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Chapter 13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes Lecture Presentation by Lee Ann Frederick University of Texas at Arlington © 2015 Pearson Education, Inc. An Introduction to the Spinal Cord, Spinal Nerves, and Spinal Reflexes • Spinal Reflexes • Rapid, automatic nerve responses triggered by specific stimuli • Controlled by spinal cord alone, not the brain © 2015 Pearson Education, Inc. Figure 13-1 An Overview of Chapters 13 and 14. CHAPTER 14: The Brain Sensory receptors Sensory input over cranial nerves Reflex centers in brain Motor output over cranial nerves Effectors Muscles CHAPTER 13: The Spinal Cord Glands Sensory receptors © 2015 Pearson Education, Inc. Sensory input over spinal nerves Reflex centers in spinal cord Motor output over spinal nerves Adipose tissue 13-2 Spinal Cord • Gross Anatomy of the Spinal Cord • About 18 inches (45 cm) long • 1/2 inch (14 mm) wide • Ends between vertebrae L1 and L2 • Bilateral symmetry • Grooves divide the spinal cord into left and right • Posterior median sulcus – on posterior side • Anterior median fissure – deeper groove on anterior side © 2015 Pearson Education, Inc. Figure 13-2 Gross Anatomy of the Adult Spinal Cord. Posterior median sulcus Dorsal root Dorsal root ganglion Cervical spinal nerves C1 C2 C3 C4 C5 C6 C7 C8 White matter Central canal Cervical enlargement Gray matter Spinal Ventral nerve root Anterior median fissure C3 T1 T2 T3 T4 T5 T6 T7 KEY Spinal cord regions Thoracic spinal nerves T8 T9 = Cervical T10 = Thoracic T11 = Lumbar = Sacral T12 L1 Posterior median sulcus T3 Lumbar enlargement Conus medullaris L2 Lumbar spinal nerves L3 Inferior tip of spinal cord L4 Cauda equina L5 L1 Sacral spinal nerves S1 S2 S3 S4 S5 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) a The superficial anatomy and orientation of the adult spinal cord. The S2 b 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. © 2015 Pearson Education, Inc. Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 1 of 4). Posterior median sulcus Dorsal root Dorsal root ganglion Cervical spinal nerves C1 C2 C3 C4 C5 C6 C7 C8 Central canal Cervical enlargement Spinal Ventral nerve root 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. © 2015 Pearson Education, Inc. White matter Gray matter Anterior median fissure C3 Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 2 of 4). T1 T2 T3 T4 T5 T6 T7 Thoracic spinal nerves T8 T9 Posterior median sulcus T10 T11 T12 T3 Lumbar enlargement Conus medullaris 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. © 2015 Pearson Education, Inc. Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 3 of 4). L1 Conus medullaris L2 Lumbar spinal nerves L3 Inferior tip of spinal cord L4 Cauda equina L5 L1 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. © 2015 Pearson Education, Inc. Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 4 of 4). Inferior tip of spinal cord Cauda equina Sacral spinal nerves S1 S2 S3 S4 S5 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) 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. © 2015 Pearson Education, Inc. S2 Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. Figure 13-3a The Spinal Cord and Spinal Meninges. White matter Ventral rootlets of spinal nerve Gray matter Dorsal root ganglion Ventral root Spinal nerve Dorsal root Dorsal rootlets of spinal nerve Meninges Pia mater Arachnoid mater Dura mater a A posterior view of the spinal cord, showing the meningeal layers, superficial landmarks, and distribution of gray matter and white matter © 2015 Pearson Education, Inc. Figure 13-3b The Spinal Cord and Spinal Meninges. Meninges ANTERIOR Subarachnoid space Vertebral body Rami communicantes