What is the name of the study of the mollusk

Mollusc nervous system

Mollusc nervous system [from Latin. molluscus = soft], E.nervous system of molluscs, the mollusc nervous system (Mollusks;see additional information 1). It is characterized by a pair of cerebral ganglion (upper throat ganglion), one of which is a throat ring and a pair of medioventral longitudinal nerves (Pedal strands) and a pair of lateral longitudinal nerves (Pleural cords or. Pleurovisceral cords) the perikarya of which are usually concentrated in ganglia. The pleural and pedal cords are medullary cords only in the original molluscs (i.e., the perikarya are distributed over the cord). The pleural cords are always connected to one another by commissures. In most molluscs, two pedal ganglia are located in the foot, while the two pleural cords each form a pleural, a parietal and a visceral ganglion. The latter innervate the lateral body wall and the intestines. There is an increasing tendency towards the displacement of ganglia in the vicinity of the cerebral ganglia in the gastropods (snails) and especially in the cephalopods (cephalopods). In the latter, most of the ganglia are so close to the cerebral ganglia that one can speak of a compact brain; in some genera there is also the formation of a head cartilage, which protects the brain - like the skull of the craniotes -. In the following, the nervous systems of the 7 classes of molluscs (see additional info 1) are discussed in more detail.
Aplacophoric nervous system (see Fig. 1): In the aplacophores, the upper pharyngeal ganglia are more or less fused together to form cerebral ganglia. To the left and right of the cerebral ganglia, three nerve tracts emerge, the buccal, lateral and ventral cords, which run through the length of the body. Three smaller nerves on both sides run from the cerebral ganglia in the rostral direction and supply the oral region, the grave shield and the anterior sensory pit. The area of ​​the foregut is nervously supplied by the buccal cords (each with a buccal ganglion) that are united with commissures to form a ring. The longitudinal nerve cords innervate the posterior regions of the body as well as the intestines. A common feature of all aplacophora is the formation of a suprarectal commissure leading over the rectum with a paired suprarectal ganglion. This sends nerve pathways to the mantle cavity and receives afferents from the dorso-terminal sensory organ.
Polyplacophoric Nervous System (see Fig. 2): In addition to the Monoplacophora, the Polyplacophora have the most primal nervous system of all molluscs. Beginning at the cerebral throat, two pleurovisceral cords pull laterally on the roof of the gill groove and two pedal cords median in the foot caudally. There the pleurovisceral cords form a suprarectal commissure. The pedal cords are connected to one another and to the respective pleurovisceral cord by commissures. The main strands are medullary cords; Ganglia are only formed as paired buccal ganglia and subradular ganglia, with connections to the subradular ganglia running from the lower pharynx to the subradular ganglia under the radula (rasp tongue). The (presumably photoreceptive) aesthetes, which are embedded in the outer shell layer, are to be mentioned as particularly characteristic sense organs. These complex sensory and secretory organs are innervated by the pleurovisceral cords.
Monoplachoric nervous system: The nervous system of the Monoplacophora shows clear similarities with that of the Polyplacophora. Here, too, two pleurovisceral and two pedal cords run caudally from the cerebral ganglia, where they connect and thus form a circumoral nerve ring. The pharynx is composed of a frontal cerebral commissure, a pair of lateral cerebral ganglia and the subcerebral commissure. It innervates the organs of the head area. Ganglia (cerebral, buccal and subradular ganglia) develop only in the area of ​​this throat ring. The pedal and pleurovisceral cords are connected by commissures that run between the dorsoventral muscles. In contrast to the gastropods, the pedal cords have only one commissure and predominantly innervate the foot muscles and the postoral tentacles. The pleurovisceral cords supply the mantle, the gills and the nephropores.
