Somatosensory

Question 1

What is the function of the first order sensory neurons?

Answer 1

First order neurons detect the stimuli and transduce it to the CNS

Question 2

Are sensory neurons part of the CNS or PNS? What is their histology?

Answer 2

1. they are part of the PNS (because their cell bodies are outside of the brain or spinal cord).
2. They are pseudobipolar neurons with an end that reaches to the central cord (dorsal horn of the spinal cord) or trigeminal ganglion (for the head innervation). The trigeminal nucleus is in the brainstem. and one end that reaches the organ innervated.

3.Their cell bodies vary in size and they are densely packed and disorganized

Question 3

What are the 3 types of sensory neurons?

Answer 3

1. Low threshold mechanoreceptors (LTM): these traduce the sensory stimuli from low frequency things (i.e gentle brushing or feeling a breeze)
2. High threshold mechanoreceptors (HTM): harmful stimuli/pain
3. Polymodal which could be either LTM or HTM: thermoreceptive or mechanoreceptive

Question 4

What are the 5 types of sensory receptors?

Answer 4

1. Merkel cells, 2. Ruffini endings, 3. Meissner’s corpuscule, 4. Pacinian’s corpuscule, 5. Free nerve endings.
2. Merkel (LTM): pressure (especially around the edges, i.e awareness of holding an object in the hand), fine touch, small receptive field
3. Ruffini’s endings (LTM): skin stretch, proprioception, large receptive field
4. Meissner’s corpuscle (LTM): low frequency vibration, very very sensitive, small receptive field, motion detection (ant on back)
5. Pacinian’s corpuscle (most sensitive LTM): high frequency vibration, huge receptive field, most sensitive
6. Free nerve ending (HTM): pain

Question 5

What are the 3 types of sensory fibers around hair cells?

Answer 5

hair is a sense organ: each hair is innervated
each hair is innervated by multiple sensory fibre types
each sensory fibre innervates many hair

there are 3 types of hair sensory fibers:
1. Abeta LTMR: directional sensitivity
2. Adelta fibers LTMR: very sensitive, high dynamic range and responsive to cooling
3. C LTMR: very sensitive to slow light touch, insensitive to rapid movement: emotional hair (Sexy)

Question 6

What are the 3 types of muscle proprioceptors?

Answer 6

there are 3 types of muscles proprioceptors: 1a,1b,II
1a = muscle spindles = dynamic stretch receptors (movement)
1b = golgi tendon organs = muscle tension (tone)
11 = Nuclear chain fibers = muscle length

Question 7

What is the function of the muscle proprioceptors?

Answer 7

the function of the muscle proprioceptors is to give feedback of the position of the muscle to coordinate muscle activities (ex. stretch reflexes)

Question 8

Function of the second order neurons

Answer 8

second order neurons process and integrate the information they received from multiple first order neurons and send the information to the thalamus (directly or indirectly (via other organs).

Question 9

Organization of the dorsal horn

Answer 9

the dorsal horn of the spinal cord is somotopically organized
the distal body parts are in the medial dorsal horn
the proximal body parts are in the lateral dorsal horn

Question 10

4 types of neurons in the dorsal horn of the spinal cord

Answer 10

it contains the terminals of the primary afferents
the local interneurons: filter and process info from the periphery before transmission
2nd order neurons: project from conscious projection
Descending projections (terminals) from the cortex: these modulate the sensory inputs (amplify or suppress)

Question 11

What are the 3 pathways of the dorsal horn?

Answer 11

there are 3 different pathways from the dorsal horn to the thalamus: 1. Spinothalamic tract, 2. dorsal column tract, 3. spinocervical tract

Question 12

What is the spinothalamic tract?

Answer 12

The spinothalamic tract:
it is the anterolateral tract from the DH to the thalamus
It carries information regarding pain and temperature
it decussates locally and ascends directly to the contralateral thalamus

Question 13

What is the dorsal column tract?

Answer 13

The dorsal column tract
it carries information regarding light touch
it is a 2 fiber tract carrying information to the brainstem nuclei it then decussates after the brainstem and brings information to the contralateral thalamus
there are 2 different tracts:
the gracile tract: carrying light touch info from the lower body
the cuneate tract: carrying light touch info from the upper body.

Question 14

What is the spinocervical tract?

Answer 14

The Spinocervical tract
the spinocervical tract is an alternative route for light touch from the dorsal column tract.
it carries information from the dorsal horn → lateral cervical nucleus (ipsi) → decussate → thalamus

Question 15

somatosensory consequene of a full spinal cord lesion?

Answer 15

a Full spinal cord lesion leads to loss of all somatosensoy input from under the lesion

Question 16

The clinical consequence (somatosensory) of a hemisection of the spinal cord?

Answer 16

1. loss of ipsilateral low threshold sensation (dorsal horn tract or spinocervical tract)
2. loss of contralateral high threshold sensation (pain and temperature) because this decussate locally (spinothalamic tract)

Question 17

What is the function of the 3rd order neurons?

Answer 17

located within the thalamus
continue the info processing and relay the output to the somatosensory cortex
the thalamic neurons process and transmit the information to the sensory cortex for processing and perception

Question 18

What is the mechanism for the modulation of pain by the CNS?

Answer 18

descending control of nociception
It is the modulation of the activity of the spinal cord sensory circuits (2nd order neurons) to suppress nociceptive activity, inhibit reflexes and inhibit pain perception.
it starts in the periaqueductal grey (PAG) in the midbrain and relays the information vis the rostral ventromedial medulla (RVM)