General Sensation Flashcards
What are the 2 different sensory pathways for the body?
What modalities do they carry?
Dorsal column pathway:
- carries information related to:
- fine touch
- pressure
- proprioception
- vibration
- it is involved in the enhancement of discrimination
Spinothalamic pathways:
- carries information related to:
- crude touch
- pressure
- pain
- temperature
- the main STT is the neospinothalamic tract, but there are others (spinoreticular, spinomesencephalic, spinotectal & spinohypothalamic)
How can spinal white matter be organised?
- spinal white matter is arranged into three longitudinally aligned funiculi
- dorsal funiculus
- lateral funiculus
- ventral funiculus
- within the funiculi, there are several fasciculi
- fasciculi are bundles of fibres that have the same origin, course and termination
What fasciculi are found within the dorsal funiculus?
Where are these present and how are they positioned?
- dorsal funiculus contains the dorsal columns
fasciculus gracilis:
- present for the entire length of the spinal cord
- located more medially
fasciculus cuneatus:
- this is only present above T6
- located more laterally
- as they are present on both sides of the cord, there are 4 dorsal columns above T6 and 2 below T6
What structures are found within the lateral and ventral funiculi?
ventral funiculus:
- anterior spinothalamic tract
lateral funiculus:
- spinocerebellar tracts
- lateral spinothalamic tract (pain & temperature)
- some motor tracts (including lateral corticospinal tract)
How is spinal cord grey matter organised?
Rexed’s laminae
- it is organised into 10 zones
- each zone contains different types of cells which have different specific functions
What nuclei are found in Rexed’s laminae I, II, III & IV?
I - Marginal zone:
- present at all levels of the spinal cord
- contains spinothalamic tract cells that are involved in mediating pain, temperature and touch sensation
II - Substantia gelatinosa:
- present at all levels of the spinal cord
- contains neurones involved in pain and temperature transmission
III, IV - Nucleus proprius:
- present at all levels of the spinal cord
- involved in general sensation
- receives inputs from the substantia gelatinosa and contributes to spinothalamic tracts mediating pain, temperature and touch sensation
What are the 3 nuclei found at Rexed’s lamina VII?
Intermediolateral nucleus:
- contains preganglionic sympathetic neurones between T1 - L3
- contains preganglionic parasympathetic neurones between S2 - S4
Clarke’s nucleus:
- contains dorsal spinocerebellar tract cells between C8 - L2
What are the 3 nuclei found at Rexed’s lamina IX?
- IX is located in the ventral horn so contains motor neurones
Motor nuclei:
- found at all levels of the spinal cord and contains motor neurones
Phrenic nucleus:
- found at levels C3 - C5 and contains motor neurones that supply the diaphragm
Accessory nucleus:
- found from the medulla - C5 and contains motor neurones that supply sternocleidomastoid and trapezius
What is the tract of Lissauer?
- it is an area of white matter that all axons entering the dorsal horn must pass through
- axons carrying sensory information MUST pass through Lissauer’s tract
What 3 things can sensory neurones do after they have passed through Lissauer’s tract?
- enter the dorsal horn and synapse at a specific lamina
- jump out of Lissauer’s tract to enter the dorsal columns
- travel up / down the spinal cord by a few levels to then synapse at a specific lamina
How can sensation be divided into 2 categories?
- sensation can be either conscious or unconscious
- if it is conscious, we are perceiving it and are aware of the sensation so this needs to be processed at the level of the cortex
How can conscious sensation be further categorised?
Exteroception:
- this describes things that we perceive that have come from the external environment
- e.g. seeing, feeling or hearing something
Proprioception:
- this describes how we perceive the position of our limbs with respect to space
- position sense describes proprioception whilst we are static
- kinesthetic sense describes proprioception whilst we are moving
How can unconscious sensation be further categorised?
enteroception:
- describes the sensation of the viscera / internal organs that we are not aware of
proprioception:
- describes the position of limbs and muscles with respect to space that we are not aware of
- e.g. the level of contraction of a muscle at any given time
What type of information is carried by the dorsal column medial lemniscus pathway?
