Traffic - Week 3 (ch. 4) Flashcards
excitable tissues
capable of producing electrical signals (temporary, rapid changes in membrane potential), nerve and muscle
resting membrane potential
the membrane potential that exists when no net changes in potential are occurring
graded potentials
local changes in membrane potential that vary in magnitude (flow of ions)
action potentials (reverse action)
brief, rapid reversals in membrane potential, which can spread throughout the membrane (flow of ions). inside positive.
voltage-gated channels (action)
Only with AP*** membrane channels that open or close in response to changes in potential
a membrane that has potential is…
polarized
depolarization (deemed for good)
a decrease in membrane potential (inside becomes more positive)
triggering event
event that initiates a depolarization (stimulus like light or touch, chemical messenger)
hyperpolarization
an increase in membrane potential (inside becomes more negative)
repolarization (action potentials) (return)
return to resting potential after a depolarization
Graded Potentials - stronger the trigger….
- stronger trigger➝greater magnitude of change in potential
current flow (graded potential)
when a graded potential occurs, a piece of the membrane (called the active area) has a different potential than the rest of the membrane (which is at resting potential, called the inactive area)
a. current flows between active area and the inactive areas (opposite charges attract)
b. previously inactive areas become active and more current flow occurs
spread of graded potential decreases…
a. as it moves along the membrane because current leaks into ECF
b. function as signals over short distances
spread of graded potential usually not an actual reversal of charges (potential)
just a reduction in potential (inside becomes less negative than before, small depolarization). graded potential important for nerve and muscle cells (e.g., postsynaptic potentials)
Action Potentials (AP) (distance)
can be transmitted over long distances without losing strength
Depolarization for AP
triggering event causes a slow depolarization until threshold is reached (about -50 to -55 mV). once threshold is reached, membrane quickly depolarizes to +30 mV
when triggering event begins depolarization (shooting salt)
some voltage-gated Na+ channels open, Na+ flows into cell (proteins that make up the channel have charged portions, shape change when charges interact with charges surrounding the membrane)
triggering events…
this further depolarizes the membrane, causing even more Na+ channels to open
at threshold
all the Na+ channels are open and there is an explosive increase in Na+ permeability (P Na+)
at peak depolarization (action potential) (goat)
the Na+ channels close (the channel is constructed so that the same depolarization that opens them also closes them)
Repolarization begins (action potential)
as Na+channels close, K+channels open (PK+ increases) due to delayed voltage-gated response to the depolarization, K+ flows out of cell
2. this restores internal negativity
as repolarization progresses…(last step)
a. Na+channels go back to closed, but can open. b. newly opened K+ channels close
(1) hyperpolarization occurs before channels close (membrane even more negative than at resting potential)
(2) resting potential restored
AP lasts…
about 1 millisecond. Ion gradient restored.
Na+-K+ pump restores ion gradients (after repolarization) (action potential) (maintenance man)
a. important for long term maintenance of gradient
b. not necessary between action potentials (AP)
(1) ion shifts during AP are not so great that they wipe out concentration gradients, so many APs can occur
parts of the neurons (nerve cells) (CAD)
3 basic parts - cell body, dendtrites and axon
cell body (house)
a. houses nucleus and organelles
b. receives signals from other cells (contains receptors for chemical messengers)
dendrites
a. projections from cell body
b. increase surface area for receiving signals
axon (nerve fiber) (tree)
long, side branches, AP away from body, hillock where AP is generated, ends in terminal, only a mm.
