3. Neurophysiology

movement of ions

• ions are always enveloped by a hydrate layer

• dipole characteristics of water molecules

Voltage(U) = R * Current(I)

Voltage = elektrische Spannung in Volt

Current = elektrischer Strom in Ampere

R = elektrischer Widerstand in Ohm

C = e0*e*A*d^-1

e0= dielectricity constant of vacuum

e = dielectricity constant of the membrane

A = membrane area

d = membrane thickness

thinner membrane -> larger membrane area & higher the dielelectricity constatn -> more changes can be stored

• major drain of energy in an organism

• for each ATP-> 3 sodium ions(Na+) to outside & 2(K+) postassium-ions inside

• maintain major chemical gradients in cells

• also called ATPase

• transmembrane proteins that form a hydrophilic pore across the membrane

• charged amino acids in center of large 3D protein that form a charged tunnel which allows ions to pass

• both

• if selective ->entrance of channel carries charges that repel the wrong charges & center of pore has diameter that restrict the passenger further

• results from a change in the 3D-dim form of the protein

1. binding of a ligand

2. mechanical forces

3. changes in electric field

System with 2 chambers & 1 semipermeable membrane

1. freely diffusable salt is put into 1 chamber -> equilibrium

2. 1 side with non-diffusable anion is added -> anion will push out other anions & kepp cations on its side -> both side neutral because still equal num of pos an dneg charges

   -> for given ion this diff. is theromdynamically bad (2.law -> all substances reach homogeneous distribution in space)

1. if k crosses membrance, it decreases its chemical gradient across membrane

2. charge gradient is built up -> voltage across membrane -> the more k ions cross membrane the higher voltage gete -> becomes major hindrance of further K ions to cross

3. given equilibrium point : chemical gradient & voltage gradient cancel each other out -> resulting in zero netto-diffusion across membrane

equate osmotic work and electrical work

Wel = nFE

Wosm = RT*ln(K+a/K+i)

-> E = (RT/nF)ln(K+a/K+i) (Nernst equitation)

R = gas costant; T = temperature in Kelvin; n = number of charges per ion; F = Faradayconstant

E = 58log(Kation+a/ Kation+i)

Voltage one ion would want the membrane to be at so its a equilibrium

The actual membrane voltage resulting from all relevant ions, weighted by permeability.

conductance

Because resting permeability of membrane for potassium is ca 100 times the resting pot. for sodium

nernst potentials of all ions & their permeability

between the nernst potential of ions

• voltage across the membrane is altered

• open at specific voltage value (threshold)

• above -> channel open, below -> closed

• possesses 2 gates

• 1 is controlled by voltage across membrane

• if gat opened -> triggers the closing of 2 gate (1ms afterwards)

• refreshing channel takes time

Because permeability change

Small permeability changes for any ion lead to small changes of the membrane potential

at certain threshould all voltage-gated sodium channel open at once & shortly afterwards

At the peak, the membrane is mostly permeable to sodium, which is why the potential is close to the sodium equilibrium potential

a = resting potential

b = depolarization

c = upstroke

d = plateau

e = repolariztaion & afterhyperpolarization

• refractory period where cell is first not excitable & theny only by strong depolarization

1. A neuron is at resting potential.

2. Sodium channels open (e.g., by binding a ligand such as a neurotransmitter)

3. A sodium influx results that depolarizes the neuron.

4. As soon as the threshold of the first voltage-gated (vg) sodium channel is reached, it opens and additional sodium flows in that depolarizes the cell further.

5. 5. A positive feedback couples opening of vg sodium channels, sodium influx and further depolarization and leads to a fast increase in sodium permeability.This eads to the upstroke of the action potential.

6. 6. The voltage-gated sodium channels close after ca 2 msec (GNa goes back to 0).

7. Now potassium permeability is increased through opening of delayed vg potassium channels that were activated in parallel to the vg sodium-channels.

8. Inactivation of vg sodium channels and delayed opening of vg potassium channels lead to a repolarization.

9. After the action potential, the membrane potential temporarily falls beneath theregular resting potential, since both leak channels and vg channels for potassium are open; this is called an afterhyperpolarization.

10. Only after the vg potassium channels have closed after a delay (closing caused by the voltage returning to resting potential, the former resting potential is reached.

Delayed closing of the v.g. potassium channels

• too few vg sodium channels are regenerated

• positive feedback can not occur

• cell is non ecxitable

• num of vg sodium channels that can again be activated is high enough to sustain pos reinforcement

• cell needs to be depolarized stronger & resulting sction potential is smaller than regular action potential

1. Strömchentheorie (Hermann, 19th century): Early idea that electrical "currents" from excited nerve areas flow to neighboring regions, triggering further excitation.

2. Modern view: This local current flow contributes to the propagation of the action potential along the axon.

3. No backward travel: The previously excited area is in the refractory period, so it cannot be re-excited immediately — ensuring one-way signal flow.

1. passive

   • not regenerative & decreases with time and distance

   • conduction velocity is fast

2. actively regenerated potential

   • gen. by vg channels of neuron

   • propagation of action potential slow (signal needs to be regenerated at each membrane)

1. Neurotransmitter binds to ligand-gated sodium channel

2. through the open channels, sodium flows into the cell

3. the current spreads through the cell

4. signal strength decreases with distance in the cell

• a long longitudinal axis, decrease of signal is observed

• decrease caused by both electric resistance along axis & membrane

• stength & time course of signal is altered over distance

• 
  

1. increase membrane resistance by isolation

2. decrease of inner resistance relative to Rm

• giant fibers

• isolation of the fiber increases Rm and decreases the capacitance of membrane

• Depolarization of next membrane compartment depends on length constant of the fiber

temperature

time constant

length constant

smaller time constant & larger lenght constant

• increases resistance across membrane

• decreases the capacity

• smaller resistance along the fiber than resistance across the membrane

in dendritic cells

• pre- and postsynaptic

• acetylcholine

• trasnmitter

• receptor for the transmitter

• ion gradients ar postsynpatic membrane

depolarization: openingn of channels for soidum or calcium

hyperpolarization: opening of channel for postassium or chloride

small computational modules where changes in synaptic strengh can take place (area of memory fomation & learning)

• spines are postsynaptic to 1-2 axonal terminals

• both biochemical and electrical compartments

1. EPSC/IPSC (Post-Synaptic Currents):

   • Represent ion flow (current), measured in picoamperes (pA)

   • Require voltage clamp to measure

   • Can be added algebraically

   • Reflects instantaneous channel activity

2. EPSP/IPSP (Post-Synaptic Potentials):

   • Represent membrane voltage changes, measured in millivolts (mV)

   • Require current clamp to measure

   • Summation is non-linear, shaped by membrane properties

   • Reflects integrated effect of current over time

• by activity

• by excitation spreading back from soma