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Nervous Systems Three Main Functions: 1. Sensory Input 2. Integration 3. Motor Output Two Main Parts of Vertebrate Nervous Systems • Central nervous system (CNS) – brain and spinal cord – integration • Peripheral nervous system (PNS) – network of nerves extending into different parts of the body – carries sensory input to the CNS and motor output away from the CNS Two Cell Types in Nervous Systems • Neurons – Cells that conduct the nerve impulses • Supporting Cells – Neuroglia Figure 48.2x Neurons Three Major Types of Nerve Cells • Sensory neurons – communicate info about the external or internal environment to the CNS • Interneurons – integrate sensory input and motor output – makes synapses only with other neurons • Motor neurons – convey impulses from the CNS to effector cells Supporting Cells - Neuroglia • provide neurons with nutrients, remove wastes Two important types in vertebrates – Oligodendrocytes – myelin sheath in CNS – Schwann cells -myelin sheath in PNS Myelin Sheath Formation Conduction of the Nerve Impulse • Membrane Potential – Voltage measured across a membrane due to differences in electrical charge – Inside of cell is negative wrt outside • Resting potential of neuron = -70 mV Figure 48.6 Measuring membrane potentials Sodium-Potassium Pump Extracellular P Intracellular 1. Carrier in membrane binds 6. Dephosphorylation of protein triggers change to original conformation, with low affinity for K+. K+ diffuses into the cell, and the cycle repeats. + ADP ATP intracellular sodium. 2. ATP phosphorylates protein with bound sodium. K+ Na+ Pi Pi Pi 5. Binding of potassium causes dephosphorylation of protein. Pi 4. This conformation has higher affinity for K+. Extracellular K+ binds to exposed sites. 3. Phosphorylation causes conformational change in protein, reducing its affinity for Na+. The Na+ then diffuses out. Excitable Cells • Neurons & muscle cells • Have gated ion channels that allow cell to change its membrane potential in response to stimuli Gated Ion Channels • Some stimuli open K+ channels – K+ leaves cell – Membrane potential more negative – hyperpolarization • Some stimuli open Na+ channels – Na+ enters cell – Membrane potential less negative – depolarization Gated Ion Channels • Strength of stimuli determines how many ion channels open = graded response Nerve Impulse Transmission Action Potentials • Occur once a threshold of depolarization is reached – -50 to –55 mV • All or none response (not graded) – Magnitude of action potential is independent of strength of depolarizing stimuli • Hyperpolarization makes them less likely 3. Top curve 2. Rising Phase Maximum voltage reached Stimulus causes above threshold voltage Potassium gate opens K+ Na+ 1. Resting Phase Equilibrium between diffusion of K+ out of cell and voltage pulling K+ into cell Voltage-gated potassium channel Membrane potential (mV) Sodium channel activation gate opens Na+ channel inactivation gate closes +50 0 –70 1 3 2 Time (ms) 4. Falling Phase Undershoot occurs as excess potassium diffuses out before potassium channel closes Potassium channel gate closes Potassium gate open Equilibrium restored Potassium channel Voltage-gated sodium channel Sodium channel activation gate closes. Inactivation gate opens. Na+ channel inactivation gate closed Refractory Period • During undershoot the membrane is less likely to depolarize • Keeps the action potential moving in one direction Propagation of Action Potential • Action potential are very localized events • DO NOT travel down membrane • Are generated anew in a sequence along the neuron resting repolarized depolarized + + + + + + + + + – – – – – – – – – + + + + + + + + + – – – – – – – – – – – + + + + + + + + + – – – – – – – Na+ + + + + + + + – – – – – – – – – + + K+ + + – – + + + + + – – + + – – – – – Na+ + + + + + – – + + – – – – – + + – – K+ K+ + + + + – – – + + – – – – + + + – – Na+ + + – – – + + + + – – + + + – – – – K+ K+ + + + + + + + – – – – – – – – – + + Na+ – – + + + + + + + + + – – – – – – – Cytoplasm Cell membrane K+ Saltatory Conduction Transfer of Nerve Impulse to Next Cell • Synapse – the gap between the synaptic terminals of an axon and a target cell Transfer of Nerve Impulse to Next Cell • Electrical synapses – Gap junctions allow ion currents to continue • Chemical synapses – More common – Electrical impulses must be changed to a chemical signal that crosses the synapse Synapses Neurotransmitters Effects of Cocaine Transporter protein Dopamine Cocaine Receptor protein Neurotransmitter Synapse Transporter protein Receptor protein 1. Reuptake of neurotransmitter by transporter at a normal synapse. Drug molecule 2. Drug molecules block transporter and cause overstimulation of the postsynaptic membrane. 3. Neuron adjusts to overstimulation by decreasing the number of receptors. 4. Decreased number of receptors make the synapse less sensitive when the drug is removed. Diversity of Nervous Systems Cnidarian Human Earthworm Central nervous system Peripheral Spinal nerves cord Nerve net Cerebrum Cerebellum Cervical nerves Thoracic nerves Arthropod Lumbar nerves Sacral nerves Echinoderm Radial nerve Nerve ribs Brain Femoral nerve Ventral nerve cords Mollusk Flatworm Nerve cords Giant axon Brain Sciatic nerve Tibial nerve Associative neurons