PSYCH 261 Week 2: Module 2.1

THE CONCEPT OF THE SYNAPSE

  • synapse —> the junction between neurons where they communicate



  • sherrington discovered the synapse

  • did this based on reflexes, automatic muscular responses to stimulii

  • he said, in a reflex of flexing your leg, what happens is a sensory neuron excites a second neuron (interneuron, connecting sensory neuron and motor neuron) , which then excites a motor neuron, which excites a muscle

    • question: so when you have your sensory neuron that excites a second neuron, the diagram says its a brain neuron. so a neuron in the central nervous system? like… why? where does the interneuron come into place? im having trouble visualizing this.

    • also question: don’t reflexes work in neural networks? they’re so fast, they don’t have to go to the brain?

  • this whole circuit is called a REFLEX ARC

    • cajal found that neurons are seperate, and so a reflex does require communication between neurons, and so if sherrington was liek “if we measure reflexes, we might figure out how they communicate”

SHERRINGTONS EXPERIMENT

  • he took a dog, put it in a harness, and pinched one of its feet

  • after less than a second, the dog flexed the pinched leg and extended the other ones.

  • he made a cut THAT DISCONNECTED THE SPINAL CORD FROM THE BRAIN, and found that the reflex still happened!! so he was like “the spinal cord controls the reflexes of extending and flexing things”(DID HE DO THIS TO RAT CHAT???????)

  • in fact, they were even more consistent after disconnecting the spinal cord from the brain cause messages from the brain modify the reflexes, changing when they are strong and when they are weak


PROPERTIES OF REFLEXES AS ASSUMED BY SHERRINGTON

  1. reflexes are slower than conduction along an axon (SLOWER???????//)

    1. QUESTION: AREN’T REFLEXES SUPPOSED TO BE FASTER???????

  2. several weak stimuli presented at nearby places or times combine their effects

    1. question: is this the whole “you put a bunch of small ESP and ISP or something and it makes the action potential go boom?”

  3. when one set of muscles becomes excited, another becomes relaxed



SPEED OF REFLEX AND DELAYED TRANSMISSION AT THE SYNAPSE

  • pinched dog’s foot, dog flexed leg.

  • during the short delay, an impulse had to travel UP AN AXON (from skin receptor to spianl cord) then an impulse had to down the leg to a muscle

    • this takes a minute!! its a circuit, it takes a bit yk

  • but then sherrington measured the distance of this entire impulse (ex. from pinch to flexing leg), and calculated the speed the impulse traveled

  • HIS FINDING: the speed of conduction throguh the reflex arc VARIED!! WASN’T THE SAME but it was never more than 15 m/s, and previoius research had measured action potential at 40 m/s

  • so sherrington was like…… hmmm okay sos omething shold be slowing conduction down through the reflex, and he said this delay must be where one neuron is communicating with another. WHICH IS THE SYNAPSE!!!

    • question: so is a reflex SLOWER? IS 15M/S SLOWER THAN 40 M/S???????

    • WAIT DHRUTHI THATS SO FUNNY SHUT UP FLAMDFAKSJDF

    • 40 METERS A SECOND IS OBVIOUSLY FUCKING FASTER THAN 15 METERS A SECOND LMFAOOOOOOOOOOOOOOOO



SHERRINGTONS EVIDENCE FOR A SYNAPTIC DELAY

  • impulse travelling through synapse in the spinal cord is SLOWER than one traveling the same distance along an AXON, as along an axon is 40 m/s, and down the spinal cord is 15 m/s

  • the speed of conduction through a reflex arc is SLOWER NAD MORE VARIABLE (at 15 m/s), so sherrington says the delay occurs AT THE SYNAPSE

    • im so confused chat, question: how does he know its at the synapse? is it the jumping from 1. sensory neuron 2. interneruon 3. motor neuron 4. muscle

/


STOP AND CHECK

  1. what evidence led sherrington to conclude that transmission at a synapse is not the same as transmission along an axon?



TEMPORAL SUMMATION

  • repeated stimuli within a brief time combine their effects

    • summation over time

  • a light pink of dog’s foot did not evoke reflex, but a few rapidly repeated pinches did

  • although a single subthreshold excitation in the postsynaptic neuron goes away with a little time, with a second excitation that fllow quickly, it combines, looking like a staircase, and with more rapidly successing ones they just keep adding, until the combo exceeds the threshold of the postsynaptic neuron and we have an action potenital!!


