Special Senses Development PDF

1. Discuss the development of the nose. In classic embryology textbooks, the rst 28 day of gesta onal life the face develops from ve swellings: The paired maxillary and mandibular p...

1. Discuss the development of the nose. In classic embryology textbooks, the rst 28 day of gesta onal life the face develops from ve swellings: The paired maxillary and mandibular processes and the unpaired frontonasal process. During the h week, the nasal placodes (i.e., nasal discs and nasal plates) develop as a result of ectodermal thickenings and can be observed on the frontonasal process. In the sixth week, infolding of the ectoderm at the epicenter of these nasal placodes ini ates the forma on of an oval pit (see below descrip on of nasal pits) resul ng in the division of the raised edge of each placode into medial and lateral nasal processes (Figure 1A). During the sixth week, the medial nasal processes fuse to form the intermaxillary process (Figure 1B), which is the primordia of the septum and bridge of the nose (Figure 1C). By the end of the seventh week, there is a lateral and inferior expansion of the medial nasal processes at their inferior ps before fusing to form the anterior roof of the oral cavity (Figure 1B). As the poles of the maxillary swellings con nue to develop they come into contact with the intermaxillary process where they fuse with each other. On the superior labial region, the intermaxillary process develops into the philtrum (Figure 1C). Forma on of the nasal passages result as the nasal pits deepens penetra ng its underlying mesenchyme during week 6 of development. Mesenchyme is loosely organized cells derived from mesodermal embryonic ssue that develops into connec ve and skeletal ssues. An ectodermally enlarged nasal sac is formed during the last days of the sixth week by the fusion of the deep endings of the nasal pits, which are topographically located superoposterior to the intermaxillary process. During the last days of the sixth week and the rst few days of the seventh week, a prolifera on of cells occurs at the posterior wall and oor of the nasal sac forming a thickened plate-like n of ectoderm origin essen ally isola ng the oral cavity from the nasal sac but s ll maintaining an epithelial con nuity between the regions. This “keel” structure is now referred to as the nasal n. The nasal sac enlarges as a result of vacuoles developing within the nasal n and then it fuses with the sac. The nasal n begins to a enuate to a thin membrane named the oronasal membrane, which demarcates the oral cavity from the nasal sac. Towards the end of the seventh week, the oronasal membrane obliterates crea ng the opening of the primi ve choana. Forma on of the nasal cavity oor, or primary palate, occurs by the backward growing of the intermaxillary process. Throughout the eighth and ninth week, the development of the de ni ve and secondary palate occurs. The main por on of the de ni ve palate develops by two shelve-like outgrowths from the maxillary processes. These two thin medial extension outgrowths are called the pala ne shelves, which appear during the sixth week of development. While these shelves are directed in a downward manner on either side of the tongue, it is during the ninth week where these shelves rotate and ascend rapidly a aining a horizontal posi on above the tongue. Fusion of the primary palate and the pala ne shelves (along the midline) assists in the forma on of the secondary palate (Figure 1D). The order of fusion rst begins at the ventral region of the pala ne shelves before proceeding dorsally. Mesenchymal condensa ons occurs when previously dispersed mesenchymal cells come together to di eren ate into a single ssue type and is considered the cri cal transi onal stage that precedes car lage forma on during embryonic development. When these mesenchymal cell condensa ons occur in the ventral region of the secondary palate endochondral ossi ca on ensues to ti tt fi ft fl ti ti ti ti ff ti ti ti ti fi ti ti ti ti ti fi fi fi ti ti ti ti ti ti fi ti ti ti tt ti ti fi ti ti ti ti ti ti ti fi fl ti ti ti ti fi ti fi fi fi  achieve the forma on the hard palate. At the dorsal region of the secondary palate, myogenic mesenchymal cells come together to form the muscular layer of the so palate. During the forma on of the secondary palate, there is a prolifera on of cells from the mesoderm and ectoderm region of the medial nasal and frontonasal processes that help form the nasal septum along its midline. As a result, the two nasal passages of the nasal cavity have now been established, communica ng with the pharynx located posterior to the secondary palate. This communica ng portal is now termed the de ni ve choana. According to classical concept, the philtrum of upper lip, the nasal dorsum, septum, and primary palate originate from the development of the intermaxillary process, whereas the lateral walls of the nasal pyramid develop from the lateral nasal processes (Figure 1A-C). 