Chapter 13 - Spinal Cord PDF

Because learning changes everything.® Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes ANATOMY & PHYSIOLOGY The Unity of Form and Function TENTH EDITION KENNETH S. SALADIN © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written c...

Because learning changes everything.® Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes ANATOMY & PHYSIOLOGY The Unity of Form and Function TENTH EDITION KENNETH S. SALADIN © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.  13.1 The Spinal Cord Expected Learning Outcomes: State the three principal functions of the spinal cord. Describe its gross and microscopic structure. Trace the pathways followed by nerve signals traveling up and down the spiral cord. © McGraw Hill, LLC 2  13.1a Functions Conduction—nerve fibers conduct sensory and motor information up and down the spinal cord Neural integration—spinal neurons receive input from multiple sources, integrate it, and execute appropriate output (for example, bladder control) Locomotion—spinal cord contains central pattern generators: groups of neurons that coordinate repetitive sequences of contractions for walking Reflexes—involuntary responses to stimuli that are vital to posture, coordination and protection © McGraw Hill, LLC 3  13.1b Surface Anatomy 1 Spinal cord—cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull Occupies the upper two-thirds of vertebral canal Inferior margin ends at L1 or slightly beyond Averages 1.8 cm thick and 45 cm long Gives rise to 31 pairs of spinal nerves Segment—part of the spinal cord supplied by each pair of spinal nerves © McGraw Hill, LLC 4  Surface Anatomy 2 Spinal cord surface anatomy (continued): Has longitudinal grooves on anterior and posterior sides Anterior median fissure and posterior median sulcus Divided into the cervical, thoracic, lumbar, and sacral regions Two areas of the cord are thicker than elsewhere: Cervical enlargement—gives rise to nerves of upper limb Lumbosacral enlargement—gives rise to nerves of pelvic region, lower limbs Medullary cone (conus medullaris)—inferior point of cord Cauda equina—bundle of nerve roots that occupy the vertebral canal from L2 to S5; resembles a horse tail © McGraw Hill, LLC 5  The Spinal Cord, Posterior View 1 Access the text alternative for slide images. Figure 13.1 © McGraw Hill, LLC 6  The Spinal Cord, Posterior View 2 Access the text alternative for slide images. Figure 13.1b © McGraw Hill, LLC 7  13.1c Meninges of the Spinal Cord 1 Meninges—three fibrous membranes that enclose the brain and spinal cord They separate soft tissue of central nervous system from bones of cranium and vertebral canal From superficial to deep: dura mater, arachnoid mater, and pia mater Dura mater: Forms loose-fitting sleeve (dural sheath) around spinal cord Tough, thick membrane composed of dense irregular connective tissue Epidural space—space between dura and vertebral bones; contains blood vessels, adipose tissue, loose connective tissue Anesthetics can be introduced into epidural space (epidural anesthesia) © McGraw Hill, LLC 8  13.1c Meninges of the Spinal Cord 2 Meninges (continued): Arachnoid mater: Consists of arachnoid membrane adhering to dura and is separated from pia by fibers spanning the subarachnoid space that is filled with cerebrospinal fluid (CSF) Inferior to medullary cone is lumbar cistern, location of spinal tap (lumbar puncture) to sample CSF Pia mater: Delicate, transparent membrane that follows contours of spinal cord and continues inferiorly as a fibrous terminal filum Fuses with dura to form coccygeal ligament Denticulate ligaments—extensions of pia that pass through arachnoid to the dura, anchoring cord © McGraw Hill, LLC 9  Cross-Sectional Anatomy of the Cervical Spinal Cord 1 Access the text alternative for slide images. Figure 13.2a © McGraw Hill, LLC 10  Cross-Sectional Anatomy of the Cervical Spinal Cord 2 Access the text alternative for slide images. Figure 13.2b © McGraw Hill, LLC 11  13.1d Cross-Sectional Anatomy 1 Spinal cord has gray matter and white matter Gray matter—dull in color (no myelin); contains neuron cell bodies, dendrites, and proximal portions of axons White matter—bright, pearly white color due to myelin; axon bundles coursing up and down cord © McGraw Hill, LLC 12  Cross-Sectional Anatomy 2 Gray matter Central core of gray matter shaped like a butterfly Two posterior (dorsal) horns—receive sensory nerve fibers, synapse with interneurons in horn Two anterior (ventral) horns—contain cell bodies of motor neurons Additional lateral horns present in segments T2 through L1, associated