Somatosensory System
•Download as PPT, PDF•
56 likes•30,298 views
An introductory lecture to the somatosensory system.
Recommended
More Related Content
What's hot
What's hot (20)
Similar to Somatosensory System
Similar to Somatosensory System (20)
More from Csilla Egri
More from Csilla Egri (14)
Recently uploaded
Recently uploaded (20)
Somatosensory System
- 1. The Somatosensory System Csilla Egri, KIN 306 Spring 2012 “Activate my mechanoreceptive free nerve endings and Pacinian corpuscles” Elmo
- 2. Outline Somatosensory system overview Sensory transduction Sensory receptors Cutaneous mechanoreceptors Thermoreceptors Nociceptors Central projections Dorsal column medial-lemniscus pathway Anterolateral pathway Pain sensations Pain disorders 2
- 3. Somatosensory system: function3 Receptors respond to mechanical, thermal & chemical stimuli within three broad categories: Proprioceptive (lecture 6,10) Body and limb position Enteroreceptive (KIN 305) Internal state of the body Exteroreceptive Touch Temperature Pain (nociception)
- 4. Somatosensory fibers 4 Afferent fibers carrying info of different modalities are of different sizes What property of large diameter axons allows for increased speed of conduction?
- 5. Somatosensory transduction 5 two classes: simple or complex mechanical deformation, heat or chemical stimulus within receptive field opens ion channels causes a local depolarization = receptor potential propagated by electrotonic conduction to axon hillock Stimulus intensity coded by number of receptors activated, and frequency of AP Receptor potential
- 6. Somatosensory transduction 6 How to convey modality of sensation? Labelled line coding Type of sensation felt when receptor is stimulated determined by where the fiber synapses in the CNS Examples of “fooled” senses: Chewing minty gum activates cold sensitive thermoreceptors sensation of cold Seeing stars after being hit on head forceful blow activates photoreceptors see spots
- 7. Cutaneous mechanoreceptors: morphology7 (rapid vibration) (slow vibration, texture (rapid vibration) (deep pressure) (movement of hairs) (sustained touch, pressure)
- 8. Cutaneous mechanoreceptors: adaptation8 receptors are classified not only on their morphology, but on their adaptation & receptive fields as well
- 9. Cutaneous mechanoreceptors: receptive fields9 small receptive fields permit high resolution of spatial detail (two point discrimination) Discrimination enhanced by lateral inhibition
- 10. Cutaneous mechanoreceptors: adaptation + receptive fields10 Kandel Fig. 21-1 Adaptation
- 11. Cutaneous mechanoreceptors: summary11 Receptor Sensation Adaptation rate Receptive field Free nerve endings Itch, tickle, pain Tonic or phasic Large or small Ruffini endings Stretching of skin, deep pressure Tonic Large Merkel discs Fine touch and pressure Tonic Small Meissner corpuscle Fine touch, pressure, slow vibration Phasic – moderate Small Hair follicle Crude touch, movement of hairs Phasic – moderate Small Krause bulbs Fast vibration Phasic - fast Small Pacinian corpuscle Pressure, fast vibration, tickling Phasic - fastest Large
- 12. Thermoreceptors 12 Free nerve endings with high thermal sensitivity Temperature change activates family of ion channels on the receptor membrane = TRP (transient receptor potential) channels Each TRP channel has a unique temperature threshold of firing, and is sensitive to various chemical agonists
- 13. Thermoreceptors: tonic responses13 Guyton Fig. 48-10 Warmth receptors: Narrow temperature range Begin firing at 30ºC, rise steeply with increased temperature, then stop abruptly Sensation of pain begins >45ºC Cold receptors: Broader temperature range Maintain steady discharge rates, with increased firing 20-30ºC <15ºC neuronal firing ceases Tonic response of thermoreceptors and thermosensitive pain fibers
- 14. Thermoreceptors: phasic responses14 Kandel Fig. 22-9 Thermoreceptors are more responsive to changes in temperature than to constant temperature A phasic response in both warm and cold receptors occurs when the temperature is changed Thermoreceptors adapt to a new steady state firing level is stimulus is maintained
- 15. Nociceptors 15 Free nerve endings that respond to intense stimuli Types: Mechanical Strong pressure, sharp objects Thermal Burning heat (>45ºC) Noxious cold (variable) Chemical pH extremes Environmental irritants Internal neuroactive substances Polymodal Sensations mediated by Aδ fibers (sharp, intense pain) and C fibers (persistent, dull pain). Check: Which fibers are myelinated?
