Evolution of the Nervous Evolution of the Nervous SystemSystem

Unifying PrinciplesUnifying Principles

Uniformity in how nerve cells function through out the animal kingdom

Great diversity in how nervous systems are organized

All nervous systems must allow for stimulus and response

The Simplest Nervous SystemsThe Simplest Nervous Systems The simplest nervous systems are found in

some cnidarians – (example: Hydra)

Have a nerve net – A loosely organized system of nerves with no

central control– Most synapses are electrical– Impulses are bi-directional – Stimulation at any point spreads to cause

movement of entire body Many radially symmetric animals such as

ctenophora (jelly fish) and echinoderms (star fish) are similar

First Nervous System First Nervous System CentralizationCentralization

Some cnidarians show the first signs of centralization

Clusters of nerve cells control the ability to perform more complex motor tasks requiring coordination, such as swimming

Centralization in the JellyfishCentralization in the Jellyfish The jellyfish (Medusa) is a cnidarian that

exhibits basic centralization The nervous system forms an

undifferentiated network and serves primarily to coordinate the animal's swimming motions.– Jellyfish's skirt must open and contract

in a coordinated manner for the animal to move through the water.

– Nervous system serves as a simple communications network so all parts of the skirt open and then contract at the same time.

Increasing CephalizationIncreasing Cephalization Bilateral animals tend to be more active

– require sense organs and feeding structures Cepahlization:

– concentration of sensory organs & feeding structures at the head or forward-moving portion of an animal

Enlargement of the anterior ganglia that receive this sensory input and control feeding gave rise to the first brains

An anterior brain connected to a nerve cord is the basic design for all organisms with central nervous systems

Invertebrate Nervous Invertebrate Nervous SystemsSystems

Invertebrates show increasing cepahalization up the evolutionary ladder

Flatworms have diffuse, ladder-like nervous systems

Annelids (segmented worms) & arthropods (insects, crustaceans) have a well defined ventral nerve cord with a brain at the anterior end– May contain ganglia in each segment to control

movement of that segment

WormsWorms The simplest organisms to have a central

nervous system. More complicated nervous system allows

worms to exhibit more complex forms of behavior.

Although there is a separate brain in worms, the brain is not the sole control of action.– even with its brain removed, worms are able to

perform many types of behaviors, including locomotion, mating, burrowing, feeding, and even maze learning

MollusksMollusks

Nervous system complexity correlates with habitat as well as phylogeny

Slow moving mollusks (e.g. clams) have little or no cephalization and simple sense organs– Nervous system is a chain of ganglia circling the

body Cepahalopods

– most sophisticated invertebrate nervous systems– Octopus – large brain & large image forming eyes– Rapid conduction along giant axons

InsectsInsectsIncreased complexity of the brain and

nervous system. Giant fiber systems (also found in worms

and jellyfish) allow rapid conduction of nerve impulses – connect parts of the brain to muscles in legs

or wings. Brain divided into three specialized

segments:– Protocerebrum, deutocerebrum,

tritocerebrum.

Variation & Adaptation in InsectsVariation & Adaptation in InsectsInsects possess a greater variety of

sensory receptors than any other group of organisms, including vertebrates. – sensitive to the odors, sounds, light

patterns, texture, pressure, humidity, temperature, and chemical composition

– concentration of sensory organs on the head provides for rapid communication with the brain located within.

Remarkable variety of behaviors

Study of Invertebrate Study of Invertebrate SystemsSystems Provide unique opportunities for study of nerve cells

Small nervous systems– approximately 1000 neurons (107 fewer than humans)

Large neurons – easy to study electrophysiologically

Identifiable neurons– can be cataloged and recognized from animal to animal

Identifiable circuits– neurons make the same connections with one another

from animal to animal Simple genetics

– small genomes, short life cycles allow genetic manipulation

Squid Giant Axon Squid Giant Axon An Important ExampleAn Important Example

Giant axons in the mantle of the north Atlantic squid, Loligo pealei, first noted by L.W. Williams in 1909

The giant axon is actually a fusion of several hundred smaller axons

The electrical properties of this structure are, however, the same as other neurons.

