Practical 3 - Somatic sensation Flashcards
describe the rough visual strcuture of the spinal cord
The spinal cord consists of a central H-shaped region of grey matter surrounded white matter.
dorsal columns carry what info?
dorsal columns, carrying fine touch and proprioceptive information
anterolateral system carries what info?
anterolateral system,
carrying pain and temperature information.
Most sensory information passes via the _______ on route to the cerebral cortex.
Most sensory information passes via the thalamus on route to the cerebral cortex.
The ventral nuclei of the thalamus are concerned particularly with ….
The ventral nuclei of the thalamus are concerned particularly with somatosensory relay and motor co- ordination
what is the internal capsule?
The internal capsule is a massive tract of white matter linking the thalamus to the cerebral cortex
Fine touch and proprioception are represented in the….
Fine touch and proprioception are represented in primary somatosensory cortex in the postcentral gyrus.
describe pains cortex representation?
Pain has more complex cortical representation, including in the anterior cingulate cortex and the insula.
how many main ascending somatosensory pathways are there?
2
T or F
The ascending somatosensory pathways comprise two very different systems for sensation from the skin, with different anatomy, different sensory receptors and different perceptual modalities
The ascending somatosensory pathways comprise two very different systems for sensation from the skin, with different anatomy, different sensory receptors and different perceptual modalities
what are the 2 afferent ascending somatosensory pathways
- One carries tactile (touch) and proprioceptive sensation,
- the other nociception (perceived as pain) and thermal sensation
describe how nociceptive and thermal sensations are carried…. (pathway)
- afferents through the dorsal roots to terminate in the spinal cord dorsal horn, close to their entry
- at least one local synaptic relay in the superficial dorsal horn
- projection cells send axons across the spinal cord in the ventral commissure
- to the contralateral anterolateral white matter where they ascend
- the ascending fibres terminate at many sites in the brainstem (medulla, pons and midbrain) as well as the several parts of the thalamus
According to the location of the ascending fibres, this is the anterolateral ascending system, sometimes referred to as the spinothalamic tract.
describe the path tactile signals take to the brain
- enter spinal cord
- ascend in dorsal columns on ipsilateral side without a synaptic relay
- fibres terminate at the rostral end of the spinal cord in the gracile and cuneate nuclei (dorsal column nuclei)
- axons from these nuclei cross the midline and ascend through the medial part of the brainstem as a large tract of fast-conducting fibres called the medial lemniscus
- These fibres project to the thalamus
- from there third order neurons carry tactile and conscious proprioceptive information to the somatosensory cortex in the postcentral gyrus of the parietal lobe
t or F
The anterolateral system and dorsal column-medial lemniscus systems have different locations in both the spinal cord and the brainstem, so losses of different types of somatic sensation can be diagnostic of lesions at specific sites.
T
the anterolateral system and dorsal column-medial lemniscus systems have different locations in both the spinal cord and the brainstem, so losses of different types of somatic sensation can be diagnostic of lesions at specific sites.
anterolateral ascending system, sometimes referred to as the spinothalamic tract carries what information?
Nociceptive and thermal sensations
dorsal column-medial lemniscus system carries inforamtion about what>
tactile and proprioception sensation
describe the centra lregion of the spinal cord in cross section
The spinal cord consists of a central region of nerve cells and processes (grey matter)
what is spinal cord grey matter surrounded by/
The spinal cord consists of a central region of nerve cells and processes (grey matter) surrounded by bundles of mainly myelinated axons (white matter) travelling to and from the brain, and between different spinal levels
T or F
the amound of white and grey matter changes at different spinal cord levels?
T
H shaped grey matter:
ventral horns correspond to what?
motor function
H shaped grey matter:
dorsal horns correspond to what?
sensory function
which regions of the spinal cord is the grey matter more extensive?
regions of the cord that innervate the brachial and lumbar/ sacral plexuses to the limbs, where the dorsal and ventral horns are enlarged (associated with limb innervation).
The thoracic cord has small _____ and ______ horns
The thoracic cord has small dorsal and ventral horns
The thoracic cord has small dorsal and ventral horns, but has two extra cell groups not found at other levels.
what are they?
