4. Somatosensory pathways Flashcards
What are the 2 types of sensation?
General and special
what is general sensation?
referring to the body wall and viscera (including parietal layer of serous membranes and mucosa of pharynx, nasal cavity and anus)
What are the 2 types of general sensation?
Somatic (conscious) and visceral (unconscious)
what is special sensation?
referring the special senses of vision, hearing, balance, taste and smell
What are the different somatic sensory modalities? (7)
- Temperature, crude touch, pain
- Fine touch, vibration, proprioception, 2 point discrimination
What are the 2 major sensory pathways?
- Dorsal column (medial lemniscus) pathway
- Spinothalamic pathway
What modalities is the spinothalamic pathway responsible for?
Modalities for survival: Temperature, pain and crude touch
What modalities is the dorsal column pathway responsible for?
Fine touch, vibration, proprioception and 2 point discrimination
describe the path of electrical activity down primary sensory neurone?
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
how does The strength of receptor activation affect action potential frequency?
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 adapting receptors? give an example.
(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
What are slowly adapting receptors? give an example.
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
What is a receptive field?
an area of skin that a single primary sensory neurone collects info from
What is the acuity proportional to?
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
why dermatomes can have ‘fuzzy’ boundaries what region is it important to test clincally?
- Due to overlap in receptive fields
- Autonomous region, region of the dermatome with no overlap
How many sensory neurones are there between receptor and primary sensory cortex?
3
First order sensory neurones
Second order sensory neurones
Third order neurones
What are the characteristics of first order sensory neurones?
- 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
What are the characteristics of second order sensory neurones?
- 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
What are the characteristics of third order sensory neurones?
- Have their cell bodies in the thalamus
- Project to the primary sensory cortex (postcentral gyrus)
Define somatotopy.
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
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?
minimises the amount of ‘wiring’ required to transmit sensory information
What is the change from dermatomal to homunculus organisation called and where does it occur?
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
what is the The dorsal column-medial lemniscus system (DCML) responsible for?
for carrying impulses concerning light touch, vibration, two point discrimination and proprioception
What are the 2 fasciculi of the dorsal column?
Gracile fasciculus and cuneate fasciculus
Where do the first order neurones synapse with the second order neurones in the dorsal column pathway?
Medulla, on the ipsilateral side (first order neurones do not decussate)
In which fasciculi of the dorsal column do the first order neurones ascend?
- Upper body (T6 and above) a the cuneate fasciculus
- Lower body (T7 and below) at the gracile fasciculus
At what nuclei do the first order neurones of the dorsal column pathway synapse?
- Upper body (T5 and above) a the cuneate nucleus in medulla (lateral)
- Lower body (T6 and below) at the gracile nucleus in medulla (medial)
Where do second order neurones in the dorsal column pathway decussate and what do they form?
Decussate at the medulla, form the medial lemniscus
where do the Neurones in the gracile and cuneate nucleus project to?
contralateral thalamus
where do third order neurones of the DCML system project?
- upper body thalamic neurones - lateral part of PSC
- lower body thalamic neurone - medial part of PSC
describe 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
what is the spinothalamic pathway responsible for?
for carrying impulses concerning pain, temperature and crude touch
which side do first order spinothalamic neurones present to and which side does the spinothalamic pathway supple?
Axons of first order neurones project to the ipsilateral dorsal cord, but the spinothalamic tract supplies the contralateral half of the body
Where do first order neurones synapse in the spinothalamic pathway?
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
Where do second order neurones in the spinothalamic pathway decussate?
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
where do third order neurones of the spinothalamic system project?
- upper body thalamic neurones - lateral part of PSC
- lower body thalamic neurone - medial part of PSC
describe 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
why is the topographical organisation of the spinothalamic tract and DCML different?
decussation of the STT second order neurones at the level of entry of the first order neurones whereas for DCML at medulla
What is the difference between position of the ascending sensory neurones in the spinothalamica and dorsal column pathway in the spinal cord?
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
what is Brown-Sequard syndrome
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.
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?
The dorsal horn The ventral horn All other cord grey matter All white matter pathways Dorsal and ventral roots
wha will be the sensory signs of Brown-Sequard syndrome
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)
What do bilateral symptoms indicate?
Indicate a midline lesion or care caused by systemic disturbances
e.g. syringomyelia
What is Lissauer’s tract?
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
describe how a central cord lesion can affect DCML and spinothalamic tracts differently
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
describe the 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
Why does rubbing a painful area of skin relieve pain?
Activation of mechanoreceptors which excite inhibitory enkephalinergic interneurones in the dorsal horn which inhibit second order sensory neurones of the spinothalamic pathway
which fibres carry impulses form mechanoreceptors in skin and which carry pain?
A fibres carry impulses form mechanoreceptors in skin
C fibres carry pain