ch.8 neurons Flashcards
(48 cards)
factors affecting ion flux across membranes
-concentration gradient; ion gradient
-ion membrane permeability
what are the two mechanisms of disruption of membrane potential
- graded potential
- action potential
describe graded potential and the three major characteristics
graded potential: change in Vm that caries in size
-found in unexitable membrane
-lacks voltage gated channels but can have ligand gated or mechanically gated channels
1. magnitude of response is directly proportional to magnitude of stimulus
2. does not transmit over long distances
3. effects can be summated, added together
describe action potential and its three characteristics
action potential: change in a membrane potential of an excitable membrane
-have voltage gated channels
-found in neurons and non-neuronal tissues like muscles
1. action potentials are all or nothing, magnitude is fixed after threshold
2. capable of transmitting over long distances
3. do not summate
brief summary of how action potentials work
- start with ligand gated channels
- concentration of ions is large enough to reach threshold
- transitions to voltage gated channels
- depolarizes
- reaches max and then repolarizes
threshold
minimum stimulus necessary to illicit a response (action potential)
-an above threshold stimulus applied to the membrane can cause a change in voltage resulting in the opening of voltage gated channels
-in the range of 15 mV
voltage gated sodium channels
has three conformations:
1. closed but capable of opening, activation gate is closed
2.opened (activated) caused by a change in vM
- activation gate opens when Vm reaches threshold
-does not stay open for long
3. closed and locked (inactivated)
- con only open at -70 mv
-repolarize: K+ leaves the cell and repolarizes the cell
voltage gated potassium channels
open in response to changes in vM but are slower than Na+ channels
-increased potassium conductance results in efflux and repolarization. inside becomes (-).
-potassium channels dont inactivate, only close when Vm returns to normal
action potential in depth
- (-70) to threshold: Vm changes caused by Ca+ influx or Na+ influx, from a graded potential (ligand gated channels)
-Phase 1: Na+ influx to the cell, from a voltage gated sodium channel, becomes activated at threshold. - peak point of action potential: VGC sodium channel closes and locks, K+ channel is late in opening: concentration and electrical gradient travel out of the cell.
-phase 2: potassium efflux repolarizes the cell.
-(-70): sodium channel is closed but unlocked. potassium channel is slow to close and causes hyperpolarization.
-cell resets back to normal by a sodium potassium pump.
at rest… the membrane is 6.5 permeable to ___
potassium
at rest the gradient is 6.5 times bigger for
sodium
describe the permeability of potassium and sodium during a action potential
- sodium permeability: increases during phase 1 and decreases at peak.
- potassium permeability: increases during phase 1.
components of efferent neuron
-nucleus: provides machinery
-dendrites: thin membrane, unexcitable membrane, has no VGC; so it is affected by graded potential
-axon hillock: transition area, gets Vm to threshold. start of VGC, known as the first start of an action potential
local current flow:
movement of action potential from one region to neighboring region, relatively slow. used by every muscle cell.
refractory period
time period after an action potential in which sodium gates may be inactive and the potassium channels may be open so that a second AP is possible in the future.
-creates one direction flow
-allows membrane to return to resting Vm, “membrane does nothing”
mechanism of local current flow
-start at the axon hillock and depolarize to threshold by a graded potential from the dendrites. need more Na going in than K going out
-VGC sodium open: inside of cell receives sodium influx which depolarizes the cell further.
-sodium diffuses to another area of membrane and depolarizes to threshold, opens VGC on that membrane. process continues in one direction down an axon. (positive feedback loop)
-meanwhile potassium VGC channels were also activated but slower to open.
-as the cell is depolarized to about +30mv, VGC sodium close and lock and VGC potassium open
-potassium concentration and electrical gradient travels out of the cell. ( falling phase of AP), which repolarizes the section of membrane. (refractory period)
action potential or graded potentials need _________ to trigger AP
disproportionate ion flux, need more sodium coming in than potassium going out.
rates of transmission of signal down a axon depend
on the diameter of the axon, wider diameters have less resistance
schwan cells
myelin forming cells of the PNS, help prevent leakage of charge
-1:1 ratio
oligodendrocytes
myelin forming cells of the CNS
-help prevent leakage of charge
-one oligodendrocyte to several axons
astrocytes
star shaped cells that surround unmyelinated cells that give structural support
what is myelin and its function
prevents leakage of charge
-wraps around axon
-made of fat
node of ranvier
portion of unmyelinated axon, where ions flow through, has VGC
saltatory conduction
action potential propogation by “jumping” from node to node in myelinated fibers
