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Aqueous humor also Puncture wounds to the eyeball are especially dangerous and provides nutrients and oxygen to the avascular lens and cornea best tadacip 20mg erectile dysfunction doctor in pakistan. Protective equipment such as goggles buy tadacip 20 mg otc erectile dysfunction emotional, shields, and shatterproof lenses should be used in haz- An estimated 5. If the eye is punctured, the the vascular epithelium of the ciliary body (fig. From its main thing to remember is to leave the object in place if it is still im- site of secretion within the posterior chamber, the aqueous paling the eyeball. Removal may allow the fluids to drain from the humor passes through the pupil into the anterior chamber. From eyeball, causing loss of intraocular pressure, a detached retina, and possibly blindness. Sensory Organs © The McGraw−Hill Anatomy, Sixth Edition Coordination Companies, 2001 Chapter 15 Sensory Organs 509 Postganglionic sympathetic axon IN DIM LIGHT From superior cervical ganglion Pupillary dilator muscle (radially arranged smooth muscle fibers of the iris) Pupillary constrictor muscle (circularly arranged smooth muscle fibers of the iris) IN NORMAL LIGHT Pupil Postganglionic parasympathetic axon Ciliary ganglion From oculomotor IN BRIGHT LIGHT nerve FIGURE 15. In dim light, the radially arranged smooth muscle fibers are stimulated to contract by sympathetic stimulation, dilating the pupil. In bright light, the circularly arranged smooth muscle fibers are stimulated to contract by parasympa- thetic stimulation, constricting the pupil. Function of the Eyeball Transmission of Light Rays The focusing of light rays and stimulation of photoreceptors of Light rays entering the eyeball pass through four transparent media the retina require five basic processes: before they stimulate the photoreceptors. In sequence, the media through which light rays pass are the cornea, aqueous humor, lens, 1. The cornea and lens are solid media composed eyeball; of tightly packed, avascular protein fibers. Refraction occurs as light Visual impairment may result if one or more of these rays pass at an oblique angle from a medium of one optical den- processes does not function properly (see Clinical Considerations). Sensory Organs © The McGraw−Hill Anatomy, Sixth Edition Coordination Companies, 2001 510 Unit 5 Integration and Coordination (a) Fibers of the optic nerve Ganglion neurons FIGURE 15. Bipolar neurons Photoreceptor neurons cortex of the occipital lobe, where the inverted image is inter- preted as right side up. Pigmented layer Choroid layer Accommodation of the Lens Accommodation is the automatic adjustment of the curvature of Sclera the lens by contraction of ciliary muscles to bring light rays into sharp focus on the retina. Contraction of the smooth muscle that light must pass through various layers of nerve cells before fibers of the ciliary body causes the suspensory ligament to relax reaching the photoreceptors (rod cells and cone cells). The lens is particularly important for refining and altering refraction. Of the refractive Constriction of the pupil occurs through parasympathetic stimula- media, only the lens can be altered in shape to achieve precise tion that causes the pupillary constrictor muscles of the iris to con- refraction. Pupillary constriction is important for two The refraction of light rays is so extensive that the visual reasons. One is that it reduces the amount of light that enters the image is formed upside down on the retina (fig. A reflexive constriction of the pupil protects the impulses of the image in this position are relayed to the visual retina from sudden or intense bright light. Sensory Organs © The McGraw−Hill Anatomy, Sixth Edition Coordination Companies, 2001 Chapter 15 Sensory Organs 511 Macula lutea Fovea centralis Optic disc Choroidal vessels (a) (b) FIGURE 15. Optic nerve fibers leave the eyeball at the optic disc to form the optic nerve. Hold the drawing about 20 inches from your face with your left eye closed and your right eye focused on the circle. Slowly move the drawing closer to your face, and at a certain point the cross will disappear. This occurs because the image of the cross is focused on the optic disc, where photorecep- tors are absent. Amblyopia (am'ble-o pe-a˘) ex anopsia, commonly called “lazy eye,” is a condition of extrinsic ocular muscle weakness. This causes a deviation of one eye, so that there is not a concurrent con- vergence of both eyeballs. With this condition, two images are re- ceived by the optic portion of the cerebral cortex—one of which is FIGURE 15. A person who has amblyopia will experience dimness of vi- The optic disc is frequently called the blind spot because it is devoid sion and partial loss of sight.

