A03 - Topography of the Major Sensory Systems

Question 1

Describe the general classifications of sensory nerve endings.

Answer 1

1 - Encapsulated - surrounded by a structural specialisation.

2 - Unencapsulated - free nerve endings.

Question 2

List the classifications of sensory nerves by location in the body.

Answer 2

1 - Exteroreceptors - detect stimuli near the outer body surface.

2 - Interoceptors - detect stimuli inside the body.

3 - Proprioceptors - detect stimuli deep inside the body.

Question 3

Which area of the brain is responsible for conscious sensation?

Answer 3

The primary somatosensory cortex in the postcentral gyrus.

Question 4

What is the sensory homunculus?

Answer 4

The representation of the somatotopic organisation of sensory information throughout the postcentral gyrus.

Question 5

In which structure of the spinal cord do sensory receptors of the periphery have their cell bodies?

Answer 5

The dorsal root ganglion.

Question 6

List the main ascending tracts that comprise conscious sensation.

List the main ascending tracts that comprise subconscious sensation.

Answer 6

Conscious tracts:

1 - Dorsal column-medial lemniscal pathway.

2 - Anterolateral system.

Subconscious tracts:

3 - Spinocerebellar tracts.

Question 7

List the components of the anterolateral system.

Answer 7

1 - Anterior spinothalamic tract.

2 - Lateral spinothalamic tract.

Question 8

What is the function of the spinothalamic tract?

Answer 8

To carry information relating to:

1 - Pain.

2 - Temperature.

3 - Non-discriminative (crude) touch.

4 - Firm pressure.

Question 9

Describe the course of the spinothalamic tract.

Answer 9

1 - The first order (sensory) neurones travel from the periphery and enter the spinal cord.

2 - They then synapse with second order neurones at the tip of the dorsal horn in the substantia gelatinosa:

• Axons carrying pain and temperature synapse and then decussate promptly within one segment of their origin, while those carrying touch and pressure may ascend for several segments before synapsing and crossing.

3 - The second order neurones form two distinct tracts that run alongside each other: the anterior spinothalamic tract (carrying crude touch and firm pressure information) and the lateral spinothalamic tract (carrying pain and temperature information).

4 - These tracts ascend the spinal cord to the brainstem, in which they run through the spinal lemniscus.

5 - Some fibres of the lateral spinothalamic tract carrying information relating to burning pain terminate in the reticular formation of the brainstem for widespread activation of the nervous system. Other fibres report to the limbic system about the nature of the stimulus.

6 - Both anterior and lateral tracts continue to ascend to the thalamus, where they synapse with third order neurones at the ventral posterolateral nucleus.

7 - The third order neurones pass through the internal capsule to synapse with the ipsilateral somatosensory cortex of the cortex.

Question 10

What is the function of the dorsal column-medial lemniscal pathway?

Answer 10

To carry information relating to:

1 - Discriminative touch.

2 - Vibration.

3 - Conscious proprioception.

Question 11

Describe the course of the dorsal column-medial lemniscal pathway.

Answer 11

1 - The first order (sensory) neurones travel from the periphery and enter the spinal cord.

2 - The first order (sensory) neurones then ascend the spinal cord in the dorsal column:

• If the information is from the upper limb (T6 and above), the first order neurones travel in the lateral part of the dorsal column known as the fasciculus cuneatus.
• If the information is from the lower limb (below T6), the first order neurones travel in the medial part of the dorsal column known as the fasciculus gracilis.

3 - First order (sensory) neurones travelling in the fasciculus cuneatus synapse in the nucleus cuneatus of the medulla oblongata. First order (sensory) neurones travelling in the fasciculus gracilis synapse in the nucleus gracilis of the medulla oblongata.

4 - Second order neurones decussate within the medulla oblongata.

5 - Second order neurones then ascend in the contralateral medial lemniscus to reach the thalamus, where they synapse with third order neurones at the ventral posterolateral nucleus.

