USMLE Step 1 Qs (6)

The spiral arteries of the endometrium (labeled A in the diagram accompanying the question) depend on specific estrogen/progesterone ratios for their development. They pass through the basalis layer of the endometrium into the functional zone, and their distal ends are subject to degeneration with each menses. The straight arteries (B) are not subject to these hormonal changes. In the proliferative phase, the endometrium is only 1- to 3-mm thick, and the glands are straight with the spiral arteries only slightly coiled. This diagram of the early secretory phase shows an edematous endometrium that is 4-mm thick, with glands that are large, beginning to sacculate in the deeper mucosa, and coiled for their entire length. In the late secretory phase, the endometrium becomes 6- to 7-mm thick.

A lesion of the spinal canal that compresses the ventral commissure (syringomyelia) would interrupt ascending fibers crossing there but would not interfere with already crossed fibers ascending in the lateral spinothalamic tracts. Pain and temperature sensation above and below the level of the cord lesion would be preserved. The cell bodies of first-order afferent (sensory) neurons are located in the dorsal root ganglia. Their central processes enter the spinal cord and ascend one segment before synapsing with a second-order neuron in the dorsal horn. The central processes of second-order neurons cross in the ventral white commissure to the opposite side of the cord and ascend in the lateral spinothalamic tract to the ventral posterior lateral nucleus of the thalamus where they synapse with third-order neurons, which relay the message to cortical neurons of the postcentral gyrus of the parietal lobe. Lesions occurring unilaterally in a peripheral nerve would result in an ipsilateral deficit, whereas lesions in a crossed ascending pathway, in the thalamus, or in the cortex would result in contralateral deficits.

Infectious mononucleosis is a virus-induced illness leading to swollen lymph nodes and spleen. The splenomegaly is evidenced by the very rounded contours of the organ. Infectious mononucleosis can exhibit liver involvement; however, the organ indicated is not the liver but the spleen in the upper left hypochondrium. The bright organ between it and the vertebra is the left kidney. The liver is on the opposite side of the abdominal cavity.

The middle cerebral artery supplies a large portion of cerebral cortex, including portions of the frontal, parietal, and temporal lobes. These regions include the Broca's and Wernicke's areas and the precentral motor and postcentral sensory regions. Decreased blood flow in these regions explains the observed motor and sensory deficits. The anterior choroidal artery is a branch of the internal carotid artery and is primarily distributed to the basal ganglia, hippocampus, and choroid plexus of the lateral ventricle. The posterior communicating artery connects the internal carotid and vertebral arterial systems. The ophthalmic artery is a direct branch of the internal carotid artery that enters the orbit along with the optic nerve. Although the anterior cerebral artery has a wide distribution and anastomoses with branches of both the middle and posterior cerebral arteries, it primarily supplies medial and superior portions of the cortex.

With the exception of the anterior surface of the right ventricle, blood returning from the coronary circulation collects in the coronary sinus, which, in turn, empties directly into the right atrium. During right ventricular systole, venous blood is stored in the right atrium, and during ventricular diastole, blood flows from the right atrium to the right ventricle.

Blockage of the foregut in the newborn produces projectile vomiting. Congenital hypertrophic pyloric stenosis, occurring in 0.5 to 1.0% of males and rarely in females, involves hypertrophy of the circular layer of muscle at the pylorus. This usually does not regress and must be treated surgically. During the fifth and sixth weeks of development, the lumen of the duodenum is occluded by muscle proliferation but normally recanalizes during the eighth week. Failure of recanalization results in duodenal atresia. Because this occurs distal to the hepatopancreatic ampulla, the vomitus will occasionally be stained with bile. Annular pancreas, rare in itself, seldom completely blocks the duodenum. Imperforate anus results in intestinal distention with bloating.

The arrow indicates the cerebral aqueduct, which is the narrow canal connecting the third and fourth ventricles. The cerebrospinal fluid produced in the lateral and third ventricles must reach the fourth ventricle to escape into the subarachnoid space through the foramina of Luschka and Magendie.

The collecting ducts are found in both the cortex and medulla of the kidney. Cortical collecting ducts are found in the medullary rays, whereas medullary collecting ducts are found in the medulla and lead into the papillary duct. The convoluted portions of the proximal and distal tubules are found exclusively in the cortex. Afferent arterioles are found adjacent to the vascular pole of the glomeruli within the cortex. Only the thin loops of Henle are found exclusively in the medulla.