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Jaramillo D buy 20 mg tadacip with visa erectile dysfunction treatment aids, Hoffer F order tadacip 20mg with amex erectile dysfunction injections treatment, Shapiro F, Rand F (1990) MR imaging CT of developmental dysplasia of the hip after reduction: di- of fractures of the growth plate. Wechsler RJ, Schwertzer ME, Deely DM, Horn BD, Pizzutillo growth plate: normal and abnormal MR imaging findings. PD (1994) Tarsal coalition: depiction and characterization with AJR Am J Roentgenol 158:1105-1111 CT and MR imaging. Leonard MA (1974) The inheritance of tarsal coalition and its AG, Strongwater A (1994) Pediatric elbow fractures: MR eval- relationship to spastic flat foot. Emery KH, Bisset GS 3rd, Johnson ND, Nunan PJ (1998) magnetic resonance imaging of the paediatric elbow. Suzuki S, Kashiwagi N, Seto Y, Mukai S (1999) Location of Chronically stressed wrists in adolescent gymnasts: MR imag- the femoral head in developmental dysplasia of the hip: three- ing appearance. Radiology 195:855-859 dimensional evaluation by means of magnetic resonance im- 49. J Pediatr Orthop 19(1):88-91 (1997) Chronic physeal fractures in myelodysplasia: magnetic 28. McNally EG, Tasker A, Benson MK (1997) MRI after opera- resonance analysis, histologic description, treatment, and out- tive reduction for developmental dysplasia of the hip. Kashiwagi N, Suzuki S, Kasahara Y, Seto Y (1996) Prediction Radiology 191:297-308 of reduction in developmental dysplasia of the hip by magnet- 51. Havranek P, Lizler J (1991) Magnetic resonance imaging in the ic resonance imaging. J Pediatr Orthop 16(2):254-258 evaluation of partial growth arrest after physeal injuries in 30. J Bone Joint Surg Am 73:1234-1241 dysplasia of the hip: three-dimensional evaluation by means 52. Disler DG (1997) Fat-suppressed three-dimensional spoiled gra- of magnetic resonance image. J Pediatr Orthop 15(6):812- dient-recalled MR imaging: assessment of articular and physeal 816 hyaline cartilage. Borsa JJ, Peterson HA, Ehman RL (1996) MR imaging of phy- BJ, Mulkern RV et al (1996) Gadolinium-enhanced MR imag- seal bars. Radiology 199:683-687 ing demonstrates abduction-caused hip ischemia and its rever- 54. AJR Am J Roentgenol 166:879-887 Pediatric knee MR imaging: pattern of injuries in the imma- 32. Jaramillo D, Villegas-Medina O, Laor T, Shapiro F, Millis MB ture skeleton. Radiology 190:397-401 (1998) Gadolinium-enhanced MR imaging of pediatric pa- 55. Fletcher BD (1991) Response of osteosarcoma and Ewing sar- tients after reduction of dysplastic hips: assessment of femoral coma to chemotherapy: imaging evaluation. AJR Am J head position, factors impeding reduction, and femoral head Roentgenol 157(4):825-833 ischemia. Bos CF, Bloem JL, Bloem RM (1991) Sequential magnetic loskeletal magnetic resonance imaging: how we do it. Sebag G, Ducou Le Pointe H, Klein I, Maiza D, Mazda K, formity secondary to brachial plexus birth palsy. J Bone Joint Bensahel H et al (1997) Dynamic gadolinium-enhanced sub- Surg Am 80(5):668-677 traction MR imaging–a simple technique for the early diagno- 58. Ruby L, Mital MA, O’Connor J, Patel U (1979) Anteversion sis of Legg-Calve-Perthes disease: preliminary results. Zurakowski D (1995) Cartilaginous abnormalities and growth Clin Orthop 120:159-163 disturbances in Legg-Calvé-Perthes disease: evaluation with 60. In: Rumack C, M, Montagne JP (1994) Legg-Perthes-Calve disease: staging Wilson S, Charboneau J (eds) Diagnostic ultrasound. Marks DS, Clegg J, al-Chalabi AN (1994) Routine ultrasound Clin N Am 6(3):627-641 screening for neonatal hip instability.