with sympathetic nervous system Left and right sides connected by gray commissure Central canal at center of commissure © McGraw Hill, LLC 13  Cross-Sectional Anatomy 3 White matter Surrounds the gray matter Three large bundles of axons—posterior, lateral, and anterior funiculus on each side of the cord Each funiculus subdivided into fasciculi or tracts © McGraw Hill, LLC 14  Cross Section of the Spinal Cord (Lumbar Region) Ed Reschke/Stone/Getty Images Access the text alternative for slide images. Figure 13.3 © McGraw Hill, LLC 15  Spinal Tap (Lumbar Puncture) Access the text alternative for slide images. Figure 13.4 © McGraw Hill, LLC 16  13.1e Spinal Tracts 1 Fibers in a given tract have similar origin, destination and function Ascending tracts—carry sensory information up Descending tracts—carry motor information down Decussation—crossing of the midline that occurs in many tracts so that brain senses and controls contralateral side of body Contralateral—when the origin and destination of a tract are on opposite sides of the body Ipsilateral—when the origin and destination of a tract are on the same side of the body; does not decussate © McGraw Hill, LLC 17  Tracts of the Spinal Cord Access the text alternative for slide images. Figure 13.5 © McGraw Hill, LLC 18  Spinal Tracts 2 Ascending tracts Sensory signals travel across three neurons from origin (receptors) to destinations in the sensory areas of the brain First-order neuron—detects stimulus and transmits signal to spinal cord or brainstem Second-order neuron—continues to the thalamus at the upper end of the brainstem Third-order neuron—carries the signal the rest of the way to the sensory region of the cerebral cortex © McGraw Hill, LLC 19  Spinal Tracts 3 The posterior funiculus consists of cuneate fasciculus and gracile fasciculus Gracile fasciculus carries signals from midthoracic and lower parts of body Cuneate fasciculus carries signals from chest and upper limbs First-order nerve fibers travel up the ipsilateral side of spinal cord, terminating at medulla; second-order cross to other side of brain Carries signals for vibration, visceral pain, deep and discriminative touch, and proprioception from lower limbs and lower trunk Proprioception—nonvisual sense of the position and movements of the body © McGraw Hill, LLC 20  Spinal Tracts 4 The spinothalamic tract is part of the anterolateral system that passes up the anterior and lateral funiculi of the spinal cord Carries signals for pain, pressure, temperature, light touch, tickle, and itch Tract is composed of axons of second-order neurons First-order neurons end in posterior horn of spinal cord Second-order neurons start in posterior horn, then decussate and form the spinothalamic tract Third-order neurons continue from there to cerebral cortex Due to cross of second-order neurons, signals are sent to cerebral hemisphere that is contralateral to site of stimulus © McGraw Hill, LLC 21  Some Ascending Pathways of the CNS 1 Access the text alternative for slide images. Figure 13.6 © McGraw Hill, LLC 22  Some Ascending Pathways of the CNS 2 Access the text alternative for slide images. Figure 13.6 © McGraw Hill, LLC 23  Some Ascending Pathways of the CNS 3 Access the text alternative for slide images. Figure 13.6 © McGraw Hill, LLC 24  Spinal Tracts 5 The spinoreticular tract travels up anterolateral system Carries pain signals resulting from tissue injury Composed of axons of second-order neurons First-order neurons enter posterior horn and immediately synapse with second-order neurons Second-order neurons decussate to opposite anterolateral system; ascend the cord and end in reticular formation— loosely organized core of gray matter in the medulla and pons © McGraw Hill, LLC 25  Spinal Tracts 6 The posterior and anterior spinocerebellar tracts travel through lateral funiculus Carry proprioceptive signals from limbs and trunk up to the cerebellum Composed of axons of second-order neurons First-order neurons originate in the muscles and tendons and end in posterior horn of the spinal cord Second-order nerves ascend spinocerebellar tracts and end in cerebellum providing it with feedback needed to coordinate movements © McGraw Hill, LLC 26  Spinal Tracts 7 Descending tracts Involve two motor neurons Upper motor neuron—originates in cerebral cortex or brainstem and terminates on a lower motor neuron Lower motor neuron—cell body is in brainstem or spinal cord; axon leads to muscle or other target organ © McGraw Hill, LLC 27  Spinal Tracts 8 Lateral and anterior corticospinal tracts carry signals from cerebral cortex for