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neurons
specialized cells of the nervous system that receive, encode and transmit information
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transduced
conversion of information from sensory cells into electrical signals
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afferent neurons
carry information into the system (to CNS)
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sensory neurons
convert input into action potentials
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efferent neurons
carry commands to effectors, like muscle cells (away from CNS)
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interneurons
store information and help with communication between neurons in the system
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nerve net
a simple network of neurons that does little more than provide direct lines of communication
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ganglia
clusters of neurons
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brain
one pair of ganglia that is larger and more central
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nerve impulses/action potentials
electrical signals generated by neurons used to conduct signals from one site down the axon to a synapse with another cell
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axon terminal
the fine nerve endings at the tip of each nerve
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Schwann cells
type of glial cells that wrap around the axons of neurons in the peripheral nervous system providing electrical insulation (myelin)
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Oligodendrocytes
similar funcions as Schwann cells
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Astrocytes
glial cells which contribute to the blood brain barrier, permeable to fat-soluble molecules (like EtOH) but not to most polar solutes
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neural networks
must contain at least afferent neuron to efferent neuron to effector
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membrane potential
voltage across a membrane
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resting potential
when the cell is at rest the interior of an electrically excitable cell is negative relative to the exterior with a potential (V) of ~-50 to -80mV
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action potential
reversal of the resting potential is periodically observed. These APs carry information in the nervous system
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Which ion is primarily responsible for resting potential
K+
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Nernst Equation
the equation used to measure a single potential (V) across an electrical gradient
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Goldman-Field equation
Takes into account multiple ion concentrations when calculating the potential difference across a membrane
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Where are Na+ Cl- and K+ hi?
- Na+ and Cl- are high in the extracellular fluid
- K+ is high in the cytoplasm
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How do electrical charges move across cell membranes?
Through charged ions, minaly through channels (mainly K, Na, Cl, Ca)
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threshold potential
the specific voltage at which voltage-gated ion channels are opened
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How does depolarization occur
- A few Na+ channels initially open allowsing some inward Na+ flux (Na+ current)
- If threshold is reached, many V-gates Na+ channels quickly change conformation and ope, permitting a large Na+ current (inward) which depolarized the cell
- *Note that these Na+ channels are "double gated"
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How does repolariztion occur?
At threshold, V-gated K+ channels slowly change conformation and open, permitting outward K+ current, which repolarized the cell
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refractory period
- the amount of time it takes for a neuron to rearm itself
- sets the upper limit on the AP frequency
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"All or None"
sub-threshold stimuli do not produce an action potential, and the small depolariztion is not propagted. Once threshold is reached, AP is made
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Two ways to increase speed of conduction
- incrase the diamter of the axon (invertebrates)
- myelination of the axons(veterbrates)
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nodes of Ranvier
- gaps in myelin that allow for electric signals to be conducted more quickly using saltaory conduction
- ion channels are clustered at these
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Multiple schlerosis
results from dmage to myelin in CNS
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synaptic cleft
the narrow space between the pre-and the post-synaptic cells
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What determines the amount of neurotransmitter relased
- the ammount of Ca++ enter the presynaptic cell
- (Recall the Ca++ enters the cell at the synaptic cleft, NOT Na+)
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How can neurotransmitter actions be terminated
- Enzymes may destroy the neurotransmitter
- Neurotransmitter may idffuse away from synaptic cleft
- Neurotransmitter may be taken back via active transport into the pre-synaptic cell for repackaging
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Fast (ionotropic) chemcial sunapses
Post-synaptic NT receptor also functions as a ligand-gated channel. The nicotinic acetylcholine receptor is an ionotropic recept
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Slow (metabotropic) Chemical synapses
Receptor (often G-protein coupled) stimulated a second messenger cascase which then affects ligand-gated channels
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If speed is so important in the nervous system, why have slow type synapses?
Improved integration, longer lasting
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What are the two types of receptors for acetylcholine and what is the difference between them?
- Nicotinic receptors - fast, tend to be excitatory in the CNS
- muscarinic receptors - slow, tend to be inhibitory in the CNS
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GABA (gamma-amino butyric acid)/glycine
- The most common inhibitory neurotransimitters in vertebrates
- The postsynaptic cells at these inhibitory synapses have ligand gated chloride channels
- when the channels are activated, they can hyperpolarize the postsynaptic membrane and make the postsynaptic cell less likely to fire an cation potential
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Excitatory Postsynaptic potential (EPSP)
depolarize the postsynaptic membrane
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inhibitory postsynaptic potential (IPSP)
hyperpolarize the postsynaptic membrane
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Summation
It takes several EPSP to generate an AP
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Electrical synapses
- Little delay at synapse
- Current passes between the electrically souples cells, in either direction, usually, with litlte, if any, attenation of current
- Not common because they cannot be inhibitted, little integration, not modifiable, large area required, do not allow for temporal summation
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Gap junctions
coupling of neurons electrically using membrane proteins called connexons
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Neural plasticity
the modification of neural fucntion resulting from experience
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synaptic plasticity
- can result from changes in the synaptic efficieny, up- or down-regulation of the ability ot elicit an EPSP (and therefore APs) resulting from history of the synapse
- Can be a form of learning
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