The Somatosensory System Flashcards
Compare the 2 classes of sensation
General;
- Referring to body wall and viscera
- Somatic (Conscious) and Visceral (Unconscious)
Special;
- Vision, Hearing, Smell, Taste, Balance
What are sensory modalities?
Different forms of sensory experience which exist due to the presence of a variety of receptors
List the sensory modalities carried in the Spinothalamic System, as well as their corresponding receptor types
- Temperature (Thermoreceptors)
- Pain (Nociceptors)
- Pressure/ crude touch (Mechanoreceptors)
List the sensory modalities carried in the Dorsal Column-Medial Lemniscus System, as well as their corresponding receptor types
- Vibration (Mechanoreceptors)
- Proprioception (Muscle spindles, Golgi tendon organs etc)
- Fine touch (Mechanoreceptors)
- Two point discrimination (Mechanoreceptors)
Which neurones receive information from receptors?
Primary sensory/ First Order/ Pseudounipolar neurones
The strength of receptor activation is converted from an Analogue to a Digital signal.
Compare the AP frequency in 1st Order neurones in strong and weak receptor activation
Strong activation: High frequency of APs
Weak activation: Low frequency of APs
Compare the 2 types of 1st Order neurones
Rapidly adapting;
- AP frequency decreases rapidly after initial stimulation
- Respond best to changes in strength of stimulation
Slowly adapting;
- AP frequency changes very little after initial stimulus
Give examples of Rapidly and Slowly adapting receptors
Rapidly;
- Mechanoreceptors
- This is why you’re not aware of clothes on skin, or after sitting on a chair for a long time you get used to it
Slowly;
- Nociceptors
- This is why pain can be persistent
What is a receptive field?
An area of skin supplied by a single 1st Order neurone
How many types of receptor is one 1st Order neurone connected to?
One
Describe the Sensory Acuity of an area of skin supplied by sensory neurones with LARGE receptive fields
Give an example
- Low sensory acuity (Poor two point discrimination where 2 points must be far apart to be distinguished)
- Skin of the back
Describe the Sensory Acuity of an area of skin supplied by sensory neurones with SMALL receptive fields
Give an example
- High sensory acuity (Good two point discrimination where 2 points could be very close together and be distinguished)
- Skin of fingertip
What’s the clinical significance of the overlap of receptive fields of 1st Order neurones?
Dermatomes can have ‘fuzzy’ boundaries, hence we test the centre of dermatomes rather than the edges/ boundaries
The somatosensory system is made up of a chain of 3 neurones.
List these 3
- 1st order neurone (In PNS, other than synapse)
- 2nd order neurone (In CNS)
- 3rd order neurone (In CNS)
Describe 1st Order neurones in the pathway of the Somatosensory system
- Communicate with a receptor
- Cell bodies are in the Dorsal Root Ganglion
- Synapse with IPSILATERAL 2nd order neurone in the CNS
Describe 2nd Order neurones in the pathway of the Somatosensory system
- Cell bodies are in the Spinal Cord Dorsal Horn or the Medulla
- Decussate
- Project onto 3rd order neurones
Describe 3rd Order neurones in the pathway of the Somatosensory system
- Cell bodies are in the Thalamus
- Project to Primary Sensory Cortex
What is Somatotopy/ Topographical representation?
The idea that:
- Every point on the surface of the body has an
- Equivalent point along the sensory pathway
With some exceptions, what can generally be said about adjacent body regions with regards to Somatotopy?
Adjacent body regions map to adjacent regions of the sensory system
(Minimises the amount of ‘wiring’ needed to transmit sensory information)
How does the pattern of information organisation change as we move from the Spinal Cord to the Thalamus
At spinal cord level: Dermatomal pattern
At thalamus level: Homuncular pattern
At what level do all the sensory modalities converge?
At the level of the sensory homunculus
Which sensory modalities are carried in the Dorsal Column-Medial Lemniscus (DCML) System?
- Fine touch
- Proprioception
- Vibration
- Two point discrimination
Describe the pathways of 1st order neurones in the Dorsal Column-Medial Lemniscus (DCML) System
Axons ascend ipsilaterally through Dorsal Columns of spinal cord;
- If from Lower Body (T7 and below), ascend through Gracile Fasciculus to synapse with Gracile Nucleus in Medulla
- If from Upper Body (T6 and above), ascend through Cuneate Fasciculus to synapse with Cuneate Nucleus in Medulla
Describe the pathways of 2nd order neurones in the Dorsal Column-Medial Lemniscus (DCML) System
Neurons from Gracile and Cuneate nuclei;
- Decussate
- Project to the Contralateral Thalamus via the Medial Lemniscus
