L. Stejnar. School of the Museum of Fine Arts, Boston.
Dopa decarboxylase By contrast generic 10mg lotrisone mastercard, the cytoplasmic decarboxylation of dopa to dopamine by the enzyme dopa decarboxylase is about 100 times more rapid (K :461074 M) than its synthesis and m indeed it is difficult to detect endogenous dopa in the CNS. This enzyme, which requires pyridoxal phosphate (vitamin B6) as co-factor, can decarboxylate other amino acids (e. Controls of synthesis It is possible to deplete the brain of both DA and NA by inhibiting tyrosine hydroxylase but while NA may be reduced independently by inhibiting dopamine b- hydroxylase, the enzyme that converts DA to NA, there is no way of specifically losing DA other than by destruction of its neurons (see below). In contrast, it is easier to augment DA than NA by giving the precursor dopa because of its rapid conversion to DA and the limit imposed on its further synthesis to NA by the restriction of dopamine b-hydroxylase to the vesicles of NA terminals. The activity of the rate-limiting enzyme tyrosine hydroxylase is controlled by the cytoplasmic concentration of DA (normal end-product inhibition), presynaptic dopamine autoreceptors (in addition to their effect on release) and impulse flow, which appears to increase the affinity of tyrosine hydroxylase for its tetrahydropteridine co-factor (see below). METABOLISM Just as the synthesis of DA and NA is similar so is their metabolism. They are both substrates for monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT). In the brain MAO is found in, or attached to, the membrane of the intraneuronal mitochondria. Thus it is only able to deaminate DA which has been taken up into nerve endings and blockade of DA uptake leads to a marked reduction in the level of its deaminated metabolites and in particular DOPAC. The final metabolite, homovanillic 142 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION acid (HVA), is one that has been both deaminated and O-methylated so it must be assumed that most of any released amine is initially taken back up into the nerve where it is deaminated and then subsequently O-methylated (Fig. Certainly the brain contains much more DOPAC (the deaminated metabolite of DA) than the corresponding O-methylated derivative (3-methoxytyramine). It is possible, however, that the high levels of DOPAC, as found particularly in rat brain, partly reflect intraneuronal metabolism of unreleased DA and it is by no means certain that the metabolism of DA to HVA is always initially to DOPAC.
The tail is continuous with the beginning por- tion of the ductus deferens; both store spermatozoa to be discharged during ejaculation lotrisone 10 mg on-line. The time required to produce ma- SPERMATIC DUCTS, ACCESSORY ture spermatozoa—from meiosis in the seminiferous tubules to storage in the ductus deferens—is approximately 2 months. From here, it penetrates the in- Objective 10 Describe the location and structure of each segment of the male duct system. The histological structure of the ductus deferens includes a Objective 12 Describe the location, structure, and functions layer of pseudostratified ciliated columnar epithelium in contact of the ejaculatory ducts, seminal vesicles, prostate, and with the tubular lumen and surrounded by three layers of tightly bulbourethral glands. Stimulation through these nerves causes peristaltic contractions of the muscular layer, Spermatic Ducts which propel the stored spermatozoa toward the ejaculatory duct. The spermatic ducts store spermatozoa and transport them from Much of the ductus deferens is located within a structure the testes to the urethra. The accessory reproductive glands pro- known as the spermatic cord (see figs. Male Reproductive © The McGraw−Hill Anatomy, Sixth Edition Development System Companies, 2001 708 Unit 7 Reproduction and Development Seminal crypt Smooth muscle FIGURE 20. Glandular epithelium plexus, nerves, the cremaster muscle, lymph vessels, and connec- tive tissue. The portion of the spermatic cord that passes anterior to the pubic bone can be easily palpated. This layer partitions the lumen pierces the capsule of the prostate on its posterior surface and into numerous intercommunicating spaces that are lined by pseu- continues through this gland (see fig. Both ejaculatory dostratified columnar and cuboidal secretory epithelia (referred ducts receive secretions from the seminal vesicles and then eject to as glandular epithelium). The seminal vesicles are innervated by shortly, serves as a passageway for both semen and urine. It nal vesicles, the prostate, and the bulbourethral glands (see is about 4 cm (1. The contents of the seminal vesicles and prostate are immediately below the urinary bladder, where it surrounds the mixed with the spermatozoa during ejaculation to form semen beginning portion of the urethra (see fig.
