Neuronal pathways and conditioning

Taste-sensitive sensory neurons that detect sucrose project from the proboscis into the suboesophageal ganglion, which is the first ganglion in the nerve cord after the brain and is also the ganglion that contains the motor neurons of the proboscis muscles. Odours are detected by sensilla on the antennae, which project into antennal lobes of the brain. After processing in the brain, information about odours is carried to the suboesophageal ganglion by neurons that originate in the forebrain, or protocerebrum. During conditioning, information about the sucrose reward must be carried to the brain from the suboesophageal ganglion and cause modification of the pathways that link olfactory sensory receptors with the protocerebral neurons. The pathways involved are shown in Fig. 9.4.

About 60 000 olfactory receptors run from each antenna to a discrete area on either side of the brain called the olfactory lobe. Each sensory neuron terminates in a spherical structure called a glomerulus, and olfactory information is processed both within and between the 160 glomeruli in an olfactory lobe before being distributed by projection neurons to other brain regions. Some neurons project directly from glomeruli to the protocerebrum, and

Suboesophageal ganglion

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Figure 9.4 The morphology of the brain and suboesophageal ganglion of the bee to show the structures and pathways involved in conditioning the proboscis-extension reflex to odours. (a) Information about odours is processed in the olfactory lobes, which send outputs both to the proto-cerebrum and to the calyces of the mushroom bodies, where Kenyon cells have their dendrites. Kenyon cell axons have branches to two separate output lobes of a mushroom body, from where interneurons travel to various brain regions, including the protocerebrum. Some interneurons travel from the protocerebrum to the suboesophageal and the segmental ganglia. Motor neurons that control proboscis muscles are situated in the suboesophageal ganglion, which is also where sucrose-detecting chemo-sensory neurons terminate. (b) A diagram to show the extent of the innervation pattern of neuron VUMmxl, whose cell body and dendrites are in the suboesophageal ganglion. (a and b after Hammer, 1993; reprinted with permission from Nature; copyright © 1993 Macmillan Magazines Ltd.)

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Figure 9.4 The morphology of the brain and suboesophageal ganglion of the bee to show the structures and pathways involved in conditioning the proboscis-extension reflex to odours. (a) Information about odours is processed in the olfactory lobes, which send outputs both to the proto-cerebrum and to the calyces of the mushroom bodies, where Kenyon cells have their dendrites. Kenyon cell axons have branches to two separate output lobes of a mushroom body, from where interneurons travel to various brain regions, including the protocerebrum. Some interneurons travel from the protocerebrum to the suboesophageal and the segmental ganglia. Motor neurons that control proboscis muscles are situated in the suboesophageal ganglion, which is also where sucrose-detecting chemo-sensory neurons terminate. (b) A diagram to show the extent of the innervation pattern of neuron VUMmxl, whose cell body and dendrites are in the suboesophageal ganglion. (a and b after Hammer, 1993; reprinted with permission from Nature; copyright © 1993 Macmillan Magazines Ltd.)

others project to another brain region, called the mushroom body. The mushroom bodies were named after their resemblance to some kinds of horn-shaped mushrooms, and have been implicated in the more complex types of insect behaviour, including learning and social behaviour. They are particularly well developed in bees, and also in some other types of insects, including cockroaches, where they may play roles in spatial memory (Mizunami, Weibrecht & Strausfeld, 1993). Each mushroom body contains a parallel array of neurons called Kenyon cells (one is drawn in Fig. 9.4a). About a third of all the neurons in the brain of a bee are Kenyon cells, with 170 000 in each mushroom body. It is extremely difficult to make recordings from them because they are small and tightly packed together. Their den-drites are arranged in structures called calyces, and olfactory information arrives at the outermost rim region of each calyx. Kenyon cell axons branch into two, one branch going into the a lobe of the mushroom body and the other branch into the p lobe. Various output neurons combine information from many Kenyon cells and deliver information from the mushroom bodies to other parts of the brain. Response properties of these output cells can alter during conditioning (Mauelshagen, 1993).

Two types of experiment suggest that both the antennal lobe and the mushroom body play roles in associative conditioning. In one, a small probe was used to cool local regions of the brain to 1-5°C for 10 s, transiently blocking or reducing activity (Erber, Masuhr & Menzel, 1980). Cooling either an olfactory lobe or a mushroom body reduced conditioning, and the effect was not simply to reduce sensory activity because conditioning was blocked if several seconds or even minutes elapsed between delivery of the odour stimulus and the start of cooling. In the second type of experiment, a small amount of the neuromodulator octopamine was injected into a brain region just after delivery of an odour pulse. The octopamine substituted for the effects of stimulation of taste receptors with sucrose, and when the odour was presented later on its own, it evoked proboscis extension. Octopamine had this effect when injected into the olfactory lobe or calyx of a mushroom body, but not elsewhere.

Essentials of Human Physiology

Essentials of Human Physiology

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