Lecture 4 - Somatosensory System Flashcards
What are the 2 types of sensation?
General and special
What are the 2 types of general sensation?
Somatic sensation
Visceral sensation
What sort of control is somatic sensation under?
What sort of control is visceral sensation under?
Somatic sensation = conscious
Visceral sensation = unconscious
What is a sensory modality?
The basic unit of sensation that correlates with receptors in the skin
What are the different type of sensory modalities and their receptors?
Temperature (thermoreceptors)
Pain (nociceptors)
Pressure/crude touch (Mechnoreceptors)
Vibration (mechanoreceptors)
Proprioception (muscle spindles)
Fine touch (Mechnoreceptors)
Two point discrimination (mechanoreceptors)
What is proprioception?
The sense where your receptors in your muscles know where your other limbs are
E.g close your eyes and touch your nose (you should be able to do it straight away)
What are the 2 systems/tracts important in sensation in the CNS?
Dorsal Columnar Medial Lemniscus system (DCML)
Spinothalamic system
What types of sensation is the spinothalamic tract responsible for?
Temperature
Pain (nocioception)
Pressure/crde touch
The more primitive sensations
What types of sensation is the dorsal column-medial meniscus responsible for?
Vibration
Fine tough
Two point discrimination
Proprioception
How is information encoded in a neurone?
Frequency of action potentials that propagate along it
How will frequency of action potentials change if a receptor is strongly stimulated?
High frequency of action potentials along axon
How will frequency of action potentials change if a receptor is weakly stimulated?
Low frequency of action potentials along the axon
What is a primary sensory neurone?
Sensory afferent that receives its information from a receptor and then ends up sysnapsing to a Secondary sensory neurone in the spinal cord
Where is the cell body for a primary sensory neurone located?
Dorsal Root Ganglion (DRG) for that sensory neurones spinal nerve
What type of neurone is a primary sensory neurone?
Psuedounipolar neurone
Why is the primary sensory neuron being a pseudounipolar neurone an advantage?
The cell body doesn’t slow down the action potential propagation
What is a rapidly adapting receptor?
Receptor which whose frequency of action potential propagation changes/decreases or decays
Frequency rapidly diminishes after the initial stimulus
What is an example of a rapidly adapting receptor?
Mechnoreceptors
Notice clothes are on you initially then dont later
Get used to smells (nose blind)
What is a slowly adapting receptor?
Receptor whose frequency of firing doesn’t really change after initial stimulus (changes very little)
What is an example of a slowly adapting receptor and what is its advantage?
Nocioceptor
Don’t get used to pain so if you have a problem/injury means you’ll go and get it checked out since you cant ignore it
What is a receptive field?
Area with which a single primary sensory neurone can detect sensation (area of skin)
How will sensory acuity differ in an area of skin that is supplied be neurones with large receptive fields to an area of skin that is supplied by neurones with small receptive fields?
Area of skin withi Large receptive fields = low visual acuity
Area of skin with small receptive fields = high visual acuity
Why can dermatomes sometimes have “fuzzy” boundaries?
Receptive fields of primary sensory neurones can overlap from adjacent dermatomes
Where are the cell bodies located for the primary sensory neurone?
Dorsal Root Ganglion
Where are the cell bodies located for the secondary sensory neurone?
Medulla
Or
Dorsal horn of spinal cord
Where are the cell bodies located for the third order sensory neurone?
Thalamus
Which order sensory neurone is always the one that decussates/crosses the midline of the spinal cord or brain ?
Second order neurone
How does the first order/primary sensory neurone run with respect to its cell body?
(Somatosensory system)
Ipsilateral side to cell body
Where do the second order sensory neurones run with respect to the primary sensory neurones cell body (DRG)?
Decussate crossing the midline running on the contralateral side of the brain or spinal cord
What structures does the second order sensory neurone run from and to?
Secondary sensory neurone cell body in the dorsal horn of the spinal cord or medulla to the third order neurones cell body in the thalamus
What do third order neurones run from and to?
Their cell body in the thalamus to the primary sensory cortex
Where is the primary sensory cortex loated?
