Peripheral Somatosensory Mechanisms PDF

Body Peripheral Somatosensory Mechanisms So sensation from the whole body September 12 , 2022 Lulwa Alhusainan 19` Aisha Al-Tunaib 20` Sonia Al-Hashimi, Ph.D. Physiology Dept. Good luck :) Objectives 1. Classification of somatic sensations. 2. Describe basis of differential sensitivity of sen...

Body Peripheral Somatosensory Mechanisms So sensation from the whole body September 12 , 2022 Lulwa Alhusainan 19` Aisha Al-Tunaib 20` Sonia Al-Hashimi, Ph.D. Physiology Dept. Good luck :) Objectives 1. Classification of somatic sensations. 2. Describe basis of differential sensitivity of sensory receptors to stimuli. 3. Explain mechanism for sensory transduction of stimuli into nerve impulses. 4. Formulate basis for receptor adaptation and functional consequences of information conveyed by tonic and phasic receptors. 5. Describe transduction in Mechanoreceptors (touch, pressure proprioception) 6. Describe transduction in Chemoreceptors 7. Describe transduction Thermoreceptors 8. Describe transduction in Nociceptros 9. Characterize the nerve fibers innervating receptors and in the dorsal column and anterolateral spinothalamic system. 1. Intro & Classification We need sensory info from the environment and internal to make any decision, whether it is reflexive or non-reflexive. Temperature: 1. Thermoreceptors Pain (nociception): 2.Nociceptors To feel warm/cold Balls Pressure/touch: 4. mechanoreceptors Which sense vibration too Itch: 3.Chemoreceptors Ex: Insect bite causes irritation due to chemical or histamine release Position sense: 5. Proprioceptors To avoid They are mechanoreceptors obstacles without looking. Found in your muscles and joints, It gives you position sense Modalities: Sensations perceived after stimuli All sensations that Somatosensation (SS): modalities that are not seeing, hearing, tasting, smelling, and vestibular balance. SS Receptors are distributed throughout the body rather than being concentrated at small, specialized locations. So all receptors that come from the body and not concentrated in specialized locations. As oppose to special organs such as the eyes, ears, eyes…etc Our focus today Outside world Exteroceptive division • Sense of direct interaction with the external world. • Cutaneous touch via mechanoreceptors, to identify objects. • Temperature (thermoreceptors), to maintain homeostasis • pain (nociceptors) to respond to harm. Somatosensations Within the body Interoceptive division Proprioceptive division • Visceral sensations (both conscious and unconscious). • Mechanoreceptors detect distention of the gut or fullness of the bladder. • Chemoreceptors monitor organ function through indicators as blood gases and pH. • Nociceptors : conscious pain warn of disease / abnormality. Referred pain. • Sense of one’s own body position • Proprioceptors are Mostly mechanoreceptors found in skin, joints, skeletal muscles, and tendons. • Receptors convey posture and movement info. to plan for future movement, and as a feedback to correct ongoing movement. 19’ SS Neuron chemical, Stimulus Like mechanical, or temperature s ing d n s r e e to erv ecep n r e Fre thout i w at DRG Close to spinal cord Axon (some axons are myelinated others are not) That recieve stimuli These are found at the end of the peripheral (the one away from the cell body in the CNS) terminal. These structures receive a stimulus. Not all of sensory neurons have those structures, and we call these free nerve endings. The job of these receptors is to receive a stimulus that need to be converted into electrical signal because this is the only language that our brain can understand, so we need a transduction process to transform the stimulus into electrical form, and this is the function of the receptor and sensory neuron. Once you have electrical signal, then you can transmit or propagate the signal along the axon to the CNS. Modalities are due to neural activity originating from receptor stimulation in the body and are processed in the CNS. Electricity is the language of neurones Sensory neurons perform⚪ 32 major functions: 1. transduction: encoding of stimuli into electrical signals 2. transmission propagation of this electric signals to CNS. 2. Receptor specificity and sensitivity Due to different structures attached Touch: So these receptors have different structures that allow them to receive different energies at different (lower) threshold. So each receptors is more selective for each stimulus, this is why they need lower threshold and so these are called adequate stimulus. For example, for the meissner’s corpuscle receptor, the adequate stimulus is light flutter, it is needed in lower threshold and it will activate this receptor. However this doesn’t mean that merkel disk can’t sense light flutter. It actually may or may not sense light flutter but it need higher threshold to do so, this is why it is called inadequate. The adequate stimulus for merkel disk is