2. What are the anatomic frameworks of the external nose? What are its components? a. Bony Framework Nasal Process of the frontal bone Nasal Bone Frontal Process of the Maxilla b. Car laginous Framework ti ti ti ti ti fi ti ft ti  3. What are the parts of the nasal septum? a. Septal car lage b. Vomer c. Perpendicular plate of ethmoid d. Maxillary crest e. Pre-maxilla 4. What are the muscles of the nose? Dilators: a. Procerus b. Dilator naris c. Quadratus labi superioris Constrictors: a. Nasalis b. Depressor sep c. Depressor aleque nasi 5. What are the structures found in the lateral nasal wall? a. Inferior turbinate i. Inferior Meatus →Nasolacrimal duct b. Middle Turbinate ii. Middle Meatus → Maxillary sinus → Anterior ethmoid sinus →Frontal sinus c. Superior turbinate iii. Superior meatus→ Posterior ethmoid sinus d. Supreme turbinate 6. Discuss the histology of the respiratory mucosa of the nose. The usually ciliated, pseudostra ed columnar epithelium of the respiratory apparatus varies considerably in di erent por ons of the nose, depending on pressure and velocity of the air streams, their temperatures, and their predominant moisture contents. Thus, the anterior ends of the turbinates and the septal mucosa for a short way past the os internum are s ll lined by a stra ed squamous epithelium without cilia – an extension of the skin of the nasal ves bule. Along the main path of the inspiratory currents the cells become columnar; the cilia are short and slightly irregular. Cells of the middle and inferior meatuses, handling most of the expiratory ow path, grow long and evenly spaced cilia. The mucous blanket in the nose is renewed about three to four mes an hour. 7. What is the blood supply of the nose? ti ti ff ti ti fi fl ti ti ti ti fi  The blood supply of the lateral nasal wall is the ethmoid arteries which are branches of the ophthalmic artery from the internal caro d artery. The sphenopala ne and greater pala ne arteries are terminal branches of the external caro d artery. The blood supply of the nasal septum, in addi on to the vessels supplying the lateral nasal wall, branches from the superior labial artery and pala ne artery reach the septum. Kiesselbach’s plexus is the most common area of epistaxis. The nose, in par cular the nasal cavity, is well supplied with blood from branches of both the internal caro d artery, and the external caro d artery. Internal caro d The main branches from the interior caro d are the anterior ethmoidal artery, and the posterior ethmoidal artery that supplies the septum, and these derive from the ophthalmic artery. One of the terminal branches of the ophthalmic artery is the dorsal nasal artery which divides into two branches. One branch crosses the root of the nose and joins with the angular artery, and the other branch joins with the lateral nasal branch of facial artery which supplies the nasal ridge and alae. External caro d Branches from the external caro d artery are the sphenopala ne artery, the greater pala ne artery, the superior labial artery, and the angular artery. The lateral walls of the nasal cavity and the septum are supplied by the sphenopala ne artery, and by the anterior and posterior ethmoid arteries. There is addi onal supply from the superior labial artery and the greater pala ne artery. ti ti ti ti ti ti ti ti ti ti ti ti ti ti ti ti ti ti  The nasal ridge is supplied by branches of the internal maxillary artery (infraorbital) and the ophthalmic arteries from the common caro d artery system. The arteries supplying the nasal cavity converge in the front lower part of the septum in a plexus known as Kiesselbach's plexus. Venous Drainage Veins of the nose include the angular vein that drains the side of the nose, receiving lateral nasal veins from the alae. The angular vein joins with the superior labial vein. Some small veins from the nasal ridge (dorsum) drain to the nasal arch of the frontal vein at the root of the nose. In the posterior region of the cavity, speci cally in the posterior part of the inferior meatus is a venous plexus known as Woodru 's plexus. This plexus is made up of large thin-walled veins with li le so ssue such as muscle or ber. The mucosa of the plexus is thin with very few structures. Lympha c drainage The nasal part of the lympha c system arises from the super cial mucosa, and drains posteriorly to the retropharyngeal lymph nodes, and anteriorly to either the upper deep cervical lymph nodes (in the neck), or to the submandibular glands (in the lower jaw), or into both the nodes and the glands of the neck and the jaw. 8. What is the innerva on of the nose? Nerve supply to the nose is from the rst two branches of the trigeminal nerve (CN V), the ophthalmic nerve (CN V1) and the maxillary nerve (CN V2). Nerve supply to the nose is from the rst two branches of the trigeminal nerve (CN V), the ophthalmic nerve (CN V1) and the maxillary nerve (CN V2). Ophthalmic