Scaphopod nervous system (see Fig. 3): The nervous system of the Scaphopoda is rather simply organized; it is a typical, symmetrically built mollusc nervous system. The cerebral and pleural ganglia are close together; The buccal ganglia lie just behind the cerebral ganglia, and an upper and a lower buccal ganglion can be distinguished. The pedal ganglia, which innervate the elongated foot, lie close together at a great distance from the cerebral ganglia. The statocysts are located at their posterior end (organ of equilibrium). Two pairs of nerves and three connectors originate from the cerebral ganglia. Dorsally, a pair of mantle nerves leave the cerebral ganglia and move to the dorsal muscles, from where they orient themselves forward and innervate the anterior dorsal mantle area. Ventrolateral to the cerebral ganglia arise the two captacular nerves around the Captacula (capture threads located on the head), each captaculum has its own nerve branch that ends in a small ganglion at the end of the capture thread. The cerebral connectives arise on the ventral sides of the cerebral ganglia in the immediate vicinity of the exit points of the captacular nerves. The cerebropleural connective originate from the posterior aspect of the cerebral ganglia. After a short distance, they merge with the Cerebropleuropedalkonnectives to form a common Cerebropleuropedal connective. From this fine nerve cords run to the statocysts. The buccal connectives descend from the front of the cerebral ganglia to the subradular ganglia. Nerve tracts run from the pleural ganglia to the cerebropleuropedal connections. Furthermore, the muscles of the body wall and the ventral anterior mantle area are supplied by these ganglia. The triangular visceral ganglia are located on either side of the anus; they innervate the intestines.
Bivalvic nervous system (see Fig. 4): Due to the extensive reduction of the head and their way of life as a filter feeder of plankton and suspended matter, the radula typical of molluscs is no longer present in the bivalvia; the nervous system is organized rather simply, whereby the reduction of the head results in modifications in the structural plan of the nervous system. The main ganglia are fused into three bilaterally symmetrical ganglia: a pair of cerebropleural ganglia, into which the buccal ganglia have also entered, a pair of pedal ganglia, and a pair of visceral ganglia. The cerebral and pleural ganglia lie close together or are fused together (cerebropleural ganglia) and lie lateral to the mouth opening. The cerebral commissure (cerebropleural commissure) is located in front of or above the pharynx. From the cerebral and pleural ganglia, a common bundle of nerves goes to the pedal ganglia in the foot and one to the particularly large visceral ganglia that lie on the posterior sphincter muscle. The visceral ganglia of the bivalvia correspond to the visceral and parietal ganglia of the gastropoda. The innervation areas of the cerebral and pleural ganglia are the anterior sphincter muscle, the mouth, the mouth lobes and the statocysts. The pedal ganglia supply the foot and, if present, the byssus retractor. In contrast, the posterior sphincter, the gills, heart, gonads, kidneys, intestines and the posterior edge of the mantle are supplied by the visceral ganglia. The large number of sensory organs on the edge of the mantle explains the size of the visceral ganglia, which accordingly have the highest number of neurons, while the number of neurons in the cerebropleural ganglia is the lowest. The ganglia usually work independently of each other. Only in reactions that affect the entire body are these coordinated by the cerebral ganglia or the cerebropleural ganglia. The Bivalvia have a multitude of different, sometimes very complex, pit, bladder and lens eyes (eye). As chemoreceptive organs (chemoreception) are i.a. Osphradia available.
Gastropod nervous system (see Fig. 5): The pleural and parietal ganglia and the mostly unpaired visceral ganglion are located one behind the other on the lateral, paired pleural cords. The contralateral ganglia belonging to one another are connected by commissures. The ipsilateral ganglia that do not belong together (originally arranged on the same side of the body) are connected via connective. The nervous system of recent gastropods essentially comprises the following main ganglia: a pair of cerebral ganglia, a pair of buccal ganglia, a pair of subradular and / or labial ganglia, a pair of pleural ganglia, a pair of pedal ganglia, a pair of parietal ganglia (subintestinal ganglia), an unpaired visceral ganglia. The cerebral, buccal, subradular, labial and pedal ganglia are connected by commissures. The pedal cords pull towards the pedal ganglia and form a pharyngeal ring with their commissures and the cerebral ganglia. Connective leads from the cerebral ganglia to the buccal, subradular, labial, pedal and pleural ganglia. The pleural ganglia are also connected to the pedal and parietal ganglia or subintestinal ganglia, as is the parietal ganglia with the visceral ganglion. However, in addition to the main