precise, finely coordinated movements Pyramids—ridges on anterior surface of medulla oblongata formed from fibers of this system Most fibers decussate in lower medulla forming the lateral corticospinal tract on contralateral side of spinal cord Some fibers form the anterior (ventral) corticospinal tract that descends in the ipsilateral side of spinal cord and decussates inferiorly (like lateral tract, they ultimately control contralateral muscles) © McGraw Hill, LLC 28  Two Descending Pathways of the CNS 1 Access the text alternative for slide images. Figure 13.7 © McGraw Hill, LLC 29  Two Descending Pathways of the CNS 2 Access the text alternative for slide images. Figure 13.7 © McGraw Hill, LLC 30  Two Descending Pathways of the CNS 3 Access the text alternative for slide images. Figure 13.7 © McGraw Hill, LLC 31  Spinal Tracts 9 Other descending tracts: The tectospinal tract begins in roof of midbrain (tectum), descends contralateral spinal cord as far as the neck Functions in reflex turning of head in response to sights and sounds Lateral and medial reticulospinal tracts Originate in the reticular formation of brainstem; control muscles of upper and lower limbs, especially those for posture and balance; contain descending analgesic pathways that regulate transmission of pain signals to brain Lateral and medial vestibulospinal tracts Begin in brainstem vestibular nuclei; receive impulses for balance from inner ear; control extensor muscles of limbs for balance control © McGraw Hill, LLC 32  Poliomyelitis and Amyotrophic Lateral Sclerosis 1 Both poliomyelitis and amyotrophic lateral sclerosis (ALS) cause destruction of motor neurons leading to skeletal muscle atrophy from lack of innervation Poliomyelitis: Caused by the poliovirus Destroys motor neurons in brainstem and anterior horn of spinal cord Signs of polio include muscle pain, weakness, and loss of some reflexes Followed by paralysis, muscular atrophy, and respiratory arrest Virus spreads by fecal contamination of water © McGraw Hill, LLC 33  Poliomyelitis and Amyotrophic Lateral Sclerosis 2 Amyotrophic lateral sclerosis (ALS) or Lou Gehrig disease: Destruction of motor neurons and muscular atrophy Also sclerosis (scarring) of lateral regions of the spinal cord Astrocytes fail to reabsorb the neurotransmitter glutamate from the tissue fluid which accumulates to toxic levels Early signs: muscular weakness; difficulty speaking, swallowing, and using hands Sensory and intellectual functions remain unaffected © McGraw Hill, LLC 34  13.2 The Spinal Nerves Expected Learning Outcomes: Describe the anatomy of nerves and ganglia in general. Describe the attachments of a spinal nerve to the spinal cord. Trace the branches of a spinal nerve distal to its attachment. Name the five plexuses of spinal nerves and describe their general anatomy. Name some major nerves that arise from each plexus and identify what they innervate. Explain the relationship of dermatomes to the spinal nerves. © McGraw Hill, LLC 35  13.2a General Anatomy of Nerves and Ganglia 1 Spinal cord communicates with the rest of the body by way of spinal nerves Nerve—a cord-like organ composed of numerous nerve fibers (axons) bound together by connective tissue May contain anywhere from a few nerve fibers to a million Smaller branches called peripheral nerves Disorders of these nerves—peripheral neuropathy © McGraw Hill, LLC 36  General Anatomy of Nerves and Ganglia 2 Wrappings and associated structures of a nerve: Nerve fibers of peripheral nervous system are surrounded by Schwann cells forming neurilemma and myelin sheath around the axon Endoneurium—loose connective tissue external to neurilemma Perineurium—layers of overlapping squamous cells that wrap fascicles: bundles of nerve fibers Epineurium—dense irregular connective tissue that wraps entire nerve Blood vessels penetrate connective tissue coverings Provide plentiful blood supply © McGraw Hill, LLC 37  Anatomy of a Nerve 1 b: PASIEKA/Science Photo Library/Getty Images Access the text alternative for slide images. Figure 13.9 © McGraw Hill, LLC 38  Anatomy of a Nerve 2 b: PASIEKA/Science Photo Library/Getty Images Access the text alternative for slide images. Figure 13.9b © McGraw Hill, LLC 39  General Anatomy of Nerves and Ganglia 3 Nerves are classified as sensory, motor, or mixed Sensory nerves—composed only of afferent fibers; carry signals from sensory receptors to the CNS Rare, include nerves for smell and vision Motor nerves—composed only of efferent fibers; carry signals from CNS to muscles and glands Mixed nerves—consist of both afferent and efferent fibers Most