Jamming Avoidance in Electric Fish

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Questions and Answers

What is the main significance of the Electric Organ Discharge (EOD) in Eigenmannia?

It facilitates sound localization in prey detection.

It serves as a means of locomotion.

It acts as a biological oscillator for understanding neural coding. (correct)

It regulates breeding behavior during the rainy season.

Which of the following correctly describes the Electric Organ in Eigenmannia?

It maintains a potential gradient via Na+/K+ pumps. (correct)

It generates electricity through the contraction of muscle fiber.

It is composed of contractile muscle cells.

It is stimulated exclusively by serotonin receptors.

What role do the electrocytes in the Electric Organ have in producing an electric field?

They contract to create movement in the water.

They reflect sound waves to detect prey.

They depolarize to generate a potential gradient. (correct)

They absorb electricity from the environment.

During which season do Eigenmannia typically breed?

During the tropical rainy season. Signup and view all the answers

What physiological mechanism primarily contributes to the depolarization of electrocytes?

Nicotinic acetylcholine receptor activation. Signup and view all the answers

Which of the following statements about the dominance of male Eigenmannia is true?

Dominant males use lower frequencies for communication. Signup and view all the answers

What best describes the nature of the Electric Organ Discharge in terms of output?

It results in a total potential averaging around 1 volt. Signup and view all the answers

What is the primary method by which Eigenmannia territories are defended?

Through biting and butting behavior. Signup and view all the answers

What is the primary role of electroreceptors in fish?

To sense electric fields produced by other fish Signup and view all the answers

Which of the following statements about the Jamming Avoidance Response is correct?

Fish increase discharge frequency if their neighbor's frequency is lower Signup and view all the answers

What does the phase shift in the mixing of electric signals indicate?

Who has the higher electric organ discharge frequency Signup and view all the answers

Which type of electroreceptor is primarily tuned to low-frequency signals?

Ampullary Receptors Signup and view all the answers

What happens when two electric fields interfere with each other?

It results in phase and amplitude modulations of the signals Signup and view all the answers

In which part of the brain is the electrosensory processing of amplitude and phase information achieved?

Midbrain Signup and view all the answers

How do fish determine the sign of the frequency difference they experience?

Through the mixing ratio of their own signal with that of a neighbor Signup and view all the answers

Which neurotransmitter is involved in inhibiting the sublemniscal prepacemaker nucleus?

GABA Signup and view all the answers

What is a key function of P-type electroreceptors?

Provide feedback on the high amplitude of signals Signup and view all the answers

What is the role of the Central Posterior Nucleus in electrosensory processing?

To encode and generate commands for adjusting EOD Signup and view all the answers

What alteration occurs in the fish's EOD frequency when there is an increase in the neighbor's frequency?

The fish decreases its EOD frequency Signup and view all the answers

Which type of synapse is primarily used by spherical cells in the electrosensory lateral line lobe?

Electric synapse Signup and view all the answers

In terms of spatial representation, how is information from electrosensory receptors organized in the brain?

Preserving the somatotopic organization of electroreceptors Signup and view all the answers

What electrical phenomenon is critical for the execution of the Jamming Avoidance Response?

Zero crossing variations and amplitude modulations Signup and view all the answers

Study Notes

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.

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Description

Explore the jamming avoidance response of Eigenmannia virescens, a unique species of weakly electric fish. This quiz covers their breeding habits, electric organ discharges, and how they use electroreception to navigate their environment. Test your knowledge on this fascinating aspect of tropical freshwater life.