Dura mater Arachnoid mater Pia mater Autonomic (sympathetic) ganglion Ventral root of spinal nerve Ventral ramus Dorsal ramus Spinal cord Adipose tissue in epidural space Denticulate Dorsal root ganglion ligament POSTERIOR b A sectional view through the spinal cord and meninges, showing the relationship of the meninges, spinal cord, and spinal nerves © 2015 Pearson Education, Inc. 13-2 Spinal Cord • The Spinal Meninges • Specialized membranes isolate spinal cord from surroundings • Functions of the spinal meninges include: • Protecting spinal cord • Carrying blood supply • Continuous with cranial meninges • Meningitis • Viral or bacterial infection of meninges © 2015 Pearson Education, Inc. 13-2 Spinal Cord • The Three Meningeal Layers 1. Dura mater • Outer layer of spinal cord 2. Arachnoid mater • Middle meningeal layer 3. Pia mater • Inner meningeal layer © 2015 Pearson Education, Inc. 13-2 Spinal Cord • The Dura Mater • Tough and fibrous • Cranially • Fuses with periosteum of occipital bone • Is continuous with cranial dura mater • Caudally • Tapers to dense cord of collagen fibers • Joins filum terminale in coccygeal ligament © 2015 Pearson Education, Inc. 13-2 Spinal Cord • The Dura Mater • The epidural space • Between spinal dura mater and walls of vertebral canal • Contains loose connective and adipose tissue • Anesthetic injection site © 2015 Pearson Education, Inc. 13-2 Spinal Cord • The Arachnoid Mater • Middle meningeal layer • Arachnoid membrane • Simple squamous epithelia • Covers arachnoid mater © 2015 Pearson Education, Inc. 13-2 Spinal Cord • The Interlayer Spaces of Arachnoid Mater • Subdural space • Between arachnoid mater and dura mater • Subarachnoid space • Between arachnoid mater and pia mater • Contains collagen/elastin fiber network (arachnoid trabeculae) • Filled with cerebrospinal fluid (CSF) © 2015 Pearson Education, Inc. 13-2 Spinal Cord • The Interlayer Spaces of Arachnoid Mater • Cerebrospinal Fluid (CSF) • Carries dissolved gases, nutrients, and wastes • Lumbar puncture or spinal tap withdraws CSF © 2015 Pearson Education, Inc. 13-2 Spinal Cord • The Pia Mater • Is the innermost meningeal layer • Is a mesh of collagen and elastic fibers • Is bound to underlying neural tissue © 2015 Pearson Education, Inc. 13-2 Spinal Cord • Structures of the Spinal Cord • Paired denticulate ligaments • Extend from pia mater to dura mater • Stabilize side-to-side movement • Blood vessels • Along surface of spinal pia mater • Within subarachnoid space © 2015 Pearson Education, Inc. Figure 13-4 The Spinal Cord and Associated Structures. Spinal cord 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 © 2015 Pearson Education, Inc. 13-3 Gray Matter and White Matter • Sectional Anatomy of the Spinal Cord • White matter • Is superficial • Contains myelinated and unmyelinated axons • Gray matter • Surrounds the central canal of spinal cord • Contains neuron cell bodies, neuroglia, unmyelinated axons • Has projections (gray horns) © 2015 Pearson Education, Inc. 13-3 Gray Matter and White Matter • Organization of Gray Matter • The gray horns • Posterior gray horns contain somatic and visceral sensory nuclei • Anterior gray horns contain somatic motor nuclei • Lateral gray horns are in thoracic and lumbar segments; contain visceral motor nuclei • Gray commissures • Axons that cross from one side of cord to the other before reaching gray matter © 2015 Pearson Education, Inc. 13-3 Gray Matter and White Matter • Organization of Gray Matter • The cell bodies of neurons form functional groups called nuclei • Sensory nuclei • Dorsal (posterior) • Connect to peripheral receptors • Motor nuclei • Ventral (anterior) • Connect to peripheral effectors © 2015 Pearson Education, Inc. 13-3 Gray