Neurophysiology Exam Two.pdf

Sensation, Motor, and Special Senses Nervous System Nervous system and endocrine system help regulate homeostasis o Endocrine (long term), nervous system (short term) Sensations: describe anything sensory. What you see, hear, smell, taste, fee...

Sensation, Motor, and Special Senses Nervous System Nervous system and endocrine system help regulate homeostasis o Endocrine (long term), nervous system (short term) Sensations: describe anything sensory. What you see, hear, smell, taste, feel. Pressure, temperature, pain, proprioception CNS o Brain, spinal cord PNS o Somatic o Autonomic (controls visceral functions like smooth muscles and glands) ▪ Sympathetic: fight/flight ▪ Parasympathetic: rest/digest ▪ Enteric: digestion, enzyme secretion Terms Definition Afferent Inward, toward the CNS Efferent Outward, away from the CNS Message Dorsal Back/posterior Transmission Ventral Anterior/front Decussation The action of crossing Ipsilateral Same side of the body Contralateral Opposite side of the body Transduction Converting sensory stimuli to electrical signal to send to CNS Transmission Conduction of sensory impulses from periphery to CNS Perception Conscious awareness of sensory modality Types of Sensations Mechanoreceptive (touch, pressure) Thermoreceptive (temperature) *All sensation involves receptor activation → neuron activation → Pain impulse transmission to the spinal cord and brain Exteroceptive (from surface of the body) Proprioceptive (position sense) Visceral (internal organs) Deep sensations (from deep tissue) Specialized Sensory Receptors Mechanoreceptors o Detect mechanical compression or stretching Thermoreceptors o detect changes in temperature Nociceptors o detect physical or chemical damage to tissue (pain) Electromagnetic o detect light on retina of eye Chemoreceptors o detect taste/smell, O2/CO2 in blood, osmolality etc. Mechanoreceptor Types [information said in class mixed with Guyton and 1) Free nerve endings Hall page number listed on PP] o Found everywhere in the skin and many o Somatic senses are classified into three physiological other tissues types: o Can detect touch and pressure o mechanoreceptive somatic senses (include both o All pain receptors are free nerve endings tactile and position) ▪ There at least six different types of tactile receptors 2) Meissner’s Corpuscle o thermoreceptive senses (detect heat and cold) o Present in the non-hairy parts of the skin o pain sense o Abundant in fingertips and lips o Adapt in a fraction of a second o Sensitive to skin touch or low-frequency vibration 3) Expanded Tip Tactile Receptors o One type of these is Merkel’s discs o Responsible for steady state signals so you can determine continuous touch of objects on skin o Initially transmit strong signal, then adapt slowly 4) Hair-end Organ o Detects movement of objects on the surface of the body and initial contact with the body o Stimulated by any slight movement of any hair 5) Ruffini’s Endings o Located in deeper layers of the skin/internal tissues o Adapt very slowly o Important for signaling continuous states of deformation of tissues (heavy prolonged touch and pressure) o Also found in joint capsules (signal degree of joint rotation) 6) Pacinian Corpuscle o Structural design: has a central nerve fiber extending through its core surrounded by multiple concentric layers o Lie immediately beneath skin and in deep fascial tissues o Stimulated only by rapid local compression; adapts very quickly o Detect tissue vibration/rapid changes in the mechanical state of tissues o Compression of the corpuscle causes mechanical deformation [causes receptor potential] o Ion channels open, Na ions enter interior of fiber → receptor potential o If threshold is reached, an action potential is elicited o Initial amplitude of receptor potential increases rapidly o With progressively stronger stimulus strength, amplitude is diminished but the frequency of repetitive action potentials increases o Action potential is transmitted along the nerve fiber to the CNS o This is how a receptor can be responsive to both weak & intense stimuli! Receptor Adaptation o Initial receptor response: HIGH o Continued/frequent stimulation: diminished response o Different types of receptors adapt at different rates o Pacinian receptors are rapid adaptors (fraction of a second) o Hair receptors adapt in 1 second o Rapid adaptor receptors are better for sensing changes o rate receptors, movement receptors, phasic receptors o Slow adaptor receptors transmit impulses to the brain if stimulus is present (better for sensing constant conditions in the body) o Arterial baroreceptors o Arterial chemoreceptors Neuron Activation and Transmission Differential sensitivity: receptors are designed for different and specific stimuli; are almost non- responsive to other types of stimuli Labeled line principle refers to specificity of nerve fibers to transmit only one