Lesson 5 - Receptor Potentials, Adaptation; PNS/CNS

Question 1

how does the stimulus for the change in potential differ between membrane potentials and receptor potentials

Answer 1

• we have learned that the membrane potential of the post synaptic neuron changes due to the binding of neurotransmitters from the presynaptic neuron
• however when it comes to the sensory system and receptor potentials, the change in membrane potential comes from the exterior sensory environment instead
• The energy from the environment will react with membrane proteins and in general this will cause depolarization
• in general this will cause depolarization of sensory receptors upon receipt of specific energy (like pressure) – much like an EPSP
• Exception: photoreceptors hyperpolarize

Question 2

where are receptor proteins found and what will happen if a signal is sent to the receptor proteins?

Answer 2

• instead of post synaptic proteins (either ionotropic or metabotropic receptors) we have receptor proteins (similar pathway)
• these receptor proteins are found in the sensory cell membrane
• if a signal is sent to these receptor proteins of the sensory cells (like a touch stimulus), the cells will change shape and generally depolarize the membrane

Question 3

what are two things that could happen when a receptor protein changes shape?

Answer 3

• Directly open ion channels (ex. cation channels –> leads to depolarization of the membrane) - similar to ionotropic
• Enzyme is activated via G protein coupling –> leading to production of 2nd messenger (cAMP, cGMP, lnP3) –> lots of 2nd messenger –> amplifying the signal – similar to metabotropic

Question 4

What happens when a chemical stimulus binds to a specific metabotropic receptor?

Answer 4

• Chemical stimulus (signal) binds to specific metabotropic receptor (G-protein coupled) > changes shape? > activation of G-protein > activate adjacent enzyme (adenyl cyclase) > produces 2nd messengers (cAMP) > cAMP activate kinases > directly interact with ion channels or phosphorylate other proteins

Question 5

how does the metabotropic effect for receptor proteins lead to the two stages of amplification?

Answer 5

• receptor proteins that are metabotropic exhibit an extra advantage: the metabotropic effect
• the advantage effect is the process of amplification

the two stages of amplification:
- G-protein can activate a number of different enzyme molecules
- each of these enzyme molecules will produce lots of 2nd messenger (cAMP)
* Thus, one stimulus molecule can produce lots of 2nd messenger (cAMP) leading to amplification

Question 6

how can the depolarizing current in your olfactory pathways lead to an AP

Answer 6

• much like PSPs, the receptors in nose produce graded potentials
• thus the depolarizing current that is produced needs to travel from the dendrites along the membrane passively to the trigger zone for the olfactory neurons
• if the current is strong enough and brings the trigger zone to threshold, an AP will fire and signal to your brain that you smell smth (ex. roses)

Question 6

use the olfactory receptor to explain how the metabotropic and amplification effect takes place when a stimulus is triggered

Answer 6

• there are olfactory neurons in the nasal epithelium and there is a coating of mucus on top of it
• the olfactory receptor cells line the mucus layer
• chemical stimuli or odorant (ex. smell of roses) dissolved in the mucus bind to the receptors in the olfactory cell membrane
• activates a G-protein –> activates adynyl cyclase –> produces cAMP –> cAMP directly binds to ion channels –> allows (Na+ and Ca++) to go through —> depolarizes the membrane

Question 7

what happens if we bind odour molecules to the olfactory receptor ionotropically instead

Answer 7

• because we are generating a current indirectly through the metabotropic system, this will result in the amplification system
• this makes the olfactory cells very very sensitive to one or two molecules in the air
• instead if the odor molecules were to bind directly to an ion channel (ionotropic pathway), then you will get one or two ion channels open and you will never be able to detect that odorant (doesnt amplify and send an AP to brain)
  (smell bypasses the thalamus – goes straight to the brain – this may be why smell is important)

Question 8

what are the two categories of sensory cell transmission?

Answer 8

1. sensory cell generates an action potential at the trigger zone aka spike generating zone
2. sensory cell releases vesicles when depolarized; impulses generated in post synaptic neuron

Question 9

explain how sensory cells like the mechanoreceptors in our skin generate an AP at the trigger zone / branch point

1st category of sensory cell transmission

Answer 9

• depolarizing current has to reach the trigger zone and bring it to threshold for an AP
• mechanoreceptors are found in your skin and respond to pressure
• the first patch of excitable membrane is located at the branch point (like the trigger zone)
• when pressure is applied, ion channels will open at the membrane and produce a depolarizing current (receptor graded potential)
• the graded potential will have to summate and travel to the branch point (like the trigger zone) so that it can reach threshold for an AP
• depolarizing current spreads to the first patch of excitable membrane, and through summation reaches an AP
• receptor potential has to summate enough at the branch point to generate an AP (like EPSPs summating at trigger zone for AP)

Question 10

what is the second category of sensory cell transmission?

