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General Morphology

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Nervous System

The nervous system is very close associated with the circulation system. The "brain", termed synganglion, is located centrally at the level of the second coxae.

The synganglion is divided into two parts by the esophagus. The pre-esophagus-part consists of the protocerebrum, the optic lobes, the cheliceral and pedipalpal ganglia, and the stomodeal pons or bridge. The post-esophageal part serves the four pair of legs.

In ticks there is a very close association between the nervous and the circulatory systems. This is demonstrated by the enclosure of the entire central nervous system within a perineural sinus of the circulatory system; this receives a dorsal aortic vessel and gives rise to vessels enclosing the major nerve trunks.

No part of the central nervous system is located within the gnathosoma of the tick, which therefore does not correspond to the head in the generalised arthropod. The brain is located centrally at the level of the second coxa. Ticks can be killed quickly by crushing this region with a hard object (ticks are very resilient: a practical method of destruction is with hot water). The central idiosomal position of the central nervous system makes it poorly accessible for direct investigations. The tick central nervous system is more condensed than in other Chelicerata.

It is a synganglion, formed by the fusion of the brain ganglia and the abdominal nerve cord into a single mass. The nerve trunks arising from the ganglia are formed by axons of both receptor and motor cells. As in other acari, the synganglion is divided into two parts by the esophagus. Cranially, the esophagus lies beneath the synganglion, then crosses obliquely through the synganglion in a ventrodorsal direction to lie dorsally on the posterior portion before joining the midgut. The cranial, preesophageal part of the synganglion consists of the protocerebrum, the optic lobes, the cheliceral and pedipalpal ganglia, and the stomodeal pons or bridge. All ticks examined have been found to possess well-developed photoreceptors, even the "eyeless'' ticks (Aponomma, Ixodes, Haemaphysalis). They also have optic nerves and optic ganglia in the brain. A set of paired nerves extends from the optic lobes, a second set of paired nerves serves the chelicerae, and a third innervates the pedipalps. The unpaired stomodeal or pharyngeal nerve in nervates the pharynx.

The postesophageal part of the synganglion gives rise to four pairs of pedal ganglia serving the four pairs of legs in the adult tick. Fine "sympathetic'' nerves connect all four pedal nerve trunks laterally on each side of the synganglion. Several pairs of opisthosomal nerves innervate the viscera. The ventral lobes of the pedal ganglia of leg 1 contain discrete areas of highly differentiated neuropile which are thought to receive olfactory fibres from pedal nerve 1, and have been called olfactory lobes.

Associative centers are represented by several bilaterally symmetrical glomerular structures.

Anterodorsal, posterodorsal, and ventral glomeruli in the preesophageal part are connected by nerve fibre trunks. A complex of nerve fibres and trunks in the postesophageal part of the synganglion forms a five-level commissure-connective system

The synganglion and all peripheral nerves are covered by a connective tissue sheath, the neurilemma, below which there is a relatively thin layer of glial cells, the perineurium. Beneath these layers, subperineural glial cells are located on both sides of a cortex layer of nerve cell bodies surrounding the central fibrous neuropile, which constitutes the greater part of the synganglion mass. There is some direct and indirect (effects of acaricidic activity) evidence that acetylcholine and catecholamines (dopamine, noradrenaline, norepinephrine) play a role as neurotransmitters in ticks. However, the role of these substances, which are known to act as neurotransmitters in other animal groups, must remain speculative until further evidence has been provided (Mehlhorn, 2001).

   

References

  • Mehlhorn H: Encyclopedic Reference of Parasitology. 2nd edn., 2001, Springer Verlag, Berlin

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