Advanced Human Physiology Chapters PDF

Advanced Human Physiology Chapters 9A Sensory Systems: Somatic Senses Lectures by Glorimar Aponte-Kline M.D. Utah Tech University Afferent Division of Nervous System Recall from Ch...

Advanced Human Physiology Chapters 9A Sensory Systems: Somatic Senses Lectures by Glorimar Aponte-Kline M.D. Utah Tech University Afferent Division of Nervous System Recall from Chapter 7 that the afferent division of the neurons convey AP’s from receptor in the somatic or special senses to CNS. In this chapter we will be emphasizing on the Sensory Afferent Division of the Nervous system. This chapter discusses sensory information that reaches the conscious and subconscious level of perception. Recall: Stimulus  Receptor  Ascending signal/pathway  Integrator Sensation Sensation is the conscious or subconscious awareness of changes in external or internal environment. The primary function of the cerebral cortex is perception. Perception is the conscious awareness and interpretation of sensations. A. Process of Sensation 1. Involves four steps a. Stimulation of the sensory receptor. 1) A structure of the nervous system that is associated with a sensory (afferent) neuron. b. Transduction of the stimulus 1) Energy in stimulus is converted to a graded potential (receptor potential) c. Generation of action potentials. 1) if graded potential (receptor potential) reaches threshold d. Integration of sensory input 1) Can be modified, allowed to continue as is or terminated Sensory Transduction converts Energy into Graded Potentials. Stimulus energy converted into information processed by CNS. Conversion of stimulus energy (chemical or physical) will open or close channels in receptor membrane producing graded potentials. “Generator potential” or “receptor potential” are simple terms meaning the graded potential recorded at the sensory receptor. Adequate stimulus: form of energy (chemical, mechanical, thermal, photo or light energy) to which a receptor is most responsive. Each sensory receptor responds best to one type of stimulus energy. Threshold: is the minimum stimulus needed to activate a receptor Receptor potential: change in sensory receptor membrane potential (graded potential) which either: 1. initiates AP to CNS 2. influences a neurotransmitter release altering electrical activity in associated sensory neuron  Different Type of Sensory Receptors Stimuli activate sensory receptors. Most stimuli are in the form of mechanical energy: pressure, sound waves, light or heat energy, or chemical energy. Five major groups of Sensory Receptors: 1.Mechanoreceptors a. Sensitive to mechanical stimuli such as the deformation, stretching, or bending of cells. 2.Thermoreceptors a. Detect changes in temperature. 3.Photoreceptors a. Detect light that strikes the retina of the eye. 4.Chemoreceptors a. Detect chemicals in the mouth (taste), nose (smell), and body fluids. 5.Nociceptors a. Respond to painful stimuli resulting from physical or chemical damage to tissues. Different Types of Sensory Receptors Sensory receptors are either: 1) Peripheral nerve endings(dendrites)Fig. (a) a) Encapsulated nerve endings - Ex. pressure, vibration, some touch sensations. b) Free nerve endings (naked) - Ex. pain, itch, thermal, tickle, some touch sensations 2) Separate cell Fig. (b) a) specialized cells that release neurotransmitter onto sensory neurons. - Ex. gustatory receptor cells, photoreceptor cells, hair cells (ear) Receptive Fields Receptive Field of a sensory neuron 1) Defined as the stimulated area, specific group of chemicals, or particular set of sound frequencies that causes a response in that neuron. 2) Depends on the type of sensory neurons involved: (a) Integumentary System – skin -adequate stimulus anywhere within the skin cause a somatic sensory neuronal response. (b) Visual System - eye – non somatic – area of visual space where light causes a response in visual neuron. (c) Olfactory System - nose - select group of chemical odorants cause olfactory receptor cells to respond. (d) Auditory System - ear – set of sound frequencies elicits response in auditory neurons innervating hair cells of inner ear. (e) Gustatory System – tongue – specific type of tastants (chemicals in food) cause response in taste neuron suppling taste receptor cell in tongue. Separate Receptive Field Neighboring Sensory neurons in separate locations have separate receptive field. A given neuron will only respond if there is a stimulus present in the receptive field associated with that neuron. Overlapping Receptive Field Neighboring sensory neuron with overlapping receptive field will respond to a stimulus that extends into the region of overlap, but the neuronal response will be proportional to the relative position of the stimulus. The presence of overlapping receptive fields enhances the ability of the brain to localize somatic sensory and visual stimuli. Sensory Coding Sensory Systems encode four attributes of a stimulus: 1. Stimulus modality a. Determined by the stimulus itself b. Modality is associated with a specific group and circuit of neurons that deliver the information to the brain 1) Label line coding 2. Stimulus location a. Ability to detect where the stimulus is 1) Two point discrimination 3. Stimulus intensity a. Determined by the frequency of action potentials 4. Stimulus duration a. Determined by the duration at which the action potentials are delivered  Sensory Coding Modality = sensation (touch, pain, hearing, vision, & taste) 1) Stimulus modality a. determined by stimulus itself b. modality is associated with a Label specific group and circuit of neurons line (label line) that deliver the information to specific parts of the brain from the peripheral receptors. The specific region of the cerebral cortex then perceives the modality. 