Images For Sensory ✏️System/ Somatosensory System Flashcards
What classification can sensation be split into?
General sensation (body wall and viscera - parietal layer of serous membrane & mucosa pharynx/ nasal cavity/ anus):
- somatic sensation (conscious)
- visceral sensation (usually unconscious)
Special sensation:
Senses
What is a modality?
A unit of sensation relying on a distinct receptor type
What two types of somatic sensation are there, what modalities are in each type and which receptors does each use?
Spinothalamic system
- temperature (thermoreceptors)
- Pain (nociceptors)
- Pressure/ crude touch (mechanoreceptors)
Dorsal column- medial lemniscus system
- vibration (mechanorecptors)
- proprioception (muscle spindles and Golgi tendon organs)
- fine touch (mechanorecptors)
- two point discrimination (mechanoreceptors)
What are other ways to say proprioception?
Joint position sense
Kinaesthetic sense
What are the other names for primary sensory neurones? What do they do? Describe their layout in the nervous system
Dorsal root ganglion neurones or primary afferents or first order secondary neurones or pseudunipolar neurones
Receive info from receptors and are responsible for initial encoding of sensory info - Each individual NEURONE receives input from a single receptor TYPE
Have their cell body in dorsal root ganglion & collect info from a single dermatome along peripheral axon - project into spinal cord along central axon
What is responsible for initial encoding of sensory information and how does it encode this?
Primary sensory neurones/ DRGN/ PA/ FOSN/ PN
Input from single receptor type - strength of receptor activation is converted from an analogue signal (ion flux during generator potential) to a digital signal (frequency of action potentials in PSN) - strong receptor activation = high frequency of APs
What’s the difference between rapidly adapting receptors and slowly adapting receptors? Give examples of each
Rapidly adapting receptors: respond best to changes in strength of stimulation, frequency of firing diminishes rapidly after initial stimulus e.g. mechanoreceptors and not being aware of clothes on your skin
Slowly adapting R: change their frequency of firing very little after initial stimulus e.g. nociceptors and persistent pain sensation
What is a receptive field? How does the size of one affect the sensation to that area?
The given area of skin supplied by a single primary sensory neurone
If an area of skin is supplied by a neurone with a large receptive field then the area would have low acuity so poor 2-point discrimination e.g. skin of back
(Low receptive field neurone gives high sensory acuity e.g. skin fingertip)
Why do dermatomes have ‘fuzzy’ boundaries?
Overlap of receptive fields of primary sensory neurones from adjacent dermatomes is one reason
Describe the chain of three neurones in the somatosensory system
- First order sensory N - receptor communicates -> cell bodies in dorsal root ganglion -> central axon ipsilateral to cell body -> communicated with ->
- Second order SN - cell bodies cord dorsal horn or medulla -> decussate -> communicate with ->
- Third order SN - cell bodies in thalamus -> project to primary sensory cortex/ postcentral gyrus
What is somatotopy/ topographical representation? What is the purpose of this?
Point for point correspondence of an area of the body to a specific point on the CNS -
Some exceptions but in general adjacent body regions map to adjacent regions of sensory system e.g. in sensory cortex the hand is represented next to the wrist
This minimises the amount of wiring required to transmit sensory info
(Motor system has similar organisation)
How is sensory information organised at the level of spinal nerves and at the levels of the thalamus and above? How do the modalities change?
At the level of spinal nerves/ spinal cord - dermatomes organisation
Thalamus and above - homuncular pattern
At the level of the sensory homunculus all modalities converge e.g. head area of sensory cortex deal with: pain, temperature, vibration etc. Simultaneously
Describe the journey of the chain of three neurones for the dorsal column- medial leminiscus system for the lower body? Which spinal level does this start from?
For T7 and below:
- first order neurones
Ascend through gracile fasciculus to gracile nucleus in medulla - 2nd order N
Project to contralateral thalamus in medial lemniscus - 3rd order N
Project to medial part of primary sensory cortex
(Axons run most medially in dorsal columns)
Describe the journey of the chain of three neurones for the dorsal column- medial leminiscus system for the upper body? Which spinal level does this start from?
