Ascending tracts and general sensory neuro stuff

Asfand Baig

• The ascending tracts carry sensory information from the peripheries to the primary somatosensory cortex in the post-central gyrus of the cerebral cortex

Sensory lesions and examination

• Sensory symptoms can be either:– Positive symptoms: pins & needles– Negative symptoms: numbness

• Abnormalities on examination:– Hypaesthesia: reduce pressure, light touch or temperature

sensation– Anaesthesia: absent pressure, light touch or temperature

sensation– Hypalgesia: reduced pain sensation– Hyperaesthesia: pain in response to touch– Hyperalgesia: severe pain in response to mildly noxious stimulus– Allodynia: perception of non-painful stimulus as painful

What are the general senses?

General Sense Receptor

Temp and pain Free nerve ending

Discriminative Touch Meissner corpuscle

Light Touch Merkel cells & disk

Touch Ruffini ending

Deep pressure Pacinian corpuscle

There are different types of mechanoreceptors in the skin:

• Rapidly adapting (Phasic) receptors– Turn on and off quickly unless there is a wound– Signal “dynamic repsosnes”– Most receptors are like this– E.g. hair follicle receptors in hairy skin, meisseners

corpuscles in glabrous skin, pacinian corpuscles in subcutaneous tissue

• Slowly adapting (Tonic) receptors– Signal “static” responses that give overall information– E.g. merkell cell endings, ruffini endings

The 3 ascending tracts are...

Anterior Spinothalamic

Crude touch, Pressure

Lateral Spinothalamic

Pain, Temperature

Dorsal ColumnsVibration, Fine Touch,

Proprioception

And there’s one more

• THE SPINOCEREBELLAR

Why is the spinocerebellar important?Unconscious proprioception

But then is that the same thing as the proprioception done by the dorsal columns?

How many sets of neurons are the main ascending tracts split into?

• 3

• 1st order, 2nd order and 3rd order

The spinal cordProprioception

VibrationFine Touch

Pressure

P+T

Dorsal ColumnsCross at Medulla

1st order neurone:From receptors in peripheryfasciculus gracilis and cuneatusascend to nucleui gracilis and cuneatus in medulla

2nd order neurone:From nuclei gracilis and cuneatus in medulla decussate in medulla become internal arcuate fibres ascend brainstem as the medial lemniscusVPL nucleus of thalamus

3rd order neurone:From VP nucleus in thalamusthough posterior limb of internal capsule cortex

Anterior Spinothalamic Tract

1st order neurone:From various receptors in periphery substantia gelatinosa in dorsal horn

2nd order neurone:From substantia gelatinosadecussates over several spinal segments and ascends in ant. Spinothalamic tractbecomes lateral then spinal lemniscus in brainstemventral posterolateral (VP) nucleus in thalamus

3rd order neurone:From VP nucleus in thalamusthough posterior limb of internal capsule cortex

Lateral Spinothalamic Tract

3rd order neurone:From VP nucleus in thalamusthough posterior limb of internal capsule cortex

2nd order neurone:From substantia gelatinosadecussates within 1 spinal segment and ascends in lat. Spinothalamic tractbecomes lateral then spinal lemniscus in brainstemventral posterolateral (VP) nucleus in thalamus

1st order neurone:From receptors in periphery (fast (A ) or δslow (C) fibres) substantia gelatinosa in dorsal horn

Spinocerebellar Tracts

1st order neurone:From receptors in periphery nucleus dorsalis

2nd order neurone:Dorsal TRACTRemains uncrossed ascend in post. Spinocerebellar tractinferior cerebellar peduncle in cerebellum

2nd order neurone:Ventral TRACTSome fibres decussate as soon as enter spinal cordascend in anterior spinocerebellar tract to superior cerebellar peduncle in cerebellum recross back over to side it entered inOther fibres remain uncrossedascend to sup. cerebellar peduncle in ipsilateral ant. Spinocerebellar tract

Spinocerebellar tracts. Remember these 2 diagrams?

Lower limb

Upper limb

Lower limb

Upper limb

Cuneocerebellar

dorsal ventral

rostral

Why doesn’t the spinocerebellar tract have a 3rd neuron?

• There is no 3rd order neurone as it is doesn’t reach consciousness i.e the cortex

What sensory modality does each branch of the spinocerebellar tract convey?

Therefore what receptors does each branch use?

• Dorsal spinocerebellar tract monitors muscle length, speed of contraction and tension from lower body and lower limb

Muscle spindles A few Golgi tendon organs• Cuneocerebellar for upper

• Ventral spinocerebellar tract monitors stretch of the tendon at the muscle-tendon interface of the lower limb - Golgi tendon organs

• Rostral for upper

Trigeminal Nerve (CN V)

Ventral Posterior n.

Ventral Posterior Lateral n.(Body)

Ventral Posterior Medial n.(Face – CN V)

1st order trigeminal nerve sensory neurons synapse in CN V nucleus

3rd order neurons pass to primary sensory cortex

2nd order neurons ascend in trigeminal lemniscus to ventral posterior medial nucleus of the thalamus