Session 3 - Lecture 1: Somatic Sensation and Ascending Pathways

Question 1

Name the two different types of sensation humans can feel?

Answer 1

General = somatic (conscious) and visceral (unconscious)
Special = modalities of various receptors e.g. pain, pressure, etc.

Question 2

What different modalities are involved in the spinothalamic pathway?

Answer 2

Temperature, pain and pressure/crude touch.

Question 3

What different modalities are involved in the dorsal column pathway?

Answer 3

Vibration, fine touch, proprioception/joint position sense and two point discrimination.

Question 4

What is a rapidly adapting receptor?

Answer 4

Activate/fire at rapid rate initially and then adapts by reducing firing rate to a more steady state. They send impulses only on the beginning and cessation of stimulation.

Question 5

What is a slowly adapting receptor?

Answer 5

Fire at same frequency throughout stimulus duration e.g. nociceptors.

Question 6

What is a receptor field?

Answer 6

An area of skin that a single sensory neurone innervates.

Question 7

What is the consequence of the overlapping of receptive fields?

Answer 7

Boundaries between adjacent dermatomes are not very accurate.

Question 8

What is the relationship between acuity and receptive fields?

Answer 8

Acuity is indirectly proportional to 1/size of receptive field, i.e. so as the size of the receptive field increases, acuity decreases.

Question 9

Where do the cell bodies of first order neurones reside and where do they project with respect to somatic pathways?

Answer 9

Cell bodies reside within the dorsal root ganglion, these neurones communicate with sensory receptors and project to the ipsilateral side in the spinal cord/medulla.

Question 10

Where do the cell bodies of second order neurones reside and where do they project with respect to somatic pathways?

Answer 10

Cell bodies are found within the spinal cord within the dorsal horn/ medulla, this is the neurone which decussates and projects up to the thalamus.

Question 11

Where do the cell bodies of third order neurones reside and where do they project with respect to somatic pathways?

Answer 11

Cell bodies reside within the thalamus and project to the primary sensory cortex which is found within the post central gyrus.

Question 12

What is the relationship between somatotopical and topographical representation?

Answer 12

Parallel relationship i.e. face neurones project to face homunculus, limb neurones project to limb homunculus.
In the dorsal column pathway, the lower body maps to the medial portion of the tract. In the spinothalamic tract, the lower body maps to the lateral/superficial parts of the tract.

Question 13

Which pathway can a central cord lesion affect?

Answer 13

This type of lesion can affect the dorsal column and spinothalamic tracts differentially.

Question 14

Describe the dorsal column pathway for lower body sensation. Classify the boundaries.

Answer 14

First order neurones enter spinal cord and project upwards towards the gracile nucleus via the gracile fasciculus where they synapse. Second order neurones then decussate and project towards the thalamus whilst passing through the medial lemniscus. The third order neurones will project to the medial part of the sensory cortex from the thalamus.
Lower body sensation is classed as T7 vertebrae level and below.

Question 15

Describe the dorsal column pathway for upper body sensation. Classify the boundaries.

Answer 15

First order neurones enter spinal cord and project upwards towards the cuneate nucleus via the cuneate fasciculus where they synapse. Second order neurones then decussate and project towards the thalamus whilst passing through the medial lemniscus. The third order neurones will project to the lateral part of the sensory cortex from the thalamus.
Upper body sensation is classes as T6 and above.

Question 16

What are they key differences between the dorsal column pathway and the spinothalamic pathway?

Answer 16

Within the STP, first order neurones synapse with second order neurones at the level at which they enter the spinal cord or one/two segments above. Lower body neurones project more laterally whereas upper body neurones project more medially. However third order neurones of the lower body project to the medial sensory cortex and upper body neurones project to the lateral sensory cortex like within the DCP. Second order neurones within the STP also project through the spinal lemniscus.

Question 17

What structures will be completely destroyed unilaterally with the destruction of a single cord segment?

Answer 17

The dorsal horn, the ventral horn, all other cord grey matter, all other white matter pathways, dorsal and ventral roots.

Question 18

What signs will be present with the destruction of a single cord segment and what is the name of this syndrome?

Answer 18

Ipsilateral complete segmental anaesthesia affecting single dermatome.
Ipsilateral loss of dorsal column modalities below the lesion.
Contralateral loss of spinothalamic modalities below the lesion.
Brown-Sequard syndrome (dissociated sensory loss).

Question 19

Why sometimes with this type of lesion can you get the sensory level of pain and temperature a couple of segments lower than the lesion?

Answer 19

First order neurones of the spinothalamic system can ascend a couple of segments in Lissauer’s tract before synapsing in the dorsal horn.

Question 20

What impulses do A fibres carry?

Answer 20

A fibres carry impulses from mechanoreceptors in the skin.

Question 21

What impulses do C fibres carry?

Answer 21

C fibres carry pain.

Question 22

Why can activation of mechanoreceptors alleviate pain?

Answer 22

As mechanoreceptor first order neurones (A fibres) can synapse onto inhibitory second order neurones which transmit pain sensation (C fibres) i.e. they excite inhibitory enkephalinergic interneurones in the spinal cord.