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Peripheral Nervous System & Reflex Activity Part A Prepared by Janice Meeking & W. Rose. Figures from Marieb & Hoehn 8th , 9th ed. Portions copyright Pearson Education Peripheral Nervous System (PNS) All neural structures outside the brain • Sensory receptors • Peripheral nerves and associated ganglia • Motor endings Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division Figure 13.1 Sensory Receptors Specialized structures which respond to changes in their environment (stimuli) Some receptors are simply ends of sensory nerve fibers. Other receptors are cells adjacent to sensory nerrve fibers. Other receptors are sensory nerve fiber endings plus specialized supporting cells and/or extracellular material Receptor activation results in graded potentials that may trigger action potentials Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain Classifying receptors Receptors have been classified according to: – The type of stimulus they detect (best) – Receptor location in the body – Structural complexity of the 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 (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals) Classification by Location Exteroceptors Respond to stimuli arising outside the body: receptors in the skin for touch, pressure, pain, and temperature; also most special sense organs (eyes, ears, etc) Interoceptors (visceroceptors) Respond to stimuli arising in internal viscera and blood vessels: chemical environment, tissue stretch, temperature Proprioceptors Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles; inform the brain of one’s movements Classification by Structural Complexity Complex receptors: Special sense organs Vision, hearing, equilibrium, smell, taste (ch. 15) Simple receptors: General sensation Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense Unencapsulated (free) dendritic endings Encapsulated dendritic endings Unencapsulated (free) dendritic endings Table 13.1 Encapsulated dendritic endings Table 13.1 From Sensation to Perception • Survival depends upon sensation and perception • Sensation: awareness of changes in the internal and external environment • Perception: conscious interpretation of those stimuli 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 Figure 13.2 Processing at the Receptor Level • Different receptors respond to different stimuli Light, pressure, temperature, chemicals, etc. • Receptive field Physical region over which each receptor can detect stimuli • Transduction occurs Stimulus energy is converted into a graded potential called a receptor potential Receptor processing in general sense receptors (receptors in which there are not separate receptor cells) Stimulus Receptor Potential (graded potential) at distal end of afferent neuron Action Potential(s) (if threshold is reached) Receptor processing in special sense receptors stimulus receptor potential in receptor cell release of neurotransmitter generator potential in first-order sensory neuron action potentials (if threshold is reached) Adaptation of Sensory Receptors Change in response in the presence of a constant stimulus • Receptor membranes become less responsive • Receptor potentials decline in frequency or stop Phasic (fast-adapting) receptors signal beginning or end of stimulus • Examples: receptors for pressure, touch, smell Tonic receptors adapt slowly or not at all • Examples: nociceptors; most proprioceptors Processing at the Circuit Level • 3-neuron pathway conducts sensory impulses upward to appropriate brain regions • First-order neurons (cell body in DRG or other peripheral ganglion): conduct impulses from receptor to second-order neurons in the CNS • Second-order neurons (in CNS): transmit impulses to thalamus or cerebellum • Third-order (thalamic) neurons: conduct impulses from the thalamus to the somatosensory cortex (perceptual level) Processing at the Perceptual Level • Identification of the sensation depends on the specific location of the target neurons in the sensory cortex • Aspects of sensory perception • Stimulus detection: requires multiple impulses • Magnitude estimation: intensity coded by frequency of APs & number of neurons active • “Spatial” discrimination: identify the site or pattern of stimulus (e.g. two-point discrimination test) Further Processing at the Perceptual Level • Feature abstraction—identification of more complex aspects and several stimulus properties • Quality discrimination: identification of submodalities of a sensation (e.g., sweet or sour tastes) • Pattern recognition: identification of familiar or significant patterns in stimuli (face, melody, etc.) 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 Figure 13.2 Nerve Structure • Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue • Connective tissue coverings, from inside to outside: – Endoneurium, perineurium, epineurium Endoneurium Axon Myelin sheath Perineurium Epineurium Fascicle Blood vessels (b) Figure 13.3b Classification of Nerves • Most nerves are mixed: afferent and efferent fibers and somatic and autonomic (visceral) fibers • Pure sensory (afferent) or motor (efferent) nerves are rare • Cranial and spinal nerves (12 pair cranial, Roman numerals; 31 pair spinal, named for the nearby vertebra, e.g. C5 or L4) Ganglion (plural: ganglia) • A group of neuron cell bodies outside the CNS (analogous to nuclei inside the CNS) • associated with nerves Examples • dorsal root ganglia (sensory, somatic; ch. 12) • autonomic ganglia, such as sympathetic trunk ganglia (motor, visceral; ch.14)