Ascending Sensory Pathways Flashcards
What does somatosensory mean?
Relating to a sensation that can occur anywhere in the body (e.g. pain), rather than localised at one organ (e.g. sight).
-I.e. senses other than 5 senses
What are the 2 groups of somaesthetic modalities?
- Modalities that are essential to survive
- Modalities that increase detail
What sort of modalities are essential to survive?
Pain, temperature, some touch & pressure.
What fibres carry modalities that are essential for survival?
Thin, unmyelinated fibres.
-relatively slow conduction
What sort of modalities increase detail?
- Discriminative touch (2-point and vibration)
- Proprioception
What fibres carry modalities that increase detail?
Large, very myelinated fibres.
-fast conduction
What is proprioception?
The body’s ability to sense movements in joints and joint positions.
What is the general mechanism of proprioception?
Golgi tendons/muscle spindles sense degree of tension/stretch in muscles»_space; CNS.
What is a pseudounipolar neuron?
Sensory neuron in PNS with a single stalk that splits into 2 processes.
(1»_space; periphery, 1»_space; spinal cord)
What is a nucleus?
Collection of cell bodies in the CNS.
What is a ganglion?
Collection of cell bodies in the PNS.
-e.g. dorsal root ganglion
How many neurons do general sensory pathways consist of?
Three.
-1/2/3*
Where do the cell bodies of sensory 1* neurons in the PNS reside?
Dorsal root ganglion.
Where do the cell bodies of sensory 1* neurons in the CNS reside?
Cranial nerve nuclei.
Where do 1* neurons synapse?
In the CNS (spinal cord/brain) on the same side as they entered the cord (ipsilateral).
Which neuron crosses the midline?
2* neuron.
Where do 2* neuron cell bodies reside?
In ipsilateral grey matter in the CNS.
What happens to 2* neurons once they cross the midline?
They ascend to the thalamus.
- VPL nucleus (from body)
- VPM nucleus (from face)
Where do 3* cell bodies reside?
In the thalamus.
Where do the 3* axons project?
3* axons project to the somatosensory area of the post-central gyrus.
-Contralateral 1* neuron origin
What does somatotopic mean?
Point-to-point correspondence.
-area of body»_space; specific point on post-central gyrus
Are general sensory pathway somatotopic?
Yes.
Is synaptic transmission from 1, 2 and 3* neurons a simple relay? Give 3 examples.
No, can be modified by other inputs.
-e.g. diverging circuit, converging circuit, descending pathways
What is a diverging circuit?
One pre-synaptic neuron stimulates a response in multiple post-synaptic neurons. Increases the amount of info to CNS, and allows brain signals to travel to various locations of the body.
What is a converging circuit?
Several pre-synaptic neurons stimulate one post-synaptic neuron. Focuses information.
What is the general pathway of sensory information to the face?
Sensory stimulus»_space; 3 neurons»_space; thalamus (VPL/VPM)»_space; post-central gyrus.
Which arteries in the medial hemisphere supplied by?
Anterior cerebral arteries.
What are the main ascending and descending fibre tracts? (4)
- Dorsal column
- Spinothalamic
- Lateral corticospinal
- Ventral corticospinal
What are the major ascending tracts? (2)
- Dorsal column pathway
- Spinothalamic pathway
What information does the spinothalamic pathway carry?
Pain, temperature, light touch and pressure.
What information does the dorsal column pathway carry?
Discriminative tough and proprioception.
plus some simple touch and pressure
Which ascending pathway synapses at the level of spinal cord entry and crosses the midline to ascend contralaterally?
Spinothalamic pathway.
Where does the 1* neuron synapse in the spinothalamic pathway?
In the dorsal grey horn (spinal cord).
What do 2* neurons of the spinaothalamic pathway cross the midline in?
Ventral white commisure.
-ventral to the central canal
How do 2* fibres ascend in the spinal cord to the thalamus in the spinothalamic pathway?