- this pathway carries information relating to:
- discriminative touch
- vibration
- conscious proprioception
- information from the lower limbs (below T6) is carried within the fasciculus gracilis
- information from the upper limbs (above T6) is carried within the fasciculus cuneatus
What is the difference in position and location of fasiculus gracilis and fasciculus cuneatus?
Fasciculus gracilis:
- carries information from the lower body
- located medially within the spinal cord
- exists for the entire length of the spinal cord, with inputs from sacral, lumbar and thoracic segments up to T6
Fasciculus cuneatus:
- carries information from the upper body
- located laterally within the spinal cord
- exists only above the level of T6
Describe the 3 neurones involved in the dorsal column medial lemniscus pathway
First order neurone:
- originates from sensory receptor in the periphery and has its cell body within the dorsal root ganglion (DRG)
- enters the spinal cord via Lissauer’s tract
- these neurones do not synapse at Lissauer’s tract - they jump out to enter the dorsal columns
- neurones from the lower limb (below T6) enter the gracile fasciculus and those from the upper limb (above T6) enter the cuneate fasciculus
- they ascend on the IPSILATERAL side to reach the caudal medulla
Second order neurone:
- first order neurone synapses with second order neurone within the cuneate / gracile nucleus
- second order neurone immediately crosses the midline to enter the medial lemniscus
- the fibres crossing the midline are the internal arcuate fibres that travel ventromedially
- the medial lemniscus ascends on the CONTRALATERAL side to reach the VPL nucleus of the thalamus
Third order neurone:
- travels from the VPL nucleus to the primary somatosensory cortex
- it ascends via the thalamocortical tract and travels in the posterior limb of the internal capsule
Somatotopic organisation is present throughout this pathway
How does the medial lemniscus change in orientation as it ascends?
Level of medulla:
- it is positioned vertically with arms represented most superiorly and legs represented most inferiorly
- it rotates in a clockwise direction as it ascends
Level of pons:
- it is positioned horizontally with legs being represented most laterally and arms most medially
Level of midbrain:
- it continues to rotate so that legs are represented most superiorly and arms more inferiorly
What are the gracile and cuneate tubercles and where are they located?
- they are bumps on the surface of the caudal (closed) medulla that overlie the gracile and cuneate nuclei
Gracile tubercle:
- located next to the midline in the caudal medulla
- underlying the gracile tubercle is the gracile nucleus, which contains cell bodies
Cuneate tubercle:
- located more laterally than the gracile tubercle
- underlying the cuneate tubercle is the cuneate nucleus, which contains cell bodies
What is the purpose of enhancement of discrimination in the gracile and cuneate nuclei?
- this works to modify the message by increasing the sensitivity of the cortex to small differences in texture and shape
What are the 3 types of neurone involved in enhancement of discrimination?
By which 2 mechanisms can this occur?
- first order neurones are excitatory ascending neurones arriving at the relay centre (gracile / cuneate nucleus)
- local interneurones are typically inhibitory
- second order neurones are travelling from the relay centre to the VPL of the thalamus
- enhancement of discrimination can occur via feedforward inhibition or feedback inhibition
Why does enhancement of discrimination need to occur?
- if you stab a pen into the skin, the area directly underneath the pen is being compressed more than the periphery
- sensory pathways travel from sensory receptors in the very centre of the skin to the gracile / cuneate nucleus via the dorsal column pathway
- there are also sensory pathways travelling from sensory receptors in the periphery of the skin
- the neurone in the centre has been stimulated more strongly** than those in the periphery, so it carries a **stronger signal
Describe the feedforward mechanism for enhancement of discrimination
- the neurone travelling from the centre of the skin sends collateral branches to stimulate local interneurones at the level of the nuclei
- local interneurones are typically inhibitory
- inhibitory interneurones dampen down the signal coming from the periphery (and travelling to the VPL nucleus)
- by dampening down the signals coming from the periphery at the level of the nuclei this allows the signal coming from the centre to be perceived as stronger
What is involved in the feedback mechanism for enhancement of discrimination?
