Somatic and visual sensation

Question 1

What is the difference between rapidly and slow adapting units?

Answer 1

Rapidly adapting units initially fire with high frequency, but then frequency of impulses decrease even though the receptor continues to be stimulated. This means we lose conscious perception of irrelevant stimulus (example of these are cutaneous mechanorecpetors).
Slow adapting units will never decrease the frequency of impulses (examples of these are pain receptors)

Question 2

What is a receptive field?

Answer 2

The area of skin that one sensory nerve innervates. Receptive fields tend to overlap- means dermatomes have ‘fuzzy’ boundaries.

Question 3

How is the size of the receptive field related to sensory acuity of that area.

Answer 3

Smaller the receptive field the greater the acuity. (acuity proportional to 1/ size of receptive field)

Question 4

Describe the dermatomal organisation of neurones in the dorsal column and spinothalamic pathways?

Answer 4

Doral column- C4 located laterally, S5 medially

Spinothalamic- C4 located proximally (to grey matter/ canal), S5 located distally

Question 5

What modalities are sensed by the dorsal column and the spinothalamic pathways?

Answer 5

dorsal column- light tough, proprioception, vibration and 2 point discrimination
Spinothalamic- pain, tempreature, crude touch

Question 6

Describe the path and locations of cell bodies of the primary, secondary and tertiary neurones of the dorsal- column- medial-lemniscus pathway

Answer 6

Primary- cell body found in dorsal root ganglion, axon travels up dorsal column (no decussation)
secondary- cell body of upper limb neurones found in cuneate nucleus, cell body of lower limb found in nucleus gracillis (both medulla). Axons follow medial lemniscus pathway to thalamus
Tertiary- cell body in thalamus projects to relevant area of sensory homunculus in parietal lobe

Question 7

Where will the sensory deficit be if there is an isolated lesion of the right dorsal column in the spinal cord?

Answer 7

the ipsilateral side- as doesn’t decussate until the medulla

Question 8

Why may some vegans get bilateral loss of light tough, proprioception, vibration and two point discrimination across 2/3 dermatomes?

Answer 8

B12 deficiency can lead to demyelination of the dorsal column, often bilateral and affects a couple of levels. the demyelination can usually be seen with an MRI.

Question 9

Describe the path of the spinothalamic pathway

Answer 9

Primary sensory neurone cell body found in DRG. Axon enters dorsal horn at same level as enters spine.
Secondary neurone cell body in dorsal horn. Axon travels anteriorly and decussates at the white commissure (white matter between the two ventral horns), axons ascends to thalamus.
Tertiary sensory neurone cell body found in thalamus. Axons travel to relevant area of motor homunculus.

Question 10

What is syringomyelia? What will it present with?

Answer 10

A cyst in the white commissure of the spinal cord. Leads to bilateral loss of pain, temp and crude touch. Number of dermatomes affected depends on how long it is and the level it appears at. If width increases it may affect other structures. They usually occur in C spine.

Question 11

Describe the difference in presentation between sharp cord transections and large lesions- eg due to crushing.

Answer 11

Sharp transections- ascending and descending neurones cut but relatively little grey matter and LMNs destroyed- means reflexes may still be intact.
Crushing injuries more devastating as grey matter lost as well as white + LMN loss = areflexia

Question 12

Explain the presentation of brown- sequard syndrome?

Answer 12

One half of spinal cord transected:

• Complete ipsilateral loss of affected dermatome as dorsal root and horn is lost)
• Ipsilateral loss of dorsal column (light tough, vibration etc) as this doesn’t decussate till medulla
• Contralateral loss of spinothalamic - as this decussates at level of associated spinal nerve
• Ipsilateral UMN signs as corticospinal tract cut, LMN signs at level of lesion and initially due to spinal shock

Question 13

What is the descending control of pain?

Answer 13

Where fibres descending from the periaqueductal grey area of the midbrain are able to inhibit transmission of pain stimuli from primary to secondary neurones via release of serotonin and enkephalin. This means you can psychologically block pain.

Question 14

Why does rubbing a sore bit stop it hurting

Answer 14

Gating of pain:
Mechanoreceptors stimulation leads to inhibition of primary to secondary transmission of pain via stimulation of an interneurone which releases enkephalin.

Question 15

Give 3 causes of insensitivity to pain?

Answer 15

diabetic neuropathy, congenital insensitivity to pain, leprosy

Question 16

Describe how light stimuli are transmitted to the optic nerve?

Answer 16

Light detected by rods and cones, which synapses with bipolar cells (infront of the rods and cones), which synapses with the ganglionic cells (infront of bipolar cells), axons of ganglionic cells travel to optic nerve.