sensation. Type of sensation felt is determined by specific fiber stimulated. Each nerve tract terminates at a specific point in the CNS Receptor Potentials: Stimuli changes the electrical membrane potential (receptor potential) causing ion channels to open. This can be caused by o Mechanical deformation: pressure causes membrane stretch, opening ion channels ▪ Ex: Pacinian corpuscle o Chemical application: chemical applied [ChatGPT Explanation of AP and Amplitude] makes ion channels open 1. When a receptor (like those in your skin) is stimulated by something (like touch), o Temperature: changes in temp alter it creates a small electrical change called a receptor potential. If this electrical change is strong enough to cross a certain threshold, it triggers action potentials. These are membrane permeability the electrical signals that nerves use to communicate. o Electromagnetic radiation: light on retina causes ion channels to open 2. The graph on the left shows that the resting membrane potential (the baseline Action Potentials electrical state of the neuron) is quite stable. Once the stimulus is strong enough to exceed the threshold, action potentials start firing. o Threshold: potential must rise to this (-40 to -50) to elicit an action potential 3. The graph on the right illustrates that as the stimulus gets stronger, the receptor o Frequency: the more the receptor potential potential initially rises sharply but then levels off even if the stimulus continues to rises above threshold, the greater the increase. This means the cell's response grows quickly at first but slows down as the stimulus becomes very strong. Amplitude in this context refers to the strength of action potential frequency (rapid firing) the receptor potential, not the external stimulus itself. This potential increases as ▪ Very intense stimulation causes the stimulus strength increases. progressively less and less additional action potentials 4. The frequency of action potentials increases as the receptor potential increases. This means that more intense stimuli cause the neuron to fire more rapidly. As the ▪ The frequency of repetitive receptor potential increases (due to a stronger stimulus), the frequency of the action receptor action potentials potentials also increases. This means that the neuron fires more rapidly when increases with the strength of the stimulated more intensely. receptor potential 5. Sensitivity to Stimuli: The system is set up so that very weak stimuli can be ▪ Allows range of sensory detected, and the neuron does not reach its maximum firing rate until the stimulus is experience extremely strong. This allows for a broad range of stimulus intensities to be encoded Amplitude: As stimulus strength increases, the by the sensory nerves. receptor amplitude increases rapidly at first then ** So, in summary, a stronger stimulus leads to a higher receptor potential (greater more slowly as intensity increases amplitude), and this higher potential causes the neuron to fire more rapidly (higher frequency of action potentials). This is how different intensities of stimuli are encoded and transmitted by sensory neurons. **  Divergence and Convergence Divergence (spread apart) o Weak signals entering a neuronal pool excite greater numbers of nerve fibers leaving the pool o Amplifying divergence → input spreads to increasing number of neurons o Divergence in multiple tracts → signal is transmitted in two directions Convergence (come together) o Signals from multiple tracts unite to excite a neuron o Either converge from a single or multiple source o Convergence allows summation of information so CNS can correlate and sort different info Reciprocal Inhibition Circuits Incoming signal causes excitatory signal in one direction, inhibitory in another Helps control opposing muscle groups o Triceps and biceps Signal Prolongation Afterdischarge: prolonged output discharge that 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 neuronal circuit feeds back to input the same circuit o Has varying degrees of complexity o Some circuits continuously emit signals (cont. intrinsic neuronal discharge/reverberatory signals) → ex: resp drive Summation Spatial Summation o Increasing signal strength is transmitted by using progressively greater number of fibers Temporal Summation o Increasing signal strength is transmitted by increasing the frequency of nerve impulse in each fiber Neuronal Pools o Definition: groups of specially organized neurons that share common inputs, outputs and functions o CNS is composed of thousands to millions of neuronal pools o Input fibers divide to create a stimulatory field o If an input fiber excitatory impulse causes a neuron to fire, it is called suprathreshold o If an input fiber excitatory impulse isn’t enough to reach threshold, it is called subthreshold (and those neurons are facilitated) o Facilitated neurons can reach threshold if stimuli are received