Postcentral gyrus which is posterior to the central sulcus
Describe the route from sensory receptor to the primary sensory cortex of an action potential regardless of whether its the Dorsal Columnar Medial Lemniscus system or the spinothalamic system:
Receptor sends AP along Primary (1st order) sensory neurone to the Ipsilateral side in the spinal cord
The Primary sensory neurone then synapses to the cell body of the second order sensory neurone that’s either in the dorsal horn of the spinal cord or the medulla
AP sent along 2nd order sensory neurone where the neurone then decussates to the opposite side of the brain/spinal cord where it then travels to the thalamus to synapse with the 3rd order neurone cell body
AP sent along axon to where it travels to the primary motor cortex
What is the somatotopy or topographical representation of sensation?
Adjacent body regions map to adjacent regions of the sensory system in CNS
Region of homunuculs/primary sensory cortex responsible for hands iis next to the arms
Which system is responsible for light touch, vibration, two point discrimination and proprioception?
Dorsal Column-medial Lemniscus system (DCML)
Which system does the morefine sensation pathway and which one does the more survival sensations?
Fine sensation = dorsal column pathway
Survival = spinothalamic pathway
Which neurone is the sensory neurone which ascends the spinal cord in the Dorsal column pathway?
Primary sensory neurone
What are the 2 areas called which the primary sensory neurones can sysnapse to the secondary sensory neurones in the medulla?
DCML system
Cuneate nucleus
Gracile nucleus
What nucleus does primary sensory neurones from T7 and below synapse to in the medulla in the DCML system
Gracile nucleus
What nucleus does primary sensory neurones from T6 and above synapse to in the medulla in the DCML system
Cuneate nucleus
What nucleus in the DCML system is important for sensation in the upper half of the body (T6 above)
What nucleus in the DCML system is important for sensation in the lower half of the body (T7 below)
T6 above = Cuneate nucleus
T7 below = Gracile nucleus
What is the name of the structure that the primary sensory neurones from T7 and below travel up to make it to the medulla to synapse to a secondary neurone?
DCML
Gracile Fasiculus to the Gracile nucleus in the medulla
What is the name of the structure that the primary sensory neurones from T6 and above travel up to make it to the medulla to synapse to a secondary neurone?
DCML
Cuneate fasciculus to the Cuneate nucleus in the medulla
What part of the primary sensory motor cortex does the 3 order sensory neurone travel to if the 2nd order neurone came from the Cuneate nucleus?
DCML
Lateral part of the primary sensory cortex
Since if the neurone came from the Cuneate nucleus it means the sensation came from the upper half of the body (T6 and up)
What part of the primary sensory motor cortex does the 3 order sensory neurone travel to if the 2nd order neurone came from the Gracile nucleus?
DCML
Medial aspect of primary sensory cortex
Since if the neurone came from the gracile nucleus it means the primary sensory neurone came from the lower half of the body (T7 and below)
How is the humunuclus arranged?
Medial most aspect of the primary sensory cortex is responsible for the lower limbs
Then the lateral aspect oof. The primary motor cortex is responsible for the upper limbs (lateral most aspect does the hands)
Think of it as a man lying across the cortex upside down lying down legs at the medial aspect and his body progresses as you go laterally (imaging standing in longitudinal fissure and leaning backwards)
Describe the propagation of an actioon potential for sensation in the L3 Dermatome in the Dorsal Column Medial Lemniscus pathway:
Receptor in L3 dermatome sends AP along the primary sensory neurone which enters the spinal cord and ascends to the Gracile nucleus in the medulla via the gracile fasiculus on the IPSILATERAL SIDE
It synapses to the 2 order sensory neurone in the gracile nucleus
2nd order sensory neurone DECUSSATES travelling to the other side of(CONTRALATERAL side) the brain to where it then travels to the thalamus where it synapses with the 3rd order sensory neurone
The 3rd order sensory neurone travels to the medial aspect of the primary sensory cortex where it synapses
Describe the propagation of an actioon potential for sensation in the C5 Dermatome in the Dorsal Column Medial Lemniscus pathway:
Receptor in C5 dermatome sends AP along the primary sensory neurone which enters the spinal cord and ascends to the Cuneate nucleus in the medulla via the Cuneate fasiculus on the IPSILATERAL SIDE
It synapses to the 2 order sensory neurone in the Cuneate nucleus
2nd order sensory neurone DECUSSATES travelling to the other side of(CONTRALATERAL side) the brain to where it then travels to the thalamus where it synapses with the 3rd order sensory neurone
The 3rd order sensory neurone travels to the Lateral aspect of the primary sensory cortex where it synapses
How are the sensory neurones that are ascending the spinal cord in the Dorsal Columnar Medial Lemniscus (DCML) system either via the gracile or Cuneate fasciculi arranged?
