20.1.2 Spinal Pathways Flashcards
How do the sensory fibres enter the spinal cord?
Through the dorsal roots
*cell bodies lie just before the dorsal root in the dorsal root ganglion.
What is the root of the mechanoreceptors and proprioceptors in the spinal cord?
Axons from mechanoreceptors and proprioceptors ascend the spinal cord in the dorsal columns. These carry fibres from the ipsilateral side of the body and continue to the dorsal column nuclei at the level of the medulla
What can the dorsal root columns be divided into?
the gracile and cuneate fascicles
What do the the gracile and cuneate fascicles contain?
contain fibres representing the lower limb and trunk, and the upper limb and neck, respectively (split at the level of T6)
What are the fascicles divided by?
The fascicles are divided by the dorsal intermediate septum
What are the dorsal column nuclei divided into?
the gracile and cuneate nuclei
What are the sensory axons doing at the level of the dorsal column nuclei?
sensory axons synapse onto internal arcuate fibres, which decussate and continue to ascend on the contralateral side in the medial lemniscus
What information do the fibres in the trigeminal lemniscus contain?
touch sensation from the face and head
Where do the medial lemniscus fibres synapse?
In the ventral posterior lateral nucleus of the thalamus (2nd to 3rd order neurons)
Where do the trigeminal lemniscus fibres synapse?
ventral posterior medial nucleus (2nd to third order neurons)
Where do all of the tertiary fibres synapse?
Primary somatosensory cortex on the postcentral gyrus
What remains throughout all levels of the dorsal column pathways?
fibres remain somatotopically arranged
How can sensory coding be achieved?
Receptive fields and lateral inhibition in dorsal column nuclei
Explain the concept of lateral inhibition in the dorsal column nuclei.
- The dorsal column nuclei are the first points of synapse for mechanoreceptive (and proprioceptive) information
- We want to be able to tell exactly where the information is coming from (e.g. in fine touch)
- Lateral inhibition allows this:
- A strongly excited second order neuron causes inhibition of neighbouring neurons
- This can occur by feed-forward or feed-back inhibition
- Therefore, only a single signal remains, allowing for high acuity
Does lateral inhibition occur in the anterolateral system?
No, because it is more important for amplification to occur (via Lissaeur’s tract), so acuity can be sacrificed.
Name some ways in which receptors can allow for diverse responses within, for example, mechanoception.
- Different receptive fields
- Different thresholds
- Adaptation vs non-adaptation
Compare the receptive fields of superficial and deep receptors in the skin.
Superficial receptors have a smaller receptive field than deep receptors.
How can increased sensitivity of receptors be achieved?
Convergence of receptive fields:
- Multiple individual receptors clustered into small hotspot can send signals to a single DRG afferent
- This means that the sensitivity is increased, since there is summation
- An example of this is with Meissner’s corpuscle mechanoreceptors
By which pathway is information from nociception and temperature sensation carried?
Anterolateral system
How do axons from the anterolateral system enter the spinal cord and where do they synapse onto?
Through dorsal horns that may ascend or descend one or two spinal segments in the tract of Lissauer, before synapsing onto secondary fibres and interneurons in the dorsal horn laminae
Where do Aδ fibres synapse?
Aδ fibres synapse in laminae I (marginal zone) and V (part of the nucleus proprius)
Where do C fibres synapse?
C fibres synapse in lamina II (substantia gelatinosa)
What is thought to cause referred pain?
Referred pain is thought to be a result of cutaneous and visceral nociceptive afferents converging on the same secondary fibres through interneurones in the dorsal horn laminae
What are the 3 tracts the secondary fibres in the anterolateral tract can decussate into on the contralateral side?
*Spinothalamic tract
*Spinoreticular tract
*Spinomesencephalic tract
What is the spinothalamic tract?
The spinothalamic tract is the largest tract, arising from dorsal horn laminae I and V–VII (the nucleus proprius and Clarke’s nucleus), and ascends to the central lateral and the ventral posterior lateral nuclei of the thalamus
What is the spinoreticular tract?
The spinoreticular tract arises from laminae VII (Clarke’s nucleus) and VIII. Some fibres terminate in the reticular formation of the pons and medulla, while others continue to the central lateral nucleus of the thalamus. Fibres that terminate in the thalamus, from both spinothalamic and spinoreticular tracts, synapse onto fibres that project to primary somatosensory cortex
What is the spinomesencephalic tract?
Fibres in the spinomesencephalic tract arise from laminae I and V, and project to the periaqueductal grey and the mesencephalic reticular formation. These projections provide inputs to the limbic system via the hypothalamus
Summarise the somatosensory thalamic nuclei. Draw their positions.
- Ventral posterior lateral (VPL) thalamus -> Main somatosensory and nociceptive nucleus receiving input from the body (via from dorsal column and spinothalamic tracts). Projects on to the primary and secondary sensory cortex for localisation of touch.
- Ventral posterior medial (VPM) thalamus -> Receives mechanoreceptive and nociceptive input from the face. Projects on to the primary and secondary sensory cortex.
- Central (intralaminar) nuclei -> Receives nociceptive inputs (from the spinothalamic tract). Projects to insular, cingulate and diffuse cortex for the affective aspects of nociception.
How do axons of neurons from the nucleus proprius get to the other side of the body?
There is the local crossing of fibres in the anterior white commissure to form the anterolateral system.
Describe transmission through the anterolateral column.