Matter and White Matter • Control and Location • Sensory or motor nucleus location within the gray matter determines which body part it controls © 2015 Pearson Education, Inc. Figure 13-5a The Sectional Organization of the Spinal Cord (Part 1 of 2). Posterior white column Posterior gray horn Dorsal root ganglion Lateral white column Lateral gray horn Anterior gray horn Anterior white column 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. © 2015 Pearson Education, Inc. Figure 13-5a The Sectional Organization of the Spinal Cord (Part 2 of 2). Posterior median sulcus Posterior gray commissure Somatic Functional Organization of Gray Matter The cell bodies of neurons in the gray matter of the spinal cord are organized into functional groups called nuclei. Sensory nuclei Visceral Visceral Motor nuclei Somatic Ventral root Anterior gray commissure Anterior white commissure Anterior median fissure 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. © 2015 Pearson Education, Inc. 13-4 Spinal Nerves and Plexuses • Anatomy of Spinal Nerves • Each spinal cord segment: • Is connected to a pair of spinal nerves • Each spinal nerve: • Is surrounded by three connective tissue layers • That support structures and contain blood vessels © 2015 Pearson Education, Inc. 13-4 Spinal Nerves and Plexuses • Three Connective Tissue Layers of Spinal Nerves 1. Epineurium • Outer layer • Dense network of collagen fibers 2. Perineurium • Middle layer • Divides nerve into fascicles (axon bundles) 3. Endoneurium • Inner layer • Surrounds individual axons © 2015 Pearson Education, Inc. Figure 13-6a A Peripheral Nerve. Blood vessels Connective Tissue Layers Epineurium covering peripheral nerve Perineurium (around one fascicle) Endoneurium Schwann cell Myelinated axon Fascicle a A typical peripheral nerve and its connective tissue wrappings © 2015 Pearson Education, Inc. Figure 13-6b A Peripheral Nerve. Blood vessels Connective Tissue Layers Perineurium (around one fascicle) Endoneurium b A scanning electron micrograph showing the various layers in great detail (SEM × 340) © 2015 Pearson Education, Inc. 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 Dorsal root Dorsal root ganglion 4 1 The ventral root of each spinal nerve contains the axons of somatic motor and visceral motor neurons. The dorsal ramus contains somatic motor and visceral motor fibers that innervate the skin and skeletal muscles of the back. The axons in the relatively large ventral ramus supply the ventrolateral body surface, structures in the body wall, and the limbs. Visceral motor nuclei To skeletal muscles of body wall, limbs Somatic motor nuclei Rami communicantes KEY = Somatic motor commands = Visceral motor commands Postganglionic fibers to smooth muscles, glands, visceral organs in thoracic cavity Preganglionic fibers to sympathetic ganglia innervating abdominopelvic viscera © 2015 Pearson Education, Inc. Sympathetic ganglion 7 Postganglionic fibers to smooth muscles, and glands of body wall, limbs 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. A sympathetic nerve contains preganglionic and postganglionic fibers innervating structures in the thoracic cavity. 1 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. 13-4 Spinal Nerves and Plexuses • Peripheral Distribution of Spinal Nerves • Sensory nerves • In addition to motor impulses: • Dorsal, ventral, and white rami also carry sensory information • Dermatomes • Bilateral region of skin • Monitored by specific pair of spinal nerves © 2015 Pearson Education, Inc. 