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Spinal Cord and Spinal Nerves Chapter 12 APR Enhanced Lecture Slides See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and animations. 12-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 12 Spinal Cord and Spinal Nerves 12-2 Spinal Cord and Spinal Nerves Spinal Nerves 31 pairs Spinal cord 12.1 Spinal Cord • Extends from foramen magnum to second lumbar vertebra Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Segmented – Cervical Brain Level of foramen – Thoracic Cervical magnum enlargement Roots of spinal nerves – Lumbar Spinal nerves – Sacral Spinal cord • Gives rise to 31 pairs of spinal nerves • Not uniform in diameter throughout length – Cervical enlargement: supplies upper limbs Lumbosacral Conus enlargement – Lumbar enlargement: supplies lower limbs medullaris Level of second lumbar vertebra Cauda • Conus medullaris: tapered inferior end. equina • Cauda equina: origins of spinal nerves extending Filum inferiorly from lumbosacral enlargement and terminale conus medullaris. Posterior view 12-4 Spinal Cord Regions Cervical Thoracic Lumbar Sacral Coccygeal Cervical & Lumbosacral Enlargements Medullary Cone & Cauda Equina Spinal Cord Conus medullaris Filum terminale Cauda equina 8 Meninges of the Spinal Cord • Connective tissue membranes surrounding spinal cord and brain – Dura mater: continuous with epineurium Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Duramater of the spinal nerves Subdural space – Arachnoid mater: thin and wispy Denticulate ligament Arachnoid mater – Pia mater: bound tightly to surface of Subarachnoid space brain and spinal cord. Forms the filum Pia mater Epineurium of terminale, which anchors spinal cord to spinal nerve coccyx and the denticulate ligaments that attach the spinal cord to the dura mater Dorsal root ganglion Spinal nerve • Spaces Ventral root – Epidural: anesthesia injected. Contains blood vessels, areolar connective tissue and fat. (a) Anterolateral view – Subdural: serous fluid – Subarachnoid: CSF and blood vessels within web-like strands of arachnoid tissue 12-9 Meninges of Spinal Cord Meninges of the Spinal Cord Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Posterior Spinous process of vertebra Transverse process of vertebra Spinal cord Dorsal root Dorsal root ganglion Denticulate ligament Spinal nerve Ventral root Body of Pia mater vertebra Subarachnoid Periosteum space Arachnoid Epidural mater space filled with adipose tissue Subdural Dura space mater Anterior 12-11 (b) Superior view Cross Section of Spinal Cord Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Posterior median sulcus Central canal Dorsal root White matter Dorsal root ganglion Dorsal (posterior) column Central canal Ventral (anterior) column Lateral column Ventral root Gray matter Posterior (dorsal) Spinal horn nerve Lateral horn Anterior (ventral) horn Ventral root Gray commissure Rootlets White commissure Anterior median fissure (a) Anterolateral view Ascending nerve tracts Descending nerve tracts (c) Anterolateral view 12-12 Spinal Cord Gray matter White matter Cross Section of Spinal Cord • Anterior median fissure and posterior median sulcus: deep clefts partially separating left and right halves • White matter: myelinated axons forming tracts – Three columns (funiculi): ventral, dorsal, lateral • Each of these divided into tracts (fasciculi; pathways) • Gray matter: neuron, cell, cell bodies, dendrites, axons – Horns • Posterior (dorsal) • Anterior (ventral) • Lateral (associated with ANS) 12-14 Cross Section of Spinal Cord Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Posterior median sulcus Central canal Dorsal root White matter Dorsal root ganglion White matter Dorsal (posterior) Dorsal (posterior) Central canal column column Ventral (anterior) Ventral (anterior) column column Lateral column Ventral root Lateral column Gray matter Posterior (dorsal) Spinal horn Dorsal root nerve Lateral horn Anterior (ventral) horn Ventral root Gray commissure Rootlets White commissure Dorsal root ganglion Anterior median fissure (a) Anterolateral view (b) Ascending nerve tracts Descending nerve tracts (c) Anterolateral view b: © Ed Reschke/Peter Arnold, Inc./Getty Images 12-15 Cross Section of Spinal Cord Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Posterior median sulcus Central canal Dorsal root White matter Dorsal root ganglion • Commissures: connections Dorsal (posterior) column Central canal between left and right Ventral (anterior) column halves Lateral column Ventral root Gray matter Posterior (dorsal) – Gray with central canal