chapter4 - IPHY 3430 Human Physiology Fall 2007 REVIEW...

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IPHY 3430 – Human Physiology Fall 2007 REVIEW TOPICS & STUDY QUESTIONS – Ch. 4: Principles of Neural and Hormonal Communication Lecture Summary 1. In response to various stimuli, electrical signals are generated in neurons. Graded potentials are relatively small changes in membrane potential that vary in amplitude and duration, depending on the quality of the stimulus. Action potentials are “all-or-none” responses of fixed amplitude and duration for a given neuron that occur when the neuronal membrane is depolarized to threshold. 2. Incoming stimuli are transduced to electrical signals (graded potentials) at the dendrites. The stimulus determines which ion channels open and, thus whether the potential has an excitatory (depolarizing; EPSP) or inhibitory (hyperpolarizing; IPSP) effect. Graded potentials spread passively away from the stimulus origin toward the axon hillock. Temporal and spatial summation of both excitatory and inhibitory potentials occurs at the axon hillock. If the summed potential (grand postsynaptic potential) reaches the neuron’s threshold potential, an action potential will occur. The amplitude and duration of graded potentials encodes stimulus strength. 3. In vivo, action potentials are initiated at the axon hillock. The impulse travels away from the hillock and is actively regenerated along the axon. Active conduction of the action potential prevents decrement of the signal as it travels toward the terminal. In unmyelinated axons, the signal is propagated along the entire length of the axon. Myelination increases conduction velocity along axons because the action potential travels via saltatory conduction and active propagation only occurs at the Nodes of Ranvier. Current flow between Nodes is passive. When the action potential arrives at the nerve terminal, depolarization causes voltage-gated Ca 2+ channels to open. Ca 2+ influx promotes mobilization and docking of synaptic vesicles at the presynaptic terminal membrane and exocytosis of neurotransmitter into the synapse. Peptide neurotransmitters are hydrophilic chemical messenger that act through receptors on the postsynaptic membrane to produce excitatory (EPSP) or inhibitory (IPSP) responses in the postsynaptic neuron. Neurotransmitter action is short-lived as the chemicals are rapidly cleared from the synapse. Integration of EPSPs and IPSPs determines the probability that the postsynaptic cell will fire. 4. Information passes between a neuron and its target cell via a synapse. There are two types of synapses: electrical and chemical. The latter is more common and involves chemical messengers. Endogenous neuromodulators, drugs and other exogenous chemicals, and multi-neuronal connections affect transmission across chemical synapses. 5. The endocrine system, like the nervous system, functions to maintain homeostasis.
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This note was uploaded on 04/14/2008 for the course IPHY 3430 taught by Professor Lynch,robe during the Fall '08 term at Colorado.

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chapter4 - IPHY 3430 Human Physiology Fall 2007 REVIEW...

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