GENERAL SENSORY PHYSIOLOGY I

1. GENERAL SENSORY PHYSIOLOGY I Dr. S. I. OGUNGBEMI Department of Physiology Faculty of Basic Medical Sciences College of Medicine University of Lagos

2. INTRODUCTION • Course Outline • Sensory receptor • Types • Properties: receptor potentials and adaptation • General sensations • Skin or cutaneous sensations • Touch • Pressure • Pain • Temperature • Vibration • Itch and tickle

3. Muscle and joint sensations • Proprioception • Joint vibration • Visceral sensations of internal environment: pH, arterial blood pressure, blood chemical sensations, thirst, hunger, volume, sensations and so forth. • Special sensation • 5 special senses: olfaction; vision; audition; gustation; rotational and linear acceleration → Sensory Physiology II • Thalamus – sensory relay station to the somatosensory cortex • Somatosensory cortex – the ultimate interpretative and integrative centre for body sensations

4. What are Sensory Receptors? • Sensory receptors are special nerve endings that convert any stimuli that excite them to receptor potentials. • Sensory receptors are gateway via which stimuli (energy) from internal and external environments are converted into action potentials and transmitted to the central nervous system via the afferent pathways.

5. CLASSIFICATION OF SENSORY RECEPTORS • Sensory receptors are classified accordingly as follow: • According to the source of their stimuli: • Interoceptor – concerned with internal stimuli • Exteroceptors – concerned with external stimuli at hand • Teloceptors – concerned with stimuli at a distant • Proprioceptors – stimuli about the position of the body in space Conscious components of proprioception is produced from stimuli from proprioceptors in and around the joints and cutaneous touch and pressure receptors.

6. According to the types of stimulus energy: • Mechanoreceptors • They detect mechanical compression or stretching of the receptors or tissues adjacent to the receptor. • Thermoreceptors • They detect changes in temperature either cold or warmth. • Nociceptors • They detect noxious stimuli owing to physical or chemical damage to the tissues from pains and extreme warmth, cold and other cutaneous stimuli. • Photoreceptors or electromagnetic receptors • They detect light on the retina in the eyes.

7. Chemoreceptors • They detect taste, smell, osmolality, arterial blood pressure, O2 and CO2 tension, pH, blood glucose and so on. • According to the types sensations: • Touch; pressure; cold; warmth; pain; olfactory; visual or light; auditory or sound; taste and vestibular receptors; proprioceptor; baroreceptor and so forth. • According to receptor adaptation: • Rapidly adapting or phasic receptors e.g. touch receptor (pacinian corpuscle) or • Slowly adapting receptor e.g. muscle spindle • According their nerve fibres types: see the tables below:

9. Mechanoreceptors • Skin tactile sensibilities (epidermis and dermis) • Free nerve endings • Expanded tip endings • Merkel’s discs • Plus several other variants • Spray endings • Ruffini’s endings • Encapsulated endings • Meissner’s corpuscles • Krause’s corpuscles • Hair end-organs

10. Deep tissue sensibilities • Free nerve endings • Expanded tip endings • Spray endings • Ruffini’s endings • Encapsulated endings • Pacinian corpuscles • Plus a few other variants • Muscle endings • Muscle spindles • Golgi tendon receptors

11. Hearing • Sound hair cells in the organ of Corti of cochlea • Equilibrium • Vestibular receptors or hair cells in the otolith organ • Arterial pressure • Baroreceptors of carotid sinuses and aorta • Thermoreceptors • Cold • Cold receptors • Warmth • Warm receptors

12. Nociceptors • Pain • Free nerve endings • Photoreceptors or Electromagnetic receptors • Vision • Rods • Cones • Chemoreceptors • Taste • Receptors of taste buds • Smell • Receptors of olfactory epithelium • Arterial oxygen • Receptors of aortic and carotid bodies

13. Osmolality • Neurons in or near supraoptic nuclei • Blood CO2 • Receptors in or on surface of medulla and in aortic and carotid bodies • Blood glucose, amino acids, fatty acids • Receptors in hypothalamus

14. Differential Sensitivity of Receptors • A sensory receptor is adapted to transduce one particular form of energy to the central nervous system at a lower threshold than other receptors. • This particular form of energy to which a receptor is most sensitive is its adequate stimulus e.g. adequate stimulus to rods and cones is light or colour. • Cones are completely nonresponsive to heat, cold, pain, pressure or chemical change in the blood. • Pain receptors are never stimulated by pressure or heat except when they cause damage to tissues.

15. The various sensory afferents transmit just only impulses to produce the different sensory modalities that we have. • To interpret simple impulses to various modalities, each nerve must ultimately terminate and activate specific areas in the sensory cortex. • When the nerve pathways from a particular sense organ are stimulated, the sensation evoked is that for which the receptor is specialised. • A pacinian corpuscle can be stimulated to produce touch at its location in the hand using an electrode via:

16. Nerve in the elbow • Brachial plexus • Dorsal column (III-VI) of the spinal cord • Areas in thalamus and • Postcentral gyrus of the sensory cortex. • Stimulation of cortical receiving areas for limbs produces sensation in the appropriate limb even in an amputee. • See laws of specific nerve energies and projection. • Intensity Discrimination • As the stimulus increases, the frequency of impulses or action potentials is also increases.