6 - The third order neurones pass through the internal capsule to synapse with the ipsilateral somatosensory cortex of the cortex.

Question 12

In the face and head, which nerves replace the function of the peripheral sensory nerves in other peripheral ascending pathways?

Answer 12

Cranial nerves.

Question 13

What is the main pathway conveying sensory information from the face and head?

From which cranial nerves does this pathway convey sensory information?

Answer 13

• The trigeminothalamic tract.
• It conveys sensory information from the:

1 - Trigeminal nerve (CN V).

2 - Facial nerve (CN VII).

3 - Glossopharyngeal nerve (CN IX).

4 - Vagus nerve (CN X).

Question 14

What is the function of the spinocerebellar tracts?

To which area of the brain is information in the spinocerebellar tracts carried?

Answer 14

• To carry information relating to subconscious proprioception.
• Information is carried to the ipsilateral hemisphere of the cerebellum.

Question 15

List 3 examples of interoceptors.

Answer 15

1 - Chemoreceptors.

2 - Stretch receptors.

3 - Internal nociceptors.

Question 16

List 4 examples of exteroceptors.

Answer 16

1 - All receptors in the skin.

2 - Olfactory receptors.

3 - Cone / rod cells.

4 - Spiral organ of Corti.

Question 17

List the sensory receptors for proprioception.

Answer 17

1 - Muscle spindle.

2 - Golgi tendon organs.

3 - Joint receptors.

Question 18

List the gross divisions of the brain.

Answer 18

1 - Forebrain (prosencephalon).

2 - Midbrain (mesencephalon).

3 - Hindbrain (rhombencephalon).

Question 19

List the subdivisions of the forebrain (prosencephalon).

List the structures contained within each subdivision.

Answer 19

• Telencephalon:

1 - Cerebral cortex.

2 - Basal ganglia.

3 - Limbic system.

• Diencephalon:

1 - Thalamus.

2 - Hypothalamus.

Question 20

List the subdivisions of the midbrain (mesencephalon).

List the structures contained within each subdivision.

Answer 20

• The midbrain has no subdivisions. It contains the:

1 - Tectum.

2 - Tegmentum.

Question 21

List the subdivisions of the hindbrain (rhombencephalon).

List the structures contained within each subdivision.

Answer 21

• Metencephalon:

1 - Cerebellum.

2 - Pons.

Myelencephalon:

1 - Medulla oblongata.

Question 22

How does the arrangement of grey and white matter differ in the spinal cord, brainstem and forebrain?

Answer 22

• In the spinal cord, white matter surrounds grey matter.
• In the brainstem and diencephalon of the forebrain, white matter surrounds discrete areas of grey matter known as nuclei.
• In the telencephalon of the forebrain, the grey matter (cerebral cortex) surrounds the white matter.

Question 23

List the areas of the sensory homunculus of the somatosensory cortex from inferolateral to superomedial.

Answer 23

1 - Abdomen.

2 - Pharynx.

3 - Mouth.

4 - Lips.

5 - Face.

6 - Fingers.

7 - Upper limb.

8 - Head.

9 - Lower limb.

10 - Toes.

11 - Genitals (cock).

*The layout is basically the same for the motor cortex but the representation of each area is different.

Question 24

List the modalities of sensation carried in the trigeminothalamic tract.

Which fibres carry which modalities?

Answer 24

• Large-diameter fibres (similar in function to the dorsal column-medial lemniscal pathway) convey information relating to:

1 - Discriminative touch.

2 - Vibration.

3 - Proprioception.

• Small-diameter fibres (similar in function to the spinothalamic tract) convey information relating to:

4 - Nondiscriminative touch.

5 - Temperature.

6 - Nociception.

Question 25

In which structure do secondary neurones of the spinothalamic tract decussate?

Answer 25

The anterior white commissure.

Question 26

Evaluation of which tract is done by assessing two-point discrimination?

What about the the tract is being assessed?

Answer 26

• The dorsal column-medial lemniscal system.

