Membrane Potentials

resting potentials
1. is the membrane potential of a "resting" neuron i.e. a neuron not sending signals
2. in mammalian neurons at resting potential there is a higher concentration of K+inside the cells
  1. the concentration of Na+ is greater on the outside
3. the concentration gradient across the plasma membrane is maintained by the sodium-potassium pump
  1. constantly moves 3 Na+ out of the neurons and 2 K+ into the neuron
    1. there is some leakage of K+ out o the cell due to the electrochemical gradient created by the pump
    2. the build up of negative charge caused by this, is the major source of the membrane potential
4. voltage of -60mV
5. a neuron at rest contains many more open K+ channels than open Na+ channels
6. the net flow of K+ stops when there are too many K+ leaving the cell, this causes an electrical imbalance as there are too many negative charges inside
  1. at this point K+ begins to be pushed back into the cell and equilibrium is reached
  2. the membrane potential at this point is called the potassium equilibrium potential or EK
    1. this is the point at which K+ diffusion out due to the concentration gradient is balanced by it's movement in due to the negative electric potential
action potential
1. is produced when the membrane becomes depolarized
  1. in order for an action potential tobe generated it must be depolarized enough to pass the thresshold
    1. the threshold is -55mV, not muh higher than that of the resting potential (-66mV)
    2. an action potential occurs if a stimulus causes the membrane voltage to pass the threshold, this reaction is a brief all-or-none depolarisation of the neuron's plasma membrane
  2. hyperpolarisation can occur when the inside of the cell becomes more negatively charged, this has an inhibitory effect on the neuron as it moves the membrane potential further away from the threshold potential
2. postsynaptic potentials (PSPs) or graded potentials
  1. fall into two categories
    1. excitatory postsynaptic potentials
      1. a single EPSP is normally too small to trigger an action potential in a postsynaptic neuron
    2. inhibitory postsynaptic potentials
  2. temporal summation
    1. when two EPSPs are produced in rapid succession
  3. spatial summation
    1. when EPSPs are produced almost simultaneously by different synapses on the same postsynaptic neuron add together
  4. through summation an IPSP can counter act the effects of an EPSP
  5. the summation of different PSPs detremines whether or not the threshold will be reached and thus generate an action potential
3. conduction
  1. after an action potential has been generated a second one cannot be initiated, this is known as the refactroy period
    1. this ensures that an impulse moves allong the axon in only one direction
    2. the result of temporary inactivation of the Na+ channels, this means that an action potential cannot be generated regardless ofthe amount of stimulation
    3. the refactory period is a period of repolarisation during which the Na+/K+ pump restores the membrane it's original polarised condition
  2. Conduction speed
    1. the speed of an action potential is affected by the diameter of the axon
      1. wider the diameter the faster the the action potential travels
    2. vertebrates the axons are insulated with myelin sheaths, this increase the action potentials speed
      1. myelin sheaths are made from glia
        Anlagen:
        Cells of the Nervous System
    3. action potentials are formed only at nodes of Ranvier
      1. these gaps in the myelin sheath are where the Na+ channels are found
      2. action potentials in myelinated axons jump between the nodes of Ranvier in a process called Saltatory conduction
  3. an action potential can travel long distances by regenerating itself along the axon
  4. at the site where the action potential is generated an electrical current depolarises the neighbouring region of the axon membrane
    1. this site is usually the axon hillock
  5. inactivated Na+ channels behind the zone of depolarisation prevent action potentialls from travelling backwards
    1. action potentials are unidirectional they travel along the axon from the cell body toward the synaptic terminals
4. signals that carry information along axons
5. caused by the opening of Na+ channels resulting in an influx of Na+ ions causing a depolarisation of the membrane
every neuron has a voltage (difference in electrical charge) across it's plasma membrane - this is the membrane potential
messages are transmitted as changes in membrane potential
ions carry the electric current in the neurons
1. sodium (Na+)
2. Potassium (K+)
3. Calcium (Ca+)
4. Chloride (Cl-)
5. each ion has a specific channel
  1. the ions can diffuse in both direction depending on 2 gradients
    1. electrical gradient
      1. the voltage difference across the membrane
    2. chemical gradient
      1. concentration difference across the membrane
6. the net movement of ions across the membrane depends upon
  1. the electro-chemical gradient
  2. whether the gates of the specific ion channels are open or not
the Nernst equation
1. Eion/ the equilibrium point of a specific ion can be calculated using the Nernst equation
2. it uses the concentrations inside and outside the cell and the charge of the ion
  1. R = universal gas constant
  2. T = absolute temperature
  3. z = the charge of the ion
  4. F = the Faraday constant
3. the Goldman equation
  1. is used to calculate the real membrane potential
  2. it takes in account:
    1. all the ions that can diffuse across the membrane
    2. the relative permeability of the membrane to those ions
      1. relative permeabilities (p) are expressed as percentages
        pK = 1.0
        pNa = 0.05
        pCl = 0.45
4. 1. this is a table of predicted membrane potentials calculated using the Nernst equation - it uses the assumption that the membrane is permeable to only one type of ion
    1. the true resting potential of a neuron is -66mV therefore the resting potential must be due to the permeability of the membrane for more than one ion
  2. the actual resting potential is closer to EK (-75mV) than ENa (+56mV) this is because there are many more open K channels than Na channels
5. when the membrane conductance for a specific ion increases the membrane potential will move towrds the Nernst potential for that ion

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Membrane Potentials

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Medienanhänge

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