Lecture 1: Somatosensory Mechanisms PDF

Lecture 1: Peripheral Somatosensory Mechanisms ● ● ● Modalities: sensations perceived after stimuli ○ are due to neural activity originating from receptor stimulation in the body & are processed in CNS Somatosensation (SS): modalities that are NOT seeing, hearing, tasting, smelling & vestibular bal...

Lecture 1: Peripheral Somatosensory Mechanisms ● ● ● Modalities: sensations perceived after stimuli ○ are due to neural activity originating from receptor stimulation in the body & are processed in CNS Somatosensation (SS): modalities that are NOT seeing, hearing, tasting, smelling & vestibular balance ○ include: movement, touch, temperature, & pain SS Receptors: distributed throughout the body rather than being concentrated at small, specialized locations ○ SS Receptor Classification: Exteroceptive division: ● Sense of direct interaction with the external world ● Mechanoreceptors → Cutaneous touch ● Thermoreceptors → maintain homeostasis ● Nociceptors → pain to respond to harm Interoceptive division: Proprioceptive division: ● Visceral sensations ● Sense of one’s own body position ● Mechanoreceptors → distention of the gut or ● Proprioceptors are mechanoreceptors fullness of the bladder found in skin, joints, skeletal muscles & ● Chemoreceptors → organ function through tendons indicators as blood gasses & pH ● Receptors convey posture & movement ● Nociceptors → conscious pain warn of info to plan for future movement & as a disease / abnormality feedback to correct ongoing movement ● Referred pain ● SS neuron: ○ perform 2 major functions: ○ transduction: encoding of stimuli into electrical signals ■ stimuli change receptor membrane permeability, allowing some ions to diffuse thro channels ■ change in membrane potential is called receptor (generator) potential ○ transmission propagation of these electric signals to CNS ● Receptor specificity & sensitivity: ○ Submodalities: ■ at peripheral terminals there are a variety of specialized receptors that respond to limited ranges of stimulus energies ○ Receptor specificity → adequate stimulus (low threshold) ○ Differential sensitivity → inadequate stimulus (higher & no threshold) ● Receptor Potential : ○ amplitude & duration of R’s potential are related to intensity & time course of R stimulation ○ ↑ amplitude of the generator potential results in ↑ in the frequency of amplitude ○ As receptors potential ↑ here, there will be ↑ in numbers of AP ■ In AP: there is depolarization & then repolarization → doesn’t change in number or amplitude as you go thro the distance if you have the same stimulus ■ In receptors potential: there is depolarization & then slow repolarization → changes with distance as it will ↓ in amplitude & also it will ↑ in amplitude with increasing the stimulus ● Adaptation: ○ Constant stimulus but receptor potential & neural response diminishes, some sensation lost = receptor adapted ○ Phasic receptors alert us to changes in sensory stimuli → responsible for our ability to pay attention to constant stimuli. This ability is called sensory adaptation Rapid: phasic receptors (dynamic) ● Respond at start & end of stimulus ● Detect change ● Ex. wearing a ring or watch. Pacinian’s corpuscle, Meissner’s corpuscle ● Slow: tonic receptors (static) ● Generate AP throughout, but diminish slowly ● Give continuous info about stimulus ● Ex. Proprioceptors, nociceptors, merkel cells No: tonic receptors ● Important for harmful pain ● Number of AP doesn’t decrease with time ● Neural response & sensation remain the same ● Ex: some nociceptors Mechanoreceptors: Check slide 13 & 14 ○ Direct pressure on skin &/or high-frequency vibration detected by Pacinian corpuscles ○ Different ways of opening the channel: ■ Direct activation thro pressure: corpuscle is filled with gel so there will be distortion when there is pressure & that will cause the opening of Na channels, depolarization generating receptors potential then there will be AP generated at the initial segment. ■ ■ ■ ● Direct activation thro lipid tension: opening channels by stretching like distention caused by changes in osmotic factors, there will be stretching