Somatosensory System Flashcards
How can sensation be classified
Sensation can be split into the following classification
o General sensation, referring to the body wall and viscera (including parietal
layer of serous membranes and mucosa of pharynx, nasal cavity and anus)
Subdivided into somatic sensation (conscious), and visceral sensation
(usually unconscious)
o Special sensation, referring the special senses of vision, hearing, balance,
taste and smell
What is a modality
Somatic sensation
o There are a variety of modalities of somatic sensation. A modality can be
thought of as a ‘unit’ of sensation, relying on a distinct receptor type
Spinothalamic vs dorsalcolumn medial lemniscus
What are the functions of the spinothalamic system
Spinothalamic system
• Temperature (thermoreceptors)
• Pain (nociceptors)
• Pressure/crude touch (mechanoreceptors)
What are the functions of the demo system
Dorsal column-medial lemniscus system
• Vibration (mechanoreceptors)
• Proprioception, or joint position sense, or kinaesthetic sense
(detected by a variety of receptors such as muscle spindles
and Golgi tendon organs)
• Fine touch (mechanoreceptors)
• Two point discrimination (mechanoreceptors)
Describe primary sensory neurones
Primary sensory neurones (aka dorsal root ganglion neurones or primary
afferents or first order sensory neurones or psueudunipolar neurones) receive information from receptors and are responsible for the initial encoding of sensory information
Each individual primary neurone receives input from a single receptor type
Primary sensory neurones 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
Describe strength of receptor activation
The strength of receptor activation is converted from an analogue signal (related to ion flux during the generator potential) to a digital signal (which is the frequency of action potentials in the primary sensory neurone)
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
What are rapidly and slowly adapting receptors
o Rapidly adapting receptors (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
o Slowly adapting receptors (e.g. nociceptors) change their frequency of firing very little after the initial stimulus. This explains why pain can be so persistent, and you never really get ‘used to’ having pain
What are receptive fields
Receptive fields
o A single primary sensory neurone supplies a given area of skin (it’s receptive
field)
o If an area of skin is supplied by sensory neurones with relatively large
receptive fields, this area will have low sensory acuity (it would have poor two-point discrimination where two points would need to be far apart to be distinguished). The skin of the back has relatively low acuity
o If an area of skin is supplied by sensory neurones with relatively small receptive fields, this area will have high 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
o The overlap of receptive fields of primary sensory neurones from adjacent dermatomes is one of the reasons why dermatomes can have ‘fuzzy’ boundaries
Wgat are the chain of 3 neurones
First, second and third order
Describe first order neurones
First order sensory neurones
Have their cell bodies in the DRG
Communicate with a receptor
Their central axon projects ipsilateral to the cell body Project onto second order neurones
Describe second order neurones
Second order sensory neurones
Have their cell bodies in the spinal cord dorsal horn or medulla Decussate
Project onto third order neurones
Describe third order neurones
Thirdorderneurones
Have their cell bodies in the thalamus
Project to the primary sensory cortex (postcentral gyrus)
What is topographical representation
Somatotopy, or topographical representation
o The principle relating to the idea that for every point on the surface of the
body, an equivalent point can be identified along the sensory pathway
o With some exceptions, 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)
o This way of organising the pathways minimises the amount of ‘wiring’
required to transmit sensory information
o The motor system has a similar organisation, but running in reverse
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
o At the level of the sensory homunculus, all modalities converge (i.e. the head
area of the sensory cortex deals with pain, temperature, vibration etc. all at the same time)
Descrbe the dcml system
The dorsal column-medial lemniscus system (DCML)
o Responsible for carrying impulses concerning light touch, vibration, two point
discrimination and proprioception
o Axons of first order neurones ascend ipsilaterally through the dorsal columns
of the spinal cord
Describe 1o neurones in the dcml system
Concerning first order neurones of the DCML system:
Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla)
Those from the upper half of the body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla
Describe 2o neurones in the dcml system
Concerning second order neurones of the DCML system:
Neurones in the gracile nucleus project to the contralateral thalamus
in the medial lemniscus
Neurones in the cuneate nucleus project to the contralateral
thalamus in the medial lemniscus
Describe 3o neurones in the dcml system
ConcerningthirdorderneuronesoftheDCMLsystem:
Thalamic neurones receiving information ultimately from the lower half of the body (via gracile nucleus) project to the medial part of the primary sensory cortex
Thalamic neurones receiving information ultimately from the upper half of the body (via cuneate nucleus) project to the lateral part of the primary sensory cortex
Describe teh topographical organisation of the dorsal columns
Topographical organisation of the dorsal columns
Axons from the lower parts of the body run most medially
Axons from progressively superior body segments are added laterally
to the dorsal columns
Gove an overview os the stt system
The spinothalamic pathway or anterolateral system or spinothalamic tract (STT)
o Responsible for carrying impulses concerning pain, temperature and crude
touch
o Axons of first order neurones project to the ipsilateral dorsal cord, but the
spinothalamic tract supplies the contralateral half of the body
Describe 1o neurones of the stt
Concerning first order neurones of the STT:
They project onto second order neurones in the ipsilateral spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root (generally)
Describe 2o neurones of the stt
Concerning second order neurones of the STT:
Their cell bodies are in the dorsal horn
Their axons decussate in the ventral white commissure of the cord
and then go on to form the spinothalamic tract The spinothalamic tract projects to the thalamus
Describe 3o neurones of the stt
ConcerningthirdorderneuronesoftheSTT:
Thalamic neurones receiving information ultimately from more
inferior parts of the body project to the medial part of the primary
sensory cortex
Thalamic neurones receiving information ultimately from more
superior parts of the body project to the lateral part of the primary sensory cortex
Describe the topographical organisation of the stt
Topographical organisation of the spinothalamic tract
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
This is the opposite of the situation for the dorsal columns, and is due
to the decussation of the STT second order neurones at the level of entry of the first order neurones
What is brown-syquard syndrome
Brown-Sequard syndrome (limited to sensory features)
o If we consider a complete cord hemisection causing destruction of one lateral
half of a single cord segment resulting from trauma or ischaemia, the following structures will be completely destroyed unilaterally:
The dorsal horn
The ventral horn
All other cord grey matter
All white matter pathways
Dorsal and ventral roots
What are th signs of brown-syquard syndrome
This will lead to the following signs (making reference to the side of the lesion):
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 – advanced, so only pursue if you are keen!)
Describe descending modulation ofpain
Descending modulation of pain
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
