1 Sensory receptors Action potential 12.11.2014. Sensory receptors Phases of the action potential Changes of ion fluxes corresponding to the different phases Sensory receptors Special type of cells which can collect and transfer different informations from the environment. Function: „translation”, signal processing. Nerve fibre Receptors 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.
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Sensory receptors Action potential - Pécsi Tudományegyetem potential 2014-… · Action potential in the cardiac muscle cells Resting membrane potential depolarisation •Na + moving
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Sensory receptorsAction potential
12.11.2014.
� Sensory receptors
� Phases of the action potential
� Changes of ion fluxes corresponding to the different phases
Sensory receptors
Special type of cells which can collect and transfer different informations from the
environment.
Function: „translation”, signal processing.
Nerve fibreReceptors 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.
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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.
� !
Local change in the receptor-potential
thresholdpotential Action-potential
frequency ~ strength of the stimulus
stimulus
„ALL OR NOTHING”
Nerve fibreReceptors Central nervous system
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Work of the receptors II.
t
Φ
Φthreshold
t
Urec
Uthreshold
t
Uaction
(Strength of the) stimulus
receptor
Nerve fibre-70 mV
0 mV
Action-potential
Below thethresholdlevel there isno sensation(no actionpotential).
�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 permeabilityand absolute concentration of the ions.
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Equlibrium potential
Nernst equation: What membrane potential (E) can compensate (balance) the
concentration gradient (X1/X2).
2
1lnX
X
zF
RTE =
The inward and outward flows of the ions are balanced(net current = zero → equilibrium = stable, balanced, or unchangingsystem).
Ionic concentrations inside and outside of a muscle cell
[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
Na+ : 120 mM
K+ : 2.5 mM
Cl- : 120 mM
Na+ : 20 mM
K+ : 139 mM
Cl- : 3.8 mM
EmV=-92mV= 30.8 mV
Adjustablepower source
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→ a membrane-potential is not equal with
any of the equlibrium potentials for thedifferent ions� EmV_K+ = -101.2 mV
� EmV_Na+ = +45.1 mV
� EmV_Cl- = +86.9 mV
→ the ions are trying to move through the
membrane ⇒ „leakage”
Passive or leakage channels
EmV= - 92mV
„LEAKAGE CHANNELS”
K+
Na+Na+
K+
• Slow movement of the ions.
• Has to be compensated somehow.
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� The passive flux of Na+ and K+ (leakage) is balanced by theactive work of Na-K pump → contribution to the membranepotential.
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.
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K+ channel (PDB: 1K4C)
membrane
Filter region
Gate
Interior of cell
K+ channel (PDB: 1K4C)
O2 K+
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Sodium channels
Bernstein,J.(1902).Untersuchungen zur Thermodynamik der bioelektrischen Strome. Pflugers Arch.ges. Physiol. 92, 521–562.
● 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 mice. Biophys J. 2000 Jun;78(6):2878-91.
Bernstein,J.(1902).Untersuchungen zur Thermodynamik der bioelektrischen Strome. Pflugers Arch.ges. Physiol. 92, 521–562.
● 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 mice. Biophys J. 2000 Jun;78(6):2878-91.
Sodium channels
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� Action potential: a momentary reversal of membrane potential (- 65mV to +40 mV) that will be followed by the restoration of the original membranepotential after a certain time period (1-400ms).
� Action potentials happens in different phases (depolarisation andrepolarisation).
� Action potentials are triggered by the depolarization of the membrane if itcan reach a critical value (voltage threshold).
� Action potentials are all or none phenomena
� any stimulation above the voltage threshold results in the same actionpotential response.
� any stimulation below the voltage threshold will not result action potentialresponse.
Action potential
Resting potential
Action potential (nerve cell)
Mem
bran
e po
tent
ial (
mV
)
time (ms)
0 1 2 3 4
Stimulus
Voltage threshold
~-70
~-50
0
~+40overshoot
Rising phase Falling phase
undershoot
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Equlibrium state
NaV channels:
� activation gate closed
� inactivation gate openResting potential
Voltage threshold
time (ms)
0 1 2 3
StimulusMem
bra
ne
po
ten
tial
(mV
)
~-70
~-50
0
~+40overshoot
Rising phase Falling phase
undershoot
Resting phase
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
Resting potential
Voltage threshold
time (ms)
0 1 2 3
StimulusMem
bra
ne
po
ten
tial
(mV
)
~-70
~-50
0
~+40overshoot
Rising phase Falling phase
undershoot
Rising phase (depolarization)
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� The movement of the Na+
will slow down
� EmV_Na+ = + 45.1 mV
(Nernst – equilibrium
potential)
� Na+ channels will start to
form an inactive conformation
� K+ channels are starting to
open-up
Resting potential
Voltage threshold
time (ms)
0 1 2 3
StimulusMem
bra
ne
po
ten
tial
(mV
)
~-70
~-50
0
~+40overshoot
Rising phase Falling phase
undershoot
Overshoot
� All the voltage gated K+
channels are open� K+ move out from the
cell� NaV channels:
� activation gate open
� inactivation gate
closed→ refractory period
Resting potential
Voltage threshold
time (ms)
0 1 2 3
StimulusMem
bra
ne
po
ten
tial
(mV
)
~-70
~-50
0
~+40overshoot
Rising phase Falling phase
undershoot
Falling phase (repolarization)
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� The movement of the K+
ions will slow down� EmV_K+ = -101.2 mV
(Nernst-equlibriumpotential)
� The K+ channels will getinto a closed conformation
� The numerous and slowlyinactivating K+ channels willcause somehyperpolarisation
Resting potential
Voltage threshold
time (ms)
0 1 2 3
StimulusMem
bra
ne
po
ten
tial
(mV
)
~-70
~-50
0
~+40overshoot
Rising phase Falling phase
undershoot
Undershoot (hyperpolarization)
Resting phase
Resting potential
Voltage threshold
time (ms)
0 1 2 3
StimulusMem
bra
ne
po
ten
tial
(mV
)
~-70
~-50
0
~+40overshoot
Rising phase Falling phase
undershoot
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Recovery after the AP
� Intracellular ion concentrations change only a small amountwith each AP (0.0001% - 1%).
� Na+/K+ ATPases will slowly restore the original ionconcentrations.
� If the Na/K ATPases of a squid giant axon is poisoned, itcan still generate 100,000 impulses while the internalsodium concentration is increased only by 10%.
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
Resting potential
Refractory period
Mem
bran
e po
tent
ial (
mV
)
time (ms)
0 1 2 3 4Stimulus
Voltage threshold
~-70
~-50
0
~+40overshoot
Rising phase Falling phase
undershoot
The cell is resistant to a stimulus. It is hard to get a response (action potential).
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Action potential in the cardiac muscle cells
Resting membrane potential
depolarisation•Na+ moving into the cell
plateau phaseEqulibrium between the Ca2+ release and the outward movement of the K+ ions