EPSP + temporal summation of 2 EPSPs, 3 combine sxceed threshold, THATS TEMPORAL


SPATIAL SUMMATION —> multiple neurons fire, the EPSPs and IPSPs from that, collective charge, if it gets to thershold, you also fire

EPSPs, small excitatory charges that go above resting potential, IPSPs, small inhibitory charges that go under resting potential


GRADED POTENTIAL:

  • depolarizations —> EXCITATORY

  • hyperpolarizations —> INHIBITORY

  • EPSP, sodium ions entering the neuron

  • IPSP —> negative charged CHLORIDE goes into cell


hyper —> goes under

  • excitatory

  • sodium ions entering the neuron (positive)

depolar —> goes above

  • inhibitory

  • chloride ions entering the neuron (negative)


SPATIAL SUMMATION

  • instead of pinching same place over and over quickly (temporal summation), pinch two places at the same time, which activates sensory neurons whose axons converged onto a neuron in the spinal cord.

    • excitation from either sensory AXON excited that one spinal neuron and a combination of both of those excitations exceeded the threshold, which produced the action potential!!


STOP AND CHECK

  1. how does temporal summation differ from spatial summation

    1. temporal summation, one neuron, EPSP happening multiiple times in rapid sucession, small bursts building up to threshold, then producing action potential

    2. spatial summation, mutliiple neurons, EPSPs, happening once but at the same time in the multiple neurons, those add up to build to threshold, then produce action potential

  2. What ion gates in the membrane open during an EPSP? What gates open during an IPSP?

    1. during an EPSP, the sodium ion gates open. Sodium ions have a positive charge (Na+), which is why the cell has a graded depolarization

    2. during an IPSP, the chloride ion gates open. Chloride ions have a negative charge (Cl-), which is why the cell has a graded hyperpolarization


INHIBITORY SYNAPSES

  • when sherrington pinched dog’s foot, leg flexed, BUT THE OTHER LEGS EXTENDED!!

  • sherrington’s explanation assumed a couple connections in the spinal cord

    • he said that a pinch on the food 1. sends a mesage to a sensory neuron 2. which then goes to an interneuron in the spinal cord 3. which then excites motor neurons connected to the flexor muscles of that leg AND the extensor muscles of the other legs

    • he also assumed that the interneuron also sneds messages to the extensor muscles in that one leg, and the felxor muscles in the other leg

      • TEHSE SIGNALS, are inhibitory synapses,

      • at tehse synapses, input from an axon hyperpolarizes the PS cell, which moves hte cell’s charge away from threshold (below the resting potential, chloride ions), and decreases the probability of an action potential

  • most neurons have a spontaneous firing rate, which is…

    • when action potentials fire even without synaptic input

    • in these cases, the EPSPs increase the frequency of action potentials ABOVE THE SPONTANEOUS RATE (question: what is the spontaneous rate), wheras the IPSPs, decrease it

    • ex. for a neuron with a spontaneous firing rate of 10 APs (action potentials) a minute, a stream of EPSPs might incrase this to 15 or more, but if we have more IPSPs, we might get 5 or fewer

    • question: does this spontaneous firing rate exists for every neuron????????

    • so the spontaneous firing rate (SFR) varies enormousl yneuron to neuron, but for a given (type?) of neuron, it stays within a narrow range

    • INHIBITORY IMPULSES ARE JUST AS IMPORTNAT AS EXCITATORY IMPULSES!!!!!!!!

stop and check

  1. what was sherington’s evidence for inhibition in the nervous system

    1. pinch a dog’s leg, it flexes, and the other legs relax. THOSE OTHER LEGS RELAXING IS INHIBITORY!! (right?)

    2. he said a reflex that stimulates a flexor muscle prevents contraction of the extensor muscles of that same limb.

      1. ex. you pinch the leg of a dog, it flexes. because its flexed, the reflex makes sure you cant extend it.

      2. then he said that an interneuron that excites one set of motor neurons stops that excitation to go to another set.

        1. weird, i dont get it im confused, we’ll worok on it. question.

  2. can an inhibitory message flow along an axon?

    1. NO!! IPSPs decrease the possibilty of an action potential. only action potentials go across neurons. and IPSPs actually just decay after a bit and as time goes on.


END OF MODULE QUIZ

  1. in what way did sherringotn’s conclusions agree with ramon y cajal’s conclusions? (d) both concluded that neurons are seperate from one another.

  2. sherrington based his conclusions on what type of evidence? (d) observations of reflexive responses

  3. why did sherrington conclude that synaptic transmission differs from conduction along an axon (a) reflexes are slower than axonal transmission

  4. an EPSP is an abbreviation for what? (a) excitatory postsynaptic potenital

  5. although one pinch did not cause a dog to flex its leg, a rapid sequence of pinches did. sherrington cited this observation as evidence for what? (a) temporal summation

  6. the extensor muscles of a leg relax when the flexor muscles contract. sherrington saw this observation as evidence for what? (c ) inhibitory synapses

  7. what channels in a neuron open during an EPSP? (c ) Sodium (Na+)

  8. In what way were Sherrington’s conclusions important for psychology as well as neurosicence (b) he demonstrated the importnace of inhibition


d, d, a, a, a, c, c, b