- It tests the ability of the dorsal column-medial lemniscal system to distinguish between two receptive fields.

Question 27

List the divisions of the spinocerebellar tracts.

What type of information does each division carry?

To which area of the brain does each division carry information?

Do any of these divisions undergo decussation?

Answer 27

1 - The posterior spinocerebellar tract carries proprioceptive information from the lower limbs to the ipsilateral cerebellum.

2 - The anterior spinocerebellar tract carries proprioceptive information form the lower limbs to the ipsilateral cerebellum by decussating twice.

3 - The cuneocerebellar tract carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.

4 - The rostral spinocerebellar tract carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.

Question 28

Which tract is unique in the number neurones it contains in chain from the periphery to the brain?

How many neurones comprise this tract, and how does this compare to other tracts?

Answer 28

• The spinocerebellar tract.

- It contains 2 neurones in chain, whereas all other tracts contain 3 neurones in chain.

Question 29

Describe the course of the spinocerebellar tracts.

What is Clarke’s column / nucleus dorsalis?

Answer 29

1 - Primary proprioceptive fibres arise in:

• The lumbar cord (anterior spinocerebellar).
• The thoracic cord (dorsal spinocerebellar).
• The cervical cord (cuneocerebellar and rostral spinocerebellar).

2 - They then synapse with second order neurons in the dorsal horn of the spinal cord in all tracts other than the cuneocerebellar tract:

• Second order neurones of the cuneocerebellar tract arise in the lateral cuneate nucleus in the caudal medulla.
• Second order neurones of the dorsal spinocerebellar tract originate from a group of cells in lamina 7 of cord segments T1-L2, known as clarke’s column / nucleus dorsalis.

2 - The anterior spinocerebellar tract decussates and ascends contralaterally. The rest ascend ipsilaterally.

3 - The tracts ascend to the cerebellum:

• The anterior spinocerebellar tract ascends to the superior cerebellar peduncle in the pons, where it decussates again and exits to terminate in the cerebellum.
• All other tracts ascend to the inferior cerebellar peduncles in the rostral medulla oblongata, where they exit to terminate in the cerebellum.

Question 30

Which cranial nerve(s) receive(s) sensory afferents for pain and temperature in the external ear?

What other nerves receive sensory afferents in this area?

Answer 30

1 - Auriculotemporal branch of CN V3.

2 - Auricular branch of CN X.

3 - Greater auricular nerve (roots C2 and C3).

4 - Lesser occipital nerve (roots C2 and C3).

Question 31

Which cranial nerve(s) receive(s) sensory afferents for pain and temperature in the lower premolar tooth?

Answer 31

The inferior alveolar nerve of CN V3.

Question 32

Which cranial nerve(s) receive(s) sensory afferents for pain and temperature in the upper canine tooth?

Answer 32

The anterior superior alveolar nerve of CN V2.

Question 33

Which cranial nerve(s) receive(s) sensory afferents for pain and temperature in the dura mater?

Answer 33

1 - Meningeal branch of CN V3.

2 - Meningeal branch of CN X.

Question 34

Describe the course of the trigeminothalamic tract.

Answer 34

1 - First order neurones (sensory cranial nerves) ascend to synapse at the principal / chief sensory nucleus in the middle pons.

• The dorsomedial division of the principal /chief sensory nucleus receives sensory information from the oral cavity, whereas the ventrolateral division receives input from all components of the trigeminal nerve.

2 - The second order sensory neurones ascend to the ventral posteromedial nucleus, where they synapse with third order sensory neurones:

• Third order sensory neurones in the ventrolateral division of the principal / chief sensory nucleus ascend contralaterally in the anterior trigeminothalamic tract.
• Third order sensory neurones in the ventrolateral division of the principal / chief sensory nucleus ascend ipsilaterally in the posterior trigeminothalamic tract.

3 - Third order sensory neurones pass through the posterior limb of the internal capsule and synapse at the somatosensory cortex in the postcentral gyrus.