Somatosensory System Flashcards
What sensory modalities are included in the somatosensory system?
Mechanosensation, proprioception, thermosensation, nociception, pruriception
How can somatic sensation be classed?
Exteroceptive
Proprioceptive
Visceral
Deep
How do primary sensory afferent fibres confer modality?
By their site of termination in the CNS (labelled line principle)
What effect do stimuli (mechanical etc) have on ion channels?
Opens them in nerve terminal to elicit a depolarising receptor (or generator) potential
What is the amplitude of a generator potential?
Graded and proportional to stimulus intensity
What do low threshold units respond to?
Low intensity (non damaging and non-painful) stimuli
What do high threshold units respond to?
High intensity (noxious, potentially damaging) stimilu, but are subject to both spinal segmental and supraspinal influences that gate input to second order neurons
What is adaptation?
A feature of primary sensory neurones that determines whether they change their firing rate only in response to a stimulus of changing intensity, or fire continuously throughout a constant stimulus
How are primary afferents classified?
Aα/I
Aβ/II
Aδ/III
C/IV
What is the receptive field (RF) of an afferent neurone?
The region that when stimulated with an adequate stimulus causes a response in that neurone
Where are Meissner’s corpuscles usually found?
Abundant in skin locations where two point discrimination is highest, not present in hairy skin
What are Merkel’s discs often grouped in?
Iggo domes
Where are Krause end bulbs found?
At the border of dry skin and mucous membranes
Where are Ruffini endings (pressure) found?
Within dermis and joint capsules
Where are Pacinian corpuscles found?
Within dermis and fascia
In the Iggo done what are multiple Merkel cells innervated by?
A single myelinated fibre
What receptor type is in the Aβ fibre group?
Meissner's Merkel Pacinian Ruffini Hair-guard (long)
What receptor type is in the Aδ fibre group?
Hair- down (short)
What receptor type is in the C fibre group?
C mechanoreceptor
What is contained within grey matter?
Cell bodies and sensory afferent terminals
What is contained within white matter?
Fibre tracts
What is the grey matter divided into?
Dorsal and ventral horns and ten distinct laminae of Rexed
Where do nociceptor afferents terminate?
Laminae I and II
Where do LTM afferents terminate?
Laminae III to VI
Where do proprioceptor afferents terminate?
Laminae VII to IX
How many primary afferent fibres cross the brainstem and decussate together in the DCML pathway?
All of them at the same point
How many sensory fibres in the spinothalamic tract cross over at the spinal cord?
All of them along the entire length of the spinal cord
Where do first order neurones in DCML pathway synapse?
Brainstem
Where do second order neurones synapse in the DCML pathway?
Thalamus
Where do first order neurones synapse in the STT?
All levels of the spinal cord
Where do second order neurones synapse in the STT?
Thalamus
What is the DCML pathway?
Dorsal column medial leminscal pathway
What is the STT?
Spinothalamic tract
What makes up the dorsal columns?
Medial fasciculus gracilis (gracile tract) and the lateral fasciculus cuneatus (cuneate tract)
Where does sensory input to T6 and below (including legs and lower trunk) travel in?
Gracile tract
Where does sensory input above T6 (including arms and upper trunk) travel in?
Cuneate tract
What is the major route by which touch and proprioceptive information ascends to the cerebral cortex?
DCML pathway
When primary afferent first order neurones enter the dorsal horn what do they form?
Synapses deep in the horn upon second order neurones
An ascending component via gracile/cuneate tract that synapses in the dorsal column gracile and cuneate nucleus
What path do axons from second order neurones of the dorsal column nuclei take?
Cross collectively in great sensory decussation and ascend in medial lemniscus to ventral posteriolateral (VPL)nucleus of the thalamus
Via what do third order neurones project to the primary somatosensory cortex(S1)?
The posterior internal capsule
What are some capabilities of the dorsal pathway?
Stereognosis-ability to recognise an object by feeling it Vibration detection Fine touch Conscious proprioception Weight discrimination
What is general somatic information from the anterior head mediated by?
Two trigeminothalamic pathways
What is included in the anterior head mediated by the trigeminal system?
Oral and nasal cavities Paranasal air sinuses Teeth Intracranial dura Cerebral arteries
Where do central terminals of the trigeminal nerve synapse upon second order neurones?
Chief sensory nucleus (general tactile stimuli) Spinal nucleus (pain, temperature information)
Where do second order neurones decussate and project via the trigeminal lemniscus to?
The ventroposteriomedial (VPM) nucleus of the thalamus
What do third order neurones relay information to the cortex via in the trigeminal system?
Thalamocortical neurones
Where is the somatosensory cortex located?
The post central gyrus of the parietal cortext immediately posterior to the central sulcus and adjacent to the posterior parietal cortex
What does the somatosensory cortex consist of?
Brodmann areas 1,2,3a,3b
What does the somatosensory cortex receive input from?
VP thalamus
What is the input and modality sensitivity for BA 3a?
Proprioceptors (muscle spindles)- body position
What is the input, modality sensitivity and lesion effect for BA 3b?
Cutaneous- SA (Merkel cells) and RA (Meissner’s) receptors, BA 3a.
Touch
Deficits in texture, shape, discrimination
What is the input, modality sensitivity and lesion effect for BA 1?
Cutaneous- RA mechanoceptors, BA 3b
Texture discrimination
Loss of texture discrimination
What is the input, modality sensitivity and lesion effect for BA 2?
Joint afferents, Golgi tendon organs, deep tissues, BA 3a,3b Object perception (size, shape, stereognosis) Impaired grasping and size and shape discrimination (astereognosis)
What are the 6 layers of the somatosensory cortex?
Molecular (Lamina I) External granular (II) External pyramidal (III) Internal granular (IV) Internal pyramidal (V) Multiform (VI)
What extends across the 6 layers of the somatosensory cortex?
Columns
What do adjacent strips of cortex map?
The same body parts in parallel to each other- not identical but mirror images
What happens if a body part is lost in an accident or amputated to the area of SI that represents it?
It is used by other sensory inputs
What happens if sensory input from a finger increases?
The cortical representation of that digit, over months, increases relative that of inputs generating less activity
What does the posterior parietal cortex do (SII)?
Receives and integrates information from SI and other cortical areas (visual, auditory) and sub-cortical areas (thalamus). Important for deeper meaning of SI information
What temperatures do thermal nociceptors respond to?
> 45°C or
