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