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B) Spinothalamic system- Anterolateral system
-Transmits noxious, thermal, crude touch-non discriminative touch (stroking of cotton wool over the skin, visceral information to the thalamus and cerebral cortex
-Receptors transmit nociceptive information consist of high threshold free nerve endings ramifying near the external surface and internal environment of the organism. Small, pseudounipolar first order neurons house somata in dorsal root ganglion. Peripheral processes of these psuedounipolar neurons consist of 2 main types of fibre
-Thin myelinated Ad fibres: Thin, small fibre diameter 1-6um, myelinated, fast conducting 436m/s High threshold mechanoreceptors-so detect mechanical stimuli in the painful threshold, sensitive to temperature >50 degrees. Glutamate-key neurotransmitter-acts on a fast 5 subunit ligand gated ion channel-so pain is sharp, short term, well localised
-Fibres transmit sensations and don't elicit an affective component associated with the experience
-C fibres: Unmyelinated C, slow conducting fibres, small diameter
-Polymodal fibres-respond to a broad range of painful stimuli (mechanical, thermal, metabolic factors)
-Glutamate and substance P- act on ligand gated channels, but on continued depolarisation act on NMDA channels
- relay dull, persistenct, poorly localised pain (E.g excessive stretching of the tendon. Fibres transmit sensations that elicit affective response.
-Sensations of heat and cold also travel by delta A, C fibres
-Central processes of pseudonunipolar neurons enter spinal cord at dorsal root entry zone via lateral division of the dorsal roots of spinal nerves. Upon entry form the dorsolateral fasciculus (tract of Lissauer) present at all spinal cord levels. in Lissauer's tract run up and down for one or two spinal cord segments before they penetrate the gray matter of the dorsal horn.
-Depending on the type of fibre (Ad / C fibres) there are different pathways the 2nd order axons travel via:
-Direct pathway of spinothalamic tract to ascend to the thalamus and sending collaterals to the reticular formation
-Indirect pathway of ALS to synapse in the reticular formation and sending some collaterals to the thalamus
-Spinomesencephalic, Spinotectal, Spinohypothalamic fibres to synapse in in several brainstem nuclei Direct pathway of the anterolateral system/ Neospinothalamic tract pathway
-Type Ad fibres of 1st order neurons (Thermal/mechanical, small receptive fields) synapse mainly with second order neuron across lamina 1 (posteromarginal nucleus) and lamina 5 (reticular nucleus) of spinal cord grey matter
--laminae 1 (receive inputs from nociceptors, thermal, visercal, itch-small diameter)
- Laminae 5 (wide dynamic range neurons),mixed information from visceroreceptors, nociceptors, low threshold mechnaoreceptors to the thalamus
-Many first order neurons synapse with spinal cord interneurons that are associated with reflex motor activity
-Axons of the 2nd order neurons immediately decussate in the anterior white commissure (same level at which they enter the cord)
-cross the midline of the spinal cord obliquely to contralateral side of spinal cord forming spinothalamic tract, ascend in the contralateral anterolateral quandrant
-Spinothalamic tract made of two parts
-Lateral spinothalamic tract- located in the lateral funiculus) -only nociceptive and thermal input
-small anterior spinothalamic tract (located in the anterior funiculus) -only non discriminative crude touch
-In the spinothalamic tract, fibres are arranged somatotopically- fibres originating in lumbar and sacral segments are located laterally, those from cervical spinal segments are positioned more medially
-As lateral spinothalamic tract ascends through medulla oblongata it lies near the lateral surface and between inferior olivary nucleus and nucleus of the spinal tract of trigeminal nerve. It is now accompanied by anterior spinothalamic tract (crude touch-non discriminative touch, pressure?) and spinotectal tract- spinal leminiscus
-Spinal leminiscus continue to ascend through posterior part of pons. In midbrain it lies in tegmentum lateral to medial lemniscus
-Spinothalamic tract transits to contralateral ventral posterior lateral nucleus of thalamus
-It also sends projection to the ventral posterior inferior and intralaminar nuclei of thalamus.
-As spinothalamic tract ascends through brainstem it also sends collaterals to the reticular formation Trigeminal pain pathway: Pain and temperature from face and head - small diameter fibres in trigeminal nerve synapse onto second order sensory neurons in spinal trigeminal nucleus of the brain stem- axons cross and ascend to the thalamus in trigeminal lemniscus -Spinothalamic trat joined by axons from the trigeminal nucleus caudalis
-Due to decussation of spinothalamic fibres in spinal cord-noxious and thermal information from each dermatome is transmitted contralatrally in anterolateral column, touch and proprioception are transmitted ipsilaterally in dorsal column
-Unilateral injury- produce deficits in tactile, proprioceptive sensations on same side of the body as the lesion, impairments in thermal or painful sensations on the side opposite the lesion
Indirect pathway of the anterolateral systemType C fibres of first order neurons (Polymodal, larger receptive fields) terminate on interneurons on laminae 2 (substantia gelatinosa) and laminae 3 of the dorsal horn
-Axons of the interneurons synapse with second order neurons in laminae 5 to 8. In lamina 5 (part of nucleus proprius-main sensory nucleus of the dorsal horn, 3, 4, 5)-many other nerve fibres also synapse with 2 nd order axons-AB, Adelta-large receptive fields, cross modality interactions Many of these axons of these second order neurons ascend ipsilaterally, however a small number of axons sweep to the opposite side of the spinal cord in the anterior white commissure
-These axons form the more prominent ipsilateral and smaller contralateral spinoreticular tracts-transmit nociceptive, thermal and non discriminatory (crude) touch signs from the spinal cord to the thalamus indirectly by forming multiple synapses in reticular formation prior to their thalamic projections
-Spinoreticular tract- paleospinothalamic pathway
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