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Y2 s1 sensory system

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Y2 s1 sensory system

  1. 1. Sensory receptors, sensory pathways & sensory cortex Prof. Vajira Weerasinghe Professor of Physiology www.slideshare.net/vajira54
  2. 2. Sensory functions • Humans do not have receptor for every possible stimuli • There are different sensory modalities that human brain can perceive • Arrival of information is sensation
  3. 3. Sensory Functions • General Sensations Physical (touch, pressure, vibration, stretch) Temperature Pain Chemical • Special Sensations Vision Hearing Taste Smell
  4. 4. Modality specificity • Stimulation of 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. 5. Sensory pathway • Once a receptor is stimulated impulse travels through a particular pathway known as sensory pathway or ascending pathway up to the brain
  6. 6. Receptor Sensory modality Sensory nerve Central Connections Ascending Sensory pathway Sensory area in the brain Touch stimulus AFFERENT Sensory pathway
  7. 7. Receptors • Receptor cells are 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. 8. Classification of receptors • Mechanoreceptors • Thermoreceptors • Nociceptors pain • Chemoreceptors taste, smell, visceral • Electromagnetic receptors visual Guyton p.496
  9. 9. Mechanoreceptors • Mainly cutaneous Touch Pressure Vibration • Crude or Fine mechanosensations • Others: auditory, vestibular, stretch, proprioceptors
  10. 10. Cutaneous mechanoreceptors • Pacinian corpuscle • Meissner’s corpuscle • Krause’s corpuscle • Ruffini’s end organ • Merkel’s disc • Hair end organ • Free nerve endings
  11. 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. 12. Mechanoreceptors • Hair end organ • Free nerve endings Crude mechanosensations (Pain, temperature)
  13. 13. 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
  14. 14. Pacinian Corpuscle Capsule Nerve fibre
  15. 15. What happens inside a 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
  16. 16. Receptor potentials • When a 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
  17. 17. Transduction Stimulus Receptor potential (Generator potential) Action potential
  18. 18. Action Potentials Threshold Resting Membrane Potential -70 - 55 +30 Stimulus Receptor potential
  19. 19. Coding of sensory stimuli • Stimulus strength is coded as the frequency of AP • Higher the stimulus more frequent are the APs • Amplitude of AP is constant
  20. 20. Stimulus Receptor potentials Action potentials
  21. 21. Sensory coding • A receptor 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
  22. 22. Transduction in different receptors • Different receptors have different ion channels • Their opening causes receptor potential
  23. 23. Receptor potential generation in the Pacinian corpuscle
  24. 24. Pacinian corpuscle
  25. 25. Resting
  26. 26. Physical Stimulus Physical stimulus causing mechanical deformation on the capsule
  27. 27. Physical Stimulus Mechanical deformation is transmitted to the inside Opens up mechanosensitive Na+ channel Causes depolarisation and thus receptor potential
  28. 28. Physical Stimulus local current Current flow through a local circuit
  29. 29. Physical Stimulus Action Potentials are generated Opening of voltage gated Na+ channels causes generation of action potentials
  30. 30. Adaptation • “getting used to” • after a period of time sensory receptors adapt partially or completely • different types Rapidly adapting receptors slowly adapting receptors
  31. 31. Adaptation • after a period of time sensory receptors adapt partially or completely • different types fast adapting receptors slowly adapting receptors
  32. 32. Pacinian corpuscle Muscle spindle Pain Time Impulsespersecond
  33. 33. Mechanism of adaptation • In the Pacinian corpuscle mechanical deformation is transmitted throughout the capsule and pressure redistributes Na+ channels inactivates after some time
  34. 34. Impulse Stimulus Redistribution of pressure inside the capsule No Impulse Stimulus
  35. 35. • Rapidly adapting receptors 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
  36. 36. 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)
  37. 37. Two ascending pathways • Dorsal column - medial lemniscus pathway fast pathway • Spinothalamic pathway slow pathway These two pathways come together at the level of thalamus
  38. 38. Dorsal root Dorsal columns Dorsal horn Dorsal root ganglion Spinothalamic tracts Posterior (dorsal) Anterior (ventral)
  39. 39. Dorsal column pathway Spinothalamic pathway Lateral Spinothalamic tract Anterior Spinothalamic tract
  40. 40. Dorsal column pathway Spinothalamic 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
  41. 41. 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
  42. 42. 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
  43. 43. 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
  44. 44. dorsal column - medial lemniscus pathway • spatial orientation in the thalamus medial: upper part of the body lateral: lower part of the body
  45. 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. 46. spinothalamic pathway • after entering 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
  47. 47. spinothalamic pathway • spatial orientation medial: upper part of the body lateral: lower part of the body
  48. 48. Dorsal column pathway Spinothalamic pathway Lateral Spinothalamic tract Anterior Spinothalamic tract
  49. 49. Thalamocortical tracts • from the thalamus 3rd order neuron ascends up through the internal capsule • up to the sensory cortex • thalamocortical radiation tracts diverge
  50. 50. Sensory cortical areas • parietal cortex • a distinct spatial orientation exists
  51. 51. Sensory cortex • Different areas of the body are represented in different cortical areas in the sensory cortex • Sensory homunculus somatotopic representation not proportionate distorted map upside down map
  52. 52. Representation •upside down •distorted concept of homunculus Map
  53. 53. Sensory homunculus
  54. 54. Brodmann areas
  55. 55. 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)
  56. 56. Somatosensory cortex •Functions To localise somatic 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
  57. 57. Somatosensory cortex •Damage to the sensory cortex results in decreased sensory thresholds inability to discriminate the properties of tactile stimuli Inability to identify objects by touch (astereognosis)
  58. 58. Secondary somatosensory cortex and 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
  59. 59. Secondary somatosensory cortex and 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
  60. 60. Receptive fields • The receptor area which when stimulated results in a response of a particular sensory neuron • Receptive fields of adjacent neurons overlap
  61. 61. Two-Point Discrimination • Whether a 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
  62. 62. Lateral Inhibition • The capacity 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
  63. 63. Lateral inhibition improves 2-point discrimination
  64. 64. Sensory abnormalities • Various types 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
  65. 65. Types of sensory abnormalities • 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
  66. 66. Localisation of the abnormality • 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
  67. 67. Examples of sensory lesions 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
  68. 68. Examples of sensory lesions 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
  69. 69. Examples of sensory lesions 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
  70. 70. Examples of sensory lesions 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
  71. 71. Examples of sensory lesions or sensory disorders • Sensory stroke  Internal capsule lesion  Numbness and sensory loss of one side of the body
  72. 72. Examples of sensory lesions or sensory disorders • Dorsal column disease (eg. Diabetes, tabes dorsalis) • Dorsal column pathways are affected • Vibration, proprioception affected early in disease process
  73. 73. Examples of sensory lesions 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