Somatosensory

Question 1

Myelin in CNS and PNS

Answer 1

From oligodendrocytes in CNS and Schwann cells in PNS

Question 2

Types of somatosensory fibres

Answer 2

Non myelinated= type C which is slowest and senses burning pain and hot temps
Small myelinated= type A delta is faster and senses sharp pain, gross touch and cold temps
Large myelinated= a alpha and a beta is fastest and sense proprioception, vibration and fine touch

Question 3

Horns of spinal cord

Answer 3

Ant is info from motor cortex of brain
Post processes external sensory info
Lateral sym

Question 4

Main parts of somatosensory pathways in white matter

Answer 4

White matter is myelinated axons of paired funiculi (bundles of nerve fibres)
Mainly medial lemniscal (post) pathway and spinothalamic (anterolateral) pathway

Question 5

Order neurones

Answer 5

1 is sensory and has receptors and convert stimuli to impulse
2 is cell body in spinal cord or brain stem
3 is cell body in thalamus
4 is cell body in sensory cortex of brain Post processes

Question 6

How does medial lemniscal pathway transport fine touch and proprioception

Answer 6

Large myelinated a alpha and a beta fibres through posterior or dorsal funiculi which is divided into lateral cuneate fascicle (somatosensory from arms and chest) or medial gracilis fascicle (trunk and legs)
1st synapse in medulla between 1st order neurone from cuneate and gracilis fascicles and the 2nd order neurones nuclei
The fibres then decussate at level of medial lemiscus then go through pons and midbrain to thalamus (2 to 3 order neurons)
To sensory cortex of brain in parietal lobe (3 to 4th order cortical neurone synapse)

Question 7

Reflex path

Answer 7

1st to post where it synapses w interneurons then axons go to ant horn where they synapse w motor neurones

Question 8

Spinothalamic tract

Answer 8

Sharp pain and cold temps, first order neurones A delta fibres
Hot temp, burning, crude touch first order neurones C fibres
To post horn where synapse w 2nd order neurones= C and A delta rise 2 vertebral segments, A delta is direct while C has to go through interneurons to synapse
Then decussate through central canal and divides into lateral (pain, pressure, temp) tract and anterior (crude touch) tract

Question 9

Lateral and ant funiculus from spinothalamic tract onwards

Answer 9

2nd synapse at thalamus to 3rd order neurone then reach sensory cortex of the brain 3 to 4 cortical neurone

Question 10

1st order / sensory neurones

Answer 10

Pseudounipolar neurones meaning theres no seperate dendrites and axons just a cell body w 2 branches
Peripheral (to peripheral tissues and receptive fields receive AP from touch eg) and central branch which takes impulse from peripheral branch to spinal cord

Question 11

Receptive fields

Answer 11

Smaller increases resolution meaning stimuli can be identified more precisely
Stimuli being distinct is shown by each neurone having to be separated by at least 1 receptive field which sends a negative signal between the 2 postive ones allowing 2 point discrimination
If strong stimulus then activates inhibitory interneurons that project onto surrounding order 1 neurones to suppress their activity called lateral inhibition which helps pin point stimulus by defining boundaries

Question 12

Stimulus strength and duration determined by what

Answer 12

Frequency of nerve firing
High fq in boiling water
Can adapt to same stimulus as sensory neurones get used to it and stop firing like phasic or fast adapting receptions which are active when stim starts but then stop firing like not feeling watch on wrist
Slow adapting or tonic on the other hand can vary length of stim as no loss of sensitivity like reading braille

Question 13

Mechanoreceptors

Answer 13

Touch to proprioception
For touch= meissners, merkel, Ruffini, pacinian

Question 14

Meissner corpuscles

Answer 14

Encapsulated in CT
In fingertips on dermis and sense vertical indentations on skin
Fast adapting and have small receptive fields to increase resolution

Question 15

Merkel discs

Answer 15

Epidermis of hairless skill
Non encapsulated
Small receptive field but are slow adapting or tonic

Question 16

Ruffini corpuscles

Answer 16

Encapsulated in dermis of hairy and hairless skin
And detects skin stretch
Also in joints to detect rotation
Slow adapting w big receptive fields w poorly defined boundaries and low resolution

Question 17

Pacinian corpuscles

Answer 17

Encapsulated
Subcutaneous or dermis of hairy and hairless skin
Sense vibration and have big receptive fields

Question 18

3 types of proprioceptors

Answer 18

Muscle spindle detect when muscle is stretched = found throughout perimysium (CT surrounding skeletal mm)
Golgi tendon organs located in tendons at location of muscle insertion and detect tendon stretch indirectly senses force exerted by muscle
Joint receptors= basically Ruffini corpuscles in joints so provide joint position and motion info by sensing stretch in articular capsules like in synovial joints = important for judging finger position

Question 19

Thermoreceptors

Answer 19

Slow adapting and detect changes in skin temperature
They have transient receptor potential channels (TRP ion channels) so allow Na and K through when activated
Transduction of heat involves subclass of TRP channels TRPV channels which r also activated by spicy food at like 32 to 48 Celsius
Transduction of cold is above class of thermoreceptors TRPM8 which are activated by menthol 10 to 40 Celsius also more superficial in dermis than TRPV
If further extreme then nociceptors become activated

Question 20

Nociceptors 3 types

Answer 20

Pain via thermal, mechanical and poly modal (both) receptors
Everyone has same threshold but tolerance is different