Somatosensation Flashcards
What kind of stimuli can somatosensation neurons detect?
- Fine discriminatory touch [light touch, pressure, vibration, flutter and stretch (mechanosensation)]
- Joint and muscle position sense (proprioception)
- Temperature (thermosensation)
- Pain (nociception)
- Itch (pruriception)
What three locations can somatosensation receptors be found?
- Exteroceptive division (cutaneous senses) surface of the body
- Proprioceptive division monitors posture and movement (sensors in muscle, tendons and joints)
- Enteroceptive division reports upon the internal state of the body and is closely related to autonomic function
How many neurons are located in the somatosensory pathway?
3- 1 peripheral and 2 central
Describe the somatosensory pathway
1st order (Sensory receptors, PNA, soma in DRG, synapse in dorsal column) 2nd order (projection, CNS, soma in dorsal horn, synapse in thalamus) 2nd order (projection, CNA, soma in thalamus, synapse in somatosensory cortex)
What does grading of potential mean?
Greater stimulation leads to firing of more AP which can add up/surmise- increase firing = increase AP release
AP frequency proportional to stimuli intensity
What is the modality of a neuron?
What kind of stimuli it transmits
When are neurons most sensitive?
At lower stimuli strength
What is the adaptation rate?
If receptor continues firing with stimuli or needs change
What do skin mechanoreceptors sense?
Touch, vibration, pressure
What do joint and muscle mechanoreceptors sense?
Propioception
What receptors sense pain?
Mechanical, thermal and polymodal nociceptors
What kind of receptors are low threshold?
Low threshold mechanoreceptors (LTMs): mediate fine discriminatory touch
Low threshold thermoreceptors: mediate cold through to hot
What kind of receptors are high threshold?
- High threshold mechanoreceptors (HTMs) (mechanical nociceptors) respond to high intensity mechanical stimuli
- Thermal nociceptors respond to extreme degrees of heat (>45°C), or cold < 10-15°C)
- Chemical nociceptors respond to substances in tissue [e.g. as in inflammation: prostaglandins, bradykinin, serotonin (5-HT), histamine, K+, H+ and ATP and many others]
- Polymodal nociceptors respond to at least two of the above
What kind of adaptations are there?
Slow adapting/tonic/satic
Fast adapting/phasic/dynamic
Very fast adapting
What kind of pattern does a slow adapting neuron give?
Fires regularly with continued stimuli and proportional to the stimuli.
Position and stretch etc
What kind of pattern does a fast adapting neuron give?
Fires when stimuli first applied but soon tails off.
What kind of pattern does a very fast adapting neuron give?
Changing stimuli- tends to just fire once when stimuli first applied.
What 4 axon types are there?
Aa/group 1
Ab/group 2
Ad/group 3
C
How do the axon types change
Aa -> C: Diameter decrease (20microm, 1.5microm), Myelination decrease (Thick, none), conduction velocity decrease (100m/s, 2m/s)
What stimuli do Aa axons conduct?
Proprioception
What stimuli do Ab axons conduct?
Mechano from skin
What stimuli do Ad axons conduct?
Pain and temp
What stimuli do C axons conduct?
Temp, pain and itch
How does two point discrimination work?
Apply 2 pointed stimuli separated by variable distance to part of body.
Patient says if can detect 1 or 2 points.
What 7 kinds of cutaneous nerve endings are there?
Free Nerve Endings Meissner’s corpuscles (encapsulated) Merkel’s discs Hair-end organs Krause end bulbs Ruffini endings (encapsulated) Pacinian corpuscles (encapsulated)
Describe free nerve endings
Everywhere
Describe Meissner’s corpuscles
Skin where 2 point discrimination needed.
Not in hairy skin.
Stroking, flutter, low frequency vibration (approx. 10 – 100 Hz)
Describe Merkel’s discs
Skin where 2 point discrimination needed.
Small number in hairy skin.
Very sensitive to touch (surface/texture/edges) and sustained pressure
Describe Krause end bulbs
Border of dry skin and mucous membranes.
Describe Ruffini endings
Dermis and joint capsules.
Strong mechanical pressure and drag/shearing force.
Describe Pacinian corpuscles
Dermis and fascia- close to bone.
Vibration (200-300Hz). Event detector…
Big in size and onion shaped.
What two types of LT MechanoR are there?
