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Y2 s1 sensory system
This document discusses the human sensory system, including sensory receptors, pathways, and cortex. It describes the different sensory modalities and sensations. The main sensory pathways are the dorsal column-medial lemniscus pathway and spinothalamic pathway, which transmit signals from receptors to the thalamus and sensory cortex. Within the cortex, different body regions are represented in a distorted map called a sensory homunculus. The document provides details on various mechanoreceptors, transduction of stimuli to action potentials, and coding of sensory information.
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Y2 s1 sensory system
- 1. Sensory receptors, sensory pathways& sensory cortex Prof. Vajira Weerasinghe Professor of Physiology www.slideshare.net/vajira54
- 2. Sensory functions • Humansdo not have receptor for every possible stimuli • There are different sensory modalities that human brain can perceive • Arrival of information is sensation
- 3. Sensory Functions • GeneralSensations Physical (touch, pressure, vibration, stretch) Temperature Pain Chemical • Special Sensations Vision Hearing Taste Smell
- 4. Modality specificity • Stimulationof a receptor usually produces only one sensation modality specific • But some receptors are stimulated by more than one sensory modality (polymodal) eg. free nerve endings
- 5. Sensory pathway • Oncea receptor is stimulated impulse travels through a particular pathway known as sensory pathway or ascending pathway up to the brain
- 6. Receptor Sensory modality Sensory nerve Central Connections Ascending Sensorypathway Sensory area in the brain Touch stimulus AFFERENT Sensory pathway
- 7. Receptors • Receptor cellsare specific cells that are sensitive to different forms of energy from the environment • These cells contain membrane receptors coupled to ion channels • They transform the stimulus into electrical signals
- 8. Classification of receptors •Mechanoreceptors • Thermoreceptors • Nociceptors pain • Chemoreceptors taste, smell, visceral • Electromagnetic receptors visual Guyton p.496
- 9. Mechanoreceptors • Mainly cutaneous Touch Pressure Vibration •Crude or Fine mechanosensations • Others: auditory, vestibular, stretch, proprioceptors
- 10. Cutaneous mechanoreceptors • Paciniancorpuscle • Meissner’s corpuscle • Krause’s corpuscle • Ruffini’s end organ • Merkel’s disc • Hair end organ • Free nerve endings
- 11. Mechanoreceptors • Pacinian corpuscle deep,pressure sensitive, fast adapting, large receptive field • Meissner’s corpuscle superficial, sensitive to touch, small receptive field • Ruffini’s end organ deep, tension sensitive, slow adapting, large receptive field • Merkel’s disc superficial, touch, pressure and texture sensitive, slowly adapting, small receptive field • Krause’s endings vibration sensitive
- 12. Mechanoreceptors • Hair endorgan • Free nerve endings Crude mechanosensations (Pain, temperature)
- 16. Pacinian corpuscles looks like onion,large receptive field, rapidly adapting Hair follicle receptor nerve endings around root of hair in hairy skin, small receptive field, either slowly or rapidly adapting Ruffini's ending looks like small Pacinian, large receptive fields, slowly adapting Merkel's disks small arrays of small disks which may have synapses to nerve endings, small receptive fields, slowly adapting Meissner's corpuscles hang under ridges of glabrous skin, small receptive fields, rapidly adapting Krause end bulbs look like knotted balls of string in skin in border between dry skin and mucous membrane in mouth, genitals, anus
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- 18. What happens insidea receptor? • TRANSDUCTION Stimulus energy is converted to action potentials Inside the nervous system signals are always action potentials Language of the nervous system contains only 1 word: action potentials • At the brain opposite happens in order to feel the sensation PERCEPTION
- 19. Receptor potentials • Whena stimulus activate a receptor initially a “receptor potential” is generated • This is also called “generator potential” • This is a graded potential • It does not follow “all-or-none law” • Its amplitude depends on the strength of the stimulus
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- 22. Coding of sensorystimuli • Stimulus strength is coded as the frequency of AP • Higher the stimulus more frequent are the APs • Amplitude of AP is constant
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- 24. Sensory coding • Areceptor must convey the type of information it is sending the kind of receptor activated determined the signal recognition by the brain • It must convey the intensity of the stimulus the stronger the signals, the more frequent will be the APs • It must send information about the location and receptive field, characteristic of the receptor
