Evolution Of The Nervous System


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Evolution Of The Nervous System

  1. 1. Evolution of the Nervous System
  2. 2. Unifying Principles <ul><li>Uniformity in how nerve cells function through out the animal kingdom </li></ul><ul><li>Great diversity in how nervous systems are organized </li></ul><ul><li>All nervous systems must allow for stimulus and response </li></ul>
  3. 3. The Simplest Nervous Systems <ul><li>The simplest nervous systems are found in some cnidarians </li></ul><ul><ul><li>(example: Hydra) </li></ul></ul><ul><li>Have a nerve net </li></ul><ul><ul><li>A loosely organized system of nerves with no central control </li></ul></ul><ul><ul><li>Most synapses are electrical </li></ul></ul><ul><ul><li>Impulses are bi-directional </li></ul></ul><ul><ul><li>Stimulation at any point spreads to cause movement of entire body </li></ul></ul><ul><li>Many radially symmetric animals such as ctenophora (jelly fish) and echinoderms (star fish) are similar </li></ul>
  4. 4. First Nervous System Centralization <ul><li>Some cnidarians show the first signs of centralization </li></ul><ul><li>Clusters of nerve cells control the ability to perform more complex motor tasks requiring coordination, such as swimming </li></ul>
  5. 5. Centralization in the Jellyfish <ul><li>The jellyfish ( Medusa ) is a cnidarian that exhibits basic centralization </li></ul><ul><li>The nervous system forms an undifferentiated network and serves primarily to coordinate the animal's swimming motions. </li></ul><ul><ul><li>Jellyfish's skirt must open and contract in a coordinated manner for the animal to move through the water. </li></ul></ul><ul><ul><li>Nervous system serves as a simple communications network so all parts of the skirt open and then contract at the same time. </li></ul></ul>
  6. 6. Increasing Cephalization <ul><li>Bilateral animals tend to be more active </li></ul><ul><ul><li>require sense organs and feeding structures </li></ul></ul><ul><li>Cepahlization : </li></ul><ul><ul><li>concentration of sensory organs & feeding structures at the head or forward-moving portion of an animal </li></ul></ul><ul><li>Enlargement of the anterior ganglia that receive this sensory input and control feeding gave rise to the first brains </li></ul><ul><li>An anterior brain connected to a nerve cord is the basic design for all organisms with central nervous systems </li></ul>
  7. 7. Invertebrate Nervous Systems <ul><li>Invertebrates show increasing cepahalization up the evolutionary ladder </li></ul><ul><li>Flatworms have diffuse, ladder-like nervous systems </li></ul><ul><li>Annelids (segmented worms) & arthropods (insects, crustaceans) have a well defined ventral nerve cord with a brain at the anterior end </li></ul><ul><ul><li>May contain ganglia in each segment to control movement of that segment </li></ul></ul>
  8. 8. Worms <ul><li>The simplest organisms to have a central nervous system. </li></ul><ul><li>More complicated nervous system allows worms to exhibit more complex forms of behavior. </li></ul><ul><li>Although there is a separate brain in worms, the brain is not the sole control of action. </li></ul><ul><ul><li>even with its brain removed, worms are able to perform many types of behaviors, including locomotion, mating, burrowing, feeding, and even maze learning </li></ul></ul>
  9. 9. Mollusks <ul><li>Nervous system complexity correlates with habitat as well as phylogeny </li></ul><ul><li>Slow moving mollusks (e.g. clams) have little or no cephalization and simple sense organs </li></ul><ul><ul><li>Nervous system is a chain of ganglia circling the body </li></ul></ul><ul><li>Cepahalopods </li></ul><ul><ul><li>most sophisticated invertebrate nervous systems </li></ul></ul><ul><ul><li>Octopus – large brain & large image forming eyes </li></ul></ul><ul><ul><li>Rapid conduction along giant axons </li></ul></ul>
  10. 10. Insects <ul><li>Increased complexity of the brain and nervous system. </li></ul><ul><li>Giant fiber systems (also found in worms and jellyfish) allow rapid conduction of nerve impulses </li></ul><ul><ul><li>connect parts of the brain to muscles in legs or wings. </li></ul></ul><ul><li>Brain divided into three specialized segments: </li></ul><ul><ul><li>Protocerebrum, deutocerebrum, tritocerebrum. </li></ul></ul>
  11. 11. Variation & Adaptation in Insects <ul><li>Insects possess a greater variety of sensory receptors than any other group of organisms, including vertebrates. </li></ul><ul><ul><li>sensitive to the odors, sounds, light patterns, texture, pressure, humidity, temperature, and chemical composition </li></ul></ul><ul><ul><li>concentration of sensory organs on the head provides for rapid communication with the brain located within. </li></ul></ul><ul><li>Remarkable variety of behaviors </li></ul>
  12. 12. Study of Invertebrate Systems <ul><li>Provide unique opportunities for study of nerve cells </li></ul><ul><li>Small nervous systems </li></ul><ul><ul><li>approximately 1000 neurons (10 7 fewer than humans) </li></ul></ul><ul><li>Large neurons </li></ul><ul><ul><li>easy to study electrophysiologically </li></ul></ul><ul><li>Identifiable neurons </li></ul><ul><ul><li>can be cataloged and recognized from animal to animal </li></ul></ul><ul><li>Identifiable circuits </li></ul><ul><ul><li>neurons make the same connections with one another from animal to animal </li></ul></ul><ul><li>Simple genetics </li></ul><ul><ul><li>small genomes, short life cycles allow genetic manipulation </li></ul></ul>
