Session 3: Somatosensory System Flashcards
This session requires you to go back and look at the notes and drawings. Flashcards are insufficient in this session.
Types of sensation
General
Special
How can general sensation further be subdivided?
Somatic (conscious)
Visceral (unconscious)
Which two systems are most clinically relevant for somatic sensation?
Spinothalamic
Dorsal column-medial lemniscus system
Give the modalities of the spinothalamic system and their associated receptors.
Temperature - thermoreceptors
Pain - nocireceptors
Crude touch - mechanoreceptors
Give the modalities of the dorsal column-medial lemniscus system and their associated receptors.
Vibration - mechano
Proprioception - muscle spindles and golgi tendon organs
Fine touch - mechano
Two point discrimination - mechano
What are analogue and digital receptor signals?
Analogue is the ion flux during generator potential
Digital is the frequency of action potential in primary sensory neuron.
What type of signal is the somatosensory system associated with?
Digital
Explain where the analogue signal can be found and where it goes into digital signal.
Found at receptor and signal is converted from analogue to digital at the synapse between receptor and 1st order neuron.
What does a strong receptor activation consist of?
High frequency of APs in PSN
What does a weak receptor activaiton consist of?
Low frequency of APs in PSN.
What is a rapidly adapting receptors?
A receptor where the frequency of firing diminishes rapidly after initial stimulus.
This means that the receptor gets used to the stimulus and adapts.
Give an example of a rapidly adapting receptor.
Mechanoreceptors in our buttocks e.g.
You realise that you sit down initially but get used to it and don’t consciously think about it afterwards.
What are slowly adapting receptors?
Frequency of firing diminished slowly after initial stimulus.
Give an example of a slowly adapting receptor.
Nocireceptors which explains why pain can be persisent and you never really get used to it.

What is a receptive field?
A single primary sensory neuron supplying a given area of skin.
What is the difference between dermatome and receptive field?
A dermatome is made up of thousands of receptive fields.
How does the size of the receptive field relate to acuity?
A large receptive field means that there is a low sensory acuity. An example being there is poor two-point discrimination.
A small receptive field means that there is a high sensory acuity.
Give examples of large and small receptive fields.
Large = back
Small = fingertips
Explain why dermatomes can have fuzzy boundaries.
Because receptive fields can overlap between spinal nerves.
This means that it is important to check sensory of a dermatome in the middle of that dermatome.

Explain the general path of the somatosensory system.
(1st order etc…)
1st order sensory with its receptor has a central axon that projects ipsilateral. It will synapse with the cell body of the 2nd order neuron.
The 2nd order neuron will decussate. The 2nd order neuron is always the one to decussate. The 2nd order neuron will then synapse with the cell body of the 3rd order neuron.
The 3rd order neuron will then synapse with the primary sensory cortex.

Where can the cell body of the 1st order neuron be found?
In the dorsal root ganglion

Where can the cell body of the 2nd order neuron be found?
In the spinal cord dorsal horn or in the medulla.
Where can the 3rd order cell body be found?
In the thalamus
Sensory of the dorsal column-medial lemniscus.
Proprioception
Fine touch
Two-point discrimination
Vibration


