Week 2A Notes

Nervous System Communication

• Neuroconduction = electrical, within neurons

  • Electrical excitation of the neurons and the combination of excitation and inhibitory signals

• Neurotransmission= chemical, between neurons

  • The release of neurotransmitters

Measuring Electrical Changes

• Use the voltmeter: V= C+ - C-

  • Placing the positive lead into the positive area and the reference into the regular solution

    • Measuring voltage between two locations which is the voltage potential between two locations

• Intracellular Patch Clamp Recording: Putting the positive lead into the neuron (axon) and reference lead into the extracellular fluid (Na+)

  • We would see a -60mv membrane resting potential of a neuron!

  • It is typically -50 and -80 V resting potential

  • All cell are usually negative in the body

Membrane Bilayer and Structures

• Proteins spanning the phospholipid bilayer allowing charged ions to pass through

• Ion Channels (pores) : A channel of proteins which allow ions to pass through the cell

  • Selectively Permeable to selective ions

• Important ions

  • sodium, potassium, calcium → positively charged cations

  • chloride, proteins→ negative anion

Why is the Interior Negative?

• More positive Na+ outside of the cells with a closed channel to enter in

  

• Sodium Potassium Pump

  

  • More sodium outside, more potassium inside

    • 3 sodium out and 2 potassiums inside (NOKIA) which decrease the positive charge

• Leak Potassium Channels → Open at all Time

  • The accumulation of potassium inside of the cell cause more positively charged potassium to exit

  • Will reach an equilibrium

• Negatively Charged Protein in the interior of the cell

Chemical “Force” → Diffusion and Electrostatic Forces

• Diffusion

  • Particles will diffuse through the solution until they are evenly distributed

  • An imaginary force that pushes the particles to low concentration

• Semipermeable Membrane

  • Only will allow certain particles to travel through and diffuse in certain regions

• Electrostatic Forces

  • Causes ions to flow towards oppositely charged areas

  • ex. Positive wants to be with a negative and not another positive charge

• Potassium Dillem a → Two opposing forces in Different Directions

  • Is attracted to the negative proteins in the inside

  • Is also wanting to exit out and diffuse on the outside of the cell

Driving Force on an Ion

• Electrical: Forces an ion to the side of the opposite charge

  • Membrane Potential (positive or negative)

  • Ionic Charge (cation and anion)

• Diffusion: Will want to travel to the side of a lower concentration

  • Concentration Gradient

• Driving Force: sum of the two forces

  • Determined by magnitude and direction of electrical and diffusion forces

  • Depends on intensity of the two forces to determine the direction of an ion

Equilibrium Potential

• Potassium

  • Electrical Force and the Chemical Force go in opposite directions which allows for the concentration of potassium to be maintained

• Sodium

  • Electrical Force and chemical force towards the inside of the cell

  • Membrane isnt permeable to sodium so thats why the sodium can’t enter into the cell

Electrochemical Equilibrium:

• Occurs when the driving force is 0

  • When both forces are zero or there is a net of 0

• There will be no net change in the direction of flow

• Equilibrium Potential: Stable Membrane voltage (reversal potential, nerst potential)

  • When the driving forces of the membrane potential are of equal size and in opposite directions

    • If the membrane is permeable to only one ion then the membrane potential will pertain to only that ion

    • If there is permeability to more than one then it will average out to the relative permeability to each ion

      • Since there is more permeability for potassium then the resting membrane potential is close to the equilibrium potential for potassium rather than for sodium

  • Calculated from the nerst equation which considers concentration and charges of ions

Spatial and Temporal Summation

Types of Ion Channels

• Leak Channels: channels that are always open; ungates

• Ligand Gated Channels: open when a specific molecule binds to them

• Voltage Gated Channels: open when the cell membrane open a voltage range

  • Crucial for action potential

• Mechanically Gated Channels: open due to mechanical pressure; physical touch, vibration

  • Like for the sense of touch

• Optical Gated Channels: open in the presence of light

Ion Channels in the Neuron

• Dendrite: Neurotransmitters are received

  • The voltage gated channel opens with the binding of the neurotransmitters to 

Excitatory Postsynaptic Potential (EPSPs)

• Most Common Excitatory Neurotransmitter: Glutamate

  • From the glutamatergic synapse, allows sodium to enter the cell by opening up the ligand gated sodium channel (depolarization)

• The sodium potassium pump will eventually pump out the excess sodium and bring the potential down again

Inhibitory Postsynaptic Potentials (IPSPs)

• Most Common Inhibitory Neurotransmitter: GABA

  • Opens up the ligand-gated chloride channel, decreasing the membrane potential (hyperpolarization)

Passive Propagation

• The cytoplasm (electrolyte) conducts electricity and propagates rapidly and will decrease electricity through travel because of electrical resistance

• At the axon hillock the IPSPs and EPSPs are accumulated

Summation

• Spatial Summation: The creation of a large EPSPs from different synapses onto twos dendrite of a cell

  

  • Will still propagate but the EPSP will be larger at the integration zone

• Temporal Summation: A single postsynaptic cell firing multiple EPSPs in successive amounts to the same dendrite

  

  • The action potential is not allowed to go back to the baseline amount and will increase the signal

• Both summations will trigger an action potential at the -55mV threshold that could reach a peak voltage of +60mV