S3) Somatosensory Pathways/System

Question 1

What are the two types of sensation?

Answer 1

• General sensation, referring to the body wall and viscera (including parietal layer of serous membranes and mucosa of pharynx, nasal cavity and anus)
• Special sensation, referring to the special senses of vision, hearing, balance, taste and smell

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/ somatic sensation e.g. pain, temperature which exists due to various types of receptors

A modality can be thought of as a ‘unit’ of sensation, relying on a distinct receptor type

Question 4

Besides pain and temperature, identify five other sensory modalities

Answer 4

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

Question 5

Different modalities travel along different trajectories via the nervous system on their way up to the brain.

What are the two different systems?

Answer 5

1. Spinothalamic system
2. Dorsal column-medial lemniscus system

Question 6

What modalities are carried by the spinothalamic system?

Answer 6

• Temperature (thermoreceptors)
• Pain (nociceptors)
• Pressure/ crude touch (mechanoreceptors)

Question 7

What modalities are carried by the dorsal column-medial lemniscus system?

Answer 7

• Vibration (mechanoreceptors)
• Proprioception, or joint position sense, or kinaesthetic sense (detected by a variety of receptors such as muscle spindles and Golgi tendon organs)
• Fine touch (mechanoreceptors)
• Two point discrimination (mechanoreceptors)

Question 8

What is two point discrimination?

Answer 8

It is the ability to resolve/ discriminate between two simultaneous stimuli

Question 9

What are primary sensory neurones aka?

Answer 9

aka dorsal root ganglion neurones or primary afferents or first order sensory neurones or psueudunipolar (one process emanating from the body) neurones

Question 10

Where are primary sensory neurones found?

Answer 10

The cell body of primary sensory neurones lies in the dorsal root ganglion.

They receive information from receptors from the skin/ elsewhere and are responsible for the inital encoding of sensory information.

And the axon transmits information to the dorsal horn of the spinal cord where the neurone synapses

Question 11

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

Answer 11

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

Question 12

Identify 3 main features of the primary sensory neurone.

Answer 12

• Each individual primary neurone receives input from a single receptor type
• Primary sensory neurones have their cell body in the dorsal root ganglion, and collect information from a single dermatome along their peripheral axon
• Primary sensory neurones project into the spinal cord along their central axon

Question 13

The strength of receptor activation is converted from one type of signal to another. What are they?

Answer 13

converted from an analogue signal (related to ion flux during the generator potential) to a digital signal (which is the frequency of action potentials in the primary sensory neurone)

Question 14

What is the relationship between the strength of the receptor activation and the frequency of the action potentials?

Answer 14

Strong receptor activation causes high frequency of action potentials in the primary sensory neurone

Weak receptor activation causes a low frequency of action potentials in the primary sensory neurone

Question 15

We have two types of primary sensory neurones.

What are they?

Answer 15

1. Rapidly adapting receptors
2. Slowly adapting receptors

Question 16

What are rapidly adapting receptors?

Answer 16

(e.g. mechanoreceptors)

respond best to changes in strength of stimulation.

However, their frequency of firing diminishes rapidly after the initial stimulus (i.e. they rapidly adapt).

Adaptation of these receptors explains why you are not aware of your clothes on your skin

Question 17

Identify an example of rapidly adapting receptors.

Answer 17

e.g. mechanoreceptors on your bottom - initially aware that you are sitting due to pressure when sat down

But eventually, no longer aware when sat down for long

Question 18

What are slowly adapting receptors?

Answer 18

e.g. nociceptors

change their frequency of firing very little after the initial stimulus.

This explains why pain can be so persistent, and you never really get ‘used to’ having pain

Question 19

What are receptive fields?

Answer 19

A single primary sensory neurone supplies a given area of skin (it’s receptive field)

Question 20

What is the relationship between the size of the receptive fields and the sensory acuity?