ganglia mentioned, there are other accessory ganglia in the area of ​​the peripheral nerves, which predominantly contain primary sensory neurons. In highly developed representatives of the gastropods, the ganglia are usually concentrated in the head area. Original representatives have relatively long pleural cords, which means that the parietal ganglia and the visceral ganglion can be found in the viscera. These are affected by a torsion of the viscera, which leads to a crossover of the pleurovisceral connective (chiastoneuria) (see additional info 2). The cerebral ganglia represent a sensory center that controls all reflexes and movements of the organs they innervate. This affects the head with the mouth region and the sensory organs there (eyes and statocysts). Buccal ganglia, which are connected to the cerebral ganglia, are responsible for supplying the oral area. Orientation and spatial memory are located in the cerebral ganglia. The pedal ganglia supply the foot and its peripheral nerve plexus and thus coordinate the movement. The mantle is innervated by the pleural ganglia. The gills, osphradia (chemosensitive organs) and the skin receive their nervous supply from the parietal ganglia. The visceral ganglia innervate the internal organs. In some gastropods, neurosecretions could be detected that control egg-laying (egg-laying hormone) and the water balance. A tendency towards the development of a closed neurosecretory system can be recognized, although real neurohemal organs, as known in arthropods and vertebrates, are not present. Neurohemal areas, which in their evolutionary differentiation stand between those of arthropods and annelids, are found in pulmonary gastropods.
Cephalopod nervous system (Nervous system, Fig. 6; see Fig. 6): Due to the advanced concentration of ganglia in the head area, one can already speak of a brain in the Cephalopoda (cephalopods). The ratio of brain weight to body weight exceeds that of most fish or reptiles. The extraordinarily large repertoire of behavior patterns as well as the high learning ability characterize the exposed position of the cephalopods within the invertebrates. The organization of the nervous system is characterized by peripheral ganglia, which are responsible for finely tuned reflexes, and a superordinate brain that exercises control over the entire organism. The main elements of the cephalopod nervous system are the cerebral, buccal, labial, subradular, pedal, and pleurovisceral ganglia. Typical new acquisitions for this class include brute, optical, olfactory, peduncular and peripheral ganglia (stellar ganglia) and axial nerve fiber bundles in the arms. In the cephalopods, the nervous system of the tetrabranchiata (Vierkiemer, the only recent genus is nautilus) can be distinguished from that of the Dibranchiata (two-gill, actual "squid") (see additional info 3, see Fig. 6).
Dibranchiata - brain: In the dibranchiaten cephalopods the concentration of nervous centers continues and reaches its highest complexity in the Octobrachia (Octopoda, eight-armed squid). The level of development and efficiency of the dibranchy brain are comparable to that of vertebrates. The development of the sensory organs corresponds to the degree of differentiation of the entire nervous system, which is also reflected in the diverse behavioral patterns. Compared to all other mollusks, the cephalopods show significantly higher reaction speeds and are able to express states of excitement through rapid changes in color and shape of the body surface as well as through special light signals. In principle, the brain consists of an upper pharynx and a sub-pharyngeal mass (the "actual" brain) and lateral lobi optici, which are often larger than the rest of the brain. For adult representatives of Octopus vulgaris one assumes approx. 40 million nerve cells in the upper and lower pharynx and approx. 130 million nerve cells in the optical praises. - The Upper and Sub-throat mass: The supra-esophageal lobes represent the higher-level brain center. Long and short-term memory, the centers of learning in its various forms and the processing of incoming sensitive information are located in the vertical, subvertical, subpeduncular, precommissural, frontal and parts of the posterior buccal lobes. The various basal lobes and the peduncular lobe represent higher motor centers. The anterior basal lobe is primarily responsible for coordinating the arms, head and eye movements. The motor skills of swimming and breathing are controlled by the median basal lobe. The lateral basal lobe controls the muscular movements of the chromatophores and skin. The motor activities of ingestion are subordinate to the superior