Honey Bee Brain Structure

Posted at: July 3, 2002 09:42 AM | Comments (0) | Edit

Brain Structure

The brain on the worker honeybee occupies a volume of around 1mm3 and weighs about 1mg. The entire body weight, by comparison, is 100mg. The total number of neurons in the brain is estimated to be 950,000 (??? 850k?). The principle parts of the brain are the optic lobes, the antennal lobes, the mushroom bodies, and the central complex. The optic and antennal lobes are responsible for processing vision and olfaction respectively. The mushroom bodies and the central complex constitute the most important centers for behaviour and instincts. Other parts of the brain include the suboesophageal ganglion, the tritocerebrum, and the ventral cord. There are various other areas of neural tissue which surround all of these structures. However the boundaries of these areas are not so clearly defined and their function is yet to be understood. Very generally, structure of the brain is a layer of cell bodies, which could be thought of as a cortex, surrounding a central mass of fibres.

Theory: complex behaviour based on brain-wide networks somehow superimposed on similar smaller networks controlling individual responses. The exact physical location for of memory storage is not known. In fact, it is suspected that there is no one specific area of the brain where memories are stored. Instead the memories are probably distributed in a holographic fashion throughout all parts of the brain. (??? This needs to be rewritten)

Optic Lobes

The optic lobes are a large pair of structures on either side of the brain adjacent to the eyes. Each lobe consists of four distinct ganglia, these are called the Lobula, the outer and inner medulla, and the lobula plate. The total number of neurons in each lobe is XXX,000 (???).

[diagram of the optic lobes]
Need to expand in considerable detail here the structure of the optic lobes. What kind of neurons are present and how they are connected to each other.

Antennal Lobes

The antennal lobes are another prominent pair of structures located at the front of the brain. In total the antennal lobes contain about 10,000(???) neurons. These neurons receive input from the chemosensors in the antennae and are responsible for the preprocessing of olfactory and chemosensory information. The lobes are globular structures containing around 156 glomeruli each. The glomeruli form the functional units of the olfactory code. Each of these units can be uniquely identified by its sensitivity to a specific chemical or set of chemicals. The two lobes are connected to each other across the midline by a bundle of nervous tissue called the "supraesophageal commissure". Another bundle, called the "olfactorio-globularis tract", connects the antennal lobes with the mushroom bodies.

Antennal motor output - via antennal nerve (same as antennal lobe?) What sort of neurons do they contain? Need more info about the ultra structure.

[diagram of the antennal lobes]
The Mushroom Bodies

The mushroom bodies are a bilaterally symmetrical pair of structures deep within the brain. They are responsible for higher-order integration of sensory information and are vitally important to memory formation. Each body is made up of four lobes, these are 2 cup-shaped structures called calyces, and a stalk which is divided into one alpha and one beta lobe. The surface of each calyx is composed of a distinct class of neurons called Kenyon cells. The axons of the Kenyon cells descend through the center of the calyx and then branch in three directions. These branches connect to the adjacent calyx and into the alpha and beta lobes. The cells of the calyces, and part of the alpha lobe, receive sensory input of all modalities. The cells of the beta lobe are primarily motor output neurons which connect to nervous tissue at the sides of the brain, to areas called the "lateral horn". A thick layer of glial cells separates the mushroom bodies from the rest of the brain. The Kenyon cells remain within this sheath.

The relative size of the mushroom bodies in different species of insects directly correlates with the behavioural complexity of the insect. In the fruitfly, for example, which has a very simple behavioural repertoire, the mushroom bodies are much smaller than in social insects such as the honeybee. The mushroom bodies of the honeybee contain a total of 170,000 neurons (Kenyon cells).