1) Label line coding- association of modality with activation of particular labeled line. Stimulus Location: Two-point discrimination Somatic sensory and visual neuron are activated by stimuli that fall within a receptive field. Acuity – the ability to precisely locate and distinguish one stimulus from another. The size of the receptive field varies inversely with the number of sensory receptors it contains. One receptive field is associated with one sensory neuron = Primary sensory neuron= first order neuron The primary sensory neuron synapses with a secondary neuron (postsynaptic neuron); one CNS neuron = Secondary sensory neuron = second order neuron. Two point discrimination – ability to perceive two points applied to the skin as two separate points. Two point discrimination test: a two point caliper is applied to the skin: (a) if the two caliper points stimulate the same receptive field, then the input is conveyed into the CNS along one pathway and only one point of touch is perceived. Ex. Back & calf (b) if the two caliper points stimulate different receptive fields and the input is conveyed into the CNS along two separate pathways, then two points of touch are perceived. Ex. Finger tips Stimulus location: Lateral Inhibition Lateral Inhibition -input from sensory receptors along the border of a stimulus is inhibited compared to input from sensory receptors at center of stimulus. Isolates location of stimulus: There is decrease in number of action potentials transmitted by peripheral areas. Center pathway is slightly inhibited and continues to transmit a higher frequency of action potentials than peripheral area. Results in the ability of the brain to determine exact location of stimulus. Stimulus intensity Intensity – Strength of stimulus. Two main factors encode intensity of stimulus. a) Frequency of AP’s generated in response to a stimulus - Frequency coding – AP frequency determines intensity of stimulus. b) The number of sensory receptors activated by the stimulus - stronger stimulus activates more receptors Nervous system interprets the increase in sensory receptor activation as an increase in stimulus intensity. Stimulus Duration: Receptor adaptation Adaptation: process whereby peripheral, sensory receptors decrease their responsiveness to a constant stimulus applied over time. This occurs because cation channels inactivate after being open for a period of time. Receptors fall into 2 classes: Tonic receptors vs Phasic receptors a) Tonic receptors are slowly adapting; When first activated, they fire rapidly, then slow and maintain their firing as long as stimulus is present. Examples include: pressure sensitive baroreceptors, irritant receptors, proprioceptors, some tactile receptors. b) Phasic receptors are fast adapting; when they receive a stimulus they fire but cease firing if the stimulus remains constant. Examples include: smell receptors Sensory Pathway Sensory pathway – a group of parallel chains of neurons that conveys sensory information from sensory receptors in the periphery to the cerebral cortex. 1) Components: a. First order neurons –sensory receptors from body which convey information to CNS. Axons synapse with second order neurons in brain or spinal cord. b. Second order neurons- interneuron- located either in brain or spinal cord. Axons ascend and synapse with third order neuron in thalamus. c. Third order neuron – interneuron- axons project to fourth order neurons in primary sensory cortex where perception of sensation occurs. d. Fourth order neurons - extend to association areas of cerebral cortex where integration of sensory information takes place. 2) Decussate – sensory pathways cross over to opposite side of body at the spinal cord or brain stem. Not all sensory pathways decussate.  The Somatic Sensory System: Tactile Sensations Tactile sensations 1. Encompass a variety of sensations: a) Touch- Result from stimulation of tactile receptors in the skin b) Pressure - Occurs with deeper deformation of the skin and subcutaneous layer. c) Vibration - Result from rapid, repetitive sensory signals from tactile receptors. d) Itch -Stimulation of free nerve endings in the skin by certain chemicals, such as histamine or bradykinin. e) Tickle -Can arise when someone else touches you (not when you touch yourself).  Somatic Sensory System: Touch Tactile sensations Tactile 2. Tactile Receptors: Receptors Rapid adapting tactile receptors: a) Meissner corpuscles- nerve ending enclosed by capsule, in upper dermis of glabrous skin, generate AP’s at onset of touch and low frequency vibrations b) Hair root plexus – nerve ending wrapped around hair follicle, in hairy skin, detect movement on skin surface. c) Pacinian Corpuscles – nerve ending enclosed by multilayer connective tissue capsule, in dermis and subcutaneous layer, and other body tissues, respond to high frequency vibrations Slowly adapting tactile receptors: d) Merkel discs- AKA Type I cutaneous mechanoreceptors, saucer shaped free nerve endings, at border of epidermis and dermis, respond to continuous touch and pressure e) Ruffini corpuscles – AKA Type II cutaneous mechanoreceptor, elongated encapsulated receptor, in subcutaneous layer and dermis, and other body tissues, sensitive to skin stretching and pressure f) Free nerve endings – naked, under surface of skin, sensitive to chemicals (histamine & bradykinin), detect itch & temperature, mediate tickle sensation.  