innerva on ▪ Nasociliary nerve – conveys sensa on to the skin area of the nose, and the mucous membrane of the anterior (front) nasal cavity. ft ti ti ti ti ti fi ff ti fi fi ti fi fi tt  ▪ Anterior ethmoid nerve – conveys sensa on in the anterior (front) half of the nasal cavity: (a) the internal areas of the ethmoid sinus and the frontal sinus; and (b) the external areas, from the nasal p to the rhinion: the anterior p of the terminal end of the nasal-bone suture. ▪ Posterior ethmoid nerve – serves the superior (upper) half of the nasal cavity, the sphenoids, and the ethmoids. ▪ Intratrochlear nerve – conveys sensa on to the medial region of the eyelids, the palpebral conjunc va, the nasion (nasolabial junc on), and the bony dorsum. Maxillary innerva on ▪ Maxillary nerve – conveys sensa on to the upper jaw, the face and the nostrils. ▪ Internal nasal branches of infraorbital nerve – conveys sensa on to the septum. ▪ Zygoma c nerve – through the zygoma c bone and the zygoma c arch, conveys sensa on to the cheekbone areas. ▪ Sphenopala ne nerve – divides into the lateral branch and the septal branch, and conveys sensa on from the rear and the central regions of the nasal cavity. The supply of parasympathe c nerves to the face and the upper jaw (maxilla) derives from the greater super cial petrosal (GSP) branch of cranial nerve VII, the facial nerve. The GSP nerve joins the deep petrosal nerve (of the sympathe c nervous system), derived from the caro d plexus, to form the vidian nerve (in the vidian canal) that traverses the pterygopala ne ganglion (an autonomic ganglion of the maxillary nerve), wherein only the parasympathe c nerves form synapses, which serve the lacrimal gland and the glands of the nose and of the palate, via the (upper jaw) maxillary division of cranial nerve V, the trigeminal nerve. 9. Discuss the physical principles of nasal air ow. During inspira on, the air stream enters the nasal ves bule in an oblique ver cal direc on. Aerodynamically, this air is in a state of laminar ow, meaning that there is no mixing of the di erent air layers. When the inspired air reaches the nasal valve located between the ves bule and nasal cavity, it passes through the narrowest site in the upper respiratory tract (limen nasi). Just past the nasal valve, the cross sec on of the airway becomes greatly expanded, crea ng a “di user e ect” that transforms most of the laminar ow of the inspired air into turbulent ow, in which di erent air layers are swirled together. Besides the velocity of the air, the degree of change in air ow characteris cs at this stage is very strongly in uenced by the specialized anatomy of the nasal cavity, which is subject to substan al individual di erences. 10. What are the protec ve func ons of the nasal mucosa? a. Mechanical defenses: The most important mechanical defense mechanism of the nasal mucosa is the mucocilliary apparatus, which physically cleanses the inspired air. The mucocilliary transport system consists of the respiratory epithelium and a mucous blanket composed of two layers: a deeper, less viscid “sol layer” in which ciliary mo on occurs, and a super cial, more viscid “gel layer”. Disturbances of mucocilliary transport can have various causes, such as increased viscosity and thickness of the periciliary sol layer, hampering ciliary movements or changes in the visco-elas city of the gel layer resul ng in ine ectual mucus transport. b. Cellular defenses The predominant phagocy c cells are neutrophilic granulocytes, monocytes and macrophages. They are accompanied by “natural killer cells” (NK cells), which comprise a small percentage of the peripheral lymphocytes and protect mainly against viral infec ons of the nasal mucosa. ti ti ti ti ti ff ti fi fl ti ti ti ti fl ti ti ti ti ti ti ti ti ti ti fl fi fl ti ti fl ti ti ti ti ti ti fl ff ff ti ti ff ti ti ti ti ff ff ti ti  c. Humoral immune response An bodies are formed in the paraglandular plasma cells. Most notably, IgA is an immunoglobulin that is characteris c of the respiratory mucosa. The plasma cells also synthesize IgM and the less common IgG. When released, the immunoglobulins (especially IgA) are absorbed by the glandular cells of the lamina propria, provided with a secretory component, and re-released as secretory an bodies (sIgA). d. Cellular immune response Representa ve of the cellular immune response of the nasal mucosa include mast cells, macrophages, various polymorphonuclear leukocytes (neutrophils, basophils, eosinophilic granulocytes), lymphocytes, and the cells of the re culoendothelial system, which occur chie y as dendri c (Langerhans) cells in the nasal mucosa. 