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Visceral motor components associated with spinal levels T1 to L2 are termed sympathetic. Those visceral motor components in cranial and sacral regions, on either side of the sympathetic region, are termed parasympathetic: |
The sympathetic system innervates structures in peripheral regions of the body and viscera. |
The parasympathetic system is more restricted to innervation of the viscera only. |
Spinal sympathetic and spinal parasympathetic neurons share certain developmental and phenotypic features that are different from those of cranial parasympathetic neurons. Based on this, some researchers have suggested reclassifying all spinal visceral motor neurons as sympathetic (Espinosa-Medina I et al. Science 2016;354:893-897). Others are against reclassification, arguing that the results only indicate that the neurons are spinal in origin (Neuhuber W et al. Anat Rec 2017;300:1369-1370). In addition, sacral nerves do not enter the sympathetic trunk, nor do they have postganglionic fibers that travel to the periphery on spinal nerves, as do T1-L2 visceral motor fibers. We have chosen to retain the classification of S2,3,4 visceral motor neurons as parasympathetic. “Parasympathetic” simply means on either side of the “sympathetic,” which correctly describes their anatomy. |
The sympathetic part of the autonomic division of the PNS leaves thoracolumbar regions of the spinal cord with the somatic components of spinal nerves T1 to L2 (Fig. 1.42). On each side, a paravertebral sympathetic trunk extends from the base of the skull to the inferior end of the vertebral column where the two trunks converge anteriorly to the coccyx at the ganglion impar. Each trunk is attached to the anterior rami of spinal nerves and becomes the route by which sympathetics are distributed to the periphery and all viscera. |
Visceral motor preganglionic fibers leave the T1 to L2 part of the spinal cord in anterior roots. The fibers then enter the spinal nerves, pass through the anterior rami and into the sympathetic trunks. One trunk is located on each side of the vertebral column (paravertebral) and positioned anterior to the anterior rami. Along the trunk is a series of segmentally arranged ganglia formed from collections of postganglionic neuronal cell bodies where the preganglionic neurons synapse with postganglionic neurons. Anterior rami of T1 to L2 are connected to the sympathetic trunk or to a ganglion by a white ramus communicans, which carries preganglionic sympathetic fibers and appears white because the fibers it contains are myelinated. |
Preganglionic sympathetic fibers that enter a paravertebral ganglion or the sympathetic trunk through a white ramus communicans may take the following four pathways to target tissues: 1. Peripheral sympathetic innervation at the level of origin of the preganglionic fiber |
Preganglionic sympathetic fibers may synapse with postganglionic motor neurons in ganglia associated with the sympathetic trunk, after which postganglionic fibers enter the same anterior ramus and are distributed with peripheral branches of the posterior and anterior rami of that spinal nerve (Fig. 1.43). The fibers innervate structures at the periphery of the body in regions supplied by the spinal nerve. The gray ramus communicans connects the sympathetic trunk or a ganglion to the anterior ramus and contains the postganglionic sympathetic fibers. It appears gray because postganglionic fibers are nonmyelinated. The gray ramus communicans is positioned medial to the white ramus communicans. |
2. Peripheral sympathetic innervation above or below the level of origin of the preganglionic fiber |
Preganglionic sympathetic fibers may ascend or descend to other vertebral levels where they synapse in ganglia associated with spinal nerves that may or may not have visceral motor input directly from the spinal cord (i.e., those nerves other than T1 to L2) (Fig. 1.44). |
The postganglionic fibers leave the distant ganglia via gray rami communicantes and are distributed along the posterior and anterior rami of the spinal nerves. |
The ascending and descending fibers, together with all the ganglia, form the paravertebral sympathetic trunk, which extends the entire length of the vertebral column. The formation of this trunk, on each side, enables visceral motor fibers of the sympathetic part of the autonomic division of the PNS, which ultimately emerge from only a small region of the spinal cord (T1 to L2), to be distributed to peripheral regions innervated by all spinal nerves. |