- 25. Transduction in differentreceptors • Different receptors have different ion channels • Their opening causes receptor potential
- 27. Receptor potential generation inthe Pacinian corpuscle
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- 30. Physical Stimulus Physical stimuluscausing mechanical deformation on the capsule
- 31. Physical Stimulus Mechanical deformationis transmitted to the inside Opens up mechanosensitive Na+ channel Causes depolarisation and thus receptor potential
- 32. Physical Stimulus local current Currentflow through a local circuit
- 33. Physical Stimulus Action Potentials aregenerated Opening of voltage gated Na+ channels causes generation of action potentials
- 34. Adaptation • “getting usedto” • after a period of time sensory receptors adapt partially or completely • different types Rapidly adapting receptors slowly adapting receptors
- 35. Adaptation • after aperiod of time sensory receptors adapt partially or completely • different types fast adapting receptors slowly adapting receptors
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- 37. Mechanism of adaptation •In the Pacinian corpuscle mechanical deformation is transmitted throughout the capsule and pressure redistributes Na+ channels inactivates after some time
- 38. Impulse Stimulus Redistribution of pressureinside the capsule No Impulse Stimulus
- 39. • Rapidly adaptingreceptors phasic or rate or movement receptors detect changes in stimulus strength eg. Pacinian corpuscle, hair end-organ • Slowly adapting receptors tonic receptors detect continuous stimulus strength eg. muscle spindles, Golgi tendon organ, baroreceptors, Ruffini endings and Merkel’s discs, pain receptors
- 40. Classification of receptors •Mechanoreceptors Cutaneous (touch, pressure, vibration) eg. Pacinian, Meissner’s corpuscle, free nerve endings Proprioceptors (joint position receptors) eg. Muscle stretch receptors, tendon organs Baroreceptors Auditory/vestibular hair cells • Chemoreceptors Taste buds and smell receptors Visceral chemoreceptors sensitive to Pco2, pH, osmolality etc • Thermoreceptors Cold and hot receptors • Nociceptors (pain receptors)
- 41. Two ascending pathways •Dorsal column - medial lemniscus pathway fast pathway • Spinothalamic pathway slow pathway These two pathways come together at the level of thalamus
- 42. Dorsal root Dorsal columns Dorsalhorn Dorsal root ganglion Spinothalamic tracts Posterior (dorsal) Anterior (ventral)
- 43. Dorsal column pathway Spinothalamicpathway Lateral Spinothalamic tract Anterior Spinothalamic tract
- 44. Dorsal column pathwaySpinothalamic pathway • touch: fine degree • highly localised touch sensations • vibratory sensations • sensations signalling movement • position sense • pressure: fine degree • Pain • Thermal sensations • Crude touch & pressure • crude localising sensations • tickle & itch • sexual sensations
- 45. Dorsal column nuclei (cuneate& gracile nucleus) Dorsal column Medial lemniscus thalamus thalamocortical tracts sensorycortex internal capsule 1st order neuron 2nd order neuron 3rd order neuron
- 46. dorsal column -medial lemniscus pathway • after entering the spinal cord lateral branch: participates in spinal cord reflexes medial branch: turns upwards • forms the dorsal columns • spatial orientation: medial: lower parts of the body lateral: upper part of the body
- 47. dorsal column -medial lemniscus pathway • synapse in the dorsal column nuclei nucleus cuneatus & nucleus gracilus • 2nd order neuron cross over to the opposite side and ascends upwards as medial lemniscus • as this travels along the brain stem fibres from head and neck are joined (trigeminal) • ends in the thalamus (ventrobasal complex) ventral posterolateral nuclei
- 48. dorsal column -medial lemniscus pathway • spatial orientation in the thalamus medial: upper part of the body lateral: lower part of the body
- 49. Dorsal column nuclei (cuneate& gracile nucleus) Dorsal column Medial lemniscus thalamus thalamocortical tracts sensorycortex internal capsule 1st order neuron 2nd order neuron 3rd order neuron
- 50. spinothalamic pathway • afterentering the spinal cord synapse in the dorsal horn • cross over to the opposite side • divide in to two tracts lateral spinothalamic tract: pain and temperature anterior spinothalamic tract crude touch
- 51. spinothalamic pathway • spatialorientation medial: upper part of the body lateral: lower part of the body
- 53. Dorsal column pathway Spinothalamicpathway Lateral Spinothalamic tract Anterior Spinothalamic tract
- 54. Thalamocortical tracts • fromthe thalamus 3rd order neuron ascends up through the internal capsule • up to the sensory cortex • thalamocortical radiation tracts diverge
- 55. Sensory cortical areas •parietal cortex • a distinct spatial orientation exists
- 61. Sensory cortex • Differentareas of the body are represented in different cortical areas in the sensory cortex • Sensory homunculus somatotopic representation not proportionate distorted map upside down map