Atrophy of the intrinsic musculature of the tongue on one side is due to a lesion of the ipsilateral hypoglossal nerve. Deviation of the tongue to the right on protrusion results from the unopposed action of the left genioglossus muscle, which is innervated by the left hypoglossal nerve. The hypoglossal nerve also innervates numerous other tongue muscles involved in deglutition.

Retinal detachment is the result of the accumulation of fluid between the retina and the RPE. In one type of detachment, rhegmatogenous retinal detachment, fluid accumulates after a break occurs in the retina. Detachments without breaks in the retina are called nonrhegmatogenous, or serous, detachments. Vitreous degeneration usually is a prerequisite for retinal detachment that results in the breaking of the retina. The breakdown of the vitreous produces traction on the retina, which may already possess an inherent area of weakness. The site of retinal detachment is the area between the inner and outer layers of the embryonic optic cup and represents a relatively weak area of adherence between the retinal and RPE layers.

Radial, or refractive, keratotomy decreases the refraction of light by the cornea. It alters corneal (not lens) anatomy to create a new shape that is more flattened in the center and higher at the periphery of the cornea. This occurs because intraocular pressure will cause a reshaping of the cornea due to induced weakness produced by the incisions. The purpose of the procedure is to reduce myopia to eliminate the need for corrective lenses. The reduction in curvature of the central portion of the cornea results in decreased refractive power of the cornea. The slits are made with a laser and the degree of correction required is estimated by computer simulations.

Retinopathy is one of the major complications of diabetes mellitus. In diabetic retinopathy pathologic changes usually begin with thickening of the basement membrane of small retinal vessels. The abnormal vessels develop microaneurysms, which leak and hemorrhage with resultant ischemia of the retinal tissue. New vessels proliferate in response to ischemia and production of angiogenic factors. Loss of phagocytic capacity of the RPE occurs in retinal dystrophy but is not a characteristic of diabetic retinopathy. Retinopathy also occurs with prematurity when retinal vascularization is disturbed

The semicircular canals, which extend from the utricle, contain the cristae ampullares and detect angular acceleration. The utricle represents the dorsal portion of the otocyst-derived inner ear; the saccule represents the ventral portion. Both the utricle and saccule contain maculae that detect linear acceleration. The maculae of the utricle and saccule are perpendicular to one another. These maculae contain type I and type II hair cells, which differ in their innervation. The hair cells have stereocilia and a kinocilium embedded in a membrane that contains otoconia (statoconia) composed of calcium carbonate. The stereocilia and kinocilia are embedded in the cupola, which does not contain the otoconia found in the maculae. The endolymph turns right when the head turns left and vice versa. Movement stimulates the stereocilia and induces depolarization. The interdental cells produce the tectorial membrane, which is essential for the development of the shearing force in the process of sound transduction in the organ of Corti. It detects sound vibration and is responsive to variation in the frequency of sound waves.

A mnemonic device for remembering the order in which the soft tissues overlie the cranium is SCALP: Skin, Connective tissue, Aponeurosis, Loose connective tissue, and Periosteum. The scalp proper is composed of the outer three layers, of which the connective tissue contains one of the richest cutaneous blood supplies of the body. The occipitofrontal muscle complex inserts into the epicranial aponeurosis, which forms the intermediate tendon of this digastric muscle. This structure, along with the underlying layer of loose connective tissue, accounts for the high degree of mobility of the scalp over the pericranium. If the aponeurosis is lacerated transversely, traction from the muscle bellies will cause considerable gaping of the wound. Secondary to trauma or infection, blood or pus may accumulate subjacent to the epicranial aponeurosis.

The nucleus ambiguus, along with special visceral efferent (SVE) components of CN IX, X, and XI, is a column of lower motor neurons that innervate muscles of the pharynx, larynx, and palate. Damage to this nucleus results in loss of the gag reflex, difficulty in swallowing, and hoarseness.

