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clinical neuroscience > Somatosensory system > Flashcards

Somatosensory system Flashcards

1
Q

What can sensation be split into? Two groups:

A

General sensation - body wall and viscera (including parietal layer of serous membranes, mucosa of pharynx, nasal cavity and anus)

Special sensation - special sense vision, hearing, balance, taste and smell

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2
Q

What can general sensation be further divided into?

A

Somatic - conscious, well localized pain

Visceral - unconscious, poorly localised pain

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3
Q

What are different types of somatic sensation known as?

A

Modalities

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4
Q

Define a modality

A

Unit of sensation - each modality has a distinct receptor type

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5
Q

What are the groups which each modality are within?

A

Spinothalamic system
or
Dorsal column - medial leminiscus system

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6
Q

Modalities within the spinothalamic system and their receptor

A

Temperature - thermoreceptors
Pain - nociceptors
Pressure/crude touch - mechanoreceptors

(The precious cord)

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7
Q

Modalities within the dorsal column - medial lemniscus system

A

Vibration - mechanoreceptors
Proprioception or joint position sense - variety much as muscle spindles and golgi tendon organs
Fine touch - mechanoreceptors
Two point discrimination - mechanoreceptors

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8
Q

What is the first step involved in encoding sensory information?

A

Primary sensory neurones (aka dorsal root ganglion neurones or primary afferents or first order neurones) receive information from receptors

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9
Q

Where is the cell body of the primary sensory neurone?

A

Dorsal root ganglion (round bit of dorsal root when you draw spinal cord)

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10
Q

Where do primary sensory neurones project to?

A

Ipsilateral side of the spinal cord

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11
Q

Where does primary sensory neurone receive its information from?

A

Single receptor type

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12
Q

What happens when there is strong stimulus at receptor vs weak stimulus (eg being pinched hard or just pinched lightly)

A

Strong receptor activation = high frequency of action potentials along primary sensory neurone

Weak receptor activation = low frequency of action potentials along primary sensory neurone

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13
Q

What are the two types of receptors?

A

Rapidly adapting receptors
Slowly adapting receptors

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14
Q

What do rapidly adapting receptors do and an example of one?

A

Eg mechanoreceptors

Respond best to changes in strength of stimulation but frequency of firing diminishes over time after initial stimulus

Adaptation of these explains why we cannot feel clothes on our skin

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15
Q

What do slowly adapting receptors do and an example?

A

eg Nociceptors - pain receptors
These change their frequency of firing very little after initial stimulus, this explains why pain is persistent, never ‘get used to it’

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16
Q

What is a receptive field?

A

A single primary sensory neurone supplies a given area of skin (small area within given dermatome for example)

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17
Q

What happens if an area of skin is supplied by a primary sensory neurone that has a large receptive field (detects touch in large area)?

A

Low sensory acuity - poor two point discrimination so two points would need to be far apart to be distinguished eg skin of the back (cannot localise touch well)

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18
Q

What happens if an area of skin is supplied by a primary sensory neurone with small receptive field?

A

High sensory acuity - great two point discrimination, points could be close together and still feel both eg fingertips

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19
Q

Are receptive fields distinct from eachother?

A

No there is some overlap - this is why dermatomes have fuzzy boundaries and you should always test centre

20
Q

What is the basic chain of neurones in order in the somatosensory system?

A

First order –> 2nd order –> 3rd order

21
Q

What are 4 facts about first order neurones in somatosensory system?

A

Have cell bodies in the dorsal root ganglion

Communicate with receptor

Central axon projects into spinal cord ipsilateral to cell body

Project onto second order neurones

22
Q

3 facts about second order neurones in somatosensory system

A

Have cell bodies in spinal cord dorsal horn or medulla

Decussate - cross midline to other side of medulla/spinal cord

Project onto third order neurones

23
Q

3 facts about third order neurones in somatosensory system

A

Have cell bodies in thalamus
Project onto primary sensory cortex (post central gyrus)

24
Q

What is somatotopy or topographical representation of brain?

A

Principle idea that for every point on the surface of the body there is an equivalent point that can be identified along the sensory pathway
Adjacent body regions map to adjacent regions of sensory system eg hand is next to wrist (there are some exceptions)

25
Q

Why is there somatotopy?

A

Minimises wiring required to transmit sensory information

26
Q

What happens to sensory input at the primary sensory cortex sensory homunculus?

A

All modalities converge (eg head area of the cortex deals with pain, temp, vibration etc all at the same time, no separate places)

27
Q

What is the dorsal column - medial lemniscus system responsible for?

A

Carrying impulses concerning:
- light touch
- vibration
- two point discrimination
- proprioception

Dorsal column likes volleyball 2 player

28
Q

What do the first order neurones do in the dorsal column - medial lemniscus?

