Ascending Pathways of the Spinal Cord and Brainstem

Question 1

Define tract.

Answer 1

Tract/fasciculus refers to a bundle of neuronal fibers of the same/similar function, travelling together within the CNS.

Question 2

A) ______________ tracts carry sensory impulses “upwards” from sensory receptors towards the cerebral cortex, hence constitute the afferent system.

B) _______________ tracts carry motor impulses “downwards” away from the brain to the effectors (muscles and glands), hence constitute the efferent system.

Answer 2

A) Ascending
B) Descending

Question 3

Most ascending pathways consist of 4 major parts. Name them.

Answer 3

1. Sensory receptor
2. 1st order (primary) sensory neuron
3. 2nd order (secondary) sensory neuron
4. 3rd order (tertiary) sensory neuron

Question 4

_____________________ are modified neurons or modified epithelial cells that detect environmental stimuli and more importantly convert the physical energies of the stimuli to bioelectric impulses (action potentials) - the language understood by the nervous system.

Answer 4

Sensory receptors

Question 5

What are 1st order sensory neurons?

Answer 5

These are the neurons that directly arise from the sensory receptor, and they may or may not be part of their sensory receptors.

Question 6

What aspects should you highlight when discussing 1st order sensory neurons of an ascending pathway?
[This is just to guide you on how to answer this particular question. Rate it a 5, don’t spend much time here. 😁]

Answer 6

• cranial or spinal nerve(s) that carry the peripheral processes from the anatomical site being discussed
• sensory ganglion that contains their cell bodies
• level of the CNS at which central processes enter the CNS. For cranial nerves, where they attach at the brain. For spinal nerves, from the dermatome, you can tell where the spinal nerve enters spinal cord e.g. if dermatome where the sensory nerve is is C7, then it enters spinal cord at C7 segment.
• tract(s) formed by the central processes once they enter the CNS. Most spinal nerves will form dorsolateral tracts of Lissauer as they enter CNS but others form tracts and end there.
• site of termination (CNS level and nucleus) of the central processes- the site where it synapses with the 2nd order sensory neuron - is nucleus in spinal cord, pons, medulla or midbrain?

Question 7

What are 2nd order sensory neurons?

Answer 7

These are neurons that begin from the site of termination of the 1st order sensory neurons and are multipolar neurons. The axonal processes usually decussate to the contralateral side, then ascend via a specified tract/lemniscus to the thalamus. They synapse with their corresponding 3rd order sensory neurons at a specified thalamic nucleus.

NOTES:
* Lemniscus: A compressed fiber tract traversing the brainstem.
* They are 4 lemniscal systems in the brainstem: medial lemniscus, lateral lemniscus, spinal lemniscus, and trigeminal lemniscus.

Question 8

What aspects should you highlight when discussing 2nd order sensory neurons of an ascending pathway?

Answer 8

• Site of origin (the CNS level and corresponding sensory nucleus)
• Site and name of the decussation of the axonal projections from the sensory nucleus. (for most, at the sensory nucleus, when the 2nd order sensory neuron arises, it decussates immediately before ascending) [MOST spinal nerves decussate at the anterior white commissure]
• tract(s) and brainstem lemniscus formed by their axonal fibers.
• thalamic nucleus of termination of the axonal fibers of the 2nd order sensory neurons

[This is just to guide you on how to answer this particular question. Rate it a 5, don’t spend much time here. 😁]

Question 9

Name the thalamic sensory relay nuclei (they are 4).

Answer 9

Ventroposterior nucleus:
- ventroposterolateral (VPL) - point of synapse of all sensations carried by spinal nerves
- ventroposteromedial (VPM) - point of synapse of all sensations that come from the head region (carried by cranial nerves) except 3 i.e.
smell [only sensation whose pathway doesn’t pass through thalamus before reaching cerebral cortex], vision and hearing

Metathalamus:
- medial geniculate body (MGB) - auditory relay
- lateral geniculate body (LGB) - visual relay

Question 10

The axons of the 3rd order sensory neurons pass through the _________________, followed by the _________________ to go to the cerebral cortex. They terminate in layer IV (inner granular layer) of their designated primary sensory cortices.

Answer 10

internal capsule, followed by the corona radiata

Question 11

What should you highlight when discussing an ascending pathway and you’re discussing the 3rd order sensory neurons in that pathway?
[This is just to guide you on how to answer this particular question. Rate it a 5, don’t spend much time here. 😁]

Answer 11

• thalamic nucleus of origin
• part of the internal capsule where their axons traverse as they go to the cortex (for most sensations that relay in the thalamus, the 3rd order sensory neurons pass through the posterior limb of the internal capsule. 3rd order sensory neurons for vision pass through the retrolenticular segment; those for hearing pass through the sublenticular segment)
• (specific) axonal radiation [known as corona radiata] formed by these axons as they go to the cortex [vision - optic radiation/geniculocalcarine radiation, hearing - auditory radiation]
• site of cortical termination (the lobe, gyrus/gyri, cortical functional area and Brodmann’s number) of the axons

Question 12

Discuss the anterolateral spinothalamic tract.

Answer 12

• Modalities it conveys: pain, temperature, crude touch, and pressure - relayed largely by spinal nerves.
• central processes of 1st order neurons ascend 2 or 3 segments in the dorsolateral tract before they synapse with the cells of the substantia gelatinosa at the dorsal grey horn of the spinal cord
• After the decussation (through the ventral white commissure), the axons of the 2nd order neurons occupy the anterolateral aspect of the spinal cord white matter.
• The lateral spinothalamic tract transmits pain and temperature sensations, anterior spinothalamic tract transmits crude touch and pressure sensations
• The spinothalamic tract forms the spinal lemniscus at the level of the brainstem.
• The axons of the substantia gelatinosa (2nd order sensory neurons) terminate in the ventral posterolateral nucleus of the thalamus
• The axons of the 3rd order sensory neurons pass through the internal capsule, followed by the corona radiata to go to the cerebral cortex
• The 3rd order neurons terminate in the postcentral gyrus of the cerebral cortex at the parietal lobe.

Notes:
- the internal capsule is a two-way tract for the transmission of information to and from the cerebral cortex
- the corona radiata is a white matter sheet that consists of afferent and efferent fibres that connect the cerebral cortex to the brainstem. It continues inferiorly as the internal capsule and superiorly as the centrum semiovale [the central area of white matter found underneath the cerebral cortex]

Question 13

Discuss the dorsal column-medial lemniscal pathway.

Answer 13

✓ pathways for proprioception, vibration sensation and fine touch
✓ The 1st order neurons enter the spinal cord just like those in the anterolateral pathway. However, they don’t synapse in the spinal cord. Instead, they occupy the dorsal white column of the spinal cord as they ascend to the brainstem as fasciculus gracilis (Tract of Goll) and fasciculus cuneatus (Tract of Burdach).
✓ Fibres from the coccygeal, sacral, lumbar and lower thoracic segments enter the tract of Goll (fasciculus gracilis) while the fibres from the upper thoracic and cervical segments run in the tract of Burdach (fasciculus cuneatus). The cervical, thoracic, lumbar and sacral fibres are arranged in that order from lateral to medial in these tracts.
✓ The 1st order neurons synapse in the medulla oblongata at their respective nuclei: nucleus gracilis and nucleus cuneatus.
✓ The axons of second-order sensory neurons from nucleus gracilis and nucleus cuneatus curve ventromedially around the central grey matter of the medulla as internal arcuate fibres to undergo decussation (internal arcuate decussation).
✓ The fibres after crossing the midline turn upwards as a flat tract called the medial lemniscus on the medial aspect of the medulla oblongata, which crosses upwards to terminate in the ventral posterolateral nucleus of the thalamus.
✓ The third order sensory neurons from thalamus project into the cerebral cortex of the cerebral hemisphere.

Question 14

Discuss spinocerebellar pathways.
[Note that these pathways carry sensations from the lower limb, not the upper limb. Similar modalities for upper limb are carried by cuneocerebellar pathway.]

Answer 14

✓ there are two principal spinocerebellar pathways: dorsal and ventral spinocerebellar pathways
✓ these occupy the peripheral aspects of the spinal cord’s lateral funiculus
✓ they carry unconscious proprioceptive sensations. [REMEMBER: Dorsal column-medial lemniscus pathway carries conscious proprioception] and cutaneous information to the cerebellum for coordination of movement

DORSAL SPINOCEREBELLAR TRACT (carries unconscious proprioception from individual muscles of the lower limb)
✓ Cell bodies of first order neurons lie in dorsal root ganglia.
✓ Central processes of these neurons enter the posterior horn of the grey matter via the posterior roots of spinal nerves and relay at Clarke’s nucleus.
✓ Axon fibres from Clarke’s nucleus convey this proprioceptive information in the spinal cord in the peripheral region of the lateral funiculus ipsilaterally.
✓ the fibers enter the cerebellum via the inferior cerebellar peduncle
✓ they terminate ipsilaterally in the rostral and caudal parts of the cerebellar vermis [Note: these fibers do not decussate]

VENTRAL SPINOCEREBELLAR TRACT [here there’s double decussation] (carries unconscious proprioception from lower limb as a whole)
✓ This tract carries proprioceptive information from the Golgi tendon organs of the ipsilateral lower limb and trunk.
✓ The central processes of first order neurons [which reside in the DRG] travel through the dorsal grey horn to form synapses in Rexed lamina VII.
✓ axons of the second order neurons decussate and travel up the spinal cord as the anterior spinocerebellar tract
✓ the fibers enter the cerebellum via superior cerebellar peduncle
✓ they terminate contralaterally in the anterior cerebellar vermis

Further notes:
The Golgi Tendon Organ is a proprioceptor that senses tension in a tendon. The GTO relays information about force levels in the muscle or tendon to the central nervous system.