- the second order neurones that have been stimulated more strongly can also send collateral branches to stimulate local interneurones at the level of the nuclei
- the inhibitory interneurones dampen down the signals of the first order neurones travelling to the nuclei from the periphery
What are the 2 different types of synapse?
Converging synapse:
- involves multiple first order neurones stimulating a single second order neurone
Diverging synapse:
- involves a single neurone stimulating multiple neurones further along the chain
What are the 2 different spinothalamic tracts and where are they found?
- lateral spinothalamic tract is found in the lateral funiculus
- anterior spinothalamic tract is found in the ventral funiculus
What must the first order neurone of the STT travel through initially?
Where can it synapse with the second order neurone and what is specific about pain & temperature information?
- first order neurones enter via and briefly travel within Lissauer’s tract
- the first order neurone can synapse with the second order neurone at either:
- the marginal zone (lamina I)
- or the nucleus proprius (laminae III & IV)
- pain & temperature information needs to travel through the substantia gelatinosa (lamina II) before it synapses with a second order neurone in lamina I, III or IV
What is the function of laminae I, II, III and IV?
- the marginal zone (I) contains spinothalamic tract cells
- the substantia gelatinosa (II) is involved in dispersing and relaying pain and temperature information
- the nucleus proprius (III, IV) contains spinothalamic tract cells
What is the difference in information carried by the anterior and lateral STTs?
Is somatotopic organisation present?
Anterior STT:
- carries information relating to crude touch (and some pressure)
Lateral STT:
- carries information relating to pain and temperature
- somatotopic organisation is present such that the cervical regions of the cord project more medially, followed by thoracic, then lumbar and sacral regions more dorsolaterally
How do sensory fibres carrying crude touch information travel when they enter the spinal cord?
- fibres carrying crude touch information have their cell bodies within the dorsal root ganglion (DRG)
- they enter the dorsal horn and split in a T-shape pattern
- the fibres will either ascend or descend for a few spinal cord segments before synapsing with a second order neurone
- the second order neurone will cross the midline via the ventral commissure and ascend in the anterior spinothalamic tract
How do sensory fibres carrying pain & temperature information travel once they have entered the spinal cord?
- sensory fibres carrying pain & temperature information have their cell bodies within the dorsal root ganglion (DRG)
- they enter the dorsal horn and synapse with the second order neurone IMMEDIATELY at the exact level of entry
- the second order neurone crosses the midline via the ventral commissure to enter the contralateral lateral spinothalamic tract
Where do pain and temperature fibres pass after entering Lissaeur’s tract and what happens here?
- after entering Lissaeur’s tract, P&T afferents pass to the substantia gelatinosa
- they will synapse with the second order neurone from laminae I, III or IV within the substantia gelatinosa
- second order neurones from laminae I, III and IV extend their dendrites into the substantia gelatinosa, allowing for synapsing to occur here
- the second order neurone then immediately crosses the midline to enter the lateral STT
- there are also local interneurones within the substantia gelatinosa that allow for modification of activity of P&T afferents
Describe the 3 neurones involved in the neospinothalamic pathway
First order neurone:
- travels from a peripheral sensory receptor and has its cell body within the DRG
- passes through Lissaeur’s tract as it enters the dorsal root of the spinal cord
- if it is a P&T afferent, it will enter the substantia gelatinosa at that spinal cord level and synapse with a second order neurone from laminae I, III or IV
- if it is a crude touch afferent, it will split into a T-shape and ascend/descend a few levels before synapsing with the second order neurone in laminae I-IV
Second order neurone:
- crosses the midline via the ventral white commissure
- P&T afferents enter the lateral STT and crude touch afferents enter the anterior STT, which both ascend to the VPL nucleus of the thalamus
Third order neurone:
- passes from the VPL nucleus of the thalamus to the somatosensory cortex via the posterior limb of the internal capsule
How is the neospinothalamic tract somatotopically organised?
