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Neurons - Coggle Diagram
Neurons
Action Potential
Propagation
Passive
- Very small neurons - enough Na+ from axon hillock -> terminal without AP
- But Na+ pumps remove Na+, Na+ spreads out
Active
- Influx of Na+ from first AP drifts -> triggers new AP
Refractory periods
- Immediately after AP, impossible/difficult to trigger AP
- Prevents Na+ moving backwards, prevents too much Na+ (toxic)
- Absolute: cannot trigger AP - Na+ channels inactivated
- Relative: difficult to trigger AP - hyperpolarisation (need more Na+)
Stages
- Depolarisation: E message from other neurons -> Na+ enters -> reach threshold => Na+ voltage-gated channels open -> Na+ enters -> depolarisation
- Overshoot: above 0
- Repolarisation: Na+ gates inactivated; K+ gates open (around peak) -> K+ out
- Hyperpolarisation: K+ gates stay open, but start to close - back to resting as elec gradient strong -> less K+ leaves
- Resting: All gates closed
All-or-none law: if threshold reached -> AP, regardless of how much Na+ / stimulus intensity
Saltatory conduction
- Myelin & Nodes of Ranvier
- AP only at Nodes of Ranvier -> less AP needed -> faster, saves energy
Other Types
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Electric: no neurotransmitters; direct mvt of ions at gap; fast; always on; used for synchronisation
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Chemical
Events
- Synthesis: neuron makes neurotransmitter (small in terminal, large in body)
- Transportation: stored in vesicles in pre-synaptic terminal
- Release: exocytosis - AP -> Ca+ channels open -> Ca+ enters -> release neurotransmitter from terminal -> diffuses across cleft
- Neurotransmitter binds to receptor in post-synaptic membrane
- E -> AP; I -> stops AP
- Summation: many synapses required to trigger AP
- Separation: from postsynaptic neuron
- Reuptake into presyaptic neuron OR diffuse away OR inactivate
- Presynaptic: autoreceptors respond to transmitter to stop further synthesis & release
- Postsynpatic: release retrograde transmitters (reverse message) to stop further release
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Ionotropic
- Receptor opens channel to let ion through
- Immediate & localised
- -> for fast events (vision, msucles)
- e.g. glutamate, GABA, plus glycine, Ach
Metabotropic
- Initiates sequence of metabolic reactions, by activating 2nd messenger in postsynaptic cell
- Slower, longer-lasting + affects most/all of cell
- -> for taste, smell, pleasure/pain, arousal, attention, emotion
- e.g. dopamine, norepinephrine, serotonin
Drugs
Agonist vs antagonist
- Agonist: block effect of neurotransmitter (high A, low E)
- Antagonist: mimics / increases effect of neurotransmitter
Affinity vs efficacy
- Affinity: how much drug binds to receptor
- Efficacy: how much drug activates receptor once bound
Types
Stimulants
- e.g. cocaine, amphetamine, methylphenidate (Ritalin)
- Inhibits dopmaine/serotonin/norepinephrine transports -> blocks reuptake -> more dopamine in cleft
- -> increased activity, counteracts fatigue
Nicotine
- In tobacco
- Binds to Ach receptors -> stimulates neurons that release dopamine
- -> increased heart rate, blood pressure, HCl in stomach, bowel activity
Opiates
- From opium poppy, e.g. morphine, heroin, methadone
- Binds to endorphin receptors - inhibit GABA + block receptors that block dopamine -> increase dopamine
- -> pain relief
Marijuana
- Cannabinoid
- Binds to reverse transmitters on pre-synaptic neuron -> less sending of glutamate, GABA; increases dopamine; stim norepinephrine receptors
-> relaxation, hallucination, reduced anxiety
Hallucinogens
- e.g. LSD, MDMA
- Binds to serotonin receptors + stimulates norepinephrine receptors; releases dopamine & serotonin; increases connectivity in brain areas not normally connected
-> hallucinations
Alcohol
- Increases dopamine receptor stimulation, blocks glutamate activity
-> Addictive
Dopamine
- Connected to addictive drugs - release in nucleus accumbens
- But doesn't correlate with pleasure/addiction (except stimulants) + blocking dopamine receptors doesn't affect opiate use -> not as essential to addiction
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Microneuroanatomy
Neuron parts
- Dendrite: branching fibres, lined with receptors
- Dendritic spines: increase surface area
- Soma: contains nucleus, ribosomes, mitochondria
- Axon: only one, conveys impulses
- Myelin: insulating sheath
- Nodes of Ranvier: interruptions in myelin
- Pre-synaptic terminal: where axon releases chemicals
- Axon hillock: start of axon, where AP calculated
Types
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-
Interneurons: in between neurons - input from neuron, output to neuron
- Intrinsic neurons: dendrites & axons in single structure
Shape
- Multipolar (e.g. motor efferent)
- Bipolar (e.g. smell)
- Unipolar (e.g. sensory afferent)
Glial cells
Oligodendrites
- In brain & spinal cord
- Builds myelin sheath - for 30-50 neurons, supplies axon with nutrients
Schwann cells
- In periphery of body
- Builds myelin sheath, supplies axon with nutrients
Astrocytes
- Regulate environment + coordinate neurons
- Wrap around synapses of functionally related axons
Radial glia
- Provides structure
- Pathway to help developing neurons migrate
- Differentiates into neurons after embryonic dev
Microglia: removes waste, immune system, learning
Basics
Gradients
Concentration
- Difference in distribution of ions b/n inside & outside membrane
- Ions move from high -> low concentrations
- Via ion channels
- Rate of crossing dependent on strength of gradient + no. open channels (permeability)
Electrical = membrane potential
- Difference in electrical charge b/n inside vs outside of cell
- Same charge repels, opposite charge attracts
- Resting = -ve (described from inside)
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Resting Potential
Established - Na+/K+ pump - 3 Na+ out, 2 K+ in
- -> many K+ leak channels -> K+ out -> -ve
- few Na+ leak channels -> little Na+ in -> slightly +ve
- Na+/K+ pumps removes 1 charge -> slightly -ve
- more -ve proteins/molecules inside -> slightly -ve
Maintained - 3 Na+ in, 2K+ out
- -ve membrane potential influences more as gets stronger
- K+: conc gradient out, elec gradient in
- Na+: conc gradient in, elec gradient in
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Graded Potential
- No Na+ gated channels - mp change in all directions, decays
- Varies in size, depending on stimulus