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NEUROPHYSIOLOGY OBJECTIVES: Describe resting membrane potential. Explain how action potentials are generated and propagated along nerves. Explain how an impulse is transmitted from one nerve to another. Clinical Application: Influence of various drugs on nerve conduction. Review of Neuron Structure ACTION POTENTIAL (Nerve Impulse) When a neuron is adequately stimulated, an electrical impulse is conducted along the length of its axon. The response is always the same, regardless of the source or type of stimulus. Analogy: Nerves serve as roadways for electrochemical signals, and myelin as their pavement. paved road vs gravel road REST POTENTIAL: The Polarized State The surface of a cell membrane is usually electrically charged or polarized with respect to the inside This is due to an unequal distribution of + and – ions between sides of the membrane At Rest Potential: More sodium is outside the cell than inside More potassium is inside the cell than outside Simple diffusion: K+ moves out Na+ moves in Every millisecond, more + charges leave the cell by diffusion than enter it. So the outside gains a slight + charge and the inside a negative charge The voltage gated Na+ and K+ channels are closed Role of Na+/K+ Pump during Rest Potential At the same time the Na+/K+ pump uses ATP to transport Na+ and K+ in opposite directions to maintain this difference It ejects 3 Na+ out and only 2 K+ in This stabilizes rest potential Generation of an Action Potential Potential Change: Typically an environmental change (temperature, light, pressure, chemicals, etc..) affects the resting membrane potential. Depolarization occurs and is followed by repolarization. 1. Na channels open (-55mV)-Na diffuses indepolarizing the membrane (close at +30mV) 2. K channels open and Na channels close (+30mV)K diffuses out- repolarizing the membrane 3. Na+/K+ pump “revs up” to restore rest potential Anatomy & Physiology Revealed Nervous System Topic: physiology Animation: Action Potential Propagation Nervous System Topic: physiology Animation: Action Potential Generation Impulse Conduction Synaptic Potentials PRESYNAPTIC NEURON POST SYNAPTIC NEURON Synaptic Transmission 1. Action potential reaches the synaptic knob 2. Knob membrane becomes permeable to Ca++ and it diffuses inward 3. Synaptic vesicles fuse with the knob membrane 4. Vesicles release neurotransmitters into the synaptic cleft 5. Neurotransmitters bind to receptors on the postsynaptic membrane causing the channels to open and allow sodium to leak in-thus setting up the action potential. Anatomy Revealed Nervous System Topic: Physiology Animation: Chemical Synapse After release: Neurotransmitters 1. are decomposed by enzymes in the synaptic cleft or 2. reuptake by the synaptic knob that released it (or by nearby neurons) This action prevents continuous stimulation of the postsynaptic neuron. Effect of Drugs on Synaptic Transmission 1. can alter re-uptake Ex: Cocaine & amphetamines When present in the brain they block the ability of the presynaptic neuron to transport the excess dopamine neurotransmitter from the synaptic cleft. The dopamine builds up and continues to excite the postsynaptic neuron. The result leads to feeling of euphoria http://science.education.nih.gov/supplements/nih2/addic tion/activities/lesson3_cocaine.htm 2. They can increase the amount of neurotransmitter that is released by binding to the presynaptic neuron. Ex: heroin & nicotine excite the dopamine containing neurons in a particular area of the brain. They produce more action potentials 3. Can slow down the release of a neurotransmitter Ex. Alcohol Slows down the release of acetylcholine which is the neurotransmitter released at the neuromuscular junction. Speech becomes slurred, reaction time slows-muscles cannot fire as quickly. 4. Some drugs decrease membrane permeability to sodium Ex. Local anesthetic drugs (like dentist uses) It interrupts impulses from passing through the affected region and reaching the brain, preventing sensations of touch and pain. 5. Some drugs can increase the effectiveness of the sodium potassium pump. Ex. During epileptic seizures neurotransmitters become exhausted Dilantin is a drug that causes more sodium ions out of neurons to stabilize membrane thresholds against too rapid stimulation 6. Some drugs mimic neurotransmitters. Ex. Morphine mimics the brains endorphins and enkephalins which are neurotransmitters that inhibit transmission (pain relievers).