4.1 Physiology of Sensation and Pain Flashcards
Identify and explain the 4 qualities of pain sensation.
- Location: where does it hurt?
- Modality: what kind of pain (ex: burning, pressure, etc.)
- Intensity: How bad does it hurt?
- Duration/Frequency: How long does it hurt, does it hurt all the time, etc.
What is the receptive field?
- The area of your body that is sensitive to the stimuli.
Can receptive fields change? If yes, how?
- Yes
- Changes as more neurons get involved
What is a 1st order neuron? 2nd order neuron? What is the interplay between these, generally?
- 1st order: small, this neuron actually gets directly excited (part of its dendritic field, or fibers extending into field). Initial response to stimulus. Sends message to 2nd order neuron.
- 2nd order: receives message from 1st order neuron, sends this message to the brain.
There are various elements coming together to make up a receptive field. Some parts are even inhibitory, and when excited they will give a negative (inhibitory) signal to the 2nd-order motorneurons. Why in the world would our bodies ever want to do this?
By doing this it helps inhibit signals being relayed from certain parts of the receptive field and therefore brings about contrast to our brains and helps more clearly define where the stimulus is coming from.
Cuddle up next to this image and get to know it. Click for the summary.
In this image you the general relay of sensory information.
This image is showing different types of nociceptors for different types of stimuli and the routes that these take.
The common part though is: 1st order neuron projects fibers and is being directly activated, signals goes into spinal cord, then to the brainstem, then finally to thalamus of the brain (cross-roads for sensory information going into brain), then into the cerebral cortex (this part of brain specifically designed to receive sensory info and process it).
What is the order of how pain is sensed? (from skin to brain).
Pain sensor.
Spinal relay neurons.
Spinal cord.
Spinothalamic (anterolateral) tract.
Thalamus.
Cerebral cortex.
Note the interplay between the motor and sensory systems of the body and how the information travels between them.
Red = sensory.
Blue = motor.
Black = in both.
What is a nociceptor, is this the only type of sensory receptor in the body?
Nociceptor: simple, bare nerve ending, sensitive to stimulus that would damage tissue. Activated by extremes of pressure, temperature, and noxious chemicals.
No, not the only sensory receptor in the body there are a ton of different ones that have all sorts of various morphologies and due to these will be more sensitive to different types of sensations (but for us right not, we only need to care about this!).
Picture a 1st order sensory neuron (even draw it out if you’d like), note what you see and how information would travel in it.
It has nerve endings that are projecting into the periphery.
These nerve endings/ fibers sense sensation.
This activates the neuron.
Information propagates down fiber to soma, then continues down the fiber to the terminal synapse and then to the relay neuron.
There are many different types of these sensory neurons (we only care about this one right now).
Where does sensory information go?
Dorsal route.
This image is very familiar. This is the RECEPTOR POTENTIAL of a sensory neuron. Receptor potentials are NOT action potentials, these increase or decrease the likelihood that an action potential will occur depending on whether depolarizing or hyperpolarizing. Their amplitude correlates with size of stimulus.
Potentials summate.
If threshold is reached, an action potential is generated.
Na+ floods cell and signal propagates.
What are these channels called? How are these channels opened in the first place?
Transient Receptor Potential (TRP) channels. Each receptor has a unique stimulus or modality that it responds to. Each is a different protein.
Variety of channels activated for various stimuli, create variety of sensitivities.
Look to image for examples.
What are the various types of nociceptors? Give examples.
Temperature: (>45deg C or <-5deg C), small diameter, thinly myelinated, 5-30 m/s. A-delta fibers.
Mechanical: intense pressure to skin. A-delta fibers.
Polymodal: high-intesity mecanical, chemical, or thermal (both hot and cold stimuli), small diameter, nonmyelinated, slow (<1.0 m/s). C fiber.
Silent: Will come to later.
How does diameter and myelination of neurons affect speed of signal?
How do sensory neurons fair?
Larger diameter = faster signal. Myelination = faster signal.
Sensory neurons are slow compared to motor.
Is it always the same receptors responding to a particular stimulus (for example, lets say temperature)?
Nope.
Check out the image.
Those lines are showing the response by various types of receptors, they all sense some type of temperature, but depending on the exact degrees they are either in action or not in action.
These different channels are called Transient Receptor Potential (TRP) channels.
What happens to a sensory receptor potential if the membrane potential is kept above threshold? Compare to motorneurons.
Unlike motoneurons you do not need to go hyperpolarize before another action potential can be fired.
If you raise the potential level then you can activate potentials repeatedly (multiple action potentials will be generated).
Since there are different types of pain fibers, can these fibers percept different types of pain? (dull vs. sharp).
Of course.
“fast pain” = sharp
“slow pain” = dull
Big signals on bigger fibers, small signals on small fibers.
Check out the image, it shows sharp vs. drawn out dull achy pain.
What is peripheral release when referring to nociceptors? Give examples.
Nerve endings can become activated by certain chemicals and cause them to directly release substances (such as substance P and calcitonin).
These chemicals cause sensitization of nociceptors to stimuli that were not previously noxious or painful. Check image.
Cox inhibitors stop this release.
Explain hyperalgesia as it pertains to the peripheral release from sensory neurons.
Explain the difference between primary and secondary hyperalgesia.
Hyperalgesia: increased pain response to a noxious stimulus.
Sensitivity is due to release of chemicals from damaged cells.
Primary hyperalgesia = increased sensitivity in the damaged area.
Secondary = increased sensitivity in areas surrounding damaged area.
What is the gate control theory of pain?
Pain transmission can be disrupted with non-noxious stimulation.
Convergence of signal paths onto a projection neuron could enable the disruption and reduce pain.
Info from C fiber and AB fibers converges on spinal cord, one of these fibers can signal to projection neuron an inhibitory signal that will reduce or weaken the signal being sent.
How does referred pain occur?
Convergence of fibers in dorsal horn lead to incorrect localization of pain.
Segmental level for visceral input is determined in embryo so localization in adult can be disparate.
Nociceptor from skin converges with nociceptor from visceral organ at spinal cord. Our bodies aren’t used to visceral input and mistake it for the skin signal and you feel referred pain.
