CHAPTER 15 INTO TO SENSORY PATHWAYS SENSORY INFO Afferent Division of Nervous System Receptors detects stimuli Sensory neurons relays sensory info towards CNS receives input from dendrite uses axon to send it to CNS Sensory pathways nerves nuclei tracts that deliver sensory info to CNS Efferent Division of Nervous System Nuclei contain cell bodies of motor neurons Motor neurons send commands to effectors thru motor tracts axons in the CNS or nerves axons in the PNS Sensory Receptors Specialized cells that monitor specific conditions In body or external environment When stimulated receptor passes info to CNS o o In form of action potentials along axon of a sensory neuron Sensory Pathways Deliver somatic visceral sensory info to their final destinations inside CNS using o Nerves bundle of axons located in PNS o Nuclei cluster of cell bodies located in CNS o Tracts bundle of axons located in CNS Ex nerves from arm bring sensory info to spinal cord o Axon terminal synapse onto interneurons in spinal cord cell bodies clustered together nucleus Located in sense organs such as eye or ear Protected by surrounding tissues Detection of Stimuli Receptor specificity Receptive field o Each receptor has a characteristic sensitivity Ex Your hands do not smell nor taste your eyes do not hear etc o Area is monitored by a single receptor cell o Larger receptive field more difficult to localize stimulus knows its somewhere w in receptive field but not specifically where Back back of arms large receptive fields Not very precise large receptive field Interpretation of Sensory Info Arriving stimulus reaches cortical neurons via labeled line link b w receptor cortical neuron o Each labeled line consists of axons carrying info about one type of stimulus modality Physical force such as pressure Dissolved chemical Sound Light Interpretation of Sensory Info Sensations o Taste hearing equilibrium vision provided by specialized receptor cells o Communicate w sensory neurons across chemical These cell bodies send info up to brain thru axon bundles now called tracts since inside CNS synapses Adaptation Somatic Motor Portion of Efferent Division Controls peripheral effectors Somatic Motor Commands Reduction in sensitivity of a constant stimulus Nervous system quickly adapts to stimuli that are painless constant Travel from motor centers in brain along somatic motor pathways of o Ex New smells pressure wearing jewelry Types of adaptation Somatic Nervous System SNS Motor neurons pathways that control skeletal muscles SENSORY RECEPTORS General Senses Describe our sensitivity to o Motor nuclei o Tracts o Nerves Pain o Temperature o o Touch Pressure o o Vibration o Proprioception monitors position movements of skeletal muscles joints Sensation Perception Special Senses The arriving info from these senses Conscious awareness of a sensation Sensory receptors are all located in head o Olfaction smell o Vision sight o Gustation taste o Equilibrium balance o Hearing Provided by special sensory receptors Special Sensory Receptors o o 1 Central inhibition of nuclei along sensory pathway occurs in the CNS Ex New smells still stimulate sensory receptor but cortical neurons no longer respond to the stimulation 2 Peripheral occurs at level of receptor PNS receptor responds initially to stimulus then stops Ex Thermoreceptors Adaptation Tonic receptors Phasic receptors o Always active o o o Remind you of an injury long after initial damage Show little peripheral adaptation Slow adapting receptors has occurred Ex Pain receptors nociceptors o Normally inactive o Become active for a short time whenever change o o occurs Fast adapting receptors Provide info about intensity rate of change of stimulus Ex Thermoreceptors detect rapid changes in temperature Adaptation Stimulation of receptor produces action potentials Along axon of sensory neuron Frequency pattern of action potentials contain info About strength duration variation of stimulus Perception of nature of that stimulus Depends on path it takes inside CNS CLASSIFYING SENSORY RECEPTORS Exteroceptors provide info about external environment Proprioceptors report positions of skeletal muscles joints Interoceptors monitor visceral organs functions Proprioceptors Provide a purely somatic sensation not internal organ movement No proprioceptors in visceral organs of thoracic abdominopelvic cavities Cannot tell where your spleen appendix or pancreas is General Sensory Receptors Divided into 4 types by nature of stimulus that excites them o o o o 1 Nociceptors pain 2 Thermoreceptors temperature 3 Mechanoreceptors physical distortion 4 Chemoreceptors chemical concentration Such as Ph oxygen carbon dioxide levels 1 Nociceptors Pain Receptors Common In Superficial portions of skin Joint capsules achy joints when older o o o W in Periostea of bones o Around walls of blood vessels Analgesia inability to feel pain Hypergesia increased sensitivity to pain May be sensitive to o Temperature extremes o Mechanical damage o Dissolved chemicals such as chemicals released by injured cells Free nerve endings w large receptive fields o Branching tips of dendrites o Not protected by accessory structures o Can be stimulated by many different stimuli o 2 types of axons Type A large myelinated Type C small unmyelinated fibers Myelinated Type A fibers o Conducted along same pathways that carry pain sensations Sent to o o Reticular formation o Thalamus o Primary sensory cortex to a lesser extent 3 Mechanoreceptors Sensitive to stimuli that distort their plasma membranes Contain mechanically gated ion channels whose gates open or close in response to Stretching o o Compression o Twisting o Other distortions of membrane Three Classes of Mechanoreceptors o 1 Tactile receptors Provide sensations of touch pressure vibration o Touch sensations provide info about shape or texture o Pressure sensations indicate degree of mechanical distortion o Vibration sensations indicate pulsing or oscillating pressure o 2 Baroreceptors o 3 Proprioceptors Detect pressure changes in walls of blood vessels in portions of digestive respiratory urinary tracts Either increase or decrease blood pressure Monitor positions of joints muscles Most structurally functionally complex of general sensory receptors 1 Tactile Receptors Mechanoreceptor Fine touch pressure receptors o Extremely sensitive o Relatively narrow receptive field o Provide detailed info about a source of stimulation Including exact location shape size texture movement
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