What are the two main categories of sensation?
Somatic sensation and special sensation
The somatosensory pathways usually consist of how many neurones?
Three
With regards to the first order neurone (i.e. the primary sensory afferent), where will the a) receptor and b) soma be found?
a) in the periphery e.g. the skin b) more central, though still in the PNS e.g. dorsal root ganglion or cranial ganglia
What is found in the dorsal root ganglion? Where is this found?
A cluster of first order neurones (i.e. primary sensory afferents) / in the dorsal root of a spinal nerve, in the intervertebral foraminae
Complete the following sentence: when a sensory receptor is activated, the information is received by ? - this carries information within a ? to the ? where it connects with the ?
The first order neurone (primary sensory afferent) / peripheral nerve / CNS / second order neurone
Complete the following sentence: a projection from a second order neurone carries sensory information to the ?, often via a ? in the ? before reaching the ? for processing
Brain / 3rd order neurone / thalamus / cortex
Where are the cell bodies of somatosensory neurones located?
In clusters outside the vertebral column in the dorsal root ganglia
The afferent projections from first order sensory neurones enter the spinal cord where?
At the dorsal horns
What is the terminal of the 1st order sensory neurone?
The receptor
Where would the cell body be found of each of the following neurones in a somatosensory pathway coming from the limbs/trunk or posterior head: a) first order? b) second order? c) third order?
a) dorsal root ganglion b) dorsal horn of spinal cord c) thalamus
Where would the cell body be found of each of the following neurones in a somatosensory pathway coming from the anterior head: a) first order? b) second order? c) third order?
a) cranial ganglia b) brainstem nuclei c) thalamus
What is the sensory unit?
The whole first order neurone, including the cell body and the receptor
Sensory units/primary afferent neurones are especially tuned to respond to a specific type of energy which normally excites them. This is known as what?
Their modality
What do low threshold units respond to?
Low intensity, non-damaging stimuli
What do high threshold units respond to? What type of receptors are these?
High, noxious stimuli only / nociceptors
Thermal nociceptors respond to extremes of heat - what would these be?
< 10 degrees or > 45 degrees
Sensory receptors can be what three main types?
Free nerve endings, encapsulated nerve endings or specialised receptors
Which type of sensory receptor will detect tactile sensation e.g. touch and pressure?
Mechanoreceptors
Which type of sensory receptor will detect temperature changes?
Thermoreceptors
Which type of sensory receptor will detect painful stimuli?
Nociceptors
Which type of sensory receptor will detect changes in the position and movement of the body?
Proprioceptors
The area of skin innervates by the left and right dorsal roots of a single spinal segment is termed what?
A dermatome
What is the only part of the body which is not innervated dermatomally?
Anterior face
If a single dorsal root is cut, the corresponding dermatome does not lose all sensation. Why is this?
There is a degree of overlap with the region of adjacent dorsal roots
Axons from 2nd order sensory neurones form what?
Sensory ascending tracts
What are the two main sensory ascending tracts and where do each of them decussate?
DCML - in the medulla / spinothalamic - in the spinal cord
Complete the sentence: axons from the ascending spinal cord tracts reach the ? where they communicate with ? which project into the ? where signals give rise to the awareness of the feeling
Thalamus / 3rd order neurones / cortex
What is stereognosis? Which ascending spinal cord pathway allows this to be possible?
The ability to recognise an object by feeling it / DCML
Complete the following sentence about trigeminal sensation: impulses originate at the terminals of two trigeminal nerves, each of which has ? divisions. The soma of sensory neurones are located in the ?
Three / trigeminal sensory ganglion
Central terminals of the trigeminal nerve synapse upon second order neurones where, for general tactile stimuli?
The main sensory nucleus
Central terminals of the trigeminal nerve synapse upon second order neurones where, for pain and temperature?
The spinal nucleus
Complete the following sentence about trigeminal sensation: central terminals of the trigeminal nerve synapse upon 2nd order neurones in the respective nucleus. These in turn decussate and project via the ? to the ? nucleus of the ?
Trigeminal lemniscus / ventroposteriomedial (VPM) / thalamus
Complete the following sentence about trigeminal sensation: 3rd order neurones relay information to the cortex via ?
Thalamocortical neurones
In general, where are the somatosensory cortices located?
In the postcentral gyrus
Where is the primary somatosensory cortex located?
Immediately posterior to the central sulcus
Where is the secondary somatosensory cortex located?
Adjacent to the posterior parietal cortex
The primary somatosensory cortex is classically described as consisting of what?
Brodmann areas
The primary somatosensory cortex receives input from where?
The ventral posterior nucleus in the thalamus
Briefly, describe the order of structures on the somatosensory homunculus from medial to lateral?
Genitals, lower limb, trunk, upper limb, face, mouth viscera
How many cell layers are there in the somatosensory cortex?
6
What happens if a finger is lost in an accident or amputated?
After months, the area that used to represent that finger will now represent the adjacent fingers
What happens if the sensory input from a finger increases?
The area of cortex associated to that finger enlarges
What is the function of the secondary somatosensory cortex?
The integrate sensory information with other cortical areas
What is agnosia?
You can see a structure but can’t tell what it is
What is astereognosia?
Can touch an object but can’t tell what it is
What is hemispatial neglect syndrome?
The patient neglects the contralateral half of their body