From T6 and above:
- 1st order neurones
Ascend through cuneate fasciculus to cuneate nucleus in medulla - 2nd order N
Project to contralateral thalamus in medial lemniscus - 3rd order N
Project to lateral part of primary sensory cortex
(From progressively superior body segments axons added laterally to the dorsal columns)
Describe the topographical organisation of axons in the dorsal column from the lower and upper parts of the body
Axons from lower part run most medially
Axons from progressively superior body segments added laterally to dorsal columns
What is Lissauer’s tract?
A white matter tract in which spinothalmic first order neurones can ascend (or descend) 2 spinal segments before synapsing onto second order neurones in the dorsal horn
Only used 15% of the time by spinothalamic neurones - usually they synapse and decussate at the level of entry
Describe the spinothalmic pathway or anterolateral system or spinothalamic tract organisation for the three chains of neurones
- first order neurones
Axons project to ipsilateral dorsal cord and (can ascend (or descend) two spinal segments via Lissauer’s tract) generally synapse in the segment at which they entered the cord through the dorsal root onto ->
-2nd order N
Cell bodies in dorsal horn, axons decussate in ventral white commissure of cord then go to form spinothalamic tract which projects to the thalamus ->
- 3rd order N
Thalamic neurones relieving info from inferior body project to media, part of primary sensory cortex OR
Info from superior part to to lateral PSC
Describe the topographical organisation of the spinothalamic tract axons from the lower and upper parts of the body. How does this compare to the organisation of the dorsal columns?
Axons from lower parts of the body run most laterally/ superficially
Axons from progressive,y superior body segments are added medially/ deeper onto spinothalamic tract
- this opposes the dorsal column because the STT 2nd order neurones decussate at the level of entry of the 1st order neurones
If you get complete cord hemisection causing destruction to one lateral half of a single cord segment which structures will be completely destroyed unilaterally?
- dorsal horn
- ventral horn
- other cord grey matter
- white matter pathways
- dorsal and ventral roots
What is Brown-Sequard syndomre?
A rare neurological condition caused by a complete cord hemisection -> destruction of one lateral half of a single cord segment (from trauma/ ischaemia)
What are the signs of Brown-Sequards syndrome?
- Ipsilateral complete segmental anaesthesia affecting a single dermatome (destruction of dorsal root and dorsal horn)
- ipsilateral loss of dorsal column modalities (fine tough, vibration, 2 point discrimination, proprioception) from destroyed segment down
- contralateral loss of spinothalamic modalities (pain, crude touch, temperature) from destroyed segment down *
- bc of Lissauer’s tract can be from one or two levels below depending on how the first order neurones ascend
Explain why rubbing a sore area relieves pain? What else relieves pain by the same mechanism?
Second order spinothalamic neurones receive nociceptive primary pain afferents as well as inhibitory interneurones which contain the endorphin ENCEPHALIN
These encephalinergic interneurones can be activated by incoming impulses from mechanorecptors (rubbing) or from the cortex (thinking about getting rid of the pain)
Where do the spinothalamic neurones second neurones decussate normally?
At the ventral white commisure at the level of entry
Syringomyelia what is it, symptoms, what part of the body would it affect first and why?
Bilateral loss of spinothalamic modalities
Would affect upper body first as axons add medially to the spinothalamic tract as we go up the body
Condition where a cyst or cavity forms in the spinal cord around the ventral white commissure
Where do the spinothalmic first order neurones usually synapse?
In the dorsal horn normally
Where is the second order cell body in spinothalmaic and dorsal column?
Dorsal horn
Medulla
Where is the third order cell body in the spinothalamic and dorsal column?
Thalamus
What are A fibres?
A mechanorecptor that sends signals to an inhibitory interneurone to prevent pain (in the dorsal horn)
What are C fibres?
Spinothalamic first order neurones that send pain signals to second neurones in the dorsal horn
