Sensory Receptors PDF

12/1/22 Sensory Receptors Sensory Receptors Assoc. Prof. Dr. Cenk Serhan Özverel [email protected] • Imput to nervous system • Detect sensory stimuli – touch, – sound, – light, – pain, – cold, – warmth. Types of Sensory Receptors and the Stimuli They Detect 1 12/1/22 • (1) mechano...

12/1/22 Sensory Receptors Sensory Receptors Assoc. Prof. Dr. Cenk Serhan Özverel [email protected] • Imput to nervous system • Detect sensory stimuli – touch, – sound, – light, – pain, – cold, – warmth. Types of Sensory Receptors and the Stimuli They Detect 1 12/1/22 • (1) mechanoreceptors, which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor; • (4) electromagnetic receptors, which detect light on the retina of the eye; • (2) thermoreceptors, which detect changes in temperature, with some receptors detecting cold and others warmth; • (3) nociceptors (pain receptors), which detect damage occurring in the tissues, whether physical damage or chemical damage; Differential Sensitivity of Receptors • How do two types of sensory receptors detect different types of sensory stimuli? • (5) chemoreceptors, which detect taste in the mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the body fluids, carbon dioxide concentration, and other factors that make up the chemistry of the body. • Example: • Rods and cones à light • Osmoreceptors à osmolality of the body fluids • Each type of receptor is highly sensitive to one type of stimulus for which it is designed • Pain receptors à NEVER by touch or pressure stimuli, BUT à severe 2 12/1/22 Modality of Sensation—The “Labeled Line” Principle • Example: • Receptor 1 • Receptor 2 Nerves Determined by the point it leads • If a p ain fiber is stimulated, the person perceiv es pain regardless of what type of stimulus excites the fiber. Th e stimulus can be electricity, overheating of the fiber, crushing of the fiber, or stimulation of the pain nerve ending by damage to thetissue cells. • Receptor 3 • Example: • Example: • if a touch fiber is stimulated by el ectrical excitation of a touch receptor or in any other way, th e person perceiv es touch because touch fibers lead to specific touch areas in the brain. • Similarly, fibers from th e retina of the ey e t erminate in th e vision areas of the brain, fibers from the ear terminate in the auditory areas of the brain, and temperature fibers t erminat e in the temperature areas. 3 12/1/22 Transduction of Sensory Stimuli into Nerve Impulses • This specificity of nerve fibers for transmitting only one modality of sensation is called the labeled line principle. • Local Electrical Currents at Nerve Endings— Receptor Potentials • All sensory receptors have one feature in common • Whatever th e type of stimulus that excites the receptor, its immediate effect is to change the membrane electrical potential of the receptor. • This change in potential is called a receptor potential. Mechanisms of Receptor Potentials Mechanisms of Receptor Potentials • Receptors excitation: – (1) by mechanical deformation of the receptor, which stretches the receptor membrane and opens ion channels; – (2) by application of a chemical to the membrane, which also opens ion channels; – (3) By change of the temperature of the membrane, which alters the permeability of the membrane; or – (4) By the effects of electromagnetic radiation, such as light on a retinal visual rec eptor, which eith er directly or indirectly changes the receptor membrane charact eristics and allows ions to flow through membrane channels. 4 12/1/22 • In all 4 situations; • Maximum Receptor Potential Amplitude. – End result is à Change in membrane potential – Ions flow – Maximum amplitude à 100 millivolts, – Only at an extremely high intensity of sensory stimulus. – thereby to change the transmembrane potential. • Relation of the Receptor Potential to Action Potentials. – Receptor potential rises above the threshold – Eliciting action potentials in the nerve fiber attached to the receptor, – Action potentials occur 5 12/1/22 • Receptor Potential of the Pacinian Corpuscle—An Example of Receptor Function – Central nerve fiber extending through its core – Surrounding this are multiple concentric capsule layers, so compression anywhere on the outside of the corpuscle will elongate, indent, or otherwise deform the central fiber • Relation Between Stimulus Intensity and the Receptor Potential 6 12/1/22 Adaptation of Receptors • pacinian corpuscle adapts very rapidly, hair receptors adapt within a second or so, and some joint capsule and muscle spindle receptors adapt slowly • Common feature • They adapt to any constant stimulus after a period of time • Continuous Sensory stimulus is applied à receptor responds at a high impulse rate at first and then at a progressively slower rate until finally th e rate of action potentials decreases to very few or often to none at all. • Mechanoreceptors adopt quick and completely Nerve Fibers That Transmit Different Types of Signals and Their Physiologic Classification – But some do not (‘Nonadapting receptors’) • Example: – Longest time 2 days – Carotid and aortic baroreceptors • Conversely, some of the non-mechanoreceptors—the chemoreceptors and pain receptors, for instance— probably never adapt completely. • Some signals need to be transmitted to or from the central nervous system extremely rapidly. Otherwise information would be useless. • Nerve fibers diameter à 0.5-20 micrometers • Larger the diameter, the greater the conducting velocity. • Conduction velocity à 0.5-120 m/sec 7 12/1/22 General Classification of Nerve Fibers. • A and C • A à alpa, beta, gamma and delta fibers. • Aà large and medium-sized myelinated fibers of spinal nerves. • Cà small unmyelinated nerve fibers that conduct impulses at low velocities. Alternative Classification Used by Sensory Physiologists. • A-alpha fibers into two subgroups. • A-beta and A-gamma cannot be distinguished. • Group Ia • Fibers from the annulospiral endings of muscle spindles (average about 17 microns in diameter; these are Alpha-type A fibers in the general classification). • Group Ib • Fibers from the Golgi tendon organs (average about 16 micrometers in diameter; these also are Alpha-type A fibers). 8 12/1/22 • Group II • Fibers from most discrete cutaneous tactile receptors and from the flower-spray endings of the muscle spindles (average about 8 micrometers in diameter; these are beta- and gamma-type A fibers in the general classification). • Group IV • Unmyelinated fibers carrying pain, itch, temperature, and crude touch sensations (0.5 to 2 micrometers in diameter; they are type C fibers in the general classification). • Group III • Fibers carrying temperature, crude touch, and pricking pain sensations (average about 3 micrometers in diameter; they are delta-type A fibers in the general classification). Transmission of Signals of Different Intensity in Nerve Tracts—Spatial and Temporal Summation Spatial Summation • Increasing signal strength is transmitted by using progressively greater numbers of fibers. • The different gradations of intensity can be transmitted either by using increasing numbers of parallel fibers or by sending more action potentials along a single fiber. • SPATIAL SUMMATION • TEMPORAL SUMMATION 9 12/1/22 Temporal Summation. • By increasing the frequency of nerve impulses in each fiber. • Threshold and Subthreshold Stimuli— Excitation or Facilitation. Relaying of Signals Through Neuronal Pools • Organization of Neurons for Relaying Signals. • “Discharge” and “facilitated” zones of a neuronal pool. 10 12/1/22 Divergence of Signals Passing Through Neuronal Pools Divergence of Signals Passing Through Neuronal Pools • Often it is important for weak signals entering a neuronal pool to excite far greater numbers of nerve fibers leaving the pool. • This phenomenon is called divergence. • Two major types of divergence occur and have entirel y different purposes. – 1-Amplifying type of divergence – 2-Divergence into multiple tracts Convergence of Signals Neuronal Circuit with both Excitatory and Inhibitory Output Signals • Multiple inputs uniting to excite a single neuron. 11 12/1/22 Reverberatory (Oscillatory) Circuit as a Cause of Signal Prolongation Continuous Signal Output from Some Neuronal Circuits THE END 12