- 16. Nociceptors: hyperalgesia of inflammation16 Nociceptors are non adapting receptors Primary hyperalgesia: damaged tissue has increased sensitivity to pain Reduced threshold to pain If normally non painful stimuli felt as painful: allodynia Increased intensity of sensation Spontaneous pain Inflammatory response releases bradykinin, prostaglandins, serotonin, substance P, K+ , H+ Substance P activates mast cells release histamine activates nociceptors B&B Fig. 13-26
- 17. Somatosensory projections: dermatomes17 Sensory neurons (dorsal root ganglion cells) enter the spinal cord through the dorsal roots Each dorsal root innervates a field of skin called a dermatome Dermatomal map used to determine level of lesion of spinal injury Epidural analgesia blocks sensations thru out several dermatomes B&L Fig. 7-4
- 18. Somatosensory projections: tracts 18 2. Anterolateral pathway1. Dorsal column-medial lemniscus pathway
- 19. 1. Dorsal column-medial lemniscus pathway19 Carry signals from mechanoreceptors of the skin, joints & muscle fine, discriminatory touch Information has high spatial & temporal resolution Large, myelinated fibers with rapid conduction velocities 1º afferents terminate & form synaptic connections with 2nd order neurons in the dorsal column nuclei within the medulla 2nd order neurons cross over at the medulla and continue to the thalamus via the medial lemniscus pathway After thalamic processing, 3rd order neurons project to the primary somatosensory cortex
- 20. 2. Anterolateral pathway 20 Pain, temperature, crude touch, tickle, itch, sexual sensations Low spatial or temporal resolution Small, myelinated/unmyelinated fibers with slower conduction velocities 1° afferents terminate upon entering spinal cord & synapse on 2nd order neurons 2nd order neurons cross to contralateral side, ascend to brain via anterolateral quadrant in spinal cord & project to thalamic nuclei After thalamic processing, 3rd order neurons project to the primary somatosensory cortex and other cortical areas
- 21. Somatosensory cortex: somatotopy21 B&B Fig. 14-11 Spatial orientation of signals form different parts of the body: somatotopy Size of somatotopic areas is proportional to density of sensory receptors in that body region Map is plastic (modifiable) size of cortical region representing particular portion of body surface can expand or contract depending on use of that body region Pain and temperature localization not as precise Integration probably happens more in the reticular formation and thalamus
- 22. Pain sensation: referred pain 22 Pain from visceral nociceptors is poorly localized, can be felt as pain on surface areas Knowledge of referred pain maps important in clinical diagnosis Somatic and visceral afferents may converge on same 2nd order neuron Guyton Fig. 48-6Kandel Fig. 24-3
- 23. Pain sensation 23 Sensation of pain intensity not necessarily linked to activation of nociceptors; under CNS control Perception of pain and pain tolerance is subjective Gate control theory of pain Activation of non-painful fibers (Aα) sends inhibitory signals to nociceptive afferents traveling to the CNS Mechanism of acupressure analgesia? Phantom limb pain Pain felt even though nociceptors no longer present in missing limb Peripheral sensitization Somatosensory reorganization Neuropathic pain Damage to Aδ or C fibers may increase sensitivity or cause spontaneous AP firing
- 24. Pain disorders: CRPS 24 Complex regional pain syndrome (CRPS) Neuropathic pain disorder involving peripheral and central mechanisms (autonomic nervous system) Changes in somatosensory systems processing thermal, tactile, noxious stimuli Local edema, altered sweating, redness, skin temperature changes, burning pain, hyperalgesia, allodynia Acute: warm, red extremities Chronic: cool, bluish extremities CRPS I – no documented nerve injury CRPS II – presence of nerve injury Surgery, fracture, crush injury, sprains, but can develop even after minimal injury WebCT readings: Complex Pain Syndrome Left arm affected by CRPS Right foot affected by CRPS