pressure, so in this case the threshold needed for pressure to activate merkel disk is not high. Touch sub modalities: Different structures = different • Submodalities: Pain: Sharp, Slow, Burning, Freezing Temp: cool, warm, cold, hot Submodalities exist because at peripheral terminals there are a variety of specialized receptors (morphologically and molecularly different) that respond to limited ranges of stimulus energies. • Receptor specificity: Each Receptor is more selective (specific) for a single stimulus energy – its adequate stimulus. • Differential sensitivity: receptors have a low threshold for the adequate stimulus, and a higher or no threshold at all to others (inadequate stimulus). Light Flutter Meissner’s corpuscle ( low threshold ) Pressure Merkel disk Vibration Pacinian corpuscle Skin stretch Ruffini corpuscle Pain, itch and tickle, temp Free nerve ending The ones without the structures mentioned above. So how can we differentiate between these and the ones with receptors? We can because we also have molecular differences (different proteins channels) for pain and temp. And they will open for that particular stimulus. Somatosensory free nerve endings vary in the populations of ion channels – this gives the neuron a specific sensory function. Example if nerve ending has thermosensitive ion channels, then they are specific for temperature sensations and considered thermoreceptors. Pain, itch tickle Differentiation through and temp: molecular changes not morphological changes Free nerve ending Without structures attachment 3.Transduction Each stimulus has a specific ion channel • Transduction: encoding of stimuli into electrical signals • Stimuli change receptor membrane permeability, allowing some ions to diffuse through channels. • Change in membrane potential is called receptor (generator) potential • If the stimulus is strong the receptor potential reaches AP threshold, and an AP is generated. Remember that this is graded response , meaning that the higher the stimulus, the higher the receptor potential and it is called generator potential because when it is high enough, it will hit the threshold and causes action potential because it opened the voltage gated sodium channels. z nan en in es es opening permeability to nng ang iions ca some s a +ve ions cause depolarisation s Receptor Potential The amplitude and duration of the receptor potential are related to the intensity and time course of stimulation of the receptor. AP doesn’t change in amplitude in response to increasing stimulus. The greater the stimulus the higher the frequency of AP He higher the amplitude of the stimulus, the higher the GENERATOR potential is. • Receptor (generator) potential is graded. Stronger the stimulus the larger the potential amplitude. • The longer the period of stimulus the longer the duration of receptor potential • Increasing amplitude of the generator potential results in increases in the frequency of that the amplitude action potentials. Remember of the AP cannot be changed, so what is changing here is the frequency. The greater the stimulus the greater the graded potential the greater the frequency of action potential B. The deeper you’re pressing, the greater the number of action potential being fired. So you can tell where the threshold is. If it starts here (*), so the threshold for this receptor is 600micrometers * No AP/spikes here in the terminal ending, at the mechano-receptors. Why? Because they are graded and they dissipate, they become less. So these are important at the beginning but they are not AP because they dissipate. They need to be high enough to open voltage-gated channels for ions influx to generate AP. These voltage-gated channels are found at the initial segment . So now along the axon we have the transmission of AP until the neurotransmitter release at the end. Receptor potential As receptors potential increases here, there will be increase in numbers of action potential. A. The number of action potentials per second recorded from a touch receptor in the hand is proportional to the amplitude of skin indentation. Each dot represents the response of the receptor to pressure applied by a small probe. The relationship between the neural firing rate and the pressure stimulus is linear. This receptor does not respond to stimuli weaker than 200 μm, its touch threshold. Can only be measured in axon hillock due to sodium gated channels B. Different recordings from different areas along the nerve The difference between receptors potential and action potential. In action potential: there is depolarization and then repolarization Doesn’t change in number or amplitude as you go through the distance if you have the same stimulus. In receptors potential: there is depolarization and then slow repolarization Changes with distance as it will decrease in amplitude and also it will increase in amplitude with increasing the stimulus. You get used to (adapt to) a certain