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These cells are important and label the diaphysis discount erectafil 20 mg overnight delivery, medullary cavity order erectafil 20 mg otc, epiphyses, articu- in bone growth, remodeling, and healing. Bone-lining cells are de- lar cartilages, nutrient foramen, periosteum, and epiphyseal rived from osteoblasts along the surface of most bones in the adult plates. These cells are thought to regulate the movement of cal- cium and phosphate into and out of bone matrix. BONE TISSUE Spongy and Compact Bone Tissues Bone tissue is composed of several types of bone cells embedded As mentioned earlier, most bones contain both spongy and com- in a matrix of ground substance, inorganic salts (calcium and phos- pact bone tissues (fig. Bone cells and ground substance the compact bone tissue, and is quite porous. Minute spikes of give bone flexibility and strength; the inorganic salts give it hardness. Spongy bone is highly vascular and pro- Objective 6 Identify the five types of bone cells and list the vides great strength to bone with minimal weight. Compact bone tissue forms the external portion of a bone and is very hard and dense. It consists of precise arrange- Objective 7 Distinguish between spongy and compact ments of microscopic cylindrical structures oriented parallel to bone tissues. These cells re- spond to trauma, such as a fracture, by giving rise to bone-forming osteoblast: Gk. Skeletal System: © The McGraw−Hill Anatomy, Sixth Edition Introduction and the Axial Companies, 2001 Skeleton Chapter 6 Skeletal System: Introduction and the Axial Skeleton 139 External circumferential lamellae Venule Periosteum Arteriole Nerve Central canal Central Canaliculi canal Medullary Lamellae cavity (c) (a) Osteocyte Lacuna Perforating fibers Canaliculi Trabeculae of Central spongy bone Blood vessels Perforating canal (d) canals (b) FIGURE 6. LA CA Canaliculi Osteocyte within a lacuna Central canal Lamella CA LA (a) (b) FIGURE 6. The lacunae (LA) provides spaces for the osteocytes, which are connected to one another by canaliculi (CA). Skeletal System: © The McGraw−Hill Anatomy, Sixth Edition Introduction and the Axial Companies, 2001 Skeleton 140 Unit 4 Support and Movement (a) (b) (c) (d) (e) (f) FIGURE 6. Metabolic activity within bone tissue occurs at the osteon Bone growth is influenced by genetics, hormones, and nutrition. Between osteons there are incomplete remnants of os- teons, called interstitial systems. Perforating (Volkmann’s) Objective 8 Describe the process of endochondral canals penetrate compact bone, connecting osteons with blood ossification as related to bone growth. In most bone development,a cartilaginous model is gradually re- Knowledge Check placed by bone tissue during endochondral bone formation (see De- velopmental Exposition,pp. Construct a sample table listing the location and function grows,the chondrocytes (cartilage cells) in the center of the shaft of each type of cell found within bone tissue hypertrophy,and minerals are deposited within the matrix in a 10. Define osteon and sketch the arrangement of osteons process called calcification (fig. At the same time,some cells of the perichondrium (dense regular con- nective tissue surrounding cartilage) differentiate into osteoblasts. Skeletal System: © The McGraw−Hill Anatomy, Sixth Edition Introduction and the Axial Companies, 2001 Skeleton Developmental Exposition certain bones of the cranium are formed this way. Sesamoid bones The Axial Skeleton are specialized intramembranous bones that develop in tendons. EXPLANATION DEVELOPMENT OF THE SKULL Development of Bone The formation of the skull is a complex process that begins dur- Bone formation, or ossification, begins at about the fourth week of ing the fourth week of embryonic development and continues embryonic development, but ossification centers cannot be read- well beyond the birth of the baby. Three aspects of the embry- ily observed until about the tenth week (exhibit I). Bone tissue onic skull are involved in this process: the chondrocranium, the derives from specialized migratory cells of mesoderm (see neurocranium, and the viscerocranium (exhibit II). The viscerocranium (splanchnocranium) is the through a hyaline cartilage stage and then it is ossified as bone.
This depressed re- Several muscles are clearly visible in the brachium (figs 20 mg erectafil overnight delivery. The axilla is clinically important because of the prominent when the elbow is flexed cheap 20mg erectafil. While the arm is in this posi- subcutaneous position of vessels, nerves, and lymph nodes in tion,the deltoid muscle can be traced as it inserts on the humerus. Two muscles form the anterior and posterior bor- The triceps brachii muscle forms the bulk of the posterior surface ders (fig. A groove forms on the medial side of the pectoralis major muscle, and the posterior axillary fold consists brachium between the biceps brachii and triceps brachii muscles, primarily of the latissimus dorsi muscle as it extends from the where pulsations of the brachial artery may be felt as it carries lumbar vertebrae to the humerus. It is also the place to apply pressure in case mary gland, which is positioned on the pectoralis major mus- of severe arterial hemorrhage in the forearm or hand. The medial and lateral epicondyles are phatic drainage pathway is toward the axilla (see fig. When the elbow is extended, these promi- nences lie on the same transverse plane; when the elbow is flexed, they form a triangle. The ulnar nerve can be palpated in the ulnar sulcus (groove) posterior to the medial epicondyle (see fig. Surface and Regional © The McGraw−Hill Anatomy, Sixth Edition Anatomy Companies, 2001 320 Unit 4 Support and Movement Deltoid m. The cubital fossa is the depression on the anterior surface Antebrachium of the elbow region, where the median cubital vein links the Contained within the antebrachium (forearm) are two parallel cephalic and basilic veins. These veins are subcutaneous and be- bones (the ulna and radius) and the muscles that control the come more conspicuous when a proximal compression is applied. The muscles of the forearm taper dis- For this reason, they are an important location (particularly the tally over the wrist, where their tendons attach to various bones median cubital) for the removal of venous blood for analyses and of the hand. Several muscles of the forearm can be identified as transfusions or for intravenous therapy (fig. Surface and Regional © The McGraw−Hill Anatomy, Sixth Edition Anatomy Companies, 2001 Chapter 10 Surface and Regional Anatomy 321 Triceps brachii m. The ulna can be palpated along its entire length from Tendon of palmaris longus m. Nerves, tendons, and vessels are close to the surface at the Styloid process of ulna wrist, making cuts to this area potentially dangerous. Tendons Thenar eminence from four flexor muscles can be observed as surface features if Hypothenar eminence the anterior forearm muscles are strongly contracted while mak- ing a fist. The tendons that can be observed along this surface, from lateral to medial, are from the following muscles: flexor carpi radialis, palmaris longus, superficial digital flexor, and flexor carpi ulnaris. The median nerve going to the hand is located under the tendon of the palmaris longus muscle (see FIGURE 10. Surface and Regional © The McGraw−Hill Anatomy, Sixth Edition Anatomy Companies, 2001 322 Unit 4 Support and Movement Tendon of extensor Nail pollicis brevis m. Distal Styloid process interphalangeal of ulna joints Anatomical Proximal snuffbox interphalangeal Tendon of extensor joints pollicis longus m. The radial artery lies along the Hand surface of the radius, immediately lateral to the tendon of the Much of the surface anatomy of the hand, such as flexion creases, flexor carpi radialis muscle.
