neurons & transmission Flashcards
membrane potential
the difference in electrical charge between the inside and outside of a cell
resting potential
the steady membrane potential of -70mV
the resting state of a neuron
here the neuron is polarized
Na & K ions in a resting neuron
more Na ions outside the cell than inside
more K ions inside the cell than outside
the unequal distribution is kept despite 2 types of pressure on Na ions to enter:
1. electrostatic - opposite charges attract - the neg charge inside attracts the pos Na ions to move inside
2. random motion - pressure for Na ions to move down their conc gradient into an area of lower Na conc (same for K)
Na channels are closed in resting - counteracts pressure
K channels are open
sodium-potassium pump
transporters that continually exchange 3 Na in for every 2 K out
depolarizing vs hyperpolarizing the receptive membrane
NTs either de or hyper polarize the receptive membrane when they bind to the postsynaptic receptors
depolarization = decrease the RP e.g. from -70 to -67
hyperpolarization = increase the RP e.g. from -70 to -72
EPSPs & IPSPs
postsynaptic depolarization = excitatory postsynaptic potential = increase likelihood that neuron will fire
postsynaptic hyperpolarization = inhibitory postsynaptic potential = decrease likelihood that neuron will fire
these are graded responses = their amplitude is proportional to the intensity of the signals that elicit them
action potential
a massive but momentary reversal of the membrane potential
from -70 to +50mV
generated in the axon initial segment only if the threshold level (usually -65mV) is reached
all-or-none response = either occur to full extent or not at all
intergration
adding/combining several individuals signals into 1 overall signal
can be done through spatial summation or temporal summation
spatial vs temporal summation
spatial: local & simultaneous PSPs sum up to form a greater PSP
- if there are simultaneous EPSPs & IPSPs they cancel out
temporal: PSPs produced in rapid succession at the same synapse integrate to form a greater signal
steps of an AP
- membrane P is depo to the threshold by a PSP
- the voltage-activated Na channels open = Na ions rush in = MP to go from -70 to +50mV
- rapid change in MP triggers opening of voltage-activated K channels = K ions to drive out of the cell
- Na channels close - ends the rising phase & begins repolarization
- repolarization occurs by continued efflux of K ions
- repolarization is achieved = K channels close gradually
- gradual closing = too many K flow out = neuron is left hyperpolarized for a bit
- sodium-potassium pump restores the neuron to the RP
axonal conduction of AP
- AP is generated
- AP travels passively along axonal membrane to the closed voltage-activated Na channels
- arrival of electrical signal open channels
- Na ions rush into neuron & generate AP at this part of the membrane
- the signal is conducted passively to the next Na channel where another AP is triggered
this is repeated several times until a full blown AP is triggered in all terminal buttons
orthodromic vs antidromic conduction
orthodromic = axonal conduction in the natural direction - from cell body to terminal button
antidromic = if electrical stimulation of sufficient intensity is applied to the terminal end of an axon - AP is generated and travels along the axon back to cell body
ways to terminate synaptic messages
reuptake by transporters: majority of NTs are almost immediately drawn back into the presynaptic buttons by transporter mechanisms
enzymatic degradation: other NTs are degraded at the synapse by enzyme
e.g. ACH broken down my acetylcholinesterase
amino acid NTs
glutamate - most prevalent excitatory NT
aspartate
glycine
gamma-aminobutyric acid (GABA) - synthesized by a simple modification in structure of glutamate - most prevalent inhibitory
monoamine NTs
dopamine
epinephrine
norepinephrine
serotonin
each synthesized from a single amino acid
present in small groups of neurons in the brain stem
