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CNS consists of brain and spinal cord PNS consists of nerves 1 Input to nervous system [Chapter 13] 2 Cordlike organ of PNS Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue 3 Connective tissue coverings include Endoneurium—loose connective tissue that encloses axons and their myelin sheaths Perineurium—coarse connective tissue that bundles fibers into fascicles Epineurium—tough fibrous sheath around a nerve 4 Peripheral nerves classified as cranial or spinal nerves Classified according to direction transmit impulses Types of fibers in mixed nerves: Somatic afferent Somatic efferent Visceral afferent Visceral efferent Sensory (afferent) nerves – impulses only toward CNS Motor (efferent) nerves – impulses only away from CNS Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers Pure sensory (afferent) or motor (efferent) nerves are rare; most mixed 5 Note that there are no neuronal cell bodies in peripheral nerves which are just myelinated axons 6 Contain neuron cell bodies associated with nerves in PNS Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons Dorsal root ganglia (sensory, somatic) (Chapter 12) Ganglia associated with efferent nerve fibers contain autonomic motor neurons Autonomic ganglia (motor, visceral) (Chapter 14 7 31 pairs of mixed nerves named for point of issue from spinal cord Supply all body parts but head and part of neck 8 cervical (C1–C8) 12 thoracic (T1–T12) 5 Lumbar (L1–L5) 5 Sacral (S1–S5) 1 Coccygeal (C0) 8 Only 7 cervical vertebrae, yet 8 pairs cervical spinal nerves 7 exit vertebral canal superior to vertebrae for which named 1 exits canal inferior to C7 Other vertebrae exit inferior to vertebra for which named 9 10 Mature neurons are amitotic but if soma of damaged nerve is intact, peripheral axon may regenerate If peripheral axon damaged Axon fragments (Wallerian degeneration); spreads distally from injury Macrophages clean dead axon; myelin sheath intact 11 Axon filaments grow through regeneration tube Axon regenerates; new myelin sheath forms Greater distance between severed ends-less chance of regeneration 11 Macrophages clean dead axon; myelin sheath intact 12 Axon filaments grow through regeneration tube 13 Axon regenerates; new myelin sheath forms Greater distance between severed ends-less chance of regeneration 14 Most CNS fibers never regenerate CNS oligodendrocytes bear growthinhibiting proteins that prevent CNS fiber regeneration Astrocytes at injury site form scar tissue containing chondroitin sulfate that blocks axonal regrowth Treatment Neutralizing growth inhibitors, 15 blocking receptors for inhibitory proteins, destroying chondroitin sulfate promising 15 Each spinal nerve connects to spinal cord via two roots Ventral roots Contain motor (efferent) fibers from ventral horn motor neurons Fibers innervate skeletal muscles 16 Dorsal roots Contain sensory (afferent) fibers from sensory neurons in dorsal root ganglia and conduct impulses from peripheral receptors 17 Dorsal and ventral roots unite to form spinal nerves, which emerge from vertebral column via intervertebral foramina 18 Spinal roots longer as move inferiorly in cord Lumbar and sacral roots extend as cauda equina 19 Spinal nerves quite short (~1-2 cm) Each branches into mixed rami Dorsal ramus Ventral ramus - larger Meningeal branch – tiny, reenters vertebral canal, innervates meninges and blood vessels Rami communicantes (autonomic pathways) join ventral rami in thoracic region 20 All ventral rami except T2–T12 form interlacing nerve networks called nerve plexuses (cervical, brachial, lumbar, and sacral) Back innervated by dorsal rami via several branches Ventral rami of T2–T12 as intercostal nerves supply muscles of ribs, 21 anterolateral thorax, and abdominal wall 21 All ventral rami except T2–T12 form interlacing nerve networks called nerve plexuses (cervical, brachial, lumbar, and sacral) 22 Within plexus fibers criss-cross Each branch contains fibers from several spinal nerves Fibers from ventral ramus go to body periphery via several routes Each limb muscle innervated by more than one spinal nerve Damage to one does not paralysis 23 Formed by ventral rami of C1–C4 Most branches form cutaneous nerves Innervate skin of neck, ear, back of head, and shoulders Other branches innervate neck muscles 24 Phrenic nerve Major motor and sensory nerve of diaphragm (receives fibers from C3–C5) Irritation hiccups 25 26 Formed by ventral rami of C5–C8 and T1 (and often C4 and/or T2) Gives rise to nerves that innervate upper limb 27 Major branches of this plexus: Roots—five ventral rami (C5–T1), which form Trunks—upper, middle, and lower, which form Divisions—anterior and posterior, which form Cords—lateral, medial, and posterior 28 Brachial Plexus: Five Important Nerves Axillary—innervates deltoid, teres minor, and skin and joint capsule of shoulder 29 Musculocutaneous—innervates biceps brachii and brachialis, coracobrachialis, and skin of lateral forearm 30 Median—innervates skin, most flexors, forearm pronators, wrist and finger flexors, thumb opposition muscles 31 