LEWIS_AHP_Sensory Physiology I_Touch, Taste and Smell_2024 (4).pptx

APPLIED HUMAN PHYSIOLOGY Sensory Physiology – Touch, Taste & Smell Chapters 16 & 17 Units IX and X Dr Anthony Lewis, SM 5.40 [email protected]; In this lecture we will be covering........  Process of sensation  Sensory receptors  Tactile, ther...

APPLIED HUMAN PHYSIOLOGY Sensory Physiology – Touch, Taste & Smell Chapters 16 & 17 Units IX and X Dr Anthony Lewis, SM 5.40 [email protected]; In this lecture we will be covering........  Process of sensation  Sensory receptors  Tactile, thermal and proprioceptive sensations  Physiology of olfaction  Physiology of gustation  Sensory Physiology Sensation is the conscious or subconscious awareness of changes in the external or internal environment Perception is the conscious interpretation of sensations and is primarily a function of the cerebral cortex Each unique type of sensation—such as touch, pain, vision or hearing - called a sensory modality (two types – general senses and special senses) A given sensory neuron carries information for only one General sensorysenses modality Somatic senses - tactile (touch, pressure, vibration, itch, and tickle), thermal (warm and cold), pain and proprioceptive sensations Vis­ceral senses - provide information about conditions within internal organs, for example, pressure, stretch, chemicals, nausea, hunger, and temperature Special senses include the sensory modalities of smell, taste, Process of Sensation Several structural and functional characteristics of sensory receptors can be used to group them into different classes; (1)microscopic structure (2)location of the receptors and the origin of stimuli that activate them (3) type of stimulus detected  (1) Microscopic structure of sensory receptors Sensory receptors may be one of the following: Free nerve endings - bare (not encapsulated) dendrites - lack any structural specialisations that can be seen under a light microscope - receptors for pain, temperature, tickle, itch, and some touch sensations A sensory receptor responds to a stimulus by generating a graded potential known as a receptor potential - if large enough to reach threshold, it triggers one or more nerve impulses in the axon of the sensory neuron  (1) Microscopic structure of sensory receptors Sensory receptors may be one of the following: Encapsulated nerve endings - other somatic and visceral sensations, such as pressure, vibration, and some touch sensations - dendrites are enclosed in a connective tissue capsule that has a distinctive microscopic structure - for example, lamellar corpuscles - different types of capsules enhance the sensitivity or specificity of the receptor  (1) Microscopic structure of sensory receptors Sensory receptors may be one of the following: Separate cells - specialised cells that synapse with sensory neurons - sensory receptors for some special senses (hearing, equilibrium, taste, sight) - olfactory sensory neurons for the sense of smell are not separate cells - they are located in olfactory cilia, which are hair-like structures that project from the dendrite of an olfactory sensory neuron  (2) Location of receptors Exteroceptors external surface of the body - sensitive to stimuli originating outside the body and provide information about the external environment sensations of hearing, vision, smell, taste, touch, pressure, vibration, temperature, and pain Interoceptors or visceroceptors blood vessels, visceral organs, muscles, and the nervous system monitor conditions in the internal environment nerve impulses produced are not normally consciously perceived Proprioceptors muscles, tendons, and joints provide information about equilibrium (balance), body position, muscle length and tension and the position and  (3) Type of stimulus detected Most stimuli are in the form of mechanical energy, such as sound waves or pressure changes; electromagnetic energy, such as light or heat; or chemical energy, such as in a molecule of glucose Mechanoreceptors are sensitive to mechanical stimuli such as the deformation, stretching, or bending of cells - provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium, monitor the stretching of blood vessels and internal organs Thermoreceptors detect changes in temperature Nociceptors respond to painful stimuli resulting from physical or chemical damage to tissue Photoreceptors detect light that strikes the retina of the eye Chemoreceptors detect chemicals in the mouth (taste), nose (smell), and body fluids Osmoreceptors detect the osmotic pressure of body fluids  Adaptation A characteristic of most sensory receptors Receptor potential decreases in amplitude during a maintained, constant stimulus Causes the frequency of nerve impulses in the sensory neuron to decrease Perception of a sensation may fade or disappear even though the stimulus persists For example, when you first step into a hot shower/bath Rapidly adapt­ing receptors are specialised for signalling changes in a stimulus i.e. receptors associated with vibration, touch, and smell Slowly adapt­ing receptors continue to trigger nerve impulses as long as the stimulus persists i.e. stimuli  Somatic Sensations Sensations that arise from stimulation of sensory receptors embedded in the skin or subcutaneous tissue; - in mucous membranes of the mouth, vagina, and anus - in skeletal muscles, tendons, and joints Sensory receptors for somatic sensations are distributed unevenly Areas with the highest