Michael Davis
Mushroom bodies are structures in the brains of insects and some crustaceans that are involved in learning and memory, particularly regarding smell. They are made up of densely packed nerve fibres called Kenyon cells, which have been found in all species investigated so far. Insects such as honeybees, flies, crickets, grasshoppers, locusts, and cockroaches are commonly studied for their mushroom bodies. The mushroom body's role in olfactory associative learning has been a focus of research, with studies suggesting that it acts as a coincidence detector, integrating multi-modal inputs and creating novel associations. Beyond insects, researchers have discovered mushroom-like benthic animals that may represent an early branch on the tree of life, potentially leading to the evolution of multicellularity.
| Characteristics | Values |
|---|---|
| Animals with mushroom bodies | Insects, Crustaceans, Honeybees, Flies (Drosophila), Crickets, Grasshoppers, Locusts, Cockroaches, Crabs, Lobsters, Crayfish, Spiders, Spiders, Crabs, Land hermit crabs |
| Function | Olfactory associative learning, Learning and memory, Motor control, Place memory, Sleep function, Spatial awareness |
| Brain structure | Kenyon cells, Serotonergic and dopaminergic neurons, GABAergic neurons, Serotonin, GABA |
| Other | The mushroom body is also found in anosmic primitive insects, The oldest pancrustaceans were locomotory adepts, not simple animals that crawled on the seabed |
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What You'll Learn
Insects such as honeybees, flies, crickets, grasshoppers, and cockroaches
Mushroom bodies are found in the brains of insects. They are composed of long, densely packed nerve fibres of Kenyon cells, the intrinsic neurons of the mushroom bodies. Fruit flies, for example, have around 2,500 Kenyon cells, whereas cockroaches have about 200,000. The Kenyon cells in the mushroom bodies of cockroaches are of class I, class II, and class III, with dendrites in a dorsal calyx and axons that bifurcate into medial and vertical lobes.
In honeybees, mushroom bodies are involved in visual learning. They are also involved in the detection of illicit drugs. In crickets, the Kenyon cells of the mushroom bodies are born from a group of neuroblasts located at the apex of the mushroom bodies.
Mushroom-related flies include the sciarid fly, the phorid fly, the house fly, the stable fly, and the cecid fly. These flies are common pests in compost wharves and feed on mushrooms.
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Crustaceans like crabs and lobsters
Mushroom bodies are structures in the brain that are known to be involved in learning and memory, particularly for smell. They are mainly composed of long, densely packed nerve fibres of the Kenyon cells. While it was historically believed that only insects had mushroom bodies, recent studies have shown that crustaceans like crabs and lobsters also possess these structures.
Studies on the brains of crustaceans, including lobsters and crayfish, have revealed that they do possess mushroom bodies. These structures are highly diverse across different crustacean species, contrasting with the evolutionary stability seen in insects. The mushroom bodies in crustaceans are believed to play a crucial role in spatial awareness, with their lobes increasing in size as they acquire more information about three-dimensional space.
Anomurans, such as land hermit crabs, provide a clear example of the role of mushroom bodies in spatial memory. In these crabs, the circuits characterizing mushroom body lobes have been integrated into the calyces, resulting in refined spatial memory for exploration and homing. This discovery sheds new light on the evolution of arthropod brains and the unity of Pancrustacea, a group that includes both crustaceans and insects.
While the presence of mushroom bodies in crustaceans like crabs and lobsters is now recognized, there are still disputes and ambiguities surrounding their exact nature and function. Some studies continue to express ambivalence about whether certain structures in the crustacean brain are true mushroom body homologues or derived from different evolutionary traits. Nonetheless, the discovery of mushroom bodies in crustaceans expands our understanding of the diversity and functionality of these structures across species.
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Hymenoptera, including ants
The mushroom bodies are a pair of structures in the brain of arthropods, including insects and crustaceans, and some annelids. They are known to play a role in olfactory learning and memory. They are involved in learning and memory, particularly for smell, and are the subject of intense research.
In larger insects, studies suggest that mushroom bodies have other learning and memory functions, like associative memory, sensory filtering, motor control, and place memory. They are also able to combine information from the internal state of the body and the olfactory input to determine innate behaviour.
The mushroom bodies are largest in the Hymenoptera, which includes ants, bees, and wasps. They are known to have particularly elaborate control over olfactory behaviours. In Hymenoptera, subregions of the mushroom body neuropil are specialized to receive olfactory, visual, or both types of sensory input. In ants, several layers can be discriminated, corresponding to different clusters of glomeruli in the antennal lobes, perhaps for processing different classes of odors.
