 Consists of circuits of neurons and supporting cells › A Neuron is 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

 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. Science/GeneralBiology/Physiology/NervousSystem/ NervousSystems/nervsys_1.gif

 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 ology/20-1.jpg

 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

ology/20-1.jpg neralBiology/Physiology/NervousSystem/NervousSystems/nervsys _1.gif /en/e/e4/Horse_nervous_system_labell ed.JPG jpg

 Three stages of processing of information by the nervous systems: sensory input, integration, and motor output › Sensory neurons transmit 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 mNQdLkkJHM&feature=related

 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 cture/LecPhysio/48_05NeuronStructure_L.jpg

 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 cture/LecPhysio/48_05NeuronStructure_L.jpg

 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 /150/neuro/c astrocytes.jpg

 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 /R2aQJvKZSXI/AAAAAAAAADc/2Iiso91 w2r0/s400/motor%2Bneruone.bmp

 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.

 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)

 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 (E ion), and is given by the Nernst equation › E ion = 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.

 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

 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 & Allot book pg. 53  Youtube video: