11-09-23 – Ascending Pathways Flashcards
Learning outcomes
- Understand how pathways connect from nerve, through spinal cord, thalamus and cortex.
- Be able to determine where a lesion might be, and conditions that may lead to clinical scenarios
- Recall the major ascending sensory pathways, particularly anterolateral, dorsal columns, and spinocerebellar.
What do nerve receptors for pain consist of?
What are the names of the 8 different receptors?
What are their modalities (in picture)?
- Nerve receptors for pain are just bare nerve endings and can be triggered by a variety of stimuli (both chemical and physical)
- 8 different receptors and their modalities (in picture)
Receptor field overlap and the concept of lateral inhibition.
What is the receptive field of each neuron?
How do neuronal dendrites differ from the centre and the periphery of a neuron?
How can different stimuli affect speed of neuronal firing?
- Receptor field overlap and the concept of lateral inhibition
- Each neuron has a receptive field around it made from dendrites
- Dendrites are dense at the centre and diffuse at the periphery
- Stimuli at the centre of the field activate more dendrites than at the periphery and so cause faster firing
Describe the neuronal firing when stimuli land on overlapping receptive fields from multiple neurons (in picture)
What is lateral inhibition amongst neurons?
What is the purpose of lateral inhibition?
What does lateral inhibition rely on?
- Lateral inhibition amongst neurons is when neighbouring neurons have an excitatory neuron that stimulates an inhibitory neuron which inhibits the neighbouring neuron
- Lateral inhibition sharpens discrimination between two points
- Lateral inhibition relies on reciprocal inhibition between two adjacent neurons where the extent of inhibition from each one is linked to the stimulus point on the neuronal receptive field overlap
How does lateral inhibition translate to sensation?
- With lateral inhibition, we end up with a much sharper peak that the brain can differentiate
What structures do different receptors possess?
What does this affect? Why is this?
- Different receptors possess specific axon types
- This affects the speed of transmission of the information to the CNS, meaning Some modalities therefore are slower at being transmitted to the brain than others
- Different axon types have different sizes and degrees of myelination, with larger axon size allowing charge to pass faster
Describe the 4 different types of axons (in picture):
* Axons from skin
* Axons from muscles
* Diameter
* Speed
* Sensory receptors
Somatic sensory input.
Describe how sensory input enters the spinal cord.
- Somatic sensory input
- Sensory information enters the dorsal root ganglion (DRG - (1st neuron cell body)
- Sensory information then enters the dorsal root (sensory - which contains 1st neuron of the sensory path that comes from a dermatome) and enters into the spinal cord
How is the white mater in the spinal cord divided up?
Describe the 6 ascending tracts of the spinal cord (from posterior to anterior)
- 6 ascending tracts of the spinal cord (from posterior to anterior):
1) Fasciculus gracilis (vertical pathway)
* Fasciculus gracilis carries information from the lower body extremities (feet and legs)
* Think G (gracilis) for ground
2) Fasciculus cuneatus (vertical pathway)
* Fasciculus cuneatus carries information from the upper body extremities above T5-T6
* Responsible for conscious proprioception e.g moving fingers and knowing where they are in space and time
* Proprioception is not only delt with by the cerebellum
3) Dorsal spinocerebellar tract (for muscle spindles)
* Dorsal and ventral spinocerebellar tracts carry proprioceptive information from muscle spindles (Dorsal) & Golgi organs (ventral)
4) Ventral spinocerebellar tract (for Golgi organs)
5) Lateral Spinothalamic
* Pain and temperature transmission etc.
6) Anterior Spinothalamic pathway (anterior pain pathway)
* The lateral spinothalamic and anterior spinothalamic pathways link together to from anterolateral pain pathway
Arrangement of the primary somatosensory cortex – Describe the topographical arrangement of the body across the surface of the sensory cortex (coronal view – in picture)
- Try to remember this pattern
General Somatosensory system layout.
What follows this pattern?
Where does this somatic sensory information end up?
Describe the 3 stages of the General Somatosensory system layout
- General Somatosensory system layout
- Most conscious sensory tracts follow the general Somatosensory system layout
- This somatic sensory information ends up at the somatosensory cortex
- 3 stages of the General Somatosensory system layout:
1) Primary afferent is a first order neuron and terminates in spinal cord or brain stem
2) The second order neuron projects to the thalamus, with the second and third order neuron synapsing in the thalamus
3) The third order neuron projects to the cortex
Where does all sensory information pass through en route to the cortex?
What is the exception to this?
What is the thalamus connected to?
- En route to the cortex, all sensory information (except olfaction) passes through the thalamus
- The thalamus is connected to all parts of the cortex, and serves as a massive gateway
What does the thalamus have connections to?
What does this allow it to do?
What do thalamic connections form?
Describe the basic thalamic circuitry in the from of the dorsal column medial lemniscus (DCML) pathway.
What are 3 ways the DCML can be inhibited due to overstimulation?
- The thalamus has reciprocal connections to all cortical regions, allowing it to relay information, receive feedback and modulate cortical activity
- Basic thalamic circuitry in the from of the dorsal column medial lemniscus (DCML) pathway:
- The DCML pathway brings joint position sense (JPS) into the thalamus
- The 2nd order neuron synapses onto the 3rd order neuron (thalamocortical neuron) in the thalamus
- This tertiary portion of the sensory pathway stimulates part of the cortex
- 3 ways the DCML can be inhibited due to overstimulation:
1) Stimulation of inhibitory corticothalamic fibre
2) The thalamocortical neuron has side chains that go into the reticular nucleus, which can then excite an inhibitory neuron
3) Collateral branches that can excite inhibitory neurons in the lobe of the thalamus
Thalamus
What are the 3 principal ascending tract systems in the spinal cord?
- 3 principal ascending tract systems in the spinal cord:
1) Dorsal column-medial lemniscus pathways
2) Spinothalamic pathways
3) Spinocerebellar pathways
Describe the 5 different pathways for ascending and descending tract systems and their modalities (in picture).
What pathways do the three principal ascending tract systems in the spinal cord use?
Why do we not use light touch as a modality to differentiate between pathways?
- 5 different pathways for ascending and descending tract systems and their modalities (in picture).
- We do not use light touch as a modality to differentiate between pathways because there are 2 different pathways light touch can take
Dorsal column - medial lemniscal (DCML) pathway - For Mechanical Stimuli.
What are 4 functions of the DCML pathway?
Describe the 6 steps in the route for the DCML pathway
- Dorsal column - medial lemniscal (DCML) pathway - For Mechanical Stimuli.
- 4 functions of the DCML pathway:
1) Conscious proprioception,
2) Light touch,
3) Vibration
4) Pressure - 6 steps in the Route for the DCML pathway:
1) Primary neuron comes in on the dorsal root of the dorsal horn, with an AB axon
2) The primary neuron (1st order) remains on the ipsilateral side (same side) and will travel into the brainstem where it will synapse with the body of the 2nd order neuron
3) The 2nd order neuron travels up the column at the back of the spinal cord, known as the Gracile or Cuneate dorsal column (gracile for lower extremities, cuneate for upper extremities – T5 and upwards
4) The 2nd order neuron (medial lemniscus) crosses to the contralateral side and ascends towards the thalamus
5) The 2nd order neuron then synapses in the thalamus
6) The 3rd order neuron goes to the cerebral cortex
How can we find a lesion blocking the Dorsal column - medial lemniscal (DCML) pathway?
Why is it difficult to make a judgement if a lesion is blocking the DCML pathway if it is high up in the pathway?
- Lesions in the DCML pathway blocks ascending JPS and mechanical stimuli originating ipsilaterally from below the lesion
- It is difficult to make a judgement if a lesion is blocking the DCML pathway if it is high up in the pathway because we can’t tell if it is ipsilateral to the side with the lost JPS, as it will be near the crossover at the medial lemniscus
What information is used for DCML pathway cortical functions?
What are 2 cortical functions of the DCML pathway?
What area of the cortex is required for the DCML pathway?
What is stereognosis and astereognosis?
- For DCML functions, fine tactile and proprioceptive information is used
- 2 cortical functions of the DCML pathway
1) To determine the shape of an object (without sight)
* Proprioception from the finger joints and fine discriminating touch provides a very accurate model of any object that is handled
* E.g handling an object behind your back
2) To determine the texture of an object (without sight)
* Texture is determined by vibration and slip receptors as well as fine discriminating touch.
* Note pleasure can be associated with this pathway
- The DCML pathway requires the somatosensory association area in the parietal lobe is functioning normally
- Stereognosis is when this pathway is working properly
- Astereognosis is when this pathway is not working
Lesions of the DCML pathway. What can lesions of the gracile fasciculus lead to?
What can lesions in the cervical cord lead to?
How do patients compensate for this? What is ataxia?
What is a classic sign of gait (or sensory) ataxia?
- Lesions of the DCML pathway.
- Lesions of the gracile fasciculus (receives input from lower extremities) can cause gait problems, as the brain (cortex and cerebellum) is deprived of information about the position of the feet.
- Lesions in the cervical cord also cause upper extremity movement problems as this would include cuneate pathways.
- Often patient is able to compensate with Vision and thus minimize the effects of sensory loss.
- Ataxia is a term for a group of disorders that affect co-ordination, balance and speech
- A classic sign of gait (or sensory) ataxia is the stamp and stick gait.
- Patient stamps down feet to enhance sensory input and maintains a broad-based stance.
What are the spinothalamic pathways composed of?
What 3 modalities do Aδ fibres detect?
What 4 modalities do C fibres detect?