from another neuron o The area of facilitated neurons is called Stimulatory field the facilitated zone (aka subthreshold zone or subliminal zone) o If incoming fibers are inhibitory (instead of excitatory), they create an inhibitory zone Nerve Fibers Transmission o Nerve fibers are categorized by function, size, degree of myelination o Nerve is a bundle of axons o Nerve diameters ▪ 0.5 micrometers to 20 micrometers o Conduction velocity ▪ 0.5-120 m/sec ▪ Influenced by size (bigger = faster), function, degree of myelination o Two classification systems ▪ General (Types A, B, C) o Type A: large/medium, myelinated, FAST o Type B: smaller, myelinated, pre- ganglionic (ANS) o Type C: small, unmyelinated, SLOW ▪ Sensory Nerve Classification (Group I-IV) Type I: large, myelinated, FAST; fibers from muscle spindles, Golgi tendon and organs II & III: myelinated Type IV: small, unmyelinated, SLOW; crude touch, pressure, tickle, aching pain, temperature o Differential Blockade: the idea that different nerve fibers block at different rates ▪ Blocks happens this order: sympathetic, sensory, and motor ▪ the belief that sensitivity to local anesthetics is inversely proportional to axon diameter ▪ When a local anesthetic interrupts nerve transmission of autonomic nerves but not sensory nerves or motor nerves (because of a variation in susceptibility), a “differential block” is said to have occurred. (Nagelhout) Sensory Pathways Almost all sensory information from somatic areas of the body enters the spinal cord through the dorsal roots of the spinal nerves Once signal enters the spinal cord, there are two sensory pathways → dorsal column-medial lemniscal system and the anterolateral. These two systems come back partially at the thalamus o Dorsal Column (Medial Lemniscal System) “ascend and then cross” ▪ Carries signals up the dorsal columns of the cord ▪ Signals cross in the medulla, then continue upward through the brain stem to the thalamus via the medial lemniscus ▪ Large, myelinated fibers ▪ Transmits mechanoreceptive sensations: fine touch, proprioception, vibration, and pressure Proprioception (position sense): located in joints, muscles, and skin. Neurons in thalamus respond to minimum and maximum joint rotation. There are two types of position sense: o Static position sense: conscious perception of body in space o Kinesthesia (dynamic position sense): rate of movement Capable of two-point discrimination Mechanoreceptors involved include: Meissner’s corpuscles, Merkel’s discs, Ruffini’s endings, Pacinian corpuscles o Anterolateral System “cross and then ascend” ▪ After signal enters spinal cord, crosses to opposite side and ascends through anterior and lateral columns; terminate at all levels of the lower brain stem and in the thalamus Anterior Spinothalamic Tract (crude touch) Lateral Spinothalamic Tract (pain and temperature) ▪ Smaller myelinated fibers ▪ Less spatial orientation. Transmits pain, thermal sensations, crude touch/pressure, tickle, itch, sexual sensations Dorsal Column Medial Lemniscal System (DCML) Anatomy (A-beta fibers) o First order neurons are in the dorsal root ganglia o After synapsing with 1st order neuron, fibers immediately divide to form the medial and lateral branches upon entering the spinal cord ▪ Medial branch: turns medially first, then upward in the dorsal column ▪ Lateral branch: enters the dorsal horn of the cord, then divides many times to provide terminals that synapse with local neurons. Some enter dorsal column, some terminate in the spinal cord (elicit spinal cord reflexes), and some give rise to the spinocerebellar tracts o Signal is passed uninterrupted up to the dorsal medulla either through fasciculus gracilis (medial, carries sensory input below T6) or fasciculus cuneatus (lateral area, carries sensory input from C2-T6). Signal synapses with a second order neuron in the dorsal column nuclei in the medulla. This is where the fiber decussates (crosses)  o After crossing, the fiber ascends via the medial lemnisci tract to the thalamus o In the thalamus, the medial leminiscal fibers terminate in the ventral posterior lateral nucleus (VPL) of the thalamus and connect with a third order neuron. Third order neuron projects to somatosensory area I Somatosensory Cortex Sensory signals terminate in the cerebral cortex o Parietal lobe: reception/interpretation of somatosensory signals o Occipital lobe: termination of visual signals o Temporal: termination of auditory signals Somatosensory Area I o Immediately behind central fissure in postcentral gyrus of cerebral cortex o Accounts for the highest degree of localization (much larger, most important) o Different regions of this area account for different parts of the body ▪ Size of region correlates with number of specialized receptors ▪ Lips have the most receptors, followed by face and tongue o Each lateral cortex receives sensory info from the opposite side of the body o The