L14: Somesthesis (mechanoreceptors) Flashcards
Tactile receptors can be divided into 2 major groups, which are?
1) slowly adapting mechanoreceptors, which respond to an enduring stimulus
2) rapidly adapting mechanoreceptors, which respond only at the onset and sometimes the termination of a long lasting stimulus
List the 4 mechanoreceptors that are found in glabrous (hairless) skin and state whether they are slowly adapting or rapidly adapting
1) Pacinian Corpuscles (lamellar corpuscles) = rapidly adapting mechanoreceptor found in deep tissues or immediately beneath skin, responsible for sensitivity to vibration & pressure
2) Meissner’s Corpuscles = encapsulated nerve ending that excites a large myelinated sensory nerve fiber. They are rapidly adapting.
3) Merkel Receptors = slowly adapting mechanoreceptors that transmit an initially strong but partially adapting signal and then a continuing weaker signal.
4) Ruffini’s end organs = multi-branched, encapusulated nerve endings that adapt very little,
Slight movement of any hair on the body stimulates what type of receptors?
another type of receptor that is made up of nerve fiber entwining the base of the hair
What is an important feature of somesthesis?
An impt feature of somesthesis is an ability to differentiate btw stimuli of the same quality that differ in place, magnitude & spatial pattern. This could be done due to relationship btw receptor density & receptive field size
In terms of somesthesis (epicritic sensation), what does receptive field refer to?
portion of skin directly innervated by the receptor terminals and the area of adjacent tissue thru which a stimulus can be conducted to the receptor
Areas of skin that have high density of receptors are more or less sensitive for fine spatial discrimination? These areas of skin have small or large receptive fields?
areas of skin with high density of receptors are most sensitive and have the greatest capacity for fine spatial discrimination. These areas have the smallest receptive fields but have the largest NUMBER of receptive fields per unit area.
receptive field size: compare fingertips to forearm to back.
fingertips are most sensitive to 2-point discrimination. Therefore, it will have the smallest receptive field size (2-3 mm). Forearm will have 20 mm and back will have 40 mm.
Areas of high sensile tactility vs areas of low sensile tactility: compare and contrast their receptor fields and receptor densities
high sensile tactility = high receptor densities, small receptive fields
low sensile tactility = low receptor densities, high receptive fields
How do mechanoreceptors give us spatial temporal discrimination ability?
Different mechanoreceptors give us different info (e.g. meissner = time; mercekl = spatial) which overall will contribute to all the somatosensory info/experience.
Describe the laminar organization of the spinal cord
the gray matter of the spinal cord has different layers. from dorsal to ventral, it’s labeled I to IX.
List and explain the 2 major ascending systems carrying somatosensory information to the cerebral cortex.
1) dorsal column/medial lemniscus system
- mediates tactile sensations including vibration sense & proprioception
2) anterolateral system aka spinothalamic system
* carries info chiefly abt pain & temperature
- mediates tactile info based on crude touch and pressure sesnations = poor localizing ability on body and little capability for fine intensity discrimination
Large myelinated fibers carrying epicritic information pass into the spinal cord in which specific region?
medially into the lateral margin of the dorsal columns
The dorsal columns are somatotopically organized. Explain
More medially = lower limb, trunk
More laterally = upper limb, neck
As the dorsal column fasiculi (gracile & cuneate fasicului) synapse at the gracile & cuneate nuclei in the caudal medulla, receptive fields of second order neurons become larger why?
receptive fields become larger b/c they receive convergent input from many primary neurons, each with a slightly different but overlapping receptive field.
In what ways are second order neurons receptive fields become more complex than the sensory receptor?
second order neurons’ receptive field has both excitatory & inhibitory regions unlike sensory receptor’s receptive field which only has a simple excitatory receptive field.
when there’s a punctuate stimulus (pin to skin), does only one region of receptors get activated?
No, several touch receptors are activated to a varying degree. Each of the primary units influences its own secondary unit and it also influences its nearest neighbors.
Gracile and cuneate fasiculi synapse in caudal medulla at gracile and cuneate nuclei. They decussate and then ascend as medial lemniscus. Where does medial lemniscus synapse in the thalaus?
medial lemniscus will synapse at the VPL nuclei of the thalamus.
epicritic and protopathic sensations of face and head synapse at which thalamic nuclei?
VPM nuclei
Within the thalamic nuclei, how are “place” and “modality” represented?
place - mediolaterally
modality -anteroposteriorly
Somatosensory info eventually will reach post-central gyrus, the primary somatosensory cortex. Explain where specifically in the primary somatosensory cortex that VPL and VPM info are sent.
VPL (contains somatosensory info from limbs and trunks) will project to the medial and superior portions of the post-central gyrus
VPM (contains somatosensory info from face and head) will project to the lateral portion of the post-central gyrus
The primary somatosensory cortex can be divided into 4 distinct cytoarchitectonic regions. List them. Where do most of the thalamic fibers terminate?
Brodmanns areas 3a, 3b, 2 and 1
3a - respond to deep tissue input
3b - respond primarily to activation of cutaneous receptors
2 - respond primarily to pressure
1- respond to activation of rapidly adapting cutaneous receptors
-Most of the thalamic fibers terminate in areas 3a and 3b. The cells in 3a and 3 b in turn project to areas 1 and 2
Explain the somatotopic representation in primary somatosensory cortex (S1)
more medially = legs, trunk, arms, hand, and face most laterally
Jacksonian seizures
seizures that affect areas of the body in the order that follows the somatotopic topography of the primary somatosensory cortex.
ex. spreading from finger to hand, then arm, down across shoulder and down ipsilateral leg (moving laterally to medially)
Are there fairly complete somatotopic maps in all 4 regions of the primary somatosensory cortex or as a whole, they form one map?
there are 4 independent and complete maps in S1, one for each of the areas 3a, 3b, 2, and 1.
In addition to architectonic regions (3a, 3b, 2, and 1) that are modality specific, there is also a columnar organization of the neocortex that is also modality specific. Within any one of the 4 architectonic areas of S1 there are, one or several interelated maps of place and modality?
several interrelated maps of place and modality
There are cells in S1 which process info at a more complex level. For example, there are cells that are not found in 3a or 3b, but in 1 and 2 that are direction-specific. What does that mean?
these vertical columns of cells located in the most posterior portion of area S1 respond only when a stimulus moves across the skin in a particular direction.
What are cells that respond to stimulation in any direction?
simple cells that are only motion sensitive
The processing of sensory info becomes even more complex farther posteriorly in the posterior parietal cortex in regions known as?
somatic association areas
What does it mean that topographic representations are use-modifiable thru out life?
practice or experience plays a role in the plasticity of somatoptic maps
For. instance, encouraging monkeys to use their middle 3 fingers at the expense of the others saw that the area of cortex devote to the middle 3 fingers was greatly expanded.