Question 15

What is key to note about the trigeminal pathways?

Answer 15

✓ pathways for general sensations from the head region - face, facial structures, anterior scalp and meninges
✓ peripheral processes of the 1st order neurons are contained within the three divisions of the trigeminal nerve - V1, V2 and V3
✓ their cell bodies are housed in the trigeminal ganglion, and their corresponding central processes enter brainstem at pontine level
✓ fibers conveying touch sensations synapse/terminate in the chief/principal sensory nucleus of trigeminal within the pons
✓ fibers conveying pain and temperature descend (forming the spinal tract of trigeminal nerve), and synapse in the spinal nucleus of V in the medulla oblongata and upper cord - continuous with the dorsal grey horn
✓ the proprioceptive and vibratory fibers ascend to the midbrain, to the mesencephalic nucleus of V, where their cell bodies are located (only 1st order sensory neurons whose collection of cell bodies is located within CNS)
✓ after the decussation, axons of 2nd order neurons ascend to the thalamus as trigeminal lemniscus to the VPM nucleus for synapse

Question 16

Discuss syringomyelia.

Answer 16

A destructive process around the central canal of the spinal cord results in increase in the size of the cavity and affects the crossing spinothalamic fibres in the anterior white commissure. So, there will be bilateral loss of pain and temperature sensations but the posterior column sensations are preserved as they do not get affected initially. This produces “dissociated sensory loss”.

Question 17

State the clinical relevance of the dorsolateral tract of Lissauer.

Answer 17

A unilateral lesion of the spinothalamic tract will result in loss of crude touch, pressure, pain and thermal sensation. The sensation will be lost on the opposite side below the level of lesion. The contralateral loss of pain and temperature sensations, however, occur 2–3 segments below the level of the lesion because pain and thermal sense rise 2–3 levels up in the tract of Lissauer before they terminate.

Question 18

The orderly arrangement of different levels of the body in a tract is ______________________ of a tract.

Answer 18

somatotopic lamination

Question 19

Describe the clinical relevance with regard to somatotopic lamination.

Answer 19

A tumour in relation to the spinal cord can either grow from outside (meningioma) or from within (neuroglioma).
If the meningioma is pressing the cervical part of spinal cord from the lateral aspect, it initially involves:
✓ The sacral and lumbar fibres of lateral corticospinal tract
causing ipsilateral monoplegia of lower limb
✓ The sacral and lumbar fibres of lateral spinothalamic tract causing contralateral loss of pain from lower limb and
perianal region
If the neuroglioma is growing in the cervical part spinal cord laterally, it initially involves:
✓ The cervical and thoracic fibres of lateral corticospinal tract
causing ipsilateral monoplegia of upper limb
✓ The cervical and thoracic fibres of lateral spinothalamic tract causing contralateral loss of pain from upper limb and trunk.
Lower limb and perianal region are last to be involved.

Question 20

Q: Trace the pathway of unconscious proprioception from the quadriceps femoris unit to the cerebellum. (8 marks) [“Such questions will be properly answered once you learn about the cerebellum, cerebrum and sensory receptors.”]

Answer 20

Proprioceptors:
* Neuromuscular spindle [1] and Golgi tendon organ [1]

Primary sensory neuron:
* Branches of the femoral nerve [1]

Synaptic point:
* Thoracic nucleus of Clark (Lamina VII) [1]

Second order neurons:
* Ventral spinocerebellar tract [1], which enter the cerebellum via the superior [1] cerebellar peduncle
* Dorsal spinocerebellar tract [1], which enter the cerebellum via the inferior [1] cerebellar peduncle

Question 21

What is the clinical relevance of the lateral spinothalamic tract?

Answer 21

1. The fibres of this tract carry pain and temperature sensations—the pain fibres being lateral to the temperature. The pain fibres become very superficial in the lateral white column of the cord in the cervical region. Therefore cordotomy can be performed safely at this level to relieve pain in the opposite half of the body.
2. The involvement of decussating fibres of this tract in the
   anterior commissure in syringomyelia leads to bilateral loss of
   pain and temperature sensations below the level of the
   lesion.

Question 22

Q: Outline the pathway of vibration from the anterior thigh. [HINT: Dorsal column-medial lemniscus pathway]

Answer 22

Modality: vibration
Receptor: Pacinian corpuscle
Dermatome: L2/L3 (depending on the exact location on the anterior thigh)

1st order neuron:
Name/peripheral process: femoral nerve
Cell body location: dorsal root ganglion
CNS entry of central process: L2/L3 segment and ascends on the ipsilateral fasciculus gracilis
Termination: gracile nucleus in medulla

2nd order neuron:
Origin: gracile nucleus
Decussation: as internal arcuate fibres at level of the medulla
Tract: medial lemniscus
Termination: VPL nucleus of thalamus

3rd order neuron:
Origin: VPL
Course: travels through thalamo-cortical radiation, posterior limb of internal capsule, corona radiata
Termination: primary sensory area, layer 4; major input layer