in the Spinal horn nerve center Lateral horn Anterior (ventral) – White horn Ventral root • Roots: spinal nerves arise as Gray commissure Rootlets White commissure rootlets then combine to Anterior median fissure form roots (a) Anterolateral view – Dorsal (posterior) root has a ganglion – Ventral (anterior) Ascending nerve tracts – Two roots merge laterally and Descending nerve tracts form the spinal nerve 12-16 (c) Anterolateral view Dorsal rootlets Spinal Nerve Ventral rootlets Dorsal root Ventral root Dorsal root Spinal nerve gangion Organization of Neurons in the Spinal Cord and Spinal Nerves • Dorsal root Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Posterior horn Dorsal root Dorsal root ganglion ganglion: collections of Sensory neuron cell bodies of Spinal nerve unipolar Interneuron sensory neurons forming dorsal Autonomic neuron Lateral horn Somatic motor neuron roots. Anterior horn Ventral root Superior view • Motor neuron cell bodies are in anterior and lateral horns of spinal cord gray matter. – Multipolar somatic motor neurons in anterior (motor) horn – Autonomic neurons in lateral horn • Axons of motor neurons form ventral roots and pass into spinal nerves 12-18 12.2 Reflexes • Basic functional unit of nervous system and simplest portion capable of receiving a stimulus and producing a response • Automatic response to a stimulus that occurs without conscious thought. Homeostatic. • Components – Action potentials produced in sensory receptors transmitted to – Sensory neuron. To-Interneurons. To-Motor neuron. To- – Effector organ which responds with a reflex 12-19 Reflex Arc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dorsal root 3 1 Interneuron Dorsal root ganglion Sensory receptor 2 Sensory neuron Spinal cord Skin 4 Motor neuron Spinal nerve Ventral root 5 Skeletal Effector organ muscle 1 A sensory receptor detects a stimulus. 2 A sensory neuron conducts action potentials through the nerve and dorsal root to the spinal cord. 3 In the spinal cord, the sensory neuron synapses with an interneuron. (An interneuron is not involved in a monosynaptic reflex arc.) 4 The interneuron synapses with a motor neuron. 5 A motor neuron axon conducts action potentials through the ventral 12-20 root and spinal nerve to an effector organ. Variety of Reflexes • Some integrated within spinal cord; some within brain • Some involve excitatory neurons yielding a response; some involve inhibitory neurons that prevent an action • Higher brain centers can influence, suppress, or exaggerate reflex responses 12-21 Stretch Reflex • Muscles contract in response to a stretching force applied to them. Unique because no interneuron. • Muscle spindle: specialized muscle cells that respond to stretch. • Innervated by specific motor neurons: gamma motor neurons (small diameter neurons). Control sensitivity of muscle spindle. • Sensory neurons innervate the noncontractile centers of the muscle spindle cells. • These sensory neurons synapse with motor neurons of the spinal cord called alpha motor neurons which in turn innervate the muscle in which the muscle spindle is embedded. 12-22 Stretch Reflex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sudden stretch of a muscle results in: From brain To brain 1 Muscle spindles detect stretch of the muscle. 2 Sensory neurons conduct action potentials to the spinal cord. 3 Sensory neurons synapse directly with alpha motor neurons. Sensory neuron 3 4 Alpha motor neurons conduct action potentials to the muscle, causing it to contract and resist being stretched. 2 Note: The muscle that contracts is the muscle that is stretched. Alpha motor Quadriceps Muscle neuron femoris muscle spindle 4 (extensor) 1 Gamma motor neuron Patellar Sensory Sensory neuron tendon neuron endings Hammer Gamma motor tap neuron endings Patellar Hamstring ligament muscles (flexor) Muscle fiber of muscle spindle Muscle fiber of muscle Stretch reflex Str et c h Muscle spindle 12-23 Golgi Tendon Reflex • Prevents contracting muscles from applying excessive tension to tendons Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Golgi tendon organ. Encapsulated Intense stretch of a skeletal muscle results in: To brain nerve endings that have at their ends 1 Golgi tendon organs detect tension applied to a tendon. 2 Sensory neurons conduct action potentials to the spinal cord. numerous terminal branches with 3 3 Sensory neurons synapse with inhibitory interneurons Sensory that synapse with alpha motor neurons. neuron small swellings associated with 4 Inhibition of the alpha motor
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