,RECEPTORS AND SENSORY PATHWAYS,LEARNING OBJECTIVES,Classify the sensory receptors Enumerate the receptors found in the skin for different sensory modalities Explain the terms receptor potentials and sensory transduction Describe the sensory pathways that mediates touch, proprioception and vibration sense Describe the organization of sensory cortex,Peripheral Nervous System,Consists of nerve fibers that carry information between the CNS and other parts of the body Afferent division Detects, encodes, and transmits peripheral signals to the CNS (sends information from internal and external environment to CNS) To maintain homeostasis Visceral afferent Incoming pathway for information from internal viscera (organs in body cavities) Sensory afferent Somatic (body sense) sensation Sensation arising from body surface and proprioception Special senses Vision, hearing, taste, smell,Receptors,Structures at peripheral endings of afferent neurons Detect stimuli (change detectable by the body), exists in various forms e.g. heat, light, sound, pressure & chemical change Each receptor is specialized to one type of stimulus ,its adequate stimulus e.g. eyes- light, ears- sound, warmth- heat Convert forms of energy into electrical signals (action potentials) Process is called transduction,Types of Receptors,Photoreceptors Responsive to visible wavelengths of light Mechanoreceptors Sensitive to mechanical energy Thermoreceptors Sensitive to heat and cold Osmoreceptors Detect changes in concentration of solutes in body fluids and resultant changes in osmotic activity Chemoreceptors Sensitive to specific chemicals Include receptors for smell and taste and receptors that detect O2 and CO2 concentrations in blood and chemical content of digestive tract Nociceptors Pain receptors that are sensitive to tissue damage or distortion of tissue,Tactile Receptors,Free nerve endings: Detect pain, touch and pressure found everywhere in the skin and viscera Meissners corpuscles: rapidly adapting (within a fraction of a second) and detect movement of light objects over skin, position sense ,dynamic fine touch (manipulation) -found on nonhairy skin, fingertips and lips Merkels discs: respond rapidly at first and then slowly adapt, detect the “steady state”, position, static touch(shape, edges, texture) -found on hairy as well a non hairy skin,Tactile Receptors,Hair end organ Adapts rapidly and detects movement over the body Ruffinis end organ Slowly adapting and respond to continual deformation of the skin and joint rotation Pacinian corpuscle Very rapidly adapting and is stimulated only by rapid movement detects vibration and other rapid changes in the skin,Receptors,May be:-Specialized ending of an afferent neuron -Separate cell closely associated with peripheral ending of a neuron -Stimulus alters receptors permeability which leads to graded receptor potential -Usually causes nonselective opening of all small ion channels -This change in membrane permeability can lead to the influx of sodium ions. This produces receptor (generator) potentials. -The magnitude of the receptor potential represents the intensity of the stimulus. -A receptor potential of sufficient magnitude can produce an action potential. This action potential is propagated along an afferent fiber to the CNS.,Conversion of Receptor and Generator Potentials into Action Potentials,Receptors,May adapt slowly or rapidly to sustained stimulation Types of receptors according to their speed of adaptation Tonic receptors Do not adapt at all or adapt slowly Ex: Muscle stretch receptors, joint proprioceptors CNS must continually get info. about degree of muscle length & joint position So, these receptors must continue to generate APs to relay information to CNS Phasic receptors Rapidly adapting receptors No longer responds to maintained stimulus Important to signal a change in stimulus intensity, rather than relay status quo information Tactile (touch) receptors in skin,When a continuous stimulus is applied, receptors respond rapidly at first, but response declines until all receptors stop firing.,Adaptation of receptors,Transmission of Receptor Information to the Brain,-The larger the nerve fiber diameter the faster the rate of transmission of the signal -Velocity of transmission can be as fast as 120 m/sec or as slow as 0.5 m/sec -Nerve fiber classification type A - myelinated fibers of varying sizes, generally fast transmission speed subdivided into a, b, d, g type C - unmyelinated fibers, small with slow transmission speed,Classification of Somatic Sensations,Mechanoreceptive - stimulated by mechanical displacement Tactile touch pressure vibration tickle and itch Position or proprioceptive static position rate of change,Classification of Somatic Sensations,Thermoreceptive: Detect heat and cold Nociceptive: Detect pain and are activated by any factor that damages tissue,Pathways for the Transmission of Sensory Information,Almost all sensory information enters the spinal cord through the dorsal roots of the spinal nerves Two pathways for sensory information:-Dorsal column-medial lemniscal system - Anterolateral system,