buccal lobe. The perioesophageal lobe represents a kind of switching station between the superordinate and subordinate motor centers. It is involved in defense and escape reactions and forms the origin of the giant fiber system (see below). The suboesophageal praises are all motor centers. - Optical praise: The optical praises process the incoming visual stimuli and regulate the behavior associated with seeing. They are the centers for visual discernment and related learning. Furthermore, they are involved in the expression of the color and shape variations of the skin and the chromatophores. A large number of substances that act as neurotransmitters or neuromodulators could be detected in the brain of dibranchiate cephalopods.
Dibranchiata - peripheral nervous system: The peripheral nervous system of the dibranchia consists of about twice as many neurons (350 million nerve cells) as the central nervous system. The ganglia belonging to it represent subordinate motor centers: The Brachial ganglia (Arm ganglia) together with the small subacetabular suction cup ganglia form a ganglion chain that coordinates the movements of the suction cups. The Octobrachia have as many brachial ganglia as there are suction cups. The inferior buccal ganglia is responsible for the innervation of the mouth region and the radula muscles. The Subradular ganglia are located in the muscles of the radula, in the region of the salivary glands: their innervation areas are the muscles of the salivary glands. The Gastric ganglia arise from the lower buccal ganglia; they form that with three nerve plexuses located in the intestinal wall stomatogastric nervous system. The fusiform ganglia (spindle-shaped ganglia) are not present in the ten-armed cephalopods (Decabrachia). They supply the auricles (auricles), ventricles (heart chamber) and the lateral area of ​​the vena cava. The Cardial ganglia each lie close to the gill hearts. It is from these ganglia that most of the nerves pull to the gill hearts. Fiber courses also supply the cordis branchialis (Sepiida) and the auricle (Sepiida). The Auricular ganglia are not present in all dibranchiate cephalopods; they are located directly on the auricles. The Branchial or Gill ganglia form a ganglion chain and supply the muscles of the gill lamellae. The Star ganglia innervate the mantle muscles. L-glutamate (glutamic acid) is considered to be the predominant neurotransmitter. The extracerebral ganglia of the neurosecretory system of the vena cava are located not far from the palliovisceral lobe. Their function is compared to that of the hypothalamus-pituitary system in vertebrates. A large number of different neurotransmitters (serotonin, FMRF-amide and catecholamines) were detected there. - All ganglia are made up of peripherally oriented perikaryen and a central neuropil. The peripheral nerves of the dibranchiata are not entirely myelinated (myelin). The system of giant fibers, which is present in Decabrachia and some Octobrachia, is a peculiarity of the nervous system of these cephalopods.The basic knowledge of the conduction of excitation in nervous tissues was gained from him. - The complexity of certain areas of the brain, the associated performance and the ability to learn of the cephalopods are closely related to their way of life. For orientation, forms living in the bottom area mainly use their sense of touch, while in the case of species living in open water the visual sense is in the foreground. Both systems, the tactile and the optical, have both promoting and inhibiting components. Due to the adjustable balance of these systems, the complicated behavior during courtship, prey capture and escape can be coordinated.

By.

Mollusc nervous system

1Mollusks (Mollusca, mollusks, E. molluscs) are a species-rich animal tribe that the Aplacophora (worm mollusks), the Polyplacophora (beetle snails), the Monoplacophora, the Scaphopoda (barnacles or barnacles), the Bivalvia (mussels), the Gastropoda (pelvis or snails) and the Cephalopoda (cephalopod) includes. They are usually compact, bilaterally symmetrical animals with head and trunk, which usually deposit a cuticle and / or a calcareous shell over their mantle. They have a ventral foot, usually feather-shaped gills in a respiratory space under the mantle fold, a radula (rasp tongue) and a heart in a pericardial space. Mollusks colonize the sea, fresh water and land in a cosmopolitan way. The Cephalopoda represent the most highly developed class within the invertebrates (non-vertebrate animals).



Mollusc nervous system

Fig. 1: Nervous system of a worm snail (Aplacophora / Solenogastres). Lateral view, middle section omitted, intestinal tube hatched.