The LAL-VPC complex

Olfactory information, including pheromone information, is relayed from olfactory receptor neurons (ORNs) to the protocerebrum (PC) via the antennal lobe (AL). Although neuronal pathways and projection areas processing pheromonal and general odorant information in the AL and in the PC show conspicuous differences (see also pheromone information pathway), the behavioural strategy of odor source localisation appears to be similar for both classes of odorants. This implies that both types of odorant information converge again at the stage of a premotor center that generates the behavioural program for odor source localisation behaviour. General odor information is processed to perform odor discrimination and include learned changes and transmitted to the premotor center, while pheromonal information is conveyed in a more simple manner in a labeled line. These differences between general odor and pheromone are possibly reflected in the differences between the neuronal pathways involved. Ultimately, motor patterns generating behaviour are generated in the more posterior ganglia.

Insect ganglia are located in the head, thorax, and abdomen. The thoracic ganglia possess motor centers that control important behaviors, such as flight, walking, and vocalization. The motor centers residing in the thoracic ganglia autonomously express and correct motor patterns in response to their own local sensory inputs largely without depending on inputs from the brain. However, the initiation and maintenance of their motor patterns is generally controlled by descending interneurons from the brain.

It is known that descending interneurons involved in odor source localisation behaviour receive synaptic inputs in the lateral accessory lobe (LAL). The LALs are a pair of bilaterally symmetrical neuropils approximately 100 μm wide (in the silkmoth) and of ellipsoidal shape. Some descending interneurons have dendrititic arborisations in the ventral protocerebrum (VPC) adjacent to each LAL. Thus LAL and VPC form a functional unit (LAL-VPC unit) (Kanzaki et al., 1994; Mishima and Kanzaki, 1999; Iwano et al., 2010). Both LAL-VPC units are connected via bilateral neurons and some descending neurons that transmit information bilaterally. The LAL-VPC complex is thought to be the major premotor center in the brain for the control of motor actions of the thoracic and possibly also abdominal ganglia. Most of the bilateral neurons are inhibitory and reciprocally inhibit the contralateral LAL-VPC unit.

The LAL-VPC unit is further subdivided into five regions, called lower LAL (lLAL), upper LAL (uLAL), outer VPC (oVPC), inner VPC (iVPC), and anterior inner VPC (aVPC). These five regions exchange information reciprocally in a complicated but apparently rigidly defined manner (Iwano et al., 2010). However, numerous details of these connections are still unclear.

How can command information for odor source localisation behavior be generated in LAL-VPC units?

Male silkmoths display a programmed behavioral sequence upon pheromone stimulation consisting of the walking patterns surge, zig-zag and looping. The command signals for this behavior are transmitted to the thoracic ganglia by descending interneurons arborizing in the LAL-VPC units. Some of the descending interneurons exhibit transient excitatory responses, others display flip-flop responses upon pheromone stimulation. The flip-flop response is a state-dependent response. Flip-flop neurons enter a state of high firing activity upon pheromone stimulation when they have been in a low firing state and by the same stimulus change to the low firing state when they have been in a high firing state before stimulation. These responses are similar to those of a commonly used type of semiconductor memory, the toggle flip-flop (T flip-flop). It is thought that the flip-flop responses are important components in the generation of the programmed behavioral sequence released upon pheromone stimulation. Flip-flop responses are likely to be generated in the LAL-VPC units.


Kanzaki R, Ikeda A, Shibuya T. (1994) Morphology and physiology of pheromone-triggered flipflopping descending interneurons of the male silkworm moth, Bombyx mori. J Comp Physiol A. 175:1-14.

Mishima T, Kanzaki R. (1999) Physiological and morphological characterization of olfactory descending interneurons of the male silkworm moth,Bombyx mori. J Comp Physiol A. 184:143-160.

Iwano M, Hill ES, Mori A, Mishima T, Mishima T, Ito K, Kanzaki R. (2010) Neurons associated with the flip-flop activity in the lateral accessory lobe and ventral protocerebrum of the silkworm moth brain.J Comp Neurol. 518(3):366-88.

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