In the spinothalamic tract on the contralateral side.
-anterolateral spinal cord
Which areas of the body (1* neuron origin) travel in which areas of the spinothalamic tract?
Medial - arm.
Middle - trunk.
Lateral - leg.
What does the spinthalamic tract become at the medulla?
Spinal lateral lemniscus.
NB. what are the 3 parts of the brainstem?
- Medulla oblongata
- Pons
- Midbrain
Which part of the thalamus does information from the body in the spinothalamic tract go to?
Ventral posterolateral nucleus (VPL).
Which part of the thalamus does information from the face in the spinothalamic tract go to?
Ventral posteromedial nucleus (VPM).
Where does the 2* neuron synapse in the spinothalamic pathway?
In the thalamus.
- VPL for body
- VPM for face
Where does the 3* neuron project to?
Post central gyrus.
-1* somatosensory cortex
Where is the 1* somatosensory cortex located?
In the lateral postcentral gyrus (parietal lobe).
What is the function of the 1* somatosensory cortex?
Main sensory receptive area for touch.
Which ascending pathway ascends in the spinal cord ipsilaterally, and synapses at the level the closed medulla?
Dorsal column pathway.
What information does the dorsal column pathway carry?
Discriminative tough and proprioception.
plus some simple touch and pressure
Which part of the spinal cord do 1* neurons of the dorsal column pathway ascend in?
- Gracile fascicles
- Cuneate fascicles
Which fascicle is located medially (gracile/cuneate)?
Gracile fascicle is medial.
Which fascicle is located laterally (gracile/cuneate)?
Cuneate fascicle is lateral.
What level of the spinal cord does the gracile fascicle run in?
Runs entire length of the spinal cord.
What level of the spinal cord does the cuneatefascicle run in?
Above T6 only.
Where does the gracile fascicle take information from?
Below T6 and the legs.
Where does the cuneate fascicle take information from?
Above T6 and the arms.
NB if 2 fascicles are present on either side»_space; must be above T6
What do the gracile and cuneate fascicles become the level of the medulla?
Gracile and cuneate tubercles.
What do the gracile and cuneate tubercle contain, and what happens in them?
Contain nuclei.
-where 1* neurons synapse before crossing the midline
At what level do 1* neurons synapse in the dorsal column pathwaty?
At the level of the closed medulla.
What do 2* neurons of the dorsal column pathway cross the midline in?
In internal arcuate fibres.
What do 2* neurons ascend in after crossing the midline?
In the medial lemniscus.
-posterior to the pyramids at closed medulla
NB what is a lemniscus?
A collection of axons (ribbon/band).
Where do the 2* neurons synapse in the dorsal column pathway?
In the thalamus.
- VPL for body
- VPM for face
Where do 3* neurons project to in the dorsal column pathway?
Post central gyrus.
-1* somatosensory cortex
Which parts of the ascending pathways are somatotopically organised? (4)
- Medial lemniscus
- Lateral lemniscus
- Thalamus
- Post-central gyrus
What do fibres travel from the thalamus to the post-central gyrus in?
-dorsal column and spinothalamic pathways
The posterior limb of the internal capsule.
CASE STUDY.
Wound to lower back»_space; spinal cord lesion:
-loss of proprioception and discriminatory touch over lower right torso and leg
-analgesia and thermoanaesthesia (no pain/temp sensation) over lower left torso and leg
Where is the lesion?
Below T6 on right hand side.
- dorsal column pathway (proprioception and discriminatory touch) = ipsilateral
- spinothalamic pathway (analgesia and thermoanaesthesia) = contralateral
What kind of neuron is a primary neuron of a general sensory pathway?
Pseudounipolar.
Do all the fibres in the spinothalamic tract decussate at the level of entry to the spinal cord?
No, most do but some ascend ipsilaterally in the spinal cord for a few vertebral levels before decussating.
-carry light touch and pressure