- 25. Pain disorders: CRPS pathophysiology25 Peripheral and central sensitization Tissue injury release of substance P and bradykinin increased excitability of nociceptive neurons in periphery and spinal cord Increased local, systemic, and CSF inflammatory factors Reduced density of Aδ and C fibers Altered SNS function and sympatho-afferent coupling Expression of adrenergic receptors on nociceptors Symptoms worsened by emotional arousal Reduced representation of affected limb in somatosensory cortex Genetic predisposition Bruehl S. Anesthesiology 2010WebCT readings: Complex Pain Syndrome
- 26. Objectives After this lecture you should be able to: List the structure and function of the various cutaneous mechanoreceptors Describe the mechanism of somatosensory transduction, including modality coding and receptor adaptation Differentiate between tonic and phasic responses of thermoreceptors Compare and contrast the function and anatomy of the dorsal column-medial lemniscus pathway with the anterolateral pathway List the factors affecting pain sensation Relate what we’ve learned in this course so far to the different theories of CRPS pathophysiology 26
- 27. 27 1. Predict on which side of the body and what sensations would be impaired if there was a spinal cord transection at T4 of: a) The left dorsal column-medial lemniscus pathway b) The left anterolateral pathway 2. Describe what would happen to both cold and warm thermoreceptors in response to an increase in temperature from 35ºC to 40ºC , sustained for 5 min, and removed. 3. Meissners corpuscles are __________________ adapting receptors with __________________ receptive fields. Test your knowledge
Editor's Notes
- Readings B&B: Chapter 13, pages 325 - 326, 352 - 357, 370 - 371; Chapter 10, pages 276, 278; Chapter 14, pages 361 & 373 BLKS: Chapter 7
- Cutaneous mechanoreceptors because we already talked about mechanically gated receptors: Hair cells in vestibular and auditory system
- System of sensory receptors and central projections that transduce, encode, and ultimately perceive information generated by stimuli arising from both the external and internal environment Next lecture, muscle proprioceptors and vestibular system Differences between somatosensory system and special senses is that somatosensory receptors are diffusely located thru out the body instead of densely distributed to a specific organ. Special senses convey information to the brain via single nerve bundle whereas somatosensory information arrives via the spinal cord dorsal roots and cranial nerves Enteroreceptive – mechanoreceptors detecting distension of gut and fullness of bladder . Although the basic senses—somatic sensation, vision, audition, vestibular sensation, and the chemical senses—are very different from one another, a few fundamental rules govern the way the nervous system deals with each of these diverse modalities. Highly specialized nerve cells called receptors convert the energy associated with mechanical forces, light, sound waves, odorant molecules, or ingested chemicals into neural signals that convey information about the stimulus to the brain. These afferent sensory signals activate central neurons capable of representing both the qualitative and quantitative aspects of the stimulus (what it is and how strong it is), and in some modalities—somatic sensation, vision, and audition—the location of the stimulus in space (where it is). The clinical evaluation of patients routinely requires an assessment of the sensory systems to infer the nature and location of potential neurological problems. Knowledge of where and how the different sensory modalities are transduced, relayed, represented, and further processed to generate appropriate behavioral responses is therefore essential to understanding and treating a wide variety of diseases. Accordingly, these chapters on the neurobiology of sensation also serve to introduce some of the major structure/function relationships in the sensory components of the nervous system.
- Increased length constant due to decrease axoplasmic resistance
- The local depolarization produced is called a generator or receptor potential. Receptor potential increases with stimulus strength. Site of AP initiation = trigger zone Stimulus intensity encoded in: firing rate of neuron (number of action potentials per second) or number of activated sensory receptors The firing rate of single receptor is frequently an exponential function of stimulus intensity.