White rami communicantes only occur in association with spinal nerves T1 to L2, whereas gray rami communicantes are associated with all spinal nerves. |
Fibers from spinal cord levels T1 to T5 pass predominantly superiorly, whereas fibers from T5 to L2 pass inferiorly. All sympathetics passing into the head have preganglionic fibers that emerge from spinal cord level |
T1 and ascend in the sympathetic trunks to the highest ganglion in the neck (the superior cervical ganglion), where they synapse. Postganglionic fibers then travel along blood vessels to target tissues in the head, including blood vessels, sweat glands, small smooth muscles associated with the upper eyelids, and the dilator of the pupil. |
3. Sympathetic innervation of thoracicPreganglionic sympathetic fibers may synapse with postganglionic motor neurons in ganglia and then leave the ganglia medially to innervate thoracic or cervical viscera (Fig. 1.45). They may ascend in the trunk before synapsing, and after synapsing the postganglionic fibers may combine with those from other levels to form named visceral nerves, such as cardiac nerves. Often, these nerves join branches from the parasympathetic system to form plexuses on or near the surface of the target organ, for example, the cardiac and pulmonary plexuses. Branches of the plexus innervate the organ. Spinal cord levels T1 to T5 mainly innervate cranial, cervical, and thoracic viscera. |
4. Sympathetic innervation of the abdomen and pelvic regions and the adrenals |
Preganglionic sympathetic fibers may pass through the sympathetic trunk and paravertebral ganglia without synapsing and, together with similar fibers from other levels, form splanchnic nerves (greater, lesser, least, lumbar, and sacral), which pass into the abdomen and pelvic regions (Fig. 1.46). The preganglionic fibers in these nerves are derived from spinal cord levels T5 to L2. |
The splanchnic nerves generally connect with sympathetic ganglia around the roots of major arteries that branch from the abdominal aorta. These ganglia are part of a large prevertebral plexus that also has input from the parasympathetic part of the autonomic division of the PNS. Postganglionic sympathetic fibers are distributed in extensions of this plexus, predominantly along arteries, to viscera in the abdomen and pelvis. |
Some of the preganglionic fibers in the prevertebral plexus do not synapse in the sympathetic ganglia of the plexus but pass through the system to the adrenal gland, where they synapse directly with cells of the adrenal medulla. These cells are homologues of sympathetic postganglionic neurons and secrete adrenaline and noradrenaline into the vascular system. |
The parasympathetic part of the autonomic division of the PNS (Fig. 1.47) leaves cranial and sacral regions of the CNS in association with: cranial nerves III, VII, IX, and X: III, VII, and IX carry parasympathetic fibers to structures within the head and neck only, whereas X (the vagus spinal nerves S2 to S4: sacral parasympathetic fibers innervate inferior abdominal viscera, pelvic viscera, and the arteries associated with erectile tissues of the perineum. |
Like the visceral motor nerves of the sympathetic part, the visceral motor nerves of the parasympathetic part generally have two neurons in the pathway. The preganglionic neurons are in the CNS, and fibers leave in the cranial nerves. |
In the sacral region, the preganglionic parasympathetic fibers form special visceral nerves (the pelvic splanchnic nerves), which originate from the anterior rami of S2 to S4 and enter pelvic extensions of the large prevertebral plexus formed around the abdominal aorta. These fibers are distributed to pelvic and abdominal viscera mainly along blood vessels. The postganglionic motor neurons are in the walls of the viscera. In organs of the gastrointestinal system, preganglionic fibers do not have a postganglionic parasympathetic motor neuron in the pathway; instead, preganglionic fibers synapse directly on neurons in the ganglia of the enteric system. |
The preganglionic parasympathetic motor fibers in CNIII, VII, and IX separate from the nerves and connect with one of four distinct ganglia, which house postganglionic motor neurons. These four ganglia are near major branches of CN V. Postganglionic fibers leave the ganglia, join the branches of CN V, and are carried to target tissues (salivary, mucous, and lacrimal glands; constrictor muscle of the pupil; and ciliary muscle in the eye) with these branches. |
