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Chapter 13

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Anatomy and Physiology
Jacqueline Carnegie

Chapter 13: The Peripheral Nervous System and Reflex Activity Nerves and Associated Ganglia Structure and Classification -a nerve is a cordlike organ that is part of the peripheral nervous system -nerves consist of parallel bundles of peripheral axons (may be myelinated or not) enclosed by successive wrappings of connective tissue -within a nerve, each axon is surrounded by endoneurium -a delicate layer of loose connective tissue that also encloses the fibre's myelin sheath or neurilemma -groups of fibres are bound into bundles or fascicles by a coarser connective tissue wrapping known as the perineurium -all fascicles are enclose by a tough fibrous sheath known as the epineurium which forms the nerve -nerves are mostly made up of myelin, the protective connective tissue wrappings, blood vessels, and lymphatic vessels; axons are only a small fraction of the nerve's bulk -nerves are classified according to the direction in which they transmit impulses -nerves containing both sensory and motor fibres and transmit impulses both to and from the central nervous system are called mixed nerves -nerves that carry impulses toward the CNS are sensory (afferent) nerves -nerves that carry impulses away from the CNS are motor (efferent) nerves -most nerves are mixed; pure sensory or motor nerves are rare -since mixed nerves carry both somatic and autonomic (visceral) nervous system fibres, the fibres in them may be classified according to the region they innervate as somatic afferent, somatic efferent, visceral afferent, and visceral efferent -ganglia are collections of neuron cell bodies associated with nerves in the PNS -ganglia associated with afferent nerve fibres contain cell bodies of sensory neurons (these are dorsal root ganglia) -ganglia associated with efferent nerve fibres contain mostly autonomic motor neurons Cranial Nerves -12 pairs of cranial nerves are associated with the brain -the first 2 pairs attach to the forebrain -the rest of the cranial nerves are associated with the brain stem -cranial nerves serve only head and neck structures (excluding the vagus nerve which extends into the abdomen) -cranial nerves are mixed nerves -the olfactory and optic nerve associated with the special sense organs ore considered purely sensory -cell bodies of the sensory neurons of the olfactory and optic nerves are located within their respective special sense organs -in sensory neurons that contribute to cranial nerves (V, VII, IX, and X) the cell bodies are located in cranial sensory ganglia just outside the brain -several of the mixed cranial nerves contain both somatic and autonomic motor fibres and so they serve both skeletal muscles and visceral organs -cell bodies of motor neurons contributing to the cranial nerves are located in the ventral gray matter regions (nuclei) of the brain stem "Some say marry money but my brother says (it’s) bad business (to) marry money" -m = motor -s = sensory -b = both -the pathways of purely or mostly sensory nerves (I, II, and VIII) are described from the receptors to the brain -the pathways of other nerves are described from the brain and distally "On occasion, our trusty truck acts funny - very good vehicle anyhow" Spinal Nerves -31 pairs of spinal nerves that contain nerve fibres arising from the spinal cord and supply all parts of the body except the head and some neck areas -all spinal nerves are mixed nerves -nerves are named according to their point of issue from the spinal cord -8 pairs of cervical spinal nerves (C1-C8) -there are 8 pairs of cervical nerves but only 7 cervical vertebrae -the first 7 pairs exit the vertebral canal superior to the vertebrae for which they are named -C8 emerges inferior to the 7th cervical vertebrae (between C7 and T1) -below the cervical level, each spinal nerve leaves the vertebral column (spinal column) inferior to the same-numbered vertebra -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) -each spinal nerve connects to the spinal cord by a dorsal root and ventral root -each root forms a series of rootlets that attach along the length of the corresponding spinal cord segment -the ventral roots contain motor (efferent) fibres that arise from the ventral horn motor neurons and extend to and innervate the skeletal muscles -the dorsal roots contain sensory (afferent) fibres that arise from sensory neurons in the dorsal root ganglia and conduct impulses from peripheral receptors to the spinal cord -spinal roots pass laterally from the cord and unite just distal to the dorsal root ganglion to form a spinal nerve before emerging from the vertebral column via their respective intervertebral foramina -the length of spinal roots increases progressively from the superior to the inferior aspect of the cord -in the cervical region, the roots are short and run horizontally -the roots of the lumbar and sacral nerves extend inferiorly through the lower vertebral canal as the cauda equina before exiting the vertebral column -a spinal nerve is short (1-2 cm) -almost immediately after emerging from its foramen, it divides into a small dorsal ramus and a larger ventral ramus and a tiny meningeal branch that re- enters the vertebral canal to innervate the meninges and blood vessels within -each ramus is mixed -special rami called rami communicates are joined to the base of the ventral rami of the thoracic spinal nerves and contain autonomic (visceral) nerve fibres Difference Between Root and Rami -roots lie medial to and form the spinal nerves -each root is strictly sensory or motor -rami lie distal to and are lateral branches of spinal nerves -carry both sensory and motor fibres like spinal nerves Innervation of Specific Body Regions -the spinal nerve rami and their main branches supply the entire somatic region of the body (skeletal muscles and skin) from the neck down -the dorsal rami supply the posterior body trunk -the ventral rami supply the rest of the trunk and the limbs -except for T2-T12, all ventral rami branch and join one another lateral to the vertebral column to form interlacing nerve networks called nerve plexuses -nerve plexuses occur in the cervical, brachial, lumbar, and sacral regions and primarily serve limbs -only ventral rami form plexuses -within a plexus, fibres from various ventral rami interlace and become redistributed -each resulting branch of the plexus contains fibre
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