common type Based on the organs they innervate, sensory and motor fibers can also be described as: Somatic or visceral; general or special © McGraw Hill, LLC 40  General Anatomy of Nerves and Ganglia 4 Ganglion—cluster of nerve cell bodies outside the CNS Enveloped in an epineurium continuous with that of the nerve Among the cell bodies are bundles of nerve fibers leading into and out of the ganglion Posterior root ganglion associated with spinal nerves © McGraw Hill, LLC 41  Anatomy of a Ganglion (Longitudinal Section) Access the text alternative for slide images. Figure 13.10 © McGraw Hill, LLC 42  13.2b Spinal Nerves 1 There are 31 pairs of spinal nerves: 8 cervical (C1 to C8) First cervical nerve exits between skull and atlas Others exit at intervertebral foramina 12 thoracic (T1 to T12) 5 lumbar (L1 to L5) 5 sacral (S1 to S5) 1 coccygeal (Co1) © McGraw Hill, LLC 43  Spinal Nerves 2 Proximal branches Each spinal nerve is formed from two roots (proximal branches) Posterior (dorsal) root is sensory input to spinal cord Posterior (dorsal) root ganglion—contains the neurosomas of sensory neurons carrying signals to the spinal cord Six to eight rootlets enter posterior horn of cord Anterior (ventral) root is motor output out of spinal cord Six to eight rootlets leave spinal cord and converge to form anterior root Cauda equina formed from roots arising from L2 to Co1 © McGraw Hill, LLC 44  The Spinal Nerve Roots and Plexuses, Posterior View Access the text alternative for slide images. Figure 13.11 © McGraw Hill, LLC 45  Spinal Nerves 3 Distal branches Beyond the vertebrae, the nerve divides into distal branches Anterior ramus: In thoracic region, each gives rise to an intercostal nerve In other regions, anterior rami form the nerve plexuses Posterior ramus—innervates the muscles and joints in that region of the spine and the skin of the back Meningeal branch—reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments © McGraw Hill, LLC 46  Branches of a Spinal Nerve in Relation to the Spinal Cord and Vertebra (Cross Section) Access the text alternative for slide images. Figure 13.12 © McGraw Hill, LLC 47  The Point of Entry of Two Spinal Nerves into the Spinal Cord ©From A Stereoscopic Atlas of Anatomy by David L. Bassett. Courtesy of Dr. Robert A. Chase, M.D. Access the text alternative for slide images. Figure 13.13 © McGraw Hill, LLC 48  Rami of the Spinal Nerves Access the text alternative for slide images. Figure 13.14 © McGraw Hill, LLC 49  Shingles Chickenpox (varicella)—common disease of early childhood caused by varicella-zoster virus Produces itchy rash that clears up without complications Virus remains for life in the posterior root ganglia; kept in check by the immune system Shingles (herpes zoster)—localized disease caused by the virus traveling down the sensory nerves by fast axonal transport when immune system is compromised Painful trail of skin discoloration and fluid-filled vesicles along path of nerve usually on chest and waist on one side of the body; pain and itching Common after age 50 Childhood chickenpox vaccinations reduce the risk of shingles later in life © McGraw Hill, LLC 50  Shingles Lesion Tracking the Dermatome of a Sensory Nerve Franciscodiazpagador/Stock/Getty Images Figure 13.19 © McGraw Hill, LLC 51  13.3 Somatic Reflexes Expected Learning Outcomes: Define reflex and explain how reflexes differ from other motor actions. Describe the general components of a typical reflex arc. Explain how the basic types of somatic reflexes function. © McGraw Hill, LLC 52  13.3a The Nature of Reflexes 1 Reflexes—quick, involuntary, stereotyped reactions of glands or muscle to stimulation Reflexes require stimulation Not spontaneous actions, but responses to sensory input Reflexes are quick Involve few, if any, interneurons and minimum synaptic delay Reflexes are involuntary Occur without intent and are difficult to suppress Reflexes are stereotyped Occur essentially the same way every time © McGraw Hill, LLC 53  The Nature of Reflexes 2 Reflexes include glandular secretion and contraction of all three types of muscle Somatic reflexes—reflexes involving the somatic nervous system, innervating skeletal muscle Visceral reflexes—reflexes involving organs such as heart and intestines © McGraw Hill, LLC 54  The Nature of Reflexes 3 A somatic reflex involves a reflex arc with the following pathway: 1. Somatic receptors in skin, muscles, or tendons 2. Afferent nerve fibers carry information from receptors to posterior horn of spinal cord or to the brainstem 3. Integrating center—a point of synaptic contact between neurons