Fast adapting (FA) Slow adapting (SA)
Describe the RF of the FA and SA LTM
FA- Small
SA- Large
What receptors do FA1 LTM have?
Meissner’s corpuscles- V small RF
What receptors do FA2 LTM have?
Pacinian corpuscles- Small RF
What receptors do SA1 LTM have?
Merkel’s discs
What receptors do SA2 LTM have?
Ruffini endings
What is the grey matter of the spinal cord divided into?
Dorsal and ventral horns
Laminae of Rexed
What lamina of Rexed do Ad and C fibres (nociception) go to?
1 and 2
What lamina of Rexed do Ab fibres (LTM) go to?
3-6
What manina of Rexed do Aa fibres (proprioception) go to?
7-9
What are the two somatosensory pathways?
Dorsal column medial leminscal (DCML) pathway
Spinothalamic tract
What sensations does the DCML transfer?
Discriminatory touch, pressure, vibration, conscious proprioception- Aa and Ab
What sensations does the STT transfer?
Pain, thermosensation, crude touch, itch, tickle- Ad and C
Where are the primary neuron soma found in the DCML and where do they synapse?
DRG and synapse in brain stem (and SC for reflexes)
Where are the primary neuron soma found in the STT and where do they synapse?
DRG and synapse in the spinal cord
Where do the secondary neurons of the DCML decussate and where do they synapse?
Decussate in brain stem and synapse in thalamus.
Where do the secondary neurons of the STT decussate and where do they synapse?
Decussate in spinal cord and synapse in thalamus.
Where do the third order neurons in both DCML and STT arise and synapse?
Thalamus to primary somatosensory cortex
What are the two parts of the DCML?
Fasciculus gracilis
Fasciculus cuneatus
What spinal level distinguished whether somatosensation is part of the fasciculus gracilis or fasciculus cuneatus?
T6- Below FG and above FC
What nucli do the FG and FC synapse in?
Dorsal column gracile nucleus (GN) or cuneate nucleus (CN)
Where can the DCGN and DCCN be found?
Medulla
How do you test stereognosis?
Ask them to ID a familiar object by touch alone
How do you test vibration detection?
128Hz tuning fork on bone and feel for buzz
How do you test conscious proprioception?
Patient closes eyes and clinician moves limb them ID where it is/up or down.
What is lateral inhibition?
When one neuron is active, it inhibits the activity of its neighbours via inhibitory interneurons.
Why do we have lateral inhibition?
To improve discrimination
Which order neurons activate inhibitory neurons in lateral inhibition?
2nd order
Which order neurons do inhibitory neurons inhibit in lateral inhibition?
Third
Which nerves supply somatic sensory info from the face?
Trigeminal
What tract does somatosensation from trigeminal nerves travel through?
Trigeminothalamic tract
Where are somas of the trigeminothalamic tract located?
Trigeminal sensory ganglion (found in dura mater near petrous part of temporal bone).
Where do trigeminal neurons synapse to second order neurons?
Chief sensory nucleus (general tactile stimuli) Spinal nucleus (pain, temperature information)
What do second order neurons of the trigeminothalamic tract do and where do they synapse?
Decussate and synapse in ventroposteriomedial (VPM) nucleus of the thalamus.
Where is the primary somatosensory cortex found?
Postcentral gyrus
What areas can the primary somatosensory cortex be broken down to?
Bodmann’s areas
Which Bodmann’s areas are found in the primary somatosensory cortex?
1, 2, 3a, 3b
Where does the primary somatosensory cortex receive input from?
Thalamus
Name the Bodmann’s areas from anterior to posterior
3a, 3b, 1, 2
What input does Bodmann’s area 3a receive?
Proprioception (body position)
What input does Bodmann’s area 3b receive?
SA1 and FA1 (Touch)
What input does Bodmann’s area 1 receive?
FA mechanoreceptors (texture discrimination)
What input does Bodmann’s area 2 receive?
Pressure and joint position: object perception (size, shape of object – stereognosis)
How many layers make up the somatosensory cortex?
6
Which layer of the somatosensory cortex receives input from the thalamus?
4
What happens to the somatosensory cortex area for a body part that is lost?
It is co opted by neighbouring areas (ie. lose middle finger then ring and index take over)
What major area can the somatosensory cortex project to?
Posterior Parietal Cortex (SII)
WHat does the Posterior Parietal Cortex (SII) do?
Receives and integrates information from SI and other cortical areas (visual, auditory) to interpret them.