Somatosensory Study Guide.docx

Lecture One - Somatosensory (we learned a lot of this last semester) ❖ The nervous system Central - brain and spinal cord Peripheral - somatic (motor/voluntary) and autonomic (sympathetic and parasympathetic) ❖ Terms ❖ Types of sensations Mechanoreceptive - touch and pressure Thermore...

Lecture One - Somatosensory (we learned a lot of this last semester) ❖ The nervous system Central - brain and spinal cord Peripheral - somatic (motor/voluntary) and autonomic (sympathetic and parasympathetic) ❖ Terms ❖ Types of sensations Mechanoreceptive - touch and pressure Thermoreceptive - temp Nociceptors - pain Exteroceptive - from surface of the body Proprioceptive - position sense Visceral - internal organs Deep sensations - from deep tissue All sensation involves: receptor activation → neuron activation → impulse transmission to the spinal cord and brain ❖ Specialized sensory receptors Mechanoreceptors - mechanical compression or stretching Free nerve endings Expanded tip receptor: sustained touch, texture discrimination Tactile hair: detects movement on surface of body/initial contact with body Pacinian corpuscle: deep vibration, movement against skin, pressure Meissner's corpuscle: Sensitive to skin touch, low frequency vibration (found on non hairy skin fingertips/lips) Krause's corpuscle: touch, pressure, temperature Ruffini's end-organ: deep and prolonged pressure, skin stretch; found in joints Golgi tendon apparatus: proprioception Muscle spindle: proprioception Thermoreceptors - changes in temperature Nociceptors - physical or chemical damage to tissue (pain) Electromagnetic - light on retina of eye Chemoreceptors - taste/smell; O2/CO2 in blood; osmolality, etc. ❖ Neuron activation and transmission Differential sensitivity - "wind vs. touch, hair follicle won't be affected by pressure since it's not supposed to detect pressure" Receptors are designed for specific types of stimuli Receptors are almost non-responsive to other types of sensory stimuli Labeled-line principle - "brain recognizes stimulation based on the 'line' it comes from" Specificity of nerve fibers to transmit only one modality of sensation Each nerve tract terminates at a specific point in the CNS Type of sensation felt is determined by the specific nerve fiber stimulated "If pain receptor is what is stimulated - you feel pain regardless of what type of stimulus it was that that activated it" ❖ Receptor potentials Stimuli change the electrical membrane potential (receptor potential) → ion channels open " → electrochemical stimulus → information is sent to the brain" - "this shows how electrolyte abnormalities can affect neural transmission" Mechanisms Mechanical deformation: mechanical pressure causes stretch of the receptor membrane; stretch of membrane opens ion channels Chemical application: chemical application to membrane opens ion channels Temperature change: change in temperature alters permeability of membrane Electromagnetic radiation: light on retina causes ion channels to open ❖ Action potentials Threshold - the potential must rise above a certain threshold to elicit an action potential (all or none response) Frequency - "the brain should recognize repetitive firing differently" The more the receptor potential rises above the threshold the greater becomes the action potential frequency Very intense stimulation causes progressively less and less additional action potentials Allows range of sensory experience (weak \> intense) Amplitude - "the bigger the stimulus → the brain should recognize that it's bigger" Increases rapidly with increased stimulus strength but progressively less less rapidly at high stimulus strength" In our words/understanding: Frequency - if your pain is a 3/10 pain, your frequency has a lot of room to increase. This "low level" stimulation, as it increases, has a lot of room to increase so it will progressively have more "additional action potentials" once it gets up to a 5 or 7, etc. However, once you're at a 9/10 the actional potential frequency can only increase so much - aka as it increases in intensity (9/10 or 10/10 pain) the room for "additional action potentials" becomes less Amplitude - the height between -70 mV and +30 mV is much higher than -30 and +30 So an initial increased stimulus has a high amplitude, but if the stimulus is intense the amplitude will continue to decrease as the frequency increases Both have big changes initially, but both the space between/frequency of additional action potentials and the amplitude/height of response to change decrease Test question from last semester: In pacinian corpuscles (touch, vibration, rapidly adapting) \_\_\_\_\_ and \_\_\_\_\_\_\_. ◆ Receptor potential changes markedly with increases in low levels of stimulation ◆ Receptor potential increases only slightly with increases in high intensity stimulation ❖ Pacinian corpuscle Structural designs - pacinian corpuscle has a central nerve fiber extending through it's core surrounded by multiple concentric layers Compression of the corpuscle