ET: Neurons Flashcards
(47 cards)
Structure of a Neuron
- Soma (cell body)
- Dendrites
- Usually a single axon
Electrical Signals
- Input
- Dendrites, cell body, axon
Chemical Signals
- Output
- Synapses
Resting Membrane Potential (RMP)
- Voltage measured across a cell membrane
- Typically -65mV
- Potential out of the cell is defined as 0
- Can be measured with intracellular micro electrodes or patch-clamp pipettes
- Based on differences in concentration of Na+ and K+ inside and outside of the cell
- The differences in permeability of cell membrane to these ions
- Electrogenic action of Na/K Pump (small)
Channels affecting permeability
- Non-gated channels (leak) that are open at rest
- In Glia cells the RMP almost entirely depends on leak K+ channels - Gated-channels (voltage or ligand) which are closed at rest
Rest Permeability of K:Na
P(K):P(Na) = 40:1
- Steady diffusion of K+ into the cell and Na+ out of the cell
Glass Micro electrode technique
Glass electrodes with a very fine tip filled with a concentrated salt solution can be inserted into a neuron membrane to read the voltage changes
Patch-Clamp technique
Patch pipette is larger and only can touch the outside of the neuron membrane
- Uses negative pressure pulse to break and then reseal the membrane to measure current and voltage
Concentration of Na+ and K+ outside the cell
Na+ outside = 150mM
K+ outside = 5mM
Concentration of Na+ and K+ inside the cell
Na+ inside = 15mM
K+ inside = 100mM
Equilibrium Potential
When net flow of ions is 0 in spite of concentration gradient and permeability
- Nernst Equation is used to calculate equilibrium potential for each individual ion that contributes to RMP
Nernst Equation
Nernst Equation is used to calculate equilibrium potential for each individual ion that contributes to RMP
- Only applicable wen the cell membrane is permeable to only one ion (e.g. glia cells only have for K+)
- No permeability included
E(ion) = 2.3 x (RT/zF) x log ([ion]o/[ion]i)
Nernst Equation for K
E(K) = 61.5mV x log ([K+]o/[K+]i)
Ek = -80mV
Nernst Equation for Na
E(Na) = 61.5mV x log ([Na+]o/[Na+]i)
Ek = +60mV
RULE 1
The higher the permeability of the cell membrane to a particular ion, the greater the ability of this ion to shift the RMP towards its equilibrium potential
Goldman Equation
Method for calculating the value of RMP taking into account both the concentration gradient and relative permeability of the resting cell membrane to K+ and Na+
Vm = 61.5mV x log{ (Pk[K+]o + Pna[Na+]o) / (Pk[K+]i + Pna[Na+]i) }
Action Potential (AP)
- Brief fluctuation in membrane potential
- Information is coded in the frequency of AP
4 Stages of an AP
- Depolarisation to threshold
- Cell membrane is depolarised from around -65mV to -55mV by a physical (electric current, light, stretch) or chemical (drug or synaptic excitation) stimulus
- Depolarisation to threshold is evoked by Excitatory Post Synaptic Potentials (EPSP’s) which spread passively from dendrites
- Voltage gated Na+ channels begin to open - Fast depolarisation (toward ENa+)
- MP moves from around -55mV to around +30mV
- Voltage gated Na+ channels open allowing fast influx of Na+ into the cell - Repolarisation (toward EK+)
- Inactivation of Na+ channels and activation of voltage-gated K+ channels
- Bringing MP down below starting (below -65mV) - After hyper polarisation (AHP)
- Returns to RMP
Absolute Refractory Period
- Stages 2 and 3 of an AP
- Has this so neuron cannot react to another stimulus straight after the first action potential
- Ensures AP travels in one direction towards axon
Relative Refractory Period
- Stage 4 of an AP
- Stops damage of ion gradients in neuron
- Another AP can be generated at this time but will have to be stronger to reach threshold
Location of AP
- Generated in the axon initial segment (axon hillock) which has the lowest threshold (i.e. trigger zone for AP)
- Transmitted actively along the axon, away from the soma
- Dendrites are covered with synaptic ending form other neutrons receiving the synaptic currents
RULE 2
When current generated by an outside source flows through the cell membrane from the outside to the inside there will be hyper polarisation (MP more negative)
When current generated by an outside source flows through the cell membrane from the inside to the outside there will be depolarisation (MP more positive) initiating an AP
Activation Gate
Voltage sensor that responds to small changes in RMP and opens allowing Na+ influx
- Middle of protein
Inactivation Gate
Voltage sensor that responses to large changes in the membrane potential and shuts the Na+ channels when the inside of the cell becomes too positive
- On the inside of the cell
