Knollenorgan Electroreceptors

Knollenorgans are tuberous electroreceptors found embedded in the skin of mormyrid electric fishes from Africa.  Knollenorgans are the time-coding electroreceptors in mormyrids.  Several hundred Knollenorgans are distributed over the body surface of these fishes, mainly on the head and in the caudal regions of the flank.  The white spots on the head of this fish are the unpigmented pores over the electroreceptors. Of these the largest ones are Knollenorgans; intermediate sized pores are mormyromasts; very small pores (not visible here) are ampullary receptors.  The Knollenorgans are specialized for detection of weak electric discharges of the electric organs of other electric fish.  

Photo copyright by C. D. Hopkins

Return to Knollenorgan Pathway

Knollenorgans have been termed "communication sensors" because:

Anatomy
Knollenorgans are composed of a set of receptor cells (RC) which may reach 40 to 60 microns in diameter.  Several receptor cells are innervated by a single, heavily myelinated nerve of large diameter.  The synapse beween receptor cell and nerve terminal is electotonic.  One to eight receptors cells are found in each organ.  There is a loose plug of epithelial cells between the tuberous organ and the skin surface.  The outer surface of the knollenorgan sensory cell is covered with dense layer of microvilli, increasing its surface area enormously.  

Sensitivity

The sensitivity is about 0.1 mV/cm to a transient step in an external electric field.  The receptors fire a single spike at the onset of the outside positive-going edge of the stimlus.  They respond only to the transient edge, not to the tonic  or D.C. component of the stimulus.  In the figures below (from Bennett, 1971)  the S trace represents the stimulus presented to the receptor pore.  RP is the receptor potential recorded at the opening of the pore.  NP is the nerve action potential.  The experiment on the left shows an annodal stimulus,  the right shows a cathodal stimulus.  Full scale time base for each trace is             `

Tuning

Knollenorgans are sensitive to high-frequency electrical stimuli, that is, frequencies between 100 Hz and 20,000 Hz.  they are insensitive to D.C. electric fields.  Knollenorgans have V-shaped tuning curves with best-frequencies centered on the peak frequency in the power spectrum of the Electric Organ Discharge of the species.  Thus,  the knollenorgan acts as a species-specific filter that enhances sensitivity to EODs compared to noise or to EODs of other mormyrid species  (reference:  Bennett, 1971;  Hopkins, 1983,  Bass & Hopkins, 1984).

Physiological Characteristics

Knollenorgans respond to outside-positive going transients.  They may show some spontaneous activity which is supressed by outside-negative-going transients.  Firing probability goes from silence to 1 to 1 firing over a dynamic range of 10 dB.  The knollenorgan spike causes the afferent nerve to spike with a short (0.2 ms) delay.  Receptor and afferent are coupled by an electrotonic synapse.

Central projections of the afferent nerve  

The knollenorgan afferents travel in the lateral line nerve to the posterior lateral line lobe of the hindbrain, also known as the electrosensory lateral line lobe (ELL).  Fibers terminate on somata of large spherical cells in the nucleus of the lateral line lobe (nELL) with large calyx like synaptic terminals.  For more on the Knollenorgan pathway,  see Knollenorgan pathway link.  The large spherical cells project to the nucleus exterolateralis pars anterior (ELA) of the midbrain via the lateral lemniscus. The spherical cells in the nELL receive inhibitory inputs from the command-associated nucleus.  This input blocks the spike in the spherical cell from Knollenorgan.


References

Bell, C., Dunn, K., Hall, C. and Caputi, A. (1995). Electric organ corollary discharge pathways in mormyrid fish: I. The mesencephalic command associated nucleus. J. Comp. Physiol. 177, 449-462.

Mugnaini, E. and Maler, L. (1987). Cytology and immunocytochemistry of the nucleus exterolateralis anterior of the mormyrid brain: possible role of GABAergic synapses in temporal analysis. Anat. Embryol. 176, 313-336.