Evolution of the nervous system
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Nerve nets in cnidarians like Hydra are a loosely organized system of nerves without central control. Stimulation at any point spreads impulses bidirectionally to cause whole body movement. Bilateral animals evolved nerve cords for more complex coordination like swimming. Centralization began with clusters of nerve cells (ganglia) controlling tasks in jellyfish and worms, concentrating sensory organs and feeding structures in the head and giving rise to the first brains connected to nerve cords.
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Evolution of the nervous system
- 2. Nerve Nets – Cnidaria Ex: Hydra - A loosely organized system of nerves with no central control - Most synapses are electrical - Impulses are bi-directional - Stimulation at any point spreads to cause movement of entire body
- 3. Nerve Cord – Bilateral Animals Ex: Worms Ventral Nerve cords – Invertebrate Ex: Insects Dorsal Nerve Cords – Chordates Ex: Fish
- 4. First Nervous System Centralization - Cnidaria - Ganglion( ganglia ) -Clusters of nerve cells control the ability to perform more complex motor tasks requiring coordination ex: swimming
- 5. Centralization in the Jellyfish – Jellyfish's skirt must open and contract in a coordinated manner for the animal to move through the water. – Nervous system serves as a simple communications network so all parts of the skirt open and then contract at the same time.
- 6. Cephalization – concentration of sensory organs & feeding structures at the head or forward-moving portion of an animal • Enlargement of the anterior ganglia that receive this sensory input and control feeding gave rise to the first brains • An anterior brain connected to a nerve cord is the basic design for all organisms with central nervous systems
- 7. Ex: earthworms • Dual nerve cord • Two ganglia -
- 8. Ex: insects • Parallel nerve cord • Series of ganglia
Editor's Notes
- The evolution of nervous systems dates back to the first development of nervous systems in animals (or metazoans). Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-celled and colonial eukaryotes.
- Simple nerve nets seen in animals like cnidaria evolved first,
- followed by nerve cords in bilateral animals - ventral nerve cords in invertebrates and dorsal nerve cords surrounded by a notochord in chordates. Bilateralization led to the evolution of brains, a process calledcephalization.relays impulses from receptors to parts of its body/ brain.
- The nervous system forms an undifferentiated network and serves primarily to coordinate the animal's swimming motions.
- The jellyfish (Medusa) is a cnidarian that exhibits basic centralization
- Bilateral animals tend to be more active require sense organs and feeding structuresBilateralization led to the evolution of brains, a process called /\\
- Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two ganglia at the head end function similar to a simple brain.Photoreceptors on the animal's eyespots provide sensory information on light and dark.The head segment contains the brain, also known as the supraesophageal ganglion.
- Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly.