Lacks sarcomeres; has actin and myosin in a way that allows stretching.

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Smooth Muscle Contraction Characteristics

• Takes longer to initiate/terminate contractions.

• Regulated by autonomic neurons; affects many cells simultaneously.

• Varies in connection degree via gap junctions.

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Types of Smooth Muscle

• Single-unit Smooth Muscle: Linked by gap junctions, few innervating neurons.

• Multi-unit Smooth Muscle: Few gap junctions, richly innervated, operates independently.

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Conclusion

• Key Takeaways:

  1. Types and structure of muscle.

  2. Mechanism of skeletal muscle contraction.

  3. Control of muscle contractions.

  4. Categories of muscle fibers.

  5. Basic understanding of cardiac and smooth muscle.

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Recap of Lecture Outcomes

• Understanding muscle contraction mechanisms.

• Preparation for upcoming topics on cardiac function.