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Download OVERVIEW OF PERIPHERAL NERVOUS SYSTEM Peripheral
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OVERVIEW OF PERIPHERAL NERVOUS SYSTEM Peripheral Nervous System • • • • • • • The Peripheral Nervous System Nervous structures outside the brain and spinal cord Nerves allow the CNS to receive information and take action Functional components of the PNS – Sensory inputs and motor outputs categorized as somatic or visceral – Sensory inputs also classified as general or special Sensory Input and Motor Output Sensory (afferent) signals picked up by sensor receptors, carried by nerve fibers of PNS to the CNS Motor (efferent) signals are carried away from the CNS, innervate muscles and glands Divided according to region they serve – Somatic body region – Visceral body region Results in four main subdivisions – Somatic sensory – Visceral sensory – Somatic motor – Visceral motor • • • • • PNS Afferent Division Afferent (sensory) division – transmits impulses from receptors to the CNS. – Somatic afferent fibers – carry impulses from skin, skeletal muscles, and joints – Visceral afferent fibers – transmit impulses from visceral organs PNS Efferent Division Motor (efferent) division – transmits impulses from the CNS to effector organs. Two subdivisions: – Somatic nervous system – provides conscious control of skeletal muscles – Autonomic nervous system – regulates smooth muscle, cardiac muscle, and glands Types of Sensory and Motor Information Sensory General somatic senses – include touch, pain, vibration, pressure, temperature Proprioceptive senses – detect stretch in tendons and muscle provide information on body position, orientation and movement of body in space Special Senses - hearing, balance, vision, olfaction (smell), gustation (taste) Motor – General somatic motor • • Signals contraction of skeletal muscles • Under our voluntary control – Visceral motor • Makes up autonomic nervous system (ANS) • Regulates the contraction of smooth and cardiac muscle, controls function of visceral organs • ANS has two divisions – Parasympathetic – Sympathetic Divisions of the ANS Sympathetic - ―fight or flight‖ – Catabolic (expend energy) – Mass activation prepares for intense activity. • Heart rate (HR) increases. • Bronchioles dilate. • Blood [glucose] increases. Parasympathetic - ―feed & breed‖, ―rest & digest‖ – – • • Maintain homeostasis Normally not activated as a whole, stimulation of separate parasympathetic nerves. – Relaxing effects: • Decreases HR. • Dilates visceral blood vessels. • Increases digestive activity. Dual innervation of many organs — having a brake and an accelerator provides more control Sympathetic Division Organization Preganglionic neurons in segments T1 to L2 – – Ganglia near the vertebral column – Sympathetic ganglia • Paired sympathetic chain ganglia • Unpaired collateral ganglia – Preganglionic fibers to adrenal medullae • Epinephrine (adrenalin) into blood stream The Autonomic Nervous System Effects of Sympathetic Activation – – – – – • • • • • • • • • • Generalized response in crises Increased alertness/energy Increased cardiovascular activity Increased respiratory activity Increased muscle tone Parasympathetic Division Organization Preganglionic neurons in brain stem and sacral spinal segment Ganglionic neurons (peripheral ganglia) in or near target organ Sacral fibers form pelvic nerves The Autonomic Nervous System Effects of Parasympathetic Activation – Relaxation – Food processing – Energy absorption – Brief effects at specific sites Basic Structural Components of the PNS Sensory receptors – pick up stimuli from inside or outside the body Motor endings – axon terminals of motor neurons innervate effectors (muscle fibers and glands) Nerves and ganglia – Nerves – bundles of peripheral axons – Ganglia – clusters of peripheral neuronal cell bodies Nerves Nerves – cablelike organs in the PNS Consists of numerous axons wrapped in connective tissue – Endoneurium – layer of delicate connective tissue surrounding the axon – Perineurium – connective tissue wrapping surrounding a nerve fascicle – Nerve fascicles – groups of axons bound into bundles – Epineurium – whole nerve is surrounded by tough fibrous sheath Axon is surrounded by Schwann cells • • • • • • • • • • Cranial Nerves Attach to the brain and pass through foramina of the skull Numbered from I–XII Cranial nerves I and II attach to the forebrain – All others attach to the brain stem Primarily serve head and neck structures – The vagus nerve (X) extends into the abdomen The 12 Pairs of Cranial Nerves Spinal Nerves 31 pairs – contain thousands of nerve fibers Connect to the spinal cord Named for point of issue from the spinal cord – 8 pairs of cervical nerves (C1–C8) – 12 pairs of thoracic nerves (T1–T12) – 5 pairs of lumbar nerves (L1–L5) – 5 pairs of sacral nerves (S1–S5) – 1 pair of coccygeal nerves (Co1) Spinal Nerves Connect to the spinal cord by the dorsal root and ventral root – Dorsal root – contains sensory fibers – Dorsal root ganglion – of afferent cell bodies – Ventral root – contains motor fibers arising from anterior gray column Branch into dorsal ramus and ventral ramus both contain sensory and motor fibers Rami communicantes connect to the base of the ventral ramus and lead to the sympathetic chain ganglia Spinal Nerves Innervation of the Back • Dorsal rami – Innervate back muscles – Follow a neat, segmented pattern – Innervate a horizontal strip of muscle and skin – In line with emergence point from the vertebral column Innervation of the Thoracic region – Ventral rami arranged in simple, segmented pattern – Intercostal nerves – supply intercostal muscles, skin, and abdominal wall • Each gives off lateral and anterior cutaneous branches Introduction to Nerve Plexuses • • Nerve plexus – a network of nerves Ventral rami (except T2 – T12) – Branch and join with one another – Form nerve plexuses • Cervical • Brachial • Lumbar • Sacral – Primarily serve the limbs – Fibers from ventral rami crisscross *************************************************************************************