Answer 20

• If an area of skin is supplied by sensory neurones with relatively large receptive fields, this area will have low sensory acuity (it would have poor two-point discrimination where two points would need to be far apart to be distinguished). e.g. The skin of the back has relatively low acuity
• If an area of skin is supplied by sensory neurones with relatively small receptive fields, this area will have high sensory acuity (it would have great two-point discrimination where two points could be very close together to be distinguished). e.g. The skin of the fingertip has relatively high acuity
• Acuity ∝ 1/size of receptive field

Question 21

Why are dermatomes said to have ‘fuzzy’ boundaries?

Answer 21

The overlap of receptive fields of primary sensory neurones from adjacent dermatomes is one of the reasons why dermatomes can have ‘fuzzy’ boundaries

Question 22

The somatosensory system conveys a system of 3 neurones.

It carries conscious sensation from the body wall e.g. skin, lining of pharynx, parietal pleura/ peritoneum

What are the 3 neurones?

Answer 22

1. First order sensory neurones
2. Secondary order sensory neurones
3. Third order neurones (thalamocortical neurones)

Question 23

Describe the three types of neurones in the somatosensory system in terms of location of their cell bodies, where they project into and their path/ communicate with.

Answer 23

– First order sensory neurones

• Have their cell bodies in the DRG
• Communicate with a receptor
• Their central axon projects ipsilateral to the cell body
• Project onto second order neurones

– Second order sensory neurones

• Have their cell bodies in the spinal cord dorsal horn or medulla
• Decussate
• Project onto third order neurones

– Third order neurones

• Have their cell bodies in the thalamus
• Project to the primary sensory cortex (postcentral gyrus)

Question 24

Describe the topographical representation in the sensory system

Answer 24

• Principle that every point on the surface of the body is equivalent to an area along the sensory pathway w some exceptions: adjacent body regions map to adjacent regions of the sensory system e.g. sensory cortex, the hand is represented adjacent to the wrist.
• This way of organising the pathways → minimises the amount of ‘wiring’ needed to transmit sensory information
• The motor system has similar organisation but running in reverse
• Info becomes re-organised as we move upwards through the neuraxis such that as the level of spinal nerves and spinal cord, we have a dermatomal organisation but at the levels of the thalamus and above, we have a homuncular pattern
• At the levels of the sensory homunculus, all modalities converge (i.e. the head area of the sensory cortex deals with pain, temperature, vibration etc. all at the same time)

Question 25

What is the purpose of the dorsal column-medial lemniscus system (DCML)?

Answer 25

* Responsible for carrying impulses concerning modalities such as **light touch, vibration, two point discrimination and proprioception**

Question 26

Observe this image and understand. - The dorsal column-medial lemniscus system

Answer 26

Question 27

We have three neurones involved in the dorsal column-medial leminscus pathway. What are they and briefly describe them.

Answer 27

* **_Concerning first order neurones of the DCML system:_** → Axons of first order neurones ascend ipsilaterally through the dorsal columns of the spinal cord → Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla) → Those from the upper half of the body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla * **_Concerning second order neurones of the DCML system:_** → Neurones in the gracile nucleus project to the contralateral thalamus in the medial lemniscus → Neurones in the cuneate nucleus project to the contralateral thalamus in the medial lemniscus * **_Concerning third order neurones of the DCML system:_** → Thalamic neurones receiving information ultimately from the lower half of the body (via gracile nucleus) project to the medial part of the primary sensory cortex → Thalamic neurones receiving information ultimately from the upper half of the body (via cuneate nucleus) project to the lateral part of the primary sensory cortex

Question 28

Just for further clarification, what is the dorsal column pathway for the **lower half of the body i.e. T7 and below?**

Answer 28

First order neurone → ascends through the **gracile fasciculus to the gracile nucleus in the medulla.** Then second order neurones in the gracile nucleus project to the **contralateral thalamus** in the **medial lemniscus** Then the third order neurone/ thalamic neurones receive info ultimately from the lower ½ of the body via (gracile nucleus) project to the **medial part of the primary sensory cortex.**

Question 29

Just for further clarification, what is the dorsal column pathway for the **upper half of the body i.e. T6 and above?**

Answer 29

First order neurone → ascends through the **cuneate fasciculus to the cuneate nucleus in the medulla.** Then second order neurones in the cuneate nucleus project to the **contralateral thalamus** in the **medial lemniscus** Then the third order neurone/ thalamic neurones receive info ultimately from the upper ½ of the body via (cuneate nucleus) project to the **lateral part of the primary sensory cortex.**

Question 30

What is the topographical organisation of the dorsal columns?

