Cutaneous receptors

pacinian, Ruffini receptors

1. CUTANEOUS RECEPTORS PRESENTOR: Dr ROHINI MODERATOR: Dr PASCHAL D’SOUZA

2. SHERRINGTON CLASSIFICATION OF RECEPTORS Depending upon the source of stimulus Exteroreceptors Interoreceptors Proprioreceptors

3. Exteroreceptors Receptors responsible for sensing stimulus coming from external environment. Present at or near the surface of the body. Includes: Touch Temperature Nociception

4. Interoreceptors Respond to stretch ,volume and pressure in the wall of viscera, excessive muscle contraction and to overstretching in the visceral organs. Essential in regulating blood flow ,pressure in the CVS ,maintaining respiration etc.

5. Proprioceptors Responds to stimulus arising in muscles ,tendons, joints etc. Essential for coordination of movements and maintenance of posture. Includes Neuromuscular Neurotendinous spindles

6. Depending upon type of stimulus energy Mechanoreceptors Cutaneous receptors Muscle and joint receptors Hair cells Baroreceptors of carotid sinus and aortic arch Thermoreceptors Nociceptors : receptors responding to pain Photorecepetors : rods and cones Chemoreceptors

7. Cutaneous receptors Sensory receptors Located in epidermis as well as dermis of skin Exteroreceptors :informs us about touch , temperature ,pain and pressure. Densly present over face ,finger tips. Includes: Mechanoreceptors Thermoreceptors Nociceptors

8. Mechanoreceptors Touch ,pressure , softness, texture of the stimulus. Includes Merkels disc and Meissners corpuscles Pacinian corpuscles Ruffini ‘s end organs Krause end bulbs Free nerve endings

9. Meissner's corpuscles Meissner's corpuscles localize in the dermis between epidermal ridges. They contain an unmyelinated nerve ending surrounded by Schwann cells . The center of the capsule contains one or more  afferent nerve fibers that generate rapidly adapting action potentials following minimal skin depression.

10. Present in glabrous (smooth, hairless) skin (the fingertips) Their afferent fibers account for about 40% of the  sensory innervation  of the human hand . Efficient in transducing information about the relatively low-frequency vibrations (30–50 Hz) that occur when textured objects are moved across the skin.

11. Pacinian corpuscles Large encapsulated endings located in the subcutaneous tissue   . Has an onion like capsule in which the inner core of membrane lamellae is separated from an outer lamella by a fluid-filled space . one or more rapidly adapting afferent axons lie at the center of this structure.  The capsule again acts as a filter, allowing only transient disturbances at high frequencies (250–350 Hz) to activate the nerve endings. 

12. Rapidly adapting Involved in the discrimination of fine surface textures or high-frequency vibrations. Provide information primarily about the dynamic qualities of mechanical stimuli.

13. Merkel's disks Located in the epidermis, where they are precisely aligned with the papillae that lie beneath the dermal ridges . Account for about 25% of the mechanoreceptors of the hand and are particularly dense in the fingertips, lips, and external genitalia. 

14. The slowly adapting nerve fiber associated with each Merkel's disk enlarges into a saucer-shaped ending that is closely applied to another specialized cell containing vesicles that apparently release peptides that modulate the nerve terminal . Selective stimulation of these receptors in humans produces a sensation of light pressure. 

15. IGGO DOME RECEPTORS Merkel's discs are often grouped together in a receptor organ called the  Iggo dome receptor, which projects upward against the underside of the epithelium of the skin. This causes the epithelium at this point to protrude outward, thus creating a dome and constituting an extremely sensitive receptor. 

16. RUFFINI  CORPUSCLES These elongated, spindle-shaped capsular specializations are located deep in the skin, as well as in ligaments and tendons . The long axis of the corpuscle is usually oriented parallel to the stretch lines in skin . Particularly sensitive to the cutaneous stretching produced by digit or limb movements. 

