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Chapter 13: The Spinal Cord, Spinal Nerves, & Spinal Reflexes
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Homeostasis Provides quick, reflexive responses to many stimuli
Pathway for sensory input to brain & motor output from brain
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Figure 13-1 An Overview of Chapters 13 and 14.
CHAPTER 14: The Brain Sensory input over cranial nerves Motor output over cranial nerves Reflex centers in brain Effectors Sensory receptors Muscles CHAPTER 13: The Spinal Cord Glands Sensory input over spinal nerves Motor output over spinal nerves Reflex centers in spinal cord Sensory receptors Adipose tissue
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Protective Structures
Meninges White matter Gray matter Dura mater Ventral rootlets of spinal nerve Arachnoid mater Dorsal root ganglion ANTERIOR Pia mater Ventral root Subarachnoid space Vertebral body Spinal nerve Autonomic (sympathetic) ganglion Dorsal root Meninges Dorsal rootlets of spinal nerve Rami communicantes Pia mater Ventral root of spinal nerve Arachnoid mater Dura mater Ventral ramus Dorsal ramus Spinal cord Adipose tissue in epidural space Denticulate ligament Dorsal root ganglion POSTERIOR b A sectional view through the spinal cord and meninges, showing the relationship of the meninges, spinal cord, and spinal nerves a A posterior view of the spinal cord, showing the meningeal layers, superficial landmarks, and distribution of gray matter and white matter
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Figure 13-4 The Spinal Cord and Associated Structures.
Anterior median fissure Pia mater Denticulate ligaments Dorsal root Ventral root, formed by several “rootlets” from one cervical segment Arachnoid mater (reflected) Dura mater (reflected) Spinal blood vessel
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Figure 13-2 Gross Anatomy of the Adult Spinal Cord.
Posterior median sulcus Dorsal root Dorsal root ganglion White matter Central canal Gray matter C1 C2 Cervical spinal nerves C3 C4 C5 Spinal nerve Ventral root C6 Cervical enlargement Anterior median fissure C7 C8 C3 T1 T2 T3 T4 T5 T6 T7 Thoracic spinal nerves T8 Posterior median sulcus T9 KEY Spinal cord regions = Cervical T10 T3 = Thoracic T11 Lumbar enlargement = Lumbar T12 = Sacral Conus medullaris L1 L2 Inferior tip of spinal cord Lumbar spinal nerves L3 L4 Cauda equina L5 L1 S1 Sacral spinal nerves S2 S3 S4 S5 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) S2 b Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. a The superficial anatomy and orientation of the adult spinal cord. The numbers to the left identify the spinal nerves and indicate where the nerve roots leave the vertebral canal. The adult spinal cord extends from the brain only to the level of vertebrae L1–L2; the spinal segments found at representative locations are indicated in the cross sections.
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Figure 13-2b Gross Anatomy of the Adult Spinal Cord.
Posterior median sulcus Dorsal root Dorsal root ganglion White matter Central canal Gray matter Spinal nerve Ventral root Anterior median fissure C3 T3 L1 S2 Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. b
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Figure 13-5 The Sectional Organization of the Spinal Cord.
Posterior median sulcus Posterior white column Functional Organization of Gray Matter Posterior gray commissure The cell bodies of neurons in the gray matter of the spinal cord are organized into functional groups called nuclei. Posterior gray horn Somatic Sensory nuclei Lateral white column Lateral gray horn Visceral Dorsal root ganglion Visceral Anterior gray horn Motor nuclei Somatic Ventral root Anterior gray commissure Anterior white commissure Anterior white column Anterior median fissure a The left half of this sectional view shows important anatomical landmarks, including the three columns of white matter. The right half indicates the functional organization of the nuclei in the anterior, lateral, and posterior gray horns. POSTERIOR Structural Organization of Gray Matter Posterior median sulcus The projections of gray matter toward the outer surface of the spinal cord are called horns. Posterior gray commissure Dura mater Posterior gray horn Arachnoid mater (broken) Lateral gray horn Dorsal root Central canal Anterior gray horn Anterior gray commissure Anterior median fissure Dorsal root ganglion Pia mater ANTERIOR Ventral root b A micrograph of a transverse section through the spinal cord, showing major landmarks in and surrounding the cord.
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Figure 13-8 Peripheral Distribution of Spinal Nerves.
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Spinal Nerves 31 pairs 8 prs cervical 12 prs thoracic 5 prs lumbar 5 prs sacral 1 pr coccygeal Formed by union of posterior & anterior roots so they are mixed nerves Consist of many axons & covered by layers of protective CT
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Figure 13-6 A Peripheral Nerve.