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 Ventral root KEY = Somatic sensations = Visceral sensations © 2015 Pearson Education, Inc. Visceral sensory nuclei 1 The sympathetic nerve carriers sensory information from the visceral organs. From interceptors of visceral organs Figure 13-7 Dermatomes. NV C2–C3 C2 C3 T2 C6 L1 L2 C8 T1 C7 L3 L4 KEY Spinal cord regions L5 C3 C4 C5 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 S2 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 L1 L2 L4 L3 L5 C4 C5 T2 C6 T1 C7 S4S3 L1 S5 C8 S1 L5 L2 S2 L3 = Cervical = Thoracic = Lumbar S1 = Sacral L4 ANTERIOR © 2015 Pearson Education, Inc. POSTERIOR 13-4 Spinal Nerves and Plexuses • Peripheral Neuropathy • Regional loss of sensory or motor function • Due to trauma or compression © 2015 Pearson Education, Inc. 13-4 Spinal Nerves and Plexuses • Nerve Plexuses • Complex, interwoven networks of nerve fibers • Formed from blended fibers of ventral rami of adjacent spinal nerves • Control skeletal muscles of the neck and limbs © 2015 Pearson Education, Inc. 13-4 Spinal Nerves and Plexuses • The Four Major Plexuses of Ventral Rami 1. 2. 3. 4. Cervical plexus Brachial plexus Lumbar plexus Sacral plexus © 2015 Pearson Education, Inc. Figure 13-9 Peripheral Nerves and Nerve Plexuses (Part 1 of 2). Cervical plexus Brachial plexus C1 C2 C3 C4 C5 C6 C7 C8 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 © 2015 Pearson Education, Inc. Lesser occipital nerve Great auricular nerve Transverse cervical nerve Supraclavicular nerve Phrenic nerve Axillary nerve Musculocutaneous nerve Thoracic nerves Figure 13-9 Peripheral Nerves and Nerve Plexuses (Part 2 of 2). T12 L1 Lumbar plexus Radial nerve L2 Ulnar nerve L3 Median nerve L4 Sacral plexus L5 S1 S2 S3 S4 S5 Co1 Iliohypogastric nerve Ilioinguinal nerve Lateral femoral cutaneous nerve Genitofemoral nerve Femoral nerve Obturator nerve Superior Inferior Gluteal nerves Pudendal nerve Saphenous nerve Sciatic nerve © 2015 Pearson Education, Inc. 13-4 Spinal Nerves and Plexuses • The Cervical Plexus • Includes ventral rami of spinal nerves C1–C5 • Innervates neck, thoracic cavity, diaphragmatic muscles • Major nerve • Phrenic nerve (controls diaphragm) © 2015 Pearson Education, Inc. Figure 13-10 The Cervical Plexus (Part 1 of 2). Cranial Nerves Accessory nerve (N XII) Hypoglossal nerve (N XII) Nerve Roots of Cervical Plexus C1 C2 C3 C4 C5 Clavicle © 2015 Pearson Education, Inc. Figure 13-10 The Cervical Plexus (Part 2 of 2). © 2015 Pearson Education, Inc. Figure 13-10 The Cervical Plexus (Part 2 of 2). © 2015 Pearson Education, Inc. 13-4 Spinal Nerves and Plexuses • The Brachial Plexus • Includes ventral rami of spinal nerves C5–T1 • Innervates pectoral girdle and upper limbs • Nerves that form brachial plexus originate from: • Superior, middle, and inferior trunks • Large bundles of axons from several spinal nerves • Lateral, medial, and posterior cords • Smaller branches that originate at trunks © 2015 Pearson Education, Inc. 13-4 Spinal Nerves and Plexuses • The Brachial Plexus • Major nerves • Musculocutaneous nerve (lateral cord) • Median nerve (lateral and medial cords) • Ulnar nerve (medial cord) • Axillary nerve (posterior cord) • Radial nerve (posterior cord) © 2015 Pearson Education, Inc. Figure 13-11a The Brachial Plexus. © 2015 Pearson Education, Inc. Figure 13-11a The Brachial Plexus. © 2015 Pearson Education, Inc. Figure 13-11b The Brachial Plexus. © 2015 Pearson Education, Inc. Figure 13-11b The Brachial Plexus. © 2015 Pearson Education, Inc. Figure 13-11b The Brachial Plexus. © 2015 Pearson Education, Inc. Figure 13-11b The Brachial Plexus. © 2015 Pearson Education, Inc. 13-4 Spinal Nerves and Plexuses • The Lumbar Plexus • Includes ventral rami of spinal nerves T12–L4 • Major nerves • Genitofemoral nerve • Lateral femoral cutaneous nerve • Femoral nerve © 2015 Pearson Education, Inc. Figure 13-12a The Lumbar and Sacral Plexuses. © 2015 Pearson