AP test 3

Calcium and Muscle Contraction

  • T-Tubules and Sarcoplasmic Reticulum (SR)

    • T-tubules are small tubular structures that penetrate deep into the muscle fiber.

    • Enlarged areas of the sarcoplasmic reticulum are referred to as terminal cisternae.

  • Role of Calcium

    • Calcium ions are stored in terminal cisternae and are released during muscle contraction.

    • Concentration Gradient: Calcium moves from high concentration in the SR to lower concentration in the sarcoplasm (cytoplasm of muscle cells).

    • Calcium floods into the vicinity of the myofibrils.

  • Myofibrils and Sarcomeres

    • Myofibrils: Bundles of myofilaments, which are structures that enable muscle contraction.

    • Sarcomeres: The basic functional units of muscle made up of repeating structures, including the Z line and thick and thin filaments.

  • Troponin and Tropomyosin:

    • Troponin: A protein that binds calcium and is part of the thin filament, attached to actin.

    • Function: Binds to calcium to expose myosin binding sites on actin.

    • Tropomyosin: A protein that covers the myosin binding sites on actin in the absence of calcium. Troponin pulls tropomyosin away when calcium is present.

  • Crossbridge Formation and Myosin Dynamics

    • Myosin is the motor protein responsible for muscle contraction.

    • Requires ATP for crossbridge attachment and detachment,

    • Myosin heads bind to actin, forming a crossbridge, and need ATP to release the attachment.

    • ATP Hydrolysis: Energy from ATP is used to recock the myosin head, preparing it for another cycle of contraction.

    • Power Stroke: The action when myosin pulls actin, shortening the sarcomere; leads to muscle contraction.

Contraction and Relaxation Mechanisms

  • Contraction: The process where myosin pulls along actin, leading to muscle shortening.

  • Termination of Contraction: To stop contraction, calcium must be removed from the sarcoplasm.

    • Calcium must be pumped back into the sarcoplasmic reticulum against the concentration gradient, requiring ATP.

    • Calcium Pump: An ATP-dependent pump that helps restore low calcium levels in the sarcoplasm.

    • Lower calcium concentration means troponin returns to its original state, and tropomyosin covers the binding sites on actin, stopping contraction.

Acetylcholine and Muscle Contraction Regulation

  • Role of Acetylcholine: A neurotransmitter released at the neuromuscular junction to stimulate muscle contraction.

  • Esterase: An enzyme in the synaptic cleft that breaks down acetylcholine to stop muscle contraction.

  • Mechanisms to control contraction aside from calcium levels include the breakdown of acetylcholine, preventing receptor activation.

Muscle Fatigue and Recovery

  • Muscle Fatigue: Occurs when repeated contractions decrease the muscle's ability to generate force, typically due to a build-up of lactic acid, which lowers pH and affects enzyme activity.

    • Lactic acid is produced from pyruvate during anaerobic respiration when muscle activity exceeds oxygen availability.

    • Recovery involves converting lactate back to pyruvate in the liver, regenerating glucose and ATP in the process.

    • The recovery period for muscle function may depend on the extent and duration of the fatigue and exercise level post-activity.

Muscle Fiber Types and Characteristics

  • Types of Muscle Fibers:

    • Fast-twitch fibers: Contract quickly, larger diameter, contain more glycogen reserves but fewer mitochondria, leading to rapid fatigue. Primarily used in sprinter-type activities or heavy lifting.

    • Slow-twitch fibers: Contract slowly, smaller in size, contain more mitochondria, myoglobin, and blood vessels, thus more efficient at using oxygen. Best suited for endurance activities like long-distance running.

    • Intermediate fibers: Have characteristics of both slow and fast fibers and can serve multiple physiological needs.

Hypertrophy and Atrophy

  • Hypertrophy: Increase in muscle size and strength due to increased cross-sectional area, more mitochondria, and enzymes involved in ATP production; typically from resistance training.

  • Atrophy: Decrease in muscle size and strength; occurs due to disuse, aging, or injury where muscle cells shrink and lose functional capacity.

Neuromuscular Pathologies

  • Poliovirus: Attacks motor neurons, leading to decreased muscle excitation and eventual atrophy.

  • Botulinum Toxin: Inhibits release of acetylcholine at the neuromuscular junction, resulting in muscle paralysis (used in Botox).

  • Myasthenia Gravis: Autoimmune disorder affecting the acetylcholine receptors at the neuromuscular junction, leading to muscle weakness due to inadequate response to stimulation.

  • Rigor Mortis: Stiffness after death due to ATP depletion; myosin heads become stuck to actin in absence of ATP, preventing relaxation.

Neuron Structure and Function

  • Components of a Neuron:

    • Neurolemma: The plasma membrane of a neuron.

    • Dendrites: Branching processes of neurons that receive signals. Contain receptors for neurotransmitters.

    • Soma (Cell body): Contains nucleus and organelles, responsible for the basic function of the neuron.

    • Axon: Transmits electrical impulses away from the neuron; the site of action potential propagation.

    • Axon Hillock: Where action potentials are initiated when depolarization reaches the threshold.

  • Membrane Potential:

    • Resting Membrane Potential: Typically around -70 mV; due to distribution of ions across the neuron membrane.

    • Depolarization: Increase in membrane potential toward a positive charge; can lead to reaching the action potential threshold (usually -55 mV).

    • Repolarization and Hyperpolarization: Returning to resting potential or moving beyond into a more negative state, respectively.

Nervous System Overview

  • Central Nervous System (CNS): Comprising the brain and spinal cord.

  • Peripheral Nervous System (PNS): Consists of all nerves outside the CNS, including cranial and spinal nerves. Divided into:

    • Afferent Division: Sensory inputs to CNS.

    • Efferent Division: Motor outputs from CNS to target organs/effectors.

    • Somatic Nervous System: Controls skeletal muscles.

    • Autonomic Nervous System (ANS): Controls involuntary functions; further divided into sympathetic (fight or flight) and parasympathetic (rest and digest).

Role of Glial Cells

  • Glial Cells: Supportive cells in the nervous system that sustain neuron function. Functions include:

    • Providing structural support

    • Forming the blood-brain barrier

    • Myelinating axons to improve signal conduction speed (e.g., Schwann cells in PNS).

  • Multiple Sclerosis (MS): An autoimmune disease that attacks the myelin sheath of neurons, impairing nervous system function and coordination.