4. Somatosensory pathways

Question 1

What are the 2 types of sensation?

Answer 1

General and special

Question 2

what is general sensation?

Answer 2

referring to the body wall and viscera (including parietal layer of serous membranes and mucosa of pharynx, nasal cavity and anus)

Question 3

What are the 2 types of general sensation?

Answer 3

Somatic (conscious) and visceral (unconscious)

Question 4

what is special sensation?

Answer 4

referring the special senses of vision, hearing, balance, taste and smell

Question 5

What are the different somatic sensory modalities? (7)

Answer 5

• Temperature, crude touch, pain

- Fine touch, vibration, proprioception, 2 point discrimination

Question 6

What are the 2 major sensory pathways?

Answer 6

• Dorsal column (medial lemniscus) pathway

- Spinothalamic pathway

Question 7

What modalities is the spinothalamic pathway responsible for?

Answer 7

Modalities for survival: Temperature, pain and crude touch

Question 8

What modalities is the dorsal column pathway responsible for?

Answer 8

Fine touch, vibration, proprioception and 2 point discrimination

Question 9

describe the path of electrical activity down primary sensory neurone?

Answer 9

 receives input from a single receptor type
 have their cell body in the dorsal root ganglion, and collect information from a single dermatome along their peripheral axon
 Primary sensory neurones project into the spinal cord along their central axon

Question 10

how does The strength of receptor activation affect action potential frequency?

Answer 10

 Strong receptor activation causes high frequency of action potentials in the primary sensory neurone
 Weak receptor activation causes a low frequency of action potentials in the primary sensory neurone

Question 11

What are rapidly adapting receptors? give an example.

Answer 11

(e.g. mechanoreceptors) respond best to changes in strength of stimulation. However, their frequency of firing diminishes rapidly after the initial stimulus (i.e. they rapidly adapt). Adaptation of these receptors explains why you are not aware of your clothes on your skin

Question 12

What are slowly adapting receptors? give an example.

Answer 12

Have a continued response to a continuous stimulus

(e.g. nociceptors) change their frequency of firing
very little after the initial stimulus. This explains why pain can be so persistent

Question 13

What is a receptive field?

Answer 13

an area of skin that a single primary sensory neurone collects info from

Question 14

What is the acuity proportional to?

Answer 14

the smaller the receptive field the higher the sensory acuity (it would have great two-point discrimination where two points could be very close together to be
distinguished)
The skin of the fingertip has relatively high acuity
The skin of the back has relatively low acuity

Question 15

why dermatomes can have ‘fuzzy’ boundaries what region is it important to test clincally?

Answer 15

• Due to overlap in receptive fields

- Autonomous region, region of the dermatome with no overlap

Question 16

How many sensory neurones are there between receptor and primary sensory cortex?

Answer 16

3
First order sensory neurones
Second order sensory neurones
Third order neurones

Question 17

What are the characteristics of first order sensory neurones?

Answer 17

• communicates with the receptor
• cell body in dorsal root ganglion
• Their central axon projects ipsilateral to the cell body into the spinal cord
• Project onto second order neurones

Question 18

What are the characteristics of second order sensory neurones?

Answer 18

• Have their cell bodies in the spinal cord dorsal horn or medulla
• decussates (at level of entry into spinal cord or medulla)
• projects onto third order neurones

Question 19

What are the characteristics of third order sensory neurones?

Answer 19

• Have their cell bodies in the thalamus

- Project to the primary sensory cortex (postcentral gyrus)

Question 20

Define somatotopy.

Answer 20

point by point correspondence between body area and CNS (PSC)

-for every point on the surface of the body, an equivalent point can be identified along the sensory pathway

Question 21

what is the advantage of adjacent body regions map to adjacent regions of the sensory system (e.g. in the sensory cortex, the hand is represented adjacent
to the wrist) with some exceptions?

Answer 21

minimises the amount of ‘wiring’ required to transmit sensory information

Question 22

What is the change from dermatomal to homunculus organisation called and where does it occur?

Answer 22

Translational convergence, occurs in the spinal cord and thalamus

o Information becomes reorganised as we move upwards through the neuraxis, such that at the level of spinal nerves and spinal cord we have a dermatomal organisation, but at levels of the thalamus and above we have a ‘homuncular’ pattern

Question 23

what is the The dorsal column-medial lemniscus system (DCML) responsible for?

Answer 23

for carrying impulses concerning light touch, vibration, two point discrimination and proprioception

Question 24

What are the 2 fasciculi of the dorsal column?

Answer 24

Gracile fasciculus and cuneate fasciculus

Question 25

Where do the first order neurones synapse with the second order neurones in the dorsal column pathway?

Answer 25

Medulla, on the ipsilateral side (first order neurones do not decussate)

Question 26

In which fasciculi of the dorsal column do the first order neurones ascend?

Answer 26

• Upper body (T6 and above) a the cuneate fasciculus

- Lower body (T7 and below) at the gracile fasciculus

Question 27

At what nuclei do the first order neurones of the dorsal column pathway synapse?

