Question

Using Figure 1, what are the functions of the 3 major classes of KCs: α/β, α’/β’, γ? Note that they are separated as medial vs vertical lobes. Do they have individual functions and if so, what?

Figure 1 Negative Reinforcement (a) PAM PPL1 B-tip (b) Positive Reinforcement PAM KC PPL1 B-tip 01 tip Water weet rient Current Opinion in Neurobiology Schematics of reinforcing dop neurons that innervate the MB. (a) The MB-MP1 PPL1-y1pedcl and M (PPL1-y2a'11 DANs in the B-MV1 protocerebral posterior lateral (PPL 1 cluster provide negative reinforcement signals. The MB-MV1 neuron projects to the lower stalk and junction region and the M neuro innervates the heel and distal peduncle. In addition, the aversive MB-M3 (PAM-B2B'2a) neuron from the B-MP1 n protocerebral anterior medial (PAM) cluster ramifies on the tip of the B lobe. All neurons shown have an identical paired neuron that primarily innervates the contralateral MB lobes. (b) DANs in the PAM cluster mostly provide positive reinforcement signals. PAM DANs representing sugar sweetness (green with yellow outline), nutritious value of sugar (darker green) and water (blue) project to discrete zones of the horizontal B, B' and y MB lobes (marked with dotted outlines for Y: sweet taste to B'2am and Y4, sugar nutrient to Y5b (tip) and a1 (and possibly B1 B2, not illustrated), and water to a subregion of y4 that appears distinct from the sweet-taste DANs. In addition, naive evaluation of water vapor in thirsty flies requires a DAN that innervates B'2p. Reward type is therefore differentially represented in the DAN population and along the MB lobes. Several of the PAM DANs also have a projection to the contralateral MB. A single y KC is shown with inset illustrating a model where adjoining segments of the KC arbor contain KC presynaptic terminals that are reinforced by DANs for water, sweet taste, or sugar nutrient value. These presynapses are assumed to wire to MBONs with a very similar zonally restricted anatomy to that of the DANs. Cell body position is not stereotyped and diagrams are not intended to be anatomically accurate. These illustrations are edited from 125].

Answer 1

Ans:

Kenyon cells are the intrinsic neurons of the mushroom body called as neuropil. Mushroom bodies are essential for olfactory learning and memory and the odor information is represented by sparse combinations of kenyon cells. Information about odors may be encoded in the mushroom body by the identities of the responsive neurons as well as the timing of their spikes. The mushroom bodies are the central integrative brain region for associative memory and they represent a paired structure of approximately 2,000 neurons per brain hemisphere and are called as the kenyon cells. Kenyon cells receive dendritic input from the antennal lobes through projection neurons in the calyx area at the posterior part of the brain and send their axons anteriorly through a parallel bundle of fibers known as the peduncle. Calyx represents the higher-order center and they receive visual and olfactory information. The mushroom bodies are composed of calyces, penduncle and alpha/beta/gamma lobes and they are involves the learning, memory and fine discrimination of odors. Alpha/beta/gamma lobes are the centers where responses are initiated/behavior is produced.

Based on their axonal morphology kenyon cells are classed into three different subtypes: axons from α/β and α′/β′ kenyon cells branch at the anterior extremity of the peduncle to form the vertical (α and α′) and medial (β and β′) lobes, whereas axons from γ neurons form a single medial γ lobe. Not only for their role in memory encoding signaling from kenyon cells are important for memory retrieval. The immediate memory requires transmission from the α/β and α′/β′ and γ. Kenyon cells both α/β and γ are also involved in memory retrieval when both ARM and MTM are present. The retrieval of LTM relies on α/β KC output.

Kenyon cells within the classes display differential labeling with antisera against neuroactive substances. Kenyon cells make synaptic contact with one another and with other neuron types in the mushroom body. Extrinsic inhibitory and putative modulatory neurons were identified. Kenyon cells exhibit broadly tuned subthreshold activation by odor stimulation and a few cells responded with action potentials to specific biologically relevant odor combinations. Kenyon cells are the axons form peduncle or the tract and they are project toward alpha/beta/gamma lobes. The kenyon cells are divided into three classes from I to III.

Class I kenyon cells: It has extensiv branching spiny dendritic arborisations in both zones of the calyx and occupy the two most posterior subdivisions of the lobes called α/β and α’/β’

Class II kenyon cells: It has narrow clawed dendritic trees in the calyx and invade the most anterior division in the lobes, called γ. Class III Kenyon cells has clawed, diffusely branching dendrites in the calyx and it also provides a separate system of axons and terminal branches, that are detached from the rest of the mushroom body called the Y tract and lobelets.