Study Notes on Electrical Signaling and Communication in Organisms

Bio 162: Electrical Signaling Beyond Animals

This document covers various aspects of electrical signaling in organisms, focusing on different communication methods, cellular processes, and the underlying mechanisms involved in action potentials in animals, plants, and fungi.

Communication Simplified

There are three main types of organismal communication:

  1. Diffusion:

    • Chemical signals are secreted by a source cell and travel via diffusion.
    • The message is received by target cells based on their receptors and competence.

  2. Bulk Flow:

    • Similar to diffusion, chemical signals diffuse into a fluid, travel via bulk flow, and diffuse out to be received by target cells, again depending on receptors and cell competence.

  3. Electrical Systems:

    • A source creates a change in charge distribution, which is then moved through conductive material, prompting a response from the target based on the change in charge distribution.

This can be illustrated as:

  • Information Source → Signal Channel → Meaning → Target
  • Example: The synaptic cleft where neurotransmitters operate.

Membrane Structure and Ion Movement

Cell Membranes

  • The cell membrane consists of a lipid bilayer.
  • Ions cannot diffuse through the lipid bilayer but can move in an aqueous solution on either side of the membrane.

Membrane Charge Distribution

Ions:
  • Defined as charged atoms or molecules, which can possess positive or negative charges.
Permeability:
  • Describes the ability of an ion to move across a membrane, influenced by:
    1. Driving Forces:
      • Whether forces drive ions in the same or opposite directions.
    2. Chemical Gradient:
      • Concentration differences across the membrane. Movement occurs from areas of high concentration to low concentration.
    3. Electrical Gradient:
      • Charge differences across the membrane. Positive charges are attracted to negative areas and vice versa.
Pathways for Ion Movement:
  1. Pumps:
    • These require energy and work against the concentration gradient.
  2. Channels:
    • Proteins that facilitate the movement of specific ions and include:
    • Leak Channels: Always open.
    • Gated Channels: Open and close under specific conditions altering the local charge distribution.
      • Ligand-Gated: Respond to chemical signals.
      • Voltage-Gated: Respond to changes in charge.
      • Mechanically-Gated: Respond to physical stimuli.
      • Photo-Gated: Respond to light stimuli.

Electrical Communication in Bacteria

  • Voltage-gated K+ channels have ancient origins and are highly conserved.
  • Bacteria can communicate over long distances within biofilm colonies through propagating waves of depolarizing K+.
  • Positive Feedback Mechanism:
    • A metabolic trigger releases K+, which then depolarizes neighboring cells stimulating them to release more K+.

Electrical Communication in Algae

  • Ca2+ signaling is a universal feature in all eukaryotes.
  • For example, brown algae respond to environmental disturbances by rapidly altering Ca2+ concentrations.
  • Diatoms utilize Na+ and Ca2+ voltage-gated channels to spontaneously generate action potentials (APs).
  • Red algae are less studied in this context, while non-plant green algae respond to light by activating light-gated ion channels resulting in Ca2+ influx.

Electrical Communication in Plants

  • Examples include the Venus Flytrap, which moves mechanically in response to signals for trapping insects, and Mimosa leaves, which react to protect themselves from damage.
  • Notably, electrical communication was first documented in the Venus Flytrap in 1873 when observed responding to temperature changes.
  • For electrical signaling, a change in voltage across a membrane is essential, signifying movement from the resting membrane potential.

Resting Potentials

Animal Neurons:
  • Utilize Na+/K+ pumps which require ATP:
    • Na+ is pumped out, K+ is pumped in.
    • K+ leak channels allow K+ to move out.
    • Resting Potential Range: -60 to -70 mV.
Plant Cells:
  • H+ pumps requiring ATP, which pump H+ out, allow K+ to leak into the cell through K+ channels.
    • Resting Potential Range: -80 to -200 mV.

Types of Potentials in Plants

  • Two primary types of potentials can be generated:

  1. Slow Wave Potential (SWP):

    • Characterized as a slow, long-lasting depolarization.
    • It is non-self-perpetuating and can trigger action potentials in response to harmful stimuli, such as wounds or flame.

  2. Action Potential (AP):

    • Described as an all-or-nothing signal, varying in size and duration depending on the species.
    • Typically triggered by non-harmful stimuli including temperature changes, light, mechanical stimuli, or electrical changes.

Steps of a Plant Action Potential

  1. Stimulus: Ca2+ ions flow into the cells.
    • This triggers a reversible shutdown of H+ pumps and activation of voltage-gated Cl- channels.
  2. Chloride Ions: Cl- ions leave the cell, leading to the action potential.
  3. Repolarization: Ca2+ pumps eject Ca2+ ions, reactivating H+ pumps for membrane repolarization.

Comparing Action Potentials

  • In animals, the inflow of Na+ initiates depolarization, while the outflow of K+ through voltage-gated channels is responsible for repolarization.
  • In plants, the outflow of Cl- leads to depolarization, with H+ pumps reactivated post-Ca2+ efflux promoting repolarization.
  • Action potentials in plants travel at speeds around 1 mm/s and are triggered by stimuli such as touch or temperature, while those in animals can travel at approximately 100 m/s responding typically to neurotransmitters.

Electrical Communication in Fungi

  • Spontaneous, action potential-like activity has been observed in fungal hyphae as a response to various stimuli.
  • The exact functions of action potentials in fungal communication remain unclear.
  • Nonetheless, such electrical communication could assist in coordinating the behavior of extensive mycelial networks or facilitate interactions with symbiotic organisms, often referred to as the 'Wood Wide Web.'

This comprehensive outline addresses the complexities of electrical signaling across various organisms, emphasizing the mechanisms and conditions underpinning their communication processes.