membrane potential, action potencial, sensory receptors · 2020. 4. 30. · dentristy 2016/2017...
TRANSCRIPT
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Dentristy 2016/2017
Membrane potential, action potencial,
sensory receptors
STRUCTURE OF THE CELL MEMBRANE, BIOLOGICAL MEMBRANES
The fundamental functional unit of the living organisms is the cell which internal milieu
divided by the cell membrane from the external environment. The structure of the cell
membrane shows similarity in every cell type.
The elemental structure of the biological membranes consist of phospholipide bilayer
(5-10 nm) The two main component of the membrane are the lipids responsible for
fluidity and the proteins which determine the rigidity. There are non-covalent, strong
interaction between the components of the membrane. The fluidity of the membrane
depends on the ratio of saturated and unsaturated fatty acid side chains. There are
strong interaction between the phospholipide molecules consisting saturated fatty acid
whereas by reason of bending at the location of the double bond which can be found in
the unsaturated fatty acids the structure will be loosening. The membrane become
disordered, but the cholesterol - consisting of sterane frame - and the membrane
proteins make the membrane more rigid.
The phospholipide bilayer consist of a polar head group (hydrophil) which is
orientated to the water phase, whereas the apolar tail group (hydrophobe) is located
farther from the water phase. The strength of the interactions between the hydrophobe
fatty acid side chains of the membrane lipids are influenced by several agent. As the
temperature is increasing the strength of the hydrophobe interaction is increasing as
well. The more the number of the carbon atoms in the apolar region the stronger is the
interaction. Multiple (second-, triple) bondings also elevate the strength of the
interaction.
The components of the phospholipid bilayer
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The glycolipids which translocated on the surface of the membrane and together with
the glycoproteins are able to determine the specific antigens. The lipids which are
turning to the inner side of the membrane can interact with other membrane proteins
responsible for cell signaling between the extra-, and intracellular space.
Shematic structure of the biological membrane
The proteins are important components of the biological membranes. They can be found
on the exterior and interior surface as well as can be integrated in the membranes.
Integrated transmembrane proteins are responsible for sensing, binding and
transporting the molecules which are important for the cells. The biological membranes
are working as semipermeable membrane.
o proteins channels (provides transport for water and ions)
o transport molecules (Carrier proteins)
o peripheral proteins linked to other proteins
o linker proteins providing interaction between the extracellular matrix and
the actin cytoskeleton
Marker proteins: peripheral proteins on the external surface which are specific for
specimen or tissue.
Receptor proteins recognise specific patterns, e.g. bacterial cell surface antigens
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A „fluid mosaic” modell
Modell of the structure of the membranes by 1972 S.J. Singer és Garth L. Nicolson
Fluid – lateral movement of the components
Mosaic – the mosaic-like arrangements of the macromolecules
THE MAIN COMPONENTS OF THE INTRA-AND EXTRACELLULAR SPACE
Water Ions
o Kations (K+, Na+, Ca2+) o Anions (Cl-, H2PO4− és HPO42− ions)
Proteins o Mainly intracellular localization o Negatively charged polyvalent (having more than one valence)
macromolecules (pH! – isoelectric point)
RESTING MEMBRANE POTENTIAL
The electrical potential difference (voltage) across a cell's plasma membrane. (V).
Its value varies in different cell types (-100 mV > Uresting < -30 mV)
Forces controlling the movements of charged particles: electro-chemical potential
Chemical potential energy:
The chemical potential of a thermodynamic system is the amount of energy (Joule) by which the system would change if an additional particle were introduced (~ number of the particles!)
Concentration gradient → diffusion: moving the particles through the permeable membrane from a high concentration area to a low concentration area → diffusion potential.
Electric potential
Energy of the charged particle in electric field. An electric field creates a force that can move the charged particles (the work of the electric field) → moving charged particles = electric current.
Electro-chemical potential the combination (sum) of the chemical and the electric potential energy.
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BERNSTEIN’A POTASSIUM HYPOTHESIS (1902)
For creation of the resting membrane potential potassium could be responsible. Mobility of potassium is possible:
o The cell membrane is selectively permeable to potassium: o Ca2+ sensitive potassium channels o Inwardly rectifying potassium channels o Voltage-gated potassium channels o “Tandem pore domain potassium channel” – “leak channel”
1952: Hodgkin and Huxley suggested the leakage of current First description: Ketchum, KA; Joiner, WJ; Sellers, AJ; Kaczmarek, LK;
Goldstein, SA. (1995) A new family of outwardly rectifying potassium channel proteins with two pore domains in tandem. Nature, 376 (6542): 690-5
The intracellular potassium concentration is high and the extracellular potassium concentration. is low.