causes mechanical deformation Ion channels open Sodium ions enter to interior of fiber (receptor potential) If threshold is reached, an action potential is elicited Initial amplitude of receptor potential increases rapidly With progressively stronger stimulus strength, amplitude is diminished but the frequency of repetitive action potentials increases Action potential is transmitted along the nerve fiber to the CNS This is how a receptor can be responsive to both weak and intense stimuli - "both light and deep pressure" ❖ Receptor adaptation Adaptation Initial receptor response: high Continued / frequent stimulation: diminished response Different types of receptors adapt at different rates Pacinian receptors are rapid adaptors (fraction of a second) Hair receptors adapt in 1 second Rapid adaptor receptors are better for sensing changes - "depolarize and repolarize" Aka rate receptors, movement receptors, phasic receptors Slow adaptor receptors Transmit impulses to the brain as long as stimulus is present Better for sensing constant conditions in the body ◆ Arterial baroreceptors ◆ Arterial chemoreceptors ◆ "Wouldn't want a quick/sudden change in the bp, etc." ❖ Nerve transmission - nerve fibers are categorized by function, size, myelination Nerve diameters: 0.5 micrometers - 20 micrometers Conduction velocity: 0.5 - 120 meters/second Two classification systems General classification (types A, B, C) A fibers → large/medium, myelinated, FAST B fibers → smaller, myelinated, preganglionic (ANS) C fibers → small, unmyelinated, SLOW Sensory nerve classification (group I - IV) Type I → large, myelinated, and FAST ◆ Fibers from muscle spindles, golgi tendon organs Type IV → small, unmyelinated, and SLOW ◆ Crude touch and pressure, tickle, aching pain, temperature ❖ \*she said specifically to know this chart\* ❖ Spatial and temporal summation: intensity Spatial summation - increasing signal strength is transmitted by using progressively greater numbers of fibers Example: painful stimulus across bigger area → more receptors are involved - bigger incisions hurt more Temporal summation - increasing signal strength is transmitted by increasing the frequency of nerve pulses in each fiber Same or nearby pre-synaptic neuron firing multiple times in close succession ❖ Neuronal pools - "think of overflow/overlap' Groups of specially organized neurons that share common inputs, outputs, and functions CNS is composed of thousands → millions of neuronal pools Input fibers divide to create a stimulatory field If an input fiber excitatory impulse causes a neuron to fire, it is called suprathreshold If an input fiber excitatory impulse isn't enough to reach threshold, it is called subthreshold (and those neurons are facilitated) Facilitated neurons can reach threshold if stimuli are received from another neuron The area of facilitated neurons is called the facilitated zone - aka subthreshold or subliminal zone If incoming fibers are inhibitory - they create an inhibitory zone ❖ Divergence - weak signals entering a neuronal pool excite greater numbers of nerve fibers leaving the pool Amplifying divergence: input signal spreads to an increasing number of neurons Divergence in multiple tracts: signal is transmitted in two directions ❖ Convergence - signals from multiple inputs excite a single neuron Convergence from a single source vs convergence from multiple sources Convergence allows summation of information so CNS can correlate, summate, and sort different types of information ❖ Reciprocal inhibition circuits Incoming signal causes output excitatory signal in one direction and an inhibitory signal going in another direction Characteristic for control of opposing muscle groups Prevent injury of weaker muscle - this is where she talked about glutes vs hamstrings One being contracted "automatically preventions contraction of the opposing muscle" ❖ Signal prolongation Afterdischarge: prolonged output discharge Can last milliseconds to minutes following the incoming signal Synaptic afterdischarge: a single input signal can cause a sustained output via a series of repetitive charges Reverberatory (oscillatory) circuit: feedback from the neuronal circuit feeds back to input the same circuit Varying degrees of complexity Some neuronal circuits continuously emit signals (continuous intrinsic neuronal discharge or reverberatory signals) ◆ Example: respiratory drive ❖ "Review questions of the first half of the content" - "examples of questions that will be on the exam" Large, myelinated, fast: A fibers → proprioception Small, unmyelinated, slow: C fibers → temperature, crude touch Specialized receptors: A beta fibers, 1a fibers ❖ Sensory pathways Most sensory input enters spinal cord through dorsal roots of spinal nerves Sensory information travels to brain via either: 1. Dorsal column (medial lemniscal system) aka DCML "Posterior column" "Enters at dorsal root, and crosses over in the lower level of the medulla" 2. Anterolateral system Anterior spinothalamic tract (crude touch) Lateral spinothalamic