Nervous system evolution Flashcards
Describe Euglena
A single cell organism which has a flagella and an eyespot- responds to light and moves away from it
What allows more complex behaviour and why?
Multicellular organisms - allows different cells to have different functions and allows a nervous system to develop
How is a sponge more developed that Euglena?
Has an epithelium which has different cell types - epithelial support cells and myocytes (muscles cells)
What are myocytes?
Mechanoreceptive cells that span the outer layer of the ectoderm (skin)
What can myocytes do in response to a signal? (daughter cells)
Proliferate to give:
- One daughter cell which ends up below the ectoderm (EFFECTOR CELL)
- One daughter cell which stays in the ectoderm and has a more complex shape - forms connections with the other daughter cell (SENSORIMOTOR)
PRIMORIDAL NERVOUS SYSTEMS
What behaviours can single cell organisms have?
Receptive and responsive
What behaviours can multicellular organisms have?
Anticipatory and response
Where did the first sensory motor system evolve from?
Myocytes spanning the ectoderm proliferating to give a daughter cell which makes connections with the daughter cells below the ectoderm
How are hydra different to sponges?
Have a 2 layered nervous system - (sensory, motor to effectors)
Where do the motor neurons in a hydra come from?
Evolve from the sensory neurons spanning the epidermis, come to sit beneath the skin, above the effector cells
Where do motor neurons have an output too?
Other motor neurons or effector cells
How is it proved that Hydra are more complex than a one way flow nervous system?
They have neuropeptidergic cells which produce neurosecretory proteins which are regulatory
Secrete SOMATOSTATIN - regulates the myocytes by sensory neurons (FEEDBACK)
2-way flow of information between sensory and motor neurons
Which cells show proof of evolution of a primordial nervous system?
Sponges - sensory neurons
Hydra - sensory, motor and interneurons. Also have 2-way flow of information between the sensory and motor neurons
Apart from sensory and motor neurons, what other neurons do hydra have?
What does this allow?
Interneurons
More complex interactions and outputs
Where do interneurons lie in the hydra?
Lie between sensory and motor neurons
Lie between neurosecretory and motor neurons
What are the first neurons to develop in the body?
Which organism are these first evident?
Sensorimotor neurons - span the epidermis
Sponges
What do sensorimotor neurons develop into?
Sensory neurons (which span the skin)
Motor neurons which come to sit underneath the skin
What can interneurons be?
Excitatory or inhibitory
How are the nerves arranged in a hydra?
In a nerve net all overt the body - joined up and connected together in a diffuse system
What is the most primitive nervous system?
Nerve net (Hydra)
How is the nervous system of flatworms more complex than in the hydra?
What does this allow?
No diffuse nerve net, there is more organisation and clustering of nerves
Can predict position of major clusters of nerves
What clustering/organisation is present in the flatworm?
- Gangliation
- Cephalization
- Bilateral symmetry of the nervous system
- Fasciculation
- Commissures
What is gangliation in the nervous system?
Clustering of neuronal cell bodies
What do ganglions/nuclei of the nervous system contain?
Cell bodies of the same neurons or cell bodies of different neurons
What is cephalization of the nervous system?
What does this form during evolution?
- Concentration of the sense organs and nervous control at the anterior end of the body
- Forms the head and the brain
How is there a clear anterior and posterior end of the flatworm?
More complex organisation of complex neurons near the head and specialise ganglia around the pharynx and mouth
What is fasciculation?
Bundling of the axons of the nervous system
What are commisures?
Neurons crossing the midline to synapse on the contralateral side of the body
What do commisures allow?
Communication between the left and right side of the body and direction of movment
In annelids (segmented worms) where is the biggest clustering of ganglia found and why?
Around the pharynx - allow to catch food
What are suprapharyngeal ganglia?
Specialised ganglia piling up dorsally to the pharynx
What does the suprapharyngeal ganglia produce?
What does this prove?
Neurotransmitters similar to that in the human brain
Very primitive brain formed above the mouth
What was the primitive brain used for?
Originally for the organism to be the best at eating and surviving
What is special about the c.elegans?
Well studied - entire nervous system has been mapped
What is a nerve ring?
Huge array of ganglia around the pharynx in c.elegans
What proves that skin and nervous system have an early lineage relationship?
Through the study of c.elegans:
- All cells develop from a single cell (P cell)
- Divides into P and AB cell
- Hypodermis and other cells of the body derive from the AB cells
Where does the nervous system lie in the drosophila and what does it show?
Ventrally
Shows:
- Gangliation
- Fasiculation
- Cephalization
How are the neurons in the head of the drosophila arranged?
Clusters above the mouth/pharynx
Complex array around the subosophageal region, extending dorsally
Signs of protocerebrum (early brain)
Eyes
In which organism is an early brain formed?
Drosophila (insect)
How do we know how the nervous system in humans develop?
Through studies in drosophila
How does the nervous system develop in drosophila?
1) Migration of neurogenic region ventrally as gastrulation occurs (ectoderm moves into the embryo, 2 areas of neurogenic region are brought together)
2) Individual cells delaminate and become neuroblasts (dividing progenitors)
3) Neuroblasts divide and give: 2nd neuroblast and a ganglion mother cell
4) GMC form neurons and ganglia
5) Neuroblast are retained as a multipotent cell
When is the nervous system set aside in development?
Very early
How is the development of the vertebrate nervous system different to invertebrates?
- Nervous system located on the dorsal side of the body (fascicles)
- NS also develops from the ectoderm as well as the neurogenic region
What are the commonalities of the nervous system between vertebrates and lower order animals?
1) Sensory and motor nerves (in something even as simple as hydra)
2) Specialised set of neurons around the mouth and pharynx, which overtime has progressed to be a part of the ventral forebrain
What is the most ancient part of the CNS and where is it found?
The hypothalamus
Found in very close proximity to the pharynx
What does the hypothalamus do?
Coordinates activities and behaviours in the ABSENCE of conscious higher order mechanisms (autonomous manner)
What are the key differences between the development of the nervous system between vertebrates and lower order animals?
1) Neurogenic region doesn’t delaminate - maintains cell-cell contacts and increased cohesion with neighbours
2) Neural cells stay as a layer called the NEUROEPITHELIUM (or neural plate)
3) Ectodermal layers are brought together and fuse over the neural tube
What is the neuroepithelium (neural plate)
A flat, single-cell layered plate-like structure of neural cells
Which organism allowed us to understand how the nervous system was set aside?
Xenopus
What do all vertebrates share?
A common body plan
When does the neural plate appear in development?
2 weeks post fertilisation
What can be identified at 4.5 weeks post fertilisation?
Midbrain, forebrain, hindbrain and spinal cord
How does the neural tube develop?
Neural plate gets longer and then rolls up and elongates along the AP axis