Question 17

What is the macula?

Answer 17

The area with the highest density of cones, there is also a dip here called the fovea centralis which is where the neural layer is thinner so light can more easily travel through it, both of these things leads to higher acuity vision.

Question 18

Why do we have a blind spot?

Answer 18

At the optic disc there are no photoreceptor cells.

Question 19

What is amaurosis fugax?

Answer 19

the result of a retinal artery occlusion, it is described as seeing a curtain come down over your vision

Question 20

Describe the visual pathway

Answer 20

• Nasal fibres decussate at optic chiasm
• Temporal fibres stay ipsilateral
• Optic tract runs into lateral geniculate nucleus
• superior nasal and temporal fibres come together to form superior radiation which travels through parietal lobe to primary visual cortex
• inferior nasal and temporal fibres come together to form inferior radiations which go through temporal lobe to primary visual cortex

Question 21

whats the difference between the optic nerve and optic tact?

Answer 21

Optic nerve is before chiasm optic tract is after chiasm

Question 22

Describe the presentation and location of lesion causing monocular blindness? Give 2 causes?

Answer 22

Complete loss of vision in one eye.
Caused by ipsilateral lesion of optic nerve, may be due to glioma or retinoblastoma in children or optic sheath meningiomas in middle aged ppl.

Question 23

Pt lost lateral half of vision in both eyes (has tunnel vision). What is name for this and what is cause

Answer 23

Bitemporal hemianopia
Lesion of optic chiasm- loss of all nasal fibres = loss of peripheral fields of vision. Usually due to pituitary tumour, raised ICP, dilation of anterior communicating artery.

Question 24

Pt has lost medial feilds of vision in right eye and lateral feilds of vision in left eye.
Name deficit and describe cause

Answer 24

Left homonymous hemianopia.
Loss of temporal fibres in right eye and nasal fibres in left eye= lesion of right optic tract. Often due to strokes, but also neoplasms and trauma.

Question 25

Pt presents with loss of both left lower visual field quadrants.
Name the deficit and describe the cause?

Answer 25

Lost superior temporal from right eye and superior nasal from left eye.
Lesion is therefor at right superior radiation

Question 26

Describe and explain the cause of a left homonymous hemianopia with macula sparing?

Answer 26

Stroke has affected the posterior cerebral artery, effectively disabling the right superior and inferior radiations.
However the occipital pole (which represents the macula) has dual blood supply from the middle cerebral artery so the macula (central vision) will be spared.

Question 27

Describe the pathway of the light reflex?

Answer 27

• light stimulates optic nerve
• axons travel to occipital lobe and synapses in pretectal area
• this gives rise to neurones going to both edinger westphal nuclei
• Each EWN is stimulated (even if light only detected by one eye) and so parasympathetics of both oculomotor nerve are stimulated.

Question 28

What are the three aspects of accommodation reflex

Answer 28

• convergence (medial rectus)
• pupil constriction (constrictor pupillae)
• convexity of lens (cillary muscles contract)

Question 29

Describe the accomodation reflex pathway?

Answer 29

• follows visual pathway to LGN where a neurone synapses and goes to visual cortex
• info is interpreted and synapses with a fibre going to the pretectal area
• which stimulates EDW nucleus
• which stimulates occulomotor nerve

Question 30

Give 3 causes of perception of excessive glare?

Answer 30

Cateracts
Uveitis
Albinism
Anything causing more scattering of light in eyeball

Question 31

what happens in MLF syndrome/ inter nuclear opthalmoplegia?

Answer 31

On looking left or right with the adducting eye ball lags behind the adbucting eyeball, presents with diplopia

Question 32

describe the pathophysiology of MLF syndrome

Answer 32

1-lesion to medial longididunal fasciculs (MLF)
2-the MLF normally connects CNVI nucleus to CNIII nucleus to coordinate movement of the lateral rectus and the medial rectus on looking left and right
3-Therefore MLF lesion = medial rectus lags behind lateral rectus

Question 33

why is the function of the medial rectus normal during convergence of someone with MLF syndrome?

Answer 33

because the medial rectus and oculomotor nerve themselves are fine

Question 34

what will be the result of a tumor arising from the cerebral aqueduct of the mid brain?

Answer 34

as it grows it will impinge EDW nucleus first impinging pupil constriction, lens flattening and pupillary light reflex. Further growth = CNIII nucleus lesion causing ptosis down and out eye and loss of convergence plus blown pupil.

Question 35

What is amaurosis fugax?

Answer 35

Occlusion of the central retinal artery or ophthalmic artery leading to transient blindness, described as curtain coming down over vision