tract (pain, temp) "Enters at dorsal root, and crosses before ascending" ❖ DCML anatomy Sensory stimuli travels via type A-beta and C fibers (A-beta is DCML, not C) and enter spinal cord at the dorsal root ganglion The first-order neurons are those located in the dorsal root ganglia These are afferent → carry sensory information to the brain Nerve fibers divide in the spinal cord to medial and lateral branch Medial branch: enters through spinal root → dorsal column → brain Lateral branch: enters through dorsal horn → further divides until some enter dorsal column and travel to the brain, some terminated in spinal cord (reflexes), and some give rise to the spinocerebellar tracts Fibers then travel up fasciculus gracilis (below T6/Medial Branch) or fasciculus cuneatus (C2-T6/Lateral Branch)) These are second-order neurons (we don't think this is right, second order start in medulla?) Fibers the decussate at the contralateral medial lemniscus (medulla) and ascend Terminate in the ventral posterior lateral nucleus of the thalamus - this is where the third order neurons are located She said specifically to know where 2nd and 3rd orders neurons are, and where they cross "1st order - dorsal root ganglia" "2nd order - gracillis or cuneatus, cross at medulla" "3rd order - thalamus" Key points Large, myelinated nerve fibers Rapid conduction velocity: 30-110 m/sec DCML carries localized touch, vibration, movement against the skin, joint sensation, proprioception, pressure Crosses over to opposite side at level of medulla Highly organized nerve fiber structure ❖ Somatosensory cortex Sensory signals terminated in the cerebral cortex Parietal lobe → reception / interpretation of somatosensory signals Occipital lobe → termination of visual signals Temporal lobe → termination of auditory signals Somatosensory area I Lies immediately behind central fissure in postcentral gyrus of the cerebral cortex Area I has the highest degree of localization, is much larger, and is most important Different parts of the body = different regions Size of region correlates with number of specialized receptors Each lateral cortex receives sensory information from the opposite side of the body Somatosensory area II Lies posterior to area I Function is not as well understood The homunculus Means "little human" Used to describe where 3rd order neurons go to transmit different anatomical impulses Used for lead placement by neurophysiologists using SSEP ❖ Proprioception - "position sense" There are two types of position sense: 1. Static position sense - conscious perception of the body in space 2. Kinesthesia (dynamic position sense) - rate of movement sense Receptors are located in joints (determine joint angulation), muscles (muscle spindles), and skin (tactile receptors) Neurons in the thalamus respond to the minimum and maximum joint rotation ❖ Anterolateral system Transmits sensory signals that do NOT require localization, discrimination Only transmission of pain, temperature, tickle, itch, and sexual sensations Conduction velocity is slower that in the DCML Spatial localization and intensity are poor Spinal cord anterolateral fibers originate in dorsal horn of laminae I, IV, V, VI Fibers cross immediately into the anterior commissure of spinal cord → anterior and lateral white columns → spinothalamic tracts → brain Spinothalamic tracts terminate in the brainstem and the thalamus Pain - reticular nuclei of the brainstem Pain - intralaminar nuclei of thalamus Tactile stimulation - ventrobasal complex of the thalamus Sensory stimuli travel from the periphery to the spinal cord before synapsing in the dorsal root ganglia The first order neurons are those located in the dorsal root ganglia These are afferents → sensory information to the brain Fibers synapse with second order neurons in the substantia gelatinosa Fibers deuces to the opposite side in the anterior white commissure, enter the anterolateral portion of the spinal cord, and ascend to the thalamus Enters the brainstem at the spinal lemniscus The third orders neuron cell body lies in the VPL (ventral posterolateral nucleus) of the thalamus ❖ Sensory pathways: review - "know the differences" DCML Consists of large, myelinated nerve fibers Rapid conduction velocity: 30-110 m/sec Carries signals to medulla via (mostly) the dorsal column Crosses over to opposite side at the level of the medulla Continues through brainstem to thalamus via the medial lemniscus Dorsal column carries localized touch, vibration, movement against the skin, joint sensation, pressure Highly organized nerve fiber structure Anterolateral Consists of smaller, myelinated or unmyelinated fibers Slower conduction velocity: 3-40 m/sec Signals enter the spinal cord from dorsal spinal nerve roots Synapse in the dorsal horns of the spinal grey matter Most cross to opposite side of the cord via anterior white commissure Ascend through anterior and lateral white columns of spinal cord Terminate at lower brainstem and thalamus Anterolateral system can transmit pain, warmth, cold, crude