Answer 10

• sensory cell here doesn’t have to generate its own AP as long as it can release neurotransmitters

–Sensory cell releases vesicles when depolarized; impulses generated in post-synaptic
neuron

Question 11

how is there a transmission of a signal in sensory cells through vesicles

2nd category of sensory cell transmission

Answer 11

• if the sensory cell can release transmitters then the next cell in line will pick this up and act as the postsynaptic neuron and fire an AP
• the release of transmitters is due to the depolarizing current which does not generate an AP but instead opens up an influx of Ca++ ions that triggers the exocytosis of vesicles
• thus sensory cell is releasing vesicles with neurotransmitters but no AP

Question 12

explain the 2nd category of sensory cell transmission using the example of the taste receptor

Answer 12

• there are specialized cells that have taste receptor proteins in the taste buds inside your mouth
• taste receptors will bind chemicals stimulus through the receptor surface of the tongue and mucus layer and produce a depolarizing current (receptor potential not AP - no axon) that will result in the release of transmitters
• this depolarizing current travels passively and reaches the other end of the current
• the depolarizing current opens the voltage gated calcium channels and leads to a calcium influx which subsequently releases transmitters (exocytosis of vesicles)
• after the release of the transmitters the next cell generates the AP so your brain knows what you tasted (no AP in the sensory cell itself)

Question 13

what is the concept of adaptation in relation to stimulus and receptor/membrane potential

what are the two types

Answer 13

• Adaptation in sensory receptors refers to how they respond to a continuous stimulus over time.
• The MP can decay over time leading to ‘Adaptation’
• The original voltage is not sustained and it’s dropped over time, even though the stimulus may be constant
• ex. when you enter a hot bath its very hot at first, but you adapt and you may not feel it to be hot over time even though the temperature is the same
• Types of adaptations
• slowly adapting
• rapidly adapting

Question 14

what is slow adaptation

Answer 14

• receptor potential is sustained for duration (slowly decayed) of a stimulus as long as the stimulus intensity remains
• it is interested in the overall magnitude of the stimulus
• Example: Merkel cells in the skin, which are slowly adapting receptors, detect sustained pressure or touch. For example, when you hold a pencil, Merkel cells keep firing while you’re holding it, providing information about the pressure and shape of the object.

Question 15

what is rapid adaption

Answer 15

• These receptors respond only when the stimulus first starts (or when it stops) and quickly stop firing even if the stimulus is still present.
• receptor potential decays to zero when stimulus is constant
• ie. if the stimulus intensity increases –> receptor potential is positive spike (depolarization)
• if the stimulus intensity decreases –> receptor potential is a negative spike (hyperpolarization)
• during the constant stimulus intensity –> receptor potential is zero
• Interested in how quickly the stimulus is being delivered, the velocity of stimulus being delivered

ex. Pacinian/cortisol corpuscles, which sense vibrations or brief touches, are rapidly adapting. When you place your hand on a vibrating surface, the Pacinian corpuscles respond quickly at first but stop firing if the vibration continues without change.

Question 16

what is habituation

Answer 16

• when we are accustomed to successive and continuous stimuli and thus pay less and less attention to
• as the stimulus successively occurs, there is a weaker and weaker receptor potential response
• Habituation response depends on the cell, some will show large degree and some won’t
• if you wait too long before you deliver the next stimulus, you wont get habituation
• ex. zoning out to the sound of someones lecture or music or a clock ticking

Question 17

how do you tell the difference between a huge stimulus and a small stimulus

Answer 17

• The receptor potential they will vary directly in proportion with the intensity of the stimulus
• Greater the stimulus intensity > greater the receptor depolarization (graded potential/ change in receptor potential) > more transmitter released and/or higher AP frequency (frequency of AP being fired will go up)
• The greater the depolarization > the faster the membrane will be brought up from hyperpolarization to generate a new spike
• we know a long lasting stimulus will release in a spike train of APs
• The Impulse frequency will always be limited by the refractory period (cannot generate like 1B APs in one second, refractory period limits the number of APs per second. lets say it takes 1 ms to restore in the refractory period, then only 1000 APs in one second)

Question 18

what if you want to code above 1000 APs per second? aka what if you get a greater stimulus and already reached 1000 APs?