- Ruffini&apos;s end organs detect tension deep in the skin. Meissner&apos;s corpuscles detect changes in texture (vibrations around 50 Hz) and adapt rapidly. Pacinian corpuscles detect rapid vibrations (about 200–300 Hz). Merkel&apos;s discs detect sustained touch and pressure. Mechanoreceiving free nerve endings detect touch, pressure and stretching Hair follicle receptors are located in hair follicles and sense position changes of hairs. Krause bulbs – innervate border areas of dry skin and mucous membranes (ie. lips external genitalia) Pacinian Corpuscle – glaborous and hair skin. vibration at 200-300Hz due to specialized capsule
- Repetitive discharge = train of action potentials Slowly adapting mechanoreceptors continue responding to a stimulus, whereas rapidly adapting receptors respond only at the onset (and often the offset) of stimulation. These functional differences allow the mechanoreceptors to provide information about both the static (via slowly adapting receptors) and dynamic (via rapidly adapting receptors) qualities of a stimulus.
- A receptive field is a region that causes activation of a sensory neuron when stimulated. Stimulus may not be perceived unless several sensory receptors are activated (spatial summation) or unless the firing rate is sufficiently high (temporal summation) to bring postsynaptic neurons to threshold. Small receptive fields are characteristic of Meissner’s corpuscles and Merkel’s disks (2-4 mm in diameter). Pacini’s and Ruffini’s corpuscles have large receptive fields. Example: Ruffini’s corpuscles respond to stretch of skin even at some distance from receptor site.
- Meissner’s Corpuscles (FA1 afferents) also rapidly adapting receptors respond best to vibration in 30-40 Hz range Meissner’s corpuscles are located in ridges of the dermis of glabrous skin; they are much smaller than Pacini’s corpuscles. Meissner’s corpuscles receive innervation from 2-5 axons, while one axon innervates about 20 corpuscles; other receptors innervated by single axon. The rapidly adapting receptors (Pacini’s and Meissner’s corpuscles) provide contact information, slip information, &contrast information (e.g. important for edge detection). Merkel’s Disks (SA1 afferents) slowly adapting receptors that respond to touch Merkel’s disks are located at border of dermis and epidermis of glabrous skin. Merkel cells seem to best encode spatial characteristics of textured surface. Pacinian Corpuscles (FA2 Afferents) respond transiently to applied pressure during sustained pressure layers slip reducing deformation of receptor membrane also respond during restitution of membrane when pressure is released respond best to vibration in 200-300 Hz range Ruffini’s Corpuscles (SA2 afferents) slowly adapting receptors respond to indentation of the skin (touch & low frequency vibration) Hair Follicle Receptors innervated by free nerve endings that wrap around or run parallel to follicle bending of hair causes deformation of follicle which deforms nerve endings and modulates their firing frequency
- Meissner’s Corpuscles (FA1 afferents) also rapidly adapting receptors respond best to vibration in 30-40 Hz range Meissner’s corpuscles are located in ridges of the dermis of glabrous skin; they are much smaller than Pacini’s corpuscles. Meissner’s corpuscles receive innervation from 2-5 axons, while one axon innervates about 20 corpuscles; other receptors innervated by single axon. The rapidly adapting receptors (Pacini’s and Meissner’s corpuscles) provide contact information, slip information, &contrast information (e.g. important for edge detection). Merkel’s Disks (SA1 afferents) slowly adapting receptors that respond to touch Merkel’s disks are located at border of dermis and epidermis of glabrous skin. Merkel cells seem to best encode spatial characteristics of textured surface. Pacinian Corpuscles (FA2 Afferents) respond transiently to applied pressure during sustained pressure layers slip reducing deformation of receptor membrane also respond during restitution of membrane when pressure is released respond best to vibration in 200-300 Hz range Ruffini’s Corpuscles (SA2 afferents) slowly adapting receptors respond to indentation of the skin (touch & low frequency vibration) Hair Follicle Receptors innervated by free nerve endings that wrap around or run parallel to follicle bending of hair causes deformation of follicle which deforms nerve endings and modulates their firing frequency
- In accordance with this functional diversity, TRP channels are implicated in a multitude of physiological processes, ranging from Ca2+ and Mg2+ homeostasis and regulation of the vascular tone to bone development, taste perception, temperature sensing and vision. These membrane receptors belong to a class (family) of proteins called the transient receptor potential proteins; these receptors bind to the chemical capsaicin, the molecule in chili peppers that gives them their heat. Other binds different chemicals including camphor, menthol, and mustard oil
- Stimulus-response curve of cold receptors is non-monotonic so temperature cannot be signaled by firing rate alone; perceived temperature is determined by relative activity of cold and warm receptors. Rapid changes in skin temperature evoke dynamic responses (transient increase in firing rate of warm receptors for increase in temperature or transient increase in firing rate of cold receptors for decrease in temperature).