The lateral spinothalamic tract passes through the medulla and carries sensory information (pain and temperature) from the contralateral extremities and trunk. Similarly, the spinal tract of CN V carries pain and temperature sensation from the ipsilateral face. Descending sympathetic pathways course through the medulla to reach the intermediolateral cell column of the spinal gray matter. Damage to these fibers would result in loss of ability to dilate the pupil (meiosis), drooping eyelid (ptosis), and loss of sweating ipsilaterally (hemianhydrosis). Damage to nerve fibers passing to and from the cerebellum via the inferior cerebellar peduncle would result in intention tremor and lack of coordination.

In the family of conditions known as spina bifida, failure of the dural portions of the developing vertebrae may expose a portion of the spinal cord and its covering. This usually occurs near the caudal end of the neural tube. If there is no projection of the spinal cord or its covering through the bony defect, the condition is generally hidden (spina bifida occulta). However, it is termed spina bifida cystica when spinal material traverses the defect. In a meningocele, this is a saclike projection formed only by the meninges. If the projection contains neural material, it is a meningomyelocele. Rachischisis is an extreme example of spina bifida cystica in which the neural folds underlying the vertebral defect fail to fuse, leaving an exposed neural plate. Anencephaly occurs when the cranial neural tube fails to fuse, thus resulting in lack of formation of forebrain structures and a portion of the enclosing cranium. Hydrocephaly results from blockage of the narrow passageways between the ventricles or between the ventricles and the subarachnoid space. Resultant swelling of the ventricles compresses the brain against the cranial vault and may cause serious mental deficits.

The peritoneal sac extends through the spermatic cord as the processus vaginalis, which continues into the scrotum as the tunica vaginalis testis. After the neonatal period, the processus vaginalis loses its patency, and thereby separates the tunica vaginalis from the peritoneal cavity. The gubernaculum is a band of connective tissue that extends between the inferior pole of the testis to the scrotal fold and directs the descent of the testis into the scrotum. The cremaster muscle is derived from the internal oblique muscle of the abdominal wall.

The stylohyoid muscle inserts onto the lesser horn of the hyoid bone (both derivatives of the second branchial arch) and raises that bone during swallowing. The distal tendon of the stylohyoid muscle is split by the digastric muscle passing through its trochlea attached to the lesser horn. The styloglossus muscle acts to retract the tongue. The sphenomandibular ligament inserts onto the lingula of the mandibular foramen; the stylohyoid ligament inserts onto the lesser horn of the hyoid bone. The stylopharyngeus muscle inserts onto the thyroid cartilage and into the middle pharyngeal constrictor.

During the uterine cycle, menstruation is initiated by the necrosis of the stratum functionale through the action of the spiral arterioles. After four to five days, proliferation begins in the endometrium in response to estrogen secretion from the granulosa cells. If 3H-thymidine were injected into animals during this period and combined with immunocytochemical staining for antiestrogen receptor, the result would be colocalization of autoradiographic grains and immunocytochemical product over the uterine epithelial and stromal cells as well as vascular endothelial cells. The peak of proliferative activity and estrogen sensitivity would occur after menses and during the proliferative phase. Maintenance of cell proliferation requires continued secretion of estrogen. As progesterone secretion increases, the secretory phase of uterine maturation occurs. The involution of the corpus luteum initiates the degeneration of the endometrial glands, which precedes menses.

The male pelvis is generally heavier than the female pelvis, with stronger bone structure and more definitive muscle markings, which reflect the larger male musculature and generally heavier male build. The generally wider and shallower female pelvis is more suited to childbearing. In the female the false pelvis tends to be shallower with flared ilia, the pelvic inlet more oval, and the pelvic outlet larger than in the male. Also, the subpubic angle between inferior pubic rami is significantly greater in the female than in the male and is perhaps the best identifying feature of the female pelvis.

The vestibular membrane, also known as Reissner's membrane, maintains the gradient between the endolymph of the scala media and the perilymph of the vestibular cavity. The middle ear contains the auditory ossicles, which transmit sound to the oval window and, therefore, serve in the transduction of sound waves to the perilymph. The helicotrema represents the opening that allows communication of the tympanic and vestibular cavities. The epithelium possesses extensive occluding junctions, which serve to maintain the concentration gradient that is essential for sensory transduction. The movement of the middle ear bones is dampened by the stapedius and tensor tympani when an individual is exposed to a loud noise.

