Session 3 Somatosensory System Flashcards
(26 cards)
What are the 2 major classifications of sensation?
Subdivision of 1 of them?
General Sensation which is subdivided into somatic (conscious) an visceral (unconscious)
Special Sensation
What does general sensation refer to?
Body wall and viscera (including parietal layer of serous membrane and mucosa of pharynx, nasal cavity and anus)
What does special sensation refer to?
Special senses of vision, taste, hearing, balance and smell
What is the meaning of a ‘modality’?
A modality can be thought of as a unit of sensation which relies on distinct receptor types
There are a variety of modalities of somatic sensation
What are the modalities of somatic sensation?
Spinothalamic system
- temperature = thermoreceptors
- pain = nociceptors
- pressure/crude touch = mechanoreceptors
Dorsal column-medial lemniscus system
- vibration = mechanoreceptors
- proprioception (or joint position sense or kinaesthetic sense) = muscle spindles and Golgi tendon organs etc
- fine touch = mechanoreceptors
- two point discrimination = mechanoreceptors
What are primary sensory neurones responsible for?
They receive information from receptors and are responsible for the initial encoding of sensory information
Each individual primary neurone recovers input from a single receptor type
Where are primary sensory neurones cell bodies and where do they collect information from?
Cell body in the dorsal root ganglion
Collect information from a single dermatome along their peripheral axon
Where do primary sensory neurones project?
Project into the spinal cord along their central axon
How is the strength of receptor activation converted and what do strong and weak receptor activation cause?
Strength of receptor activation is converted from an ANALOGUE signal to a DIGITAL signal
Strong receptor activation causes a high frequency of action potentials in the primary sensory neurone
Weak receptor activation causes a low frequency of action potential in the primary sensory neurone
What do rapidly adapting receptors response best to?
Changes in strength of stimulation
But their frequency of firing diminishes rapidly after the initial stimulus
Adaptation of these receptors explains why you are not aware of clothes on your skin
(E.g. mechanoreceptors!)
What happens with slowly adapting receptors?
They change their frequency of firing very little after the initial stimulus
This explains why pain can be so persistent and why you never really get used to having pain
(E.g. nociceptors)
What is a receptive field?
A single primary sensory neurone supplies a given area of skin = it’s receptive field
What happens if an are of skin is supplied by sensory neurones with relatively large receptive fields?
This are will haha low sensory acuity (poor two-point discrimination so two points need to be far away to distinguish)
E.g. skin of the back
What happens if an area of skin is supplied by sensory neurones with relatively small receptive fields?
Area will have High sensory acuity
Greater two-point discrimination
E.g. skin of fingertips
What is one of the reasons why dermatomes can have ‘fuzzy’ boundaries?
The overlap of receptive fields of primary sensory neurones from adjacent dermatomes
Explain the chain of the 3 neurones in the somatosensory system
First order sensory neurones
- cell bodies in the DRG
- communicate with a receptor
- central axon projects ipsilateral to the cell body
- project onto second order neurones
Second order sensory neurones
- cell bodies in the spinal cord dorsal horn OR medulla
- they decussate
- project onto third order neurones
Third order neurones
- have their cell bodies in the thalamus
- project to the primary sensory cortex (postcentral gyrus)
What is somatotopy
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
With some exceptions - adjacent body regions map to adjacent regions of the sensory system
What is good about somatotopy
This way of organising the pathways, minimises the amount of wiring required to transmit sensory information
(Motor is similar but runs in reverse)
DCML
First order neurones
Second order neurones
Third order neurones
Topographical organisation
AXONS of the first order neurones ascend ipsilateral through the DORSAL columns of the spinal cord
Concerning 1st order neurones:
- those from lower body (T7 and below): ascend through the GRACILE fasciculus to the GRACILE nucleus in the MEDULLA
- those from upper body (T6 and above): ascend through the CUNEATE fasciculus to the CUNEATE nucleus in the MEDULLA
2nd order neurones
- neurones in the GN project into the contra lateral thalamus in the medial lemniscus
- neurones in the CN project into the contra lateral thalamus in the medial lemniscus
3rd order
* thalamic neurones receiving information ultimately from the lower part of the body
Outline the DCLM system (in terms of neurones, where they ascend, nucleuses etc)
First order neurones
- their axons ascend ipsilateral (on the same side) through the dorsal columns of the spinal cord
- T7 and below: ascend through the gracile fasciculus TO the gracile nucleus (in the medulla)
- T6 and above: ascend through the CUNEATE fasciculus to the CUNEATE nucleus (in the medulla)
T6 and up = cuneate
T7 and down = gracile
Second order neurones
- neurones in the gracile nucleus project to the contralateral thymus in the medial lemniscus
- neurones in the cuneate do the same as above
Third order neurones
- thalamic neurones receiving information from:
- lower half of body (gracile nucleus) project to the medial part of the primary sensory cortex
- upper half of body (cuneate nucleus) project to the LATERAL part of the primary sensory cortex
Lower half
- gracile fasciculus - gracile nucleus - contralateral thalamus in medial lemniscus - medial part of the primary sensory cortex
Upper half
- cuneate fasciculus - cuneate nucleus - contralateral thymus in the medial lemniscus - lateral part of the primary sensory cortex
What is the topographical arrangement of the dorsal columns?
From the lower parts of the body, the axons run the most medially but then as you progress upwards, axons here are added laterally to the dorsal columns
Outline the arrangement of the spinothalamic pathway (axons, nucleuses etc)
Axons of the first order neurones project to the ipsilateral dorsal cord BUT the spinothalamic tract supplies the CONTRAlateral half of the body!
First order neurones
* project onto 2nd order neurones in the IPSILATERAL spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root
Second order neurones
- cell bodies = IN THE DORSAL HORN
- axons decussate in the ventral white commissure of the cord and then go on to form the spinothalamic tract
- spinothalamic tract projects to the thalamus
Third order neurones
- thalamic neurones receiving information from more INFERIOR parts of the body will project to the MEDIAL part of the primary sensory cortex
- thalamic neurones receiving information from more SUPERIOR parts of the body will project to the LATERAL parts of the primary sensory cortex
Outline the topographical organisation of the spinothalamic tract
- axons from the lower parts of the body run the most laterally and superficially
- axons from the more superior body segments are added medially and deeper onto the spinothalamic tract
NB: 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-Sequard syndrome?
What structures are completely destroyed unilaterally?
What will the signs be?
= a rare neurological condition characterised by a lesion in the spinal cord
* if we have a complete cord hemisection which causes destruction of one lateral half of a single cord segment (from trauma or ischaemia) the following structures will be completely destroyed unilaterally: Dorsal horn and root Ventral horn and root All other cord grey matter All white matter pathways
