Psych b sensory system

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Psych b sensory system

  1. 1. Sensory Functions • General Sensations Mechanical – touch, pressure, vibration, stretch Chemical • Special Sensations Vision Hearing Taste Smell
  2. 2. Sensory area in the brain FF A NT RE E Touch stimulus Receptor Sensory modality Ascending Sensory pathway Central Connections Sensory nerve
  3. 3. • stimulation of a receptor usually produces only one sensation modality specific • But some receptors are stimulated by more than one sensory modality eg. free nerve endings
  4. 4. Classification of receptors • Mechanoreceptors • Thermoreceptors • Nociceptors pain • Chemoreceptors taste, smell, visceral • Electromagnetic receptors visual Guyton p.496
  5. 5. Mechanoreceptors • Mainly cutaneous Touch Pressure Vibration • Crude or Fine mechanosensations • Others: auditory, vestibular, stretch
  6. 6. Mechanoreceptors • Pacinian corpuscle • Meissner’s corpuscle • Krause’s corpuscle • Ruffini’s end organ • Merkel’s disc • Hair end organ • Free nerve endings Guyton p.496
  7. 7. Mechanoreceptors • Pacinian corpuscle deep, pressure sensitive, fast adapting • Ruffini’s end organ deep, tension sensitive, slow adapting • Merkel’s disc superficial, sensitive to deformation of skin, slowly adapting Guyton p.496
  8. 8. Mechanoreceptors • Meissner’s corpuscle superficial, sensitive to sideways movements • Krause’s endings • Hair end organ • Free nerve endings Guyton p.496
  9. 9. Pacinian Corpuscle Nerve fibre Capsule
  10. 10. 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
  11. 11. Transduction Stimulus Receptor potential (Generator potential) Action potential
  12. 12. Receptor potentials • are graded • do not follow all-or-none law • amplitude depends on the strength • of the stimulus when reaches the threshold: triggers the action potentials
  13. 13. Action Potentials Threshold Resting Membrane Potential
  14. 14. Resting
  15. 15. Receptor potentials • mechanical transformation on the capsule
  16. 16. Physical Stimulus
  17. 17. Receptor potentials • Opens up Na+ channels • Na+ influx
  18. 18. Physical Stimulus
  19. 19. Receptor potentials • Membrane inside the capsule: • depolarisation thus receptor potential is generated
  20. 20. Physical Stimulus local current
  21. 21. Physical Stimulus Action Potentials are generated
  22. 22. • Stimulus strength is coded as the • • frequency of AP Higher the stimulus more frequent are the APs Amplitude of AP is constant
  23. 23. Action potentials Receptor potentials Stimulus
  24. 24. Adaptation • “getting used to” • after a period of time sensory receptors adapt partially or completely • different types fast adapting receptors slowly adapting receptors
  25. 25. Adaptation • after a period of time sensory • receptors adapt partially or completely different types fast adapting receptors slowly adapting receptors
  26. 26. Impulses per second Pain Muscle spindle Pacinian corpuscle Time
  27. 27. Mechanism of adaptation • In the Pacinian corpuscle mechanical deformation is transmitted throughout the capsule and pressure redistributes Na+ channels inactivates after some time
  28. 28. Stimulus Impulse Redistribution of pressure inside the capsule Stimulus No Impulse
  29. 29. • 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
  30. 30. Sensory area in the brain Ascending Sensory pathway Central Connections Sensory nerve Touch stimulus Receptor Sensory modality
  31. 31. Two ascending pathways • Dorsal column - medial lemniscus pathway fast pathway • Spinothalamic pathway slow pathway These two pathways come together at the level of thalamus
  32. 32. Posterior (dorsal) Dorsal root ganglion Dorsal root Dorsal columns Dorsal horn Spinothalamic tracts Anterior (ventral)
  33. 33. Spinothalamic pathway Dorsal column pathway Lateral Spinothalamic tract Anterior Spinothalamic tract
  34. 34. Dorsal column pathway • • • • • • touch: fine degree highly localised touch sensations vibratory sensations sensations signalling movement position sense pressure: fine degree Spinothalamic pathway • • • • • • Pain Thermal sensations Crude touch & pressure crude localising sensations tickle & itch sexual sensations
  35. 35. n se x te or internal capsule c ry so thalamocortical tracts 3rd order neuron thalamus Medial lemniscus Dorsal column nuclei 2nd order neuron (cuneate & gracile nucleus) Dorsal column 1st order neuron
  36. 36. Dorsal column medial lemniscus pathway sensory cortex thalamocortical tracts thalamus M ed ul la Medial lemniscus 3rd order neuron cross over Dorsal column nuclei 2nd order neuron (cuneate & gracile nucleus) Dorsal column Dorsal root Dorsal root ganglion 1st order neuron
  37. 37. 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
  38. 38. 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
  39. 39. dorsal column - medial lemniscus pathway • spatial orientation in the thalamus medial: upper part of the body lateral: lower part of the body
  40. 40. n se x te or internal capsule c ry so thalamocortical tracts 3rd order neuron thalamus Spinothalamic tracts 2nd order neuron 1st order neuron
  41. 41. Spinothalamic pathway sensory cortex thalamocortical tracts 3rd order neuron thalamus anterior lateral 2nd order neuron Spinothalamic tracts cross over Dorsal horn Dorsal root Dorsal root ganglion 1st order neuron
  42. 42. 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
  43. 43. spinothalamic pathway • spatial orientation medial: upper part of the body lateral: lower part of the body
  44. 44. Note the spatial orientation of
  45. 45. Spinothalamic pathway Dorsal column pathway Lateral Spinothalamic tract Anterior Spinothalamic tract
  46. 46. thalamocortical tracts • from the thalamus 3rd order neuron • • ascends up through the internal capsule up to the sensory cortex thalamocortical radiation tracts diverge
  47. 47. Sensory cortical areas • parietal cortex • a distinct spatial orientation exists
  48. 48. 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
  49. 49. Sensory homunculus
  50. 50. Brodmann areas
  51. 51. Sensory cortical areas • primary somatosensory cortex (SI) postcentral gyrus (Brodmann areas 3a, 3b, 1, 2) • secondary somatosensory cortex (SII) parietal cortex behind postcentral gyrus (Brodmann areas 5, 7)
  52. 52. somatosensory cortex Functions • Localisation of somatic sensations • to judge critical degree of pressure • identify objects by their weight, shape, form - stereognosis • to judge texture of materials • localisation of pain & temperature
  53. 53. Abnormalities • Sensory loss • Anaesthesia absence of sensation • Paraesthesia abnormal sensation • Hemianaesthesia Loss of sensation of one half of the body • Astereognosis
  54. 54. Localisation of the abnormality • Peripheral nerve part of a limb is affected • Roots dermatomal pattern of sensory loss • spinal cord a sensory level • internal capsule one half of the body

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