These are carbonyl-trapping agents that react with the aldehyde group of PLP; as many other enzymes use PLP as a co-factor buy 20 mg erectafil with amex,these agents are not specific for GAD 20 mg erectafil visa. Two other agents,allylglycine and 3-mercaptopropionic acid, are competitive inhibitors of GAD. In general,GAD inhibitors reduce the level of GABA in the brain and cause seizures in experimental animals that,in the case of the hydrazides, can be overcome by application of vitamin B6,the precursor of PLP. Similarly,in humans an inherited defect in pyridoxine metabolism is characterised by a low concentration of GABA in the cerebrospinal fluid,and intrauterine or neonatal seizures that also respond to treatment with vitamin B6. These findings support the notion that maintained synthesis of GABA is an important factor in the control of overall brain excitability. STORAGE OF GABA Within nerve terminals,GABA,like other classical non-peptide neurotransmitters,is stored in synaptic vesicles into which it is accumulated by active transport. The uptake of GABA from the cytosol (where it is present at a concentration of a few millimolar) into the vesicle lumen (where it may reach several 100 millimolar) is dependent on a vesicular protein that transports cytosolic GABA in exchange for lumenal protons. The proton electrochemical gradient that drives this uptake is generated by a H-ATPase located in the vesicle membrane. Agene(unc-47) encoding a vesicular GABA transporter was first identified from experiments on the simple nervous system of the nematode worm C. Mammalian homologues were subsequently cloned from rat and mouse; these were named VGAT (for vesicular GABA transporter; McIntire et al. These essentially identical clones have sequences predicting proteins of approximately 520 amino acids with ten transmembrane domains and,when expressed in mammalian cell lines,confer vesicular GABA and glycine transport. Immunohistochemical studies showed that VGAT/VIAAT is concentrated not only in the terminals of GABAergic neurons but also in those of neurons known to use glycine as a neurotransmitter (Gasnier 2000). As yet,no specific blockers or modulators of VGAT/VIAAT activity have been identified. UPTAKE OF GABA Once released from a vesicle,GABA molecules are able to activate receptors located on the pre- or postsynaptic membrane before rapidly diffusing out of the synaptic cleft. The ultimate removal of GABA from the extracellular space,and the maintenance of a low extracellular GABA concentration (low micromolar),is achieved by the high- affinity Na- and ClÀ-dependent uptake of GABA into both GABAergic neurons and glial cells. Like the accumulation of GABA into vesicles,this is a secondary active AMINO ACIDS: INHIBITORY 231 transport mechanism,but in this case GABA uptake is coupled to the movement of Na down its electrochemical gradient into the cell. Drugs which block the uptake of GABA may be beneficial in conditions of reduced GABA function,as they are likely to prolong the action of synaptically released GABA (Thompson and Gahwiler 1992). The uptake of GABA is inhibited by a variety of simple GABA analogues,including nipecotic acid,b-alanine,2,4-diaminobutyric acid (DABA), cis-3-aminocyclohexane-carboxylic acid (ACHC), 4,5,6,7-tetrahydroisoxazolo [4,5-c]pyridin-3-ol (THPO) and guvacine (Fig. In early studies,a number of compounds were suggested to preferentially inhibit GABA uptake into neurons (DABA and ACHC) or glia (b-alanine and THPO),while others were clearly non-selective (nipecotic acid and guvacine). Cloned GABA transporters This simple distinction between glial and neuronal uptake has required revision following the molecular cloning of a family of four Na-andCl -dependentÀ GABA transporters,each encoded by a different gene: GAT-1,GAT-2,GAT-3 and BGT-1 (reviewed by Palacin et al. The nucleotide sequence of GAT-1 predicts a protein of 599 amino acids with a presumed structure containing twelve membrane-spanning regions. The transport of each GABA molecule into the cell is coupled to the movement of 2 Na and 1 ClÀ. All the GABA transporters share a similar structure,with approximately 50% amino acid identity. GAT-1 appears to be mainly neuronal in origin as its mRNA is found in neurons and it is inhibited more effectively by neuronal than by glial uptake inhibitors.
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