Homunculus (“little human”) is used to describe where 3rd order neurons go to transmit different anatomical impulses ▪ Used for lead placement for SSEP Somatosensory Area II o Lies posterior to Area I o Function not understood Anterolateral System (A-delta and C fibers) Transmits signals that don’t require localization/discrimination (i.e. pain, temp, tick, itch, sexual sensations) Conduction is slower, spatial localization/intensity is poor Sensory (afferent) information travels from periphery to spinal cord before synapsing with first order neurons in the dorsal root ganglion These fibers then synapse with second order neurons in the substantia gelatinosa (laminae II, III, and IV) o Anterolateral fibers originate is dorsal laminae I, IV, V, and VI Fibers cross (decuss) through the anterior white commissure to the anterolateral portion on opposite side; they then turn upward and travel toward brain via anterior spinothalamic or lateral spinothalamic tracts Spinothalamic tracts terminate in brainstem (RAS) and thalamus; When spinothalamic tracts enter the brainstem, they merge back together and are called the spinal lemniscus o Termination in Brainstem ▪ Reticular nuclei (pain) o Termination in Thalamus ▪ Ventrobasal complex (tactile stimuli) ▪ Intralaminar nuclei (pain) ▪ 3rd order neurons lie in the VPL (ventral posterolateral nucleus) of the thalamus ▪ After termination in the thalamus, they travel with dorsal column fibers to somatosensory cortex Pain Receptors o Acute pain in protective! ▪ promotes withdrawal from pain stimuli, teaches avoidance of pain stimuli, allows injuries to heal o Adapt very little (if at all) o Pain receptors (nociceptors) are free nerve endings; defined as sensory receptors that detect signals of tissue damage or threats of damage; located in the skin and other tissues (periosteum, arterial walls, joint surfaces, and the tentorium in the cranial vault) ▪ Can be elicited by multiple types of stimuli (mechanical, chemical, thermal) Chemical o Chemicals that excite/stimulate pain are bradykinin, serotonin, histamine, potassium ions, acids, acetylcholine, proteolytic enzymes o Prostaglandins and Substance P enhance the sensitivity of pain endings but don’t excite them o Excitation of pain fibers can progressively increase pain for protective purposes o Hyperalgesia: unusually severe pain in situation where pain is normal (enhanced pain in responses to noxious stimuli) ▪ Average person perceives pain at 45º C (temperature tissues start to be damaged by heat) ▪ Pain often correlates with rate at which damage to tissues is occurring, NOT with tissue damage that occurred o Classified as either fast pain or slow pain o Dual pathways for pain signals to the CNS (fast-sharp and slow-chronic) Pathways o After crossing, pain fibers take one of two pathways → neospinothalamic or paleospinothalamic ▪ Neospinothalamic tract (fast type, direct) Fiber: A-Delta Felt: within 0.1 seconds Termination (Spinal Cord): Lamina I (lamina marginalis); second order neurons cross at anterior commissure, ascend through anterolateral columns Pain Type: mechanical, acute thermal pain, fast pain; described as sharp, prickling, acute, or electrical pain Stimuli: usually elicited by mechanical or thermal Neurotransmitter: glutamate Localization: more accurate o Localization relies on sensory information from touch receptors Termination (Brain): thalamus → cortex ▪ Paleospinothalamic tract (slow-chronic, indirect) Fiber: mainly C fibers, some A-delta Felt: Within 1 second or more Termination (Spinal Cord): laminae II and III (substantia gelatinosa); some fibers synapse with fibers in laminae V before crossing and then ascending Pain Type: slow, contributes to emotional/autonomic aspects of pain, described as slow burning, aching, throbbing, or chronic pain. Usually associated with tissue destruction Stimuli: elicited by mechanical, thermal, and chemical Neurotransmitter: glutamate (instant) and substance P (slow) Localization: poor, imprecise Termination (Brain): lower regions of brain (medulla, pons, mesencephalon)  Pain Sensations Referred Pain: personal feels pain in a part of the body that is remote from the tissue causing the pain; mechanism of referred pain considers that branches of visceral pain fibers synapse in the spinal cord on the same second-order neurons that receive pain signals from the skin, making the person have the feeling that the sensation/pain originated in 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 o Transduction: converting painful stimulus into an electrical signal that is transmitted to the CNS o Transmission: conduction of pain impulses along the Aδ and C fibers (primary-order neurons) →dorsal horn o 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 o 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 one 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 inactivate pain pathways Dermatomes Clavicle: C4 Nipples: T4 