- The intermediolateral nucleus
- Clarke’s nucleus (Clarke’s column)
describe the The intermediolateral nucleus
composed of sympathetic pre-ganglionic neurons, the axons of which run to the chain of sympathetic ganglia.
describe clarkes nucleus
Clarke’s nucleus (Clarke’s column) has relay cells for proprioception from the lower limbs.
Shape of spinal cord at main levels
Shape of spinal cord at main levels
describe how the white matter is organised?
3 basic bundles, columns or fasciculi: dorsal, ventral and lateral.
The major somatic sensory pathways comprise two large ascending tracts with very different function:
what are they? where do they run in the spinal cord?
the anterolateral system conveys noxious and thermal (“pain and temperature”) information, running as its name suggests anteriorly (ventrally) in the lateral column.
The dorsal column conveys discriminative (“fine”) touch sensation, and proprioception.
whre can the dorsal column be further subdivided?
At cervical and upper thoracic levels
At cervical and upper thoracic levels the dorsal column can be further subdivided into…..
a lateral cuneate - fibres from upper limb
medial gracile division - fibres from lower limb
describe the dorsolateral and ventral columns in the white matter
The dorsolateral and ventral columns contain a mixture of ascending and descending tracts (many of which you will see in the next practical)
Overall the surrounding white matter is thicker in the _______ segments, where all of the ascending axons have entered the cord, and few descending fibres from the brain have terminated
Overall the surrounding white matter is thicker in the cervical segments, where all of the ascending axons have entered the cord, and few descending fibres from the brain have terminated
Features at different spinal levels
describe how the spinal cord grey matter can be divided?
series of layers (Rexed’s laminae)
they represent functional specialisations
most important division is the substantia gelatinosa (layer II), a critical site for processing noxious information, and therefore a therapeutic target.
do you know what lissauers tract is?
Overlying the substantia gelatinosa (layer 2 of rexeds laminae) is Lissauer’s tract which contains incoming axons carrying pain and temperature information that travel up or down the cord to an adjacent segment before entering the dorsal horn.
substantia gelatinosa (layer II), a critical site for processing ……
substantia gelatinosa (layer II), a critical site for processing noxious information, and therefore a therapeutic target.
Cross-section through the mid cervical spinal cord
Deeper layers of dorsal horn (IV, V) contain…
Deeper layers of dorsal horn (IV, V) contain relay neurons many with axons that cross the mid- line in the ventral commissure to ascend in the anterolateral column.
he largest cells in the spinal cord are…..
where are they found?
he largest cells in the spinal cord are the motorneurons that project to muscles, and these are found in groups in the ventral horn.
The major region in the centre of the cord (lamina VII) contains ….
The major region in the centre of the cord (lamina VII) contains spinal interneurons concerned with local processing (e.g. reflexes).
The brainstem comprises the…
The brainstem comprises the medulla, pons and midbrain.
this prac is mainly dorsal brainstem btw
col
Features on the dorsal surface of the brainstem
- Thalamus
2 .Superior colliculus
3 Inferior colliculus
4.Superior cerebellar peduncle - Middle cerebellar peduncle (cut)
- Inferior cerebellar peduncle
- Cuneate swelling or tubercle
- Gracile swelling or tubercle
The Spinal nucleus and tract of the trigeminal nerve (V) are important for pain and temperature sensation from,….
The Spinal nucleus and tract of the trigeminal nerve (V) are important for pain and temperature sensation from the head and neck (trigeminal nerve territory),
The Spinal nucleus and tract of the trigeminal nerve and are also prominent in the low medulla, where they are continuous with (and homologous to) the……
substantia gelatinosa and Lissauer’s tract of the spinal cord.
Section through lower (caudal) medulla (at the spinal cord junction
- The dorsal columns (DC) terminate in the dorsal column nuclei, a little higher (more rostral) in the medulla. The gracile fasciculi are more medial to the cuneate fasciculi.
- Note the location of the anterolateral tract (AL), although it is not a separate fibre bundle.
- The Spinal nucleus of the trigeminal nerve (V) is prominent.
