CH 4: Neural Conduction & Synaptic Transmission Flashcards
Define MEMBRANE POTENTIAL - describe how it’s recorded.
- = Difference in electrical charge b/w the inside & outside of a cell
- Recorded by placing an electrode tip inside the neuron & another outside neuron in extracellular fluid
Define RESTING POTENTIAL.
- Steady membrane potential of -70mV
- -70mV indicates that the potential inside the resting neuron is 70mV less than the outside
Define ION CHANNELS.
- Pores on neural membrane
- Each channel specialized to pass either Na+ or K+
- -> maintains uneven distribution of Na+ & K+
Describe the resting membrane potential & its ionic basis.
Describe the 2 factors that allow more Na+ outside and more K+ inside neuron.
- Resting membrane potential = -70mV
- Na+ channels are closed thus ^Na+ outside than inside
- K+ channels are open but few K+ exit cell bc (-) resting potential (-70mV - opposites attract)
Describe 2 reasons why theres more pressure on Na+ to enter resting neurons.
- Electrostatic pressure: -70mV inside - opposites attract
2. Random motion pressure: Na+ ^likely to move down its [ ] gradient (go where there’s less of it)
List the 2 types of postsynaptic potentials.
- Depolarization
2. Hyperpolarization
Describe how DEPOLARIZATION is conducted.
- NT binds to postsynaptic receptor on postsynaptic neuron & decreases the resting membrane potential (ie. -70mV –> -67mV)
- aka EPSPs
Define EPSPs.
Excitatory postsynaptic potentials = ^likelihood that neuron will fire
Describe how HYPERPOLARIZATION is conducted.
- NT binds to postsynaptic receptor & ^ resting membrane potential (ie. -70mV –> -72mV)
- aka IPSPs
Define IPSPs.
Inhibitory postsynaptic potentials = decrease likelihood that neuron will fire
Describe how postsynaptic potentials summate, & how action potentials are generated.
- Whether a neuron fires depends on balance b/w excitatory & inhibitory signals reaching its axon.
- ACTION POTENTIALS (AP) are generated when sum of depolarizations & hyperpolarizations reaching the axon at any time is sufficient to depolarize the membrane to a level = THRESHOLD OF EXCITATION = ~65mV
- AP’s are all-or-nothing response
- -> Stimulating a neuron more intensely does not ^speed or amplitude of resting action potential
- As a neuron is stimulated, it becomes less polarized until the threshold of excitation is reached & firing occurs
Define INTEGRATION.
- Combining a number of individual signals into 1 overall signal
Define SPATIAL SUMMATION.
- Integration of signals that originate at diff sites on the neuron’s membranes
- ie) EPSP + EPSP = greater EPSP
- ie) IPSP + EPSP = 0
etc.
Define TEMPORAL SUMMATION
- Integration of neural signals that occur at diff times at the same synapse
Define VOLTAGE-ACTIVATED ION CHANNELS
- Open/close in response to changes in level of membrane potential
Explain the ionic basis of an action potential.
- Resting neuron = ~impermeable to Na+ bc those that do pass in are passed out
- When EPSP excitation:
–> membrane potential of axon is depolarized
–> voltage-activated Na+ channels open wide
–> Na+ rush into cell
–> drive membrane potential from -70 to +50mV
–> K+ driven out
–> Na+ channels close
= end of rising phase of action potential & beginning of depolarization by continued efflux of K+ - Once achieve depolarization:
–> K+ channels close
–> many K+ flow out of neuron
–> neuron = hyper-polarized
Define ABSOLUTE REFRACTORY PERIOD
- Brief period after initiation of an AP during which it’s impossible to elicit another AP in the same neuron
Define RELATIVE REFRACTORY PERIOD
- After absolute refractory period when a higher-than-normal amount of stimulation is necessary to make a neuron fire
Explain how the refractory period is responsible for 2 important characteristics of neural activity.
(1) Responsible for how AP’s travel along axons in only 1 direction
- bc portions of axon over which the AP has just travelled are left momentarily refractory, an AP can’t reverse direction
(2) Responsible for how the rate of neural firing is related to the intensity of stimulation
- If neuron under HIGH level of continuous stimulation, it fires & then fires again ASAP as absolute refractory period is over
Describe how AP’s are conducted along UN-MYELINATED axons.
- Once AP generated, it travels passively along atonal membrane to the voltage-activated Na+ channels which have yet to be open
- The arriving electrical signal opens these channels
- -> allows Na+ into neuron
- -> generate full blown AP at this portion of membrane
- -> signal then conducted passively to net Na+ channels
- -> AP actively triggered
- -> repeated full blown AP triggered in all terminal buttons
Describe how AP’s are conducted along MYELINATED axons.
- Ions pass through the axonal membrane only at the NODES OF RANVIER (=gaps b/w adjacent myelin segments)
- -> Na+ channels ^[ ] at nodes of Ranvier
- -> AP conducted passively along axon to next node
- -> another full blown AP elicited, etc.
- Myelination ^speed of axonal conduction
- Passive conduction = instant = signal jumps along axon from node to node
Explain the shortcomings of the Hodgkin-Huxley model when applied to neurons in the mammalian brain.
- Model provided simple intro to what we understand about ways in which neurons conduct signals
- Problem = simple neurons & mech of model do NOT rep the variety, complexity & plasticity of many neurons in mammalian brain
- -> Model based study on squid motor neurons = large & ^simplistic than mammalian neurons
Define AXODENDRITIC Synapses
- Synapses of axon terminal buttons on dendrites
Define AXOSOMATIC Synapses
- Synapses of axon terminal buttons on somas (cell bodies)
