17 Somatosensory Pathways

17 Somatosensory Pathways - PSL302Y: Lecture 16, by Prof....

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Unformatted text preview: PSL302Y: Lecture 16, by Prof. MacKay# Fri., Oct.22, 2010 Somatosensory Pathways 1) Dorsal columns-medial lemniscus - Fast - Solely mechanoreceptive: large conducting afferents - A- afferents; muscle spindle, Golgi tendon organ, joint receptor - Small receptive fields: sensitive, detecting small stimuli - Exception: Pacinian corpuscle (huge r.f. but detects small stimuli) 2) Spinothalamic tract - Many modalities: mechanoreceptive thermoreceptive, nociceptive - `Wide dynamic range' neurons: polymodal; but also... - Pain- and temperature-specific neurons - Goes directly from spinal cord to thalamus 3) Spinoreticulothalamic pathway - Slowest: lots of synapses - Mixed modalities - Pain-related neurons - Goes thru interm. zone in spinal cord -> retic.form in brainstem -> thalamus 1) a. Dorsal column - Somatotopic organization: "in the order of the body" - As afferents enter s.c. from diff parts of body, kept segregated = mapping - Afferents enter via dorsal root UP s.c. = separation of lower and upper body - Bottom of cord: afferent has most medial location - Go up cord: packed into lateral regions - Segregation of cutaneous (superficial) and proprioceptive (deep) afferents - Cutaneous afferents on superficial parts of dorsal column - Proprioceptive afferents relegated to deeper parts Somatosensory Pathways - Separation of slowly & rapidly adapting afferents - Afferent axons branch upon entering via dorsal roots (no synapse) - One branch goes up dorsal column to brainstem - Other goes into dorsal horn of s.c. = local processing - First synapse at dorsal column nuclei (set of nuclei where s.c. joins to medulla) - 3 separate nuclei: segregate leg and arm - Nucleus Z gets input from proprioceptors only from lumbar-sacral lvl; synapse in own special dorsal column region b/c no space in other nuclei - After synapse: post-synaptic axon up to thalamus -> cross midline: L half of body cross to R half of brainstem = terminate in R half of thalamus - Visual system causes everything to cross 1 of 6 Cerebral Cortex central sulcus Parietal lobe October Dorsal Column Nuclei PSL302Y: Lecture 16, by Prof. MacKay# Nuclei Fri., Oct.22, 2010 Dorsal Column b. Specific nucleus in thalamus that receives input: - Tracts from d.c. nuclei to thalamus known as medial lemniscus - Lemniscus: "ribbon-like" tract - Close to midline, runs thru brainstem (organized somatotopically) - Terminates in ventral caudal nucleus in thalamus Somatosensory Pathways (organized somatotopically) - V: ventral tier of nuclei in thalamus; caudal: posterior on sulcus - Info projected into somatosensory cortex on posterior bank of central sulcus - Cross-section of central sulcus (at red line): every fxnal part of cerebral cortex given a histological # - Anterior bank: area 4 - Posterior bank: area 3 - Area 3 is primary somatosensory region: VC nucleus projects into here - Proprioceptive component goes to 3a - Cutaneous signalsSomatosensory Pathways go to 3b - Parallel mapping of body in each (somatotopic representation Cerebral Cortex central sulcus Parietal lobe Frontal lobe Occipital lobe lateral fissure Temporal lobe left hemisphere Octobe 2 c. Primary somatosensory cortex (S1, or area 3) - Divided into 2 parts: both have somatotopic' representation # Area 3a: proprioreceptive map of body Primary somatosensory cortex # Area 3b: cutaneous map of body right hemisphere - Creation of `ON' & `OFF' responses in cortex to stimulation of or `area 3' divided into 2 parts: `S1' r.f. in primary afferents area 3a: proprioceptive map of body - ON-OFF response refer to delivering the stimulus to 3b: cutaneous map of body; area the application of stimulus `somatotopic' representation - But many cells in S1 will not respond to application`ON' and `OFF' responses to stimulation of of receptive p stimulus (even if w/i r.f.): they'll be inhibited by stimulus field Cortex - Once stimulus removed, burst of activity in these cells has 6 layers of neuronal cell bodies - Cortex has 6 layers of neuronal cell bodies Characteristic neuron is `pyramidal cell' - Characteristic neuron is `pyramidal cell' - Schematic mapping of S1, 3b-cutaneous (diagram) - Area 3a along distal edge (bottom of sulcus) - Mapped somatotopically but upside down representation - Foot & leg repped medially; face repped laterally - Rep is distorted b/c rep of population of afferents: more afferents from fingers, lips, tongues, toes, vs. trunk & leg - Parts of body not contiguous to one another can be contiguous in S1 Hypercolumn 2 of 6 Functional unit of cerebral cortex Somatosensory Pathways October PSL302Y: Lecture 16, by Prof. MacKay# Somatosensory Pathways Fri., Oct.22, 2010 October 22, 2010 d. Hypercolumn - Fxnal unit of cerebral cortex: not anatomical but physiological - Def'n: "block of cortex containing all of the neurons responsive to same pt on skin" - Given an area on skin, find a hypercolumn in map of S1 Somatosensory Pathways where all its neurons are congregated - Hypercolumns are arranged mainly in somatotopic order October 22, 2010 Illustration: Association areas - Activate # of diff afferents whose r.f. overlap this point - Activate diff cells in cerebral cortex = block of S1 Series of somatosensory association areas are posterior to S1 in parietal lobe - Hypercolummn: contains all neurons that respond to integrate S1 responses into more complex that pt patterns example: recognition of moving stimulus p g g - R.f.'s for each of the neurons in hypercolumn would across a skin territory in a specific direction all be slightly diffs - Segregation of rapidly- and slowly-adapting neurons - fMRI of S1: diff hypercolumns in 3D fMRI of S1: middle finger fMRI of S1: finger From `Cerebral middle Cortex', Sept/2003 e. Association areas - POSTERIOR to S1: series of somatosensory association areas in parietal lobe - Primary cortical regions followed by association areas where elements are put together into larger constructs - Integrate S1 responses into more complex patterns - Ex.: Recognition of moving stimulus across a skin territory in a specific direction - W/i S1, neurons responding to application/ removal of stimulus (ON/OFF responses) Body Image - In assoc cortex: response to MOVING stimulus neurons in parietal areas 2 & 5 respond to postural configurations of limbs, or to - Location: S1 in posterior bank of central sulcus specific vectors of limb motion representation of body parts and their - Series of assoc areas that make more and more complex response patterns current state lesion: - Area 7: somatosensory response integrated w/ visual system loss of feeling that body part(s) belong to you - W/i areas 2/5: body schema representation -> actual body posture/image Somatosensory Pathways 5 From `Cerebral Cortex', Sept/2003 October 22, 2010 6 f. Body Image - Neurons in parietal areas 2 & 5 respond to postural configurations of limbs, or to specific vectors of limb motion - Representation of body parts and their current state - Integrate input (cutaneous & proprioceptive) - Determines how cells w/i region respond to body positions/limb mvts - Internal perception of where body parts are - Lesion: loss of feeling that body part(s) belong to you, i.e. due to stroke in A2/5 - Lines stand for impulse firing; change position = another neuron starts firing 7 3 of 6 PSL302Y: Lecture 16, by Prof. MacKay# deep STT neurons are commonly "wi Fri., Oct.22, 2010 dynamic range" neurons 2) a. Spinothalamic tract (STT) - Multimodal: mechano-, thermo- and nociceptive - Synapse in dorsal horn: either in lamina I or in deep layers - Lamina I contains projection neurons specific for pain, temperature, or itch - Pain neurons project in distinct tract via contralateral (opposite side) cord to thalamus - Deep STT neurons are commonly "wide dynamic range" neurons (mechanoreceptive, nociceptive) - Afferent enter dorsal horn - Synapse in lamina I or deep layers - Axon of post-synaptic neuron intercross and project up spinal cord in ventral-lateral white-matter - Terminate in thalamus: overlap w/ dorsal-columnmedial-lemniscus pathway - Precise input overlaps w/ d-c-m-l - Pain & temp goes to edge (not VC region) - Subsystem w/i STT conveys info from head & mouth via spinal trigeminal nucleus: input from Somatosensory Pathways October 22, caudalis) cranial sensory complex (subnucleus2010 - Maintain somatotopic order of afferents - Pain+temp segregated to bottom edge of VC b. Wide Dynamic Range Cells - Located deep in dorsal horn - Common type of spinothalamic neuron Wide Dynamic Range cells - Convergent inputs from low threshold common type of spinothalamic neuron mechanoreceptors (A afferents from skin terminate directly here) and high threshold convergent inputs from low threshold nociceptors (group C afferents terminate superficially -> deeply via interneurons) mechanoreceptors (direct) and high threshold in r.f.