Answer 30

* Axons from the **lower parts of the body run most medially** * Axons from progressively **superior body segments are added laterally** to the dorsal columns * i.e.CTL (from lateral to medial)

Question 31

What is the purpose of the spinothalamic pathway or antero-lateral system or spinothalamic tract (STT).

Answer 31

Responsible for carrying impulses concerning modalities e.g pain, temperature and crude touch

Question 32

Observe this image and understand. - The spinothalamic system

Answer 32

Question 33

We have three neurones involved in the spinothalamic pathway. What are they and briefly describe them.

Answer 33

Overall: Axons of first order neurones project to the ipsilateral dorsal cord, but the **spinothalamic tract** supplies the **contralateral half of the body** – **_Concerning first order neurones of the STT:_** * They project onto second order neurones in the ipsilateral spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root (generally) – **_Concerning second order neurones of the STT:_** * Their cell bodies are in the dorsal horn * Their axons decussate in the ventral white commissure of the cord and then go on to form the spinothalamic tract * The spinothalamic tract projects to the thalamus – **_Concerning third order neurones of the STT:_** * Thalamic neurones receiving information ultimately from more **inferior parts** of the body project to the **medial part of the primary sensory cortex** * Thalamic neurones receiving information ultimately from more **superior parts** of the body project to the **lateral part of the primary sensory cortex**

Question 34

What is the topographical organisation of the spinothalamic tract?

Answer 34

* Axons from the **lower parts of the body** run most **laterally/superficially** * Axons from progressively **superior body segments** are added **medially/deepe**r onto the spinothalamic tract * This is the **opposite of the situation for the dorsal columns**, and is **due to the decussation of the STT second order neurones** at the level of entry of the first order neurones * i.e. LTC (from lateral to medial)

Question 35

In terms of lesions, if there was damage to DCLM vs STT, which side of body affected?

Answer 35

Damage to **DCLM** → Ipsilateral sensory loss below level of lesion .: **IPSILATERAL FEATURES** Damage to **STT** → Contralateral sensory loss below level of lesion due to decussation of the second order neurones .: **CONTRALATERAL FEATURES**

Question 36

What is the overall somatotopic organisation of spinal cord tracts?

Answer 36

Question 37

What is Lissauer's tract? (aka Fasciculus proprius)

Answer 37

It enables axons of 1st order neurones to ascend a couple of segments before synapsing onto a second order neurone within the spinal cord/ grey matter.

Question 38

What is the concept of the Lissauer's tract? What is its clinical relevance?

Answer 38

* 1st order sensory neurones of the STT have a no. of options - some synapse immediately at the level of the dorsal root + some of them ascend in Lissauer's tract to synapse higher up * Clinical relevance: It allows us to **bypass certain types of spinal cord lesions** because we can **ascend up a couple of segments in Lissaeur's tract to synapse with a second order neurone**.: we typically see that the **_sensory level for STT modalities is a couple of segments lower down than_** _what we see for the_ **_sensory level for dorsal column modalities._** (allows the ability to bypass hemisections of the spinal cord) – AKA Brown-Sequard syndrome.

Question 39

In Brown-Sequard syndrome (limited to sensory features), if we considered a complete cord hemisection causing destruction of one lateral half of the spinal cord segment resulting from trauma/ ischaemia, what structures will be completely destroyed unilaterally?

