L2 - Nervous System Evolution

Question 1

Single cell organisms

Answer 1

Receptive and responsive
Euglena – responds to light photons transduced by pigment localised to eyespot
Triggers more movement

Question 2

Multicellular organisms

Answer 2

Show specialisation
Primitive appearance of a nervous system
Allows more complex anticipatory and responsive behaviour

Question 3

Sponges

Answer 3

Water flows in through the body wall
Water flows out via osculum
Flow regulated by myocytes - specialised muscle cells that respond to stretch

Question 4

Stretch receptive myocytes

Answer 4

Evolved into primordial nervous systems

Myocytes – mechanoreceptive cells than span the outer epithelial ectoderm layer

Question 5

What do myocytes evolve into?

Answer 5

First neurons

• Sensorimotor cells that span from exterior to interior effector cells
• Transduce stimulus from exterior to interior

Question 6

Hydra

Answer 6

Some neurons evolved to be located beneath the outer ectoderm
See different types of neurons
- Sensory neurones span ectoderm
- Other neurones lie beneath it

Question 7

Hydra nervous system

Answer 7

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

Question 8

Hydra have interneurons

Answer 8

Lie between

• Sensory and myocytes/motor neurones
• Neurosecretory and myocytes/motor neurones
• Inhibition or excitation allows more complex interaction
• Diffused nerve net

Question 9

Hydra role

Answer 9

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

Question 10

Flatworms show?

Answer 10

Clustering and organisation of nerves showing

• Gangliation
• Cephalization
• Bilateral symmetry
• Fasiculation
• Commissures

Question 11

Gangliation

Answer 11

Coming together of neurones into a recognisable structure

Question 12

Cephalization

Answer 12

Clear posterior and anterior ends, head important

Specialised ganglia evolved around the pharynx – importance of eating

Question 13

Fasiculation

Answer 13

Nerves come together and sit in big bundles

Question 14

Commissures

Answer 14

Nerves synapse with contralateral nerves

Question 15

C.elegans organisation

Answer 15

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

Question 16

Drosophila provide evidence for?

Answer 16

Clusters of neurones above the mouth/pharynx

Question 17

Drosophila life cycle

Answer 17

Fertilisation 
Embryonic development 
Hatching – three larval stages separated by molts 
Pupation 
Metamorphis

Question 18

Drosophila major ventral cord

Answer 18

This is the main nerve system

This complexity starts at pharynx and extends dorsally

Question 19

Drosophila embryo development

Answer 19

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

Question 20

Drosophila delamination

Answer 20

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

Question 21

Vertebrates

Answer 21

Have a common body plan
Early nervous system is similar across families
The nervous system forms from neuroectoderm
But the nervous system is dorsal

Question 22

Vertebrate commonalities with lower order animals

Answer 22

Hypothalamus – ancient part of vertebrate. Coordinates behaviour without consciousness
CNS – autonomous integrating centre

Question 23

In both vertebrates and insects

Answer 23

Neurogenic region next to ectoderm
Neurogenic region migrates downwards
Neurogenic region is next to mesoderm that involutes

Question 24

In vertebrates not insects

Answer 24

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

Question 25

Neurons are derived from?

Answer 25

Surface layers from the most primitive times

Derived from precursors which migrate from their site of origin