1. Receptors and Sense Organs Dr Fawzia ALRoug, MBBS, Master, Ph.D Assistant Professor, Department of Physiology, College of Medicine, King Khalid University Hospital, Riyadh, Saudi Arabia

2. A sensory System It is a system which gives information about sensory stimuli through skin and the body adjusts its responses through the motor system.

3. Components of Sensory System Receptors Peripheral nerves Spinal cord Tracts Thalamus Thalamocortical projection Somatosensory cortex

4. Receptor Peripheral Nerve Peripheral Nerve Dorsal Horn Of Spinal cord Dorsal Horn Of Spinal cord Tracts Medullary Nuclei Tracts Medullary Nuclei Thalamus Sensory Cortex SI & SII Sensory Cortex SI & SII (Localization & Perception of sensation) Stimulus 3 rd order 2nd order 1 st order

5. How information about internal & external environment reaches the CNS? Via sensors i.e., Receptors that are connected to the CNS by different cables [i.e., ascending tracts].

6. What are the receptors? They are transducers that convert various forms of energy in the environment [internal or external] into action potentials in the afferent neurons.

7. What is the difference between a sense organ and a receptor? A sensory receptor is a specialized structure or a cell that generates Ap in neurons. A sense organ forms when the receptor is associated with non-neural cells surrounding it.

8. What are the forms of energy converted by receptors? MechanicalThermal Electromagnetic [light] Chemical energy [odor, taste, O2 content of blood]

10. How receptors respond to stimuli [different forms of energies]? By converting them into electrical response, generator potential which when reach a threshold value will cause as AP. This is what is called receptor transduction.

11. Do all receptors respond to the same stimulus? NO. Because each receptor is most sensitive to a particular form of energy called its adequate stimulus. The receptor respond to one particular form of energy at a much lower threshold than other receptors respond to this form of energy.

12. Do receptors respond to forms of energy other than their adequate stimulus? YES. But the threshold for these non-specific responses is much higher, e.g. pressure on the eyeball will stimulate rods & cones, but the threshold of these receptors to pressure is higher than threshold of pressure receptors in the skin.

13. Differential Sensitivity of Receptors: How do two types of sensory receptors detect different types of sensory stimuli? This depends on the differential sensitivity of receptors, i.e., each type of receptor is highly sensitive to one type of stimulus for which it’s designed & is not responsive to normal intensities of other types of stimuli. Normally pain receptors are not stimulated by touch or pressure in normal skin, under normal condition

14. Sensory Modalities. What is this? A sensory modality is the type(s) of sensation(s) we experience when a receptor is adequately stimulated, e.g., pain, touch, sound. There is what we call submodality, e.g.: 4 different submodalities of taste: sweet, salt, sour, bitter. Each of them is subserved by distinct type of receptors.

15. Classification of sensory receptors: 1.Teleceptors: (distance receivers), detect events at distance e.g. rods & cones  light; sound: hair cells 2.Exteroceptors: concerned with information of the ext. environment e.g. skin: pain, touch, temperature 3.Interoreceptors: concerned with internal environment, e.g. chemoreceptors, baroreceptors, osmoreceptors 4.Proprioceptors: provide information about body position in space, e.g. receptors in muscle, tendons & joints: muscle spindles, Goli tendon organs 5.Nociceptors: these are pain receptors

17. Free nerve ending Epithelium of Skin Non adapting Pain

18. Another Classification Cutaneous receptors Mechanoreceptors: terminal of AB myelinated fibers Thermoreceptors nociceptors Musculoskeletal receptors proprioceptors Visceral receptors e.g., chemoreceptors, baroceptors, mechanoreceptors

19. Transduction of sensory stimuli into nerve impulses: The generator potentials [=receptor potential]: it’s the change in the membrane potential of the receptor when excited by a stimulus. It’s a non-propagated depolarizing potential resembling EPSP. When the magnitude of the generated potential is 10mV  AP is generated in the sensory nerve.

20. Transduction (receptor potential)  It is a change in membrane potential of a receptor.  Due to: 1) Mechanical deformation of receptor membrane. 2) Application of chemical which binds to the receptor. 3) Change in temperature. 4) Application of electromagnetic energy.  These lead to the opening of selective channels and depolarization of receptor membrane.

21. Generator Potential (Receptor Potential) Non propagated depolarizing potential. It is produced in un-myelinated nerve terminal.

22.  Depolarization of receptor membrane leads to local circuit which initiates action potential at the first node of Ranvier on the nerve attached to the receptor.

24. RECEPTOR POTENTIAL & ACTION POTENTIAL

26. Sensory Transduction

27. Source of generator potential: The generator potential is produced in the unmyelinated nerve terminal of the receptor When pressure stimulus is applied  the terminal fibers of the corpuscle will be deformed  opening of Na+ channels  Na+ influx (to the interior of the fiber)  creates high positivity inside the fiber  receptor potential  then the generator potential depolarizes the sensory nerve at the 1 st node of Ranvier. Once the firing level is reached, action potential is produced.

28. Once the firing level is reached, action potential is produced: The node converts the graded response of the receptor into action potentials The frequency of AP is proportionate to the intensity (magnitude) of the applied stimuli In other way: the more the receptor potential rises above threshold level, the greater becomes the action potential frequency

29. Ionic basis of receptor potentials: Opening of channels of receptor membranes, e.g. Na+ channels as mentioned G proteins: as in rods & cones

30. Properties of receptors: 1- Adaptation:= desensitization It’s the decrease in the frequency of APs in a sensory nerve overtime when a maintained stimulus of constant strength is applied to its receptor. This phenomenon depends on the type of the receptor or sense organ.