Xiphoid: T6 Umbilicus: T10 Tibia: L4-L5 Perineum: S2-S5 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 Symptoms: Pain and paresthesia localized to the affected dermatome o Thoracic or lumbar dermatome are most common but can appear on any dermatome o If Ophthalmic branch of the trigeminal nerve is affected, it should be considered a medical emergency as it is sight threatening Peripheral Nerve Injuries Classification o Transection: partial or complete destruction of a nerve o Compression: pressure on nerve from bony prominence pressing against an internal or external surface o Traction: stretching of a nerve against immobile surface Injury mechanisms o Ischemia o Structural disruption o Transection Special Senses: Vision Eye Anatomy o Sclera: white of the eye, connective tissues; continuous with dura mater around CN II o Retina: cones/rod, inner layer of the eye; macula is the reddish circle around the fovea o Cornea: curves transparent outer layer o Iris: colored muscular portion which elicits mydriasis (dilation) and miosis (constriction) o Lens: behind the cornea, viscous gel; aqueous and vitreous humors Vision o 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 o Nerve impulses pass through the optic nerves (CN II) to the optic chiasm o Fibers of the optic tracts terminate in the primary visual cortex in the occipital lobe of the brain o Some fibers terminate in the hypothalamus and are involved in circadian regulation/ sleep-wake cycle Glaucoma o Eye is filled with vitreous humor and aqueous humor (gives it shape, keeps it from collapsing) ▪ Aqueous humor: formed by ciliary body, flows through the pupil to anterior chamber of the eye; exits via the Canal of Schlemm ▪ Maintains intraocular pressure (normal= 12-20 mmHg) o Glaucoma, IOP can be as high as 60-70 mm Hg ▪ Open angle glaucoma: Most common, arises slowly and is non-painful, mismatch in aqueous humor production and drainage ▪ Closed angle glaucoma: Can arise slowly or suddenly; “Sudden variety” (accounts for 10% of cases) is a medical emergency, and the patient will lose sight if not surgically managed  Retrobulbar Block Apnea Syndrome (result of block) o Injection of local anesthetic for a retrobulbar block that enters the optic nerve sheath can spread centrally and produce unconsciousness and apnea o Treatment is supportive ▪ Intubation / ventilation ▪ Manage cardiac arrhythmia ▪ Can get hemorrhage in the eye Anesthesia and the Eye o Increase IOP ▪ Succinylcholine (5-10mmHg for 5-10mins) ▪ Ketamine ▪ Intubation o Decreased IOP ▪ Inhalation anesthetics ▪ Propofol ▪ Opioids ▪ Benzodiazepines o N2O should not be used in eye surgery especially if surgeon placing a gas bubble o Corneal abrasions are the most common ocular injury in the perioperative period o Oculocardiac Reflex (Aschner Reflex, Trigeminovagal Reflex) ▪ Traction on extraocular muscles, pressure on eyeball, and administration of a retrobulbar block, can elicit a wide variety of cardiac dysrhythmias (bradycardia, ventricular ectopy, VF) ▪ Prevention and treatment strategies Tell them to stop/remove stimulus Deepen anesthetics Atropine/glyco ▪ [From Cranial Nerve lecture] ▪ Aka “5 & Dime”, described by Aschner in 1908 ▪ Pressure applied to eyeball (on globe or traction to eyeball) caused a 10% decrease in HR ▪ Afferent is Trigeminal, efferent is Vagus ▪ Triggers of reflex: traction/manipulation on extraocular muscle or tissue in orbital apex, direct pressure on globe, ocular pain, retrobulbar block, ocular trauma ▪ Manifestations: SB, junctional rhythm, ectopic beats, AV block, VT, asystole o Treatment: notify surgeon to stop stimulation, deepen anesthetic/prevent light anesthesia, optimize O2 and ventilation, atropine or glycol Visual Evoked Potentials o Performed by neurophysiologists during surgery; high potential for CNII damage ▪ Transsphenoidal hypophysectomy ▪ Craniotomy for disease near CNII o Flashes of light are emitted, and electrodes placed over the occipital lobe, verify the integrity of the circuit o 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 Hearing and Balance The ear is composed of external, middle, and inner structures. o External ear ▪ Structures: pinna, auditory canal, and tympanic membrane. ▪ Only involved in hearing o Middle ear ▪ Composed of the tympanic cavity (containing three bones: the malleus, the incus, and the stapes), oval window, eustachian tube, and fluid. ▪ Only involved in hearing o Inner ear ▪ Includes the bony and membranous labyrinths (transmit sound waves) and the semicircular canals and vestibule (help maintain balance) ▪ Involved in both hearing and equilibrium Vestibular System o Accounts for balance and spatial orientation o Small tract of neurons descend from the vestibular system to form the vestibulospinal tract o The vestibulospinal tract again forms a DIRECT connection between position sense and efferent motor neurons (bypassing cognition) to keep the body in balance