of membrane & ions will enter. ● This is the type that you get from your gut when you eat a lot. Direct activation thro structural proteins: opening of channels mechanically, sensed by a structure, either extracellular or intracellular Indirect action thro membrane structural proteins: mechanical energy will cause another protein to change in structure & open the channel Sensory transduction in slow adapting mechanoreceptors: merkel cell-neurite complex: ○ Skin deformation (pressure) activates Piezo2 channels (nonspecific cation permeable channels) in the Merkel cell, depolarizing it & allowing voltage-gated CaV channels to open & release NT continuously. Binding of nt further depolarizes neurite, producing sustained firing in axon ■ in this case there will not be adaptations, there will not be less response with time Touch & pressure receptors: RECEPTOR SUBMODALITY RECEPTOR TYPE ADAPTATION EXAMPLE Merkel Sustained touch (edges/points) mechanoreceptor slow Blind ppl Meissner corpuscle Changes in light touch, stroke, flutter mechanoreceptor fast feather Pacinian corpuscle Fast vibration mechanoreceptor fast phone Ruffini ending Skin stretch, sustained pressure mechanoreceptor slow apple Hair follicle Flutter, light touch mechanoreceptor fast *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. ● Proprioception: ○ perception of joint & muscle movement as well as position of the body in space ○ Romberg test: pt should stand still & close their eyes, if they’re still then their proprioception is fine but if they’re not balanced then there is problem ○ proprioceptive mechanoreceptors located in muscles, joints & the skin to control movement Muscle spindle: Golgi tendon organ: ● Intrafusal fibers inside will stretch & pull the sensory ● Encapsulated structures at the junction between skeletal muscle fibers & tendon endings, so you will open stretch sensitive channels ● Each capsule has braided collagen fibers connected in series to a group of that will cause the receptor potential & firing of AP muscle fibers in the afferent ● Each tendon organ is innervated by a single Ib axon that branches into many endings inside the capsule intertwined with the collagen fascicles ● When stretched (bcz of muscle contraction), the Ib afferent axon is compressed *check lecture 2 for an extensive explanation :) by collagen fibers & its rate of firing ↑ (ull experience pressure & tension not stretch) ○ ○ α motor efferent: from CNS For muscle contraction γ motor Efferent: from CNS control spindle sensitivity Chemoreceptors: Pain: Visceral sensations: ● Chemoreceptors detect molecules spilled into ● PO2, PCO2 receptors extracellular fluid by tissue injury & molecules that are ● Hunger: food molecules activate hypothalamic part of the inflammatory response chemoreceptors ● Lactic acid when exercising open H+ gated ion ● Thirst: osmoreceptors channels on nociceptive neural endings Itch Histamine: ● released by our immune system ● Thermoreceptors: ○ 6 types of Transient receptor potential channel (TRP): protein channels on free nerve endings are gated by temperature (thermoreceptors) ○ Each type responds to different temp ranges & have diff activation thresholds (differential sensitivity) ○ Channels may also open with chemical ligands ■ 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 ↑ concentration (higher threshold) ○ Ex: TRPM8: nonselective cation channel expressed in small diameter trigeminal & DRG neurons in which cooling & menthol evoke inward depolarizing currents & intracellular calcium rises Nociceptors: Free nerve endings respond selectively to stimuli that can damage tissue Thermal nociceptors: ● TRPV1 receptor ● extreme temp (hot, very chilli, acid) ● In peripheral endings of small diameter, thinly myelinated axons (fast) Mechanical nociceptors: ● intense pressure to skin ● In endings of thinly myelinated axons (stabbing, squeezing, pinching of the skin) Polymodal nociception: ● Noxious stimuli: ↑-intensity mechanical, chemical, or thermal (both hot & cold) stimuli ● At ends of small-diameter, unmyelinated C axons that conduct more slowly (dull, burning pain, diffusely localized, poorly tolerated)