Blood vessels Connective Tissue Layers Epineurium covering peripheral nerve Perineurium (around one fascicle) Endoneurium Schwann cell Myelinated axon Fascicle b A scanning electron micrograph showing the various layers in great detail (SEM × 340) a A typical peripheral nerve and its connective tissue wrappings
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Figure 13-8 Peripheral Distribution of Spinal Nerves (Part 1 of 2).
From interoceptors of back From exteroceptors, proprioceptors of back 4 The dorsal root of each spinal nerve carriers sensory information to the spinal cord. 3 The dorsal ramus carries sensory information from the skin and skeletal muscles of the back. Somatic sensory nuclei 2 The ventral ramus carries sensory information from the ventrolateral body surface, structures in the body, wall, and the limbs. Dorsal root ganglion From exteroceptors, proprioceptors of body wall, limbs From interoceptors of body wall, limbs Rami communicantes Visceral sensory nuclei Ventral root KEY = Somatic sensations 1 = Visceral sensations The sympathetic nerve carriers sensory information from the visceral organs. From interceptors of visceral organs
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Figure 13-8 Peripheral Distribution of Spinal Nerves (Part 2 of 2).
To skeletal muscles of back Postganglionic fibers to smooth muscles, glands, etc., of back 2 The spinal nerve forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite. 3 The dorsal ramus contains somatic motor and visceral motor fibers that innervate the skin and skeletal muscles of the back. Dorsal root ganglion Dorsal root 4 The axons in the relatively large ventral ramus supply the ventrolateral body surface, structures in the body wall, and the limbs. 1 The ventral root of each spinal nerve contains the axons of somatic motor and visceral motor neurons. Visceral motor nuclei To skeletal muscles of body wall, limbs Somatic motor nuclei Rami communicantes KEY Postganglionic fibers to smooth muscles, and glands of body wall, limbs = Somatic motor commands Sympathetic ganglion = Visceral motor commands 5 The white ramus communicans is the first branch from the spinal nerve and carries visceral motor fibers to a nearby sympathetic ganglion. Because these preganglionic axons are myelinated, this branch has a light color and is therefore known as the white ramus. White rami are only found between T1 and L2. Postganglionic fibers to smooth muscles, glands, visceral organs in thoracic cavity 7 A sympathetic nerve contains preganglionic and postganglionic fibers innervating structures in the thoracic cavity. 6 The gray ramus communicans contains preganglionic fibers that innervate glands and smooth muscles in the body wall or limbs. These fibers are unmyelinated and have a dark gray color. Gray rami are associated with each spinal nerve. Preganglionic fibers to sympathetic ganglia innervating abdominopelvic viscera 1
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Plexuses Axons of anterior rami form networks by joining with other axons of anterior rami
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Figure 13-10 The Cervical Plexus.
Nerves of the Cervical Plexus Spinal Segments Nerve and Distribution Great Auricular Cranial Nerves C2–C3 Skin over the posterior aspect of the ear and the neck Accessory nerve (N XII) Lesser Occipital Hypoglossal nerve (N XII) C2 Skin of the neck and the scalp posterior and superior to the ear Transverse Cervical Nerve Roots of Cervical Plexus C3–C4 Skin of the anterior triangle of the neck C1 C2 Ansa Cervicalis C3 C1–C4 Five of the extrinsic laryngeal muscles by way of cranial nerve XII C4 C5 Geniohyoid muscle Thyrohyoid muscle Omohyoid muscle Sternothyroid muscle Sternohyoid muscle Clavicle Phrenic C3–C5 Diaphragm Supraclaviculars C3–C4 Skin of the neck and shoulder
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Figure 13-11 The Brachial Plexus.
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Figure 13-12 The Lumbar and Sacral Plexuses.
