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Nervous System FUNCTION: Senses, processes, interprets, and determines the response to stimuli from the environment • Central Nervous System (CNS) - made of the brain and spinal chord • Peripheral Nervous System (PNS) - all nerve cells outside of the CNS Divisions of the Nervous System (see pages 388-89 for more detail on each) Sensory division receives Information (senses Motor division - relays information to organs/glands QuickTime™ and a TIFF (Uncompressed) decompressor involuntary are needed to see this picture. “Fight or flight” “rest & digest” voluntary Cells of the Nervous System • Neurons - cells that conduct nerve impulses (action potentials) to communicate with organs and glands • Neuroglia (glial cells) support, protect and nourish neurons (do not send nerve impulses QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Structure of a Neuron • Axon - transmits nerve impulses to • • • • • communicate with other cells and organs Dendrites - receive signals from other neurons Myelin sheath - fatty coating on axon that speeds up action potential Nodes of Ranvier - gaps in the myelin sheath where the axon is exposed Cell body - part of neuron from which dendrites arise (also contains nucleus of cell) Axon terminals - end of axon/part that releases neurotransmitters to communicate with other neurons QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Functional Regions of the Neuron QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Receptive zone -Receives input from other neurons Conducting zone -generates action potential (nerve impulse) Secretory zone -Releases neurotransmitters Functional Classification of Neurons Sensory (afferent) neurons • Receive information from the environment (senses) Motor (efferent) neurons • Send signals to muscles/glands/organs to carry out response Interneurons • Relay signals between sensory and motor neurons QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Nerves vs. Neurons Nerves are bundles of neurons QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Synapse - area where two or more neurons communicate with each other QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Action Potentials • A brief reversal in charge • • across a membrane Happens in the axon membrane at Nodes of Ranvier (Saltatory conduction) Voltage gated ion channels for Na+ and K+ open and close in response to changes in membrane potential (charge) Action potential is initiated by the axon hillock QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Stages of an Action Potential 1) More Na+ outside cell/more 2) 3) 4) 5) K+ inside Na+ enters the axon (charge becomes more positive depolarization) K+ leaves axon (charge becomes more negative) Too much K+ has left (hyperpolarization - more negative than resting) Na/K pump restores original conditions QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. View action potential stages in action Neurotransmitters • Chemical messengers that cross the synapse allowing one neuron to communicate with another • Can be excitatory (cause postsynaptic neuron to depolarize (become more positive)) • Can be inhibitory (cause post synaptic membrane to hyperpolarize (become more negative)) Neurotransmitters (cont.) • Stored in vesicles in axon terminals • Ca2+ rushes into the terminal in response to arriving • • • action potentials Ca2+ causes vesicles to release neurotransmitters into synaptic cleft (example) Neurotransmitters bind to their receptors on the postsynaptic membrane Neurotransmitters are broken down by enzymes in the synaptic cleft or are taken back up by the pre-synaptic neuron via transporter proteins Neurotransmitters (cont) Examples: • GABA (inhibitory) • Glutamate (excitatory) • Dopamine, serotonin, norepinephrine (excitatory or inhibitory depending on the nature of the synapse) • Over 50 identified IPSP vs. EPSP • Inhibitory post-synaptic potentials (IPSP) decrease the likelihood of the post-synaptic neuron sending an action potential (hyperpolarizes post-synaptic neuron: lets Cl- in or lets K+ out) • Excitatory post-synaptic potentials (EPSP) increase the likelihood of the post-synaptic neuron sending an action potential (depolarizes post-synaptic neuron: lets Na+ in) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Summation • Additive effect of the inputs of all pre-synaptic neurons • If there are more excitatory than inhibitory signals, then depolarization may occur and an action potential may be sent by the post-synaptic neuron QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Long-term Potentiation (LTP) • LTP is the long-lasting strengthening of synapses • • between two neurons Post-synaptic neurons become more sensitive to neurotransmitters coming from the pre-synaptic neuron(s) by: 1) making more receptors 2) increased sensitivity of existing receptors Involved in learning and memory formation QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. BEFORE: Glutamate (excitatory neurotransmitter) stimulates NMDA receptors (in green) at a high frequency AFTER: Because of the frequency of stimulation, there is an increase in the number and sensitivity of receptors on the post-synaptic neuron (increasing the strength of the synapse) What causes this to happen?