- Neuroactive substances include histamine and bradykinin in sites of infection Adelta: fast, myelinated C: slow, unmyelinated
- Increased pain following sunburn. Secondary hyperalgesia describes pain that occurs in surrounding undamaged tissue.
- large 1 afferent axons bifurcate in dorsal column and branches: rostrally to dorsal column nuclei (DCN) caudally, over a distance of several spinal segments Collateral branches of primary afferents project ventrally into grey matter of spinal cord where they synapse on interneurons and motoneurons. The person is on all fours because this is how dermatomes develop during development, they are distorted during embryological development. Epidural from L3-4 blocks sensation in uterus and lower dermatomes. Inserted into epidural space because this is the location of the dorsal root ganglia
- Carry signals from mechanoreceptors of the skin, joints & muscle fine, discriminatory touch Large, myelinated fibers with rapid conduction velocities Information has high spatial & temporal resolution 1º afferents terminate & form synaptic connections with 2nd order neurons in the dorsal column nuclei within the medulla 2nd order neurons cross over at the medulla and continue to the thalamus via the medial lemniscus pathway Pain, temperature, crude touch, tickle, itch, sexual sensations Small, myelinated fibers with slower conduction velocities Low spatial or temporal resolution 1 afferents terminate upon entering spinal cord & synapse on 2nd order neurons 2nd order neurons cross to contralateral side, ascend to brain in ventral part of lateral funiculus & project to thalamic nuclei 3rd order neurons project to somatosensory cortex & other cortical areas
- START! Mechanoreceptors of the skin, joints and muscle provide tactile, discriminative, and proprioceptive information. A high spatial and temporal resolution refers to the ability to discriminate details of spatial location and stimulus intensity. Fasciculus gracilis: fibers conveying information from lower limbs are in the medial region Fasciculus cuneatus: fibers from upper limbs, trunk and neck are in the lateral region DCN = gracile and cuneate nuclei
- most prominent ascending pathway for pain and thermal sensations, also carries crude tactile sensation thalamic nuclei include VPL nucleus & intralaminar complex & others cortical projections of 3rd order neurons are involved in affective responses such as the cingulate gyrus and insula, which have limbic system functions
- Somatotopic order in the human primary somatic sensory cortex. (A) Diagram showing the region of the human cortex from which electrical activity is recorded following mechanosensory stimulation of different parts of the body. The patients in the study were undergoing neurosurgical procedures for which such mapping was required. Although modern imaging methods are now refining these classical data, the human somatotopic map first defined in the 1930s has remained generally valid. (B) Diagram along the plane in (A) showing the somatotopic representation of body parts from medial to lateral. (C) Cartoon of the homunculus constructed on the basis of such mapping. Note that the amount of somatic sensory cortex devoted to the hands and face is much larger than the relative amount of body surface in these regions. A similar disproportion is apparent in the primary motor cortex, for much the same reasons (see Chapter 17). (After Penfield et al., 1953, and Corsi, 1991.) after denervation, representation of neighboring skin regions expands to occupy that of denervated region (may enhance sensitivity to stimuli or ability to process) - signals from neighboring regions to compensate for missing input {Fig. 14-13} excessive use of one digit at expense of others results in expansion of its cortical representation
- Allodynia: different form hyperalgesia in that a normally non painful stimulus, such as light touch, is felt as extrememley painful, (A fiber mediated) whereas in hyperalgasia the painful stimulus intensity is heightened (A or C fiber mediated)
- Reduced density not known if it is a cause or a result of CRPS
- Left side of body fine discriminatory touch below T4 Right side of the body, pain temp heat below T4 3. Moderately adpating phasic receptors with small receptive fields.