`s - Diffcesnociceptors (via interneurons) r.f. for light touch is much smaller than r.f. - gR.f. for light touch is much smaller (direct synapses: not too many inputs) for noxious stimuli - R.f. for noxious stimuli is very large b/c so many synapses in btwn -> convergence at every synapse - R.f.'s get bigger w/ every subsequent post-synaptic nuclei = summation 9 4 of 6 matosensory Pathways October 22, 2010 PSL302Y: Lecture 16, by Prof. MacKay# Fri., Oct.22, 2010 Diagram: record from one wide-dynamic range cell deep in dorsal horn Light pressure input: -conveyed by L group A afferents (mechanoreceptors) -# of them synapse directly on w-d-r-cell -Depending on how many synapse on that cell = how lg the r.f. is -> two fingers Nociceptive/thermoreceptive input: i.e. Somatosensory Pathways Pinching/heating/freezing skin -Conveyed by S group A+C afferents -Terminate superficially in dorsal horn -Synapse w/ interneurons (# synapses) = summation -> conveyed to w-d-r-cell -R.f. is much larger -> whole hand! Somatosensory Pathways October 22, 2010 October 22, 2010 c. Spinoreticulothalamic cells Somatosensory Pathways - Oldest somatosensory pathway - Projection neurons begin in intermediate zone (s.c.) - Bilateral (or contralateral) projection to reticular formation Spinoreticulothalamic cells (extension of interm zone in brainstem) - Reticular formation projects to centromedian (CM) nucleus Referred pain of thalamus neurons generally in intermediate Projection Artifact of convergence unto spinothalamic - Does not project specifically to S1in cortex zone (and spinoreticulothalamic) neurons - Diffuse projection to modulate wide area of cortex same projection neuron receives nociceptor bilateral projection to reticular formation input from several body loci - Major target is basal ganglia highly convergent input, all large r.f.'s brain interprets g g - Highly convergent input:verymodalities converge (except p signal as coming from locus which is most often injured signals behavioral states pain remains specific) - Very large formation projects tosummation reticular r.f.`s due to lots of centromedian - Signals behavioral states: harmful stimulus=get away? nice nucleus of thalamus stimulus=want more? d. Referred pain - Artifact of convergence into spinothalamic (and spinoreticulothalamic) neurons (conveying pain modality) 10 - Same projection neuron receives nociceptor input from several body loci - Post-synaptic convergence = summation - How does the brain distinguish where? - Brain interprets signal as coming from locus which is most often injured, (i.e. heart attack) - B/c summation, pain location mixed up! - Diagram: group A+C afferents convey pain -> all affect same post-synaptic cell - Interneurons relaying signals in lamina II & III called substantia gelatinosa (tiny cells mixed up w/ lots of axons=gelatinous appearance) 5 of 6 October 22, 2010 11 11 12 mechanism of massage therapy PSL302Y: Lecture 16, by Prof. MacKay# Fri., Oct.22, 2010 E. `Gate control' of pain: Regulation at spinal cord - Regulated b/c have such powerful effects on body - Can't let pain control you when in trouble - Mech: Large mechanoreceptor afferents presynaptically inhibit small nociceptor afferents - Location: Inhibitory interneurons located in substantia gelatinosa of dorsal horn - I.e. Mech of massage therapy - Strong continuous signal in large mechanoreceptor afferents - Activates inhibitory inputs to shut down small nociceptor afferents Diagram: Large mechanoreceptors directly activate deep projection neurons - Also send branch into substantia gelatinosa to excite inhibitory interneurons - Presynaptically inhibit nociceptor afferents 6 of 6 ...
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