The vagus nerve [X] gives rise to visceral branches along its course. These branches contribute to plexuses associated with thoracic viscera or to the large prevertebral plexus in the abdomen and pelvis. Many of these plexuses also contain sympathetic fibers. |
When present, postganglionic parasympathetic neurons are in the walls of the target viscera. |
motor fibers.Visceral sensory fibers follow the course of sympathetic fibers entering the spinal cord at similar spinal cord levels. However, visceral sensory fibers may also enter the spinal cord at levels other than those associated with motor output. For example, visceral sensory fibers from the heart may enter at levels higher than spinal cord level T1. Visceral sensory fibers that accompany sympathetic fibers are mainly concerned with detecting pain. |
Visceral sensory fibers accompanying parasympathetic fibers are carried mainly in IX and X and in spinal nerves S2 to S4. |
Visceral sensory fibers in IX carry information from chemoreceptors and baroreceptors associated with the walls of major arteries in the neck, and from receptors in the pharynx. |
Visceral sensory fibers in X include those from cervical viscera, and major vessels and viscera in the thorax and abdomen. |
Visceral sensory fibers from pelvic viscera and the distal parts of the colon are carried in S2 to S4. |
Visceral sensory fibers associated with parasympathetic fibers primarily relay information to the CNS about the status of normal physiological processes and reflex activities. |
The enteric systemThe enteric nervous system consists of motor and sensory neurons and their support cells, which form two interconnected plexuses, the myenteric and submucous nerve plexuses, within the walls of the gastrointestinal tract (Fig. 1.48). Each of these plexuses is formed by: ganglia, which house the nerve cell bodies and associated cells, and bundles of nerve fibers, which pass between ganglia and from the ganglia into surrounding tissues. |
Neurons in the enteric system are derived from neural crest cells originally associated with occipitocervical and sacral regions. Interestingly, more neurons are reported to be in the enteric system than in the spinal cord itself. |
Sensory and motor neurons within the enteric system control reflex activity within and between parts of the gastrointestinal system. These reflexes regulate peristalsis, secretomotor activity, and vascular tone. These activities can occur independently of the brain and spinal cord, but can also be modified by input from preganglionic parasympathetic and postganglionic sympathetic fibers. |
Sensory information from the enteric system is carried back to the CNS by visceral sensory fibers. |
Nerve plexuses are either somatic or visceral and combine fibers from different sources or levels to form new nerves with specific targets or destinations (Fig. 1.49). Plexuses of the enteric system also generate reflex activity independent of the CNS. |
Major somatic plexuses formed from the anterior rami of spinal nerves are the cervical (C1 to C4), brachial (C5 to T1), lumbar (L1 to L4), sacral (L4 to S4), and coccygeal (S5 to Co) plexuses. Except for spinal nerve T1, the anterior rami of thoracic spinal nerves remain independent and do not participate in plexuses. |
Visceral nerve plexuses are formed in association with viscera and generally contain efferent (sympathetic and parasympathetic) and afferent components (Fig. 1.49). These plexuses include cardiac and pulmonary plexuses in the thorax and a large prevertebral plexus in the abdomen anterior to the aorta, which extends inferiorly onto the lateral walls of the pelvis. The massive prevertebral plexus supplies input to and receives output from all abdominal and pelvic viscera. |
Specific information about the organization and components of the respiratory, gastrointestinal, and urogenital systems will be discussed in each of the succeeding chapters of this text. |
Fig. 1.1 The anatomical position, planes, and terms of location and orientation. |
Feet togethertoes forwardHands by sidespalms forwardFace looking forwardInferior margin of orbit level withtop of external auditory meatusSagittal planeCoronal planeSuperiorAnteriorPosteriorMedialLateralInferiorTransverse, horizontal,or axial plane |