Education, Inc. Figure 13-12a The Lumbar and Sacral Plexuses. © 2015 Pearson Education, Inc. Figure 13-12b The Lumbar and Sacral Plexuses. Nerve Roots of Sacral Plexus Lumbosacral trunk Sacral Plexus L4 Spinal Segments Nerve and Distribution Superior Gluteal L4–S2 L4–S2 S1 Gluteus maximus muscle S2 Sacrum Skin over perineum and posterior thigh and leg Semimembranosus, semitendinosus, and adductor magnus muscles; branches into tibial and fibular nerves Pudendal S2–S4 S3 S4 Sciatic L4–S3 L5 Inferior Gluteal Posterior Femoral Cutaneous S1–S3 L5 Gluteus minimus, gluteus medius, and tensor fasciae latae muscles The sacral plexus is formed by a branch from L4 and ventral rami of L5–S4. S5 Co1 Muscles of the perineum; skin over external genitalia, bulbospongiosus and ischiocavernosus muscles Sacral plexus, anterior view © 2015 Pearson Education, Inc. Figure 13-12c The Lumbar and Sacral Plexuses. Iliohypogastric nerve Ilioinguinal nerve Genitofemoral nerve Lateral femoral cutaneous nerve Femoral nerve Obturator nerve Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve (cut) Sciatic nerve Saphenous nerve Common fibular nerve Superficial fibular nerve Deep fibular nerve Nerves of the lumbar and sacral plexuses, anterior view © 2015 Pearson Education, Inc. Figure 13-12d The Lumbar and Sacral Plexuses. Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve Sciatic nerve Tibial nerve Common fibular nerve Sural nerve Nerves of the sacral plexus, posterior view © 2015 Pearson Education, Inc. 13-5 Neuronal Pools • Functional Organization of Neurons • Sensory neurons • About 10 million • Deliver information to CNS • Motor neurons • About 1/2 million • Deliver commands to peripheral effectors • Interneurons • About 20 billion • Interpret, plan, and coordinate signals in and out © 2015 Pearson Education, Inc. 13-5 Neuronal Pools • Neuronal Pools • Functional groups of interconnected neurons (interneurons) • Each with limited input sources and output destinations • May stimulate or depress parts of brain or spinal cord © 2015 Pearson Education, Inc. Figure 13-13a Neural Circuits: The Organization of Neuronal Pools. a Divergence A mechanism for spreading stimulation to multiple neurons or neuronal pools in the CNS © 2015 Pearson Education, Inc. Figure 13-13b Neural Circuits: The Organization of Neuronal Pools. b Convergence A mechanism for providing input to a single neuron from multiple sources © 2015 Pearson Education, Inc. Figure 13-13c Neural Circuits: The Organization of Neuronal Pools. c Serial processing A mechanism in which neurons or pools work sequentially © 2015 Pearson Education, Inc. Figure 13-13d Neural Circuits: The Organization of Neuronal Pools. d Parallel processing A mechanism in which neurons or pools process the same information simultaneously © 2015 Pearson Education, Inc. Figure 13-13e Neural Circuits: The Organization of Neuronal Pools. e Reverberation A positive feedback mechanism © 2015 Pearson Education, Inc. 13-6 Reflexes • Reflexes • Automatic responses coordinated within spinal cord • Through interconnected sensory neurons, motor neurons, and interneurons • Produce simple and complex reflexes © 2015 Pearson Education, Inc. 13-6 Reflexes • Neural Reflexes • Rapid, automatic responses to specific stimuli • Basic building blocks of neural function • One neural reflex produces one motor response • Reflex arc • The wiring of a single reflex • Beginning at receptor • Ending at peripheral effector • Generally opposes original stimulus (negative feedback) © 2015 Pearson Education, Inc. 13-6 Reflexes • Five Steps in a Neural Reflex • Step 1: Arrival of stimulus, activation of receptor • Physical or chemical changes • Step 2: Activation of sensory neuron • Graded depolarization • Step 3: Information processing by postsynaptic cell • Triggered by neurotransmitters • Step 4: Activation of motor neuron • Action potential • Step 5: Response of peripheral effector • Triggered by neurotransmitters © 2015 Pearson Education, Inc. Figure 13-14 Spinal Reflexes (Part 1 of 4). 