Jamming Avoidance in Electric Fish

Chapter 8: Jamming Avoidance Response of Weakly Electric Fish Eigenmannia

• Eigenmannia virescens is a teleost (bony fish) belonging to the order Gymnotiformes, found in South and Central America.
• It breeds during tropical rainy seasons.
• Females defend nesting sites, while males are fiercely territorial.
• They produce electric fields using an electric organ in their tail.
• Electric organ discharges (EOD) are 200-500 times/sec (200-500 Hz) and species-specific, resembling a train of sine waves.
• EOD is generated by electrocytes, modified muscle cells, that maintain a negative charge inside.
• Upon stimulation, these cells depolarize, causing a discharge.
• Electroreceptors, located throughout the fish's body, detect electric discharges from their own EOD and others.
• Tuberous and ampullary electroreceptors are two types, containing 25-35 small receptor cells.

Introduction

• Model systems (e.g., toads, bats, barn owls) focus on sensory processing and motor function.
• Jamming Avoidance Response (JAR) in Eigenmannia is a critical model for understanding neural coding, especially temporal coding of spike time variability, information transmission, and neural pathway sensory based behavior.
• Electric Organ Discharge (EOD) in Eigenmannia is a stable biological oscillator.

Knifefish (Eigenmannia)

• They belong to the order Gymnotiformes and have 61 different species
• Electric Organ is positioned in their tails and is used for electrolocation and communication.
• Males and females have different nesting behavior during breeding season.

The Electric Organ

• Composed of modified muscle cells (electrocytes).
• EOD is generated via synchronized depolarization of electrocytes.
• Electrocytes are arranged in series, producing tens of millivolts discharge, and synchronous depolarization of tens to ~1V.
• The cells are electrically excitable and do not contract.
• The positive charge inside electrocytes is maintained by actively transporting ions across the membrane, using a sodium-potassium pump.
• Electrocytes use nicotinic acetylcholine receptors to initiate the process of depolarization.

Electric Organ Discharges

• Discharge frequency varies between 200 and 500 times per second (200-500 Hz).
• The waveform of the EOD is species-specific and often resembles a train of sine waves in knifefish
• The EOD is generated by endogenous oscillator within the medulla, specifically the pacemaker nucleus.

Electroreceptors

• Distributed throughout the body, often concentrated around the head.
• Electroreceptors can sense their own and other fish's electric discharges via electroreceptors.
• Two types: tuberous and ampullary.
• Each type contains 25-35 receptor cells.
• Synapse with sensory neurons.

Electrolocation

• Fish use their own electric field to "see" objects and neighboring fish by noticing changes in the electric field distortions.
• Changes in their electroreceptor responses indicate objects around them.
• Objects, or other fish, in the vicinity change the electric field, altering current patterns on the fish's body. This alteration is perceived as an "electric image".

Jamming Avoidance Response

• When two fish with similar EOD frequencies come close, interference occurs. This is detrimental if the frequency difference is only 20 Hz.
• JAR is a critical mechanism to avoid jamming of electric signals.
• Fish adjust their EOD frequency. If neighbour's frequency is higher, the fish lowers its frequency, and vice-versa.

Determination of Sign of Frequency Difference Without Internal Reference

• Fish do not need an internal reference for frequency, they use a mixture of its own signal and an interfering signal from other fish on parts of its body—with different geometries.
• Variation in mixing ratio is critical for jamming avoidance response.

Modulation of Amplitude and Phase

• Mixing signals (Sfish, Sneighbor) causes amplitude and phase modulation relative to Sfish, its degree of modulation dependent on amplitude ratio of interfering signals.
• Frequency difference is determined by electrosensory processing.

Electrosensory Processing I: Electroreceptors

• Ampullary receptors are tuned to DC and low-frequency signals, unclear role in JAR.
• Tuberous receptors are tuned to variable-frequency AC signals.
• Two subtypes: P-type sensors ("probability coders," detect changes in stimulus amplitude) and T-type sensors ("time coders," fire one spike per cycle).

Electrosensory Processing II: Electrosensory Lateral Line Lobe

• Tuberous electroreceptors project to four maps in the electrosensory lateral line lobe (ELL).
• Information processing proceeds in parallel to identify the sign and magnitude of the difference in EOD frequencies.
• T-type receptors provide phase information, and P-type receptors provide amplitude information. This is encoded as the difference in the phase of its own EOD and that of a neighbor's, which is used for JAR.

Electrosensory Processing III: Torus Semicircularis

• Located in the midbrain.
• Divided into distinct laminae. Phase coding in spherical cells of electrosensory lateral line lobe and amplitude coding in pyramidal cells.
• Phase coding is important information.
• Amplitude and phase information is achieved by vertical connections between different layers.
• Sign-selective neurons determine if the neighbor's EOD frequency is higher or lower.

Electrosensory Processing IV: Nucleus Electrosensorius

• Located in the diencephalon.
• Codes a sign from frequency differences between signals.
• Amplifies the response to phase modulations to improve JAR.
• Critical for executing jamming avoidance responses.

Motor Control via the Prepacemaker Nucleus

• Final command for adjusting fish's EOD is generated here.
• Central Posterior Nucleus (CP) of the Prepacemaker (PPnG) nucleus is activated if the needed increase in EOD frequency is detected.
• Sublemniscal Prepacemaker Nucleus (SPPn) is inhibited if the needed decrease in EOD frequency is detected.
• This leads to adjustment of frequency and allows the fish to avoid jamming.

Motor Control via the Pacemaker Cells and Relay Cells

• Pacemaker cells in the medulla oblongata drive the relay cells.
• Relay cells transmit the commands to spinal motor neurons that lead to adjusting the discharge of the electric organ.