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The Peripheral Nervous System (PNS) Part A Human Anatomy & Physiology, Sixth Edition Elaine N. Marieb 13 Peripheral Nervous System (PNS) Neural structures outside CNS, although soma may be w/in CNS sensory receptors, peripheral nerves, ganglia, & motor endings Divisions of PNS Afferent division (sensory) Efferent division (motor) Somatic – innervate skeletal muscle Autonomic – innervate smooth, cardiac, glands, visceral organs Sympathetic – soma in spinal cord Parasympathetic – soma in midbrain or sacrum Efferent Division Motor neurons of somatic division Terminate at motor end plate neuromuscular junction Motor neurons of autonomic division Varicosities in smooth & cardiac muscle & glands Afferent Division - Sensory Neurons Sensory neuron termini (dendritic processes) specialized to respond to stimuli Activation triggers impulses to CNS Perception in brain Receptor Classification by Stimulus Type Mechanoreceptors – change in neuron shape Thermoreceptors - temperature Photoreceptors - light Chemoreceptors - chemicals Nociceptors – pain-causing stimuli (chemicals) Receptor Classification by Location: Exteroceptors Near body surface Respond to stimuli arising outside body Include special sense organs Interoceptors Respond to stimuli arising w/in body Found in internal viscera & blood vessels Proprioceptors Respond to stretch In skeletal muscles, tendons, joints, ligaments, & connective tissue Receptor Classification by Structure Simple Dendritic process triggered directly by stimulus Encapsulated or unencapsulated nociceptors touch/pressure Complex Receptor cells w/in special sense organs Sensory neuron stimulated by bipolar neuron Photoreceptors Mechanoreceptors Olfactory receptors Gustatory receptors Table 13.1.1 Table 13.1.2 Adaptation of Sensory Receptors sensory receptors subjected to unchanging stimulus Receptor membranes become less responsive Receptor potentials decline in frequency or stop Pressure, touch, & smell receptors adapt quickly Merkel’s discs, Ruffini’s corpuscles, & interoceptors for blood chemicals adapt slowly Pain receptors & proprioceptors do not adapt Structure of a Nerve Classification of Nerves by Directionality Sensory (afferent) – signals TO CNS Motor (efferent) – signals FROM CNS Mixed – both sensory & motor most common somatic & autonomic signals Regeneration of Nerve Fibers Mature neurons are amitotic If soma remains intact, neuron can regenerate Figure 13.4 Cranial Nerves 12 pairs of nerves directly from brain Sensory, motor, or mixed I - XII according to anterior level of origin Named by to innervated organs/function Four cranial nerves carry parasympathetic fibers serving muscles & glands Cranial Nerves Summary of Cranial Nerves Spinal Nerves Figure 13.6 Spinal Nerve Roots (ANS) Figure 13.7a Nerve Plexuses Interlacing nerve networks cervical brachial lumbar sacral Branches of plexus contain fibers from several nerves Every muscle innervated by multiple spinal nerves Cervical Plexus Figure 13.8 Brachial Plexus Figure 13.9a Brachial Plexus: Distribution of Nerves Figure 13.9c Spinal Nerve Innervation: Back, Anterolateral Thorax, & Abdominal Wall Figure 13.7b Lumbar Plexus Figure 13.10 Sacral Plexus Figure 13.11 Reflexes Rapid, predictable motor response to a stimulus Reflexes: Intrinsic or acquired Involve only PNS & spinal cord Can relay to higher brain centers Somatic reflexes – skeletal muscle Autonomic reflexes – smooth muscle, glands Reflex Arc Components Receptor Sensory neuron Integration center Motor neuron Effector Spinal cord (in cross-section) Stimulus 2 Sensory neuron 1 3 Integration center Receptor 4 Motor neuron Skin 5 Effector Interneuron Stretch & Deep Tendon Reflexes Proprioceptors in tendons & muscle continually maintain postural contractions & muscle tone These effects are via spinal reflex arcs Muscle Spindles – Stretch Receptors Intrafusal muscle fibers lacking myofilaments in central regions Wrapped by type Ia & type II fibers afferent fibers Innervated by efferent fibers Operation of Muscle Spindles Stimulates action potential in postsynaptic neuron Does not stimulates action potential in postsynaptic neuron Stretch Reflex Figure 13.17 Flexor & Crossed Extensor Reflex Figure 13.19