Describe the route of the spinothalamic pathways
- The spinothalamic pathways are composed of smaller diameter (slow either Aδ or C fibres) fibres with no specialized sensory endings
- They are lightly or unmyelinated fibers which are ‘Quick to cross’
- 3 Modalities Aδ fibres detect:
1) Discriminative sensation (strong stimuli that does not damage the skin)
2) Heat or cold
3) Sharp pain - 4 modalities do C fibres (arrive later at the cortex) detect:
1) Dull aching pain
2) Itch (histamine sensitive)
3) Thermal
4) Mechanical - Route of the spinothalamic pathways:
- 1st order neurons synapse with 2nd order neurons in the dorsal horn, which then decussate (cross-over) and ascend in either the lateral or anterior spinothalamic tract depending on the sensory modality
- At the thalamus, the 2nd order neurons synapse with 3rd order neurons, which then projects to the sensory cortex in the same way as the DCML
What are all spinothalamic pathways together referred to as?
How many spinothalamic pathways are there?
What are the 2 different types of spinothalamic pathways?
What fibres do they consist of?
What are their sensory modalities?
- All the spinothalamic pathway routes together are referred to as the anterolateral (pain) pathway
- There are four main spinothalamic pathways, each having a specific function
- There are 2 lateral spinothalamic and 2 anterior spinothalamic pathways, all routes however follow the same basic pattern of connectivity:
1) Spinothalamic lateral route
* Mixture of 2 pathways, one for mainly Aδ fibres (destination is somatosensory cortex) and the other for C fibres which feeds into the emotional and memory processes.
* They carry information about pain and temperature via Aδ heat or cold activated fibres and C fibres (dull pain and itch)
* This route links an emotional aspect to pain and memory
2) Spinothalamic anterior or ventral route
* Only C fibres carrying information about coarse, non-discriminating touch via mechanosensitive receptors, destination brainstem areas
What will lesions in the lateral spinothalamic tract block?
- Lesion in the lateral spinothalamic tract blocks ascending pain and temp originating contralaterally from below the lesion (couple of dermatomes below lesion)
Anterolateral spinothalamic system. Where does the Neospinothalamic tract (lateral pathway) terminate?
What is it composed of?
Where does the Palaeospinothalamic tract (anterior pathway) terminate? What is it composed of?
How do these pathways differ in terms of generalised and localised sensation?
What is the limbic system association cortices associated with?
Describe a diagram of the anterolateral system (in picture).
- Anterolateral spinothalamic system (anterior and lateral spinothalamic tracts together) aka anterolateral pain pathway
- The Neospinothalamic tract goes to the ventral posterior lateral nucleus (VPL), then the primary somatosensory cortex, and is mainly composed of Aᵟ fibres.
- The Palaeospinothalamic tract goes to the dorsomedial (DM) and intra laminar nuclei, then the limbic association cortices and is composed of C fibres.
- As the VPL is somatotopic, there is locational discrimination along this pathway whereas the dorsomedial (DM) nucleus and intralaminar areas only provide a generalised location for pain.
- Limbic system association cortices are associated with emotion, pain and other association cortices, which allows us to develop a fear of pain etc
- Diagram of the anterolateral system (in picture)
How do anterolateral tract lesions affect sensation?
How can this be used as a treatment for terminal pain?
- Lesions in the neo part of the anterolateral tract cause decreased perception of pain and temperature on the contralateral side of the body, always one or two dermatomes below the level of the lesion (Lissauers tract).
- This neo pathway is lesioned using electrical current in a surgical procedure (cordotomy) for terminal disease pain, but pain returns after about 1 year.
What 3 structures does the spinocerebellar pathway carry information from?
How does the cerebellum utilise this pathway?
How does the cerebellum do this?
How many neurons are in each pathway?
Where are these neurons located?
Where do they terminate?
What do dorsal, cuneate, and anterior tracts provide information on?
How is the spinocerebellar pathway most obviously affected?
- 3 structures the spinocerebellar pathway carries information from:
1) Muscle spindles (muscle length)
2) Golgi organs (proprioception)
3) Touch receptors - This pathway is used by cerebellum for the control of posture and co-ordination of movement
- The cerebellum does this via feedback to motor cortices and refining information leaving these areas
- Each pathway (anterior or posterior) contains only two neurons along its length (1st and 2nd order).
- In both cases the cell bodies of the second order neurons are located in the dorsal horn of the spinal cord, and terminate in the vermis of the cerebellum
- Dorsal tracts provide information on proprioception (unconscious proprioception) and movement (cutaneous receptors) of upper and lower body.
- Anterior tract provides a live comparison between the intended movements (descending motor) and actual movements (ascending proprioceptive - sensory).
- This pathway is the most obviously affected one during alcohol consumption e.g slurred speech, lack of coordination etc
Describe how to test the following modalities (in picture):
* Joint and position sense
* Pain and temperature
* Light touch
* Vibration sense
What modalities will a brown sequard lesion block???
- Brown sequard lesion blocks ipsilateral JPS and vibration (DMCL pathway) and contralateral pain and temp below the lesion (anterolateral spinothalamic pathway)
