Download Sensory receptors Action potential

Sensory receptors
Action potential
26.09.2012.
Sensory receptors
Special type of cells which can collect and transfer different
informations from the environment.
Function: „translation”, signal processing.
Receptors
(sensory organs)
Nerve fibre
Central nervous system
stimulus: (physico-chemical) effect from the environment.
→ metabolic changes inside the cells induced by a stimulus.
sensation: will involve the work of the CNS.
Classification of sensory receptors
Type of stimulus
• light-photoreceptors
• temperature-thermoreceptors
• pressure-mechanoreceptors…
Localisation of the receptors
• head, ear…
The origin of the information
• Exteroceptor (informations form the environment)
• Interoceptor (informations from the body)
• Proprioceptor (position of the different parts of the body)
Work of the receptors I.
!
Receptors
(sensory organs)
stimulus
Nerve fibre
Central nervous system
Local change of the receptor-potential
thresholdpotential
Action-potential
frequency ~ strength of the stimulus
„all or nothing”
Work of the receptors II.
Φ
(Strength of the) stimulus
Φthreshold
Below
the
threshold
level there is
no sensation
(no
action
potential).
t
Urec
receptor
Uthreshold
t
Uaction
Action-potential
0 mV
-70 mV
t
Nerve fibre
The origin of the resting membrane
potential
Bernstein potassium hypothesis
Nernst-equlibrium potential (electro-chemical potential)
Donnan equlibrium: the membrane is impermeable for
some components (e.g. intracellular proteins).
Goldman equation: The membrane potential is the result
of a „compromise” between the various equlibrium
potentials, each weighted by the membrane permeability
and absolute concentration of the ions.
Equlibrium potential
Nernst equation: What membrane potential
(E) can compensate (balance) the
concentration gradient (X1/X2).
RT X 1
E=
ln
zF X 2
The inward and outward flows of the ions are balanced
(net current = zero → equilibrium = stable, balanced, or unchanging
system).
Ionic concentrations inside and
outside of a muscle cell
battery
Na+ : 120 mM
Na+ : 20 mM
K+ : 2.5 mM
K+ : 139 mM
Cl- : 120 mM
Cl- : 3.8 mM
[K+] ⇒ EmV = -58/1 log (139/2.5) = - 101.2 mV
[Na+] ⇒ EmV = -58/1 log (20/120) = + 45.1 mV
[Cl-] ⇒ EmV = -58/1 log (3.8/120) = + 86.9 mV
= 30.8 mV
EmV=-92mV
Passive or leakage channels
→ a membrane-potential is not equal with
any of the equlibrium potentials for the
different ions
EmV_K+ = -101.2 mV
EmV_Na+ = +45.1 mV
EmV_Cl- = +86.9 mV
EmV= - 92mV
→ the ions are trying to move through the
membrane ⇒ „leakage”
„Leakage channels”
K+
K+
Na+
+
Na
• Slow movement of the ions.
• Has to be compensated somehow.
Na-K ATPase
The passive flux of Na+ and K+ (leakage) is
balanced by the active work of Na-K pump
→ contribution to the membrane potential.
3 Na+ move out vs. 2 K+ move in (exchanger)
ATP (energy source) is needed
Na-K ATPase
Sodium/potassium pump
active transport → ATP hydrolysis - transport the ions against
their concentration gradient (antiport transport ↔ symport).
Antiport transport mechanism: a coupled movement of two compounds in opposite direction.
Symport transport mechanism: a coupled movement of two compounds in the same direction.
http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.html
Potassium channels
●
Ca2+ sensitive potassium channels (KCa)
●
Inwardly rectifying potassium channels (KIR)
●
“Tandem pore domain potassium channel” – “leak channel” (K2p)
●
Voltage-gated potassium channels (KV)
Sensitive (dependent) to voltage
changes in the membrane potential
Islas LD, Sigworth FJ. Voltage sensitivity and gating charge in Shaker and Shab family potassium channels. J Gen Physiol. 1999 Nov;114(5):723-42.
Sodium channels
● Ligand gated sodium channels
● Voltage gated (sensitive, dependent) sodium channels
contains a voltage sensor
Sensitive (dependent) to voltage
changes in the membrane potential
Planells-Cases R, Caprini M, Zhang J, Rockenstein EM, Rivera RR, Murre C, Masliah E, Montal M. Neuronal death and perinatal lethality in voltage-gated sodium channel alpha(II)-deficient
Bernstein,J.(1902).Untersuchungen
zur Thermodynamik der bioelektrischen Strome. Pflugers Arch.ges. Physiol. 92, 521–562.
mice.
Biophys J. 2000 Jun;78(6):2878-91.