11. What are the paranasal sinuses? Discuss its anatomy (include the drainage system). The paranasal sinuses are air- lled cavi es. All but the sphenoid sinus are already present as outpouchings of the mucosa during embryonic life, but except for the ethmoid air cells, they do not develop into bony cavi es un l a er birth. The frontal sinus and sphenoid sinus reach their de ni ve size in the rst decade of life. The maxillary sinus is present at birth but remains very small un l the second den on, because the presence of tooth germs in the maxilla limits the extent of the sinuses. All of the sinuses are lined with a modi ed respiratory epithelium capable of producing mucus and having cilia, of emptying secre ons into the nasal cavi es. The sinuses are essen ally air- lled. a. Maxillary sinus i. Aka Antrum of Highmore ii. Pyramidal in shape located in the maxillary bone iii. Largest paranasal sinus iv. Volume: 15mL each v. Present at birth vi. Floor is formed by the alveolar process of the maxilla b. Ethmoid Sinus i. Bony labyrinth of small air cells ti ti ti ti fi ti ti ti ti fi ft ti fi ti ti ti ti fi ti fi fl ti ti  ii. Between the medial wall of the orbit and the middle turbinate iii. Present at birth iv. Develop as outpocke ng of the medial meatus v. Quan ty: 7-15 per side, 14mL per volume vi. Anterior ethmoids drains to Middle Meatus vii. Posterior ethmoids drains to Superior Meatus viii. Lateral wall: Lamina Papyracea ix. Medial wall: Middle turbinate c. Frontal Sinus i. Located in the frontal bone ii. Arise from an expansion of an anterior air cell from the ethmoids iii. Not present un l 6 years of age iv. Vol: 6 – 7mL per sinus v. Drainage via the middle meatus d. Sphenoid Sinus i. Develop as excava on into the sphenoid bone at around 4 mos of age ii. Vol. 7.5mL per sinus iii. Os um open into the superior meatus through the sphenoid recess iv. Borders: Anterior: Posterior Ethmoids Posterior: Pons and Basila artery Superior: Op c chiasm Inferior: Nasal cavity and nasopharynx Lateral: Op c nerves, cavervous sinuses, internal caro d artery 12. What are the func ons of the paranasal sinuses? a. Humidica on b. Vocal resonance c. Mucus produc on d. Increased olfactory area e. Absorbs shock to the g=head f. Regula on of intranasal 13. What comprises the osteomeatal complex? Osteomeatal complex contains the narrow channels that provide for mucociliary clearance and ven la on of the anterior ethmoid, maxillary, and frontal sinuses. Drainage and ven la on of the major paranasal sinuses are dependent on the patency of the osteomeatal complex. a. Uncinate process b. Maxillary Os um c. Middle turbinate b. Bulla ethmoidalis c. Ethmoid infundibulum d. Hiatus semilunaris e. Basal Lamella 14. Describe the olfactory membrane? The olfactory membrane, the histology of which is shown in Figure 53–3, lies in the superior part of each nostril. Medially, the olfactory membrane folds downward along the surface of the superior septum; laterally, it folds over the superior turbinate and even over a small por on of the upper surface ti ti ti ti ti ti ti ti ti ti ti ti ti ti ti ti ti ti  of the middle turbinate. In each nostril, the olfactory membrane has a surface area of about 2.4 square cen meters. 15. What are the olfactory cells? The receptor cells for the smell sensa on are the olfactory cells (see Figure 53–3), which are actually bipolar nerve cells derived originally from the central nervous system itself. There are about 100 million of these cells in the olfactory epithelium interspersed among sustentacular cells, as shown in Figure 53– 3. The mucosal end of the olfactory cell forms a knob from which 4 to 25 olfactory hairs (also called olfactory cilia), measuring 0.3 micrometer in diameter and up to 200 micrometers in length, project into the mucus that coats the inner surface of the nasal cavity. These projec ng olfactory cilia form a dense mat in the mucus, and it is these cilia that react to odors in the air and s mulate the olfactory cells. Spaced among the olfactory cells in the olfactory membrane are many small Bowman’s glands that secrete mucus onto the surface of the olfactory membrane. 16. Discuss the s mula on of the olfactory cells and the neural pathway into the olfactory cortex. Mechanism of Excita on of the Olfactory Cells The por on of each olfactory cell that responds to the olfactory chemical s muli is the olfactory cilia. The odorant substance, on coming in contact with the olfactory membrane surface, rst di uses into the mucus that covers the cilia. Then it binds with receptor proteins in the membrane of each cilium. Each receptor protein is actually a long molecule that threads its way through the membrane about seven mes, folding inward and outward. The odorant binds with the por on of the receptor protein that folds to the outside. The inside of the folding protein, however, is coupled to a so-called G-protein, itself a combina on of three sub-units. On excita on of the receptor protein, an alpha subunit breaks away from the G-protein and immediately ac vates adenylyl cyclase, which is a ached to the inside of the ciliary membrane near the receptor cell body. The ac vated cyclase, in turn, converts many molecules of intracellular adenosine triphosphate into cyclic adenosine