The superior ophthalmic vein drains the region of the paranasal sinuses and is directly connected with the cavernous sinus although blood flow is normally away from the brain. The pterygoid venous plexus communicates with the cavernous sinus via the petrosal sinuses. The frontal emissary vein communicates with the superior sagittal sinus via the foramen cecum. The basilar venous plexus communicates with the inferior petrosal sinus. The parietal emissary vein also communicates with the superior sagittal sinus.

The symptoms indicate that the lesion is at the level of the midbrain. The spastic paralysis, hyperreflexia, and positive Babinski reflect an upper motor neuron lesion. The corticobulbar and corticospinal tracts pass through the cerebral peduncles (basis pedunculi). Those originating in the right cortex will pass through the right peduncle and then cross to the contralateral side in the pyramidal decussation resulting in left-side hemiplegia. It is of interest that the lower motor neurons innervating muscles of facial expression located below the eye receive upper motor neurons (corticobulbar tract) only from the contralateral cortex, whereas lower motor neurons innervating facial muscles above the eye (e.g., frontalis) receive input from both sides of the cortex. This explains why only the lower portion of the left face was paralyzed. The deficit in movement of the right eye indicates damage to the ipsilateral oculomotor nerve (CN III), which passes through the cerebral peduncle en route to the interpeduncular fossa. The "down and out" direction of the right eye is explained by unopposed contraction of the lateral rectus (CN VI) and superior oblique (CN IV) muscles. Because there were no sensory deficits, neither the thalamus nor sensory cortex were involved. The sensory tracts are arranged dorsolaterally in the midbrain and do not pass through the affected area.

The obturator nerve runs on the lateral surface of the pelvic wall and exits the pelvis through a canal in the obturator membrane. It innervates the adductor muscles of the medial thigh (a. magnus, a. longus, a. brevis, pectineus, and gracilis). The pudendal nerve exits the pelvic cavity via the greater sciatic foramen and enters the perineum via the lesser sciatic foramen. It innervates the perineal muscles and the skin of the genitalia. The genitofemoral nerve supplies skin over the femoral triangle and scrotum/labia majora. The superior gluteal nerve innervates the gluteus medius and gluteus minimus, which are powerful abductors of the thigh. The femoral nerve innervates the quadriceps muscles, which extend the leg at the knee joint.

Information from baroreceptors in the vascular wall passes via cardiac depressor nerves to the glossopharyngeal nerve (CN IX). The cell bodies of these neurons are located in the petrosal ganglion, and their central processes terminate on second-order interneurons in the nucleus of the solitary tract. These interneurons project to preganglionic parasympathetic cardioinhibitory neurons in the nucleus ambiguus, which reach ganglia on the surface of the heart via CN X (vagus nerve) and branches of the cardiac plexus. CN V (trigeminal) carries general somatic afferents from the face and innervates muscles involved in mastication. CN VII (facial) innervates the muscles of facial expression. CN XI primarily innervates the trapezius and sternocleidomastoid muscles.

The ilioinguinal nerve innervates the portion of the internal oblique muscle inserting in the lateral border of the conjoint tendon. Paralysis of these fibers would create weakness in the conjoint tendon, allowing herniation to occur medial to the inferior epigastric vessels. The genitofemoral nerve supplies sensory innervation to the skin of the femoral triangle and scrotum/labia majora. The subcostal nerve (T12) supplies lower portions of the external abdominal oblique muscle. The pelvic splanchnic nerves supply autonomic (parasympathetic) innervation to the pelvic viscera. The tenth thoracic spinal nerve (T10) supplies abdominal muscles superior to the inguinal region.

The splenic artery originates from the celiac trunk and courses tortuously along the posterior aspect of the pancreas. The left gastric artery is a separate branch of the celiac trunk and courses along the lesser curvature of the stomach where it anastomoses with the right gastric artery, a branch of hepatic artery. The right gastro-omental (gastroepiploic) artery is a branch of the gastroduodenal artery and courses along the greater curvature of the stomach.

