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Benefits of Adaption in touch sensations - ️️1) Helps us to ignore constant, innocuous stimuli, reducing distraction 2) Helps to avoid saturation of neuronal firing rates, allowing us to detect changes in stimulus intensity over a larger ranger of intensities. Merkel Cells and Mechanotransd...

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  • September 17, 2024
  • 14
  • 2024/2025
  • Exam (elaborations)
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PNB 2XB3 Midterm 2 Review
Benefits of Adaption in touch sensations - ✔️✔️1) Helps us to ignore constant,
innocuous stimuli, reducing distraction
2) Helps to avoid saturation of neuronal firing rates, allowing us to detect changes in
stimulus intensity over a larger ranger of intensities.



Merkel Cells and Mechanotransduction - ✔️✔️When skin is presed:
1) Mechanically-gated ion channels in the A beta membrane open, letting in ions (Na)
2) This causes the first ("dynamic phase") action potential
3) Mechanically-gated ion channels in the Merkel cell membrane also open, letting in
ions (Na)
4) This depolarizes the Merkel cell and results in the opening of voltage gated Ca
channels
5) As a result, a neurotransmitter (norepinephrine) is released onto the A beta axon
terminal, which contains transmitter receptors
6) This causes the second ("static") action potentials


Dorsal Column Medial Lemniscus Pathway - ✔️✔️Pathway that responds to light touch
A delta axons go up lumbar and cervical spinal cord, then the dorsal column nuclei, then
medial lemniscus in the caudal medulla (base of brain).
From there to pons, midbrain, thalamus and eventually parietal cortex using
thalamocortical axons.
Decussates at caudal medulla.


A beta Fibers - ✔️✔️Large sensory fibers involved in rapidly transmitting sensation.
Responds to touch
(around 50 m/s)

A delta Fibers - ✔️✔️Slower fiber that responds to sharp pain ( around 10m/s)

C fibers - ✔️✔️Slow fiber that is associated with dull pain (around 1m/2s)

Spinothalamic Tract - ✔️✔️Pathway that responds to painful touch. (A delta and C
axons)
Travels in ventral matter of the spinal cord. Decussates immediately.

Brown-Sequard Syndrome - ✔️✔️Damage to half of the spinal cord.

, Results in loss of touch on the ipsilateral side, and on the contralateral side, no pain
sensations.
Upper body still has normal sensation.

4 Types of Touch Receptors - ✔️✔️Meissner corpuscle, Merkel cell-neurite complex,
Ruffini ending, Pacinian corpuscle.

Meissner Receptors - ✔️✔️Small receptive field size
Responds best to low frequency vibration (2 to 50Hz)
Located near surface (around 1mm deep into the skin)

Merkel Receptors - ✔️✔️Small receptive field size.
Responds best to static indentation (pressure
Located near surface (around 1mm deep into the skin)

Ruffini Receptors - ✔️✔️Large receptive field size
Responds best to skin stretch
Located deeper in skin

Pacinian Receptors - ✔️✔️Large receptive field size
Responds best to high frequency vibrations
(greater than 50Hz)

What does the sensation of vibrations help us do? - ✔️✔️Allows us to perceive the
texture of materials, the "slippage" between the skin and grasped object and the small
vibrations of tools.
EX Detecting change in amplitude when you're running you fingers across an etched
design.
4 Sections of the spinal cord - ✔️✔️1. Cervical
2. Thoracic
3. Lumbar
4. Sacral



Stimulus Response Spike Train - ✔️✔️Merkel and Ruffini cells have an onset response
(dynamic phase) and an ongoing response (static phase)
Meissner and Pacinian cells respond to the first part of the stimulus

2 different ways to get an action potential along the A beta axon - ✔️✔️1) Deforming
the A beta axon
2) Deforming the Merkel cell
Both the A beta axon and the Merkel cell have voltage gated channels. Responds
mainly to the ongoing part of the stimulus (ongoing indentation)

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