Electrical and Synaptic Signaling in Neurons

Electrical and Synaptic Signaling in Neurons

Nervous System Overview

  • The nervous system transmits impulses through specialized plasma membranes of nerve cells.
    • Central Nervous System (CNS): Comprises the brain and spinal cord.
    • Peripheral Nervous System (PNS): Includes sensory and motor components.
  • Two main types of cells:
    • Neurons: Send and receive electrical impulses (nerve impulses).
    • Glial Cells: Supportive cells with various functions.

Types of Neurons

  1. Sensory Neurons: Specialized for detecting stimuli (e.g., light, sound).
  2. Interneurons: Process signals and transmit information within the nervous system.
  3. Motor Neurons: Transmit signals from the CNS to muscles and glands (innervation).

Types of Glial Cells

  1. Microglia: Fight infections and remove debris.
  2. Oligodendrocytes & Schwann Cells: Form the insulating myelin sheath around neurons in CNS and PNS.
  3. Astrocytes: Control the access of blood-borne components to extracellular fluid, forming the blood-brain barrier.
  4. Ependymal Cells: Specialized ciliated epithelial cells filled with cerebrospinal fluid.

Neurons and Electrical Signaling

  • Appearance: Cell body (contains the nucleus), dendrites (receive signals), and axons (conduct signals).
  • Synapse: The junction between a nerve cell and another cell (e.g., muscle or gland).

Membrane Potential

  • Fundamental property of all cells:
    • Cells at rest have excess negative charge inside and excess positive charge outside.
    • Resting Membrane Potential (Vm): The electrical potential difference across the membrane when the neuron is not transmitting signals.
  • Potassium and Sodium Ion Distribution:
    • Higher concentration of K+ inside and Na+ outside the cell.
    • This creates a gradient critical for action potential generation.

Mechanisms of Membrane Potential

  1. Leak Channels: Always open channels allowing ions to diffuse based on concentration gradients.
    • K+ Leak Channels: More numerous than Na+ channels, leading to a negative resting potential as K+ leaves the cell.
  2. Na+/K+ Pump: Actively transports sodium out and potassium into the cell (typically ejects 3 Na+ for every 2 K+ brought in).

Action Potentials

  • Definition: Rapid changes in membrane potential due to changes in sodium and potassium permeability.
  • Stages of Action Potential:
    1. Depolarization: Na+ channels open, influx of Na+ makes inside positive (+40 mV).
    2. Repolarization: K+ channels open, and Na+ channels inactivate, causing K+ efflux.
    3. Hyperpolarization: K+ channels remain open too long, causing the membrane potential to dip below resting level.
  • Propagation: Action potentials propagate along axons without losing strength, driven by voltage-gated channels that open in response to membrane depolarization, enabling positive feedback cycles (Hodgkin cycle).

Refractory Periods

  • Absolute Refractory Period: No action potential can be triggered; Na+ channels are inactivated.
  • Relative Refractory Period: Action potential can be triggered, but it is more challenging; occurs during undershoot when the membrane potential is very negative.