in gray matter of cord or brainstem Determines whether efferent neurons issue signal to muscles 4. Efferent nerve fibers carry motor impulses to muscles 5. Effectors—the muscles that carry out the response © McGraw Hill, LLC 55  A Representative Reflex Arc Access the text alternative for slide images. Figure 13.23 © McGraw Hill, LLC 56  13.3b The Muscle Spindle 1 Many somatic reflexes involve muscle spindles—stretch receptors embedded in skeletal muscles Serve as proprioceptors—specialized sense organs to monitor position and movement of body parts Inform the brain of muscle length and body movement Enable brain to send motor commands back to the muscles that control coordinated movement, corrective reflexes, muscle tone, and posture © McGraw Hill, LLC 57  A Muscle Spindle and Its Innervation Access the text alternative for slide images. Figure 13.24 © McGraw Hill, LLC 58  The Muscle Spindle 2 Structure of a muscle spindle: Intrafusal fibers—modified muscle fibers within the spindle Rest of the muscle’s fibers (those generating force for movement) are extrafusal fibers A gamma motor neuron innervates the ends of an intrafusal fiber and keeps it taut (alpha motor neurons supply the extrafusal fibers) The midportion of the intrafusal fiber contains sensory nerve fibers Primary afferent fibers—monitor fiber length and speed of length changes Secondary afferent fibers—monitor length only Example of spindle function: help you to keep upright when standing on a boat © McGraw Hill, LLC 59  13.3c The Stretch Reflex 1 Stretch (myotatic) reflex—when a muscle is stretched, it “fights back” and contracts Helps maintain equilibrium and posture Head starts to tip forward as you fall asleep, muscles contract to raise the head Stabilize joints by balancing tension in extensors and flexors, smoothing muscle actions Stretch reflex is mediated primarily by the brain, but its spinal component can be more pronounced if muscle is suddenly stretched by a tendon tap (knee jerk) © McGraw Hill, LLC 60  The Stretch Reflex 2 Monosynaptic reflex arcs—one synapse between the afferent and efferent neurons; little synaptic delay, so very prompt response Patellar (knee-jerk) reflex is a monosynaptic reflex Testing somatic reflexes helps diagnose many diseases Reciprocal inhibition—reflex phenomenon that prevents muscles from working against each other by inhibiting antagonist when agonist is excited © McGraw Hill, LLC 61  The Patellar Tendon Reflex Arc and Reciprocal Inhibition of the Antagonistic Muscle Access the text alternative for slide images. Figure 13.25 © McGraw Hill, LLC 62  13.3d The Flexor (Withdrawal) Reflex Flexor reflex—the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus Triggers contraction of the flexors and relaxation of the extensors in that limb Polysynaptic reflex arc—pathway in which signals travel over many synapses on their way to the muscle © McGraw Hill, LLC 63  13.3e The Crossed Extension Reflex Crossed extension reflex—contraction of extensor muscles in limb opposite of the one that is withdrawn Maintains balance by extending other leg Flexor reflex uses an ipsilateral reflex arc, (stimulus and response on same side) whereas crossed extension reflex uses a contralateral reflex arc (input and output are on opposite sides) Intersegmental reflex—one in which the input and output occur at different levels (segments) of the spinal cord Pain in foot causes contraction of abdominal muscles © McGraw Hill, LLC 64  The Flexor and Crossed Extension Reflexes Access the text alternative for slide images. Figure 13.26 © McGraw Hill, LLC 65  13.3f The Tendon Reflex Tendon organs—proprioceptors in a tendon near its junction with a muscle; involved in the tendon reflex Is 0.5 mm long, consists of encapsulated bundle of collagen fibers and one or more nerve fibers Tendon reflex—response to excessive tension on the tendon Inhibits muscle from contracting strongly Moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone © McGraw Hill, LLC 66  A Tendon Organ Access the text alternative for slide images. Figure 13.27 © McGraw Hill, LLC 67  Spinal Cord Trauma Complete transection—complete severance of cord Immediate loss of motor control below level of injury Above C4 poses the threat of respiratory failure Spinal shock Paralysis, three forms: Paraplegia—paralysis of both lower limbs Quadriplegia—paralysis of all four limbs Hemiplegia—paralysis on one side of the body Paresis—partial paralysis or weakness of the limbs © McGraw Hill, LLC 68  Because learning changes everything. ® www.mheducation.com © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.