The cell labeled in the transmission electron micrograph of the renal corpuscle is a mesangial cell that synthesizes extracellular matrix including the basement membrane of the glomerulus for the support of the capillary wall. Mesangial cells are morphologically similar to pericytes found in association with other systemic blood vessels. The mesangial cells surround glomerular capillaries as illustrated in this electron micrograph. Other proposed functions for the mesangial cells include phagocytosis and regulation of glomerular blood flow. A podocyte with its processes in close association with the glomerular capillary is observed below the mesangial cell. The podocytes form the visceral layer of the Bowman's capsule and possess processes that interdigitate to form the pedicels. The outer layer of the Bowman's capsule is formed by parietal cells, one of which is located in the lower left corner of the micrograph. An endothelial cell within a glomerular capillary is also shown below the mesangial cell. Endothelial cells synthesize vasoactive substances (e.g., endothelin) and are antithrombogenic. The urinary space and the blood are separated by the glomerular basement membrane formed from the fusion of the capillary and podocyte-produced basal laminae.

The vertebral arteries usually arise from the subclavian arteries and ascend through the transverse foramina of the sixth to the first cervical vertebrae but not the seventh. They enter the cranium through the foramen magnum after which they join to form the basilar artery. The basilar artery terminates by bifurcating into the posterior cerebral arteries. The hypoglossal nerves (CN XII) leave the cranium via the anterior condylar (hypoglossal) canals, whereas the posterior condylar canals transmit emissary veins.

The muscle indicated is attached to the pubic bone and extends around the rectum. It is the puborectalis portion of the levator ani (pelvic diaphragm). The puborectalis is responsible for fecal continence. The urogenital diaphragm is positioned inferior to the pelvic diaphragm and includes the deep transverse perineus muscle. The obturator internus covers the lateral wall of the lesser pelvis. The obturator externus is found in the deep thigh.

An MRI (magnetic resonance imaging) revealed a hemorrhage in the right subthalamic nucleus that is interconnected with the basal ganglia. The basal ganglia participate in feedback regulation of the motor system. The basal ganglia receive information from all regions of the cortex and project back to premotor and association cortex of the frontal lobe via the ventrolateral and ventroanterior thalamic nuclei. A direct pathway facilitates or excites cortical motor activity, whereas an indirect pathway inhibits cortical motor activity. Damage to the subthalamic nucleus decreases the inhibition of excitatory relay neurons in the thalamus, thus increasing the activation of cortical motor neurons, resulting in a characteristic form of jerky movement called ballism. Damage to an appropriate region of the cerebellum would result in loss of fine motor control but not in ballism. Damage to the neurons in the ventral horn of a specific level of the spinal cord would result in a localized flaccid paralysis. The hypothalamus is involved in vegetative functions, not voluntary movement. Damage to a localized portion of primary sensory cortex could not be expected to result in feedback deficits over both upper and lower extremities without other neurologic deficits.

The location of the glossopharyngeal nerve in the tonsillar bed places it in jeopardy during tonsillectomy. The facial nerve lies superficial on the face, whereas the hypoglossal and lingual nerves pass well inferior to the tonsillar bed.

Branches of the arteries of the sixth aortic arches form the pulmonary arteries. In addition, the left sixth arch artery forms the ductus arteriosus. The blood supply to the right side of the heart is primarily derived from the right and left coronary arteries and are derived from the truncus arteriosus. The face and thyroid gland receive blood primarily from the facial and superior thyroid arteries, respectively. These are branches of the common and external carotid arteries which, in turn, are derivatives of the second and third aortic arch arteries. The upper digestive tract is supplied by the celiac and superior mesenteric arteries, derivatives of the vitelline arteries.

The confluence of sinuses is formed by the superior sagittal sinus, both transverse sinuses, the occipital sinus, and the straight sinus. The inferior sagittal sinus and the great cerebral vein join to form the straight sinus. The superior and inferior petrosal sinuses both drain the cavernous sinuses, the former connecting with the ipsilateral transverse sinus and the latter with the origin of the internal jugular vein.

Malrotation by failure of either the ascending or descending portions of the abdominal wall to fixate to the posterior peritoneal wall and become secondarily retroperitoneal increases the probability of the bowel twisting about itself (volvulus). This not only produces intestinal obstruction but also may strangulate the vascular supply with intestinal ischemia and life-threatening intestinal infarct. Situs inversus (normal, but reversed) does not predispose to volvulus. The transverse and sigmoid colons are both supported by mesenteries of variable length. A retrocecal appendix is very common.