touch, tickle, itch, sexual sensations Much less organized nerve fiber structure ❖ Dermatomes Know the landmarks C4 - clavicle T4 - nipples T6 - xiphoid T10 - umbilicus L4-5 - tibia S2-5 - perineum ❖ Herpes Zoster (shingles) Caused by herpes virus - spread via airborne droplets or direct contact with actively viral shedding lesions Virus remains latent in trigeminal and dorsal (sensory) root ganglia, followed years later by reactivation to cause herpes zoster S/sx: pain and paresthesia localized to affected dermatome Thoracic or lumbar dermatome are most common If ophthalmic branch of trigeminal nerve is affected - it is considered a medical emergency as it is sight-threatening ❖ Peripheral nerve injuries Transection - partial or complete destruction of a nerve Compression - pressure on a nerve from bony prominence pressing against an internal or external surface Traction - stretching of a nerve against an immobile surface Injury mechanisms: ischemia, structural disruption, transection (cut) ❖ Pain Acute pain is protective Promotes withdrawal from painful stimuli Allows injury to heal Teaches the avoidance of painful stimuli Pain receptors are free nerves endings ("are everywhere") that are located in skin and other tissues including: periosteum, arterial walls, joint surfaces, and tentorium in the cranial vault Respond to mechanical, thermal, or chemical stimuli Pain is classified into two types: fast pain and slow pain Dual pathways for pain transmission → fast-sharp pain pathway vs slow-chronic pain pathway Fast pain → fast/sharp pain pathway → neospinothalamic Transmitted by small type A-delta fibers Felt within 0.1 seconds Describes as sharp, pricking, acute, or electric pain Usually elicited by mechanical and thermal stimuli Slow pain → slow/chronic pain pathway → paleospinothalamic Transmitted primarily by type C fibers Felt after 1 second or more, then increases Described as slow burning, aching, throbbin, or chronic pain Usually associated with tissue destruction Can be elicited by mechanical, thermal, and chemical stimuli ❖ Pain receptors Nociceptors Sensory receptors that detect signals from damaged tissue or the threat of damage Pain receptors are free nerve endings Pain can be elicited by multiple types of stimuli (mechanical, chemical, thermal) Stimulated by bradykinin, serotonin, histamine, potassium ions, acids, acetylcholine, proteolytic enzymes Enhanced sensitivity by prostaglandins and substance P Average person receives pain at 45 degrees C ("aka the temp when tissues begin to be damaged") Pain often correlates with rate at which damage to tissues is occurring - NOT with tissue damage that occurred Fast pain → mechanical and thermal stimuli Slow pain → mechanical, chemical, and thermal stimuli Pain receptors adapt very little (if at all) Excitation of pain fibers sometimes actually progressively increases (protective purpose) Hyperalgesia: unusually severe pain in situations where pain is normal, but pain is worse than it "should" be "If fibers/tissue is already damaged it can alter the perception/response to pain" ❖ Pain pathways: fast vs slow pain Pain fibers enter the spinal cord from the dorsal spinal roots The first-order neurons are those located in the dorsal root ganglia Within spinal cord, they take one of 2 pathways to the brain Neospinothalamic tract (fast-type pain) Pain fibers enter the spinal cord from the dorsal spinal roots The first-order neurons are those located in the dorsal root ganglia Fast type A-delta fibers transmit mainly mechanical and acute thermal pain Glutamate is excitatory transmitter Fast-type pain can be localized better than slow-type pain ◆ Localization relies on sensory information from touch receptors Terminate mainly in lamina I (lamina marginalis) of dorsal horn where they excite second-order neurons of neospinothalamic tract Second-order neurons cross to opposite side of cord through the anterior commissure → anterolateral column → brain Paleospinothalamic tract (slow-chronic pain) Pain fibers enter the spinal cord from the dorsal spinal roots First-order neurons are those located in the dorsal root ganglia Transmits pain mainly via type C pain fibers plus some type A-delta fibers Peripheral fibers terminate in the spinal cord in lamina II and III (substantia gelatinosa) Most signals pass through 1 or more short fiber neuron before entering at lamina V Join fibers of the fast pain pathway Discuss at anterior commissure Follow anterolateral pathway to brain Most terminate in lower regions of the brain Pain is poorly localized type C pain fibers release both glutamate and substance P ❖ Pain sensations Referred pain: branches of visceral pain fibers synapse in spinal cord on the same second-order neurons that receive pain signals from the skin Visceral pain: pain from organs in abdomen and chest, highly localized types of damage to viscera seldom cause severe pain Diffuse visceral pain can be severe Nociception: processing of painful stimuli, involves four phases Transduction - converting painful stimulus into an