stimulus, for example, sitting on an uncomfortable chair, as time passes you “adapt” to this situation and don’t feel that the chair is uncomfortable alsocaneddynamic nesewintensonaboutchange Rapid adaptation by phasic receptors Dynamic receptors 4.Adaptation stimulus pressures Adaptation at the NERVE ENDING NOT BRAIN! • Constant stimulus but receptor potential and neural response diminishes, some sensation lost = receptor adapted amisspacinia corpacsieatine endoenermenaing mesearereceptorma g p.m have polarised re immediately v nowtnestiman butter isstinmere response.me no east tailback rapidly they adapted rapidadaptation Lyonweararing not comfortablea namenonce itsin.youdonitee.im a Na+s You sit in an uncomfortable chair, at first it hurts your back but then you get used to it. You adapted it so you don’t feel the pain anymore, you forget it. Chair is still there but neuro-response diminished. So the receptor potential decreased and the number of AP also decreased. • Phasic receptors alert us to changes in sensory stimuli and are in part responsible for our ability to cease paying attention to constant stimuli. This ability is called sensory adaptation. miscausesopening oeananneisanason receptor Potential youwinon a nymore ring eeeiitagainwnenyoutrytotacrit.ee soitonignappensauringenange Slow adaptation Tonic/static receptors like pain receptors (ex: painful shoes) Emeyremoveait susterenerve ending itaiminisnession v Slow adaptation like proprioceptive receptors inionisgonautozerownenstimains unlike removed dynamic where wenttozerownen stimulus is one stir was Receptor adaptation No adaptation Slow adaptation By Tonic receptors By Tonic receptors (static) This is important for harmful pain. Rapid Adaptation By Phasic receptors (Dynamic) Constant stimulus when wearingtight ring Receptor Potential butdon't decreases slowly the Throughout disappear Action Potential • • AP here decreased because receptors potential decreased. Notice here that the number of AP doesn’t decrease with time Time Time Neural response and sensation remains the same. Some Nociceptors don't impforpainso you hurtyourself • • • the at day Generate AP throughout, but diminish slowly. Give continuous info about stimulus eg. Proprioceptors, nociceptors, merkel cells Time • • • Respond at start and end of stimulus. Detect change E.g. wearing ring or watch. Pacinian’s corpuscle, Meissner’s corpuscle 5. Mechanoreceptors 1,2,3, and 4 are different ways of opening the channel. 2Opening the channels by stretching like if there is distention caused by changes in osmotic factors, there will be streching of membrane and ions will go through. This is the type that you get from your gut when you eat a lot. The corpuscle is filled with gel so there will be distortion when there is pressure and that will cause the opening of Na channels, depolarization generating receptors potential then there will be AP generated at the initial segment. Opening of the channels mechanically, sensed by a structure, either extracellular or intracellular. This will be attached to the membrane. If in extracellular for example and there is some movement in the extracellular fluid, it would open the channel. E.g. Direct pressure on skin and/or high-frequency vibration detected by Pacinian corpuscles. imp Removal of stimulus relieves mechanical stress on receptor and allows channels to close. Through change in the extracellular fluid Mechanical energy will cause another protein to change in structure or it move closer to the channel and then it will open the channel (this is Touch and pressure receptors 1. Merkel receptor Merkel cells Located the most superficial Glabrous skin Sensory neuron Merkel disc Important for blind people to read in Braille So for blind people, they use this receptor in order to read 2 Receptor Submodality 1. Merkel Sustained touch, edges/points, Receptor type Adaptation Mechanoreceptor Slow Bcuz of the NS being released E.g. A model of sensory transduction in slow adapting mechanoreceptors. found inneuron one the Piezo2 On the disk that is attached to the segment Merkel cell–neurite complex. Pressure on skin opens Piezo2 channels (nonspecific cation permeable channels) that are activated by mechanical stimuli in Merkel cell and in the fiber that receives synaptic input from the Merkel cell. Skin deformation activates Piezo2 channels in the Merkel cell, depolarizing it and allowing voltage-gated CaV channels in the Merkel cell to open and release neurotransmitter continuously. Binding of the neurotransmitter further depolarizes the neurite, producing sustained firing in the axon. So in this case there will not be adaptations, there will not be less response with time. tic changes that sense aresuper Glabrous skin 2.Meissner Found in the dermis, it is the most superficial and it is corpuscle second the most sensitive. capsule Collagen fibers nerve terminal Schwann cells Receptor Submodality Receptor type 2. Meissner Corpuscle