Transcript Slide 1
بنام یزدان
Membrane Potential
At rest
* A membrane potential at rest is defined
as the potential at which the flow of ions
out (mostly K+) is equal to the flow of ions
in (mostly Na+)
* To maintain this in a steady state, the
cell must utilize pumps
agents most important in membrane potential
•ions electrical Charge
• membrane
permeability
• ion concentration in both side membrane
• ion channels and pomp Na-K
Na
K
K+ “leak” channels •
ACTION
POTENTIAL
1. RESTING PHASE
- Voltage gated Na+ channels are in resting state
(CLOSED Activation gate and OPEN inactivation gate
i.e. NO Na+
passing through)
- Voltage gated K+ channels are closed
2. DEPOLARISATION
- Membrane depolarizes ( e.g. from an incoming stimulus) to threshold
(~ -55mV) and opens Na+ activation
gate
Na+ inflow (down its concentration gradient)
- Na+ inflow further depolarizes membrane until polarity reverses.
3. REPOLARISATION BEGINS
- At reverse membrane polarity (more depolarization), voltage gated K+
channels opens
K+ outflow (down its concentration gradient)
- Na+ inactivation gates close
4. REPOLARISATION CONTINUES
- K+ outflow restores RMP.
- Na+ inactivation gates open and K+ gates closes.
NOTE : At this stage (4) RMP is restored, but not the electrochemical
gradient.
Calculating the equilibrium potential of
an ion across a membrane
We use the Nernst equation to determine
this value for each ion (X):
Ex = (RT/zF) loge{Xo/Xi}
– R=Gas constant
– z=valence of ion (X)
Reduces to:
Ex =+_ 61 (log10{Xi/Xo})
T=°Kelvin
F=Faraday’s constant
Examples of normal concentration values and
equilibrium potentials
Ion
Na+
outside
145 mM
inside
12 mM
K+
4 mM
155 mM
Cl-
123 mM
Ca++
1.5 mM
4.2 mM
10-7 mM
ratio(o/i)
12
Ex (mV)
+67
0.026
-98
29
-90
15,000
+129
(the net effect is a membrane potential of -90mV - how?)
Membrane potential.
*Membrane potential is the ‘sum’ of all of
the different ions and their concentration
differences across the membrane
*Generally, we focus on the dominant ions
for purposes of simplifying this calculation
– Na+, K+, Cl- (sometimes calcium is included)
Action Potentials
Phases
– Depolarization
Inside plasma
membrane becomes
less negative
– Repolarization
Return of resting
membrane potential
All-or-none principle
– Like camera flash system
Propagate
– Spread from one location
to another
Frequency
– Number of action
potential produced per
unit of time
ALL-OR-NONE PRINCIPLE
-
i.e. Means that each time an AP arises, it always have a constant
and maximum strength and will therefore will be conducted along
the axon.
ANALOGY – A long row of dominoes, once you push the first
domino, all the dominos along the row will fall. So it either falls or
remains standing.
Calculating the membrane potential
Goldman equation
Vm=+_ 61 log (X)
X = P [K] +P
K
P
o
Na[Na]o
+PCl[Cl]i
PK[K]i +PNa[Na]i +PCl[Cl]o
permeability
Frequency of Action Potentials
Figure 8-13: Coding for stimulus intensity
Generator Potentials
In response to stimulus,
sensory nerve endings
produce a local graded
change in membrane
potential.
Potential changes are
called receptor or
generator potential.
– Analogous to EPSPs.
Phasic response:
Figure 10-2
Generator potential increases with increased stimulus, then
as stimulus continues, generator potential size diminishes.
Tonic response:
Generator potential proportional to intensity of stimulus.
cardiac action potential :
Fast
slow
dendrite
dendritddndrite
receives information
axon
Axon
transmits nerve impulse
Direction of nerve impulse
cell
Cell body
body
contains nucleus &
organelles
axon
Axonterminal
terminal
transmits to next neuron
synapse
junction between two neurons
synapse
Figure 44.2
Refractory Period
Two types
Absolute
When Na+ channels close, at peak of
AP, they do not reopen for a time
Relative
Membrane hyperpolarized
Some Na+ channels still refractory
ACTION POTENTIAL
PROPERTIES OF ACTION POTENTIAL;
1. REFRACTORY PERIODS (Resting periods)
2. CONDUCTION PATTERN
PROPERTIES OF ACTION POTENTIAL
REFRACTORY PERIOD – is the period of time that an excitable
tissue can not generate another AP
2 types of refractory period;
1. ABSOLUTE REFRACTORY PERIOD
2. RELATIVE REFRACTORY PERIOD
REFRACTORY PERIODS
1. ABSOLUTE REFRACTORY PERIOD
Refers to the time period during which a 2nd AP can not be initiated
even with a very strong stimulus
REASON;
Na+ inactivation gates still closed – they must first return to resting
state before they can get activated.
REFRACTORY PERIOD
2. RELATIVE REFRACTORY PERIOD
- Refers to the period during which a second AP can be initiated, but
only by a stimulus larger than threshold strength.
REASON:
- Voltage gated k+ channel are still open
- The inactivated Na+ gates have returned to their resting state – so
they are ready to be opened again as long as the stimulus is
strong enough (i.e. larger than threshold ~55mV)
Regulating the AP
Figure 8-12: Refractory periods
برای انسان های بزرگ بن بست وجود ندارد ،چون بر این باورند که :
یا راهی خواهند یافت ،یا راهی خواهند ساخت .
بسیاری در پیچ وخم یک راه مانده اند و همواره از خویشتن می پرسند :
ما چرا ناتوان از ادامه راهیم .بدانها باید گفت
می دانی در کجا مانده ای؟ همانجای که خود را پرمایه دانسته ای.
SALTATORY CONDUCTION
i.e. Conduction pattern where current leaps from node to node as each
nodal area depolarizes to threshold.
REASON:
- When a nerve impulse propagates along a myelinated fiber
the
current is carried by the flow of ions through the extracellular fluid
surrounding the myelin sheath
and through the cytosol from one
node to the next.
- But current flows across the membrane only at the nodes.
Synapse
• site of communication between two cells
• formed when an axon of a presynaptic cell
“connects” with the dendrites of a
postsynaptic cell
NERVE CELL
PARTS OF A NERVE CELL;
1. CELL BODY;
- Contains the nucleus of the nerve cell.
- Control center of the nerve cell
2. AXON;
- longest cytoplasmic extensionof nerve
- Conducts impulses away from nerve
3. DENDRITES;
- Short cytoplasmic extensions coming off
the nerve cell body.
- Receives incoming impulses
4. MYELIN SHEATH
- Neuroglia around nerve
- Provides metabolic, structural support
to nerve fiber