Fig. 1.2 Cathode ray tube for the production of X-rays. |
Fig. 1.3 Fluoroscopy unit.Fig. 1.4 Barium sulfate follow-through.Fig. 1.5 Digital subtraction angiogram.Fig. 1.6 Ultrasound examination of the abdomen.Fig. 1.7 Computed tomography scanner.Fig. 1.8 Computed tomography scan of the abdomen at vertebral level L2. |
Fig. 1.9 A T2-weighted MR image in the sagittal plane of the pelvic viscera in a woman. |
Fig. 1.10 T1-weighted (A) and T2-weighted (B) MR images of the brain in the coronal plane. |
Fig. 1.11 A gamma camera.Fig. 1.12 The axial skeleton and the appendicular skeleton.Fig. 1.13 Accessory and sesamoid bones. A. Radiograph of the ankle region showing an accessory bone (os trigonum). |
B. Radiograph of the feet showing numerous sesamoid bones and an accessory bone (os naviculare). |
Fig. 1.14 A developmental series of radiographs showing the progressive ossification of carpal (wrist) bones from 3 (A) to 10 (D) years of age. |
Fig. 1.15 T1-weighted image in the coronal plane, demonstrating the relatively high signal intensity returned from the femoral heads and proximal femoral necks, consistent with yellow marrow. In this young patient, the vertebral bodies return an intermediate darker signal that represents red marrow. There is relatively little fat in these vertebrae; hence the lower signal return. |
Fig. 1.16 Radiograph, lateral view, showing fracture of the ulna at the elbow joint (A) and repair of this fracture (B) using internal fixation with a plate and multiple screws. |
Fig. 1.17 Image of the hip joints demonstrating loss of height of the right femoral head with juxta-articular bony sclerosis and subchondral cyst formation secondary to avascular necrosis. There is also significant wasting of the muscles supporting the hip, which is secondary to disuse and pain. |
Normal left hipBladderAvascular necrosisWasting of gluteal muscleFig. 1.18 Joints. A. Synovial joint. B. Solid joint.Fig. 1.19 Synovial joints. A. Major features of a synovial joint. B. Accessory structures associated with synovial joints. |
Fig. 1.20 Various types of synovial joints. A. Condylar (wrist). B. Gliding (radio-ulnar). C. Hinge (elbow). D. Ball and socket (hip). E. Saddle (carpometacarpal of thumb). F. Pivot (atlanto-axial). |
Fig. 1.21 Solid joints.Fig. 1.22 This operative photograph demonstrates the focal areas of cartilage loss in the patella and femoral condyles throughout the knee joint. |
Fig. 1.23 This radiograph demonstrates the loss of joint space in the medial compartment and presence of small spiky osteophytic regions at the medial lateral aspect of the joint. |
OsteophytesLoss of joint spaceFig. 1.24 After knee replacement. This radiograph shows the position of the prosthesis. |
Fig. 1.25 This is a radiograph, anteroposterior view, of the pelvis after a right total hip replacement. There are additional significant degenerative changes in the left hip joint, which will also need to be replaced. |
Fig. 1.26 Axial inversion recovery MR imaging series, which suppresses fat and soft tissue and leaves high signal intensity where fluid is seen. A muscle tear in the right adductor longus with edema in and around the muscle is shown. |
Fig. 1.27 Photograph demonstrating varicose veins.Fig. 1.28 Lymphatic vessels mainly collect fluid lost from vascular capillary beds during nutrient exchange processes and deliver it back to the venous side of the vascular system. |
Fig. 1.29 Regions associated with clusters or a particular abundance of lymph nodes. |
Cervical nodes(along courseof internaljugular vein)Axillary nodes(in axilla)Deep nodes(related to aortaand celiac trunkand superior andinferior mesentericarteries)Pericranial ring(base of head)Tracheal nodes(nodes related totrachea and bronchi)Inguinal nodes(along course ofinguinal ligament)Femoral nodes(along femoral vein) |
Fig. 1.30 Major lymphatic vessels that drain into large veins in the neck. |
Fig. 1.31 A. This computed tomogram with contrast, in the axial plane, demonstrates the normal common carotid arteries and internal jugular veins with numerous other nonenhancing nodules that represent lymph nodes in a patient with lymphoma. B. This computed tomogram with contrast, in the axial plane, demonstrates a large anterior soft tissue mediastinal mass that represents a lymphoma. |
Fig. 1.32 CNS and PNS.Fig. 1.33 Arrangement of meninges in the cranial cavity.Fig. 1.34 Differentiation of somites in a “tubular” embryo.Fig. 1.35 Somatic sensory and motor neurons. Blue lines indicate motor nerves and red lines indicate sensory nerves. |
