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Flashcards in Ascending pathways Deck (27):

Which ascending pathway is more susceptible to modulation



What does anterolateral system convey

Nociception, temperature and crude tough


Describe the first order axons for anterolateral system

A delta fibres synapse in lamina 1 and 5
C fibres synapse in lamina 2
First order ascend/descend coupel segments in tract of Lissauer before synapsing onto interneurone/secondary fibre in dorsal horn laminae


What do second order neurones do in anterolateral system

Decussate at same spinal segment (forming anterior white commissure)


What lamina does spinoreticular tract arise from

Medial lamina 6-8


WHat lamina does spinomesencephalic tract arise from

Lamina 1-5


What lamina does spinothalamic tract arise from

New system: lamina 1
Old system 5-7


Compare ALS and dorsal system in terms of thalamic/cortical connections

Thalamic/cortical connections of anterolateral system cover wider, more diffuse territory than dorsal column


Describe old system of spinothalamic tract

C and A delta fibres with large receptive fields influence WDR neurones in nucleus proprius.
WDR project to central lateral thalamus (receives mainly nociceptive inputs) --> --> diffuse emotional areas of cortex e.g. anterior insula and cigulate


Describe new system of anterolateral tract

A delta fibres with small fields terminate on nociceptive specific lamina 1 --> VPL nucleus of thalamus --> Primary somatosensory cortex --> localisation of pain


Where does spinoreticular tract terminate

Reticular formation of pons + medulla (e.g. raphe nucleus -5HT) --> higher centres and alert cerebral cortex
Central lateral nucleus of thalamus --> S1
Also influences descending reticulospinal tract, --> increase stretch reflex activity and flexor withdrawal activity for posture and gait


What does spinomesenphalic tract project to

PAG --> pain contral area
Mesencephalic reticular formation --> provides input to limbic system via hypothalamus --> emotion
Superior colliculus --> unconscious orientation in response to tactile, visual and auditory stimuli --> influences tectospinal descending system


Describe course of A alpha and A delta fibres entering dorsal horn

Give off collateral branch which ascends in ipsilateral dorsal column compromising gracile fascicle and cuneate fascicle


What lamina do A beta fibres innervate

Lamina 3-5


What happens at dorsal column nucleus

Axons of dorsal column terminate, synapse onto internal arcuate fibres which decussate and ascend contralaterally in medial lemniscus


What happens to medial lemniscus fibres

Synapse in VPL nucleus of thalamus onto tertiary fibres continuing to primary/secondary somatosensory cortex for localising touch via posterior limb of internal capsule


At what level does lateral inhibition act

On dorsal column nuclei upwards


Describe how lateral inhibition works in dorsal column nuclei

Inhibitory interneurons project laterally from primary afferent to neighbouring secondary internal arcuate fibres --> if primary afferent has high firing rate --> excites its secondary neurone and inhibits neighbouring secondary neurone


Why is lateral inhibition important

Amplifies difference in activity in neighbouring neurones --> contrast enhancement improves localisation


Instead of lateral inhibition, what happens in anterolateral column

Transmission is broadened and amplified by convergent stimuli --> stimulus converges onto many targets of Tract of Lissauer and interneurons fibres more to amplify signals --> i.e. anterolateral system not interested in localising source of harm but just amplifying nociception


Explain how lateral inhibition maintains good two point discrimination

At each synaptic relay, several presynaptic neurones converge on one post synaptic neurone --> receptive field of neurone grows. But with lateral inhibition, keeps central excitatory zone of neurone's receptive field small


Describe course of primary afferent in trigeminal pathway for innervating face/head

Axons travel to ipsilateral trigeminal nucleus (A beta --> mechanoreceptors to principal nucleus, A delta send axons to spinal nucelus for pain/temp, proprioceptive input via A alpha --> mesencephalic nucleus then to motor nucleus trigeminal) --> synapse onto second order neurones


Describe second order neurones in trigeminal pathway

Decussate and ascend in trigeminal lemniscus to ventral posterior medial nucleus of contralateral thalamus --> synapse onto tertiary fibres continuing to cerebral cortex


Summaries inputs into VP medial and lateral

VP medial receives input from face
VP lateral receives inputs from rest of body


How would you characterise cortical columns

Penetrate cortical layers, columns contain neurones with same receptive field and columns alternate between rapidly and slowly adapting receptors


How are cellular responses of neurones in each column input specific

Layer 4 cells --> target of thalamic input
Layer 5 --> projects to brainstem and spinal cord
Layer 6 --> project to thalamus --> modulates information coming in


What carries output of somatosensory cortex

Corticospinal tract --> provides descending control of ascending sensory system and motor control