PNB 2250 L7 2/4 Myocytes

Cytoskeletal Elements

  • Microfilaments, Intermediate Filaments, Microtubules:

    • Microtubules are the largest cytoskeletal elements.

    • Microfilaments are crucial for muscle contraction and cell movement.

    • Intermediary filaments provide structural support.

  • Analogy to a Straw:

    • Microtubules are compared to a straw, emphasizing their cylindrical shape.

Muscle Structure and Function

  • Unit of Muscle:

    • Sarcomere: The basic structural and functional unit of muscle.

    • In muscle histology, the striated nature is observable under a microscope by its banding pattern.

  • Control Mechanisms:

    • Neurotransmitters (like acetylcholine) play a pivotal role in muscle contraction through interaction with calcium ions.

    • Understanding muscle diversity, predominantly red muscle (slow oxidative) and white muscle (fast glycolytic).

Cytoskeletal Elements Formation

  • Formation of Microtubules:

    • Composed of alpha and beta tubulin monomers that polymerize to form sheets, which coil into cylindrical shapes due to their positive and negative charges (polarity).

    • Self-Assembly: Driven by the attractive forces between positively charged (alpha) and negatively charged (beta) tubulin.

  • Microtubule Functions:

    • Acts as a transport highway within cells.

    • Involved in mitosis by creating spindles that assist in chromosome movement.

    • Constitutes components of cell appendages like flagella.

Motor Proteins

  • Kinesin and Dynein:

    • Motor proteins that move along microtubules.

    • Kinesin: Transports vesicles towards the axon terminal.

      • Functions as a ‘dump truck’ for neurotransmitter transport to synapses.

    • Dynein: Moves towards the cell body to retrieve materials.

    • Both utilize ATP hydrolysis to power their movements.

Microfilaments (Actin) Structure and Function

  • Actin Filaments:

    • Comprised of actin monomers that assemble into twisted filaments (f-actin).

    • Actin has polarity, allowing it to polymerize effectively to form filaments necessary for muscle contraction.

  • Role in Fertilization:

    • Actin is crucial in sperm cells for the penetration of egg cells during fertilization.

Sarcomere Structure

  • Thick and Thin Filaments:

    • Thick Filament (Myosin): Appears beige in structural diagrams.

    • Thin Filament (Actin): Typically shown in red.

    • Z Discs: Provide boundary and anchoring points for thin filaments.

  • Sliding Filament Model:

    • Describes muscle contraction via increased overlap between thick and thin filaments resulting in muscle shortening.

    • Demonstrates the importance of myofibril arrangements in contraction efficiency.

Recruitment of Muscle Fibers

  • Motor Units:

    • Defined as groups of muscle fibers innervated by a single motor neuron.

    • Recruitment occurs based on the intensity of required force

    • Greater demand increases recruitment of additional motor units.

Fiber Types in Muscle Tissue

  • Slow versus Fast Twitch Fibers:

    • Slow Oxidative (Red):

      • Utilize aerobic respiration, fatigue resistant, better for endurance activities.

      • Contain myoglobin for oxygen storage, leading to color differences in muscle.

    • Fast Glycolytic (White):

      • Rely on anaerobic glycolysis, leading to faster fatigue.

      • Efficient for short bursts of speed and strength.

Muscle Cell Physiology

  • Calcium Regulation:

    • Troponin and Tropomyosin regulate actin’s binding sites. Only when calcium is present can the binding sites be exposed for myosin heads to attach and initiate contraction.

    • Calcium influx leads to muscle contraction, while its removal allows for relaxation by pumping calcium back into storage (sarcoplasmic reticulum).

  • Sarcoplasmic Reticulum and Ion Channels:

    • Sarcoplasmic reticulum stores calcium; upon depolarization by acetylcholine, voltage-gated calcium channels open, flooding the cytoplasm with calcium.

    • Action potentials initiate this process through synaptic transmission at neuromuscular junctions.

Types of Muscle Tissue

  • Skeletal Muscle:

    • Voluntary muscle that exhibits striations, controlled through conscious thought.

  • Cardiac Muscle:

    • Involuntary muscle with unique plateau-shape action potentials that prevent rapid re-contraction, regulating heart function.

  • Smooth Muscle:

    • Involuntary muscle arranged in bundles, involved in regulatory processes in the gut and blood vessels without striations.

    • Example includes bivalve retractor muscles utilizing a catch state to minimize energy use during contraction.

Comparison of Muscle Types

  • Skeletal Muscles Staining:

    • Staining can reveal striations and characteristics of fast versus slow twitch muscles based on their contraction mechanisms and energy utilization rates.

  • Functional Adaptations:

    • Adaptations of muscle fibers are related to their specific functions in animal behavior, influencing endurance and sprint capability based on ecological niches and lifestyle.