Physiology Wk 2 Flashcards
What is the potential related to
The uneven distribution of sodium, potassium and large intracellular protein anions between the intracellular and extracellular fluid and to the differential permeability of the plasma membrane to these ions
What is the resting potential
The constant membrane potential present when a cell is electrically at rest and not producing electrical signals
Which tissues are excitable
Nerve and muscle tissues
What is polarisation
Charges are separated across the plasma membrane, so the membrane has potential
- anytime the membrane potential is other than 0 mv the membrane is in a state of polarisation
What is mV for the membrane at resting potential
-70mV
What happens during depolarisation
The membrane becomes less polarised and the inside becomes less negative than the resting potential with potential moving closer to 0mV (eg -70mV to -60mV).
Sodium ions rush into the cell via voltage gated ion channels
What is repolarisation
The membrane returns to its resting potential after having been depolarised
- due to the flowing out of potassium ions
What is hyperpolarisation
The membrane has become more polarised, the inside becomes more negative than the resting potential so farther away from the 0mV (-70mV to -80mV)
- due to the continuous outflow (leak) of potassium ions via potassium channels
Give an example of what happens to membrane potential
If sodium ions move in (positively charged ions) the membrane depolarises (becomes less negative)
If the net outward flow of positively charged ions increases (potassium ions moves out) the name range hyperpolarises) becomes more negative inside
What happens to gate opening and closing channels via gated channels
It occurs through a triggering event that causes a conformational change in shape of the protein that forms that gated channel
What are the 4 types of gated channels
(1) Voltage-gated channels open or close in response to changes in membrane potential,
(2) chemically gated channels change shape in response to binding of a spe- cific extracellular chemical messenger to a surface membrane receptor,
(3) mechanically gated channels respond to stretching or other mechanical deformation, and
(4) ther- mally gated channels respond to local changes in tempera- ture (heat or cold).
What are graded potentials
- short distance signals
are local changes in membrane potential that occur in varying grades or degrees of magnitude or strength. For example, membrane potential could change from 270 to 260 mV (a 10-mV graded potential) or from 270 to 250 mV (a 20-mV graded potential).
What is the statement used for the triggering of graded potentials
The stronger a triggering event, the larger the
resultant graded potential.
For example - The stronger the triggering event is, the more gated Na1 channels open. As more gated Na1 channels open, more positive charges in the form of Na1 enter the cell. The more positive charges that enter the cell, the less negative (more depolarized) the inside becomes at this specialized region. This depolarization is the graded potential.
What is the temporarily depolarised area called
Active area
What happens to the active area inside the cell and outside the cell
Inside the cell- is relatively more positive than the neighbouring inactive areas that are still at resting potential
Outside the cell, the active area is relatively less positive than the adjacent inactive areas
What is resistance
The hindrance to electrical charge movement - The greater the difference in potential, the greater the current flow; by contrast, the lower the resistance, the greater the current flow.
Explain the process of current flow during a graded potential ( hint: triggering event)
The membrane of an excitable cell at resting potential
A triggering event opens ion channels, usually leading to net Na + entry that depolarises the membrane at this site and the adjacent inactive areas are still at resting potential
Local current flows between the active and adjacent inactive areas resulting in depolarisation of the previously inactive areas- so depolarisation spreads away from the point of origin
Graded potentials die out over a short distance
Describe this
Current is lost across the plasma membrane as charge carrying ions in the form of K+ leak out through the ‘uninsulated’ parts of the membrane that is by diffusing outward down their electrochemical gradient through opening K+ leak channels
What happens to the graded potential because of this
Due to the current loss the magnitude of the local current and graded potential progressively dismisses the farther away it moves from the initial area
What are insulators
have high resistance and greatly hinder movement of charge
What is decremental
Gradually decreasing
What are action potentials
Are brief, rapid large changes in the membrane potential during which the potential actually reverses so that the inside of the excitable cell becomes more positive than the outside
What happens when action potentials are conducted
Throughout the entire membrane nondecrementally - that is they do not diminish in strength as they travel form their sight of initiation throughout the remainder of the cell membrane
What is the threshold potential
Depolarisation from the resting potential of -70mV proceeds slowly until it reaches a critical level known as the threshold potential - typically between -50-> -55mV
What three formations can the Na+ gates exist
Closed but capable of opening - activation gate closed, inactivation gate open
Open or activated (both gates are open)
Closed and not capable of opening- (activation open, inactivation closed
Describe the permeability changes and ion fluxes during an action potential
- resting potential: all voltage gated channels are closed
- at threshold Na+ activation gate opens and PNa+ rises
- Na+ enters the cell, causing explosive depolarisation to +30mV which generates rising phase of action potential
- At peak of action potential, Na+ inactivation gate closed and Pna+ falls, ending net movement of na + into cell. At the same time K+ activation gate opens and PK+ rises
- K+ leaves cell, causing its depolarisation to resting potential, which generates falling phase of action potential
- on return to resting potential, Na+ activation gate closes and inactivation gate oceans, resetting channel to respond to another depolarising triggering event
- further outward movement of K+ through still open K+ channel briefly hypeorpolarises membrane, which generates after hyperpolarisation
- K+ activation gate closes, and membrane returns to resting potential
What is the Na+ K+ pump
Restores these ions to their original locations in the long run.
The movement of relatively few Na+ and K+ ions causes the large swings in the membrane potential that occur during the AP.
LOOK AT GOODNOTES BOOK- ANATOMY OF NEURON
LOOK AT GOODNOTES BOOK
What is the refractory period
Prevents the “backward” current flow. During an action potential and slightly afterward, an area cannot be re stimulated by normal events to undergo another AP
Thus the RP ensures the AP can be propagated only in the forward direction along the axon
What happens during the absolute refractory period
The portion of the membrane that has undergone an action potential cannot be re stimulated . The period corresponds to the time during which Na+ gates are not in their resting conformation
What happens during relative refractory period
The membrane can be re stimulated only a strong stimulus than is usually unnecessary. This period corresponds to the time during which the K+ gates opened during the action potential has not closed yet, - there is also inactivation of voltage gated Na+ channels
What is the all or none law
An excitable membrane either responds to a triggering event with a maximal action potential that spreads nondecrementally throughout the membrane or does not respond with a AP at all
What are myelinated fibres
- in a myelinated fibre the impulse jumps from node to node, skipping over the myelinated sections of the axon (saltatory conduction)
- a myelinated fibre is surrounded by myelin at regular intervals
Unmyelinated areas are
Nodes of ranvier
In the PNS each patch of myelin sheath is formed by a
Separate SCHWANN CELL
Why does saltatory conduction propagate APs more rapidly than contiguous conduction
Because APs does not have to be regenerated at myelinated sections but regenerated where unmyelinated sections of the axon are
Myelinated fibres conduct impulses 50 times faster than unmyelinated fibres of comparable size
What is contiguous conduction
Local current flow between the active area at the peak of an action potential and the adjacent inactive area is still at resting potential reduces the resting potential un this contiguous inactive area which triggers an AP in the previously inactive area. The original active area returns to resting potential and the new active area induces an AP in the next adjacent inactive area by local current flow as the cycle repeats itself down the length of the axon
Name the four zones of the neurone
Input zone- receives incoming signals from other neurones
Trigger zone- initiates action potentials
Conducting zone- conducts APs in undiminishing fashion, often over long distances
Output zone- releases the NT that influences other cells
what are oligodendrocytes
in the CNS and provide insulation