Section through the medulla at level of the sensory decussation
- At this level the gracile (G) and cuneate (C) nuclei (dorsal column nuclei) are clear.
- The second order sensory fibres leave the dorsal column nuclei, decussate (SD) and form the MEDIAL LEMNISCUS (ML), on route to the thalamus.
- A large spinal trigeminal nucleus is still obvious (V), together with its overlying fibre tract.
Section through the upper (rostral) medulla
- At the upper end of the medulla all of the ascending sensory fibres have terminated, and the ascending tactile pathway is represented by the Medial Lemniscus (ML).
- There is still a large spinal trigeminal nucleus (V), although not as obvious as in the lower medulla.
Section through the upper pons
- The ventral or basilar part of the pons appears as a large swelling of the brain stem (the dorsal part is hidden below the cerebellum). Along the ventral mid-line is a well- marked groove, the site of the basilar artery (B).
- The medial lemniscus (ML) continues through the pons towards the thalamus.
Section through lower midbrain
A prominent feature of the midbrain is the roof of the midbrain, composed of two pairs of small swellings, the superior and inferior colliculi (SC & IC). Together they constitute the tectum (“roof”) - the part of the midbrain dorsal to the narrow aqueduct (A) running through the midbrain, linking third and fourth ventricles. These are all useful landmarks. In the lower midbrain the inferior colliculus (IC) is visible. Note that the ascending fibres of the medial lemniscus (ML) are now further lateral in the brainstem, and the fibres of the anterolateral tract have joined them – sometimes called the spinal lemniscus
Section through the upper midbrain
In the upper midbrain, the superior colliculi (SC) are visible.
The thalamus is the major input route to…
The thalamus is the major input route to the cerebral cortex,
The thalamus is the major input route to the cerebral cortex, relaying information from the…
The thalamus is the major input route to the cerebral cortex, relaying information from the medulla, cerebellum and brain stem, and from other areas of the hemisphere.
what are the ventral nuclei of the thalamus concerned with?
The ventral nuclei of the thalamus are concerned particularly with somatosensory relay and motor co-ordination
what is the internal capsule?
The internal capsule is a massive tract of white matter linking cortex and thalamus.
It also contains descending fibres and fibres running between the nuclei of the thalamus and between different cerebral cortical regions.
summarise the ascending pathways for Fine touch and proprioception
- Primary sensory axons enter the spinal cord in the dorsal roots
- ascend the spinal cord in the dorsal columns to the dorsal column nuclei in the lower medulla.
- There is no synapse at the spinal level
- Above the mid-thoracic level each dorsal column consists of a medial gracile fasciculus containing fibres from the lower body and a lateral cuneate fasciculus, containing fibres from the upper body
- Second order neurons arising in the gracile and cuneate nuclei send their heavily myelinated axons as a ribbon-like bundle (the medial lemniscus) across the mid-line at the sensory decussation and through the brainstem to the thalamus
- to synapse on third order neurons that project to the primary somatosensory cortex via the internal capsule
describe the ascending pathway for peripheral nociceptors & temperature receptors
- enter the spinal cord in the dorsal roots,
- do not ascend to the brain
- terminate in the dorsal horn of the spinal cord
- primary afferents bifurcate into short ascending and descending branches that run for about a spinal segment in Lissauer’s tract and give rise to branches that enter the spinal cord dorsal horns
- 1st synapse in dorsal horn same side
- 2nd order neurones arise on same side
- cross the midline in the ventral (anterior) commissure immediately below the central canal
- ascend contralaterally in the anterolateral white matter – note that this is not a distinct tract
- terminate in thalamus
- third order neurons ascend via the internal capsule to sensory processing areas of the cerebral cortex.
as well as the cerebral cortex - where do many of the anterolateral fibres terminate?
brainstem - especially in the reticular formation, where they influence the level of arousal through actions on the sympathetic system (medulla) and other ascending systems.
is the anterolateral system subject to modulation?
from where?
This system is subject to considerable modulation by descending pathways from “higher centres”, such as the raphe nuclei embedded in the medullary reticular formation (not shown), and the periaqueductal grey of the midbrain
Ascending pathway for fine touch and proprioception
fine touch and proprioception
Ascending pathway for pain and temperature
pain and temperature
is pain associated with a specific area of the brain?
there are multiple regions of cerebral cortex activated by noxious stimulation
which part of the brain contributes to the localisation of pain
describe the role of the insula in nociceptive pathways?