Transduction of Tactile Stimuli Transduction in a Pacinian corpuscle involves the opening of cation channels in response to a mechanical stimulus (vibration) Transduction of tactile stimuli 1. Mechanically gated channels are open due to pressure. 2. Creates ion influx 3. Creates depolarizing receptor potential and if it reaches threshold will produce an AP  Thermal Sensations Free nerve endings Transduction in warm and cold Terminate in subcutaneous layers receptors Detected by thermoreceptors: Cold receptors Activated by temperatures between 10-35 O C Warm receptors Fewer than cold Activated by temperatures between 30-45 O C Thermoreceptors use cation channels called transient receptor potential (TRP) channels to initiate AP’s. Warm receptors have TRPV3 channels open via warm temperatures Cold receptors have TRPV8 channels open via cold temperatures Chemical(camphor) stimuli opens TRPV3 channels causing a warm sensation Chemical(menthol) stimuli opens TRPM8 channels causing a cold sensation TRPV3 =Transient Receptor Potential, Vanilloid subfamily member 3 TRPM8 = Transient Receptor Potential Melastatin subfamily Pain Sensations Protect the body from stimuli that can cause tissue damage a. Types of Nociceptors (pain receptors) 1. Mechanical – respond to intense mechanical stimuli, such as a pinch or puncture 2. Thermal – respond to extreme thermal stimuli such as temperatures above 45oC or below 10oC 3. Polymodal – respond to a variety of stimuli, including intense mechanical  Pain Sensation: Transduction of Nociceptors Transduction of nociceptors Free nerve endings Contains TRPV1 channels in membrane of polymodal nociceptors Cation channels open in response to extreme heat or to a chemical - capsaicin (in chili peppers). TRPV1 = Transient Receptor Potential Vanilloid  Fast and Slow Pain Pain Subjective perception Fast pain Sharp, pricking sensation that is localized; rapidly transmitted to CNS by A (A- delta) fibers Small myelinated axons with conduction velocities ranging from 12- 30m/sec. Associated with mechanical and thermal nociceptors Slow pain Dull, aching and diffuse; slow transmitted to CNS by C fibers Small unmyelinated axons with conduction velocities ranging from 0.5-2 m/sec. Associated with polymodal nociceptors Pain Pathways Nociceptors activate two pathways: 1) spinal reflex Integrated in spinal cord provide unconscious protective responses Withdrawal reflex – rapid and unconscious response ex. Touch hot stove, step on tack Pain Pathways Nociceptors activate two pathways: 2) Ascending pathways to cerebral cortex. Primary sensory fibers synapses in spinal cord One ascending pathway provides conscious awareness of pain via thalamus Other ascending pathways convey input to reticular formation (pain increases level of arousal), limbic system (pain evokes emotional responses), hypothalamus (pain causes an autonomic nervous system response). First order neuron releases glutamate and substance P activating second order neuron Somatic Sensory Pathways Relay information from somatic sensory receptors to primary somatosensory cortex. First order neuron synapses on secondary order neuron either in spinal cord or brain stem. Two major pathways: 1) Dorsal Column Pathway 2) Anterolateral (spinothalamic) pathway Somatic Sensory Pathways  Referred Pain Somatic Pain – from stimulation of nociceptors in skin, skeletal muscles and joints. Visceral Pain -When pain is felt in a place other than origin of pain. Referred pain- visceral and somatic sensory inputs converge on single ascending tract. Since pain signals from the skin are more common than pain signals from internal organs, the brain may incorrectly interpret pain from a visceral organ as having a somatic origin. Ex. Heart attack Gate Control Theory of Pain The gate control theory of pain states that mechanical stimulus such as touch, pressure, or vibration can suppress pain sensations. Normally interneurons within spinal cord inhibit second-order neurons of ascending pathway to brain. Thus, prevents pain signal to brain. (not shown) (a) C fibers (from nociceptors) synapse on (Ascending inhibitory interneuron, blocking the activity of the pathway) inhibitory neuron, and exciting ascending pathway, allowing pain signal from C fiber to reach brain. (b) Gate Control Theory – Part of the body experiences a painful stimulus along with mechanical stimulus. C fibers inhibit interneuron activity and activate ascending pathway. Aβ fibers (carry sensory information about mechanical stimuli) synapse on inhibitory interneurons and excite interneuron. Integrated response from Aβ and C fibers (Ascending decreases the perception of pain by the pathway) brain. Proprioceptive Sensations Provide information about muscles and joint position Three types of receptors a. Muscle spindle – detects muscle stretch and length. b. Golgi tendon – change in muscle tension c. Joint kinesthetic – sensitive to stretch of the joint capsule and ligaments. Will be covered in Chapter 12  Pain Modulation Three main ways to suppress pain: 1) Drug therapy a) Analgesics- causes pain relief (analgesia) Ex. Aspirin & Ibuprofen -Inhibits prostaglandins, decreases inflammation, and slows transmission of pain to site of injury. 2) Mechanical stimulus applied to painful area – Pain relief via activation of Aβ fibers. Transcutaneous Electric Nerve Stimulation (TENS) applied to painful area of skin. 3) Endogenous analgesia – descending pathways that can suppress the transmission of incoming pain signals from nociceptors. Ex. Endogenous opioids – block pain perception by suppressing pain signals at at synapse between primary and secondary neuron of pain pathway. See fig 9.18