a. single long projection
b. conducts APs away from cell body c. often has collaterals (side branches)
d. axon hillock (part of cell body and first part of axon) is area where APs generated in most neurons
e. ends in branches called axon terminals that release chemical messengers
f. maybe less than a mm or more than a m
Propagation of an AP (how AP moves down axon) (local bar)
Conduction by local current flow (contiguous conduction)
- AP at axon hillock
a. local current flow between this active area and adjacent inactive area causes new AP
b. AP passed section by section along axon
Saltatory conduction
Occurs in myelinated fibers - special cells and nodes of Ranvier. AP “jumps” from node to node a. much faster
b. conserves energy
(1) fewer ions move so Na+-K+ pump uses less ATP restoring gradients
special cells (saltatory) hot dog
Special cells form a barrier that is impermeable to ions
(1) wrap around fiber
(2) mostly lipids
(3) formed by oligodendrocytes in central nervous system (CNS), by Schwann cells in peripheral nervous system (PNS)
nodes of Ranvier (bare salty french)
(1) bare spaces between myelin
(2) contain Na+ and K+ channels
Fiber diameter…
influences speed of propagation
- larger fiber diameter➝ faster conduction
- large myelinated fibers where speed is critical (e.g., fibers innervating skeletal muscle)
- smaller unmyelinated fibers found in areas where speed not critical (e.g. fibers innervating digestive tract)
Refractory period (factory assembly line)
- ensures APS propagated in only one direction
2. membrane that just had AP not very sensitive to further stimulation. lasts for one to a few msec
absolute refractory period
(1) no amount of stimulation will induce another AP
(2) time between when Na+ gates first opened and when they are in their “ready to open” conformation
relative refractory period
(1) needs stronger than usual stimulation to
produce another AP
(2) only some Na+ gates ready to open, some K+ gates still open
All-or-none law
- a membrane responds with a maximal AP or it doesn’t respond at all
a. a stimulus that doesn’t reach threshold never initiates an AP - nervous system differentiates between relatively weak or strong stimuli by frequency of APs
a. stronger stimuli ➝ more APs/sec
neuron terminates at…
mag
- muscle
- gland
- another neuron
Synapse basics
- junction between axon terminal of one neuron and dendrites or cell body of next neuron
- most neurons have thousands of synaptic inputs
presynaptic neuron
(1) conducts APs toward synapse
(2) ends in swelling called synaptic knob, which contains synaptic vesicles that store a specific neurotransmitter
postsynaptic neuron
(1) conducts APs away from synapse
(2) portion at synapse called subsynaptic membrane
synaptic cleft
space between pre and postsynaptic neurons
synapse (one direction)
generally operates in one direction (changes in membrane potential in presynaptic neuron bring changes in membrane potential of postsynaptic neuron)
basic synapse function
a. AP reaches presynaptic neuron axon terminal
b. voltage-gated Ca2+ channels in synaptic knob open c. Ca2+ flows in, release of neurotransmitter from synaptic vesicles by exocytosis
d. neurotransmitter diffuses across cleft and binds with receptor sites on subsynaptic membrane
e. triggers opening of specific ion channels in subsynaptic membrane (chemically-gated channels)
Two types of synapses
excitatory and inhibitory. a given synapse is either always excitatory or always inhibitory
inhibitory synapse
a. opens channels for K+orCl-
b. K+➝out,orCl-➝in
c. results in small hyperpolarization called an IPSP (inhibitory postsynaptic potential)
d. each IPSP moves membrane further away from threshold
excitatory synapse (party)
a. Na+ and K+ channels open
b. net movement of Na+ in
c. results in small depolarization called an EPSP
(excitatory postsynaptic potential), a kind of
graded potential
d. takes many EPSPs to bring membrane to
threshold and generate an AP (each one brings membrane closer to threshold)
the same neurotransmitter is generally released at a given synapse
(1) some nts are always excitatory
(2) some nts are always inhibitory
(3) some nts produce an IPSP at one synapse and
an EPSP at another synapse
Synaptic delay
- takes time for signal to cross synapse (.5-1 msec)
1. the more synapses a signal must cross, the longer the time needed for response to occur (total reaction time)
Removal of nt from the synaptic cleft
- as long as the nt is bound to receptor, IPSP or
EPSP continues - quickly removed from synapse
a. diffuses away from synaptic cleft
b. inactivated by enzymes in subsynaptic membrane
c. actively transported back to presynaptic neuron
Neurotransmitters - what types of channels
- most nts change conformation of chemically-gated channels
1. some use second messenger systems (cAMP),which work in short term and long term changes like memory
Grand postsynaptic potentials (GPSP)
composite of all EPSPs and IPSPs occurring on a cell at the same time
temporal summation (this is grand postsynaptic)
A single synapse. total potential occurring very close together in time from same synaptic input
spatial summation
Many synapses. potentials from many different synaptic inputs at the same time
classical neurotransmitters (mozart)
a. small, fast-acting
b. made in axon terminal
c. amino acids or related compounds
neuropeptides
a. large (many amino acids)
b. made in cellbody, stored in dense-core vesicles in
the axon terminal
(1) one or more may be released along with a
classical nt
c. some act like classical nts d. most are neuromodulators
(1) don’t cause EPSPs/IPSPs
(2) cause long term changes in synapse (3) often use second messenger systems
Presynaptic inhibition and facilitation
a third neuron may influence the effect of one neuron
on another
a. probably involves Ca2+
b. can influence inputs from certain neurons while
not affecting other inputs
Convergence and divergence
- in convergence a single neuron is influenced by many other neurons synapsing on it
- in divergence a single neuron influences many other neurons
important for nerve and muscle cells
graded potential
never mylenated
dendrites
voltage-gated Na channels -inactivation gate
ball and chain