density of somatic sensory receptors are the tip of the tongue, the lips, and the fingertips Somatic sensations that arise from stimulating the skin surface are cutaneous sensations  Tactile Sensations Include touch, pressure, vibration, itch, and tickle Arise by activation of some of the same types of receptors Several types of encapsulated mechanoreceptors attached to large-diameter myelinated A fibres mediate sensations of touch, pressure, and vibration Other tactile sensations, such as itch and tickle sensations, are detected by free nerve endings attached to small-diameter unmyelinated C fibres  Tactile Sensations - Touch Sensations of touch generally Two types of rapidly result from stimulation of tactile receptors in the skin or adapting touch subcutaneous tissue receptors: 1) Tactile (Meissner) corpuscles - located in the dermal papillae of hairless skin - generate nerve impulses mainly at the onset of a touch - abundant in the fingertips, hands, eyelids, tip of the tongue, lips, nipples, soles, clitoris, and tip of the penis 2) Hair root plexuses - found in hairy skin - they consist of free nerve endings wrapped around hair follicles  Tactile Sensations - Touch Sensations of touch generally Two types of slowly result from stimulation of tactile adapting touch receptors in the skin or receptors: subcutaneous tissue 1) Nonencapsulated sensory corpuscles (Merkel discs) - saucer-shaped, flattened free nerve endings - contact tactile epithelial cells (Merkel cells) of the stratum basale - plentiful in the fingertips, hands, lips, and external genitals - respond to continuous touch 2) Bulbous (Ruffini) corpuscles - elongated, encapsulated receptors  Tactile Sensations – Pressure Sensations of pressure generally result Pressure from stimulation of lamellar and bulbous corpuscles - a sustained sensation that is felt over a larger area than touch - occurs with deeper deformation of the skin and subcutaneous tissue - lamellar and bulbous corpuscles - respond to a steady pressure stimulus because they are slowly adapting  Tactile Sensations – Vibration Sensations of vibration generally Vibration result from stimulation of Pacinian Sensations result from and tactile corpuscles rapidly repetitive sensory signals from tactile receptors Lamellar (Pacinian) corpuscles - consist of a nerve endings surrounded by a multi- layered connective tissue capsule that resembles a sliced onion - adapt rapidly - found in the dermis, subcutaneous tissue - respond to high-frequency vibrations Tactile corpuscles - respond to low-frequency  Tactile Sensations – Itch and Tickle Sensations of itch and tickle Itch (pruitus) generally result from stimulation of - stimulation by certain free nerve endings chemicals such as bradykinin, histamine - scratching usually alleviates itching by activating a pathway that blocks transmission of the itch signal through the spinal cord Tickle - arises when someone else touches you, not when you touch yourself - nerve impulses that conduct to and from the cerebellum when you are moving your fingers and touching yourself don’t occur when someone else  Tactile Sensations – Thermal Thermal sensations result from Cold receptors stimulation of free nerve endings - located in the stratum (receptive fields about 1 mm in basale of the epidermis diameter on the skin surface) - activated by temperatures between 10° and 35°C Warm receptors - located in the dermis - activated by temperatures between 30° and 45°C Both adapt rapidly at the onset of a stimulus, but they continue to generate nerve impulses at a lower frequency throughout a prolonged stimulus Temperatures below 10°C and above 45°C primarily stimulate pain receptors,  Proprioception Sensations allow us to recognize that parts of our body belong to us and their position in space Kinesthesia is the perception of body movements proprioceptors embedded in muscles (especially postural muscles) and tendons inform the - degree to which muscles are contracted - amount of tension on tendons - positions of joints - the weight of an object (weight discrimination) Hair cells of the inner ear monitor the orientation of the head relative to the ground and head position during movements Proprioceptors adapt slowly so the brain continually receives nerve impulses related to the position of different body parts and makes adjustments to ensure coordination  Proprioception Muscle spindle - monitor changes in skeletal muscle length - sensory nerve endings wrap around the central portion of intrafusal muscle fibres - participate in stretch reflexes - allows brain to set an overall level of mus­cle tone - plentiful in  Proprioception Tendon organ - monitor the force of muscle contraction - sensory nerve endings are activated by increasing tension on a tendon - located at the junction of a tendon and a muscle - protect tendons and their associated Physiology of Olfaction – sense of smell  Both smell and taste are chemical senses; the sensations arise from the interaction of molecules with smell or taste receptors  The stimulating molecules must be dissolved  Impulses for smell and taste propagate to the limbic system (and to higher cortical areas as well), certain odours and tastes can evoke strong emotional responses or a flood of memories  Anatomy of olfactory receptors Located in the olfactory epithelium of the nose Total area of 5 cm2 occupies the superior part of the nasal cavity  Anatomy of olfactory receptors Extending from the