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The mode in which action because the K channels open more slowly in response to potentials propagate and the speed with which they are depolarization 20mg tadacip for sale impotence quiz. This increase in Na permeability com- conducted along an axon depend on whether the axon is pared to that of K causes the membrane potential to move myelinated discount tadacip 20 mg visa erectile dysfunction protocol formula. The diameter of the axon also influences the toward the equilibrium potential for Na. The net effect of inactivating channels are distributed uniformly along the length of the Na channels and opening additional K channels is the axonal membrane. An action potential is generated when repolarization of the membrane (Fig. The hillock acts as a “sink” where Na afterhyperpolarization is a result of K channels remaining ions enter the cell. The “source” of these Na ions is the ex- open, allowing the continued efflux of K ions. The entry of way to think about afterhyperpolarization is that the mem- Na ions into the axon hillock causes the adjacent region brane’s permeability to K is higher than when the neuron of the axon to depolarize as the ions that entered the cell, is at rest. Consequently, the membrane potential is driven during the peak of the action potential, flow away from the even more toward the K equilibrium potential (Fig. This local spread of the current depolarizes the adja- The changes in membrane potential during an action cent region to threshold and causes an action potential in potential result from selective alterations in membrane that region. These membrane conductance of the axon, the action potential propagates or moves along changes reflect the summated activity of individual volt- the length of the axon from point to point, like a traveling age-gated sodium and potassium ion channels. This in- phases to both the decrease in sodium conductance and the crease in ion flow in the cytoplasm causes greater lengths increase in potassium conductance, and afterhyperpolariza- of the axon to be depolarized, decreasing the time needed tion to the sustained increase of potassium conductance. Recall CHAPTER 3 The Action Potential, Synaptic Transmission, and Maintenance of Nerve Function 43 +50 Depolarizing Repolarizing phase phase 0 E (mV) B C m Resting Resting state Afterhyper- state -50 A polarization A D -100 Time Voltage-gated Na+ Channel Voltage-gated K+ Channel Na+ A Resting state Resting state K+ Na+ B Active state Resting state FIGURE 3. A, At the resting membrane potential, both channels are in a closed, resting state. B, Dur- ing the depolarizing phase of the K+ action potential the voltage-gated sodium channels are activated Na+ (open), but the potassium channels C Inactive state Active state open more slowly and, therefore, have not yet responded to the depo- larization. C, During the repolariz- ing phase, sodium channels become inactivated, while the potassium channels become activated (open). D, During the afterhyperpolariza- tion, the sodium channels are both closed and inactivated, and the K+ potassium channels remain in their Na+ active state. Eventually, the potas- Closed and sium channels close and the sodium D Active state inactive state channel inactivation is removed, so that both channels are in their rest- ing state and the membrane poten- tial returns to resting membrane po- tential. Note that the voltage-gated potassium channel does not have an inactivated state. In this case, the local stimulus is the inward cause the internal axoplasmic resistance, Ra, decreases, al- sodium current that accompanies the action potential. The lowing the current to spread farther down the inside of the larger the space constant, the farther along the membrane axon before leaking back across the membrane. Affected individuals generally have no neurons and muscle cells and in synaptic transmission. The defect in membrane repolar- nel properties derived from biophysical studies of isolated ization could be a result of a prolonged inward sodium cells or their membranes. The advent of molecular ap- current or a reduced outward potassium current. In fact, proaches resulted in the cloning of the genes for a variety mutations in potassium channels account for two differ- of channels and the subsequent expression of these genes ent LQT syndromes, and a third derives from a sodium in a large cell, such as the Xenopus oocyte, for further char- channel mutation. Myotonia is a condition characterized by a delayed re- This approach also allowed experimental manipulation laxation of muscle following contraction. There are several of the channels by expressing genes that were altered in types of myotonias, all related to abnormalities in muscle known ways. Some myotonias are associated with a skele- which parts of channel molecules were responsible for tal muscle sodium channel, and others are associated with particular properties, including voltage sensitivity, ion a skeletal muscle chloride channel. This genetic understand- and spinocerebellar ataxias, some forms of epilepsy, and ing of the control of channel properties led to the realiza- familial hemiplegic migraine.