Changes in light touch, stroke, Flutter Mechanoreceptor There you can see myelin and then once you enter the receptor (nerve ending) they don’t have myelin. This structure is encapsulated. Collagen fibers are attached to epidermis, so slight movement of epidermis will move the structure, open the channel and you will have AP. Adaptation Fast They don’t even have neurotransmitter function here Lamellae Separated by gel 3. Pacinian Glabrous skin corpuscle This is a deep structure, big, and looks like onion under the microscope. bright field light microscopy Deep but large thus sensitive Receptor Submodality Receptor type Adaptation 3. Pacinian Corpuscle Fast Vibration Mechanoreceptor Fast Elongation Glabrous skin 4. Ruffini Example: Stretching of the skin when holding an apple, gives you an idea of how big the thing you’re holding Receptor Submodality Receptor type 4. Ruffini Ending Skin stretch, sustained pressure Mechanoreceptor Adaptation Slow • Hairy skin has all of the mechanoreceptor organs of the glabrous skin except the Meissner corpuscle. • hair follicle afferents serve a function similar to that of Meissner corpuscles.So slight movement of hair can be felt • Hair follicle afferents innervate 10 to 30 hairs spread over an area of 1 to 2 cm2 and are sensitive to hair movement. Receptor Submodality Receptor type Adaptation 5. Hair follicle Flutter, light touch Mechanoreceptor Fast So it doesn’t relate to vision it’s inside the muscle Proprioception: Position sense So without seeing you can do that right movements The perception of joint and muscle movement as well as position of the body, or body segments, in space. Eyes closed: Up or down? Here you ask the person to close his eyes and then move the toe up and down and if there is nothing wrong with his propioception he will be able to tell you whether you moved it up or down without looking. The pt will sway Romberg test Here you ask the patient to stand still and close their eyes, if they’re still then their propioception is fine but if they’re not balanced with their eyes closed then there is problem with their propioception so they need to balance out their posture. To control movement, the brain has to integrate information from proprioceptive mechanoreceptors located in muscles, joints and the skin. This is alongside your skeletal fibers. It is encapsulated, there are fibers inside and they are called intrafusal fibers and there are nerve endings that wrap around the fibers. So when the muscle stretches, this structure will also stretch, the fibers inside will stretch as well and it will pull the nerve ending , causing the channels to open and action potential will fire on these afferents. γ motor Efferent: from CNS α motor Afferents: control spindle signal muscle efferent: sensitivity from CNS stretch to CNS For muscle Ia and II contraction ‫عبدالس"م‬ .‫نوتة د‬ ّ 1. Muscle spindle In the belly of the muscle Ib Afferents: signal muscle tension to CNS Found between the muscle and the tendon and its afferent is lb 2. Golgi Tendon Organ: Sensory Nerve endings Senses tension Tendon fascicles Extrafusal Muscle fibers Intrafusal Muscle fibers Sensory Nerve endings containing ion channels Transduction in muscle spindle You have intrafusal fibers inside the muscle spindle. these will stretch and pull the sensory endings, so you will open stretch sensitive channels that will cause the receptor potential and firing of AP in the afferent. Transduction in GTO • Golgi tendon organs are encapsulated structures at the junction between skeletal muscle fibers and tendon. • Each capsule has braided collagen fibers connected in series to a group of muscle fibers. • Each tendon organ is innervated by a single Ib axon that branches into many endings inside the capsule intertwined with the collagen fascicles • When the Golgi tendon organ is stretched (usually because of contraction of the muscle), the Ib Here we are sensing afferent axon is compressed by pressure and tension collagen fibers and its rate of and not stretch. firing increases. So it senses tension (contraction) 6. Chemoreceptors Examples of Visceral sensations: If you hold your breath long enough you’ll have • PO2 , PCO2 receptors the urge to breathe • Hunger: food molecules activate hypothalamic chemoreceptors • Thirst: osmoreceptors Pain: • chemoreceptors detect molecules spilled into extracellular fluid by tissue injury and molecules that are part of the inflammatory response. • Lactic acid when exercising open H+ gated ion channels on nociceptive neural endings. Itch: is unpleasant, we attempt to eliminate it by scratching. Histamine bythe system Released by immune the immune system released 7. Thermoreceptors • Transient receptor potential channel (TRP): protein channels on free responsible for thermal transduction at the nerve endings.peripheral ends of somatosensory neurons. Moderate temperature Warm/cool nothing extreme Stimulus You need to know how they work • At least 6 types