Answer 18

• not great stimulus = 20-50 APs per sec
• great stimulus = up to 1000 APs per sec

to code more APs per second:
* The strategy is to recruit additional neurons
* As stimulus intensity increases, we recruit higher threshold sensory neurons
- higher threshold sensory neurons require a greater stimulus before they even deliver a receptor potential

Question 19

explain how higher threshold sensory receptors/neurons are used to generate a greater frequency of APs with greater stimulus intensity even after it reaches its maximal discharge rate (1000 APs)?

Answer 19

Receptor A: has a lower threshold stimulus strength and once reached by the stimulus input, there is an increase of AP frequency at the excitable membrane until it reaches its maximal discharge rate (plateaus)
(inc. intensity of stimulus > inc frequency of AP [receptor A])

• to generate more APs for a great stimulus intensity, receptor B is activated only if the receptor reaches the greater threshold for stimulus strength
• once this is reached, receptor B is also able to generate APs until the maximal discharge rate (1000 APs)
• the number of APs generated is the sum between receptor A and B (since they are both activated with a great stimulus, over 1000 APs can be generated)

Question 20

how do we code for different types/modality/quality of stimulus?

Answer 20

• we use a ‘labeled line strategy’
• the pathway or system is labelled (ex. visual pathway/system)
• each quality of stimulus has a different system devoted to it (ex. light, color, sound)
• This means that activity in one pathway means a particular stimulus quality and nothing else
  ex. if you press your eye hard you see flashes of light. the visual pathway is activated only.

Question 21

what is the variation we see within a modality?

what is the problem with assigning a different receptor protein for each quality in a modality?

Answer 21

• Variety: All sensations have sub-modalities that you could distinguish (ex. different colors in the visual pathway, touch can be smooth, rough, oily etc.)
• If you had to devise receptor proteins for ALL these qualities, there wont be enough space in your brain and it won’t be efficient (ex. if you had to had different receptors for all the different colors, your eyes will explode)
• Is there a better way?

Question 22

what is the population code

Answer 22

• Population coding is coding using the ratio of activity from a restricted number of different receptor types (instead of having a receptor for say every shade of every color)
• Specific stimulus is coded by ratio of activity across the population of receptors

Question 23

how does the population code work
ex of three receptors and a stimulus

Answer 23

• the ratio of activity across a population of receptors
• A given receptor (e.g. A), type will respond to a wide range of sensory space, but it has a peak response that is different from other receptors
• Thus, any given stimulus in a sensory space (dotted line) will activate the closest receptor peak the strongest (ex. C) but other receptors (A, B) more weakly

see the graph page 36

Question 24

what is receptive field

Answer 24

• receptive field refers to what territory a sensory neuron covers
• Receptive Field: Each sensory neuron is going to respond to a particular spatial area (e.g. skin, it’s the territory on the skin)
• Receptive Field of a given sensory neuron is the territory in which you could activate that neuron
• Receptive Field is always defined in relation to a given sensory neuron, each sensory neuron will have a different Receptive Field
  ex. the sensory neurons on your hand will respond to stimulus provoked on that specific region on your hand

Question 25

explain convergence in large receptive fields

Answer 25

• you have one sensory neuron which corresponds to a specific secondary receptive field
• the secondary receptive field has different sections that corresponds to different primary neurons that overlap to create one large secondary receptive field from their own individual receptive fields
• all of these primary neurons converge to the one sensory neuron thus two stimuli that fall within the same secondary receptive field are perceived as a single point because only one signal goes to the brain. there is no two point discrimination

Question 26

explain sensitivity in small receptive fields

Answer 26

• regions that are more sensitive ie your fingertips, have smaller receptive fields
• this means a secondary sensory neuron only corresponds to one primary sensory neuron instead of three for instance, and will thus have the same small receptive field the primary sensory neuron creates (not converged to create a big secondary receptive field - same size primary and secondary receptive field)
• these sensitive areas have many sensory neurons present so that if two stimuli are stimulated, then separate pathways are activated to the brain. the two points are perceived as distinct stimuli and hence there is two-point discrimination

Question 27

ex of small sensitive vs. large convergent receptive fields

2 pencils poking you

Answer 27

if two pencils poke you at the:

• fingertip –> can distinguish, small sensitive receptive field
• back –> cannot distinguish usually, big convergent receptive field