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Chapter 48-Overview: Lines of CommunicationThe nervous system functions to:1) Respond rapidly to changes in the environment (inside and outside the body)2) Coordinates body activates3) Store knowledge (memories)Neurons: are nerve cells that transfer information with in the body2 signals to communicate1) Electrical (Long distance)2) Chemical (Short Distance)Ganglia: simple cluster of neuronsBrain: complex organization of neurons-Introduction to information processingNervous systems process information in 3 stages: sensory input, integration, and motor outputSensors detect external stimuli and internal conditions and transmit information along sensory neuronsSensory information is sent to the brain or ganglia, where interneurons integrate the informationMotor output leaves the brain or ganglia via motor neurons, whichtriggers a muscle or gland.Many animals have complex nervous systemsCentral Nervous System (CNS): where integration takes place, thisincludes the brain or spinal cord.Peripheral Nervous System (PNS): Which carries info in and out of CNS, neurons in PNS, when bundled together form nerves.-Neuron Structure and functionMost of neurons organelles are in the cell bodyMost neurons have dendrites, highly branched extension that receive signals from other neurons or sensory cellsThe Axon is typically a much longer-extension that transmits signals to other cells at synapsesThe cone-shaped base of an axon is called the axon-hillockConnects the axon to the soma and is the area where electrical signals called action potentials are storedThe Synaptic (Axon) terminal: of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters.A synapse is a junction between an axon and another cellNeurotransmitters are released from the axon terminal to signal thetarget cell (muscle cells, glands or other neurons)Information is transmitted from a presynaptic cell (neuron) to a postsynaptic cell (neuron, muscle, or gland)Transmission of information is usually in one direction, from the presynaptic cell to the postsynaptic cellMost neurons are nourished or insulated by cells called glia (Glial cells)Glial cells create myelin sheath around axon-Concept 48.2: Ion pumps and ion channels establish the resting potential of a neuronEvery cell has a voltage across the plasma membrane called a membrane potential. Message are transmitted as changes in this membrane potentialResting Potential: membrane of a neuron not sending signals-Formation of a resting potentialAt resting potential K+ is highest in the cell, and Na+ is highest outside the cell.Sodium potassium pumps: Use ATP to maintain K+ and Na+ gradientsThese concentration gradients represent chemical potential energy.Opening of Ion Channels in the plasma membrane converts the chemical potential to electrical potentialA neuron at resting potential contains many open K+ ion channels, and fewer open Na+ ion channels.Resulting build up of negative charge within the neuron is the major source of membrane potentialAnions such as protein contribute to the (-) charge of cells-Concept 48.3: Action potentials are signals conducted by AxonsChanges in membrane potential occur because neurons contain gated ion channels that open or close in response to stimuli1) Ligand-Gated: Responds to chemical stimuli2) Voltage-Gated: Responds to a change in membrane potential3) Mechanically-Gated: Responds to physical stimuli-Hyperpolarization and DepolarizationWhen gated K+ channels open, K+ diffuses out, making the inside of the cell more negativeThis is hyperpolarization, an increase in magnitude of membrane potential, becomes (-)Opening other types of ion channels such as Na+ channels, triggers depolarization, a reduction in the magnitude of the membrane potential (+), Na+ flows in.-Graded Potentials and Action PotentialsGraded Potentials: are changes in polarization where the magnitude of the change varies with the strength of the stimulusMore channels that become open cause a greater change in membrane potentialIf a depolarization shifts the membrane potential sufficiently, it results in amassive change in membrane voltage called an action potentialAction potentials are all-or-none, and transmit over long distancesThey arise because some ion channels are voltage-gated, opening or closing when the membrane potential reaches a certain level.-Generation of an Action PotentialAn action potential as a series of stepsAt resting potential1) Most voltage-gated sodium (Na+) and Potassium (K+) channels are closed When an Action potential is generated2) Voltage-gated Na+ channels open first, sodium flows into the cell3) During the rising (depolarization) phase, the threshold (is crossed and the membrane potential increases as more voltage-gated sodium channels are opening during the depolarization phase.4) During the falling (repolarization) phase voltage-gated sodium channels close, voltage-gated potassium channels open, and potassium flows out of the cell5) During undershoot (Hyperpolarization) membrane permeability to potassium is at first higher than the rest, then voltage-gated potassium channels close and resting potential is restoredA Neuron can produce hundreds of action potentials per second (1-2 msecs each)During refractory period after an action potential a second action potential can not be initiatedDue to temporary inactivation of sodium channels-Conduction of Action PotentialsAt the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membraneAction potentials travel in one direction: toward synaptic (axon) terminalsAction potential can regenerate itself by depolarizing the region of the axon membraneInactivated sodium channels behind the zone of depolarization prevents the action potential from going backwards-Evolutionary Adaption of Axon StructureSpeed of an action potential increases with Axon’s diameterMyelin Sheath speeds up action potential; even through it is made mostly of lipids, which are poor conductors of electricityMade by glia: oligodenrocytes in the CNS and Schwann cells in the PNSAction potentials are formed only at nodes of Ranvier gaps in the myelin sheath where voltage gated sodium channels found,Action potentials in myelinated axons jump


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FSU BSC 2010 - Chapter 48

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