monophosphate (cAMP). Finally, this cAMP ac vates another nearby membrane protein, a gated sodium ion channel that opens its “gate” and allows large numbers of sodium ions to pour through the membrane into the receptor cell cytoplasm. The sodium ions increase the electrical poten al in the posi ve direc on inside the cell membrane, thus exci ng the olfactory neuron and transmi ng ac on poten als into the central nervous system by way of the olfactory nerve. The importance of this mechanism for ac va ng olfactory nerves is that it greatly mul plies the excitatory e ect of even the weakest odorant. To summarize: (1) Ac va on of the receptor protein by the odorant substance ac vates the G-protein complex. (2) This, in turn, ac vates mul ple molecules of adenylyl cyclase inside the olfactory cell membrane. (3) This causes the forma on of many mes more molecules of cAMP. (4) Finally, the cAMP opens s ll many mes more sodium ion channels. Therefore, ti ti ti ff ti ti ti ti ti ti ti ti ti ti ti ti ti tti ti ti ti ti ti ti ti ti ti ti ti ti ti ti tt ti ti ti fi ti ff  even the minutest concentra on of a speci c odorant ini ates a cascading e ect that opens extremely large numbers of sodium channels. This accounts for the exquisite sensi vity of the olfactory neurons to even the slightest amount of odorant. In addi on to the basic chemical mechanism by which the olfactory cells are s mulated, several physical factors a ect the degree of s mula on. First, only vola le substances that can be sni ed into the nostrils can be smelled. Second, the s mula ng substance must be at least slightly water soluble so that it can pass through the mucus to reach the olfactory cilia. Third, it is helpful for the substance to be at least slightly lipid soluble, presumably because lipid cons tuents of the cilium itself are a weak barrier to non-lipid-soluble odorants. Membrane Poten als and Ac on Poten als in Olfactory Cells The membrane poten al inside uns mulated olfactory cells, as measured by microelectrodes, averages about –55 millivolts. At this poten al, most of the cells generate con nuous ac on poten als at a very slow rate, varying from once every 20 seconds up to two or three per second. Most odorants cause depolariza on of the olfactory cell membrane, decreasing the nega ve poten al in the cell from the normal level of –55 millivolts to –30 millivolts or less — that is, changing the voltage in the posi ve direc on. Along with this, the number of ac on poten als increases to 20 to 30 per second, which is a high rate for the minute olfactory nerve bers. Over a wide range, the rate of olfactory nerve impulses changes approximately in propor on to the logarithm of the s mulus strength, which demonstrates that the olfactory receptors obey principles of transduc on similar to those of other sensory receptors Transduc on of Olfactory S muli Odorant molecules bind reversibly to the diverse receptor membrane proteins which are coupled to a G-s group of proteins called Golf. Ac va on of adenylyl cyclase leads to the forma on of cAMP with the ac va on of Ca2+/ Na+ ca on channels. The primary e ect of in ux of these ions is depolariza on and the genera on of a generator poten al (Figure 9.9). Generated ionic currents are graded in response to the ow rate of the odorant molecules and to their concentra on. Sites of summated generator poten als occur across the olfactory mucosa to produce speci c spa al pa ern of ac vity for each s mula ng odorant molecules, which may contribute to neural coding of odors. These spa al responses across the olfactory mucosa can be recorded (electro-olfactograms) with surface electrodes. ti ti ti ti ti ti ti ff ti ti ti ti ti ti ti ti ti ti fl ti ti ti ti ti ti ti ti ti ti ti fi ti ti ti ti ti ti ti ti fi ti ti ff ti ff ti ti fl fi ti ti ti ti ti ff tt ti ti  Propaga on of Ac on Poten als and Convergence upon the Olfactory Bulb The resul ng in ux of Na+ and Ca2+ produces a depolarizing generator poten al that spreads to the axon hillock. There, ac on poten als are generated, which are propagated to the synap c endings in the olfactory bulb (Figure 9.9). The ac on poten al frequency is propor onal to the concentra on of speci c odorant molecules. However, ac on poten al frequency will be a enuated by adapta on or desensi za on of the receptor and reduc on in the produc on of cAMP. Rapid adapta on and removal of the odorants permit con nued recogni on and discrimina on of new aromas that are inhaled in the next respiratory cycle. Ac on poten als generated in the axon terminals of ac vated neurons are propagated into the glomeruli within the olfactory bulb. The olfactory bulbs have many di erent types of neurons and these have a laminar distribu on. On the ventral side of the olfactory bulbs is a layer of glomeruli. This is a site at which axon terminals of several thousand olfactory neurons synapse with numerous dendrites from large mitral cells and tu ed cells. Interneurons such as