Somatosensory Pathways PDF

1 Somatosensory pathways Tracey Langfield School of Biomedical Sciences Faculty of Medicine [email protected] 2 Learning Objectives 1. Describe the location, organisation, & modalities of the 2 main spinal somatosensory systems:...

1 Somatosensory pathways Tracey Langfield School of Biomedical Sciences Faculty of Medicine [email protected] 2 Learning Objectives 1. Describe the location, organisation, & modalities of the 2 main spinal somatosensory systems: Anterolateral Dorsal column 2. Make a brief comparison between the spinal somatosensory systems and the trigeminothalamic system.  Somatosensations: discrete touch = perception of light touch, localisation of touch, 2-point discrimination, texture pressure = distortion of the skin receptors vibratory sense proprioception = awareness of position and movement of body parts (conscious & subsconscious) crude touch = poorly localised touch nociception = indicates tissue damage or potential tissue damage - pain itch = chemical irritation of skin provoking urge to scratch to remove source temperature = perception of hot & cold Somatosensory information from the periphery to the cerebral cortex is transmitted by a 3-neuron circuit. face = Trigeminal N body = spinal nerves Crossman & Neary 2010 Somatotopy a systematic relationship exists between the position of axons in tracts and neurons in nuclei & cortex. Spinal somatosensory systems versus trigeminal somatosensory systems: Somatosensory systems: All first order neurons are pseudounipolar neurons cell body located in the spinal (dorsal root) ganglion (limbs and trunk) or trigeminal ganglion (face).  Somatosensory receptors The peripheral axon terminal of the 1st order neuron is the receptive part of the neuron. simple free nerve endings specialised receptors The type (modality) of the stimulus determines the type of 1st order neuron: 1st order neuron conduction modality Receptor/s fibre type velocity mechanoreceptors, A-α (1), A-β (2) proprioception muscle spindles, myelinated golgi tendon organs A-β (2) touch mechanoreceptors myelinated A-δ (3), C (4) temperature cold & warmth receptors myelinated, unmyelinated A-δ (3), C (4) fast pain free nerve endings - nociceptors myelinated, unmyelinated A-δ (3), C (4) slow pain free nerve endings - nociceptors myelinated, unmyelinated C (4) itch Pruritic receptors unmyelinated Different types of stimuli (modalities) are transmitted via different systems / pathways: dorsal column system: 1. discrete touch 2. pressure 3. vibratory sense 4. conscious proprioception anterolateral system: 1. crude touch 2. nociception / pain 3. itch 4. temperature Dorsal column system: 1. discrete touch 2. pressure 3. vibration 4. conscious proprioception the 1st order neuron enters via dorsal root & ascends in ipsilateral dorsal column fasciculus gracilis - LL & lower trunk fasciculus cuneatus – upper trunk and UL also refer to Fig 7.3 Krebs The dorsal column / funiculus below T6 = Fasciculus gracilis above T6 = Fasciculus gracilis & Fasciculus cuneatus Funiculus = column Fasciculus = tract Nolte 2009  Dorsal column system: thalamus 2nd order neuron decussates in caudal medulla & ascends through brainstem as the medial lemniscus 1st order neuron synapses with the 2nd order neuron in the caudal medulla nucleus gracilis - LL & lower trunk nucleus cuneatus – upper trunk and UL 1st order neuron enters via dorsal root & ascends in ipsilateral dorsal column fasciculus gracilis - LL & lower trunk fasciculus cuneatus – upper trunk and UL also refer to Fig 7.3 Krebs 1st order neuron enters the dorsal column & ascends ipsilaterally to medulla, at successive rostral levels new fibres are added to the lateral edge of the existing tract Fasciculus gracilis if < T6 Fasciculus cuneatus if > T6 Fasciculus gracilis Fasciculus cuneatus Cervical C1 Cervical cord Cervical C8 Thoracic Lumbar Sacral Schuenke 2010 Sacral axons medially, cervical axons laterally in the dorsal columns Anterolateral system: 1. crude touch 2. nociception anterior white 3. itch commissure 4. temperature The 2nd order neuron ascends contralaterally in the anterior part of the lateral column Schuenke 2010  Anterolateral system: 1st order neurons enter via the dorsal root. Some 1st order neurons will enter the dorsal horn at the level they entered the spinal cord & synapse. Other neurons divide into ascending & descending branches which travel up or down 1-3 segments in the dorsolateral / Lissauer’s tract before entering the dorsal horn & synapsing. anterior white commissure L L Lissauer’s tract SG SG substantia gelatinosa dorsal root Schuenke 2010 Organisation of the spinal cord grey matter: Rexed laminae: Subnuclei: Krebs 2018 p87-89  Anterolateral system: 1st order neurons synapse primarily in laminae I & II / substantia gelatinosa This is a potential site of pain modulation. Vanderah 2017 Somatotopic organisation of the anterolateral system: new fibres join the anterolateral tract at its anteromedial edge sacral axons are lateral, cervical axons are medial ventral White commissure Schuenke 2010  Somatotopy of the dorsal column versus the anterolateral pathway 1 1 2 Anterolateral pathways Anterolateral pathways: spinothalamic (to thalamus) conscious awareness of nociception (pain) localising source of pain spinomesencephalic (to PAG) descending pain modulation spinoreticular (to RF) level of attention in response to nociception spinohypothalamic (to hypothalamus) autonomic responses to nociception also refer to Fig 7.7 Krebs somatosensory 2nd order neurons ascend in the tegmentum of the brainstem R thalamus R frontal lobe midbrain R occipital lobe pons medulla midsagittal section somatosensory 2nd order neurons ascend in the tegmentum of the brainstem thalamus tegmentum midsagittal section 2nd order neurons project to the thalamus & synapse with the 3rd order neuron C B Lateral I I sulcus T T T B B 3 coronal section transverse section 3rd order neurons from the thalamus project to the postcentral gyrus via the posterior limb of the internal capsule coronal section Somatotopic organisation of the posterior limb of the internal capsule 1. Anterior limb 2. Genu 3. Posterior limb 4. Retrolenticular part C 5. Sublenticular part L Posterior limb T = somatosensory & somatomotor Head & neck (near the genu) Upper limb Trunk Lower Limb  Somatotopic organisation in the postcentral gyrus: Somatoensory homunculus lower limb Primary somatosensory cortex trunk post central gyrus parietal lobe upper limb face internal capsule Martin 2012 Secondary Somatosensory cortex: Integrates and interprets aspects of touch, proprioception, and pressure. Deficit = somatosensory agnosia inability to identify objects by feeling shape, size, texture, weight. location: parietal operculum and cortex extending into the lateral sulcus toward, & possibly including, the posterior insular cortex Tertiary multimodal sensory area for integration of multiple sensory modalities location: Posterior parietal cortex Somatosensations from the face & oral cavity = Trigeminothalamic pathways Summary of conscious pathways head = Trigeminal N body = spinal nerve Crossman & Neary 2010 p17 Blood supply of the spinal cord relate to pathways Each dorsal column is supplied by a posterior spinal artery The rest is supplied by the anterior spinal artery and segmental branches off the aorta ipsilateral loss of discrete touch and limb position sense at level of lesion and below contralateral loss of pain and temperature senses beginning a few levels below the lesion