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Invertebrate Sensory Systems
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Covers the variations in CNS in different orders of invertebrates.
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Comparative Anatomy and Physiology of AnimalsLecture 25 Invertebrate Sensory Systems 16/02/21
General terminology
- Neurone: a type of cell. It is part of the nervous system and consists of a soma that gives rise
to dendrites which conduct electric excitation in a directed way. A neurone communicates
with other cells via synapses. Most neurones synthesise and secrete neuroactive substances.
- Axon: a long, slender projection of a neurone, that conducts electrical impulses away from
the neurone’s cell body. Myelinated axons are known as nerve fibres. The function of the
axon is to transmit information to different neurones, muscles, and glands.
- Brain: a brain is a cluster of specialised groups of neurones. It is part of a nervous system. It
is the most prominent anterior condensation of neurones and may also include further types
of cells.
- Nerve cord: a nerve is a cluster of neurones. It is the most prominent longitudinally
extending condensed part of a nervous system. In invertebrates, it runs ventrally.
- Ganglion: a ganglion is a group of specialised neurones. It is part of a CNS. The neurones are
arranged in a specific constellation: neuronal somata are concentrated at the surface, thus
forming a cell cortex, and neurites are concentrated in the centre of the ganglion to form the
neuropil. A ganglion is a distinct unit, but several ganglia may be anterio-posteriorly joined
by connectives or transversally by commissures.
- Nervous system: a nervous system is a cluster of neurones. It comprises all neurones of an
organism and may include additional glial cells. It may also include accessory cells, which, for
example, serve as supportive structures, stimulus guiding structures, or protective
structures.
Origin of the nervous system
- >95% of 116 genes involved in brain or neural
morphogenesis, were commonly shared among:
Chordata, Echinodermata, Lophophorates,
Arthropoda, Annelida, and Mollusca.
Cephalisation: The Annelid nervous system
- The central nervous system contains the brain and
the nerve cord.
- The peripheral nervous system contains the nerves
that travel to the brain, and the nerve cord to
specific body areas. The PNS main job is to send
information gathered by the body’s sensory
receptors to the CNS as quickly as possible.
Cephalisation
- Cephalisation is the process by which nervous tissue, over
many tissues, becomes concentrated towards one end of
an organism.
Variation of the CNS in Chelicerata (Arachnids)
- Spiders exhibit maximum concentration if the nervous
system. The whole series of ganglia are aggregated
together, and fused into one great central brain, from
whence nerves radiate to all parts of the body.
, Variations of the CNS in Crustaceans
Variations of the CNS in Insects
- In insects, the evolutionary trend has been towards a reduction in the number of abdominal
ganglia and most insects have fewer.
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