information by certain techniques is limited by time and space → overview of techniques is
needed to see what happens in the brain in different times and spaces
Cajal: neuron drawing, a neuron is a unity
→ information can flow between neurons
→ with this the brain can be studied as a network
timeline:
- single cell recordings were very important for Cajal in the beginning
- now we see more multicellular recordings, because we see things as a network
- 1873: Golgi method invention
- 1888: neuron doctrine
- 1929: development of EEG
Membrane potential (how does the neuron work)
- Hodgkin and Huxley
- squid, because of it’s long axons
what causes a membrane potential:
1. Separation between ions in and outside the cell
2. Excess negative charge inside the cell at rest
3. The ion fluxes across the membrane
4. The lipid bilayer structure of the membrane
- Membranes are lipid bilayers
- Membrane potential arises from a separation of positive and negative charges
across the cell membrane
- Excess charges are concentrated directly at the membrane (in- and outside)
- Each excess is only a tiny fraction of the total number of ions in/outside the cell
- More potassium (K) inside the cell, more sodium (Na) outside
- negative charges inside the cell
- positive charges outside the cell
- membrane potential is used for speed in the communication; rapid transport of
signals through CNS → you only have to open the gates to use the ion gradients.
- membrane potential: Vm = Vin - Vout
- resting membrane potential: -85 mV tot -60mV
- Polarization: potential at which Vin –Vout is not zero.
- Depolarization: loss of (negative) polarization, so Vm moves towards 0 mV
- Hyperpolarization: reinforced (negative)polarization, so Vm moves further away from
0 mV, becomes more negative than before
- Convention: direction of current flow is defined as direction of net movement of
positive charge.
2
,Ion channels
Ion channels are the gateway through the membrane (because membranes are lipid
bilayers, they are not permeable to molecules or water)
- have a selectivity for size and binding site
- open and close in response to electrical, mechanical
and chemical signals
- is very rapid
- is passive
- direction and magnitude of flux are dependent on
- electrostatic forces (across membrane)
- concentration difference (in vs outside cell)
- ion channels can be:
- open
- closed
- inactive / refractory: is open, but blocked. A way to control channels
different types
1. ligand gated: sensitive to chemical substance, e.g. transmitter or olfactory stimulus
2. phosphorylation gated: ATP needed
3. voltage gated: sensitive to transmembrane voltage
4. mechanically gated: sensitive to pressure or stretch / volume change
5. resting channels: non-gated, normally open as leak current to keep the neuron at a
resting membrane potential
Equilibrium potential
Equilibrium potential: potential at which chemical driving force == electrical driving force →
no net flux of ions (Nernst equation)
Resting potential is determined by a weighted sum of the equilibrium potentials, where the
weight is determined by the relative permeability of the ion
3
, Steady-state condition with passive current flow balanced by active ion transport by the
Na+/K+ pump.
steady-state: no net current flow into or from cell (but ATP needed to maintain) → stable
membrane potential
Local signalling
3 passive electrical properties:
1. input resistance
a. change in electric current and transmembrane voltage
2. membrane capacitance
a. Phospholipid bilayer has the ability to (transiently) “store” electrical charges
depending on membrane potential.
3. axial resistance
a. in cytoplasm
myelination: glial cell membranes are tightly wrapped around axons, the thicker the
insulation, the lower the capacitance → faster spreading to neighboring regions
node of ranvier: amount of current flowing down axon from axon hillock is not enough to
discharge capacitance along the entire length of the axon → nodes of Ranvier prevent the
action potential from dying out.
Action potentials
Time course of events:
1) Initial depolarization needed
2) Na+ channels open
3) Further depolarization of membrane
4) More Na+ channels open → more depolarization (positive feedback loop, regenerative)
5) K+ channels start to open: repolarizing effect
6) Na+ channels inactivate
7) K+ conductance outlasts Na+ conductance: hyperpolarization
8) Refractory period → impossible or very difficult to excite membrane
- two things need to happen at the same time to reach hyperpolarization;
- sodium (Na) channels close
- potassium (K) channels open
K+ conductance involved in AP:
- Grows with stronger depolarization
- Does not inactivate
4
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