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PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College CHAPTER 13 The Peripheral Nervous System and Reflex Activity: Part A Copyright © 2010 Pearson Education, Inc. Peripheral Nervous System (PNS) • All neural structures OUTSIDE the brain and those starting in and ending outside of the spinal cord • Sensory nerves contain • Neurons receiving info in senses, skin, organs, muscles • Motor nerves contain • neurons going to muscles and glands Copyright © 2010 Pearson Education, Inc. Structure of a nerve Endoneurium Axon Myelin sheath Perineurium Epineurium Fascicle Blood vessels (b) Copyright © 2010 Pearson Education, Inc. Figure 13.3b How is a nerve different than a neuron? • A nerve is a group of axons • Most nerves have both Sensory and Motor neurons • Cranial nerves carry information to and from the head and shoulders and brain • Spinal nerves carry info to and from the body and the spinal cord. Copyright © 2010 Pearson Education, Inc. Sensory “Receptors” • Are just modified dendrites of sensory neurons that respond to things other than just chemical neurotransmitters • They respond to changes in their environment (stimuli) rather than NT. Copyright © 2010 Pearson Education, Inc. Perception: Modified dendrites respond to many things • 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 (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals) Copyright © 2010 Pearson Education, Inc. Receptors can be classified by Location 1. Exteroceptors -Respond to stimuli arising outside the body on skin and in eyes/ears/mouth/nose 2. Interoceptors (visceroceptors) -Respond to stimuli from organs and blood vessels (low pressure=empty, high pressure= full) 3. Proprioceptors -Respond to stretch in skeletal muscles, tendons, joints -they Inform the brain of where your arms and legs are. -How do you know where your hand is with your eyes closed? Proprioceptors! Copyright © 2010 Pearson Education, Inc. Receptors can be classified by how they are made 1. Unencapsulated dendrites -free dendrites -temperature, pain, chemicals 2. Encapsulated dendrites -have specialized coverings - to distinguish deep touch, light touch Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Table 13.1 How do we perceive the environment? • Levels of neural integration in sensory systems: 1. Receptor level—the sensory receptors (dendrites or special receptors) (1st order) 2. Circuit level—ascending pathways through spinal cord/brainstem (2nd order) 3. Perceptual level—neuronal circuits in the cerebral cortex (3rd order) Copyright © 2010 Pearson Education, Inc. Perceptual level (processing in cortical sensory centers) 3 Motor cortex Somatosensory cortex Thalamus Reticular formation Pons 2 Circuit level (processing in Spinal ascending pathways) cord Cerebellum Medulla Free nerve endings (pain, cold, warmth) Muscle spindle Receptor level (sensory reception Joint and transmission kinesthetic to CNS) receptor 1 Copyright © 2010 Pearson Education, Inc. Figure 13.2 Adaptation of Sensory Receptors • Have you ever gotten “used to” an itchy sweater, tight necktie or bra? • Adaptation is a change in sensitivity in the presence of a constant stimulus • Some receptors become less responsive over time and may turn off. Copyright © 2010 Pearson Education, Inc. Adaptation of Sensory Receptors • Phasic receptors signal adapt quickly • only signal the beginning or end of a stimulus • receptors for pressure, touch, and smell • Tonic receptors adapt slowly or not at all • Nociceptors (you want to be aware of pain) • Proprioceptors (you want to know where your limbs are) Copyright © 2010 Pearson Education, Inc. How does the brain know what stimuli are coming in? • Different senses GO to different places in the brain • Eye nerves go to back of brain (vision) • Touch nerves go to side of brain (feeling) • So different parts of the brain expect different information • The more intense the stimulus • the more action potentials are received • The bigger the area it is received from Copyright © 2010 Pearson Education, Inc. Processing a the perceptual level • When information gets to the brain it needs to decode it: • By the frequency of APs sent by one or more neurons (how loud you shout!) • By the number of neurons sending information from a narrow or broad area of the body (how many people are shouting) Copyright © 2010 Pearson Education, Inc. Perceptual level (processing in cortical sensory centers) 3 Motor cortex Somatosensory cortex Thalamus Reticular formation Pons 2 Circuit level (processing in Spinal ascending pathways) cord Cerebellum Medulla Free nerve endings (pain, cold, warmth) Muscle spindle Receptor level (sensory reception Joint and transmission kinesthetic to CNS) receptor 1 Copyright © 2010 Pearson Education, Inc. Figure 13.2 Pain is an important sense • Warns of actual or impending tissue damage • Stimuli include extreme pressure and temperature, and chemicals • Glutamate and substance P are NT that relay pain info • Some pain impulses are blocked by inhibitory NT called endorphins Copyright © 2010 Pearson Education, Inc. Visceral Pain • Organs have pain, but there are very few sensory receptors in organs. Stimulation of visceral organ receptors is: • Felt as vague aching, gnawing, burning • Activated by tissue stretching, ischemia, chemicals, muscle spasms Copyright © 2010 Pearson Education, Inc. Referred Pain • Referred pain • Pain from one body region perceived from different region • Since visceral and somatic pain fibers travel in same nerves the brain assumes stimulus from commonly felt areas, like skeletal muscles or skin • Ex., left arm pain (commonly felt in life) is perceived during heart even though the pain is actually from the heart (hardly ever felt in life!). Copyright © 2010 Pearson Education, Inc. Figure 13.3 Map of referred pain. Lungs and diaphragm Heart Gallbladder Appendix Liver Stomach Pancreas Small intestine Ovaries Colon Kidneys Urinary bladder Ureters Copyright © 2010 Pearson Education, Inc. Regeneration of a nerve axon in a peripheral nerve. (1 of 4) Endoneurium Schwann cells Droplets of myelin Fragmented axon Site of nerve damage Copyright © 2010 Pearson Education, Inc. 1 The axon becomes fragmented at the injury site. Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (2 of 4) Schwann cell Copyright © 2010 Pearson Education, Inc. Macrophage 2 Macrophages clean out the dead axon distal to the injury. Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (3 of 4) Aligning Schwann cells form regeneration tube Fine axon sprouts or filaments Copyright © 2010 Pearson Education, Inc. 3 Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells. Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (4 of 4) Schwann cell Single enlarging axon filament Copyright © 2010 Pearson Education, Inc. New myelin sheath forming 4 The axon regenerates and a new myelin sheath forms. Regeneration of damaged axons:summary • If a cell body of a damaged nerve cell is still intact, peripheral axon might regenerate • The axon fragments (Wallerian degeneration) • Macrophages clean dead axon • The Schwann cells are still in place and form a “regeneration tube” or tunnel. • Axon filaments grow through regeneration tube • Axon regenerates; new myelin sheath form Copyright © 2010 Pearson Education, Inc. Regeneration of CNS neurons • Most CNS neurons never regenerate if damaged • CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration • Astrocytes at injury site form scar tissue containing chondroitin sulfate that blocks axonal regrowth • Treatment • Neutralizing growth inhibitors, blocking receptors for inhibitory proteins, destroying chondroitin sulfate promising Copyright © 2010 Pearson Education, Inc.