Nervous Systems

1. Nervous Systems • Neuron… basic unit • Human brain contains roughly 1011 interconnected neurons that communicate with each other to store memories and effect thought

2. 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

3. 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

4. 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

5. 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

6. 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 Ca2+ contributing to potential • Nernst Equation • Eion = 62mV(log ([ion] outside/[ion]inside) • Maintained through ion channels by the Na/K pumps

7. 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

8. 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

9. 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