Somatic sensory neurondeveloping from neural crest cellsEpaxial (back) musclesHypaxial musclesAxon of motor neuronprojects to muscle developingfrom dermatomyotomeSomatic motor neuroncell body in anterior regionof neural tube |
Fig. 1.36 Dermatomes.C6 segment of spinal cordSpinal ganglionDermatomyotomeAutonomous region(where overlap ofdermatomes isleast likely)of C6 dermatome(pad of thumb)Skin on the lateral side of the forearm and on thethumb is innervated by C6 spinal level (spinal nerve).The dermis of the skin in this region develops from the somiteinitially associated with the C6 level of the developing spinal cordCaudalCranialSomite |
Fig. 1.37 Myotomes.C6 segment of spinal cordMuscles that abduct the arm are innervated by C5 and C6 spinal levels (spinal nerves) and develop from somites initially associated with C5 and C6 regions of developing spinal cordC5 segment of spinal cordDermatomyotomeSomite |
Fig. 1.38 Dermatomes. A. Anterior view. B. Posterior view.Fig. 1.39 Development of the visceral part of the nervous system. |
Motor nerve endingassociated withblood vessels,sweat glands,arrector pili musclesat peripheryPart of neural crest developinginto spinal gangliaVisceral motor ganglionMotor nerve ending associated with visceraDeveloping gastrointestinal tractSensory nerve endingBody cavity(coelom)Visceral sensory neuron developsfrom neural crest and becomespart of spinal ganglionVisceral motorpreganglionicneuron in lateralregion of CNS(spinal cord)Postganglionic motor neuron is outside CNS.An aggregation of postganglionic neuronal cellbodies forms a peripheral visceral motor ganglion. |
Fig. 1.40 Basic anatomy of a thoracic spinal nerve.Fig. 1.41 Parts of the CNS associated with visceral motor components. |
SympatheticT1 to L2spinal segmentsBrainstemcranial nervesIII, VII, IX, XS2 to S4spinal segmentsParasympathetic |
Fig. 1.42 Sympathetic part of the autonomic division of the PNS. |
Abdominal visceraHeartOrgansPeripheralSympathetic nerves followsomatic nerves to periphery(glands, smooth muscle)Pelvic visceraGanglion imparEsophageal plexusPrevertebral plexus |
Fig. 1.43 Course of sympathetic fibers that travel to the periphery in the same spinal nerves in which they travel out of the spinal cord. |
Gray ramus communicansT10 spinal nervePosteriorramusAnteriorramusPeripheral distribution of sympatheticscarried peripherally by terminal cutaneousbranches of spinal nerve T1 to L2Motor nerve to sweat glands,smooth muscle of bloodvessels, and arrector pilimuscles in the part of T10dermatome supplied by theanterior ramusT10 spinal segmentWhite ramus communicans |
Fig. 1.44 Course of sympathetic nerves that travel to the periphery in spinal nerves that are not the ones through which they left the spinal cord. |
Sympathetic paravertebral trunksPeripheral distribution ofascending sympatheticsPeripheral distribution ofdescending sympathetics(C1) C2 to C8T1 to L2L3 to CoWhite ramus communicansGray ramus communicansPosterior rootGray ramus communicansGray ramus communicansAnterior root |
Fig. 1.45 Course of sympathetic nerves traveling to the heart. |
Sympathetic cardiac nervesSympathetic cardiac nervesSympathetic trunkCardiac plexusT1 to T4CervicalWhite ramuscommunicansGray ramuscommunicans |
Fig. 1.46 Course of sympathetic nerves traveling to abdominal and pelvic viscera. |
White ramus communicansGray ramus communicansSacral splanchnic nervesLumbar splanchnic nervesLeast splanchnic nervesLesser splanchnic nervesGreater splanchnic nervesPrevertebral plexusand gangliaParavertebralsympathetic trunkAbdominalandpelvic visceraAortaT5 to T9T12T9 to T10(T10 to T11)L1 to L2 |
Fig. 1.47 Parasympathetic part of the autonomic division of the PNS. |
Thoracic visceral plexusPrevertebral plexusAbdominal visceraSynapse with nerve cellsof enteric systemErectile tissues of penisand clitorisS2 to S4Sacral parasympatheticoutflow via pelvicsplanchnic nervesCranial parasympatheticoutflow via cranial nervesHeartSubmandibularganglionPterygopalatineganglionOtic ganglionCiliary ganglion[III][VII][IX][X]Pelvic visceraPupillary constrictionTransition from supply by [X]to pelvic splanchnic nervesSalivary glandsLacrimal glandParotid gland |