- the legs are represented dorsolaterally
- the arms are represented ventromedially
- the trunk is located between them
What are the roles of the indirect spinothalamic tracts in the tranmission of pain?
How do they ascend>
- indirect pathways are involved in inhibiting ascending pain sensation
- they also have roles in the autonomic, endocrine and arousal components of pain, temperature & simple touch
- they ascend bilaterally with poor somatotopic organisation
- they synapse with diffuse areas of cortex, reticular formation, hypothalamus and the limbic system
What is the role of the spinoreticular pathway and how does it travel?
- some fibres leave the spinothalamic tract at the level of the reticular formation
- they travel via the spinoreticular tract to synapse within the medullary or pontine reticular formation
- from the reticular formation, fibres travel to the thalamus via the reticulothalamic tract
- fibres travelling from the thalamus to the cortex allow us to become more alert in response to pain
- the reticular formation has roles in arousal / keeping us awake
- fibres are also sent to the limbic system, which allows us to deal with the emotional aspects of pain
What is involved in the spinomesencephalic pathway and what is its role?
- some fibres will stop at the level of the midbrain and synapse within the periaqueductal gray (PAG)
- at the level of the midbrain there are structures that can send descending fibres to the level of the original synapse
- the descending fibres are able to modify and dampen down the pain signals
What is the reticular formation and where is it found?
What are some roles of the reticular formation?
- it is a loosely defined network of diffuse neurones that spans the entire length of the brainstem (within tegmentum)
Roles of the RF:
- contains vital autonomic centres (including respiratory and cardiovascular)
- has roles in maintaining consciousness
- has roles in pain transmission and modulation
- sleep wake cycle
- arousal
What is the role of descending pain fibres?
What are the 2 sources of descending pain fibres?
they are able to modulate pain by influencing ascending pain fibres
- serotonergic neurones from the periaqueductal gray / nucleus raphe magnus
- noradrenergic neurones from the locus ceruleus
- both of these pathways can be further stimulated by higher centres such as the hypothalamus, amygdala and cortex
Describe the pathway and action of descending pain fibres from the periaqueductal gray
- some fibres of the pain pathway travel to the midbrain via the spinomesencephalic tract and synapse within the periaqueductal gray
- from the periaqueductal gray, there are descending serotonergic neurones that travel to the nucleus raphe magnus of the medulla
- from the nucleus raphe magnus, there are descending projections to the level of the original synapse
- these fibres are capable of modulating and dampening down the pain input at the level of the original synapse
How is the locus ceruleus involved in modulating pain transmission?
- the locus ceruleus is a defined network of neurones within the pons
- it gives rise to descending noradrenergic neurones that travel to the level of the original synapse
- they are capable of modulating and dampening down the pain response at the level of the original synapse
How do the descending pain pathways actually work to modify pain transmission at the level of the original synapse?
- primary afferent pain fibre reaches the substantia gelatinosa to synapse with the second order neurone
- at the level of this synapse, inhibitory local interneurones are present
- descending serotonergic neurones from the periaqueductal gray stimulate serotonergic neurones from the nucleus raphe magnus
- both descending serotonergic neurones from the nucleus raphe magnus and noradrenergic neurones from the locus ceruleus can activate the inhibitory interneurones
- the activity of the second order neurone in the pain pathway is decreased due to the effects of inhibitory interneurones
What are the differences in the ways that the dorsal column pathway and spinothalamic tract travel in the spinal cord?
Dorsal column:
- information ascends IPSILATERALLY within the spinal cord via the fasciculus gracilis / cuneatus
- 1o neurone synapses with 2o neurone within the gracile / cuneate nucleus at the level of the closed (caudal) medulla
- 2o neurone crosses the midline at the level of the caudal medulla to enter the medial lemniscus
Spinothalamic tract:
- information synapses at the level of the spinal cord and 2o neurone immediately crosses the midline via the ventral white commissure
- information ascends to the thalamus on the CONTRALATERAL side of the spinal cord