electrical signal that is transmitted to the CNS Transmission - conduction of pain impulses along the a-delta and c fibers (primary-order neurons) → dorsal horn Perception - conscious awareness of pain, occurs primarily in the reticular and limbic systems and cerebral cortex Influenced by genetics, culture, sex roles, age, level of health, and past pain experiences Modulation - mechanisms that increase or decrease the transmission of pain signals (neurotransmitters, analgesic drugs, anesthesia, and nonpharmacologic interventions such as transcutaneous nerve stimulation, acupuncture, hypnosis, PT) ❖ Endorphins and enkephalins There are approximately 1-dozen naturally-occurring opiate like substances in the CNS Derived from pro-opiomelanocortin, proenkephalin, and prodynorphin Most important are beta-endorphin, met-enkephalin, leu-enkephalin, dynorphin Not completely understood Can activate analgesia and/or inactive pain pathways ❖ Special senses - vision, hearing, balance, gustation, smell Vision Eye anatomy Sclera - white of the eye, connective tissue; continuous with the dura mater around CN II Retina - cones/rods inner layer of the eye; macula is the reddish circle around the fovea Cornea - curved transparent outer layer Iris - colored muscular portion which elicits mydriasis and miosis Lens - behind the cornea; viscous gel; aqueous and vitreous humor Retina contains rods and cones - special photoreceptors that convert light energy into nerve impulses Rods mediate peripheral and dim light vision Cones are color and detail receptors Nerve impulses pass through the optic nerves (CN II) to the optic chiasm Fibers of the optic tracts terminate in the primary visual cortex in the occipital lobe of the brain Some fibers terminate in the hypothalamus and are involved in circadian regulation / sleep-wake cycle Glaucoma Eye is filled with vitreous humor and aqueous humor to give it shape, keep it from collapsing Aqueous humor: formed by ciliary body, flows through the pupil → anterior chamber of the eye → exits via the canal of schlemm ◆ Maintains IOP (normal: 12-20 mmHg) ◆ Glaucoma, IOP can be as high as 60-70 mmHg Open angle glaucoma: most common, arises slowly and is non-painful, mismatch in AH production and drainage Closed angle glaucoma: can arise slowly or suddenly, "sudden variety" (10% of cases) is a medical emergency and the patient will lose their sight if it's not surgically managed Retrobulbar block Retrobulbar block apnea syndrome - a complication of these blocks ◆ Injection of LA for a retrobulbar block that enters the optic nerve sheath can spread centrally and produce unconsciousness and apnea ◆ Treatment is supportive Intubation / ventilation Manage cardiac arrhythmia ◆ "Can cause hemorrhage" and "yes, CRNAs will do this" "Anesthesia and the eye" Intraocular pressure and anesthesia Increase IOP ◆ Succinylcholine (5-10 mmHg for 5-10 minutes) ◆ Ketamine? ("source varies") ◆ Intubation - "SNS stimulation" Decrease IOP ◆ Inhalation anesthetics ◆ Propofol ◆ Opioids ◆ Benzodiazepines N2O should not be used in eye surgery especially if surgeon is placing a gas bubble Oculocardiac reflex - "five and dime" and Aschner reflex aka trigeminovagal reflex Traction on extraocular muscles, pressure on eyeball, and administration of a retrobulbar block - can all elicit a wide variety of cardiac dysrhythmias (bradycardia, ventricular ectopy, VF) Prevention and treatment strategies ◆ "Remove stimulus, deepen anesthetics, atropine/glycopyrrolate" "Common in pediatrics - strabismus repair" Visual evoked potentials (VEP) Performed by neurophysiologists during surgery with high potential for CN II damage Transsphenoidal hypophysectomy Craniotomy for disease near CN II Flashes of light are emitted, and electrodes placed over the occipital lobe, verify the integrity of the circuit VERY sensitive to anesthetic agents Anesthetic agents lower the amplitude and latency of the signal making it hard for the neurophysiologist to accurately assess integrity of the pathway Corneal abrasions Most common peri-op period ocular injury "One of my biggest pet peeves" - Dr. S ❖ Hearing and balance The ear is composed of external, middle, and inner structures External ear structures are the pinna, auditory canal, and tympanic membrane - only involved in hearing Middle ear is composed of the tympanic cavity, oval window, eustachian tube, and fluid - only involved in hearing Tympanic cavity contains three bones: malleus, incus, and stapes Inner ear is involved in hearing and equilibrium Includes the bony and membranous labyrinths that transmit sound waves Includes the semicircular canals and vestibule - help maintain balance Vestibular system: balance and spatial orientation, balance Small tract of neurons descend from the vestibular system to form the vestibulospinal tract The vestibulospinal tract again forms a DIRECT connection between position sense and efferent motor neurons (bypassing cognition) to keep the body in balance ❖ Gustation - "Some people are taste blind" ❖ Smell - "Least understood" "CN I"