Evolution Of The Nervous System

Evolution of the Nervous System

Unifying 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 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 Centralization
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 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 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 Systems
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

Worms
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

Mollusks
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

Insects
Increased 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 Insects
Insects 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 Systems
Provide unique opportunities for study of nerve cells
Small nervous systems
approximately 1000 neurons (10 7 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 An 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 Axons
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 Research
1952 –
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 Systems
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 Brain
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 Brains Fish Amphibian Reptile Mammals

Three Trends in Brain Evolution
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

Convolutions
Convolutions increase surface area
Surface area is more important than volume in determining complexity because cell bodies are in the cortex
Greatest in primates & cetaceans (whales & porpoises)

Convolutions in Mammals

Increase in Relative Brain Size

Mammals
Brain keeps its three major components
Two 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 Brainstem
Present in all mammalian brains
Oldest part of brain
Evolved ~ 500 million yrs ago
Called “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 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 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 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.

Plasticity
Increase in plasticity
The brain's ability to modify itself as a result of experience
Makes memory and the learning of new perceptual and motor abilities possible.

Evolution Of The Nervous System

by Edward Tsien

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