Iliohypogastric nerve Lumbar Plexus Nerve Roots of Lumbar Plexus Ilioinguinal nerve Superior gluteal nerve Spinal Segments Nerve and Distribution Genitofemoral nerve Inferior gluteal nerve Lateral femoral cutaneous nerve Iliohypogastric Pudendal nerve Femoral nerve Posterior femoral cutaneous nerve External and internal oblique and transverse abdominis muscles; skin over the inferior abdomen and buttocks Obturator nerve Superior gluteal nerve Sciatic nerve Ilioinguinal Inferior gluteal nerve Abdominal muscles (with iliohypogastric nerve); skin over superior, medial thigh and portions of external genitalia Pudendal nerve Genitofemoral Posterior femoral cutaneous nerve (cut) Skin over anteromedial thigh and portions of external genitalia Sciatic nerve Lateral Femoral Cutaneous Skin over anterior, lateral, and posterior thigh Saphenous nerve Tibial nerve Femoral Common fibular nerve Quadriceps, sartorius, pectineus, and iliopsoas muscles; skin of the anteromedial thigh, and medial surface of the leg and foot Obturator Lumbosacral trunk Common fibular nerve Gracilis, and adductor magnus, brevis and longus muscles; skin from the medial surface of the thigh a Lumbar plexus, anterior view Superficial fibular nerve Sural nerve Deep fibular nerve Lumbosacral trunk Nerve Roots of Sacral Plexus Sacral Plexus Spinal Segments Nerve and Distribution c Nerves of the lumbar and sacral plexuses, anterior view Superior Gluteal d Nerves of the sacral plexus, posterior view Gluteus minimus, gluteus medius, and tensor fasciae latae muscles Inferior Gluteal Gluteus maximus muscle Posterior Femoral Cutaneous Sacrum Skin over perineum and posterior thigh and leg Sciatic Semimembranosus, semitendinosus, and adductor magnus muscles; branches into tibial and fibular nerves Pudendal Muscles of the perineum; skin over external genitalia, bulbospongiosus and ischiocavernosus muscles b Sacral plexus, anterior view
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Intercostal Nerves Anterior rami of T2-T12
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N V C2–C3 C2 C3 C4 C5 T1 T2 T7 T3 T4 T5 T6 T8 T9 T10 T11 T12 C6 C7 C8 L1 L2 L3 L4 L5 S2 S1 S3 S4 S5 ANTERIOR POSTERIOR KEY Spinal cord regions = Cervical = Thoracic = Lumbar = Sacral Dermatomes Area of skin that provides sensory input to CNS via 1 pair of spinal nerves Allows us to locate damage
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Clinical Note 13-2 Sensory Innervation in the Hand.
Radial nerve Ulnar nerve Median nerve Anterior Posterior
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Clinical Note 13-3 Sensory Innervation in the Ankle and Foot.
Saphenous nerve Sural nerve Fibular nerve Tibial nerve Saphenous nerve Sural nerve Saphenous nerve Sural nerve Tibial nerve Fibular nerve
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Sensory Tracts Dorsal root ganglion Posterior column pathway
Ventral root Fasciculus gracilis Fasciculus cuneatus Posterior spinocerebellar tract Anterior spinocerebellar tract Lateral spinothalamic tract Anterior spinothalamic tract Spinothalamic pathway Spinocerebellar pathway Posterior column pathway
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Anterior corticospinal tract
Motor Tracts Lateral corticospinal tract Anterior corticospinal tract Rubrospinal tract Reticulospinal tract Tectospinal tract Vestibulospinal tract Corticospinal pathway Lateral pathway Medial pathway
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Reflexes Fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus
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Figure 13-15 The Classification of Reflexes.
can be classified by development response complexity of circuit processing site Innate Reflexes Somatic Reflexes Monosynaptic Spinal Reflexes • Genetically determined • Control skeletal muscle contractions • One synapse • Processing in the spinal cord • Include superficial and stretch reflexes Acquired Reflexes Visceral (Autonomic) Reflexes Polysynaptic Cranial Reflexes • Learned • Control actions of smooth and cardiac muscles, glands, and adipose tissue • Multiple synapses (two to several hundred) • Processing in the brain
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Reflex Arc Components Sensory receptor Sensory neuron
Integrating center Motor neuron Effector Arrival of stimulus and activation of receptor Activation of a sensory neuron Dorsal root Sensation relayed to the brain by axon collaterals Spinal cord Receptor Stimulus Effector Response by a peripheral effector Activation of a motor neuron Ventral root REFLEX ARC KEY Sensory neuron (stimulated) Excitatory interneuron Motor neuron (stimulated) Information processing in the CNS 3 2 4 5 1
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Stretch Reflex Skeletal muscle contracts in response to stretching to prevent overstretching Monosynaptic & ipsilateral Response REFLEX ARC Stimulus Stretch Receptor (muscle spindle) Contraction Effector Spinal cord Sensory neuron (stimulated) Motor neuron (stimulated) KEY
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Distribution within gray horns to other segments of the spinal cord
Flexor Reflex Aka withdrawal reflex Polysynaptic & ipsilateral Pulls body part away from source of possibly damaging stimulus Painful stimulus Flexors stimulated Extensors inhibited Distribution within gray horns to other segments of the spinal cord
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Crossed Extensor Reflex
To motor neurons in other segments of the spinal cord Polysynaptic & contralateral Helps you maintain your balance Usually paired with flexor reflex Extensors inhibited Flexors stimulated Extensors stimulated Flexors inhibited KEY Sensory neuron (stimulated) Motor neuron (inhibited) Excitatory interneuron Inhibitory interneuron Motor neuron (stimulated) Painful stimulus
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The Babinski Reflex a The plantar reflex (negative Babinski reflex), a curling of the toes, is seen in healthy adults. b The Babinski sign (positive Babinski reflex) occurs in the absence of descending inhibition. It is normal in infants, but pathological in adults.
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