Answer 39

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

Question 40

In Brown-Sequard syndrome (limited to **sensory features**), if we do have a complete cord hemisection (destruction of one lateral half of a spinal cord segment), what signs will you see?

Answer 40

Destruction of the dorsal root and dorsal horn → Ipsilateral complete segmental anaesthesia affecting a single dermatome Contralateral loss of spinothalamic modalities at and below the destroyed segment (although level can be up to a couple of segments lower due to ascent of some primary afferents in Lissauer’s tract –advanced, so only pursue if you are keen!)

Question 41

Which fibres are involved in the descending modulation of pain?

Answer 41

* **C fibres** project into the dorsal horn of the spinal cord - they **carry the pain impulses from the nociceptors** * **A fibres (A-beta fibres)** carry **impulses from the mechanoreceptors** in the skin (e.g. rubbing the site of pain) and they stimulate mainly the **encephalingeric inhibitory interneurones** which **inhibits the 2nd order neurone i**n the pain pathway .: **switching off pain transmission.** Second order neurones of the spinothalamic system dealing with pain receive nociceptive primary afferents as well as inhibitory interneurones which contain the endorphin encephalin These encephalinergic interneurones can be activated by incoming impulses from mechanoreceptors (hence explaining why rubbing a sore area relieves the pain)

Question 42

How else can the encephalingeric interneurones be activated?

Answer 42

Activated by **descending inputs from higher centres** such as the **periaqueductal grey matter or the nucleus raphe magnus** e.g. Hypnosis - They activate **cortical neurones** known to project down into the **midbrain**. These are excitatory neurones .: they **stimulate a 2nd order neurone** which sits in the **Peri-aqueductal grey matter of the midbrain**. Then these neurones will project down into the medulla where they stimulate **neurones in the Reticular formation of the medulla** called **Nucleus Raphe magnus.** These neurones descends down the cord and **inhibits the 2nd order sensory neurone** in the STT by **stimulating the inhibitory interneurone.**

Question 43

The modality of the stimulus is dependent on the type of receptor activated. What are the different types of receptors?

Answer 43

- Nociceptor - Mechanoreceptors - Thermoreceptors

Question 44

Different types of receptors adapt in different ways. What is a tonic receptor?

Answer 44

**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 45

Different types of receptors adapt in different ways. What is a phasic receptor?

Answer 45

**- 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 46

What is a nociceptor?

Answer 46

**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 47

What is a mechanoreceptor?

Answer 47

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

Question 48

What is a thermoreceptor?

Answer 48

**Thermoreceptors** are tonic receptors that respond to warmth and cold

Question 49

What is a dermatome?

Answer 49

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

Question 50

Where are second order sensory neurones found?

Answer 50

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

Question 51

Where are third order sensory neurones found?

Answer 51

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

Question 52

Where are the majority of the ascending tracts found?

Answer 52

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

Question 53

What is the sensory homunculus?

Answer 53

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 54

Explain how CNS lesions can show varied patterns

Answer 54

- **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 55

Describe the somatotropic organisation of the spinal cord tracts

Answer 55

- 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 56

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

Answer 56

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

Question 57

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 57

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

Question 58

Information for which sensory modalities is relayed by the DCML?

Answer 58

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

Question 59

How do isolated lesions of the DC pathway present?

Answer 59

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

Question 60

Describe the significance of the following in the dorsal column pathway: - Gracile/cuneate nucleus - Medial leminiscus

Answer 60

- 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 61

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

Answer 61

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

Question 62

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

Answer 62

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

Question 63

How do isolated lesions of spinothalamic tract present?

Answer 63

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

Question 64

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

Answer 64

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 65

Illustrate the ascension of axons in the spinothalamic pathway

Answer 65

Question 66

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

Answer 66

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

Question 67

What is Brown-Sequard syndrome?

Answer 67

- **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 68

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 68

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

Question 69

How does Brown-Sequard syndrome present?

Answer 69

- 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 70

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

Answer 70

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

Question 71

Explain how the activation of mechanoreceptors alleviates pain

Answer 71

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