Anatomic features of nerve plexuses in the honeybee brain

Hiroyuki Ai (Fukuoka University)

External view

Perspective view

Honeybee Standard Brain (HSB) was donated
from Dr. Rybak and and Prof. Dr. Menzel,
Freie Univ. Berlin (Brandt et al., 2005)

1. Brain

a. Structure and function of the brain

An insect brain is comprised of a protocerebrum, a deutocerebrum, and a tritocerebrum. In honeybees, the tritocerebrum is contiguous and fused to the back of the protocerebrum and the deutocerebrum; therefore, the boundary becomes ambiguous. In addition, the suboesophageal ganglion is fused to the brain; thus, the ventral nerve cord between them has been lost. The following outlines the structure and function of the nerve plexuses in the honeybee’s brain.

Mushroom body (MB)
A pair of mushroom-shaped nerve plexuses are located, one on each side of the protocerebrum. Each mushroom body is comprised of a calyx, a peduncle, and an α and β lobe; the calyx has separate input regions for visual, olfactory, and mechanical sensing. Endogenous neurons of mushroom body (Kenyon cells) form a layered structure, and individual layers are considered to process specific types of sensory information. Within the output regions of the mushroom body (α and β lobes), there are branching mushroom body exogenous neurons, which have been shown to change their mode of response upon learning.

Optic lobe (OL)
A pair of optic lobes are located, one on each side of the protocerebrum. They extend inside the compound eyes. An optic lobe comprises of three nerve plexus regions (lamina, medulla, and lobula), and the respective regions conduct three-step information processing on color/shape identification and movement vision. The lamina is involved in detection of light/dark contrast (determination of the contour of an object), the medulla is involved in the signaling of spatial and color vision information, and the lobula is involved in motion detection.

Central body (CB)
This is an unpaired nerve plexus in the center of the protocerebrum. It is considered to be pivotal in movement, but its full function remains unknown. In grasshoppers, the central body comprises layered neurons whose characteristics vary in response to polarized light.

Antennal lobe (AL)
These paired structures, one on each side of the deutocerebrum, consist of 156–166 glomeruli. Antennal olfactory, temperature, and humidity receptor neurons end therein, forming synapses with interneurons. Electrophysiological techniques and imaging with a calcium indicator have revealed an information processing olfactory mode for the antennal lobe, and an olfactory response map of glomeruli has been created. Considering the fact that sensory neurons responding to different smells converge in different glomeruli, the antennal lobe helps identify smells during olfactory information processing.

Dorsal lobe (DL)
This is a paired spherical structure located on each side of the deutocerebrum. Antennal mechanical, vibration, and taste receptor neurons end therein, forming synapses with interneurons. Unlike the antennal lobe, the dorsal lobe has no glomerular structures. However, because mechanical, vibration, and taste receptor neurons end in different regions, different types of sensory information are considered to be processed in distinct regions of the dorsal lobe.

b. Interneuron
In the foregoing brain structures, interneurons involved in information processing has been identified. These represent an important clue in determining the functions of individual nerve plexuses in the brain, and understanding the neural network for information processing. The current status of studies on each sensory information processing is outlined below.

Olfactory interneurons
In honeybees, the olfactory discrimination mechanism has been revealed via identification and imaging of antennal lobe output neurons that responsible for the signaling of olfactory information from the antennal lobe to the protocerebrum. In addition, honeybees are known to be capable of olfactory learning via smell conditioning based on the proboscis stretch reflex. The mechanism of olfactory learning has been elucidated at the neuron level.

Visual interneurons
Several neurons have been identified to receive input from the optic lobe, and have axons spreading to other regions of the brain and subordinate ganglia. In particular, several interneurons involved in motion detection have been identified with their dendrites branched into the lobula. Furthermore, brain descending neurons are branched towards the terminal area of these neurons; several of these have been identified in the protocerebrum, where they respond to movement in different directions depending on the manner of their branching.

Auditory interneurons
The auditory organ in the honeybee is the Johnston’s organ located in the second segment of antenna. Honeybees detect and signal the air-borne vibration component of a sound with this organ. Primary centers of vibration receptor neurons derived from the auditory (vibratory, to be precise) organ, are known to be distributed broadly in the protocerebrum, deutocerebrum, and suboesophageal ganglion. Research on the identification of interneurons receiving vibration information in each region has advanced, and it has been shown that vibration information is processed in parallel in the individual regions.

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