Evolution Of The Nervous System
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Evolution Of The Nervous System

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Evolution Of The Nervous System Evolution Of The Nervous System Presentation Transcript

  • 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.
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