NERNST EQUATION
Equilibrium potential: What membrane potential (E) can compensate (balance) the concentration gradient (X1 /X2).
The inward and outward flows of the ions are balanced, are in dynamic equilibrium.
Results does not match the experimental results: the ions are not independent of each other and are not a closed system.
𝐸=𝑅𝑇
𝑧𝐹ln
𝑐1
𝑐2
DONNAN POTENTIAL
Donnan equilibrium or Donnan distribution
Diffusion of ions with altering mobility (K+, Cl-) through a semipermeable membrane results in diffusion potential. If one of the charged particle (intracellular proteins) cannot diffuse through the membrane equilibrium concentration difference will be created between the two sides of the membrane
GOLDMAN-HODGKIN-KATZ EQUATION
To determine the potential across a cell's membrane taking into account all of the ions with different PERMEABILITIES through the membrane. The membrane potential is the result of a „compromise” between the various equilibrium potentials, each weighted by the membrane permeability and absolute concentration of the ions.
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Considering number N positive and number N negative ions:
.
•Em = membrane potential
•Pion = permeability concerning each ions
•[ion]out = extracellular concentration
•[ion]in = intracellular concentration
•R= universal gas constant
•T= absolute temperature
•F= Faraday constant
NA-K ATPASE
Developing the resting membrane potential the main roles have K+ and Na+ ions (different distribution and permeability). The concentration of Na+ is higher extracellularly, while the concentration of K+ is higher intracellularly. The Na-K pumps works against these concentration gradients using ATP (3 Na+ out; 2 K+ into).
Its work plays essential role establishing membrane potential.
TYPES OF SODIUM AND POTASSIUM CHANNELS
The ion channels composed of transmembrane transport proteins which can be divided into two groups.
Types of sodium channels
● Ligand-gated sodium channels
● Voltage-gated (sensitive, dependent) sodium channels
o contains a voltage sensor (Sensitive, dependent) to voltage changes in the membrane potential)
o activation gate (extracellular side)
o inactivation gate (intracellular side)
● Ca2+ sensitive (KCa)
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● Inwardly rectifying (KIR)
● “Tandem pore domain potassium channel” – “leaking channel” (K2p)
● Voltage-gated potassium channels (KV )
o Sensitive (dependent) to voltage changes in the membrane potential
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).
It is a result of ions moving through the membranes.
The local membrane potential changes upon stimulus. If the change reaches a threshold (threshold potential), then action potential occurs.
Action potentias happen in different phases (depolarisation and repolarisation).
Action potentials are „all or none” phenomena: any stimulation above the voltage
threshold results in the same action potential response. In any stimulation below the
voltage threshold will not result action potential response.
Phase of the action potential
Resting potential, condition of the voltage-gated Na channels (NaV):
o Aktivation gate is closed
o Inactivation gate is opened
Depolarization (increasing phase)
o Due to the stimulus over the threshold potential the voltage-gated Na
channels open
Activation gate is opened
Inactivation gate is closed
o Na+ influx occurs in the cell followed by positively charged extracellular milieu
Peak phase
o Na+ influx become slower
alteration of the membrane potential is getting closer to the value of
the equilibrium potential of Na+ (EmV_Na+ = +45,1 mV it can be
calculated by Nernst-equation)
part of the Na+ channels are inactivated
o K+ channels start to open
Repolarization (decreasing phase)
o Voltage-gated K+ channels totally open leading to great K+ out flow from the
cell
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o Condition of the voltage-gated Na+ channels:
Activation gate is opened
Inactivation gate is closed refracter periode
Undershoot (hyperpolarization)
o K+ influx become slower
alteration of the membrane potential is getting closer to the value of
the equilibrium potential of K+ (EmV_K+ = -101,2 mV it can be
calculated by the Nernst equation)
K+ channels totally close
o the slowly inactivating great number of K+ channels lead to hyperpolarization
Regeneration after action potential
o alteration of the absolute value of the intracellular ion concentration is low
during the action potential (0.0001% - 1% in thick and thin axons).
o Na-K ATPase set back the resting potential but not immediatly
Refracter periode
During the refracter periode emerging of a new action potential is partly inhibited
o Absolute refrakter periode – the occurance of a new action potential is totally
inhibited since the Na+ channels are inactive state
o Relative refrakter periode – the greater the depolarization than the threshold
potential can trigger the action potential
Source:
Biofizika előadás jegyzet
http://www.tankonyvtar.hu/hu/tartalom/tamop425/0019_1A_Elettani_alapismeretek/ch02.
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http://sotepedia.hu/aok/targyak/orvosi_elettan_a-d/jelatvitel_v
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