the inhibitory periglomerular cells synapse with the nerve endings within adjacent glomeruli. Millions of axon bers converge upon only a few thousand glomeruli within each bulb to synapse with about 75,000 mitral cells (see Figure 9.10) and about twice this number of tu ed/ periglomerular cells. Mitral cells are 2nd order sensory neurons whose axons enter the olfactory tract and ascend to the olfactory cortex. This convergence/divergence between the axons of olfactory neurons and the specialized cells of the olfactory bulb generate excitatory postsynap c poten als (EPSPs) in the dendrites of mitral cells and subsequent ac on poten als. Lateral inhibi on by the periglomerular cells modulates ac vity in adjacent glomeruli innervated by other mitral and tu cells. A complex pa ern of neuronal integra on for discrimina on of various odorant molecules is indicated by the mechanisms of convergence/ divergence with excita on/inhibi on of these 2nd order sensory neurons. This complexity is related to the recogni on that no single odor s mulates a speci c group of olfactory neurons. Rather a neural code is created from the ac va on of mul ple receptors and neurons. NEURAL PATHWAY INTO THE OLFACTORY CORTEX Axons from mitral and tu cells project caudally into the olfactory tract. Fibers diverge and synapse with neurons of the anterior olfactory nucleus (AON). Axons from the AON cross to the opposite side of the hemisphere through the anterior commissure. The majority of the axons from the olfactory bulb diverges laterally and forms the lateral olfactory tract which synapses with nuclei of the olfactory cortex. These are the piriform cortex (pc), the periamygdaloid cortex, part of the amygdala, and hippocampus. There are no direct relays from the olfactory bulb into the thalamus, but a few bers synapse with 3rd order sensory neurons in the thalamic dorsomedial nucleus which are projected to the ipsilateral cerebral hemisphere (Figure 9.11). ti ti ti ti ti ti ti ft ti ti ti ti fl fi ti ti ti ff ti ti ti ti ti ft ti fi ti tt ti ti ti ti ti ti ti ft ti ti tt ti ti ti ti ti ti ti ti ft fi ti ti ti ti fi  17. What does threshold for smell means? One of the principal characteris cs of smell is the minute quan ty of s mula ng agent in the air that can elicit a smell sensa on. For instance, the substance methylmercaptan can be smelled when only one 25 trillionth of a gram is present in each milliliter of air. Because of this very low threshold, this substance is mixed with natural gas to give the gas an odor that can be detected when even small amounts of gas leak from a pipeline. 18. How does the central nervous system receive smell signals? Transmission of olfactory signals into the olfactory bulb The olfactory nerve bers leading backward from the bulb are called cranial nerve I, or the olfactory tract. However, in reality, both the tract and the bulb are an anterior outgrowth of brain ssue from the base of the brain; the bulbous enlargement at its end, the olfactory bulb, lies over the cribriform plate, separa ng the brain cavity from the upper reaches of the nasal cavity. The cribriform plate has mul ple small perfora ons through which an equal number of small nerves pass upward from the olfactory membrane in the nasal cavity to enter the olfactory bulb in the cranial cavity. Figure 53–3 demonstrates the close rela on between the olfactory cells in the olfactory membrane and the olfactory bulb, showing short axons from the olfactory cells termina ng in mul ple globular structures within the olfactory bulb called glomeruli. Each bulb has several thousand such glomeruli, each of which is the terminus for about 25,000 axons from olfactory cells. Each glomerulus also is the terminus for dendrites from about 25 large mitral cells and about 60 smaller tu ed cells, the cell bodies of which lie in the olfactory bulb superior to the glomeruli. These dendrites receive synapses from the olfactory cell neurons, and the mitral and tu ed cells send axons through the olfactory tract to transmit olfactory signals to higher levels in the central nervous system. Some research has suggested that di erent glomeruli respond to di erent odors. It is possible that speci c glomeruli are the real clue to the analysis of di erent odor signals transmi ed into the central nervous system. fi ti fi ti ti ti ti ft ti ff ti ft ff ti ff ti ti ti tt ti  19. What are the di erences between the medial olfactory area from the lateral olfactory area? The Very Old Olfactory System—the Medial Olfactory Area The medial olfactory area consists of a group of nuclei located in the midbasal por ons of the brain immediately anterior to the hypothalamus. Most conspicuous are the septal nuclei, which are midline nuclei that feed into the hypothalamus and other primi ve por ons of the brain’s limbic system. This is the brain area most concerned with basic behavior. The importance of this medial olfactory area is best understood by considering what