Ulnar—supplies flexor carpi ulnaris, part of flexor digitorum profundus, most intrinsic hand muscles, skin of medial aspect of hand, wrist/finger flexion 32 Radial—innervates essentially all extensor muscles, supinators, and posterior skin of limb 33 34 35 Arises from L1–L4 Innervates thigh, abdominal wall, and psoas muscle Femoral nerve—innervates quadriceps and skin of anterior thigh and medial surface of leg Obturator nerve—passes through obturator foramen to innervate adductor muscles 36 37 38 Cremaster muscle innervated by the genitofemoral nerve 39 Arises from L4–S4 Serves the buttock, lower limb, pelvic structures, and perineum Sciatic nerve Longest and thickest nerve of body Innervates hamstring muscles, adductor magnus, and most muscles in leg and foot Composed of two nerves: tibial and common fibular 40 41 42 43 Ventral rami in thorax in simple segmental pattern Form intercostal nerves that supply intercostal muscles, muscle and skin of anterolateral thorax, most abdominal wall Give off cutaneous branches to skin along course Dorsal rami innervate posterior body trunk 44 Dermatome - area of skin innervated by cutaneous branches of single spinal nerve All spinal nerves except C1 participate in dermatomes Extent of spinal cord injuries ascertained by affected dermatomes Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness 45 Provides links from and to world outside body All neural structures outside brain Sensory receptors Peripheral nerves and associated ganglia Efferent motor endings SENSORY RECEPTORS Specialized to respond to changes in environment (stimuli) Activation results in graded potentials that trigger nerve impulses Sensation (awareness of stimulus) and perception (interpretation of meaning of stimulus) occur in brain Classification of receptors: Based on Type of stimulus they detect Location in body Structural complexity 47 Classification by Stimulus Type Mechanoreceptors—respond to touch, pressure, vibration, and stretch 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) 48 Classification by Location Exteroceptors Respond to stimuli arising outside body Receptors in skin for touch, pressure, pain, and temperature Most special sense organs Interoceptors (visceroceptors) Respond to stimuli arising in internal viscera and blood vessels Sensitive to chemical changes, tissue stretch, and temperature changes Sometimes cause discomfort but usually unaware of their workings 49 Interoceptors (visceroceptors) Respond to stimuli arising in internal viscera and blood vessels Sensitive to chemical changes, tissue stretch, and temperature changes Sometimes cause discomfort but usually unaware of their workings 50 Proprioceptors Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles Inform brain of one's movements 51 Simple receptors for general senses Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense Modified dendritic endings of sensory neurons Either nonencapsulated (free) or encapsulated Nonencapsulated (free) nerve endings Abundant in epithelia and connective tissues Most nonmyelinated, small-diameter group C fibers; distal endings have knoblike swellings Respond mostly to temperature and pain; some to pressure-induced tissue movement; itch 52 Thermoreceptors Cold receptors (10–40ºC); in superficial dermis Heat receptors (32–48ºC); in deeper dermis Outside those temperature ranges nociceptors activated pain 53 Nociceptors Player in detection – vanilloid receptor Ion channel opened by heat, low pH, chemicals, e.g., capsaicin (red peppers) Respond to: Pinching, chemicals from damaged tissue, capsaicin 54 Light touch receptors Tactile (Merkel) discs Hair follicle receptors 55 56 Encapsulated Dendritic Endings All mechanoreceptors in connective tissue capsule Tactile (Meissner's) corpuscles— discriminative touch Lamellar (Pacinian) corpuscles—deep pressure and vibration Bulbous corpuscles (Ruffini endings)— deep continuous pressure 57 Encapsulated Dendritic Endings Muscle spindles—muscle stretch Tendon organs—stretch in tendons Joint kinesthetic receptors—joint position and motion 58 Somatosensory system – part of sensory system serving body wall and limbs Receives inputs from Exteroceptors, proprioceptors, and interoceptors Input relayed toward head, but processed along way 59 Sensation - the awareness of changes in the internal and external environment To produce a sensation Receptors have specificity for stimulus energy Stimulus must be applied in receptive field Transduction occurs Stimulus changed to graded potential Generator potential or receptor potential Graded potentials must reach threshold Action Potential In general sense receptors, graded potential called generator potential Stimulus Generator potential in afferent neuron Action potential 60 Receptors have specificity for stimulus energy Stimulus must be applied in receptive field 61 Transduction occurs Stimulus changed to graded potential Generator potential or receptor potential Graded potentials proportional to stimulus 62 Receptor and generator potentials are GRADED potentials [effect on adjacent ionic concentrations noted as blue