Sensory System (Lecture 3) PDF

Assafa College Department of Physiology NERUROPHYSIOLOGY Dr. Rayan Khalid, MBBS, M.Sc, MD Lecture No (3) SENSORY SYSTEM  Sensory system provides Information about ( internal & external) environment autonomic afferent...

Assafa College Department of Physiology NERUROPHYSIOLOGY Dr. Rayan Khalid, MBBS, M.Sc, MD Lecture No (3) SENSORY SYSTEM  Sensory system provides Information about ( internal & external) environment autonomic afferent Visceral e.g. : BP , ECF volume Sensation Somatic Somatic afferent  RECEPTORS Sensory receptors are detectors and transducers that convert various form of energy (light, sound, chemical ,mechanical ) in to action potentials in neuron. Anatomically : receptor are specialized structures present at peripheral termination of afferent nerve fiber. CLASSIFICATION OF RECEPTORS Traditional classification :  According to the Site of Event Distance events Teleceptors Visual receptors exteroceptors Immediate external environment Touch receptors Interoceptors internal environment e.g. chemoreceptors Change in body position Proprioceptors Joints , tendons , ligaments According to the degree of adaptation Slowly Rapidly adapting adapting Moderately non adapting adapting According to the type (Energy) of stimulus Mechanoreceptors Pressure, stretch, sound. information about the total Chemoreceptors solute concentration of a solution. Thermo receptors hot, cold. Nociceptors pain receptor stimulated by any energy form that cause tissues damage. Baroreceptors elevation of BP photoreceptor  According to morphology 1- Free nerve endings : The terminal branches of the neuron are unmyelinated e.g pain and temperature receptors 2- Encapsulated receptors: Meissner corpuscles Dendrites encapsulated in connective tissue. Respond to change in texture and slow vibration.  Pacinian corpuscles Unmyelinated dendretic endings of sensory nerve fiber 2 micro m in diameter, encapsulated by connective tissue. Respond to deep pressure and fast vibration  Ruffini corpuscles Expanded dendritic endings with elongated capsule Respond to sustained pressure. Meissner corpuscles Dendrites encapsulated in connective tissue. Respond to change in texture and slow vibration.  3- Expanded end: eg Merkel’s discs and Ruffini endings 4- Sense Organ Organs that translate certain forms of energy into nerve impulses that are perceived as special sensations. Consist of sensory receptors & associated with non neural cells. E.g. eye , nose, ear PROPERTIES OF RECEPTORS 1. Specificity: Each type of receptor is most sensitive to a specific form of energy called its adequate stimulus, and is non- responsive to the other normal intensities of other form of energy. 2. Excitability : Different types of receptors are excited in different ways. The receptor transform these various stimuli into a local electrical change, called generator or receptor potential. If firing level is reached, an action potential is produced. Mechanism of generation of receptor potential: Stimuli → change in membrane permeability of the receptor → opening of ion channel → change in transmembrane potential.  Generator potential Characteristic: Localized (not propagated along axon of neuron) Graded (increase or decrease in amplitude depending on strength of stimuli ) Can be summated to reached sufficient magnitude(threshold )and cause action potential. Then action potential propagated along the sensory nerve. the frequency of AP is related to the intensity of the stimulus. 3. Adaptation: sensory receptors are subjected to an unchanged stimulus , Receptor membranes become less responsive. Gradual decline in frequency of action potential in the afferent nerve fiber when the receptor is stimulated continuously by constant strength. The rate at which it occurs varies in different types of receptor. A- Slowly adapting : Tonic receptor Continue to transmit impulses to the brain at constant rate during continuous stimulation. Useful in detecting changes in the body.eg ; Muscle spindle receptor, joint receptor, baroreceptor. B- Not adapting receptor : Eg: pain receptor Useful because pain serves as protective mechanism of the body. C- Rapidly adapting : Phasic receptor, movement receptor, rate receptor. Useful in transmitting information about event which under go rapid changes. E.g. touch receptor. D- Moderately adapting: E.g. temperature receptor, the rods and cones STEPS IN SENSORY TRANSDUCTION FOR CUTANEOUS RECEPTORS  SENSORY CODING Definition: Converting a receptor stimulus to a recognizable sensation. All sensory systems code for four elementary attributes of a stimulus: modality, location, intensity, and duration. MODALITY Modality is the type of energy transmitted by the stimulus. There is a specific stimulus for each receptor (the adequate stimulus) Law of specific nerve energies: stimulation of the receptor or its nerve pathway give rise to one type of sensation, regardless of the method of stimulation. Each nerve fiber in the pathway ends at specific area in the CNS. LOCALIZATION Location is the site on the body or space where the stimulus originated. sensory unit: is a single sensory axon and all its peripheral branches. Receptive field??  INTENSITY The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor. The increased intensity of stimulus is coded to the brain in two ways : increase in frequency of action potential along the sensory nerve. Increase in number of receptor s stimulated.  SENSORY CORTICAL AREAS The cortical areas associated with processing of somatic sensory information collectively form somatosensory cortex. somatosensory cortical areas are: Primary somatosensory area I & II Somatosensory association cortex Supplementary sensory area CONNECTIONS OF SOMATOSENSORY AREAS Somatosensory areas are interconnected to each other & Have reciprocal and specific point to-point connection with the thalamus. TOPOGRAPHIC ORGANIZATION Entire body is represented in Somatosensory areas The topographic representation is called the sensory homunculus  SENSORY PATHWAYS (ASCENDING TRACTS) 1. Spinothalamic tracts ventral (anterior). lateral 2. Dorsal column  SPINOTHALAMIC TRACTS Ventral (anterior) spinothalamic tract : transmits crude touch and pressure as well as the itch and tickle sensations. Afferent: A-delta and C afferent nerve fibers.  lateral spinothalamic tract: transmits pain, thermoceptive sensations. Afferent are: type A delta nerve fiber (for fast pain and cold) type C fibers (for slow pain & heat) THE DORSAL COLUMN OR LEMNISCAL SYSTEM It consists mainly of A-alpha & A-beta nerve fibers These tracts transport: 1. Fine tactile sensations 2. Stereognosis and texture of material sensation 3. Fine pressure and muscle tension sensations 4. The vibration sense 5. The proprioceptive sensations  LAW OF PROJECTION Conscious perception of a particular sensation is always projected to the locality of the receptor irrespective of where the sensory pathway is stimulated. Phantom limb.