17. In addition, higher threshold receptors are recruited to increase the number of receptor stimulated. • So, the increases in receptor stimulated and frequency of impulses determine the intensity of the stimulus. • Similarly, the magnitude of sensation felt is proportionate to the log of intensity of stimulus → R = KSA: • R = the sensation felt; S = stimulus intensity and K & A = constants • The density of receptors of any tissue is proportional to the sensitivity of that tissue in the body. • touch receptors are more numerous in the hand than on the trunk.

19. PROPERTIES OF SENSORY RECEPTORS • Production of Receptor Potential • Receptor potentials are produced by adequate stimulus in various sensory receptors by various mechanisms. • Application of a sub-threshold stimulus to a sensory receptor gives rise to receptor potential (RP). • RP is a positive transmembrane potential • This RP is a nondepolarising subthreshold potential (like EPSP) when recorded on a sensory nerve of any receptor. • The magnitude of RP increases with increase in stimulus strength or frequency.

20. RPs then fire action potentials (AP) or impulses down the sensory neurone at RP ≥ 10 mV. • As stimulus strength increases, RP increases to maximum (≈ 100 mV) and maximum frequency of AP is produced. • This increase in frequency “f” is a logarithmic function of stimulus intensity → i.e. f α log Ia . • The mechanisms of propagating RP varies from one sensory receptor to another. See a few examples below

21. In mechanoreceptors, mechanical force deforms: • Membranes of sensory dendrites of touch and pressure receptors • Hair cells of sensory nerve endings in vestibular and Corti organs. • In nociceptors, pain mediators (from damaged tissues) irritate membranes of free nerve endings for pain. • In photoreceptors, photochemical reaction affects ionic permeability of rod and cone membrane in the retina. • In chemoreceptors, dissolved metabolites have interactions with receptor membranes and alter ionic permeability. • e.g. in carotid body, olfactory and gustatory cells.

22. Ionic Bases for Receptor Potential • Receptor potential (RP) is then generated by increase in membrane conductance to cations by increase in: • Influx of Na⁺ and Ca²⁺ in most sensory receptors and influx of K⁺ in vestibulo-cochlear hair cells generate positive potentials. • Closure of Na⁺ channel in rods and cones lead to influx of cations in afferent neurones (i.e. ganglion cells) • Closure of K⁺ and Cl⁻ channels in some sensory receptors to generate positive membrane potentials and • Inactivation of Na⁺- K⁺ ATPase pumps • These ionic changes produce protomotive force that generate action potential as the case may be.

25. Receptor Adaptation • Application of maintained stimulus of constant strength to a receptor declines the frequency of APs in its sensory nerve over time as if the stimulus were not there. • Degree of adaptation of different sensory receptors varies, giving rise to rapidly adapting and slowly adapting receptors. • Rapidly adapting receptors detect change in stimulus strength following an initial adaptation, because they are phasic. • They include tactile receptors (e.g. pacinian corpuscles), olfactory receptors, cone cells in the retina.

26. They don’t transmit continuous signals. • e.g., sudden pressure application on pacinian corpuscle produces APs for a few msec. • Then, the APs decay in spite of continuous pressure application until there is change in pressure strength before it transmit another APs. • Slowly adapting receptors are tonic receptors that transmit signals for many hours. • They include receptors of the macula in the vestibular apparatus; pain receptors; baroreceptors of the arterial tree and chemoreceptors of the carotid and aortic bodies, muscle spindle

27. Importance: • Pacinian corpuscle is important in apprising the CNS of rapid tissue deformations. • So, light touch would be distracting if it were persistent. • Slow adaptation of muscle spindle and Golgi tendon input is needed to maintain, status of muscle contraction, load on the muscle tendon and posture at each instant. • Inputs from nociceptors are needed in apprising the CNS of the damaged tissues of the body for conscious protection from further injury.

28. Ionic Bases or Mechanism for Adaptation • The following ionic changes explain receptor adaptation. • Closure of Na⁺ and Ca²⁺ channels • Opening of K⁺ and Cl⁻ channels and • Activation of Na⁺- K⁺ ATPase pumps • These ionic changes reduces protomotive force across receptor or afferent membranes, resulting in adaptation.

29. The height of the curve in each case indicates the frequency of the discharge in afferent nerve fibers at various times after beginning sustained stimulation.

30. Other Properties of Afferent Nerves • Divergence and Convergence • In divergence, an afferent neurone has several branches and synapses on several efferent neurones. • In convergence, several efferent neurones synapse on the same, single or common efferent neurone. • Convergence and divergence allow phenomenon like summation, facilitation, occlusion and inhibition in the central nervous system. • Summation: classified as spatial and temporal summation

31. Spatial Summation: subthreshold stimuli in 2 or more afferents add up at the same time to cause excitation of the post-synaptic membrane. • Temporal Summation: stimuli in the same afferent coming in close succession in time lead to summation and excitation of the post-synaptic membrane. • Facilitation: if 2 afferent neurones, ‘a’ and ‘b’, are stimulated separately to give responses ‘x’ and ‘y’ respectively. • Then stimulation of ‘a’ and ‘b’ at the same time would produce a facilitation (x + y + n) than just summation (x + y) owing to overlap of subliminal fringes.