Fig. 1.48 Enteric part of the nervous system.Fig. 1.49 Nerve plexuses.C7C6C5C4C3C2C1T1T2T3T4T5T6T7T8T9T10T11T12L1S1S2S3S4S5L2L3L4L5C8GreaterLeastLesserSOMATIC PLEXUSESVISCERAL PLEXUSESCervical plexusanterior rami C1 to C4Brachial plexusanterior rami C5 to T1Lumbar plexusanterior rami L1 to L4Sacral plexusanterior ramiL4 to S4Parasympathetic [X]S2 to S4 pelvic splanchnic nerves(parasympathetic)Pulmonary branchPulmonary branchesCardiac branchesCardiac plexusThoracic aortic plexusEsophageal plexusPrevertebral plexusVagal trunkGanglion imparSacral splanchnic nervesSplanchnicnervesLumbar splanchnicnerves |
Fig. 1.50 Mechanism for referred pain from an inflamed appendix to the T10 dermatome. |
Table 1.1 The approximate dosage of radiation exposure as an order of magnitude |
In the clinicThese are extra bones that are not usually found as part of the normal skeleton, but can exist as a normal variant in many people. They are typically found in multiple locations in the wrist and hands, ankles and feet (Fig. 1.13). These should not be mistaken for fractures on imaging. |
Sesamoid bones are embedded within tendons, the largest of which is the patella. There are many other sesamoids in the body particularly in tendons of the hands and feet, and most frequently in flexor tendons of the thumb and big toe. |
Degenerative and inflammatory changes of, as well as mechanical stresses on, the accessory bones and sesamoids can cause pain, which can be treated with physiotherapy and targeted steroid injections, but in some severe cases it may be necessary to surgically remove the bone. |
In the clinicDetermination of skeletal ageThroughout life the bones develop in a predictable way to form the skeletally mature adult at the end of puberty. In western countries skeletal maturity tends to occur between the ages of 20 and 25 years. However, this may well vary according to geography and socioeconomic conditions. Skeletal maturity will also be determined by genetic factors and disease states. |
Up until the age of skeletal maturity, bony growth and development follows a typically predictable ordered state, which can be measured through either ultrasound, plain radiographs, or MRI scanning. Typically, the nondominant (left) hand is radiographed, and the radiograph is compared to a series of standard radiographs. From these images the bone age can be determined (Fig. 1.14). |
In certain disease states, such as malnutrition and hypothyroidism, bony maturity may be slow. If the skeletal bone age is significantly reduced from the patient’s true age, treatment may be required. |
In the healthy individual the bone age accurately represents the true age of the patient. This is important in determining the true age of the subject. This may also have medicolegal importance. |
In the clinicThe bone marrow serves an important function. There are two types of bone marrow, red marrow (otherwise known as myeloid tissue) and yellow marrow. Red blood cells, platelets, and most white blood cells arise from within the red marrow. In the yellow marrow a few white cells are made; however, this marrow is dominated by large fat globules (producing its yellow appearance) (Fig. 1.15). |
From birth most of the body’s marrow is red; however, as the subject ages, more red marrow is converted into yellow marrow within the medulla of the long and flat bones. |
Bone marrow contains two types of stem cells. Hemopoietic stem cells give rise to the white blood cells, red blood cells, and platelets. Mesenchymal stem cells differentiate into structures that form bone, cartilage, and muscle. |
There are a number of diseases that may involve the bone marrow, including infection and malignancy. In patients who develop a bone marrow malignancy (e.g., leukemia) it may be possible to harvest nonmalignant cells from the patient’s bone marrow or cells from another person’s bone marrow. The patient’s own marrow can be destroyed with chemotherapy or radiation and the new cells infused. This treatment is bone marrow transplantation. |
In the clinicFractures occur in normal bone because of abnormal load or stress, in which the bone gives way (Fig. 1.16A). Fractures may also occur in bone that is of poor quality (osteoporosis); in such cases a normal stress is placed upon a bone that is not of sufficient quality to withstand this force and subsequently fractures. |
In children whose bones are still developing, fractures may occur across the growth plate or across the shaft. These shaft fractures typically involve partial cortical disruption, similar to breaking a branch of a young tree; hence they are termed “greenstick” fractures. |