Evolution of the nervous system

when did the nervous system first develop

650 million years ago

what animals do not have a nervous system

placozoa and sponges

first nervous systems

CNIDARIANS

Cnidarians

includes hydra, sea anemones, corals and jelly fish

• with some organisation in jelly fish

simple never nets in hydra

starfish

start to see more organisation in nervous system

nerve ring, surrounding oesophagus, with a radial never in each arm

evidence of sensory and motor neurones

can detect food and move towards it

no clustering of neurones

Bilaterians

Have bilateral symmetry- head, tail, back and belly

bilateral symmetry in nervous system

head with specialised functions- resulted in clustering of neurons at this end

Cephalisation

results in brain

simplest forms

• flatworms and roundworms

bilateral symmetry

a body plan in which an organism or object can be divided into identical left and right halves along a single central axis, creating mirror images.

cephalisation

the concentration of sense organs, nervous control, etc., at the anterior end of the body, forming a head and brain, both during evolution and in the course of an embryo's development.

Roundworms

Nematodes

4 nerve cords running length of body

dorsal associated with motor control

2 lateral associated with sensory input

ventral associated with both

nerve ring and ganglia in head surrounding pharynx

sensory nerves extending forwards towards the anterior end

roundworm neurons

Do not produce a voltage-sensitive sodium current

seem to mange with just voltage-gated calcium channels

may pre-date voltage-gated sodium channels

Caenorhabditis elegans

very well characterised system

first animal to have its genome sequenced

adult hermaphrodites and males

all hermaphrodites have 959 cells

have 302 neurons of know lineage and wiring diagram

can be genetically manipulated with relative ease

lots of behavioural mutants

Caenorhabditis elegans map of nervous system

flatworms

nervous system has bilateral symmetry

has a pair of lateral never cords with numerous ganglia connected by transverse nerves

large cerebral ganglia= primitive brain

coincide with eyespots

sensory projections at front

Annelids

segmented worms (earthworms/leaches)

More complex NS than other worms

paired ventral nerve cord with ganglia in every segment

segmental nerves

more active annelids have bristles that require coordination for movement

leeches have anterior and posterior suckers also requiring coordinated control

the more complex the annelids have a simple brain

• large cerebral ganglia

• nerve ring (encircling the oesophagus)

Molluscs

very diverse in terms of body plan and types of nervous system design

main groups are bivalves, gastropods and cephalopods

molluscs- cephalopods

marked step in organisation and cephalisation

head ganglia are fused to form a multi lobed brain

well developed eyes- important for communication

ganglia

a structure containing a number of nerve cell bodies, typically linked by synapses, and often forming a swelling on a nerve fibre.

• a network of cells forming a nerve centre in the nervous system of an invertebrate.

• a well-defined mass of grey matter within the central nervous system.

molluscs- octopus

particularly well developed

very good visual function- lots of data to process

octopus can be trained to solve puzzles

Chromatophores

allow cephalopods to change skin colour

cells contain pigment or have light reflecting properties

chromatophores subtypes

•Xanthophores – yellow (pteridines)

•Erythrophores – red (carotenoids)

•Iridophores – reflective/iridescent (guanine)

•Leucophores – white (purines)

•Melanophores – black/brown (eumelanin)

•Cyanophores – blue (?)

chromatophores structure

chromatophore organs, composed of a single chromatophore cell and numerous muscle, never, glial and sheath cells.

chromatophore cell contains pigment granules in an intracellular sac, the cytoelastic sacculus that has elestic walls.

4-24 radically arranged muscle cells, associated with never and glial cells, attach to cell membrane where the latter is anchored to the cytoelastic sacculus around its equator

muscle cells contract stretching the lenticular sacculus into a thin, flat disk with serrated edges.

The sacculus diameter expands up to 7 times, increase of area by bout 50 times. Chromatophores retract due to the elastic nature of the sacculus walls. Primary infoldings and pouches of the chromatophore appear in its upper and lower surfaces during chromatophore retraction and disappear during chromatophore expansion.

folding’s are anchored to sacculus at various points on its surface.

The structureless sheath cells enable slippage of the chromatophore organs within the dermis of the skin

Arthropods

A huge group of animals diverse body plans and nervous system

insects, crustaceans, arachnids, myriapods

tend to have a well defined head with eyes and cephalisation- they have a basic brain

have thorax and abdomen

more complex behaviours- including social interactions

Arthropods- structure

Have ventral nerve cord with paired ganglia supporting each body segment

• like annelids pairing isn’t always obvious due to lateral fusion

sometimes the segmental arrangement is also lost due to antero-posterior fusion of ganglia

• fly and spider

Vertebrates

pronounced cephalisation

• distinct brain

complex spinal cord acting as a wiring interface between CNS and PNS

CNS is encased in bone or cartilage- skull and vertebrae

first appeared in fish about 520 million years ago

vertebrates summary

complex spinal cord acting as a wiring interface between CNS and PNS

Vertebrates peripheral NS

autonomic vs somatic system

autonomic split into sympathetic and parasympathetic

vertebrate evolution

brain becoming enlarged

most obvious cerebrum

handles much of brains conscious actions

• senses

• language

• working memory

• personality

• movement

• learning, logic and reasoning

Bird skills

mimicry

use tools

weave

Mammals

extensive cephalisation

lots of cool behaviour

especially primates

learning/training

reasoning

strategy, e.g. hunting

emotions

curiosty

play

Encephalisation quotient

a comparison of the ratio of brain weight to body weight against a typical animal of that group

Encephalisation quotient calculation

C= E/S^r

C= EQ

E= brain weight

S= body weight

r= exponential constant