17. Ruffini corpuscle from original slide sent by Ruffini to Sir Charles Sherrington

18. HAIR FOLLICLE RECEPTOR Unencapsulated Primary afferent spiral around hair follicle base. Runs parallel to the hair shaft to form a lattice like pattern. Signals the direction and velocity of movement. Rapidly adaptating

19. Krause end bulb Type of meissners receptor Mainly present in glabrous skin such as skin of genitalia, papillae of lips,conjunctiva etc Afferent fibres belong to A delta group Detects fine touch and pressure

20. Adaptation in cutaneous receptors Rapid adaptation means that there is no response to sustained pressure, only to changes in pressure (either an increase or a decrease). Slow adaptation means that the receptor continues to respond to pressure for as long as it is sustained, within some reasonable time frame. ADAPTATION RAPIDLY SLOWLY

21. RAPIDLY ADAPTING PACINIAN CORPUSCELS MEISSNER'S CORPUSCLES High frequency vibration , pressure Light touch SLOWLY ADAPTING MERKELS DISC RUFFINI Low frequency vibration and texture Skin stretching

22. Receptive fields of cutaneous receptors Portion of the skin which, when stimulated, affects the activity or state of the receptor.

23. Receptive Fields — the typically oval areas of skin along which a mechanical stimulus (e.g., touch) leads to a change in that neurons action potential firing rates. In particular, Pacinian corpuscles have fairly large receptive fields, making them more sensitive (if you consider the goal of a mechanoreceptor to sense touch anywhere on the body) Meissner’s corpuscles have fairly small receptive fields, making them more specific 

24. The  A-beta fibers  are large, fast, myelinated afferents. The  Meisnner's corpuscles  and  Pacinian corpuscles  belong to this class. These are rapidly adapting touch afferents. 

25. Pathways from Skin to Cortex Two major pathways in the spinal cord: – Medial lemniscal pathway consists of large fibers that carry proprioceptive and touch information . Spinothalamic pathway consists of smaller fibers that carry temperature and pain information . These cross over to the opposite side of the body and synapse in the thalamus, and then on to the Somatosensory cortex

26. Signals travel from the thalamus to the somatosensory receiving area (S1) and the secondary receiving area (S2) in the parietal lobe . Body map (homunculus) on the cortex shows more cortical space allocated to parts of the body that are responsible for detail .

27. THERMORECEPTORS Thermoreceptors are free dendrite endings in skin, and thus are primary sensory organs. They have no specialized epithelial cells or supporting cells. The nerves of skin branch from musculocutaneous nerves that arise segmentally from spinal nerves. The pattern of nerve fibers in skin is similar to the vascular patterns—nerve fibers form a deep plexus, then ascend to a superficial, subpapillary plexus.

28. The penicillate fibers are the primary nerve fibers found subepidermally in haired skin. Rapidly adapting receptors that function in the perception of touch, temperature, pain, and itch. Overlapping innervation,leads discrimination to be generalized. FREE NERVE ENDING Penicillate Papillary

29. Papillary nerve endings are found at the orifice of a follicle and are thought to be particularly receptive to cold sensation. Hair follicles also contain , slow-adapting receptors that respond to the bending or movement of hairs.

30. A very high concentration of thermosensitive TRP (Transient Receptor Potential ) ion channels are found in keratinocytes in the epidermis. Each TRP has a unique temperature threshold of firing.

31. THERMORECEPTIVE NEURONS C-FIBRES 1.Heat sense neurons 2.Unmyelinated 3.Less velocity of transduction 4.Innervated epidermis 5.Fewer dendrites per neuron ,small receptive fied A DELTA FIBRES 1.Cold sense neuron 2.Myelinated 3.More velocity of transduction 4.Innervated layers between epidermis and dermis 5.High receptive field

32. Cell bodies of the afferent fibers of cutaneous thermoreceptors reside in the dorsal root ganglion (DRG) or the trigeminal ganglion on the dorsal horn of the spinal cord. The trigeminal neuron is particularily sensitive to cold due to its high expression of cold-activated TRP ion channels.

33. NOCICEPTORS Mediate pain Terminal branches of thin myelinated A delta and unmyleniated C fibres Somatic nociceptors : free nerve endings in skin Visceral nociceptors : not well known Nociceptors are generally electrically silent and transmit all-or-none action potentials only when stimulated.

34. Nociceptive fibers have been classified on the basis of their conduction velocity and sensitivity and threshold to noxious mechanical (M), heat (H), and cold (C) .  C fibres :(C-MH, C-MC, C-MHC) A-fiber nociceptors are predominately heat- and or mechanosensitive (A-MH, A-H, A-M)

35. Excitatory neurons and release glutamate as their primary neurotransmitter as well as other components including peptides (e.g., substance P, calcitonin gene-related peptide [CGRP], somatostatin .)