The omental bursa (lesser sac) is the remnant of the right coelomic cavity, which, owing to rotation of the gut and differential growth of the liver, lies behind the stomach. A posterior gastric perforation or an inflamed pancreas could lead to abscess formation in the lesser sac. The right subhepatic space might become secondarily involved via communication through the omental foramen (of Winslow). The pouch of Morison, which is the combined right subhepatic and the hepatorenal spaces, may be the seat of abscess formation related to gallbladder disease or perforation of a duodenal ulcer. The right subphrenic space is between the liver and the diaphragm and communicates with the pouch of Morison. All these spaces are in communication with the greater sac of the peritoneal cavity.

The kidney forms in three stages. The pronephric, metanephric, and mesonephric kidneys all form from the urogenital ridge, an extension of intermediate mesoderm into the coelomic cavity. Mesoderm derived from the somites (somitic) gives rise to components of the axial skeleton and associated muscle and connective tissues. Splanchnic lateral plate mesoderm gives rise to the smooth muscle and connective tissue tunics of the abdominal viscera. Somatic lateral plate mesoderm contributes substantially to the skeleton, connective tissue, and muscle mass of the appendages.

The posterior cricoarytenoid muscles rotate the arytenoids laterally, which swings the vocal process of that cartilage outward to abduct the vocal cords and open the glottis. These are the sole abductors of the vocal folds. The lateral cricoarytenoid muscles and the unpaired transverse arytenoid muscle adduct the vocal folds. The thyroarytenoid muscle and its innermost portion, the vocalis muscle, act to tense the cords. The cricothyroid muscle lengthens the vocal cords.

Shearing of the hair cell stereocilia against the tectorial membrane results in depolarization, and the release of neurotransmitter onto afferent endings of the auditory cranial nerve leads to initiation of an action potential. Sound waves are directed toward the tympanic membrane by the pinna and the external auditory canal of the external ear. The vibration of the tympanic membrane is transmitted to the oval window by way of the ossicles of the middle ear. Induction of waves in the perilymph results in the movement of the basilar and vestibular membranes toward the scala tympani and causes the round window to bulge outward. The movement of the hair cells is facilitated because the tectorial membrane is rigid and the pillar cells form a pivot. The stabilization of the pressure between the middle ear and the nasopharynx is not directly related to the mechanism of sound transmission.

The parietal pleura associated with the cupola extends superior to the first rib into the cervical region and may drain directly into axillary lymph nodes. Lymph from the visceral pleura and parenchyma of the right lung drains via superficial and deep plexuses, pulmonary nodes, bronchopulmonary nodes, and tracheobronchial nodes, to the right bronchomediastinal trunk, which may terminate in a short lymphatic duct before reentering the venous system at the juncture of the right subclavian and jugular veins. Lymph from the visceral pleura and superior lobe of the left lung follows a similar pathway on the left, terminating in the thoracic duct. Lymph from the visceral pleura and parenchyma of the inferior lobe of the left lung may enter the right-side pathway at the right tracheobronchial nodes. Lymph from most of the parietal pleura drains via intercostal, parasternal, mediastinal, and phrenic nodes of the thoracic wall.

The inferior thyroid artery arises from the thyrocervical trunk, a branch of the subclavian artery. The superior thyroid artery arises from the external carotid artery. An inconsistent thyroid ima artery, when present, may arise from the aortic arch, the innominate artery, or the common carotid artery. There are no branches of the internal carotid artery and infrequent branches of the common carotid artery in the neck. The transverse cervical artery supplies the posterior triangle of the neck. The vertebral arteries give off spinal and muscular branches in the neck

The puborectalis, pubococcygeus, and iliococcygeus comprise the levator ani, the main muscular component of the pelvic floor. The pubococcygeus is the part of the levator ani most frequently damaged during parturition. Because the pubococcygeus surrounds and supports the neck of the bladder and the proximal urethra, urinary leakage is a common result, particularly during increased abdominopelvic pressure, e.g., during coughing. Damage to the puborectalis would result in fecal incontinence under similar situations. Both the obturator internus muscle and the piriformis are parts of the lateral wall of the pelvis and assist in lateral rotation of the thigh.