The neural mechanisms that underlie the variety of behavioural traits of the hymenopteran insects remain largely unknown. However, research has shown that mushroom bodies generally act as a sort of coincidence detector, integrating multi-modal inputs and creating novel associations, thus suggesting their role in learning and memory.
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Kenyon cells, which are found in all species that have been investigated
Kenyon cells are the intrinsic neurons of the mushroom body, a neuropil found in the brains of most arthropods and some annelids. They were first described by F. C. Kenyon in 1896. The number of Kenyon cells in an organism varies greatly between species. For example, in the fruit fly, Drosophila melanogaster, there are about 2,500 Kenyon cells per mushroom body, while in cockroaches there are about 230,000. While the exact features of Kenyon cells can vary between species, there are enough similarities to define their general structure.
Kenyon cells have been found in the mushroom bodies of all species that have been investigated, though their number varies. Fruit flies, for example, have around 2,500, whereas cockroaches have about 200,000. The Kenyon cells are the long, densely packed nerve fibres of the mushroom bodies. They have dendritic branches that arborize in the calyx or calyces, cup-shaped regions of the mushroom body. At the base of the calyces, Kenyon cell axons come together and form a bundle known as the pedunculus. At the end of the pedunculus, Kenyon cell axons bifurcate and extend branches into the vertical and medial lobes.
In most insects, the major inputs to the calyces arise in the antennal lobes and convey information about the odour world of the insect. However, the social Hymenoptera are an exception to this rule of olfactory dominance in the mushroom bodies because their calyces also receive substantial inputs from the optic lobes. In the honeybee, the MBs are also involved in sensory integration. Honeybee MBs are paired structures, each of which has two cup-shaped calyces. Somata of the MB intrinsic neurons, termed Kenyon Cells (KCs), are distributed to the outer surface and inside of the MB calyces.
Several decades of behavioural and genetic experiments in fruit flies and honeybees have established that the MB is required for associative learning and recall. The intrinsic MB neurons are known as Kenyon cells (KCs). Their small cell bodies are packed tightly above the calyx (as grapes in a wine glass); the dendrites of each KC occupy a small fraction of the volume of the calyx. The Kenyon cells in the adult mushroom bodies of crickets are characterized by substantial adult neurogenesis. A cricket can add as many as 20 Kenyon cells per day to each calyx, resulting in the addition of hundreds of new Kenyon cells to the mushroom bodies of the adult.
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The role of mushroom bodies in learning and memory
The mushroom body, a structure found in the brain of arthropods, including insects and crustaceans, plays a crucial role in learning and memory, especially in the context of smell or olfactory processing.
The mushroom body is composed of densely packed nerve fibres known as Kenyon cells, which are intrinsic neurons. These cells have been identified in the mushroom bodies of various species, with their number varying—for example, fruit flies have around 2,500, while cockroaches have approximately 200,000.
Studies have revealed that the mushroom body is integral to olfactory associative learning. It receives olfactory signals from neurons outside the mushroom body, including dopaminergic, octopaminergic, cholinergic, serotonergic, and GABAergic neurons. The largest mushroom bodies are found in Hymenoptera, which exhibit sophisticated control over olfactory behaviours. However, their role likely extends beyond olfactory processing, as they are also present in anosmic primitive insects.
In larger insects, the mushroom body has been implicated in other learning and memory functions, such as associative memory, sensory filtering, motor control, and place memory. It acts as a coincidence detector, integrating multiple inputs and forming novel associations, thus contributing to learning and memory processes. Additionally, the mushroom body plays a role in innate olfactory behaviours through interactions with the lateral horn, utilizing the partially stereotyped sensory responses of mushroom body output neurons (MBONs).
Research on Drosophila has provided valuable insights into the role of mushroom bodies in learning and memory. Experiments have shown that ablation of mushroom body structures impairs olfactory learning and memory in these organisms. Drosophila are often used as model organisms to study learning and memory due to the ease of manipulating their relatively discrete mushroom bodies. Olfactory learning assays typically involve exposing flies to two different odors, one paired with electric shock pulses (conditioned stimulus, CS+) and the other unpaired (unconditioned stimulus, US). The percentage of flies that subsequently avoid the CS+ is considered an indicator of learning and memory retention.
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Frequently asked questions
Insects are known to have mushroom bodies, including honeybees, flies, crickets, grasshoppers, locusts, cockroaches, and fruit flies.
Crustaceans, such as crabs and lobsters, also have mushroom bodies.
Mushroom bodies are structures in the brain that are involved in learning and memory, particularly for smell.
Understanding the genetic basis of mushroom body functioning is important for research in learning and memory, as well as brain function.