32. If neurone ‘a’ innervates 4 and ‘b’ innervates 6 afferents. • Stimulation of ‘a’ and ‘b’ will result to innervations of 4 and 6 afferents respectively. • Each time ‘a’ or ‘b’ is stimulated, an area of subthreshold excitation of subliminal fringe is created. • Simultaneous stimulation of ‘a’ and ‘b’ produces overlap of the 2 subliminal fringes, creating additional 3 neurones. • Thus, making facilitation responses (4+6+3 = 13) to exceed summation of individual responses of ‘a’ and ‘b’ (4+6 = 10). • Occlusion: when 2 afferents are stimulated simultaneously to produce less responses than the sum of the individual responses – then occlusion has occurred.

33. Occlusion occurs due to sharing of the same efferent neurones by 2 or more afferents. • If neurones ‘a’ and ‘b’ innervate 4 and 6 efferents respectively and share 2 of these efferents, then simultaneous stimulation of ‘a’ and ‘b’ produces less (4+6-2 = 8) efferents than the addition (4+6 = 10) of their separate stimulations at any instance. • After-Discharge is the persistence of the response or sensation after the stimulus is over owing to reverberatory circuits (followings an initial response). • The reverberatory circuits produce longer, continuous, and persistent excitatory responses or sensation.

34. Post-Tetanic Facilitation is increase in response of post-synaptic membrane following a brief but rapid tetanic series of impulses. • It occurs as a result of excessive Ca²⁺ or neurotransmitter release during stimulation. • Fatigue occurs due to excessive and continuous stimulation at a high frequency, leading to the exhaustion of the neurotransmitters, Ca²⁺, cations and water. • During fatigue, the neurone in question produces diminished responses in spite of the presence of adequate stimulus or excitation.

39. GENERAL SENSORY PATHWAYS • Tactile Sensations • Tactile sensations: touch, pressure, vibration, (apart from tickle) are detected by similar tactile receptors. • Light touch sensation results from stimulation of tactile receptors in the skin or tissue beneath the skin. • Pressure sensation results from deformation of deeper tissues and fascia. • Vibration sensation results from rapidly repetitive sensory signals on the skin and deeper tissues.

40. Itch and are relatively mild stimulation from movement of objects across the skin and repeated local mechanical stimulation. • Mediator substance for itch is kinnin. • Histamine is found in the itch area but not responsible for it. • It occurs only in the skin, eye, mucous membrane but not in deep tissue of viscera. • Its spot can be identified on the skin by careful mapping. • High frequency stimulation of itch spots on the skin increases the intensity of itching without producing pain.

41. Itch sensation is transmitted via C fibres • Tickle is pleasurable, whereas itching is annoying, but pain is unpleasant. • Tactile Receptors • Free nerve endings found everywhere in the skin and in many other tissues can detect touch and pressure. • Messner’s corpuscle is found in non-hairy parts of the skin, particularly abundant in finger tips and lips. • Merkel’s disc is found on the finger tips is an example of expanded tip tactile receptors.

42. Merkel’s disc and Meissner’s corpuscle play important roles in localising touch sensation to specific surface areas of the body and determining the texture of what is felt. • Hair end organ adapt rapidly and detects movement of objects on the surface of the body or initial contacts with the body. • Ruffinis end-organ is located in deeper layers of the skin and adapts very little, and therefore important for signaling continuous state of deformation of skin and deeper tissues.

43. Pacinian corpuscle lies immediately beneath the skin and in deep tissues of the body. • It is stimulated by very rapid movement and detects tissue vibration. • Transmission of Tactile Sensations • Meissner’s corpuscle, Pacinian corpuscle, Merkel’s disc, hair receptor, Ruffinis nerve endings produce signals that are transmitted through Aβ fibres of 30-70 m/sec velocity. • Free nerve endings signals are via Aδ fibres of 6-30 m/sec velocity and type C unmyelinated fibres of 2 m/sec velocity.

48. Streognosis • Ability to identify objects with sizes and shapes by handling without looking at them. • Normally, people readily identify objects (keys, coins, biros, pins, buttons, stones) of various size and shape. • The ability depends on intact touch and pressure (proprioception?) sensations. • The ability is lost when the dorsal column is damaged, leading to astreognosis – inability to identify objects.

49. Proprioceptors • Proprioceptors are also stretch receptors. • 2 types of stretch receptors associate with skeletal muscle. • Tendon Receptors • They are found in the tendon region of the muscle. • They respond to increasing muscle force with an increase in the firing rate. • It is suited to monitor total force extended by the muscles.

50. Muscle Spindles • They determine joint angulation in mid-ranges of motion. • They function to maintain muscle length. • Joint Receptors • They are a class of proprioceptors. • They sense the degree of flexion of various skeletal joints. • They are slowly adapting