The scala tympani contains perilymph. Endolymph is similar to intracellular fluid (high K+, low Na+). It is found in the utricle, saccule, semicircular canals, and scala media (cochlear duct), which are parts of the membranous labyrinth. Endolymph is synthesized by the highly vascular stria vascularis in the lateral wall of the scala media. The endolymphatic sac and duct are responsible for absorption of endolymph and the endocytosis of molecules from the endolymph.

The maxillary division of the trigeminal nerve (CN V2), which passes through the foramen rotundum, is entirely sensory. Damage to this nerve results in sensory deprivation over the maxillary region of the face and loss of the sneeze reflex. The mandibular division of the trigeminal nerve, which passes through the foramen ovale, innervates the masticatory muscles responsible for clenching the jaw as well as the tensor palatini muscle, which assists in the establishment of the velopharyngeal seal. The other muscles of the soft palate are innervated by the pharyngeal branch of the vagus nerve (CN X), which transits the jugular foramen. The orbicularis oris and buccinator muscles are innervated by the facial nerve (CN VII), which transits the stylomastoid foramen. The inferior oblique muscle of the eye is innervated by the inferior branch of the oculomotor nerve, which enters the orbit through the superior orbital fissure.

After closure of the neural tube, cells proliferate and establish three primitive layers: (1) the ventricular zone adjoining the central canal and ventricles; mitoses of neuronal and glial precursors continue in this zone; (2) a mantle zone consisting of cell bodies of neurons and glia that have migrated out of the ventricular zone; and (3) a marginal zone on the periphery containing the myelinated nerve processes characteristic of white matter. Closure of the neural tube begins near the midpoint of its length and proceeds in both directions simultaneously. The neurectoderm of the neural tube will give rise to neurons and some glial cells (astrocytes, oligodendroglia, and ependymal cells), but the precursors of microglia (the monocyte-macrophage lineage) migrate into the nervous system from the blood. The sensory ganglia are formed by neural crest cells that migrated before the development of mature neurons

Because the superior segmental bronchi of the left and right lower lobes are the most posterior and, therefore, most dependent when the patient is supine, they are most frequently involved in aspiration pneumonia. This is easily ascertained by percussive dullness over the superior segment posteriorly, and diminished breath sounds over the same area on auscultation.

The lobe indicated by the * is the left upper (or superior) lobe. The general orientation when viewing CTs is that the observer is looking up from the patient's feet. Therefore, the patient's left is on your right. In addition, on the left, the inferior (lower) lobe begins relatively high in the thoracic cavity and is posterior to the upper lobe.

The patient has facial paralysis, which indicates injury to the facial nerve. A problem in the internal auditory meatus usually affects hearing and balance. That the superior salivatory nucleus is normal is indicated by normal lacrimation. Hence, the lesion must be distal to the origin of the greater superficial nerve at the genu of the facial nerve. However, absence of hyperacusis indicates that the branch to the stapedius muscle is functioning normally, and this fact suggests that the lesion is close to the stylomastoid foramen. Loss of taste and diminished salivation locate the lesion proximal to the origin of the chorda tympani nerve. If the lesion were distal to the stylomastoid foramen, taste and salivation would have been normal with facial paralysis as the only sign.

Sympathetic outflow occurs between the first thoracic and second lumbar segments of the spinal cord. The presynaptic sympathetic neurons originate in the lateral cell columns of the spinal cord and pass along the thoracic and upper lumbar spinal nerves. These myelinated neurons leave the respective spinal nerves by white rami communicantes, which convey the sympathetic neurons to the chain of sympathetic ganglia. Some of the sympathetic neurons pass through the chain as splanchnic nerves to reach prevertebral ganglia associated with the viscera. Others may ascend in the cervical portion of the chain or descend to the lower lumbar or sacral portions before synapse occurs. Visceral afferent neurons conveying sensation from the viscera reach the sympathetic chain via the splanchnic nerves and then continue along the white rami communicantes to reach the spinal nerves.

The pterygomandibular raphe extends from the hamulus of the medial pterygoid plate to the mandible and marks the junction of the buccinator and the superior constrictor muscles. The buccinator, a muscle of facial expression, is innervated by the facial nerve (CN VII). The superior constrictor is innervated by the pharyngeal branch of the vagus nerve (CN X).