- 62. Representation •upside down •distorted concept ofhomunculus Map
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- 65. Sensory cortical areas •Primary somatosensory cortex (SI) postcentral gyrus (Brodmann areas 3a, 3b, 1, 2) • Secondary somatosensory cortex and Somatosensory association cortex Posterior parietal areas (Brodmann areas 5, 7)
- 66. Somatosensory cortex •Functions To localisesomatic sensations To judge critical degree of pressure To identify objects by their weight, shape, form - stereognosis To judge texture of materials To localise pain & temperature
- 67. Somatosensory cortex •Damage tothe sensory cortex results in decreased sensory thresholds inability to discriminate the properties of tactile stimuli Inability to identify objects by touch (astereognosis)
- 68. Secondary somatosensory cortexand Somatosensory association cortex • Located directly posterior to the sensory cortex in the superior parietal lobes • Consists of areas 5 and 7 • Receives synthesized connections from the primary and secondary sensory cortices • Neurons respond to several types of inputs and are involved
- 69. Secondary somatosensory cortexand Somatosensory association cortex • Damage can cause Tactile agnosia inability to recognize objects even though the objects can be felt Spatial neglect This typically happens with non-dominant hemisphere lesions Neglect can be so severe that the individual even denies that their left side belongs to them
- 70. Receptive fields • Thereceptor area which when stimulated results in a response of a particular sensory neuron • Receptive fields of adjacent neurons overlap
- 72. Two-Point Discrimination • Whethera stimulus feels like one sensation or two distinct sensations depends on the size of the receptive fields of the sensory receptors • Different areas of the body have sensory receptors with different sized receptive fields • Smaller receptive fields result in greater sensitivity • Fingers are more sensitive than backs
- 74. Lateral Inhibition • Thecapacity of an excited neuron to reduce the activity of its neighbors • When the skin is touched by an object several sensory neurons in the skin next to one another are stimulated neurons that are firing suppress the stimulation of neighbouring neurons only the neurons that are most stimulated and least inhibited will fire so the firing pattern tends to concentrate at stimulus peaks • Lateral inhibition increases the contrast and sharpness • Weaker signals get weaker, stronger signals get stronger
- 76. Lateral inhibition improves2-point discrimination
- 77. Sensory abnormalities • Varioustypes of sensory abnormalities can occur when the sensory pathways are damaged • Sensory loss, altered sensations or pain could occur as a result • In addition, motor pathways could also be affected resulting in motor weakness
- 78. Types of sensoryabnormalities • Sensory loss • Anaesthesia absence of sensation • Paraesthesia (numbness or pins-needles- sensation) altered sensation • Neuropathic pain • Hemianaesthesia Loss of sensation of one half of the body • Astereognosis • Spatial neglect
- 79. Localisation of theabnormality • Peripheral nerve innervated area affected • Roots dermatomal pattern of sensory loss • Spinal cord a sensory level • Internal capsule one half of the body • Cortical areas Other features
- 80. Examples of sensorylesions or sensory disorders •Carpal tunnel syndrome Median nerve lesion at the wrist Numbness of thumb, index and middle fingers Pain in the hand Pain could radiate upwards
- 81. Examples of sensorylesions or sensory disorders •Polyneuropathy All sensory nerves of both upper and lower limbs are degenerated Numbness of hands and feet Glove and stocking type of sensory loss Diabetic or nutritional neuropathy
- 82. Examples of sensorylesions or sensory disorders • Cervical radiculopathy Cervical root lesion Compression of nerve root as it comes out through intervertebral foramina Numbness and sensory loss of relevant dermatomes Commonly affected are C56 dermatomes
- 83. Examples of sensorylesions or sensory disorders • Spinal cord lesion (cervical myelopathy) • Damage to the spinal cord • Sensory loss or numbness below the level of the spinal cord lesion • eg. Sensory loss at T10
- 84. Examples of sensorylesions or sensory disorders • Sensory stroke Internal capsule lesion Numbness and sensory loss of one side of the body
- 85. Examples of sensorylesions or sensory disorders • Dorsal column disease (eg. Diabetes, tabes dorsalis) • Dorsal column pathways are affected • Vibration, proprioception affected early in disease process
- 86. Examples of sensorylesions or sensory disorders • Syringomyelia Spinal cord central canal lesion Dissociated sensory loss Temperature and pain sensations affected in early in disease process Touch and dorsal column functions not affected