The Nervous System

1. The Nervous System Dani, Niral, and Sammy

2. Structure of the Nervous System • Consists of circuits of neurons and supporting cells • A Neuronis a nerve cell; the fundamental unit of the nervous system, having structure and properties that allow it to conduct signals by taking advantage of the electrical charge across its cell membrane • In the simplest animals with a nervous system (ex. cnidarians), the neurons controlling the contraction and expansion of their gastrovascular cavity are arranged in diffuse nerve nets

3. Structure (cont'd) • More complex animals have nerve nets as well as nerves,which are bundles of fiber-like extensions of neurons • Ex. Sea stars have a nerve net in each arm, connected by radial nerves to a central nerve ring. This organization is better suited than a diffuse nerve net for • controlling more • complex movements. http://www.cartage.org.lb/en/themes/Sciences/LifeScience/GeneralBiology/Physiology/NervousSystem/NervousSystems/nervsys_1.gif

4. Greater complexity of nervous systems and more complex behavior evolved with cephalization • Clustering of neurons in a brain near the front end in animals with elongated, bilaterally symmetrical bodies • http://img.sparknotes.com/101s/biology/20-1.jpg

5. A small brain and longitudinal nerve cords constitute the simplest clearly defined central nervous system (CNS) • Simplest nervous system exhibited in flatworms, such as the planarian • In more complex invertebrates, behavior is regulated by more complicated brains and ventral nerve cords containing segmentally arranged clusters of neurons called ganglia • Nerves that connect the CNS with the rest of an animal’s body make up the peripheral nervous system (PNS). • Nervous system organization correlates with animal’s lifestyle

6. http://www.saburchill.com/images04/291007005.jpg • http://img.sparknotes.com/101s/biology/20-1.jpg http://upload.wikimedia.org/wikibooks/en/e/e4/Horse_nervous_system_labelled.JPG http://www.cartage.org.lb/en/themes/Sciences/LifeScience/GeneralBiology/Physiology/NervousSystem/NervousSystems/nervsys_1.gif

7. Information Processing • Three stages of processing of information by the nervous systems: sensory input, integration, and motor output • Sensory neuronstransmit info from sensors that detect external stimuli, internal conditions, or muscle tension • This info is sent to the CNS, where interneurons integrate (analyze&interpret) the sensory input • Motor output leaves the CNS via motor neurons, which communicate with the effector cells (muscle cells or endocrine cells) • Example: Reflexes http://www.youtube.com/watch?v=QmNQdLkkJHM&feature=related

8. Neuron Structure • Most of a neuron’s organelles are located in the cell body. • Two types of extensions from cell body: • Dendrites- highly branched extensions that receive signals from other neurons • Axon- a typically much longer extension that transmits signals to other cells • Axon hillcock- conical region of an axon where it joins the cell body, typically the region where the signals that travel down the axon are generated http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecPhysio/48_05NeuronStructure_L.jpg

9. Neuron Structure (cont’d) • Many axons are enclosed by a layer called the myelin sheath • Near its end, an axon usually divides into several branches, each of which ends in a synaptic terminal • Snapse- the site of communication between a synaptic terminal and another cell • Information is passed from the transmitting neuron to the receiving cell by means of chemical messengers called neurotransmitters • The complexity of a neuron’s shape is reflects the number of synapses it has with other neurons http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecPhysio/48_05NeuronStructure_L.jpg

10. Supporting Cells (Glia) • Gila are supporting cells that are essential for the structural integrity of the nervous system and for the normal functioning of the neurons • Astrocytes- provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters • Helps create the blood-brain barrier, which restricts the passage of most substances into the CNS http://porpax.bio.miami.edu/~cmallery/150/neuro/c7.48.7.astrocytes.jpg

11. Supporting Cells (cont’d) • Radial Glia- form tracks along which newly formed neurons migrate from the neural tube (the structure that gives rise to the CNS) • Both radial glia and astrocytes can act as stem cells, generating neurons and other glia • Oligodendrocytes- glia that form the myelin sheaths around the axon in the CNS • Schwann cells- same thing, but in the PNS • Myelin sheath provides • electrical insulation of the axon http://4.bp.blogspot.com/_TFshpEsf4xM/R2aQJvKZSXI/AAAAAAAAADc/2Iiso91w2r0/s400/motor%2Bneruone.bmp

12. Membrane Potential • All cells have an electrical potential difference (voltage) across their plasma membrane. • This voltage is called the membrane potential • In neurons, the membrane potential is typically between -60 and -80 mv (millivolts) when the cell is not transmitting signals. The (-) indicates that the inside of the cell is negative relative to the outside.

13. The Resting Potential • The membrane potential of a neuron that is not transmitting signals is called the resting potential • Resting potential depends on the ionic gradients that exist across the plasma membrane. • Example: In mammals, the extracellular fluid has a sodium ion concentration of 150 mM (millimolar). In the cytosol, the Na concentration is 15 mM. Therefore, the Na concentration gradient is 150/15 = 10. • (Outside concentration/Inside Concentration)

14. When the electrical gradient exactly balances the concentration gradient, an equilibrium is established. • The magnitude of the membrane voltage at equilibrium is called the equilibrium potential (Eion), and is given by the Nernst equation • Eion = 62 mV [log ([ion] outside/ [ion] inside)] • The Nernst equation applies to any membrane that is permeable to a single type of ion. • The resting potential results from the diffusion of K and Na ions channels that are always open.

15. Gated Ion Channels • Neurons also have gated ion channels, which open or close in response to three kinds of stimuli.. • Stretch-gated ion channels- are found in cells that sense stretch and open when the membrane is mechanically deformed • Ligand-gated ion channels are found at synapses and open or close when a specific channel when a specific chemical binds to the channel • Voltage-gated ion channels- are found in axons and open or close when the membrane potential changes • Gated ions are responsible for generating the signals of the nervous system

16. Action Potential • Action Potential- the reversal and restoration across the plasma membrane of a cell, as an electrical inpulse passes along it (depolarization and repolarization). • A stimulus strong enough to produce a depolarization that reaches the threshold triggers the action potential • When an impulse passes along the neuron, sodium and potassium ions diffuse across the membrane through voltage-gated ion channels • The electrical potential is initially reversed and then restored. This is called an action potential. • Look in AP book for detail on pg. 1018 & Allot book pg. 53 • Youtube video: http://www.youtube.com/watch?v=SCasruJT-DU