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Neurons Dendrite branching fibers their surface is lined with synaptic receptors responsible for bringing information into the neuron some contain dendritic spines that further branch to increase the surface area because the greater the surface area the more information it can receive Soma cell body contains the nucleus mitochondria ribosomes and other cell structure it is responsible for metabolic work Axon thin fiber of a neuron responsible for transmitting nerve impulses toward other neurons organs or muscles some are insulated with myelin sheath o Afferent axon bringing information into the structure o Efferent axon carrying information from the structure Presynaptic axon terminals end points of an axon where chemicals are released to communicate with other neurons Neurons vary in size shape and function the shape determines its connection with other neurons and contribution to the nervous system the function is closely related to its shape Interneuons intrinsic neurons bipolar dendrites and axons are completely contained within a single structure Motoneuron multipolar the soma is in the spinal cord Sensory neurons unipolar specialized at one end to be highly sensitive to a particular type of stimulation and its soma is in the middle Pyrimidal cell widely branching dendrites of some cerebral cells that enable up to 20 000 inputs opposed to retnal cells that only have a few inputs Signaling Dendrites receive the signal the current travels to the soma triggers signal at the base of the axon travels down the axon to axon terminal releases neurotransmitters into the synaptic cleft REPEAT Astrocytes help to synchronize the activity of the axon by wrapping around the presynaptic terminals and taking up chemicals released by the axon o Branches of the astrocyte in the center surround the presynaptic terminals of related axons o If a few of them are active at once the astrocytes absorb some chemicals released o It temporally inhibits all the axons to which it is connected o When inhibition ceases all of the axons are primed to respond again in synchrony Neuro glia make the brain function at a microscopic level mircoglia remove waste material and other microorganisms that could prove harmful to the neuron Oligodendrocytes CNS and Schwann cells PNS build the myelin sheath that surrounds and insulates certain vertebrate axons Radial glia guide the migration of neurons and the growth of their axons and dendrites during embryonic development Neurons communicate by a combination of electrical and chemical signals electricity carries signals quickly down a long axon to the synapse chemical transmission are at the synapse to the next neuron Nerve impulses are like dominos that relay messages from one neuron to the next The brain is not set up to register small differences in the time of arrival of touch messages but it does matter with audition At resting potential the membrane maintains an electrical gradient by being selectively permeable where the inside is more negative and the outside is positive stable until stimulated Polarization is the difference in the electrical charges inside and outside of the cell The membrane has ion channels to modify the flow of Na and K in and out of the cell at rest the K channel is partially open For every 2 K ions in the cell there are 3 Na ions transported out of the cell For active transport sodium is 10 times greater outside of the cell than in and potassium is greater in the cell Na follows the electrical and concentration gradient to move into the cell where K s electrical gradient puts it into the cell and the concentration gradient forces it out Neurons fire when neurotransmitters bind to their receptors and the cell is depolarized using electricity In neurons during depolarization Na channels open sodium rushes in and potassium leaves so there is more sodium and less potassium in the cell During the absolute refractory period sodium channels close and the cell with not fie again for 1ms Action Potential Loewi 1920 Synapse During the relative refractory period it requires more than normal stimulation to fire the cell for 2 4ms because it is an all or nothing response All or nothing is the amplitude and intensity of an action potential is independent of the stimulus that initiated it Threshold of excitement is the level above which any stimulation produces a massive depolarization Action potentials can be altered by blocking ion channels and changing the threshold like anesthetics blocking sodium channels Hatian zombies given tetrodotoxin a Na blocker buried dug up given atropine and scopolamine Hyperpolarization is increasing the polarization to become more negative and less positive like alcohol on GABA Myelination Signals in thin axons conduct action potentials at 1m s and thick axons can increase to 10m s Myelin sheath and saltatory conduction is only in vertebrates Myelin is a fat and protein layer In multiple sclerosis nerves lost myelination so there are no ion channels left and potentials die out between the nodes Sherrington 1906 Reflex arc he cut connections to the brain and the action is faster and more reliable but it was slower than he expected The reflex arc was never faster than 15 m s and myelinated axons can move 40m s He postulated that the slowing had to be happening in between neurons at the synapse He believed that cells were communicating to make movement happen the presynaptic cell excites the postsynaptic cell Temporal summation several impulses from one neuron over time Spatial summation impulses from several neurons at the same time Elliot 1905 chemical communication in the nervous system If you stimulate the vagus nerve you can decrease a frogs heartbeat transfer fluid from a frog with a stimulated vagus to a new from then the new frog will also have a decreased heartbeat 1 The neuron synthesizes chemicals that serve as neurotransmitters Amino acids glutamate GABA glycine aspartate Monoamines serotonin dopamine norepinephrine epinephrine Gasses nitric oxide Neuropeptides endorphins NPY substance P small peptides are synthesized in the axon large ones in Acetylcholine acetylcholine Purines ATP adenosine the cell body they are all stored in small vesicles and hanging out in the presynaptic terminal 2 Action potentials travel down the axon Calcium ion channel opens in the presynaptic terminal calcium enters and causes exocytosis the release of neurotransmitters 3 Released molecules diffuse across the cleft attach to


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FSU PSB 2000 - Neurons

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