Answer 27

• Upper body (T5 and above) a the cuneate nucleus in medulla (lateral)
• Lower body (T6 and below) at the gracile nucleus in medulla (medial)

Question 28

Where do second order neurones in the dorsal column pathway decussate and what do they form?

Answer 28

Decussate at the medulla, form the medial lemniscus

Question 29

where do the Neurones in the gracile and cuneate nucleus project to?

Answer 29

contralateral thalamus

Question 30

where do third order neurones of the DCML system project?

Answer 30

• upper body thalamic neurones - lateral part of PSC

- lower body thalamic neurone - medial part of PSC

Question 31

describe Topographical organisation of the dorsal columns

Answer 31

 Axons from the lower parts of the body run most medially

 Axons from progressively superior body segments are added laterally to the dorsal columns

Question 32

what is the spinothalamic pathway responsible for?

Answer 32

for carrying impulses concerning pain, temperature and crude touch

Question 33

which side do first order spinothalamic neurones present to and which side does the spinothalamic pathway supple?

Answer 33

Axons of first order neurones project to the ipsilateral dorsal cord, but the spinothalamic tract supplies the contralateral half of the body

Question 34

Where do first order neurones synapse in the spinothalamic pathway?

Answer 34

in the dorsal horn, on the ipsilateral side (first order do not decussate) in the segment at which they enter the cord through the dorsal root

Question 35

Where do second order neurones in the spinothalamic pathway decussate?

Answer 35

At the ventral white commissure of the cord at the segment where the first order neurone entered (which is anterior to the grey commissure), and head to the spinothalamic tract which projects to thalamus

Question 36

where do third order neurones of the spinothalamic system project?

Answer 36

• upper body thalamic neurones - lateral part of PSC

- lower body thalamic neurone - medial part of PSC

Question 37

describe Topographical organisation of the spinothalamic tract

Answer 37

 Axons from the lower parts of the body run most
laterally/superficially
 Axons from progressively superior body segments are added medially/deeper onto the spinothalamic tract

Question 38

why is the topographical organisation of the spinothalamic tract and DCML different?

Answer 38

decussation of the STT second order neurones at the level of entry of the first order neurones whereas for DCML at medulla

Question 39

What is the difference between position of the ascending sensory neurones in the spinothalamica and dorsal column pathway in the spinal cord?

Answer 39

Upper body (T5 above): lateral in the dorsal column and medial in the spinothalamic pathway
Lower body (T6 below): medial in the dorsal column and lateral in the spinothalamic pathway

Question 40

what is Brown-Sequard syndrome

Answer 40

Brown-Sequard syndrome (BSS) is a rare neurological condition characterized by a lesion in the spinal cord which results in weakness or paralysis (hemiparaplegia) on one side of the body and a loss of sensation (hemianesthesia) on the opposite side.

Question 41

If we consider a complete cord hemisection causing destruction of one lateral half of a single cord segment resulting from trauma or ischaemia (Brown-Sequard syndrome ), what structures will be completely destroyed unilaterally?

Answer 41

 The dorsal horn
 The ventral horn
 All other cord grey matter 
 All white matter pathways 
 Dorsal and ventral roots

Question 42

wha will be the sensory signs of Brown-Sequard syndrome

Answer 42

 Ipsilateral complete segmental anaesthesia affecting a single dermatome (due to destruction of dorsal root and dorsal horn)
 Ipsilateral loss of dorsal column modalities below the destroyed segment
 Contralateral loss of spinothalamic modalities at and below the destroyed segment (although level can be up to a couple of segments lower due to ascent of some primary afferents in Lissauer’s tract - some tracts below or at level of the damage may be spared)

Question 43

What do bilateral symptoms indicate?

Answer 43

Indicate a midline lesion or care caused by systemic disturbances
e.g. syringomyelia

Question 44

What is Lissauer’s tract?

Answer 44

Posterolateral tract near the dorsal horn that contains first order neurones of the spinothalamic pathway that are able to ascend 2/3 spinal levels before entering the dorsal horn and synpasing with the second order neurones

Question 45

describe how a central cord lesion can affect DCML and spinothalamic tracts differently

Answer 45

dissociated sensory loss:

initially, central lesion will affect the dorsal column modalities of lower limb more as they run more medially than upper limb

initially, central lesion will affect the spinothalamic modalities of upper limb more as the run more medially that lower limb fibres

Question 46

describe the Descending modulation of pain

Answer 46

o Second order neurones of the spinothalamic system dealing with pain receive nociceptive primary afferents as well as inhibitory interneurones which contain the endorphin encephalin
o These encephalinergic interneurones can be activated by incoming impulses from mechanoreceptors (hence explaining why rubbing a sore area relieves the pain)
o Additionally, these encephalinergic interneurones can also be activated by descending inputs from higher centres such as the periaqueductal grey matter or the nucleus raphe magnus

Question 47

Why does rubbing a painful area of skin relieve pain?

Answer 47

Activation of mechanoreceptors which excite inhibitory enkephalinergic interneurones in the dorsal horn which inhibit second order sensory neurones of the spinothalamic pathway

Question 48

which fibres carry impulses form mechanoreceptors in skin and which carry pain?

Answer 48

A fibres carry impulses form mechanoreceptors in skin

C fibres carry pain