S3) Somatic Sensation and Ascending Pathways

Question 1

What are the two types of sensation?

Answer 1

• General sensation
• Special sensation

Question 2

What are the two types of general sensation?

Answer 2

• Somatic sensation (conscious)
• Visceral sensation (unconscious)

Question 3

What are sensory modalities?

Answer 3

Sensory modalities are different forms of sensory experience e.g. pain, temperature which exists due to various types of receptors

Question 4

Besides pain and temperature, identify five other sensory modalities

Answer 4

• Pressure (crude touch)
• Vibration
• Distension
• Proprioception (kinesthetic sense)
• Fine touch

Question 5

Where are primary sensory neurones found?

Answer 5

The cell body of primary sensory neurones lies in the dorsal root ganglion and the axon transmits information to the dorsal horn of the spinal cord where the neurone synapses

Question 6

Describe the relationship between the cell body and axon for primary sensory neurones

Answer 6

For primary sensory neurones, the axon runs ipsilaterally to the cell body

Question 7

The modality of the stimulus is dependent on the type of receptor activated.

What are the different types of receptors?

Answer 7

• Nociceptor
• Mechanoreceptors
• Thermoreceptors

Question 8

Different types of receptors adapt in different ways.

What is a tonic receptor?

Answer 8

Tonic receptors are slow adapting receptors, respond to the stimulus as long as it persists, and produce a continuous high frequency of action potentials e.g. all nociceptors

Question 9

Different types of receptors adapt in different ways.

What is a phasic receptor?

Answer 9

- Phasic receptors are rapidly adapting receptors and respond quickly to stimuli but stop responding upon continual stimulation

• Action potential frequency decreases during prolonged stimulation and the receptor remains sensitive to a change/removal of stimulus

Question 10

What is a nociceptor?

Answer 10

Nociceptors are tonic receptors, which respond to noxious stimuli (stimuli that would cause tissue injury if they were to persist) and result in the sensation of pain

Question 11

What is a mechanoreceptor?

Answer 11

A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion by the means of pressure, touch, vibration or stretch

Question 12

What is a thermoreceptor?

Answer 12

Thermoreceptors are tonic receptors that respond to warmth and cold

Question 13

What is a dermatome?

Answer 13

A dermatome is an area of skin supplied by a single spinal nerve

Question 14

Where are second order sensory neurones found?

Answer 14

The cell body of second order sensory neurones lies in the dorsal horn of the spinal cord

Question 15

Where are third order sensory neurones found?

Answer 15

The cell body of third order sensory neurones is located in the thalamus and the axon extends into the somatosensory cortex

Question 16

Where are the majority of the ascending tracts found?

Answer 16

The destination of the majority of the ascending tracts is the somatosensory cortex in the postcentral gyrus of the parietal lobe

Question 17

What is the sensory homunculus?

Answer 17

The somatosensory cortex corresponds to the sensory homunculus, which is a map of brain areas dedicated to sensory processing for different anatomical divisions of the body

Question 18

Explain how CNS lesions can show varied patterns

Answer 18

• Dermatomal pattern of sensory loss suggests a lesion at the level of the spinal nerve
• Homuncular pattern of sensory loss suggests a lesion at the level of the cortex

Question 19

Describe the somatotropic organisation of the spinal cord tracts

Answer 19

• 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 portion of the tract

Question 20

What is the significance of the varying somatotropic organisation of the spinal cord tracts?

Answer 20

A central cord lesion can affect dorsal column and spinothalamic tracts differentially

Question 21

Different ascending tracts are found in specific locations in the spinal cord, and each of the ascending pathways carry information about specific sensory modalities.

Identify two ascending tracts of interest and their location

Answer 21

• Dorsal column-medial lemniscus (DCML)
• Spinothalamic tract

Question 22

Information for which sensory modalities is relayed by the DCML?

Answer 22

The dorsal column-medial lemniscus relays information relating to conscious proprioception of the limbs, fine touch and vibration

Question 23

How do isolated lesions of the DC pathway present?

Answer 23

Isolated lesions of DC pathway in the cord lead to ipsilateral signs below the lesion

Question 24

Describe the significance of the following in the dorsal column pathway:

• Gracile/cuneate nucleus
• Medial leminiscus

Answer 24

• The cell bodies lie in the gracile/cuneate nucleus
• The fibres decussate to the contralateral side at the great sensory decussation in the medulla oblongata, and then ascend via the medial leminiscus and terminate at the thalamus

Question 25

Describe and illustrate the organisation of the homunculus in the dorsal column pathway

Answer 25

In the **sensory** and **motor homunculus,** the lower limb is represented medially and the upper limb and face are represented laterally

Question 26

Information for which sensory modalities is relayed by the spinothalamic tracts?

Answer 26

The spinothalamic tract relays information relating to **pain**, **temperature**, and **pressure**

Question 27

How do isolated lesions of spinothalamic tract present?

Answer 27

Isolated lesions of spinothalamic tract in cord lead to **contralateral signs** below the lesion

Question 28

What is the ventral white commissure and what is its significance in the spinothalamic pathway?

Answer 28

The axons of the tract cells decussate to the other side of the spinal cord via the **anterior/ventral white commissure** and to the anterolateral corner of the spinal cord

Question 29

Illustrate the ascension of axons in the spinothalamic pathway

Answer 29

Question 30

Describe and illustrate the organisation of the homunculus in the spinothalamic pathway

Answer 30

In the sensory and motor homunculus, the lower limb is represented laterally and the upper limb and face are represented medially

Question 31

What is Brown-Sequard syndrome?

Answer 31

- **Brown-Sequard syndrome** is the clinical syndrome that emerges when one destroys one half of the spinal cord - It demonstrates the rare occurrence of a hemisection (only right or left is severed) of the spinal cord

Question 32

In Brown-Sequard syndrome, consider a complete cord hemisection causing destruction of a single cord segment resulting from trauma or ischaemia and identify the five structures that will be completely destroyed unilaterally

Answer 32

- The dorsal horn - The ventral horn - All other cord grey matter - All white matter pathways - Dorsal and ventral roots

Question 33

How does Brown-Sequard syndrome present?

Answer 33

- Ipsilateral complete **segmental anaesthesia** affecting a single dermatome (due to destruction of dorsal root and dorsal horn) - Ipsilateral **loss of dorsal column modalities** below the lesion - Contralateral **loss of spinothalamic modalities** below the lesion

Question 34

Describe the respective functions of A fibres and C fibres in analgesia

Answer 34

- **A fibres** carry impulses from mechanoreceptors in the skin - **C fibres** carry pain

Question 35

Explain how the activation of mechanoreceptors alleviates pain

Answer 35

Activation of mechanoreceptors alleviates pain (i.e. rubbing a painful area helps) by **exciting inhibitory enkephalinergic interneurones** in the cord