The accessibility of several centimeters of giant axon up to 1 mm in diameter & its viability for several hours in physiological solution made many neurophysiology experiments possible

Research on Squid Giant Research on Squid Giant AxonsAxons 1936 –

The first measurement of the resting potential in any living cell.

1943 – Goldman equation derived using squid giant

axons. 1945 – First recording of resting potential in a living cell.

– A section of squid giant axon several cms long was dissected free and placed in a physiological solution.

– A minute capillary tube filled with KCl was inserted down the central axis of one end of the axon and the voltage was recorded relative to the bath.

And More ResearchAnd More Research1952 –Hodgkin & Huxley propose

equations to describe currents measured with voltage clamps in squid giant axons.– These equations could account for the

action potential. – This turned out to have very wide

applicability for neurophysiological phenomena in many species

Vertebrate Nervous Vertebrate Nervous SystemsSystems

Simplest vertebrates: – fish, reptiles & amphibians

Brain:– becomes much larger and more complex– composed of a series of swellings of the

anterior end of the spinal cord The spinal cord

– protected by the vertebrae– Serves as a two-way path of

communication – fibers segregated into descending motor

pathways and ascending sensory pathways

Evolution of the Vertebrate Evolution of the Vertebrate BrainBrain

The vertebrate brain began as 3 bulges at the anterior end of the spinal cord:– Prosencephalon (forebrain)– Mesencepahlon (midbrain)– Rhombencephalon (hindbrain)

These are present in all vertebrates In more complex brains, they are

further subdivided for integration of complex tasks

Complex behaviors due to increased brain complexity

Comparing Vertebrate Comparing Vertebrate BrainsBrains

Fish

Amphibian

Reptile

Mammals

Three Trends in Brain Three Trends in Brain EvolutionEvolution

Relative size of the brain increases– Brain size is a constant proportion of body

weight in fishes, amphibians, and reptiles– Increases relative to body size in birds &

mammals Increased compartmentalization of function Increasing complexity of the forebrain

– Transition from water to land of amphibians & reptiles made vision & hearing more important, favoring enlargement of the midbrain & hindbrain

More complex behaviors parallel growth of cerebrum

ConvolutionsConvolutions

Convolutions increase surface areaSurface area is more important

than volume in determining complexity because cell bodies are in the cortex

Greatest in primates & cetaceans (whales & porpoises)

Convolutions in MammalsConvolutions in Mammals

Increase in Relative Brain SizeIncrease in Relative Brain Size

MammalsMammalsBrain keeps its three major componentsTwo new structures:Neocerebellum ("new cerebellum")

added to the cerebellum, at the base of the brain

Neocortex ("new cortex") at the front of the forebrain. – In most mammals, these structures are not

particularly large relative to the brain stem. – In primates they are much larger

The BrainstemThe BrainstemPresent in all mammalian brainsOldest part of brainEvolved ~ 500 million yrs agoCalled “reptilian brain” because it

resembles the entire brain of a reptile

Handles basic functions for survival : breathing, heart rate, etc.

Determines alertness & detects incoming info

Limbic SystemLimbic System

Group of structures located between the brainstem and the cortex

Evolved between 200 & 300 million years ago

Called the “mammalian brain” because it is most highly developed in mammals

The Human BrainThe Human Brain Continues four major evolutionary trends: Increasingly centralized in architecture Trend toward Encephalization (concentration

of neurons at one end of the organism) Size, number, and variety of elements of the

brain increased Increase in plasticity Humans have the largest ratio of brain

weight to body weight of any of earth's creatures.

Centralized ArchitectureCentralized Architecture Evolved from a loose network of nerve

cells (as in the jellyfish) to a spinal column and complex brain with large swellings at the hindbrain and forebrain.

Increasingly hierarchical– Newer additions to the human brain are

involved in control– The initiation of voluntary behavior, the

ability to plan, engage in conscious thought, and use language depend on neocortical structures.

PlasticityPlasticity

Increase in plasticityThe brain's ability to modify itself

as a result of experience Makes memory and the learning of

new perceptual and motor abilities possible.