circles above] 63 Note: inactivation gates of voltage-gated sodium channels are CLOSED until point 5 64 Graded potential gets membrane potential to threshold 65 graded potential lasts for many, many milliseconds action potential lasts for less than 5 mS 66 Persistent graded potential combines with falling and undershoot phases of action potential 67 In general sense receptors, graded potential called generator potential Stimulus Generator potential in afferent neuron Action potential 68 In special sense organs: Stimulus Graded potential in receptor cell called receptor potential Affects amount of neurotransmitter released Neurotransmitters generate graded potentials in sensory neuron 69 Adaptation is change in sensitivity in presence of 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, and smell Tonic receptors adapt slowly or not at all Examples - nociceptors and most proprioceptors 70 Phasic (fast-adapting) receptors signal beginning or end of stimulus Examples - receptors for pressure, touch, and smell Tonic receptors adapt slowly or not at all Examples - nociceptors and most proprioceptors 71 Twelve pairs of nerves associated with brain Two attach to forebrain; rest with brain stem Most mixed nerves; two pairs purely sensory Each numbered (I through XII) and named from rostral to caudal 72 Twelve pairs of nerves associated with brain Two attach to forebrain; rest with brain stem Most mixed nerves; two pairs purely sensory Each numbered (I through XII) and named from rostral to cauxdal 73 Make up your own mnemonic 74 Cranial nerves with sensory modalities: Olfactory and optic nerves Neuron cell bodies within special sense organs Other nerves with sensory information (V, VII, VIII, IX, X) Neuron cell bodies in cranial sensory 75 ganglia 75 PS = parasympathetic [only I and II are pure sensory nerves] 76 Sensory nerves of smell Run from nasal mucosa to olfactory bulbs Pass through cribriform plate of ethmoid bone Fibers synapse in olfactory bulbs Pathway terminates in primary olfactory cortex Purely sensory (olfactory) function 77 Arise from retinas; really a brain tract not a peripheral nerve Pass through optic canals, converge and partially cross over at optic chiasma Optic tracts continue to thalamus, where they synapse Optic radiation fibers run to occipital (visual) cortex Purely sensory (visual) function 78 Fibers extend from ventral midbrain through superior orbital fissures to four of six extrinsic eye muscles Function in raising eyelid, directing eyeball, constricting iris (parasympathetic), and controlling lens shape 79 Fibers from dorsal midbrain enter orbits via superior orbital fissures to innervate superior oblique muscle Primarily motor nerve that directs eyeball 80 81 Largest cranial nerves; fibers extend from pons to face Three divisions Ophthalmic (V1) passes through superior orbital fissure Maxillary (V2) passes through foramen rotundum Mandibular (V3) passes through the foramen ovale 82 Convey sensory impulses from various areas of face (V1) and (V2) Supply motor fibers (V3) for mastication 82 Note location of the ganglion containing the cell bodies of the sensory neurons of Cranial nerve V 83 Shown to emphasize the unipolar characteristics of Peripheral Nerve sensory cells in Cranial Nerve. From article about the very pretty stain. 84 Just like Dorsal Root Ganglia 85 Fibers from inferior pons enter orbits via superior orbital fissures Primarily a motor, innervating lateral rectus muscle 86 87 Fibers from pons travel through internal acoustic meatuses, and emerge through stylomastoid foramina to lateral aspect of face Chief motor nerves of face with 5 major branches Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands Sensory function (taste) from anterior two-thirds of tongue 88 Afferent fibers from hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from inner ear through internal acoustic meatuses, and enter brain stem at pons-medulla border Mostly sensory function; small motor component for adjustment of sensitivity of receptors Formerly auditory nerve 89 Fibers from medulla leave skull via jugular foramen and run to throat Motor functions - innervate part of tongue and pharynx for swallowing, and provide parasympathetic fibers to parotid salivary glands Sensory functions - fibers conduct taste and general sensory impulses from pharynx and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors 90 Only cranial nerves that extend beyond head and neck region Fibers from medulla exit skull via jugular foramen Most motor fibers are parasympathetic fibers that help regulate activities of heart, lungs, and abdominal viscera Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx 91 Formed from ventral rootlets from C1–C5 region of spinal cord (not brain) Rootlets pass into cranium via each foramen magnum Accessory nerves exit skull via jugular foramina to innervate trapezius and sternocleidomastoid muscles Formerly spinal accessory nerve 92 Fibers from medulla exit skull via hypoglossal canal Innervate extrinsic and intrinsic muscles of tongue that contribute to swallowing and speech 93 94 95 96