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Neurons are
- - electrically active
- - chemically active
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Neurons transduce (convert one type f a signal into another, one type of energy into another) information
- - sensory information
- - chemical/biochemical signals form other neurons
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Neurons also transmit information
- - neurons are electrically active
- - electrical impulses are generated in response to stimulation: firing an action potential
- - chemical signals are sent to other neurons
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Resting Membrane Potential
- at rest, inside of the nerve cell membrane is about
- - 70 mV
- relative to the outside
- - stores charge like a capacitor
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Important Ions For Electrophysiology of Neurons
- Anions
- Cations
- Na+
- K+
- CL-
- Ca2+
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Forces that move Ions
- 1. Movement along a chemical gradient (from area of high concentration to area of low concentration)
- 2. Movement along an electrical gradient (feels like a charge repel, opposite charges attract)
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Membrane-Bound Proteins
- Channels - pores that are open or close, allowing ions to pass through
- Receptors - detect the presence of the neurotransmitters; transmitter binds to their receptors
- Enzymes - biological catalysts that promote chemical reactions; they synthesize transmitters and intracellular signals as "second messangers"
- Transport mechanisms - pump substances across the membrane; e.g. "Na pump" or "Na-K pump"
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Resting Membrane Potential
- Why is it negative?
- - Something positive(Na+) is actively pumped out
- - the positive Na+ ions cannot get back in because Na+ channels are closed
- - this sets up an electromotive force for sodium; i.e. a 'sodium-powered batter' or capacitor
- How does the neuron become excited?
- - anything that makes membrane more permeable to Na+ will 'discharge' the sodium powered battery
- - excitation results from increase in Na+ influx into cell
- - inside of cell moves in positive direction
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Electrical Activity of Neurons
- EPSP - excitatory post synaptic potential; small, transient movement in the positive direction; "depolarization"; can vary in size
- IPSP - inhibitory postsynaptic potential; small, transient movement in the negative direction; "hyperpolarization"; can vary in size
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Action Potential
- very large, rapid change in positive direction, followed by return to baseline; neurons "fire", also known as a "spike"; action potential fires when voltage crossed the threshold
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Propagation of electrical Impulses
- EPSPs and IPSPs are propagated in a graded and decremental fashion
- graded - they vary in size, can be larger or smaller depending upon the size of the inputs
- decremental - they are the largest at the point (i.e. synapse) of initials stimulation, and decrease in size as they move away from that point
- Ex: EPSPs and IPSPs are like ripples on the surface of a pond
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Propagation of Action Potentials
- Action potentials are not graded or decremental
- Action Potentials...
- - are ALL or NONE (either they fire or don't)
- - remain the same size as they travel down the axon
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Examples of Mechanisms that produce EPSPs and IPSPs
- Excitatory neurotransmitter: GlutamateInhibitory neurotransmitter: GABA
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Chemically Gated Channels
- - linked to receptors
- - detection of transmitter by receptor opens channel
- EPSP: receptors detect glutamate, receptor linked to a positive-ion channel, Na+ goes into cell
- IPSP: receptors detect GABA, receptor linked to a negative-ion channel, Cl- goes into cell
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Mechanisms underlying Action Potential
- - voltage shoots up in a positive direction (ascending limb) and then shoots down in a negative direction (descending limb)
- - these effects are mediated by opening of voltage-gated channels
- -descending limb: voltage gated K+ open, K+ goes out
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Information Processing by Neurons
- Each neuron receives many excitatory and inhibitory inputs
- These inputs are integrated (i.e. summed)
- Over time - temporal summationAnd space - spatial summationIf the integrated excitatory input crosses the threshold at any moment, an action potential is triggered
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Information Processing by Neurons
- EPSPs and IPSPs are graded and decremental.
- Action Potentials are not graded and not decremental.
- Action potentials….
- - are ALL OR NONE (they either fire, or they don’t)
- - remain the same size as they travel down the axon
- If they cannot vary in height to convey information, how can action potentials encode for information?
- - They convey information by their pattern & frequency
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Chemical Transmission
- Neurotransmitter release is triggered by:
- -The action potential, when it reaches the terminal, opens
- - voltage-gated Ca++ channel, and Ca flows into the terminal
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Examples of Neurotransmitters
- Glutamate(Glu)GABADopamine(DA)Serotonin(5-HT)Norepinephrine(NE)
- Acetylcholine(ACH)
Important neuro modulator: Adenosine
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Stages of Chemical Transmission
- Synthsis - transmitter made by enzyme
- Storage - stored in vesicles
- Release - released from terminals
- Post-synaptic Action - moves across aynapse, acts on receptors
- Inactivation - by enzymes or uptake
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Drugs modify the Process of Chemical Transmission in the Nervous System
- - Alter neurotransmitter synthesis
- - block storage of transmitter
- - stimulate or reduce release
- - stimulate or block receptors
- - block inactivation (enzymatic breakdown or uptake) of transmitter
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Antidepressant Drugs
- Prozac
- Zoloft
- Block inactivation of serotonin(5-HT)- -
other antidepressants can have actions on other transmitters(NE)
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Antipsychotic Drugs
- HaldolThorazine
- Clozapine
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Block the receptors for dopamine(DA)
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Antianxiety Drugs
- ValiumLibriumXanax- Facilitate the inhibitory actions of GABA on a type of GABA receptor (GABAa)
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Major Stimulant Drugs
- CocaineMethamphetamineRitalin- Block the inactivation of dopamine (DA), or stimulate release of DA; also act on NE and 5-HT
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Minor Stimulant Drugs
- CaffeineTheophylline (components of tea, coffee, energy drinks)
- - Block adenosine receptors
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