Images For Sensory ✏️System/ Somatosensory System

Question 1

What classification can sensation be split into?

Answer 1

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

Question 2

What is a modality?

Answer 2

A unit of sensation relying on a distinct receptor type

Question 3

What two types of somatic sensation are there, what modalities are in each type and which receptors does each use?

Answer 3

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)

Question 4

What are other ways to say proprioception?

Answer 4

Joint position sense

Kinaesthetic sense

Question 5

What are the other names for primary sensory neurones? What do they do? Describe their layout in the nervous system

Answer 5

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

Question 6

What is responsible for initial encoding of sensory information and how does it encode this?

Answer 6

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

Question 7

What’s the difference between rapidly adapting receptors and slowly adapting receptors? Give examples of each

Answer 7

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

Question 8

What is a receptive field? How does the size of one affect the sensation to that area?

Answer 8

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)

Question 9

Why do dermatomes have ‘fuzzy’ boundaries?

Answer 9

Overlap of receptive fields of primary sensory neurones from adjacent dermatomes is one reason

Question 10

Describe the chain of three neurones in the somatosensory system

Answer 10

1. First order sensory N - receptor communicates -> cell bodies in dorsal root ganglion -> central axon ipsilateral to cell body -> communicated with ->
2. Second order SN - cell bodies cord dorsal horn or medulla -> decussate -> communicate with ->
3. Third order SN - cell bodies in thalamus -> project to primary sensory cortex/ postcentral gyrus

Question 11

What is somatotopy/ topographical representation? What is the purpose of this?

Answer 11

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)

Question 12

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?

Answer 12

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

Question 13

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?

Answer 13

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)

Question 14

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?

Answer 14

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)

Question 15

Describe the topographical organisation of axons in the dorsal column from the lower and upper parts of the body

Answer 15

Axons from lower part run most medially

Axons from progressively superior body segments added laterally to dorsal columns

Question 16

What is Lissauer’s tract?

Answer 16

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

Question 17

Describe the spinothalmic pathway or anterolateral system or spinothalamic tract organisation for the three chains of neurones

Answer 17

• 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

Question 18

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?

Answer 18

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

Question 19

If you get complete cord hemisection causing destruction to one lateral half of a single cord segment which structures will be completely destroyed unilaterally?

Answer 19

• dorsal horn
• ventral horn
• other cord grey matter
• white matter pathways
• dorsal and ventral roots

Question 20

What is Brown-Sequard syndomre?

Answer 20

A rare neurological condition caused by a complete cord hemisection -> destruction of one lateral half of a single cord segment (from trauma/ ischaemia)

Question 21

What are the signs of Brown-Sequards syndrome?

Answer 21

• 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

Question 22

Explain why rubbing a sore area relieves pain? What else relieves pain by the same mechanism?

Answer 22

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)

Question 23

Where do the spinothalamic neurones second neurones decussate normally?

Answer 23

At the ventral white commisure at the level of entry

Question 24

Syringomyelia what is it, symptoms, what part of the body would it affect first and why?

Answer 24

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

Question 25

Where do the spinothalmic first order neurones usually synapse?

Answer 25

In the dorsal horn normally

Question 26

Where is the second order cell body in spinothalmaic and dorsal column?

Answer 26

Dorsal horn

Medulla

Question 27

Where is the third order cell body in the spinothalamic and dorsal column?

Answer 27

Thalamus

Question 28

What are A fibres?

Answer 28

A mechanorecptor that sends signals to an inhibitory interneurone to prevent pain (in the dorsal horn)

Question 29

What are C fibres?

Answer 29

Spinothalamic first order neurones that send pain signals to second neurones in the dorsal horn