Topic 3 Flashcards
Neurons
when stimulated usually on cell body or dendrites an electrical impulse may be generated and propagated along the axon = nerve impulse
Electrical properties of cell is due to
- ionic concentration differences across membrane
- permeability of cell membrane to ions
Important ions
K, Na, Cl, Ca, large negatively charged organic ions (org)
Large negatively charged organic ions are..
non diffusible proteins
Only ion that is higher on the inside of the cell
K
Na and K due to and maintained by..
activity of Na/K ATPase pump in cell membrane
Ca due to..
various transporters in cell endoplasmic reticulum membranes
Cl repelled by ..
org so is higher outside of the cell than inside
Org stay where..
inside the cell
Permeability of cell membrane to ions determined by..
ion channels - ions diffuse through them down conc. gradients
2 types of ion channels
- gated
- non gated
Non gated ion channels
always open, more K than Na therefore cell membrane more permeable to K at rest (no stimulus).
What channels are import in establishing the resting membrane potential
non gated ion channels, especially K (more numerous)
Gated channels
not involved at rest, open in response to stimuli
4 types of gated channels and their stimuli
- voltage gates: membrane voltage changes
- chemical gates: chemicals, binding of hormone/nt
- thermal gates: temperature
- mechanical gates: mechanical deformation
Resting membrane potential (RMP)
at rest, MP= -70 mV (inside of cell is more -ve)
Membrane potential
a charge difference (potential difference) exists just across the cell membrane
Factors establishing RMP (-70 mV)
- Na/K ATPase
- Org inside cell can’t cross membrane
- more non gated K channels than non gated Na channels (membrane more permeable to K than Na at rest, so K is major determinant of RMP)
K diffuses out of the cell down the concentration gradient therefore..
cell loses positive charge and inside becomes more negative
What increases due to increasing attraction to -‘ve cell interior
Na diffusion into cell
Until -70 mV is reached..
positive out of the cell (K) is greater then positive in the cell (greater K permeability)
Once at -70mV ..
amount of positive (k) moving out equals the amount of positive (Na) moving in because force on Na much higher than on K, therefor the net movement of charge (ions) is 0
Electrically excitable cells are ONLY
muscle and nerve cells
Electrically excitable cells are capable of producing..
departures from RMP in response to stimuli
When a neuron is stimulated
- gated ion channels open
- MP changes = producing graded potential if the threshold is reached
- triggers an action potential
Graded potentials
stimulis causes a small change in RMP, usually on dendrite or cell body by opening gated channels (changed membrane permeability)
Possible results from graded potentials
- more positive than RMP = depolarization
- more negative than RMP = hyperpolarization
In GP ions move ..
passivley (unlike charged attract) = current flow causing deplo. or hyperpol. on adjacent membrane
GPs are..
short distance signals, die away quickly
Magnitude and distance travelled by GP varies..
directly with the strength of the stimulus
How do GPs summate
1st GP present when 2nd stimulus occurs, these add together to create the resulting GP
After GP..
repolarization, return t the RMP after deplo. or hyperpol.
GPs are essential to..
initiating a nerve impulse (AP)
What leads to an action potential
GP that causes depolarization and is large enough
Steps in a GP action potential
- critical stimulus (summating stimuli)
- GP reaching threshold
- action potential
Action potential
nerve impulse, large change in MP that propagates along an axon w/ no change in intensity
Action potentials initiates at..
trigger zone
Phases of action potential
- depolarization
- repolarization
- after-hyperpolarization
Depolarization phase
voltage gated Na channels respond to MP change and open. as gates open more Na diffuses in cause even more Na gates to open (positive feedback) Na diffuses in causing depolarization to 30 mV
Repolarization phase
Na channels close, become inactivated and Na movement returns to resting levels, voltage gated K channels open therefore K diffuses out
After-hyperpolarization phase
K channels are slow to close and remain open longer than necessary, Na channels are reactivated, can response to stimuli at this point
Once K channels close..
MP returns to RMP
Na/K ATPase always working ..
maintain gradients
Takes ____ of APs to cause a…..
10 000; measurable change in ion in the cell
Absolute refractory period prevents..
AP summation
During the absolute refractory period no..
AP can be generated, regardless of stimulus size
Absolute refractory period results from either..
- all Na channels being open (region b)
- all Na channels being inactivated (can’t open until MP reaches RMP, region c)
Relative refractory period
period when an AP can be generated but only by a greater than normal stimulus, Na channels are reactivated when MP passes RMP, they are closed but can be reopened if theresfolg reached