It carries afferent nociceptive (and some mechanoreceptive) information:
- Wide dynamic range neurons in the nucleus proprius send axons across the midline (via the anterior white commissure)
- The fibres then travel via the anterior and lateral spinothalamic tracts to the medulla, where the spinal leminiscus
- This carries the information to the thalamus
- Relay neurons take the information from the thalamus to the somatic sensory cortex
What is the effect of enkephalins on pain?
[IMPORTANT]
- Enkephalins are endogenous opioid peptides
- They can be released into the synapse between a sensory nociceptive fibre and a projection neuron that travels up the spinal cord
- They inhibit nociception (e.g. in descending control of nociception)
Describe the descending control of nociceptive pathways.
- The pariaqueductal grey sends fibres to the locus coeruleus and nucleus raphe magnus.
- The locus coeruleus releases noradrenaline onto the inhibitory enkephalin interneurons in the spinal cord.
- The nucleus raphe magnus also releases 5-HT and enkephalins onto inhibitory enkephalin interneurons in the spinal cord.
- The inhibitory enkephalin interneurons release enkephalins that are inhibitory to a synapse between the first and second order nociceptive neurons in the spinal cord. (Check this cos it doesn’t quite agree with what Trev-dog says)
Describe the gate theory of pain.
- The gate theory of pain states that when a nociceptive C fibre fires, if an Aβ fibres simultaneously, the Aβ fibre will stimulate an inhibitory interneuron
- The inhibitory interneuron releases enkephalin [IMPORTANT]
- This inhibits the second order nociceptive neuron, so that the nociception is reduced
- Thus, it can be seen that pain can be reduced by providing some gentle touch stimulation to the same area (e.g. rubbing a wound after falling over)
What are the 3 main tracts involved in taking pain information to the brain? What is the role of each?
- Spinothalamic -> The big one, responsible for conscious perception of pain
- Spinoreticular -> Arousal in response to pain “wow yeah I should do something about this”
- Spinomesencephalic -> Via the brainstem, involved in descending control of nociception “this is a bit hardcore, I should modulate it”
For Aα fibres, describe where they originate and terminate.
- Originate in muscle spindles, from where they carry sensory proprioception information
- Terminate at lamina 6 to 9, which includes interneurons in the dorsal horn and motor neurons in the ventral horn
For Aβ fibres, describe where they originate and terminate.
- Originate from cutaneous receptors and static proprioceptors
- Terminate in the laminae 3 to 6 of the dorsal horn (a.k.a. the nucleus proprius)
- They synapse onto wide dynamic range neurons (interneurons involved in polysynaptic reflexes)
Where do Aδ and C fibres (nociceptive fibres) synapse?
- Aδ -> At the marginal zone of the dorsal horn (lamina 1) and nucleus proprius (laminae 3-5)
- C -> At the substantia gelatinosa of the dorsal horn (lamina 2)
Note: Each of these fibres can synapse at multiple heights in the spinal cord.
What type of lesion occurs in Brown-Sequard syndrome?
Lateral lesion, knocking out one side completely
What functions are lost in Brown-Sequard syndrome?
Ipsilateral side of lesion: loss of mechanoreception and proprioception
Contralateral side of lesion: loss of nociception and temp sensation
Draw the position of the main ascending and descending white matter columns.
Ascending (sensory):
- Dorsal column
- Spinocerebellar tracts
- Anterolateral system
Descending (motor):
- Corticospinal tracts (lateral + anterior)
Describe transmission through the spino-cerebellar tracts.
It carries afferent proprioceptive information from the lower limb:
- Dorsal root ganglion axons at the lumbar region, which receive information from the lower limb, have branches that ascend up to the thoracic region
- At the thoracic region, they terminate at Clarke’s column (a.k.a. thoracic nucleus)
- This sends out relay neurons to the dorsal spino-cerebellar tracts
- These take the information to the cerebellum on the same side of the body -> This is important for unconscious perception and response to the stimulus
Describe somatotopy throughout the dorsal columns and in the medial lemniscus.
Dorsal columns:
- From medial to lateral, the innervation becomes more superior (i.e. medial = legs, lateral = arms)
- This is because low down we start with just the gracile fascicles and then the cuneate fascicles join
Medial lemniscus:
- In medulla -> Arms to legs in a dorsal to ventral direction
- In pons -> Arms to legs in medial to lateral direction
- In mibrain -> Arms to legs in ventral to dorsal direction
(In the medial lemniscus it is as if the person is doing a sommersault)
Draw the somatotopy of the primary somatosensory cortex in the post-central gyrus.
Describe the columnar organisation of the primary somatosensory cortex (and the neocortex in general).
Inputs to the primary somatosensory cortex (from the thalamus) travel in columns from each receptor. These columns are divided into layers:
- Layer 4 -> This receives information from the thalamus, then passes it up to layers 2/3
- Layers 2/3 -> Passes the information to S2 or back down to layers 5/6
- Layer 5 -> Cells project down to brainstem and spinal cord
- Layer 6 -> Cells project down to thalamus
It is also worth noting that layer 1 is composed of just axons that can travel to different parts of the cortex (e.g. between Brodmann areas).
What are some causes of lesion of the primary somatosensory cortex (S1)? How do they differ?
They are frequently caused by stroke:
- Anterior cerebral artery -> Foot, leg and trunk somatosensation lost
- Middle cerebral artery -> Arm, hand and face somatosensation lost
This is because of the parts of S1 that each artery supplies.
What is the descending contribution of the somatosensory cortex?
The somatosensory cortex uses the corticospinal (CST) tract to provide control of the ascending sensory systems (in addition to motor control).