are gated by temperature (thermoreceptors). • Each type responds to different temperature ranges and have different activation thresholds (differential sensitivity). >35°C >25–35°C You will not be asked to know the names and temperature but you should know on general that moderate temperature changes will cause certain channels to open. <25°C when thisisactivated sensation oe nave win you co na • Channels may also open with chemical ligands • E.g. TRPM8: nonselective cation channel expressed in small diameter trigeminal and dorsal root ganglion neurons in which cooling and menthol evoke inward depolarizing currents and intracellular calcium rises. This is Inadequate stimuli, it can open the same channel for cold so you will feel cold even tho it is not something cold. But it needs high concentration (higher threshold) if TRP Channels may also open with chemical ligands • E.g., TRPM8: nonselective cation channel expressed in small diameter trigeminal and dorsal root ganglion neurons in which cooling and menthol evoke inward depolarizing currents and intracellular calcium rises. menthol effect cooling 8. Nociceptors acid that iswhyapplying tee m ake you s kin w in me to not 43C Free nerve endings respond selectively to stimuli that can damage tissue. 1. trout like p Thermal nociceptors activated 2. directly by extreme temp. In peripheral endings of small diameter, thinly myelinated axons east _R t Mechanical nociceptors activated tg TEST directly by intense pressure to skin. In endings of thinly myelinated axons. (stabbing, squeezing, pinching of the skin)damagingstimuli difference soggy It opens during extreme hot temperature, but it can also open with something that is not hot. Like when you eat this (something chilli) you will feel hot. Slower than thickly myelinated 3. oA Polymodal nociception: Nerve fiber endings have receptors for highintensity mechanical, chemical, or thermal (both hot and cold) stimuli. E At ends of small-diameter, unmyelinated C axons that conduct slow because they are non-myelin. Transmitted by polymodal more slowly (dull, burning pain, Dull, receptors , it has 3 nerve endings. The worst is combination of all kinds of receptors, there are temp. receptors (v. High and v. Low) you have diffusely localised, poorly acids, and mechanical receptors. So you have this dull pain and you don’t know what is causing it tolerated). because there are many different receptors. __ ENNN <18°C >42°C off t >52°C Stimuli that can cause harm 2. Mechanical nociceptors activated directly by intense pressure to skin. In endings of thinly myelinated axons. (stabbing, squeezing, pinching of the skin) have That's dull pain why we polymodainerreending It'sactivatedbydifferentstimulilikeacid 3. Polymodal nociception: Nerve fiber endings have receptors for high-intensity mechanical, chemical, or thermal (both hot and cold) stimuli. At ends of smallslow diameter, unmyelinated C axons that conduct more slowly (dull, burning pain, diffusely localised, poorly tolerated). It’s activated by different stimuli like acid Ee Summary of the above 1 2 3 TRPV2 chemical Mechanical peripheral endings of neurons contain specialized ion channels and receptors (for example, nociceptors have transient receptor potential vanilloid 1 (TRPV1) that are capable of transducing noxious stimuli into an electrical signal). Specific voltage-gated sodium channels (VGSC) may contribute to the amplification of these signals until the threshold for action potential generation has been reached. VGSC and VGPC channels cause propagation of action potentials to the central terminals within the spinal cord. Depolarization of the presynaptic terminal activates voltage-gated calcium channels, leading to release of neurotransmitters, which activate CNS neurons via binding to postsynaptic ion channels and receptors. Stimulus intensity coding asyouincreastnestinuus meamplitudewinincrease asyouinareaceduration increase amplitude in win the an increase mere's innumber an witimeimenumberoear increase a win AttheendthemoreAP wehave themore Nsare The greater the stimulus the greater the graded potential the greater the frequency of action potential and the greater the amount of neurotransmitters secreted beingreleased 9. Characterising nerve fibers: moremyelin fastersignals a and b Touch forfasttouin fastpain forsiontemporpain slowest one The different speeds of fibres (conduction velocity) are due to diameter and degree of myelination. From Dr. Heba’s lecture Anterolateral Dorsal Column-Medial lemniscus pathway Spinothalamic pathway Primary SS cortex Primary SS cortex 3 3 Thalamus (Ventral Posterior Nucleus) Dorsal Column nuclei 2 Dorsal Column 1 Aα Aβ Spinothalamic tract Decussation: Medial Medulla lemniscus Ia, II Ib frommuscletendon 1 Spinal cord muscle east myelinated A, C III, IV sun Thinlymyelinated 2 Decussation Somearemyelinatedsomearenot y Nobel prize of 2021 in Physiology/ Medicine David Julius Ardem Patapoutian Thank you