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)
Define DIRECTED Synapses
- Synapses at which the site of NT release & site of NT reception are in close proximity
Define NON-DIRECTED Synapses
- Synapses at which the site of release is at some distance from the site of reception
Describe how SMALL NTs are synthesized & packaged in vesicles.
- Syn in cytoplasm of terminal button
- -> Packaged in synaptic vesicles by button’s Golgi complex
- NT-filled vesicles are stored in clusters next to presynaptic membrane
Define & describe how LARGE Its are synthesized & packaged in vesicles.
- Large NTs = NEUROPEPTIDES = short AA chains = short proteins
- Assembled in ribosomes in cytoplasm
- -> packaged in vesicles by cell body’s Golgi complex
- -> transported by microtubules to terminal buttons
- Neuropeptide-filled vesicles are larger than small NT-filled vesicles
Define EXOCYTOSIS
- Process of NT release
Explain the process of NT exocytosis.
- Neuron at rest: small NT-filled synaptic vesicles congregate near sections of presynaptic membrane that are rich in voltage-activated Ca2+ channels
- Neuron stimulated by AP
- -> Ca2+ channels open
- -> Ca2+ enter button
- -> cause synaptic vesicles to fuse w/ presynaptic membrane & enter their contents into synaptic cleft
- -> small NTs released in a pulse each time an AP triggers momentary influx Ca2+ through presynaptic membrane
- -> neuropeptides (large NTs) release gradually IRT general ^ in level of intracellular Ca2+
Explain 2 ways that NTs are removed from a synapse.
(1) REUPTAKE = once NTs released, are imm drawn back into presynaptic buttons via transported mech
- most common mech for deactivated a released NT
(2) ENZYMATIC DEGENERATION = enzyme break down NT in synapse
Define GAP JUNCTIONS
- Narrow spaces b/w adjacent cells that are bridged by fine, tubular cytoplasts-filled protein channels
Describe the role of glia & gap junctions in synaptic transmission.
- Connects the cytoplasm of 2 adjacent cells
- -> allows electrical signals & small molecules to pass from one cell to the next
- aka electrical synapses
- -> transmits signals ^rapidly than chemical synapses
Name the 3 classes of NTs
(1) Conventional small-molecule NTs:
- AA’s
- Monamines
- Acetylcholine
(2) Unconventional small-molecule NTs
(3) Large-molecule NTs:
- Neuropeptides
