Chapter 48-Overview: Lines of CommunicationThe 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 body2 signals to communicate1) Electrical (Long distance)2) Chemical (Short Distance)Ganglia: simple cluster of neuronsBrain: complex organization of neurons-Introduction to information processingNervous systems process information in 3 stages: sensory input, integration, and motor outputSensors detect external stimuli and internal conditions and transmit information along sensory neuronsSensory information is sent to the brain or ganglia, where interneurons integrate the informationMotor output leaves the brain or ganglia via motor neurons, whichtriggers a muscle or gland.Many animals have complex nervous systemsCentral 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 functionMost of neurons organelles are in the cell bodyMost neurons have dendrites, highly branched extension that receive signals from other neurons or sensory cellsThe Axon is typically a much longer-extension that transmits signals to other cells at synapsesThe cone-shaped base of an axon is called the axon-hillockConnects the axon to the soma and is the area where electrical signals called action potentials are storedThe 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 cellNeurotransmitters 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 cellMost 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 neuronEvery cell has a voltage across the plasma membrane called a membrane potential. Message are transmitted as changes in this membrane potentialResting Potential: membrane of a neuron not sending signals-Formation of a resting potentialAt 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+ gradientsThese concentration gradients represent chemical potential energy.Opening of Ion Channels in the plasma membrane converts the chemical potential to electrical potentialA 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 potentialAnions such as protein contribute to the (-) charge of cells-Concept 48.3: Action potentials are signals conducted by AxonsChanges 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 DepolarizationWhen gated K+ channels open, K+ diffuses out, making the inside of the cell more negativeThis 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 PotentialsGraded Potentials: are changes in polarization where the magnitude of the change varies with the strength of the stimulusMore channels that become open cause a greater change in membrane potentialIf a depolarization shifts the membrane potential sufficiently, it results in amassive change in membrane voltage called an action potentialAction potentials are all-or-none, and transmit over long distancesThey arise because some ion channels are voltage-gated, opening or closing when the membrane potential reaches a certain level.-Generation of an Action PotentialAn action potential as a series of stepsAt 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 restoredA 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 initiatedDue to temporary inactivation of sodium channels-Conduction of Action PotentialsAt the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membraneAction potentials travel in one direction: toward synaptic (axon) terminalsAction potential can regenerate itself by depolarizing the region of the axon membraneInactivated sodium channels behind the zone of depolarization prevents the action potential from going backwards-Evolutionary Adaption of Axon StructureSpeed of an action potential increases with Axon’s diameterMyelin Sheath speeds up action potential; even through it is made mostly of lipids, which are poor conductors of electricityMade by glia: oligodenrocytes in the CNS and Schwann cells in the PNSAction 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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