The muscle in the wall of the right atrium contracts at just the right time to help fill the IVC right ventricle order lotrisone 10mg line. Contractions of the right ventricle propel blood through the lungs where oxygen and carbon dioxide are exchanged in the pulmonary capillaries. Pressures are much lower in the pulmonary circulation than in the sys- temic circulation. Blood then flows via the pulmonary vein to the left atrium, which functions much like the right atrium. The thick muscular wall of the left ventricle devel- ops the high pressure necessary to drive blood around the systemic circulation. The mechanisms that regulate all of the above anatomic elements of the circulation are the subject of the next few chapters. In this chapter, we consider the physical princi- ples on which the study of the circulation is based. In con- trast, the circulations of the organs other than the lungs are in Hemodynamics is the branch of physiology concerned parallel; that is, each organ receives blood from the aorta and re- with the physical principles governing pressure, flow, re- turns it to the vena cava. Exceptions are the various “portal” circu- sistance, volume, and compliance as they relate to the car- lations, which include the liver, kidney tubules, and hypothala- diovascular system. SVC, superior vena cava; IVC, inferior vena cava; RA, right few chapters to explain the performance of each part of the atrium; RV, right ventricle; LA, left atrium; LV, left ventricle. Smooth muscle in the relatively thick walls Between Pressure and Flow of small arteries and arterioles can contract or relax, causing large changes in flow to a particular organ or tissue. Pressure is of their ability to adjust their caliber, small arteries and ar- force applied over a surface, such as the force applied to terioles are called resistance vessels. The prominent pres- the cross-sectional surface of a fluid at each end of a rigid sure pulsations in the aorta and large arteries are damped by tube.
In slow-twitch skeletal muscles lotrisone 10 mg without prescription, which DERMAL CIRCULATION can easily increase oxidative metabolic requirements by The Skin Has a Microvascular Anatomy to more than 10 to 20 times during heavy exercise, it is not hard Support Tissue Metabolism and Heat Dissipation to imagine that whatever causes metabolically linked vasodi- lation is in ample supply at high metabolic rates. The structure of the skin vasculature differs according to lo- During rhythmic muscle contractions, the blood flow cation in the body. In all areas, an arcade of arterioles exists during the relaxation phase can be high, and it is unlikely at the boundary of the dermis and the subcutaneous tissue that the muscle becomes significantly hypoxic during sub- over fatty tissues and skeletal muscles (Fig. Studies in humans and animals arteriolar arcade, arterioles ascend through the dermis into indicate that lactic acid formation, an indication of hypoxia the superficial layers of the dermis, adjacent to the epider- and anaerobic metabolism, is present only during the first mal layers. These arterioles form a second network in the several minutes of submaximal exercise. Once the vasodila- superficial dermal tissue and perfuse the extensive capillary tion and increased blood flow associated with exercise are loops that extend upward into the dermal papillae just be- established, after 1 to 2 minutes, the microvasculature is neath the epidermis. CHAPTER 17 Special Circulations 285 hands and feet and, to a lesser extent, the face, neck, and ears to lose heat efficiently in a warm environment. Skin Blood Flow Is Important in Body Temperature Regulation The skin is a large organ, representing 10 to 15% of to- tal body mass. The primary functions of the skin are pro- tection of the body from the external environment and dissipation or conservation of heat during body temper- ature regulation. The skin has one of the lowest metabolic rates in the body and requires relatively little blood flow for purely nu- tritive functions. Consequently, despite its large mass, its resting metabolism does not place a major flow demand on the cardiovascular system. However, in warm climates, body temperature regulation requires that warm blood from the body core be carried to the external surface, where heat transfer to the environment can occur.