Sodium channels
● Ligand gated sodium channels
● Voltage gated (sensitive, dependent) sodium channels
contains a voltage sensor
Sensitive (dependent) to voltage
changes in the membrane potential
activation gate (extracellular side)
inactivation gate (intracellular side)
Planells-Cases R, Caprini M, Zhang J, Rockenstein EM, Rivera RR, Murre C, Masliah E, Montal M. Neuronal death and perinatal lethality in voltage-gated sodium channel alpha(II)-deficient
Bernstein,J.(1902).Untersuchungen
zur Thermodynamik der bioelektrischen Strome. Pflugers Arch.ges. Physiol. 92, 521–562.
mice.
Biophys J. 2000 Jun;78(6):2878-91.
Action potential
Action potential: a momentary reversal of membrane potential (- 65mV
to + 40 mV) that will be followed by the restoration of the original
membrane potential after a certain time period (1-400ms).
Action potentials happens in different phases (depolarisation and
repolarisation).
Action potentials are triggered by the depolarization of the membrane if it
can reach a critical value (voltage threshold).
Action potentials are all or none phenomena
any stimulation above the voltage threshold results in the same action
potential response.
any stimulation below the voltage threshold will not result action
potential response.
Membrane potential (mV)
Action potential (nerve cell)
overshoot
~+40
0
Falling phase
Rising phase
Voltage threshold
~-50
Resting potential
~-70
undershoot
Stimulus
0
1
2
time (ms)
3
4
Equlibrium situation
NaV channels:
activation gate closed
inactivation gate open
Membrane potential (mV)
Resting phase
overshoot
~+40
0
Rising phase
Falling phase
Voltage threshold
~-50
Resting potential
~-70
Stimulus
0
1
undershoot
2
time (ms)
3
The voltage-gated sodium
channels will open-up if the
voltage threshold reached by a
stimulus
NaV channels:
activation gate open
inactivation gate open
→ Na+ will move into the cell
→ the inner surface of the cell
will be positively charged
Membrane potential (mV)
Rising phase (depolarization)
overshoot
~+40
0
Rising phase
Falling phase
Voltage threshold
~-50
Resting potential
~-70
Stimulus
0
1
undershoot
2
time (ms)
3
The movement of the Na+
will slow down
EmV_Na+ = +45.1 mV
(Nernst
–equlibrium
potential)
Na+ channels will start to
form
an
inactive
conformation
K+ channels are starting to
open-up
Membrane potential (mV)
Overshoot
overshoot
~+40
0
Rising phase
Falling phase
Voltage threshold
~-50
Resting potential
~-70
Stimulus
0
1
undershoot
2
time (ms)
3
All the voltage gated K+
channels are open
K+ move out from the cell
NaV channels:
activation gate open
inactivation gate closed
→ refractory period
Membrane potential (mV)
Falling phase (repolarization)
overshoot
~+40
0
Rising phase
Falling phase
Voltage threshold
~-50
Resting potential
~-70
Stimulus
0
1
undershoot
2
time (ms)
3
The movement of the K+
ions will slow down
EmV_K+ = -101.2 mV
(Nernst-equlibrium
potential)
The K+ channels will get
into a closed conformation
The numerous and slowly
inactivating K+ channels
will
cause
some
hyperpolarisation
Membrane potential (mV)
Undershoot (hyperpolarization)
overshoot
~+40
0
Rising phase
Falling phase
Voltage threshold
~-50
Resting potential
~-70
Stimulus
0
1
undershoot
2
time (ms)
3
Membrane potential (mV)
Resting phase
overshoot
~+40
0
Rising phase
Falling phase
Voltage threshold
~-50
Resting potential
~-70
Stimulus
0
1
undershoot
2
time (ms)
3
Recovery after the AP
Intracellular ion concentrations change only a
small amount with each AP (0.0001% - 1%).
Na+/K+ ATPases will slowly restore the original
ion concentrations.
If the Na/K ATPases of a squid giant axon is
poisoned, it can still generate 100,000 impulses
while the internal sodium concentration is
increased only by 10%.
Propagation of the action potential
Action potentials can propagate:
the AP at one site on the membrane can itself
initiate an AP at a neighboring region.
Unidirectional movement because of the refractory
period – inactivated Na+ channels.
The electrical signal can propagate down the axon
without a decrease in amplitude.
Refractory period
The cell is resistant to a stimulus. It is hard to get a response (action potential).
Membrane potential (mV)
Absolute refractory period:
The formation of a new AP is totaly
blocked
Relative refractory period: larger
depolarisation is needed than the
threshold to initialize an AP
overshoot
~+40
0
Rising phase
Falling phase
Voltage threshold
~-50
Resting potential
~-70
0
Stimulus
1
time (ms)
undershoot
2
3
4
Action potential in the cardiac muscle cells
K+ and Cl- ion movement
Membrane potential
plateau phase
Equlibrium between the Ca2+ release and
the outward movement of the K+ ions
~+35mV
depolarisation
•Na+ moving into the cell
K+ ion moving
out from the cell
~-90mV
0 ms
350 ms
Resting membrane potential
Plateau: can help to direct the flow of the APs.
time (ms)
Resting membrane potential
The end!!!