[diagram P140 (kuhlenbeck?)]
As is to be expected of a brain area which is responsible for memory formation, the neurons of the mushroom bodies display a large degree of structural plasticity. It seems that memory traces are initially transferred from the antennal and optic lobes to various parts of the mushroom bodies. Processing in the mushroom bodies continues for a while even after stimulation has ended. A few minutes later the memories are then transferred to long term memory. This was demonstrated by experiments during which different parts of the brain were cooled whilst the bee was still alive and performing training exercises. In another experiment an insect was genetically engineered so that it contained less than 90% of it's normal quota of Kenyon cells. The deficient fly's behaviour was mostly normal, but it suffered severe amnesia during odor conditioning expermiments. Although the insect used in this experiment was the fruitfly, which is more amenable to genetic modification than the honeybee, the brain structure of these two insects are similar and the same functional principles are thought to apply.

Shape and size of the Kenyon cell neurons. kenyon cells two synapses from olfactory receptors bundles of long thin fibers might be involved in temporal integration of sensory signals

The Central Complex

The central complex is a group of structures located in the center of the brain. The function of the group is not known for certain as its neural circuitry is much more complex and less intensively studied than, for example, the mushroom bodies. The apparent function is the high-level control of behavioural activity and the regulation of the state of arousal of the insect. It is probably also a center for the integration of multimodal sensory information from the left and right hemispheres, as well as from other parts of the brain. Each of the central complex substructures is composed so-called "large-field" neurons which collect information from a large array of sources. These neurons form parallel strata within each substructure and are internonnected by complex sets of columnar "small-field" interneurons. This structure gives the central complex the appearance of a repetitive and modular architecture.

The central complex has four main substructures. These are the fan shaped body, the protocerebral bridge, the ellipsoid body, and the noduli. The largest of these is the fan shaped body. Its principle role is probably in arousal generation, i.e. it readies the animal for response to stimuli. We can guess that it is not involved in complex behaviour because its size is not related to the behavioural complexity of the animal. The fan shaped body is connected to other parts of the central complex, namely the protocerebral bridge and the eliposid body as well as to lateral parts of the brain. There are no direct connections to the mushroom bodies, the optic, or the antennal lobes. As are the other parts of the central complex, the fan shaped body contains layers of neurons through which runs a complex system of crossed axons.

The protocerebral bridge connects the optic system, the antennal lobes, and other parts of the brain to the fan-shaped body. This structure is a center for the higher control of walking behaviour. Mutant fruitflies which had a disrupted protocerebral bridge were unable to flie and had difficulties walking and making turns. The ellipsoid body is a small nucleus adjacent to the fan-shaped body. The noduli are a pair of ganglia located on the underside of the brain. The function of these last two parts of the central complex are not known.

The Sub-Oesophageal Ganglion

The sub-oesophageal ganglion is a relay station between the main brain and the ventral cord. It has been suggested that it is composed of six discrete glomerular structures. These are probably the nuclei for several nerves including the labial and pharyngeal nerves, as well as nerves reaching the mouthparts. These nerves are both sensory and motor. The motor neurons for the proboscis and mandible project from this ganglion. Sensory input from the many chemo and mechano-receptors on the tip of the proboscis is also processed here.

The Tritocerebrum

The tritocerebrum is composed of two, relatively small, bilateral lobes at the base of the brain. These are adjacent to the sub-oesophageal ganglia. The two lobes are connected by a bundle of nerve fibres called a commisure. The components of the tritocerebrum are not so well defined, but they are known to have both sensory and motor connections to the mouth and the digestive tract (stomatogastric nerves). The tegumental nerve also originates here. This nerve receives sensory input from the hairs on the top of the head.

The Ventral Cord

All insects have a ventral nerve cord which extends down the length of the ventral midline from the base of the brain to almost the tip of the abdomen. Along the ventral cord are a number of ganglia connected together by their longitudinal axons. These can display varying degrees of fusion. In the fruitfly, for example, the ganglia fuse into just one large thoraco-abdominal ganglia. The larva of the honeybee initially has 11 ganglia in its ventral cord, but these fuse to become just 7 in the mature adult.

Present in the ventral cord are two large bilaterally symmetrical interneurons which are part of what is called the giant fibre circuit. These two fibres relay excitation from the protocerebrum to the muscles of the thorax. The fibres are larger than normal and so have faster transmission times. This circuit is used for evasive responses.