happens in animals when the lateral olfactory areas on both sides of the brain are removed and only the medial system remains. The answer is that this hardly a ects the more primi ve responses to olfac on, such as licking the lips, saliva on, and other feeding responses caused by the smell of food or by primi ve emo onal drives associated with smell. Conversely, removal of the lateral areas abolishes the more complicated olfactory condi oned re exes. The Less Old Olfactory System—the Lateral Olfactory Area The lateral olfactory area is composed mainly of the prepyriform and pyriform cortex plus the cor cal por on of the amygdaloid nuclei. From these areas, signal pathways pass into almost all por ons of the limbic system, especially into less primi ve por ons such as the hippocampus, which seem to be most important for learning to like or dislike certain foods depending on one’s experiences with them. For instance, it is believed that this lateral olfactory area and its many connec ons with the limbic behavioral system cause a person to develop an absolute aversion to foods that have caused nausea and vomi ng. An important feature of the lateral olfactory area is that many signal pathways from this area also feed directly into an older part of the cerebral cortex called the paleocortex in the anteromedial por on of the temporal lobe. This is the only area of the en re cerebral cortex where sensory signals pass directly to the cortex without passing rst through the thalamus. 20. Brie y describe the neck, its anatomy, muscles and blood vessels, especially the caro d system. NECK The neck is divided by the sternocleidomastoid muscle into an anterior and posterior triangle. The anterior triangle contains vascular structures (caro d artery and internal jugular vein), cranial nerve (CN) X, and the respiratory (trachea and larynx) and diges ve ti ti ti ti fl ti ff ti ti ti fl ti fi ti ff ti ti ti ti ti ti ti ti ti ti ti (pharynx and esophagus) visceral structures. The posterior triangle contains the muscles associated with the cervical vertebrae, CN XI, cervical plexus, and the origins of the brachial plexus. Thoracic Outlet The thoracic outlet is the space bounded by the manubrium, the rst rib, and T1 vertebra. The interval between the anterior and middle scalene muscles and the rst rib (scalene triangle) transmits the structures coursing between the thorax, upper limb and lower neck. The triangle contains the trunks of the brachial plexus andthesubclavian artery (Figure II-6-1). Note that the subclavian vein and the phrenic nerve (C 3, 4, and 5) are on the anterior surface of the anterior scalene muscle and are not in the scalene triangle. CAROTID AND SUBCLAVIAN ARTERIES Clinical Correlate Thoracic Outlet Syndrome Thoracic outlet syndrome results from the compression of the trunks of the brachial plexus and the subclavian artery within the scalene triangle. Compression of these structures can result from tumors of the neck (Pancoast on apex of lung), a cervical rib or hypertrophy of the scalene muscles. The lower trunk of the brachial plexus (C8, T1) is usually the rst to be a ected. Clinical symptoms include the following: Numbness and pain on the medial aspect of the forearm and hand Weakness of the muscles supplied by the ulnar nerve in the hand (claw hand) Decreased blood ow into the upper limb, indicated by a weakened radial pulse Compression can also a ect the cervical sympathe c trunk (Horner’s syndrome) and the recurrent laryngeal nerves (hoarseness). fl ff ti fi fi fi ff  The most signi cant artery of the external caro d system is the middle meningeal artery. It arises from the maxillary artery in the infratemporal fossa and enters the skull through the foramen spinosum to supply skull and dura. Lacera ons of this vessel result in an epidural hematoma. 21. Describe the topographic anatomy of the neck? The neck is bounded above by the inferior border of the mandible, the p of the mastoid process, and the external occiput protuberance. The lateral contours of the neck are de ned by the palpable sternocleidomastoid muscles and the borders of the trapezius muscles. Palpable medial structures are the hyoid bone, thyroid car lage, cricoid car lage, and when enlarged, the thyroid gland. 