Nervous Systems Neuron… basic unit – Human brain contains roughly interconnected neurons that communicate with each other to store memories and effect thought

Complexity of Nervous Systems The complexity of the organism is directly related to the complexity of the nervous system – Sponge… only animal without a nervous system – Hydra… nerve net (no centralized processing of information) – Flatworms and annelids (cephalization) – The size of the brain or amount of cephalization increases with increasing complexity

Organization of the Nervous System Central Nervous System (CNS)… brain and spinal cord – Peripheral Nervous System (PNS)… nerves and ganglia Sensory neurons… bring information form the sensor to the CNS Interneurons… transmit information from CNS to the motor neurons – Analyze and interpret based on current context and past experience Motor neurons… send information out from the CNS to the effector Reflex arcs can bypass the input of interneurons

Neuron Structure Cell body… contains the nucleus Dendrite… receives signals from the effector (sensor) or other neurons Axon… moves the nerve impulse away from the nerve body – Many wrapped in a myelin sheath Insulates the axon and increases conduction speed Synaptic Terminal… site of communication – Presynaptic to postsynaptic cell through neurotransmitters Interneurons generally have far more synapses than motor neurons

Supporting Cells Glia… supporting cells that provide structural and nutritional support. – Astrocytes… structure and regulation of ions Cause vessel dilatation in areas of active neurons facilitating oxygen and glucose transport – Radial Glia… active in embryo formation in the development of the CNS Act as stem cells along with astrocytes during formation – Oligodendrites & Schwann Cells… form the myelin sheath Primarily of lipid material that is non- polar and provides insulation during conduction of the nerve impulse

Membrane Potential Resting membrane potential is established across the plasma membrane through maintenance of ion gradients – -60 to -80mV in most mammals – Na +, K +, & Cl - are the primary ions with some charged amino acids in the cytosol and Ca 2+ contributing to potential Nernst Equation – E ion = 62mV(log ([ion] outside/[ion]inside) Maintained through ion channels by the Na/K pumps

Gated Ion Channels Stretch-gated channels – Open when a membrane is deformed… signal after being touched Ligand-gated channels – Open when a specific chemical is encountered… often found near synapses Voltage-gated channels – Found on axons and dendrites and are used to transmit a nerve impulse Causes either hyperpolarization, depolarization, or reestablishment of resting potential

Action Potentials An action potential is an all-or- nothing response to depolarization once a threshold has been met – The threshold is a voltage created by a depolarization event strong enough to start an action potential – The action potential is graded… the amount of action potential is determined by the strength of depolarization or hyperpolarizing stimulus – Once started the action potential are carried along the length of axons and dendrites Propagated by voltage-gated channels

Conduction of Action Potentials Na+ influx after stimulation depolarizes cell – Creates a stimulus for neighboring cell once threshold has been reached – K+ ion gates opens to repolarize membrane Prevents action potential from traveling backward Speed is increased through saltatory conduction across the nodes of Ranvier – Impulse jumps to the gaps between Schwann cells Transmission from toes to spinal cord ~1-2ms Extracellular fluid is only in contact at the nodes