L2 - Nervous System Evolution Flashcards
Single cell organisms
Receptive and responsive
Euglena – responds to light photons transduced by pigment localised to eyespot
Triggers more movement
Multicellular organisms
Show specialisation
Primitive appearance of a nervous system
Allows more complex anticipatory and responsive behaviour
Sponges
Water flows in through the body wall
Water flows out via osculum
Flow regulated by myocytes - specialised muscle cells that respond to stretch
Stretch receptive myocytes
Evolved into primordial nervous systems
Myocytes – mechanoreceptive cells than span the outer epithelial ectoderm layer
What do myocytes evolve into?
First neurons
- Sensorimotor cells that span from exterior to interior effector cells
- Transduce stimulus from exterior to interior
Hydra
Some neurons evolved to be located beneath the outer ectoderm
See different types of neurons
- Sensory neurones span ectoderm
- Other neurones lie beneath it
Hydra nervous system
Until recently believed to have a 2 layered nervous system
- Information flow from sensory to motor outputs
- Motor output to effector cells and other motoneurons
Hydra have interneurons
Lie between
- Sensory and myocytes/motor neurones
- Neurosecretory and myocytes/motor neurones
- Inhibition or excitation allows more complex interaction
- Diffused nerve net
Hydra role
Have neurosectory cells
- Secrete regulatory peptides belonging to hormone families insulin, somatostatin, glucagon
- Early cell type that supports evolution of a primordial nervous system
They can regulate myocytes
- 2 way flow of information
Flatworms show?
Clustering and organisation of nerves showing
- Gangliation
- Cephalization
- Bilateral symmetry
- Fasiculation
- Commissures
Gangliation
Coming together of neurones into a recognisable structure
Cephalization
Clear posterior and anterior ends, head important
Specialised ganglia evolved around the pharynx – importance of eating
Fasiculation
Nerves come together and sit in big bundles
Commissures
Nerves synapse with contralateral nerves
C.elegans organisation
Nervous system mapped
302 neurones
56 glia
Ventral, dorsal and lateral nerve cords
As they develop we can trace the progeny of cells as they divide and differentiate
Most neurones derived from AB cells - share lineage with hypodermis
Ganglia arranged in a nerve ring around the pharynx
Drosophila provide evidence for?
Clusters of neurones above the mouth/pharynx
Drosophila life cycle
Fertilisation Embryonic development Hatching – three larval stages separated by molts Pupation Metamorphis
Drosophila major ventral cord
This is the main nerve system
This complexity starts at pharynx and extends dorsally
Drosophila embryo development
Can see the nervous system develop
Neurogenic region (next to ectoderm) will become nervous system
Migration of neurogenic region during gastrulation
Individual neuroblasts delaminate and migrate inwards then coalesce
Drosophila delamination
Results in neuroblasts
These are dividing progenitors
They divide to give a second neuroblast and a ganglion mother cells
Ganglion mother cells form neurones and glia
Vertebrates
Have a common body plan
Early nervous system is similar across families
The nervous system forms from neuroectoderm
But the nervous system is dorsal
Vertebrate commonalities with lower order animals
Hypothalamus – ancient part of vertebrate. Coordinates behaviour without consciousness
CNS – autonomous integrating centre
In both vertebrates and insects
Neurogenic region next to ectoderm
Neurogenic region migrates downwards
Neurogenic region is next to mesoderm that involutes
In vertebrates not insects
Neural circuits do not delaminate
Neural cells stay as neuroepithelium/neural plate - a flat single layered plate like structure
- This elongates along anterior/posterior axis
- This rolls up to give rise to the neural tube
Nervous system is dorsal
- Major nerve fascicles are ventral in insects
Neurons are derived from?
Surface layers from the most primitive times
Derived from precursors which migrate from their site of origin