31. According to this we have:- 1- Phase receptors:- These are rapidly adapting receptors. e.g. touch receptors 2- Tonic receptors:- These are slowly adapting receptors: its generator potential is prolonged and decays very slowly. e.g. muscle spindles, cold & pain receptors, lung inflation receptors Mechanism of adaptation: Accommodation of the sensory nerve fiber to the generator potential.

32. RESPONSE OF PHASIC & TONIC MECHANORECEPTOR

33. ADAPTATION OF RECEPTOR

34. 2- Excitability: Response of a receptor to an adequate stimulus by producing a non-propagated potential = receptor potential. If magnitude is sufficient  AP will propagate along the sensory nerve.

35. How a receptor be stimulated? 1.Mechanical deformation  open ion channels 2.Application of a chemical to the receptor membrane  open ion channels 3.Change of temperature  change permeability 4.Effect of electromagnetic radiation  (+) G protein Give examples for receptors!

36. 3.Adequate stimulus Each type of receptor is most sensitive to a specific form of energy, called adequate stimulus, the receptor is almost non-responsive to the normal intensities of other forms of energy. e.g. rods & cones are stimulated by light not heat What is the difference between generator potential and action potential? Generator potential Graded Doesn’t obey all or none rule Can be summated Unpropagated Action potential Not Obeys all or non rule Not summated Propagated

37. Sensory receptors in the skin: [Mechanoreceptors] 1- Expanded ends of sensory nerve fibers: - Ruffini endings - Ruffini endings - Merkel’s discs - Merkel’s discs 2- Encapsulated endings: - Meissner’s corpuscles - Meissner’s corpuscles - Pacinian corpuscles - Pacinian corpuscles - Krause’s end-bulbs - Krause’s end-bulbs 3- Naked nerve endings between cells in tissues.

38. Coding of sensory information If we know that: APs in the nerve from a touch receptor are essentially identical to those in the nerve from warmth receptor. So, why Stimulation of a touch receptor causes a sensation of touch and not warmth? And how possible to tell that touch is light or heavy?

39. 1- Doctrine of specific nerve energies: [Modality discrimination] The sensation evoked (=produced) by impulses generated in a receptor depends on the specific part of the brain they ultimately activate. Because specific sensory pathway are discrete, from sense organ to cortex, when nerve pathways from certain sense organ are stimulated  the sensation produced is that for which the receptor is specialized no matter how or where along the pathway the activity is initiated. This is the doctrine of specific nerve energies. So, if a sensory nerve from Pacinian corpuscle in the hand is stimulated at elbow, sensation produced is touch. If stimulated at dorsal column of sp. cord  touch.

40. 2- Projection: [Locality discrimination] This principle is that: No matter where a particular sensory pathway is stimulated along its course to the cortex, the conscious sensation produced is referred to the location of the receptor. Examples: When the cortical receiving area for impulses from the left hand is stimulated  patient reports sensation in the hand not the head.

41. Phantom limb: amputee complain of pain and proprioceptive sensation in the absent limb (phantom limb) as if it were still in place. Mechanisms: The ends of nerves cut at time of amputation often forms neuromas. They may discharge spontaneously or when pressure put on them, impulses are generated in nerve fibers that previously come from sense organs in the amputated limb & sensation produced is projected to where receptors used to be.

42. 3- Intensity discrimination Intensity (strength) discrimination depends on: 1- Number of receptors stimulated: stronger stimuli 1- Number of receptors stimulated: stronger stimuli stimulate more receptors stimulate more receptors 2- Frequency of action potentials (generated by 2- Frequency of action potentials (generated by activity in a given receptor) reaching the cortex. activity in a given receptor) reaching the cortex. R = KS A R = sensation S = intensity of the stimulus K & A = constants

43. N.B. Tactile localization: - one-point localization: ability to localize one-point of touch [locality discrimination]. - tactile discrimination: two-point discrimination, ability to feel two points touched simultaneously as two separate points depends on:- 1- Number of receptors stimulated 1- Number of receptors stimulated 2- degree of overlap in receptive field 2- degree of overlap in receptive field  Overlap   discrimination It’s accurate on tips of fingers, lips & tongue, WHY? 1- greater number of receptors 2- greater number of afferents 3- less convergence 4- more area of cortical representation

44. Note: Labeled line principle: The specificity of nerve fibers for transmitting only one modality of sensation is called “labeled line principle” e.g. if a touch fiber is stimulated by exciting a touch receptor electrically or in any other way, the person perceives touch because touch fibers leads to specific touch areas in the brain. So, the sensation felt depends on which area in the NS the fibers leads [ultimately stimulated].

45. RECEPTIVE FIELD

46. Sensory Pathways

47. DORSAL COLUMN PATHWAY CARRIES FINE TOUCH, POSITION, PRESSURE, VIBRATION, TWO POINT DESRIMINATION stereognosis AFFERENT SENSORY FIBERS Aβ TYPE. VERY FAST VELOCITY 30 – 70 m/s 3 NEURON SYSTEM (SEE THE DIAGRAM) ANTEROLATERAL PATHWAY CARRIES PAIN & TEMPRATURE (lat. Sp.Th) CRUDE TOUCH & PRESSURE ( VENT, Sp. Th) AFFERENT SENSORY FIBERS Aδ (MYELINATED) FAST PAIN C FIBERS( UNMYELINATED) SLOW PAIN RELATIVELY SLOW VELOCITY Aδ – 6 – 30 m/s. C – 0.5 – 2 m/s. 3 NEURON SYSTEM (SEE THE DIAGRAM)

49. Spinal Tracts