Sensory neurones from the lower part of the body are closest to the midline whereas the sensory neurones from the upper part of the body are more lateral
As you ascend the cord it gets layered more and more (axons from progressively superior body segments are added laterally to the dorsal columns)
Think of it as a Motor way system for the dcMl system from the bottom to the brain
Like your driving up to the brain, if you’re the lowest neuronal level your closest to the midline (if you’re not leaving the motor way anytime soon you go in Lane 3)
If your just joining the Motorr way (high sensory neurones) you’re in the outer lane since you’ve just joined
How does the primary 1st order sensory neurone in the Spinothalamic system run?
Runs to the Ipsilateral part of the spinal cord and then immediately synapses to the second order sensory neurone in the dorsal horn of the spinal cord
How does the second order neurone of the spinothalamic system run?
Cell body in dorsal horn of spinal cord
It the Decussates to the contralateral side of the spinal cord which ascends all the way to the thalamus still on the contralateral side
How does the third order sensory neurone of the spinothalamic system run?
Cell body in the thalamus, axon then projects and synapses to the primary sensory cortex (post central gyrus)
What is the name of the point at which the Second order sensory neurone of the spinothalamic system decussates/crosses the midline to the contralateral side?
Ventral white comissure
Describe how the spinothalamic tract carries pain, temp or crude touch sensation from receptor to primary sensory cortex:
Receptor sends signal along primary sensory neurone which enters the spinal cord and immediately synapses at that spinal level to the secondary sensory neurone in the dorsal horn
Secondary sensory neurone decussates/crosses the midline to the contralateral side of the spinal cord at the ventral white commissure and then ascends up the spinal cord to the thalamus where it synapses to the 3rd order neurone
3rd order neurone then travels to the primary sensory cortex
Which sensory neurone ascends the spinal cord in the Dorsal Columnar Medial Lemniscus pathway (DCML) and which neurone ascends the spinal cord in the Spinothalamic pathway?
DCML = 1st order neurone ascends
Spinothalamic = 2nd order neurone ascends
How are the ascending secondary sensory neurones arranged in the spinal cord?
Axons from the lower part of the body are most lateral
Axons from the upper part of the body are most medial
How does the arrangement of the sensory neurones that are ascending in the spinal cord differ between the DCML pathway and the Spinothalamic pathway?
DCML = lower limb axons are medial, upper limb axons are lateral
Spinothalamic = lower limb axons are lateral , upper limb axons are medial
What is Lissaeurs tract?
A tract allowing 1st order neurones in the spinothalamic tract to ascend on the Ipsilateral side 2 levels higher and then synapse with a 2nd order sensory neurone and then decussate to the contralateral side
What is the advantage of having Lissaeurs tract in the spinothalamic system?
Helps preserve spinothalamic sensation the event of hemisections (lesions affecting only a segment on 1 half/side of the cord)
E.g if there’s a hemisection of the right half of the C5 cord segment it means that they’ll still be spinothalamic sensation from C7 and above
What is Brown-Sequard syndrome?
How does it present?
When there is an injury/hemisection/lesion to 1 half/side of the spinal cord segment e.g at C5 on the right
This leads to loss of Dorsal Columnar modalities/sensation on the Ipsilateral side at the level of the lesion and below
Loss of Spinothalamic modalities at and below the destroyed section on the contralateral side (however can sometimes preserved Spinothalamic function 2 levels below the lesion due to Lissaeurs tract so spinothalwmic modalities can be preserved to C7)
What are the different fibres in the descending control of pain?
A fibres
C fibres
What is the function of A fibres int he descending control of pain?
Carry impulses from Mechnoreceptors in the skin
What is the function of C fibres int he descending control of pain?
Carry pain
How does rubbing a painful area of skin helps alleviate pain?
Activation of mechanoreceptors alleviates pain by exciting inhibitory encephalinergic internuerones in the cord
Draw the Dorsal Columnar Medial Lemniscus (DCML) pathway using examples of both C5 and L3 as levels where the receptors are:
Draw the spinothalamic pathway using examples of both C5 and L3 as levels where the receptors are
