Lecture 2 + Assignment 2 Flashcards
Perception according to Aristotle vs. Oscar Wilde
A:
- brain isn’t responsible for any sensations
- like heartbreak
OW:
- in the brain everything takes place
Brain relation to perception
- wherever we feel something, it’s due to brain activity
sensory stimuli -> electrical impulses/action potentials -> brain
Johannes Muller conduction belief
- thought we’ll never be able to measure the velocity of a nervous action because it’s faster than the speed of light
Hermann von Helmholtz conduction response
+ 2 experiments
- Muller’s student
Motor nerves
- measured conduction speed in frogs by shocking nerve and measuring time till muscle contraction
Sensory nerves
- did the same with humans to see how long it took participants to perceive the signal
= response time till teeth clamped
Experiment in class results - stimulus to perception
close to brain
= perception occurs with little delay
far from brain
= perception occurs with longer delay
why?
- finite travel speed
- larger travel distance
Sensory and motor neurons = afferent or efferent
Also formulas for calculating
Sensory axon = afferent
(distance from ankle to brain - distance from shoulder to brain) / (ankle time - shoulder time)
Motor neurons = efferent
Receptor potentials causing action potentials
- receptor potentials are graded
- if the receptor potential is big enough, an action potential will start
- they need to make it depolarize enough
action potentials = all-or-none
Parts of the action potential graph
-65 mV
resting potential
depolarization
-50 mV
threshold
rising phase
falling phase
takes ~1 sec
< -65 mV
afterhyperpolarization / AHP
(undershoot)
Goldman-Hodgkin-Katz equation
Vm = 58log[(Pk[K]out + PNa[Na]out)/(Pk[K]in + PNa[Na]in)]
- permeability not constant, changes during action potential
- no units
- shows ions relative to each other
Rising vs. falling phase feedback loops
Fast positive feedback loop
- Signal depolarizes membrane potential
- Voltage-gated Na+ channels open
- Na+ rushes in
Slow negative feedback loop
- Voltage-gated K+ channels open
- K+ rushes out
- Hyperpolarizes neuron
Three states of the voltage-gated sodium channel
- Closed
ㄱ
- at rest - Open
—- ⊦–
- initial depolarization - Inactivated
_」
- top of action potential
- repels positive charges
End of rising phase
- Na+ channels inactivate
- Na+ stops rushing in
- no more depolarization
- fast positive feedback loop ends
Falling phase
- voltage-gated K+ channels open with a delay in response to depolarization
- K+ channel opens and K+ leaves through the creaky / slow door
Two states of the voltage-gated potassium channels
- Closed
- almost always — - —- - Open
- during falling phase
__ \___
Action potential shape in space vs. in time (sticky note/him in Hawaii example)
- Velocity goes opposite direction as time
- Action potential has the same shape in space as it does in time
Pros and cons of the refractory period
Pros
- ensures unidirectional a.p. conduction away from point of origin
Cons
- places limit on the firing rate of a neuron (could be good or bad)
Relative vs. absolute refractory periods
Relative
- relatively more difficult to elicit another action potential
- due to hyperpolarization as K+ channel takes longer to close (so K+ keeps leaving the cell)
- not all Na+ channels have gone from inactivated to closed state
Unidirectional conduction
- action potentials only move in one direction along an axon
- due to the refractory period / refractory wake
Voltage clamp technique
- internal electrode measures membrane potential Vm and is connected to voltage clamp amplifier
- voltage clamp amplifier compares membrane potential with desired (command) potential
- if they’re different, clamp amplifier injects current into the the axon through a second electrode
= membrane potential the same as the command potential - current measured
Squid giant axon experiment + results
1963 Hodgkin + Huxley
- used voltage clamp to discover ionic basis of action potential
Membrane potential graphs
- start at the resting -65 mV potential
- goes to the voltage they set it to
Membrane current graphs
- current needed to keep it there
- slight dip as sodium enters the cell (early inward current)
- rises as potassium leaves the cell (late outward current)
- IF membrane potential set to the Na equilibrium potential (52 mV in squid), no early inward current
When is current positive or negative
Positive charge entering the cell = negative/downward current
Permeability change graphs during an a.p.
aka conductance
Na+ peaks just before 1sec
K+ peaks just after 1sec
- conductance only gets half as high as Na+ was
Local anesthetics
- block voltage-gated Na+ channels
—– >——
block in-between gates
—– +>——
ex. lidocaine
- eventually dissociates / pops out of the channel
Tetrodotoxin (TTX)
- in pufferfish ovaries and liver
- produced by bacteria that live in the fish
- 1200 times deadlier than cyanide
- could kill 30 adult humans
- “fugu” delicacy in japan
- blocks Na channels
- dissociates much slower than lidocaine
- death by paralysis of respiratory muscles (going to diaphragm)
- fish themselves have TTX-resistant Na channels and so do the hearts of mammals
Saxitoxin (STX)
- in butter clam (saxidomus giganteus)
- in other shellfish too
- from contact with toxic algal blooms
= toxic dinoflagellates found in red tides - butter clams consume the algal bloom and store in their fatty tissue for a year
- heat stable = not destroyed by cooking
- causes paralytic shellfish poisoning
symptoms:
- start 1hr after consumption
- paresthesia
- pins and needles
- numbness
- paresis = difficulty moving
- respiratory difficulty
Why is the voltage at the top of the rising phase never as high as the sodium equilibrium potential
- because it’s always permeable to other ions
- could only reach if zero permeability to K+
- but there is always some K+ permeability due to leak channels
If voltage-gated sodium channels blocked - in the case of TTX and lidocaine
- no action potentials
If voltage-gated potassium channels blocked
- slower falling phase and no hyperpolarization undershoot AHP
The firing rate code
- maximum rate 500 AP/second for typical neurons
- even when stimulus is very intense
