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Sensory system

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Sensory system

  1. 1. Sensory System
  2. 2. General senses • Receptors that are widely distributed throughout the body • Skin, various organs and jointsSpecial senses • Specialized receptors confined to structures in the head • Eyes, ears, nose and mouth 2
  3. 3. Receptors, Sensation, and Perception• Sensory receptors • Specialized cells or multicellular structures that collect information from the environment • Stimulate neurons to send impulses along sensory fibers to the brain• Sensation • A feeling that occurs when brain becomes aware of sensory impulse• Perception • A person’s view of the stimulus; the way the brain interprets the information 3
  4. 4. Receptor Types• Chemoreceptors • Respond to changes in chemical concentrations• Pain receptors (nociceptors) • Respond to tissue damage• Thermoreceptors • Respond to changes in temperature• Mechanoreceptors • Respond to mechanical forces• Photoreceptors • Respond to light• Osmoreceptors • Respond to changes in solute concentration 4
  5. 5. Modalities Heat Light sound pressurechemical change
  6. 6. • Receptors transform an external signal into a membrane potential• Receptor Potential - separate receptor• Generator Potential - a specialized ending of an afferent neuron
  7. 7. Sensory Impulses• Stimulation of receptor causes local change in its receptor potential• A graded electrical current is generated that reflects intensity ofstimulation• If receptor is part of a neuron, the membrane potential may generatean action potential• If receptor is not part of a neuron, the receptor potential must betransferred to a neuron to trigger an action potential• Peripheral nerves transmit impulses to CNS where they are analyzedand interpreted in the brain 7
  8. 8. Sensory Adaptation The ability to diminish the extent of their depolarization despite sustained stimulus strengthTypes of receptors according to their speed ofadaptation• Tonic(Do not adapt or adapt slowly e.g. muscle stretchreceptors and joint proprioceptors)• Phasic(Rapidly adapting receptors e.g. Tactile receptors) 8
  9. 9. Mechanism of adaptation
  10. 10. Nerve Fiber ClassificationGeneral classification scheme – A fibers: Myelinated • Subtypes: some overlap in ranges • Fastest conducting and largest diameter – m/sec, – B fibers: Slower myelinated – C fibers: Un myelinated • Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 )
  11. 11. Nerve Fiber ClassificationSensory nerve classification – I, II, III fibers: Myelinated • Subtypes: Ia, Ib • Fastest conducting and largest diameter – Ia – IV fibers: Unmyelinated • Slower conducting than IIIs and smallest diameter
  12. 12. Summation• Spatial• TemporalRelaying of signals through neuronal pools• Threshold---Subthreshold stimuli• Excitation or FacilitationDivergence and Convergence of signals passing through neuronal poolAfter Discharge• Synaptic after discharge• Reverberatory circuit• Continuous signal output• Rhythmical signal output
  13. 13. General Senses• Senses associated with skin, muscles, joints and viscera• Three (3) groups: • Exteroceptive senses (exteroceptors) • Senses associated with body surface such as touch, pressure, temperature, and pain • Visceroceptive senses (interoceptors) • Senses associated with changes in the viscera such as blood pressure stretching blood vessels and ingestion of a meal • Proprioceptive senses • Senses associated with changes in muscles and tendons such as at joints 13
  14. 14. Touch and Pressure SensesFree nerve endings Tactile (Meissner’s) corpuscles • Common in epithelial tissues • Abundant in hairless portions of • Simplest receptors skin and lips • Sense itching • Detect fine touch; distinguish between two points on the skin •Adapt rapidlyExpanded Tip receptors (Merkel’sdiscs---- Iggo dome receptors • Initially strong but partially Hair end organ adapting •Adapts readily •Localize touch and determine •Touch (movement of object on texture surface of body + initial contact)Lamellated (Pacinian) corpuscles Ruffini’s endings • Common in deeper • Slow adaptation subcutaneous tissues, tendons • found in joint capsules and ligaments • Detect heavy pressure and vibrations 14
  15. 15. Touch and Pressure Receptors Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Free nerve endings Section of skin Epithelial cellsEpidermis Sensory (a) nerve fiber Epithelial cellsDermis Tactile (Meissner’s) corpuscle (touch receptor) Sensory nerve fiber (b) Lamellated (Pacinian) corpuscle (pressure receptor) Connective tissue cells Sensory nerve fiber (c) b, c: © Ed Reschke 15
  16. 16. Summary of Receptors of the General Senses 17
  17. 17. Dorsal column Medial leminiscal system Anterolateral System• Fine touch • Pain• Vibration • Temperature• Position • Crude touch• Pressure • Tickle and itch • Sexual sensations
  18. 18. Layers of somatosensory cortex• Incoming sensory signals excite layer VI• I & II Receive signals from lower centers of brain(Controls excitability of respective area stimulated)• II &III send axons to related portions on opposide side of cortex through corpus callosum• V & VI Send axons to deeper parts of brainV ( more distant areas)VI ( Thalamus)
  19. 19. Functions of somatosensory Area I• Localize• Determine Pressure against body• Judge weights of objects• Judge shapes or forms• Judge texture
  20. 20. • Two point discrimination• Vibratory sense• Position sense
  21. 21. Two point discrimination
  22. 22. Vibratory sense Position sense Multiple types of receptors for joint angulation Spindle (mid ranges of motion)30-800 cycles/sec (Pacinian) Pacinian, Ruffinis, Golgi tendon2- 80 cycles/sec(Meissners) Extremes of joint angulation