BIOL 1412 Lecture Notes - Lecture 3: Resting Potential, Ibm System I, Threshold Potential
1 May 2018
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TOPIC III: NERVOUS SYSTEM I – NEURONS, IMPULSE GENERATION & TRANSMISSION
A) Neurons Review fig.11.5
- Neurons are excitable (responsive to stimuli) we can do something to allow it to get
eited
- when stimulated (usually on cell body or dendrites) an electrical impulse may
be generated and propagated along the axon = nerve impulse
B) Electrical Properties of Cells:
- due to: (all the steps that are involved)
1) ionic concentration differences across the cell membrane (gradients) (you
have to think about the charge as well as the concentration)
a) important ions: look at drawing in notebook
- Na+, K+, Cl-, Ca++
- large, negatively charged organic ions (org-) – are non-diffusible
proteins
- [K+] are always higher in the inside of the cell than the outside =
meaning it always wants to escape
b) [Na+] & [K+] due to and maintained by activity of Na+/K+- ATPase
(pump) in cell membrane
- for every 2 K+ that is pumped in, 3 Na+ is pumped out
c) [Ca++] due to various transporters in cell and ER membrane
d) - Cl- repelled by org- , so is higher outside the cell than inside
e) - org- stay inside the cell = cannot get outside the cell membrane
2) permeability of cell membrane to ions
- determined by ion channels - ions diffuse through them down conc.
gradient
- ion channel types:
a) non-gated fig.11,1
- always open
- more K+ than Na+ in a neuron ∴ cell membrane more
permeable to K+ at rest (no stimulus)
- because there is a higher [K+] of the inside than out, K+
will want to move out the neuron.
- these channels (especially K+ - more numerous) are
important in establishing the resting membrane
potential (RMP)
b) gated
- NOT involved at rest
- open in response to stimuli: e.g.
i) membrane voltage changes = voltage gates (they
change the membrane potential allowing action potential to happen)
ii) chemical e.g. binding of hormone (like ligand) or
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neurotransmitter (nt) = chemical gates
iii) temperature = thermal gates
iv) mechanical deformation (changing of shape) = mechanical
gates
C) Neuronal Physiology
1) Resting Membrane Potential (RMP): fig.11,8
- at rest (not simulated) a charge difference (potential difference)(which is energy
stored) exists just across the cell membrane = membrane potential (balance between
molecule number and charge) (when its at rest)
- ≈ -70 mV - i.e. inside of cell = 70 mV more negative than outside
- the inside of the neuron is more negative because there is more ions (K+) that
is leaving than there is more ions coming in (Na+) making it more negative
- factors establishing RMP:
a) Na+/K+-ATPase (Na+/K+ pump) - not a channel (keep in mind where you got
your transport and where you got your diffusion)
- breaks down 1 ATP and uses energy to pump 3 Na+ out and 2 K+
in → both ions are pumped against their concentration gradients
∴ active transport (for every Potassium that goes out, it is becoming
more and more negative)
- effects:
i) maintains concentration gradients of Na+ and K+
ii) contributes a little (a few mV) to RMP (pumping more
positive (+ve) ions out than in)
b) org- inside cell e.g. proteins - cannot cross membrane
c) more non-gated K+ channels than non-gated Na+ channels (membrane
more permeable to K+ than Na+ at rest ∴ K+ is major determinant of RMP)
i) K+ diffuses out of cell down concentration gradient∴ cell loses
+ve charge (inside becomes more negative (–ve))
ii) unlike charges attract and K+ diffusion slows as inside becomes
increasingly –ve (bc + and – attrat so its harder to separate
iii) Na+ diffusion into cell increases due to increasing attraction to
–ve cell interior because + and - attract
iv) until – 70 mV reached, +ve moving out (K+) is greater than +ve moving
in (Na+) – greater K+ permeability
v) at -70 mV, the amount of +ve (K+) moving out equals the
amount of +ve (Na+) moving in
vi) ∴ The net movement of charge (ions) is 0 (equal in both
directions): RMP = - 70 mV
2) Electrically excitable cells
- ONLY muscle and nerve cells
The
potassium
ions and
channels are
not the main
things that is
involved in
reaching
RMP= -
70mV
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- capable of producing departures from RMP in response to stimuli (= changes in
the external or internal environment) (they have to stimulated in order to open
the channels)
- when a neuron is stimulated:
a) gated ion channels open
b) Membrane Potential (MP) changes, producing a graded potential. If the
threshold potential is reahed…
c) triggers an action potential
3) Graded Potentials (GPs) fig.11.9
- stimulus causes a small change in MP, usually on dendrite or cell body (no longer at
rest) by opening gated channels (changes membrane permeability) = GP
- more opening of gated channels mean that there is more movement of ions making
it more graded
- possible results:
a) more +ve than RMP = depolarization
e.g. -70 mV to -65 mV (closer to zero)
b) more -ve than RMP = hyperpolarization (more polarized than it was at rest)
e.g. from -70 mV to -75 mV
- characteristics: fig 11.10
a) ions move passively (unlike charges attract (+, -)) = current flow,
causing depol. or hyperpol. on adjacent membrane
b) GPs are short distance signals – die away quickly (short-lived)
c) magnitude and distance traveled by potential varies directly with the
strength of the stimulus stimu. also affects the amount of channels that open
- i.e. larger stimulus → larger graded potential that travels further
across the membrane (think if you throw a small rock into a pond,
there will have small rings that are formed around it, now if you throw a much bigger
rock the rings with be bigger and more and goes further)
d) GPs can summate - 1st GP present when 2nd stim occurs → these add
(sum) to create the resulting larger GP fig.11.9
- after a GP:
- repolarization = return to RMP after depolarization or hyperpolarization
- back to resting ^
4) GPs → Atio Potetial AP fig 11.9 + 11.12
- GPs are essential in initiating a nerve impulse (AP) without GP there wot e AP
- if the GP causes depol. and if depol. is large enough or multiple GPs sum to be large
enough (i.e. there is a critical stimulus) → leads to an action potential
- steps:
a) critical stimulus (large GP or summation of multiple GPs)
↓
b) GP reaches threshold
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Document Summary
Topic iii: nervous system i neurons, impulse generation & transmission: neurons review fig. 11. 5. Neurons are excitable (responsive to stimuli) we can do something to allow it to get (cid:858)e(cid:454)(cid:272)ited(cid:859) When stimulated (usually on cell body or dendrites) an electrical impulse may be generated and propagated along the axon = nerve impulse: electrical properties of cells, ionic concentration differences across the cell membrane (gradients) (you. Large, negatively charged organic ions (org-) are non-diffusible proteins. [k+] are always higher in the inside of the cell than the outside : important ions: look at drawing in notebook. Due to: (all the steps that are involved) have to think about the charge as well as the concentration) meaning it always wants to escape: permeability of cell membrane to ions (pump) in cell membrane. Because there is a higher [k+] of the inside than out, k+ Determined by ion channels - ions diffuse through them down conc. gradient.
