Somatosensory System Flashcards
What is the somatosensory system concerned with?
Conscious sensation
What sensory modalities are carried by the somatosensory system?
- Temperature
- Pain
- Vibration
- Pressure
- Stretch
- 2 point discrimination
- Proprioception
What do the different sensory modalities carried in the somatosensory system require?
Different types of receptor
What is the clinical relevance of the different sensory modalities in the somatosensory system travelling through different CNS pathways?
Clinically assessing functions can help localise damage
What is the quality of a sensation determined by?
- The way a receptor is activated
- The receptor subtype
How is the quality of sensation determined by receptor subtypes?
Different proteins fire at different rates of stimulation
How does sensory information in the somatosensory system begin?
With detection of a stimuli at its receptor
What happens once a stimuli has been detected in the somatosensory sysetm?
The information passes down the first order neurone, to the spinal cord
Where is the cell body of the first order neurone found?
Always in the dorsal root ganglia, or the trigeminal ganglia if it supplies the head and neck
What does the first order neurone synapse with?
The second order neurone
Where does the second order neurone pass?
Up the spinal cord to the thalamus
What does the second order neurone always do?
Cross the midline
What is the thalamus considered to be in the somatosensory system?
The sensory relay station
What happens to the second order neurone in the thalamus?
It synapses with the third order neurone
Where does the third order neurone go from the thalamus?
It projects from the thalamus to the primary sensory cortex in the post-central gyrus
How is sensory information encoded?
The amount of stimulation is proportional to the frequency of action potentials propagated- higher stimulation results in more APs
What does the way that the frequency of the APs change with levels of stimulation depend on?
The receptor types
What are the two broad classes of receptor types?
- Phasic receptors
- Tonic receptors
Give an example of a sensation picked up by phasic receptors
Pressure
What happens with phasic receptors?
The initial stimulatus is rapid firing, but the rate of firing then decreases as receptors adapt. The frequency gets so low that you become unaware of the stimulus
Give an example of a sensation picked up by tonic receptors
Pain
What happens with tonic receptors?
Frequency of APs doesn’t change with prolonged stimulation
What is sensory acuity proportional to?
1/The size of the receptive field
What is meant by the receptive field?
The area of skin provided by 1 primary afferent
What does a large RF mean for sensory resolution?
Poor sensory resolution
Can receptive fields overlap?
Yes
What is the importance of the potential for sensory fields to overlap?
it can be clinically important when assessing dermatomes - if one neurone has been damaged, you would still get reduced sensation provided by the other
In what parts of the sensory system does lateral inhibition occur?
All parts of the sensory system
When happens when the skin is depressed, e.g. by a sharp object?
More than one sensory neurone is stimulated, however the receptor closest to the depression is stimulated the most
What is the purpose of lateral inhibition when considering skin depression by a sharp object?
It is a mechanism to localise the depression
How does lateral inhibition occur?
The first order neurone synapse with inhibitory interneurones, which synapse with the second order neurones of adjacent neurones, inhibiting them, and therefore maximising contrast
What is a tract?
A white matter connective between 2 areas of grey matter
What does the dorsal column medial lemniscus pathway detect?
- Light touch
- Joint position sense
- Vibration
- Two point discrimination
What are the names of the dorsal columns?
- Fasciculus gracilis
- Fasciculus cuneatus
What does the fasciculus gracilis supply?
The lower half of the body
What does the fasciculus cuneatus supply?
The upper 1/2 of the body
Where does the first order neurone enter the dorsal column pathway?
In the spinal cord
What happens to the first order neurone once it has entered the dorsal column pathway in the spinal cord?
It enters the white matter of the dorsal column, and ascends up the cord
Does the first order neurone in the dorsal column cross the midline?
No, it travels close to it, but does not cross it
Where does the first order neurone synapse with the second order neurone in the dorsal column medial lemiscus pathway?
In the medulla
Where does the second order neurone synapse with the third order neurone in the dorsal column medial lemiscus pathway?
In the thalamus
Which fibres travel closest to the midline in the dorsal column medial lemiscus pathway?
Those from the lower half of the body
Why are fibres from higher levels added to the outside in the dorsal column medial lemiscus pathway?
It economises myelin and axon
What does the spinothalamic pathway detect?
- Pain
- Temperature
- Crude touch
Where does the first order neurone synapse with the second order neurone in the spinothalamic pathway?
In the spinal cord
Where does the second order neurone synapse with the third order neurone in the spinothalamic pathway?
In the thalamus
Which neurones run on the outside in the spinothalamic pathway?
Those entering lower
Which fibres carry pain?
C fibres
What fibres carry impulses from the mechanoreceptors in the skin?
A-fibres
What effect does activation of mechanoreceptors have on pain?
It alleviates pain
How does activation of mechanoreceptors alleviate pain?
It excites inhibitory enkephalinergic interneurones in the cord