ANP1106 – SENSORY Physiology of the Nervous System L2 March 15 Define sensation and discuss the underlying processes Sensory receptors Explain signal transduction; define generator and receptor potentials, and describe the patterns of nerve impulses that they generate Explain the concept of adaptation; describe phasic and tonic receptors Classify the sensory receptors according to their structure, location and function Somatic sensation Describe the properties and location of tactile receptors Describe the properties and location of thermoreceptors Describe the properties and location of nociceptors; compare somatic and visceral pain; explain the concept of referred pain Describe the location and functions of proprioceptors Sensory properties of intensity coding, acuity, and adaptation Peripheral Nervous System (PNS) • PNS – all neural structures outside the brain and spinal cord • Includes sensory receptors, peripheral nerves, associated ganglia, autonomic nervous system and motor endings • Provides links to and from the external environment The function of the CNS is critically dependent on input from the PNS •The PNS receives inputs from peripheral sensory receptors •Sensation = awareness of changes in the internal and external environment •Perception = conscious interpretation of those stimuli •Receptor level = sensory receptors /Circuit level = ascending pathways / Perception level = cerebral cortex interpretation of sensory input The peripheral nervous system is divided into sensory input and motor output divisions. The motor output is divided into somatic (neuromuscular) and autonomic divisions. The outputs within the autonomic division are further subdivided into sympathetic and parasympathetic divisions Table 13.1_1 General Sensory Receptors Classified By Structure and Function © 2019 Pearson Education, Inc. Table 13.1_2 General Sensory Receptors Classified By Structure and Function © 2019 Pearson Education, Inc. Figure 13.2 Three basic levels of neural integration in sensory systems. Not meant to show details of ascending sensory pathways See next lecture instead. Perception level = cerebral cortex, interpretation of input into primary and association sensory cortex Circuit level = ascending pathways (second order sensory neuron, spinothalamic or medial lemniscus PLUS third order thalamocortical Receptor level = sensory receptors and primary sensory neuron (first order sensory neuron, dorsal horn or dorsal column) © 2019 Pearson Education, Inc. Figure 13.4b Structure of a nerve. endo meaning within, inner peri, “enclosing” or “surrounding epi on, upon, above, upper, outermost, © 2019 Pearson Education, Inc. There are active mechanisms promoting axon regrowth after damage in the PNS. BUT there are active mechanisms inhibiting axon regrowth after damage in the CNS Neuron 89 March 2, 2016 881 Inside Out: Core Network of Transcription Factors Drives Axon Regeneration http://www.cell.com/neuron/current Sensory territories on the surface of the body are organized into band-like sensory territories called dermatomes Sensory Receptors • Structures specialized to respond to stimuli • Activation of sensory receptors results in graded depolarizations that trigger action potential impulses along the nerve axon to the CNS • Reflex activity takes place in the spinal cord. Sensation and perception, occur in the cerebral cortex The key to the physiological process of sensation is transduction – the conversion of an environmental physical property (stimulus) into a neuronal electrochemical property. A receptor potential, also known as a generator potential, a type of graded potential, is the transmembrane potential difference produced by activation of a sensory receptor. A receptor potential is often produced by sensory transduction. It is generally a depolarizing event resulting from inward current flow Pain Perception • nociception is the perception of stimuli (thermal, mechanical or anoxic) that have the potential to produce tissue damage. We avoid these stimuli by both reflex and conscious responses • histamine, K+, ATP, acid and/or bradykinin are released by stressed or damaged tissue and they depolarize the free nerve endings of pain sensory endings • We use many mechanisms to achieve pain relief: – Electrical stimulation of specific areas of the central nervous system – Pharmacological agents (NSAIDs like Tylenol® inhibits prostoglandin synthesis) and morphine (opioids) – Some of the neurons in these inhibitory pathways release morphinelike endogenous opioids. – Acupuncture (seems to be linked to activation of the endogenous opioid pathways) 15 Figure UN 13.1 Referred pain Receptor Class by Location: Exteroceptors • Respond to stimuli arising outside the body • Found near the body surface • Sensitive to touch, pressure, pain, and temperature • Include the special sense organs Interoceptors • Respond to stimuli arising within the body • Found in internal viscera and blood vessels • Sensitive to chemical changes, stretch, and temperature changes Receptor Classification by Stimulus Type • Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch • Thermoreceptors – sensitive to changes in temperature • Photoreceptors – respond to light energy (e.g., retina) • Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry) • Nociceptors – sensitive to pain-causing stimuli Thermoreceptors - Transient Receptor Potential (TRP) family of proteins free nerve endings with membrane channels that change their permeability (and therefore axon firing rates) across specific temperature ranges menthol capsaicin Receptor Class by Location: Proprioceptors • Respond to degree of stretch of the organs they occupy • Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles • Constantly “advise” the brain of one’s movements • Give information concerning movements and position of the body Proprioceptors Proprioception means "sense of self". In the limbs, the proprioceptors are sensors that provide information about joint angle, muscle length, and muscle tension, which is integrated to give information about the position of the limb in space. The muscle spindle is one type of proprioceptor that provides information about changes in muscle length. The Golgi tendon organ is another type of proprioceptor that provides information about changes in muscle tension. kinesthesia, muscle spindle, intrafusal vs extrafusal, motor neuron (alpha vs gamma), tendon organ Figure 11.25 Receptor Classification by Receptor Structure • Receptors are structurally classified as either simple or complex • Most receptors are simple and include encapsulated and unencapsulated varieties (free nerve endings) • Complex receptors are special sense organs in contact with a free nerve ending Figure 7-1 Sensory receptors are either specialized endings of afferent neurons or separate cells that signal the afferent neuron. Complex receptors photoreceptors and retinal bipolar cell axons hair cells and auditory axons nerve taste cells and taste nerve axons Figure 7-5 The number of action potentials generated by a hypothetical, pressure-sensitive, sensory afferent neuron, as shown here, is directly proportional to stimulus intensity. Figure 7-11 The off Response Is shown. MNT Different sensory units vary in their speed of adaptation to stimuli. Adaptation of Sensory Receptors • Receptors responding to pressure, touch, and smell adapt quickly (phasic) • Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles, and interoceptors that respond to chemical levels in the blood • Pain receptors and proprioceptors do not exhibit adaptation (tonic) Figure 7-6 Sensory afferent neuron (a) has a finer, more spatially limited receptive field than sensory afferent neuron (b). Acuity. * Figure 7-8 Where you feel the sensation depends on the location of the responding sensory endings on the body surface (or internally). Overlapping stimulation between neighboring receptive fields provides general information about the location of a stimulus.