22. What are the triangles of the neck and its subdivisions? What are the contents on each triangle? f. Posterior Triangle Boundaries: Super cial Covering - Super cial fascia, Platysma, Inves ng Layer fi fi fi ti ti ti ti ti ti fi  Deep (Floor) - Prevertebral layer of fascia Contents: Occipital artery Great auricular nerve Less occipital nerve Accessory nerve Supraclavicular nerve Super cial branches cervical plexus External Jugular vein Phrenic nerve (deep) Branchial plexus roots & trunks g. Anterior Triangle Boundaries: Anterior - Midline of neck Posterior - Sternocleidomastoid muscle Superior - Mandible (Lower margin) Contents: Caro d sheath (Internal and Common caro d artery, Jugular vein, Vagus nerve) Hypoglassal nerve & Ansa cervicalis i. Caro d triangle Boundaries: Superior: Posterior belly of the digastric muscle. Lateral: Medial border of the sternocleidomastoid muscle. Inferior: Superior belly of the omohyoid muscle. The main contents of the caro d triangle are the common caro d artery (which bifurcates within the caro d triangle into the external and internal caro d arteries), the internal jugular vein, and the hypoglossal and vagus nerves. ti fi ti ti ti ti ti ti  ii. Submental triangle Boundaries: Inferiorly – Hyoid bone. Medially – Imaginary sagi al midline of the neck. Laterally – Anterior belly of the digastric. The base of the submental triangle is formed by the mylohyoid muscle, which runs from the mandible to the hyoid bone. iii. Submandibular triangle Boundaries: Superiorly: Body of the mandible. Anteriorly: Anterior belly of the digastric muscle. Posteriorly: Posterior belly of the digastric muscle. iv. Muscular triangle Boundaries: Superiorly: The hyoid bone. Medially: Imaginary midline of the neck. Supero-laterally: Superior belly of the omohyoid muscle. Infero-laterally: Inferior por on of the sternocleidomastoid muscle. 23. What are the zones of the neck? For descrip ve and clinical management purposes, the neck is divided into three zones: zones 1, 2, and 3. In penetra ng trauma, zone designa ons have anatomic, diagnos c, and management implica ons. Since the zone system is helpful in guiding management decisions, it is preferable to employ the zone system when describing trauma c injuries. Understanding of the anatomy of the neck, especially the loca on of important structures, is essen al to providing op mal care. ▪ Zone 1: This is the area between the clavicles and the cricoid car lage. This zone contains vital structures which include the innominate vessels, the origin of the common caro d artery, the subclavian vessels and the vertebral artery, the brachial plexus, the trachea, the esophagus, the apex of the lung, and the thoracic duct. Furthermore, surgical exposure and access can be di cult in this zone, because of the presence of the clavicle and bony structures of the thoracic inlet. ▪ Zone II: This is the area between the cricoid car lage and the angle of the mandible. The following structures are located here: the caro d and vertebral arteries, the internal jugular veins, trachea, and the esophagus. This zone has compara vely easy access for clinical examina on and surgical explora on. It is the largest zone and the most commonly injured in the neck. ▪ Zone III: This is the area between the angle of the mandible and the base of the skull. This area contains the distal caro d and vertebral arteries and the pharynx. Since it is very close to the base of the skull, this area is less amenable to physical examina on and di cult to explore during surgical evalua on. Anatomically, the neck is also described in triangles. The sternocleidomastoid muscle separates the neck into two triangles. The anterior triangle contains most of the major anatomic structures of the neck including larynx, trachea, pharynx, esophagus, and major vascular structures. The posterior triangle contains muscles, the spinal accessory nerve, and the spinal column. 24. What are the fascial compartments in the neck? ffi ti ti ti ti ti ti ti tt ti ti ti ti ti ti ti ti ti ti ti ffi ti ti  FASCIA OF NECK One of the earliest lessons in anatomy is that there are two types of fascia in the body: the super cial and the deep. In the region of the abdominal wall, super cial fascia consists of two layers—a fa y layer (Camper’s fascia) and a deeper, membranous layer (Scarpa’s fascia). The deep fascial layer of the abdominal wall is not subdivided but simply envelops the abdominal muscles. In the neck the super cial fascia is very thin and is not divided into layers, whereas the deep fascia is divided into three layers. The names of these layers vary with di erent authors, resul ng in a somewhat chao c terminology. Regardless of the terminology used, the divisions are arbitrary at best. A simple approach can provide a workable solu on for either the anatomist or the surgeon. a. Super cial Layer of the Cervical Fascia The super cial layer of cervical fascia is a single layer of fascia underlying the skin. It usually is thin, except in the obese person, in which case it is thickened by adipose ssue. Its primary surgical signi cance is that it provides a fascial pad that protects underlying structures when a skin incision is made. In excep onally lean people, however, the paucity of this layer may not protect underlying structures, such as the accessory nerve, so the surgeon should be wary when opera ng on such pa ents. It underlies the platysma and subcutaneous fat, invests the en re neck, and encases the sternocleidomastoid and trapezius muscles. This fascial layer is a ached to the hyoid bone and stretches superiorly to the mandibular border inferiorly to the manubrium sterni and clavicle.