LEC Nervous System LEC

Overview of Nervous System Motor Divisions

  • Visceral Motor Division (Autonomic)
    • Targets: Smooth muscles and cardiac muscles.
  • Somatic Motor Division
    • Targets: Skeletal muscles.

Axonal Transport

  • Fast Axonal Transport
    • Characterized by ATP-dependent transport of vesicles along microtubules.
  • Other Transport Methods
    • Movement of cytoskeletal proteins via diffusion.
    • Passive migration of ions towards axon terminal.
    • Reverse transport of neurotransmitters into dendrites.

Action Potentials

  • Definition: A rapid depolarization and repolarization of the membrane potential of a neuron, initiating a signal that can be transmitted to other neurons, muscle fibers, or glands.

Key Steps in Action Potentials

  1. Trigger Zone

    • Location: Initial segment of the axon, where action potential begins.
    • Local potentials occur in dendrites and cell bodies, leading to generation of action potentials only at the trigger zone.

  2. Voltage-gated Channels

    • Potassium Ion Channels:
      • Two states:
      • Resting state: Closed.
      • Activated state: Open.
    • Sodium Ion Channels:
      • Three states:
      • Resting state: Activation gate closed, inactivation gate open.
      • Activated state: Both gates open.
      • Inactivated state: Activation gate open, inactivation gate closed.

  3. Local Anesthetic Drugs

    • Example: Lidocaine
    • Function: Blocks voltage-gated sodium ion channels, inhibiting action potential transmission and causing numbing effects.

Phases of Action Potential

  1. Depolarization Phase

    • Triggered when local potential reaches threshold of approximately -55 mV, opening sodium channels (activated state).
    • Sodium ions rapidly enter the cell, causing the membrane potential to rise towards +30 mV.

  2. Repolarization Phase

    • Sodium channels inactivate, stopping Na+ influx.
    • Potassium channels activate (open), allowing K+ to exit, restoring negative membrane potential.

  3. Hyperpolarization Phase

    • Membrane potential overshoots resting potential, reaching around -90 mV due to continued K+ outflow.
    • Resting membrane potential reestablished by sodium-potassium pumps and leak channels.

Refractory Periods

  • Absolute Refractory Period
    • No additional stimulus can generate another action potential.
    • Sodium channels inactivated.
  • Relative Refractory Period
    • A stronger-than-normal stimulus can generate an action potential.
    • Membrane is hyperpolarized; potassium channels still open.

Differences between Local and Action Potentials

  • Local Potentials
    • Graded, reversible changes in membrane potential.
  • Action Potentials
    • All-or-none response, irreversible once triggered.

Propagation of Action Potentials

  • Action potentials are self-propagating, traveling in one direction along the axon, triggered by sequential opening of voltage-gated sodium channels.
  • Continuous Conduction: Occurs in unmyelinated axons, slower propagation.
  • Saltatory Conduction: Occurs in myelinated axons, faster due to myelin sheath insulating segments; action potentials jump between nodes of Ranvier.

Types of Nerve Fibers

  1. Type A Fibers
    • Largest, myelinated, fastest conduction speed.
    • Found in skeletal muscle.
  2. Type B Fibers
    • Intermediate diameter, lightly myelinated.
  3. Type C Fibers
    • Smallest, unmyelinated, slowest conduction speed.
    • Associated with pain, temperature, and pressure sensations.

Neuronal Synapses

  • Definition: Junction where a neuron communicates with its target cell.
  • Types of Synapses:
    • Axodendritic: Between an axon and dendrite.
    • Axosomatic: Between an axon and cell body.
    • Axoaxonic: Between axons.
  • Presynaptic Neuron: The neuron sending the signal.
  • Postsynaptic Neuron: The neuron receiving the signal.

Synaptic Transmission

  1. Electrical Synapses: Allow direct, instantaneous current flow via gap junctions.

    • Bidirectional transmission; fast communication.
    • Used in synchronized activities (e.g., cardiac function).

  2. Chemical Synapses: Most common type, transmitting signals via neurotransmitters.

    • Synaptic Delay: 0.5 ms gap between action potential arrival and postsynaptic response.
    • Unidirectional; neurotransmitters released from presynaptic neurons and bind to postsynaptic receptors.
    • Varied signal size; can lead to local potentials or action potentials based on neurotransmitter binding.

Phases of Chemical Synaptic Transmission

  1. Action Potential Arrival: Triggers calcium ion channel opening leading to calcium influx.
  2. Neurotransmitter Release:
    • Synaptic vesicles fuse with presynaptic membrane to release neurotransmitters via exocytosis into the synaptic cleft.
  3. Neurotransmitter Binding: Neurotransmitters bind to postsynaptic receptors, opening ion channels and generating local potentials or action potentials.

Types of Neurotransmitters and Functions

  • Examples include acetylcholine, dopamine, norepinephrine, serotonin.
  • Functions encompass voluntary movement, cognition, sensation, and emotion.

Conclusion

  • Understanding the detailed mechanisms of action potentials and synapses is imperative for applications in health care, particularly in neurology and pharmacotherapy.