1 Arrival of stimulus and activation of receptor 2 Dorsal root Activation of a sensory neuron Sensation relayed to the brain by axon collaterals Spinal cord Stimulus 5 Response by a peripheral effector Effector Receptor Information processing in the CNS REFLEX ARC 4 Ventral root Activation of a motor neuron © 2015 Pearson Education, Inc. 3 KEY Sensory neuron (stimulated) Excitatory interneuron Motor neuron (stimulated) 13-6 Reflexes • Four Classifications of Reflexes 1. 2. 3. 4. By early development By type of motor response By complexity of neural circuit By site of information processing © 2015 Pearson Education, Inc. 13-6 Reflexes • Development of Reflexes • Innate reflexes • Basic neural reflexes • Formed before birth • Acquired reflexes • Rapid, automatic • Learned motor patterns © 2015 Pearson Education, Inc. 13-6 Reflexes • Motor Response • Nature of resulting motor response • Somatic reflexes • Involuntary control of nervous system • Superficial reflexes of skin, mucous membranes • Stretch or deep tendon reflexes (e.g., patellar, or “knee-jerk,” reflex) • Visceral reflexes (autonomic reflexes) • Control systems other than muscular system © 2015 Pearson Education, Inc. 13-6 Reflexes • Complexity of Neural Circuit • Monosynaptic reflex • Sensory neuron synapses directly onto motor neuron • Polysynaptic reflex • At least one interneuron between sensory neuron and motor neuron © 2015 Pearson Education, Inc. 13-6 Reflexes • Sites of Information Processing • Spinal reflexes • Occur in spinal cord • Cranial reflexes • Occur in brain © 2015 Pearson Education, Inc. Figure 13-15 The Classification of Reflexes. Reflexes can be classified by development response complexity of circuit processing site Innate Reflexes Somatic Reflexes Monosynaptic Spinal Reflexes • Genetically determined Acquired Reflexes • Learned © 2015 Pearson Education, Inc. • Control skeletal muscle contractions • Include superficial and stretch reflexes Visceral (Autonomic) Reflexes • Control actions of smooth and cardiac muscles, glands, and adipose tissue • One synapse Polysynaptic • Multiple synapses (two to several hundred) • Processing in the spinal cord Cranial Reflexes • Processing in the brain 13-7 Spinal Reflexes • Spinal Reflexes • Range in increasing order of complexity • Monosynaptic reflexes • Polysynaptic reflexes • Intersegmental reflex arcs • Many segments interact • Produce highly variable motor response © 2015 Pearson Education, Inc. 13-7 Spinal Reflexes • Monosynaptic Reflexes • A stretch reflex • Have least delay between sensory input and motor output • For example, stretch reflex (such as patellar reflex) • Completed in 20–40 msec • Receptor is muscle spindle © 2015 Pearson Education, Inc. Figure 13-14 Spinal Reflexes (Part 2 of 4). Stretch Receptor (muscle spindle) Spinal cord REFLEX ARC Stimulus Contraction Effector KEY Response © 2015 Pearson Education, Inc. Sensory neuron (stimulated) Motor neuron (stimulated) 13-7 Spinal Reflexes • Postural Reflexes • Stretch reflexes • Maintain normal upright posture • Stretched muscle responds by contracting • Automatically maintains balance © 2015 Pearson Education, Inc. 13-7 Spinal Reflexes • Polysynaptic Reflexes • More complicated than monosynaptic reflexes • Interneurons control more than one muscle group • Produce either EPSPs or IPSPs © 2015 Pearson Education, Inc. 13-7 Spinal Reflexes • The Tendon Reflex • Prevents skeletal muscles from: • Developing too much tension • Tearing or breaking tendons • Sensory receptors unlike muscle spindles or proprioceptors © 2015 Pearson Education, Inc. 13-7 Spinal Reflexes • Withdrawal Reflexes • Move body part away from stimulus (pain or pressure) • For example, flexor reflex • Pulls hand away from hot stove • Strength and extent of response • Depend on intensity and location of stimulus © 2015 Pearson Education, Inc. Figure 13-14 Spinal Reflexes (Part 3 of 4). Distribution within gray horns to other segments of the spinal cord Painful stimulus Flexors stimulated Extensors inhibited © 2015 Pearson Education, Inc. 13-7 Spinal Reflexes • Reciprocal Inhibition • For flexor reflex to work • The stretch reflex of antagonistic (extensor) muscle must be inhibited (reciprocal inhibition) by interneurons in spinal cord © 2015 Pearson Education, Inc. 13-7 Spinal Reflexes • Reflex Arcs • Ipsilateral reflex arcs • Occur on same side of body as stimulus • Stretch, tendon, and withdrawal reflexes • Crossed extensor reflexes • Involve a contralateral reflex arc • Occur on side opposite stimulus © 2015 Pearson Education, Inc. 13-7 Spinal Reflexes • Crossed Extensor Reflexes • Occur simultaneously, coordinated with flexor reflex • For example, flexor reflex causes leg to pull up • Crossed extensor reflex straightens other leg • To receive body weight • Maintained by reverberating circuits © 2015 Pearson Education, Inc. Figure 13-14 Spinal Reflexes (Part 4 of 4). To motor neurons in other segments of the spinal cord Extensors inhibited Flexors stimulated Extensors stimulated Flexors inhibited KEY Sensory neuron (stimulated) Motor neuron (inhibited) Excitatory interneuron Inhibitory interneuron Motor neuron (stimulated) © 2015 Pearson Education, Inc. Painful stimulus 13-7 Spinal Reflexes • Five General Characteristics of Polysynaptic Reflexes 1. 2. 3. 4. Involve pools of interneurons Are intersegmental in distribution Involve reciprocal inhibition Have reverberating circuits • Which prolong reflexive motor response 5. Several reflexes cooperate • To produce coordinated, controlled response © 2015 Pearson Education, Inc. 13-8 The Brain Can Alter Spinal Reflexes • Integration and Control of Spinal Reflexes • Reflex behaviors are automatic • But processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord © 2015 Pearson Education, Inc. 13-8 The Brain Can Alter Spinal Reflexes • Voluntary Movements and Reflex Motor Patterns • Higher centers of brain incorporate lower, reflexive motor patterns • Automatic reflexes • Can be activated by brain as needed • Use few nerve impulses to control complex motor functions • Walking, running, jumping © 2015 Pearson Education, Inc. 13-8 The Brain Can Alter Spinal Reflexes • Reinforcement of Spinal Reflexes • Higher centers reinforce spinal reflexes • By stimulating excitatory neurons in brain stem or spinal cord • Creating EPSPs at reflex motor neurons • Facilitating postsynaptic neurons © 2015 Pearson Education, Inc. 13-8 The Brain Can Alter Spinal Reflexes • Inhibition of Spinal Reflexes • Higher centers inhibit spinal reflexes by: • Stimulating inhibitory neurons • Creating IPSPs at reflex motor neurons • Suppressing postsynaptic neurons © 2015 Pearson Education, Inc. 13-8 The Brain Can Alter Spinal Reflexes • The Babinski Reflexes • Normal in infants • May indicate CNS damage in adults © 2015 Pearson Education, Inc. Figure 13-17a The Babinski Reflexes. a The plantar reflex (negative Babinski reflex), a curling of the toes, is seen in healthy adults. © 2015 Pearson Education, Inc. Figure 13-17b The Babinski Reflexes. b The Babinski sign (positive Babinski reflex) occurs in the absence of descending inhibition. It is normal in infants, but pathological in adults. © 2015 Pearson Education, Inc.
