Section 6 Flashcards
What is proprioception?
Refers to our capacity to know the position of the different parts of our bodies
What do somatosensory receptors detect?
Cutaneous or muscular changes produced by mechanical, thermal, or sometimes chemical stimuli, and to convey this information to the CNS
Where are cell bodies of somatosensory receptors located?
In the dorsal root ganglion
What do we call the region of the skin that is innervated by a specific nerve?
Dermatoma
Cutaneous or subcutaneous receptors have receptive fields that are defined by ____________
The area of the skin where a stimulation can elicit a response in that receptor
How do you achieve a greater capacity to identify the precise location of an object touching the skin?
By having a high concentration of receptors in that area which have small receptive fields
How does the size of a somatosensory neurons receptive field vary based on where the receptor is?
Receptors located deeper in the skin will tend to have larger receptive fields than the ones located on the surface
Explain why several somatosensory receptors can be simultaneously activated even if the stimulus is very small?
The receptive fields of somatosensory receptors overlap
What is tactile acuity?
The capacity to detect the most delicate features of a stimulus
What technique would you use to assess tactile acuity?
Two-points threshold technique
Describe the two-points threshold technique
Typically assessed by applying two points of a drawing compass on the skin and asking participants if they perceive one of two points
The smaller the distance required to discriminate two points, the _______ (smaller/greater) the tactile acuity
greater
Regions such as the face and hands have much _______ (smaller/greater) tactile acuity compared to other regions such as the back and legs
greater
What are the two general types of somatosensory receptors?
Fast-adapting (FA) and slow-adapting (SA)
Describe fast-adapting somatosensory receptors
Respond strongly to the onset of a mechanical stimulus, but then quickly decrease their firing rate even if the stimulus is maintained
List three characteristics of FA type I receptors
- attached to Meissner corpuscles
- have smaller receptive fields
- mostly found in the fingertips and are extremely sensitive to movement and low frequency vibration, which is important for perceiving slip and maintaining grip when manipulating objects
List three characteristics of FA type II receptors
- Attached to Pacinian corpuscles that absorb much of the mechanical energy applied on the skin
- Located deeper in the skin and therefore have large receptive fields
- Most sensitive to high frequency vibration on the skin, which is important for perceiving fine textures or manipulating tools
Describe slow-adapting somatosensory receptors
Their response is sustained throughout the application of the mechanical stimulus
List three characteristics of SA type I receptors
- attached to Merkel cells
- located closer to the surface of the skin and therefore have small receptive fields
- play an important role in perceiving pattern, texture, and shape
List three characteristics of SA type II receptors
- attached to Ruffini endings
- have large receptive fields
- responsive to stretching of the skin, which can be useful for perceiving hand conformation
What are proprioceptors?
Convey information on the mechanical forces originating from body movements and posture in order to inform on the position of limbs and other body parts
Where are proprioceptors located?
muscle spindles, Golgi tendon organ, and ligaments
What are nociceptors?
They respond to pain and temperature throughout the body, and have never endings that are free from any specialized endings
Why do nociceptors have a high response threshold?
Because there is no reason for them to be activated by stimulus intensities that are too low to cause injury
List all the nerve cell types that would be thickly myelinated
tactile and proprioceptive A-beta and A-alpha fibers
List all the nerve cell types that would be thinly myelinated
nociceptors (A-delta fibers)
What myelination do C-fibers have?
None at all
What causes congenital insensitivity to pain?
A genetic mutation that causes impairments in the development of small-diameter fibers such as A-delta and C fibers
How is it possible that we have first and second pain?
We have two different types of pain fibers
Describe first and second pain
First: the initial feeling of pain that causes you to move that body part away from the source
Second: The more diffuse pain you feel a few seconds after removing yourself from the source
What happens to the different somatosensory fibers when your limb goes numb?
Myelinated fibers get impaired pretty quickly, but C fibers stay in tact
The axons of the peripheral somatosensory receptors enter the CNS through the _______ (dorsal/ventral) part of the spinal cord
dorsal
Describe the spinothalamic pathway
Temperature and pain fibers will enter the dorsal horn of the spinal cord and make a synapse onto projection neurons. Then, the axon of the projection neuron will cross to the anterolateral quadrant on the opposite side of the spinal cord and ascend vertically towards the brain
Describe the dorsal column-medial lemniscal (DCML) pathway
Instead of synapsing in the spinal cord, the axons of tactile and proprioceptive nerve fibers will directly ascend vertically upwards towards the brain through the dorsal columns on the ipsilateral part of the brain
Dorsal columns are bundles of _____ (white/gray) matter
white
At what point do axons of tactile and proprioceptive nerve fibers cross over to the other side of the brain?
When the axons reach the medulla
Describe the Brown-Sequard syndrome
Because tactile and thermoalgesic axons have different pathways, spinal cord lesions will result in having a tactile deficit on the same side of the body as the lesion, but a thermoalgesic deficit on the contralateral side of the body relative to the lesion
Which location is one of the few where nociceptive and tactile information interact?
dorsal horn
Describe the gate control theory of pain
It was hypothesized that a large-diameter tactile afferent could stimulate an inhibitory neuron (interneuron) in the spinal cord, which would then go on to inhibit the spinal projection neurons that would have conveyed nociceptive information to the brain, thereby blocking the transmission of nociceptive signals coming from the small-diameter nociceptors
What is the intensity theory of pain?
Proposed that pain resulted from excessive activation in any kind of nerve fiber
What is the specificity theory of pain?
Proposed that pain was a specific sense distinct from touch
What is the only type of somatosensory neuron that does not make a synapse in the thalamus?
Proprioceptive fibers
Although proprioceptors do not project to the thalamus, what other brain region do they project to?
Cerebellum
Describe the pathway tactile information takes once at the thalamus
Will first project to the ventral posterior nucleus of the thalamus, and from there will reach the post-central gyrus which houses the primary somatosensory cortex
Describe the asymmetry in the primary somatosensory cortex
It is asymmetric in the way the different body parts are represented, with the size of the cortical area dedicated to the face and hands being the largest relative to the rest of the body