Neurophysiology (Chapter 2)

History (Jose Delgado):

• Radio-controlled bulls → radio frequency to stimulate the brain + control bulls charging at the stimulus
  • Founding early work for TMS
  • What does it mean if a device is controlling your behavior?

Key Vocab:

• ==Neurophysiology==::study of life processes of neurons
• ==Intracellular fluid==::inside of our cells have fluid, negative charge usually
• ==Extracellular fluid==::surrounding cells, positive charge usually
• ==Microelectrode==::tool used to measure charge of the inside/outside of a neuron
• ==Ion==::an electrically charged atom or molecule
  • Anion = neg; Cation = pos
• ==Cell membrane==::a phospholipid bilayer
  • Phosphate groups w/inactive arms naturally align themselves in tight rows - fat soluble things can pass through, otherwise molecules must have a channel to go through (very specific channel)
• ==Threshold of excitation==::the amount of depolarization required to initiate an action potential

Resting and Action Potential:

• Resting - what does the cell look like when the neuron is not firing?
  • Intracellular fluid has high levels of
  • Organic anions (A-) - big, lots of molecular weight and potassium ions (K+)
  • Naturally neg because A- is so prevalent
  • Extracellular fluid has higher levels of
  • Chloride (Cl-) ions and sodium (Na+) ions
• What will cause the inside of the cell to become more positive?
  • Calcium (Ca++) ions predominantly on the outside - has a double charge - every ion that enters the intracellular fluid has double the charge of a sodium ion
• ==Depolarization==::cell becomes less neg on the inside (closer to 0)
• ==Hyperpolarization==::cell becomes more neg on the inside (further from 0)

Forces that Guide Molecules:

• ==Concentration gradient (diffusion)==::from high concentration to low concentration - natural, passive process
• ==Voltage gradient (electrostatic forces)==::in high positive concentration, they will be drawn to neg charge; in high neg concentration, they will be drawn to pos charge → opposites attract
  • Potassium (K+) wants to leave cell via concentration; stay inside via voltage
  • Chloride (Cl-) wants to enter cell via concentration; stay outside via voltage
  • Sodium (Na+) wants to enter cell via concentration; enter cell via voltage
  • Calcium (Ca++) wants to enter cell via concentration; enter cell via voltage
• If you need depolarization, big players are calcium and sodium entering
• If you need hyperpolarization, big players are potassium (leaving) and chloride (entering)

Sodium-Potassium Pump:

• Actively pulls sodium out of the cells and potassium into the cell to re-establish resting potential
  • Huge metabolic expense for running the brain
• Occurs b/w every action potential so the neurons can fire again

Ion Channels:

• Ion channels are on the dendrites/dendritic spines, cell body, every surface that grows
• Open and close depending on what they’re told to do by NTs, allowing certain molecules in depending on the channel

Membrane Permeability:

• Ability of the molecules to pass thru the membrane
  • Smaller ions can sneak through
  • Ex: sodium prefers to go thru a channel, but can sneak thru membrane a little bit
• How much sneaks in depends on the concentration gradient
  • Sodium and calcium have greater potential to get thru the membrane

Equilibrium Potential:

• The point where the electrostatic pressure pulling ions in one direction is offset by the diffusion force pushing them in the opposite direction (-40mV to -80mV)
  • Chloride and sodium are sitting equilibrium at RP

Action Potential:

• Toilet analogy
• Voltage-gated channels only open when the membrane potential is at the threshold of excitation
• Concentration gradient only gradient working when 0mV (no voltage acting on the sodium)
• ==Absolute refractory period==::neuron cannot fire again
• ==Relative refractory period==::neuron can potentially fire again, sodium-potassium pump working super hard
  • Think of double flushing a toilet
  • Think of sodium like a tense hunting bow, potassium drifting and gradually picking up speed
• ==Tonic==::baseline firing rate; ==Phasic==::burst firing rate, brain codes as important

Myelination:

• Hearing and touch pathways begin myelinating in the womb - majority occurs after birth
• Tied to experience - neurons that fire more regularly will be more myelinated
  • Speeds it up from base level to its conduction velocity (150 m/s → 300mph)
• Unmyelinated axons is like doing the wave at a Cowboys game; myelinated axons is like standing in a circle squeezing palms
  • “Saltatory conduction”
  • At each node of ranvier there are big clusters of Na+ and K+ channels

Neural Integration:

• ==EPSP (Excitatory Post Synaptic Potential)==::depolarization; Ca++, Na+ will eventually reach threshold of excitation

• ==IPSP (Inhibitory Post Synaptic Potential)==::hyperpolarization; Cl-, K+

• To consider

  1. Are there enough EPSPs to create an action potential?
  2. IPSPs are very important to balance and modulate the neurons

End of the Axon:

• Action potential comes down and reaches the terminal ending, which have voltage-gated calcium channels
  • They respond to the change in electrical activity and open
  • Calcium enters due to the concentration gradient and turns on second messengers
    • Messengers will bind to the vesicles on the terminal endings to make them mobilize → go to their docking station to fuse w/the membrane to open → NTs drift away (concentration gradient)

Tools:

• ==EEG==::single electrode or complex electrodes; read the electrical charge of potentially millions of neurons; no specificity for cell by cell actions - looking for gross activity of neurons acting together
  • Good for distinguishing phases of sleep and identifying seizure disorders
• ==ERP (Evoked Response Potential)==::EEGs configured in a new way, give a stimulus and look how neurons respond immediately before or after - measured over time

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