dendrite of an olfactory sensory neuron (OSN) are several nonmotile olfactory cilia - sites of olfactory transduction (conversion of stimulus energy into a graded potential in a sensory receptor) Within the plasma membranes of the olfactory cilia are olfactory receptors that detect inhaled chemicals (odorants) OSNs respond to the chemical stimulation of an odorant molecule by producing a receptor potential, thus initiating the olfactory response Axons of OSNs expressing the same odorant receptors  Anatomy of olfactory receptors Supporting epithelial cells are columnar epithelial cells of the mucous membrane lining the nose - provide physical support, nourishment, and electrical insulation for the OSNs and help detoxify chemicals that come in contact with the olfactory epithelium Basal epithelial cells are stem cells located between the bases of the supporting epithelial cells - continually undergo cell division to produce new olfactory sensory neurons - which live for only about two months before being replaced Olfactory glands (Bowman’s glands) - produce mucus that is carried to the surface of the epithelium by ducts - moistens the surface of the olfactory epithelium and dissolves odorants so that transduction can occur  Olfactory transduction Binding of an odorant molecule to an olfactory sensory receptor protein (GPCR) Activates G protein signalling pathway and adenylyl cyclase ↑ cyclic AMP - opens cation channels – influx of Na+ and Ca2+ ions  Olfactory physiology Nose contains about 10 million olfactory receptors 400 different functional types Only one type of receptor is found in any given OSN Ability to recognise about 10,000 different odours depends on patterns of activity in the brain that arise from activation of many different combinations of the olfactory receptor cells Odour threshold is LOW Natural gas contains methyl mercaptan - sensed at 1/25 billionth mg/ml Adaptation is RAPID (↓ sensitivity by 50% in 1 s) - complete insensitivity after ~1 min posmia – reduced ability to smell – head injury, AD, PD, smo Physiology of Gustation – sense of taste Much simpler than olfaction in that only five primary tastes can be distinguished: - salty (Na+ ions) - sour (H+ ions from acids) - sweet (sugars/artificial sweeteners) - bitter (caffeine, quinine and many poisonous substances) - umami (amino acids – MSG – monosodium glutamate) Complex flavours are combinations of the five primary tastes Most taste is via stimulation of olfactory sensory neurons Anatomy of taste buds Found in elevations on the tongue called lingual papillae: 1. About 12 very large, circular vallate papillae or circum vallate papillae form an inverted V-shaped row at the back of the tongue. Each houses 100– 300 taste buds 2.Fungiform papillae are mushroom- shaped elevations scattered over the entire surface of the tongue - contain about 5 taste buds each 3.Foliate papillae located in small trenches on the lateral margins of the tongue - taste buds degenerate in early childhood In addition, the entire surface of the tongue has filiform papillae - pointed, threadlike structures contain tactile receptors (no taste buds)  Anatomy of taste buds ~ 10,000 taste buds (tongue, soft palate, pharynx, epiglottis) Oval body consisting of three kinds of epithelial cells: 1) Supporting epithelial cells 2) Gustatory epithelial cells (contain gustatory microvilli (gustatory hairs) which project to the external surface through the taste pore (opening) - lifespan ~10 days - at their base synapse with dendrites of the first order neurons. Dendrites contact many gustatory epithelial cells in several taste buds 3) Basal epithelial cells, stem cells found at the periphery of the taste bud near the connective tissue layer produce  Physiology of Gustation Chemicals that stimulate gustatory epithelial cells are known as tastants  Physiology of Gustation Na+ enters gustatory epithelial cells via Na+ channels ↑ Na+i causes depolarization and release of neurotransmitter G proteins activate enzymes that produce the second messenger inositol trisphosphate (IP3) IP3 causes depolarisation of the gustatory epithelial cell and release of neurotransmitter H+ in sour tastants flow into gustatory epithelial cells via H+ channels Depolarization and the liberation of neurotransmitter Tastants dissolve in saliva - contact with the plasma membranes of the gustatory microvilli, which are the sites of taste transduction Results in depolarising receptor potential - stimulates exocytosis of neurotransmitter - triggers graded potentials - nerve impulses in the first-order sensory neurons that synapse with gustatory receptor cells  Gustatory physiology Individual gustatory epithelial cells respond to only one type of tastant Thus, each gustatory epithelial cell is “tuned” to detect a specific primary taste But each taste bud contains gustatory epithelial cells for each type of tastant, allowing all of the primary tastes to be detected in all parts of the tongue Different tastes arise from activation of different combinations of gustatory epithelial cells Threshold for bitter substances, such as quinine, is lowest (high sensitivity) and may is a protective function Complete adaptation to a specific taste can occur in 1–5 minutes of continuous stimulation  Summary You should now be familiar with:  Process of sensation  Sensory receptors and their classification  Tactile, thermal and proprioceptive sensations  Physiology of olfaction  Physiology of gustation