  13. 13. Squid Giant Axon An Important Example <ul><li>Giant axons in the mantle of the north Atlantic squid, Loligo pealei , first noted by L.W. Williams in 1909 </li></ul><ul><li>The giant axon is actually a fusion of several hundred smaller axons </li></ul><ul><li>The electrical properties of this structure are, however, the same as other neurons. </li></ul><ul><li>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 </li></ul>
  14. 14. Research on Squid Giant Axons <ul><li>1936 – </li></ul><ul><li>The first measurement of the resting potential in any living cell. </li></ul><ul><li>1943 – </li></ul><ul><li>Goldman equation derived using squid giant axons. </li></ul><ul><li>1945 – </li></ul><ul><li>First recording of resting potential in a living cell. </li></ul><ul><ul><li>A section of squid giant axon several cms long was dissected free and placed in a physiological solution. </li></ul></ul><ul><ul><li>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. </li></ul></ul>
  15. 15. And More Research <ul><li>1952 – </li></ul><ul><li>Hodgkin & Huxley propose equations to describe currents measured with voltage clamps in squid giant axons. </li></ul><ul><ul><li>These equations could account for the action potential. </li></ul></ul><ul><ul><li>This turned out to have very wide applicability for neurophysiological phenomena in many species </li></ul></ul>
  16. 16. Vertebrate Nervous Systems <ul><li>Simplest vertebrates: </li></ul><ul><ul><li>fish, reptiles & amphibians </li></ul></ul><ul><li>Brain: </li></ul><ul><ul><li>becomes much larger and more complex </li></ul></ul><ul><ul><li>composed of a series of swellings of the anterior end of the spinal cord </li></ul></ul><ul><li>The spinal cord </li></ul><ul><ul><li>protected by the vertebrae </li></ul></ul><ul><ul><li>Serves as a two-way path of communication </li></ul></ul><ul><ul><li>fibers segregated into descending motor pathways and ascending sensory pathways </li></ul></ul>
  17. 17. Evolution of the Vertebrate Brain <ul><li>The vertebrate brain began as 3 bulges at the anterior end of the spinal cord: </li></ul><ul><ul><li>Prosencephalon (forebrain) </li></ul></ul><ul><ul><li>Mesencepahlon (midbrain) </li></ul></ul><ul><ul><li>Rhombencephalon (hindbrain) </li></ul></ul><ul><li>These are present in all vertebrates </li></ul><ul><li>In more complex brains, they are further subdivided for integration of complex tasks </li></ul><ul><li>Complex behaviors due to increased brain complexity </li></ul>
  18. 18. Comparing Vertebrate Brains Fish Amphibian Reptile Mammals
  19. 19. Three Trends in Brain Evolution <ul><li>Relative size of the brain increases </li></ul><ul><ul><li>Brain size is a constant proportion of body weight in fishes, amphibians, and reptiles </li></ul></ul><ul><ul><li>Increases relative to body size in birds & mammals </li></ul></ul><ul><li>Increased compartmentalization of function </li></ul><ul><li>Increasing complexity of the forebrain </li></ul><ul><ul><li>Transition from water to land of amphibians & reptiles made vision & hearing more important, favoring enlargement of the midbrain & hindbrain </li></ul></ul><ul><li>More complex behaviors parallel growth of cerebrum </li></ul>
  20. 20. Convolutions <ul><li>Convolutions increase surface area </li></ul><ul><li>Surface area is more important than volume in determining complexity because cell bodies are in the cortex </li></ul><ul><li>Greatest in primates & cetaceans (whales & porpoises) </li></ul>
  21. 21. Convolutions in Mammals
  22. 22. Increase in Relative Brain Size
  23. 23. Mammals <ul><li>Brain keeps its three major components </li></ul><ul><li>Two new structures: </li></ul><ul><li>Neocerebellum (&quot;new cerebellum&quot;) added to the cerebellum, at the base of the brain </li></ul><ul><li>Neocortex (&quot;new cortex&quot;) at the front of the forebrain. </li></ul><ul><ul><li>In most mammals, these structures are not particularly large relative to the brain stem. </li></ul></ul><ul><ul><li>In primates they are much larger </li></ul></ul>
  24. 24. The Brainstem <ul><li>Present in all mammalian brains </li></ul><ul><li>Oldest part of brain </li></ul><ul><li>Evolved ~ 500 million yrs ago </li></ul><ul><li>Called “reptilian brain” because it resembles the entire brain of a reptile </li></ul><ul><li>Handles basic functions for survival : breathing, heart rate, etc. </li></ul><ul><li>Determines alertness & detects incoming info </li></ul>
  25. 25. Limbic System <ul><li>Group of structures located between the brainstem and the cortex </li></ul><ul><li>Evolved between 200 & 300 million years ago </li></ul><ul><li>Called the “mammalian brain” because it is most highly developed in mammals </li></ul>
  26. 26. The Human Brain <ul><li>Continues four major evolutionary trends: </li></ul><ul><li>Increasingly centralized in architecture </li></ul><ul><li>Trend toward Encephalization (concentration of neurons at one end of the organism) </li></ul><ul><li>Size, number, and variety of elements of the brain increased </li></ul><ul><li>Increase in plasticity </li></ul><ul><li>Humans have the largest ratio of brain weight to body weight of any of earth's creatures. </li></ul>
  27. 27. Centralized Architecture <ul><li>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. </li></ul><ul><li>Increasingly hierarchical </li></ul><ul><ul><li>Newer additions to the human brain are involved in control </li></ul></ul><ul><ul><li>The initiation of voluntary behavior, the ability to plan, engage in conscious thought, and use language depend on neocortical structures. </li></ul></ul>
  28. 28. Plasticity <ul><li>Increase in plasticity </li></ul><ul><li>The brain's ability to modify itself as a result of experience </li></ul><ul><li>Makes memory and the learning of new perceptual and motor abilities possible. </li></ul>