One is called the insula, the buried cortex that lies deep in the lateral fissure, and which can be seen on coronal sections of the brain. The insula seems to be a major relay for processing noxious information.
(1 of 2 cortical areas buried deep below the cortical surface - recieved projections from the thalamus)
describe how the cingulate gyrus is involved in the nociceptive pathway
the anterior part of the cingulate gyrus, which lies on the medial part of the frontal lobe, inside the longitudinal fissure, and can be seen on a brain hemisphere. Functional brain imaging suggests that this region is particularly important for the “affective” aspects of pain sensation.
Insula and anterior cingulate cortex
Somatic sensation from the head and neck are primarily mediated by the….
Somatic sensation from the head and neck are primarily mediated by the trigeminal nerve
somatic sensation for head and neck: the tactile and nociceptive systems are anatomically together or separate?
separate - like for body
describe the path of sensory pathways for pain and temperature in the brain for the head and neck
- The trigeminal nerve enters the brainstem in the pons
- Incoming primary afferent fibres for pain and temperature from the face turn caudally and descend through the brainstem forming a fibre bundle, the spinal tract of V, which is prominent in the low medulla (and extends into the upper cervical spinal cord)
- fibres synapse in the adjacent spinal nucleus of V located in the caudal medulla (pale in colour)
- Second-order afferents from this nucleus cross the midline and ascend to the thalamus (trigeminothalamic fibres) with the anterolateral system fibres from the spinal cord.
how is the tactile fibres path from the head and neck different from the pain and temperature
- incoming tactile fibres terminate in the chief sensory nucleus of V, which lies in the pons, at the same level as the incoming trigeminal nerve fibres
- From here fibres cross to join the medial lemniscus and ascend to the thalamus
Schematic diagram of Trigeminal sensory pathways
describe how other ascending pathways are involved in ‘Unconscious’ proprioception
there are many other ascending pathways carrying information from sensory receptors
largest of these project to the cerebellum
there is a particularly heavy projection of proprioceptors (sensory fibres from muscle and joints)
what provides the pathway from lower limb proprioceptors to the cerebellum
Clarke’s column
describe Clarke’s column
the characteristic nucleus in the dorsal horn of the spinal cord in the thoracic segments, provides the pathway from lower limb proprioceptors to the cerebellum.
how do proproceptors from the upper limb get to the cerebellum?
Proprioceptors from the upper limb pass via part of the cuneate nucleus to the cerebellum (unconscious) and thalamus/ cortex (conscious).
what is Brown – Sequard syndrome
after spinal cord hemisection tactile
sensation from below the lesion site is lost on the same side (the dorsal columns are uncrossed),
whereas nociception and thermal sensation will be normal on the side ipsilateral to the lesion.
On the contra-lateral side, nociception and thermal sensation will be lost below the lesion,
but tactile sensation will be normal.
what is Tabes dorsalis
Tabes dorsalis is seen in late stage cases of syphilis, during which the nervous system becomes involved, and means, literally, ‘dorsal wasting’. The disease causes a degeneration of central projections from the dorsal root ganglia, especially in the fasciculus gracilis and fasciculus cuneatus.
(loss of procrioception)
what are Spinal dural arteriovenous fistulae
relatively rare vascular malformations which are a consequence of an abnormal connection between a meningeal branch of a segmental artery (which normally constitutes the vascular supply of the spinal dura) and an intradural radiculomedullary vein (which normally constitutes segmental venous drainage of the spine)
The lesion causes high pressure blood to flow retrograde into the coronal venous plexus of the spinal cord causing venous congestion. This causes oedema and injury to the spinal cord. Spinal dural arteriovenous fistulae typically present with sensory disturbances due to their dorsal location but if untreated they progress to cause motor weakness. Treatment involves dividing the abnormal connection inside the dura. Typically this connection is located at the dural sleeve covering the nerve root where an abnormal vein with arterialised blood can be found.
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