Lecture 11: Ascending Pathways I Flashcards
What modalities does the spinothalamic pathway carry?
Pain, Temperature, Pressure and Crude Touch
What modalities does the Dorsal column Medial Lemniscus Pathway carry?
Discriminative Touch, Vibration, Pressure and Proprioception.
What is a dermatome?
Dermatomes: A specific region of skin on the body innervated by sensory fibres of a single segmental spinal nerve e.g. around the level of the nipple is supplied by T4. For the sensory innervation of the face, there is a similar concept, by the cranial nerves. This by definition is not called a dermatome (as the cranial nerves do not supply the body).
What is the difference between modalities that are essential compared to those that increase detail?
Modalities that are essential such as pain, temperature, some touch and pressure is carried by thin, poorly myelinated or unmyelinated fibres. Conduction is relatively slow.
Modalities that increase details such as discriminative touch, vibration and proprioception are carried via large diameter, heavily myelinated fibres. Conduction is fast.
Generally, how are sensory pathways laid out?
Consists of a 3 neurone pathway:
- 1 Order neurone - Brings information from the periphery to send sensation using the axon into the CNS. In the case of spinal nerves, the first order neurone is psuedounipolar and has its ganglion in the dorsal root ganglion.
- 2 Order neurone crosses the midline and travels to the VPL (ventral posterior lateral nucleus of the thalamus).
- 3 Order neurone - Has its cell body in the thalamus. The axons sent into the primary sensory cortex.
Describe the pathway for pain, temperature, crude touch and some pressure.
Spinothalamic pathway:
- The receptors are located in the periphery. The fibres are sent through the spinal nerve. The cell body is in the dorsal root ganglion. They travel through the Lissauer’s tract to enter the dorsal horn. At the dorsal horn the cell bodies synapse and collect with the second order neuron. This may happen immediately at the level of entry or may go up a few segments.
- The second order neurone sends an axon. The crossing over occurs at the ventral white commissure. The axon is now in the white matter of the cord and enters the spinothalamic tract. The axon then travels up the spinothalamic tract on the contralateral side to reach the contralateral side in the thalamus.
- The third order neurone synapses with the third order neurone at the VPL to send an axon to the somatosensory cortex.
Which Rexed’s laminae contain synapses form the spinothalamic tract?
I, III and IV - Crude Touch
II - Pain and Temperature
What is the substantia gelatinosa?
Lamina II.
Fibres that carry pain and temperature travel in the DRG and those synapse at the level of entry in lamina II.
What is the difference in the pathway for crude touch and pain/temperature in the spinothalamic pathway?
- Fibres that carry pain and temperature travel in the DRG and those synapse at the level of entry in lamina II - substantia gelantinosa
- For the fibres that carry crude touch and pressure, when they enter the cord, the fibres ascend a couple of levels and then synapse at level I, III, or IV.
We can then differentiate if it is crude touch or pain and temperature
How is the post-central gyrus organised?
The post central gyrus is somatotopically organised. Signals coming from the face are projected most lateral. After which is signals from the arm, trunk and then the legs.
Describe the Dorsal Column Medial Lemniscus Pathway.
We have on either side of the midline a gracile fascicle throughout the whole length of the spinal cord. They carry sensory information from below T6 (generally from the legs). The cuneate fascicle is more lateral - this only exists form above T6. To generalise, information from the arms.
The first order neurone, with its cell body in the DRG, enters the white matter. It then jumps to the dorsal white column and travels ipsilaterally. The fibres are somatotopically organised. Above T6, fibres travel in the cuneate fascial. Below T6, fibres join the gracile fascicle.
The fibres from above T6 synapse at the cuneate nucleus, and those below at the gracile nucleus, synapse with the second order neurone. Here the second order neurons send an axon to cross the midline to travel in the medial lemniscus.
This axon synapses at the VPL with the third order neurone to project information to the somatosensory cortex.
How is the Medial Lemniscus organised?
Somatopically organised - Legs, trunk and then arms from inferior to superior.
How do synapses in the sensory pathways integrate information from many neurones?
There is opportunity to for modification by other input such as convergence/divergence or descending pathways. Synaptic transmission isn’t a simple relay.
How do ascending sensory axons travel to the post-central gyrus?
The spinothalamic tract and the medial lemniscus travel in the internal capsule in the posterior limb. This is located between the thalamus an the lentiform nuclei. They then reach the thalamus, to synapse and the specific neurones then synapse out to get to the somatosensory cortex.
What is the name of the spinothalamic tract in the medulla?
At the medulla the spinothalamic tract changes name - it travels in a ribbon of axon know as the spinal lemniscus. It reaches the contralateral thalamus in the VPL. From there the third order neurone will leave and reach the post central gyrus.
