PRIMITIVE NERVOUS SYSTEM IN COLENTRATES

Presented by:Anirudh Acharya

Department Of ZoologyUniversity of Mysore

CONTENTS

Introduction to Coelentrates

Sensory Structures

Organization of Sensory Structures in Different Classes

References

INTRODUCTION TO COLENTRATES

Cnidarians - Primitive animals such as Hydra, Sea Anemones, Jellyfishes & Corals

Ctenophores – Comb Jellies

Classes of Cnidarians:-Hydrozoa – HydraCubozoa – Box JellyfishesScyphozoa – True JellyfishesAnthozoa - Sea Anemones

INTRODUCTION TO NERVOUS SYSTEM IN CNIDARIANS

Cnidarians have simple diffused nervous system called “NERVE NET”.

There is no “central nervous system” in cnidarians

The neurons usually are arranged in two reticular arrays, one between the epidermis and the mesenchyme and another between the gastrodermis and the mesenchyme .

The subgastrodermal net is generally less well developed than the subepidermal net, and is absent altogether in some species. Some hydrozoans possess one or two additional nerve nets.

At some locations they condense to form longitudinal tracts or circular nerve rings.

Eg: Longitudinal tract – Physalia Circular tract- Hydrozoan Medusae

A few nerve cells and synapses - polarized (bipolar) - transmission in only one direction

But most cnidarian neurons and synapses are nonpolar that is, impulses can travel in either direction along the cell or across the synapse.

In general body presence of minute hair like structure – Mechanoreceptors & Chemoreceptors

Some appear to be associated specifically with discharged cnidae, such as the ciliary cone apparatus of anthozoan polyps.

Motile medusae have more sophisticated nervous systems and sense organs than do the sessile polyps

In many groups, especially the hydromedusae, the epidermal nerve net of the bell is condensed into two nerve rings near the bell margin.

Axons end on other nerve cells at synapses or neural junctions with sensory cells or effector organs.

One way transmission between neurons in higher animals occurs because of the relative activity of the protein channels for sodium and potassium in the axons that generate the electrical signals (action potentials). In addition, synaptic vesicles are only found in the nerve cell terminal (synaptic button).

Cnidarian nerve nets are comprised of cells that have synaptic vesicles on both sides.

In cnidarians, there can be both one-way and two-way synapses with other neurons.

Cnidarian neurons lack the myelin sheaths which we will see in other animals. Thismyelin is a fatty acid membrane (in fact, a cell membrane) that will increase the speed of action potentials in larger, more complex animals.

SENSORY STRUCTURES

Ocelli - patches of pigment and photoreceptor cell organized as a disc or a pit.

Statocysts may be in the form of pits or closed vesicles, the latter housing a calcareous statolith adjacent to regime by directed swimming behaviors.

RHOPALIA

Rhopalium are the sensory structures of scyphozoan jellyfish. They include specialized structures for sensing light (eyespots) and movement or direction with respect to gravity (statoliths). The 'eyes' are most complex in the Cubozoa.

A RHOPALIUM (SECTION) HAS VARIOUS

SENSORY REGIONS.

A RHOPALIUM OF AURELIA (DIAGRAMMATIC).A PORTION OF THE GASTROVASCULAR CANAL HAS BEEN CUT

AWAY

A PIGMENTCUPOCELLUS (CROSS SECTION) OF A HYDROZOAN

MEDUSA.

A CUBOZOAN RHOPALIUM.

Most complex well developed Rhopalium. Cubomedusae possess as many as 24 well-developed eyes located near the bell

margin. One pit is located on the exumbrellar side of the hood of the rhopalium, the other

on the subumbrellar side.

THE EYE OF A CUBOZOAN (CARYBDEA) (CROSS SECTION).

They have a true epidermal cornea, a spherical cellular lens, and a retina.

Retina- multilayered, containing a sensory layer, a pigmented layer, a nuclear layer, and a region of nerve fibers.

There are, roughly 11,000 sensory cells in each of these remarkable eyes.

Cytoplasmic conducting System-

Similar in nature to that of sponges. Epidermalcells and muscle elements appear to be the principal components of the system. Impulse conduction, whichtravels very slowly and in a highly diffuse fashion.

REFERENCES:

Invertebrates by Brusca – 2003

The nervous systems of invertebrates: an evolutionary and comparative approach By Wolfram Kutsch

ReviewCnidarians and the evolutionary origin of thenervous system -2005 by Hiroshi Watanabe, Toshitaka Fujisawa and Thomas W. Holstein

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