Ascending Pathways of the Spinal Cord and Brainstem Flashcards
Define tract.
Tract/fasciculus refers to a bundle of neuronal fibers of the same/similar function, travelling together within the CNS.
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.
A) Ascending
B) Descending
Most ascending pathways consist of 4 major parts. Name them.
- Sensory receptor
- 1st order (primary) sensory neuron
- 2nd order (secondary) sensory neuron
- 3rd order (tertiary) sensory neuron
_____________________ 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.
Sensory receptors
What are 1st order sensory neurons?
These are the neurons that directly arise from the sensory receptor, and they may or may not be part of their sensory receptors.
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. 😁]
- 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?
What are 2nd order sensory neurons?
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.
What aspects should you highlight when discussing 2nd order sensory neurons of an ascending pathway?
- 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. 😁]
Name the thalamic sensory relay nuclei (they are 4).
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
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.
internal capsule, followed by the corona radiata
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. 😁]
- 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
Discuss the anterolateral spinothalamic tract.
- 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]
Discuss the dorsal column-medial lemniscal pathway.
✓ 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.
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.]
✓ 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.
What is key to note about the trigeminal pathways?
✓ 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