A

Ascend ipsilaterally through dorsal columns of the spinal cord

29
Q

Where do the first order neurones of the dorsal column ascend to?

A

From lower body - T7 and below - ascend through gracile fasciculus to gracile nucleus in medulla

From upper body - T6 and above - ascend through cuneate fasciculus to cuneate nucleus in medulla

30
Q

What do the second order neurones in the dorsal column medial lemniscus do?

A

Neurones in gracile nucleus project to contralateral thalamus in the medial lemniscus

Neurones in cuneate nucleus project to the contralateral thalamus in the medial lemniscus

(second order neurones always are the ones which decussate REMEMBER)

31
Q

What do the third order neurones of the dorsal column medial lemniscus system do?

A

Thalamic neurones receiving info from lower half of body (from gracile nucleus) project onto medial sensory cortec

Thalamic neurones receiving info from upper half body (from cuneate nucleus) project to lateral part of sensory cortex

32
Q

What is the topographical organisation of the dorsal columns for the dorsal column medial lemniscus system?

A

Axons from lower part of body are more medial
Axons from upper part of body are added laterally

(imagine as they ascend to medulla that the primary sensory neurones are added to the outside from each spinal level)

33
Q

What is the spinothalamic pathway responsible for?

A

Carrying impulses concerning:
Pain
Temperature
Crude touch

(think Spinothalamic need to Survive, primal)

34
Q

What do the first and second order neurones of the spinothalamic tract supply?

A

The contralateral half of the body - they project to ipsilateral dorsal cord as cell body

(2nd order neurone then immediately crosses to the other side)

35
Q

What do the first order neurones do in the spinothalamic tract?

A

Project onto second order neurones in the ipsilateral spinal cord in the dorsal horn at the same segment which they enter the cord through dorsal root

(basically they have cell body in dorsal root ganglion, enter dorsal root and enter dorsal horn)

36
Q

What do the second order neurones do in the spinothalamic system?

A

The cell bodies are in the dorsal horn

Axons decussate (cross midline) in the ventral white commissure of the cord and then go on to form spinothalamic tract (ascending up)

Spinothalamic tract projects to thalamus

37
Q

What do third order neurones do in the spinothalamic pathway?

A

Thalamic neurones receiving information from inferior body project to medial part of the primary sensory cortex

Thalamic neurones receiving information from superior parts of the body project to lateral part of primary sensory cortex

38
Q

What is the topographical organisation of the spinothalamic tract in cord

A

Axons from lower part of body run most laterally/superficially

Axons from progressively superior body segments are added medially/deeper onto the spinothalamic tract

This is the opposite to dorsal columns and is due to the decussation of the STT second order neurones at the level of entry of the first order neurones

39
Q

What is brown sequard syndrome?

A

Complete cord hemisection causing destruction of one lateral half of a single cord segment resulting from trauma/ischaemia

40
Q

What structures are destroyed completely unilaterally in brown sequard syndrome?

A

Dorsal horn
Ventral horn
Other cord grey matter
All white matter pathways
Dorsal and ventral roots

41
Q

What signs will occur in Brown-Sequard syndrome?

A

Ipsilateral complete segmental anaesthesia affecting single dermatome (due to destruction of dorsal root and horn)

Ipsilateral loss of dorsal column modalities (vibration, light touch, two point, proprioception) below segment - dorsal columns cannot ascend (they ascend on same side up to medulla) these will be blocked at level of lesion eg if C5 was lesion, L4 dorsum modalities could not travel up so would be lost

Contralateral loss of spinothalamic modalities at and below segment (contralateral as these cross at level of spinal cord so those below on ipsilateral side will not be affected as they will have crossed to other side, but those on contralateral side will cross and try to ascend)

42
Q

Why can contralateral loss of spinothalamic modalities in Brown-Sequard syndrome sometimes start a bit lower than lesion level? (preserve some function at lesion level, why?)

A

Lissauers tract - some primary sensory neurones can ascend so are spared as they travel up a couple segments (eg lesion at C5 can sometimes spare C5 and C5 spinothalamic contralateral pathways)

43
Q

How can pain receptors be inhibited in spinal cord?

A

Second order neurones of spinothalamic system dealing with pain receive nociceptive primary afferents (first order neurones) and inhibitory interneurones which contain endorphin encephalin

These encephalingergic interneurones can be activated by incoming impulses from mechnoreceptors (eg rubbing a sore area relieves pain)

They can also be activated by descending input from higher centres

44
Q

Examples of things that can activate inhibitory interneurones at 2nd order neurones in spinothalamic pathway

A

Higher centres - periaqueductal grey matter, nucleus of raphe

Mechanoreceptors

45
Q

Draw it out following Steve Jacques Lecture

A

:)

clinical neuroscience (24 decks)

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