By S. Boss. Brevard College.

The nerve fibers within the tracts are generally myelin- either corticospinal or extrapyramidal buy elavil 25mg lowest price. Corticospinal (pyramidal) tracts descend directly, without synaptic interruption, from the cerebral cortex to the lower motor neurons. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 Chapter 11 Nervous Tissue and the Central Nervous System 387 Cerebrum Sensory cortex Motor cortex of cerebrum of cerebrum Cerebrum Corticospinal tract Thalamus Midbrain Sections of brain Midbrain Sections of brain Pons Pons Sensory fibers decussate Medulla oblongata Medulla Motor fibers oblongata decussate Section of cervical spinal cord Section of cervical spinal cord Motor impulse to skeletal muscle Sensory impulse from skin receptors FIGURE 11. Most (about 85%) of The corticospinal tracts appear to be particularly important in the corticospinal fibers decussate in the pyramids of the medulla voluntary movements that require complex interactions between the motor cortex and sensory input. The remaining 15% do not cross from ample, is impaired when the corticospinal tracts are damaged one side to the other. The fibers that cross compose the lateral in the thoracic region of the spinal cord, whereas involuntary breath- corticospinal tracts, and the remaining uncrossed fibers compose ing continues. Damage to the pyramidal motor system can be de- the anterior corticospinal tracts. Because of the crossing of fibers tected clinically by the presence of Babinski’s reflex, in which stimulation of the sole of the foot causes extension (upward move- from higher motor neurons in the pyramids, the right hemisphere ment) of the great toe and fanning out of the other toes. Babinski’s re- primarily controls the musculature on the left side of the body, flex is normally present in infants because neural control is not yet whereas the left hemisphere controls the right musculature. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 388 Unit 5 Integration and Coordination TABLE 11. Nervous Tissue and the © The McGraw−Hill Anatomy, Sixth Edition Coordination Central Nervous System Companies, 2001 Chapter 11 Nervous Tissue and the Central Nervous System 389 FIGURE 11. The remaining descending tracts are extrapyramidal tracts that originate in the brain stem region. Electrical stimulation of the cerebral cortex, the cerebellum, and the basal nuclei indi- rectly evokes movements because of synaptic connections within extrapyramidal tracts. Neurostimulation of the reticular formation by the cerebrum or cerebellum either facilitates or inhibits the ac- tivity of lower motor neurons (depending on the area stimu- lated) (fig.

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If thyroid hormones are deficient during these prenatal and postnatal periods of differentiation and maturation of the brain generic 25 mg elavil otc, mental retardation occurs. Thyroid hormone therapy must be given to a T4 T3 thyroid hormone-deficient child during the first few Coactivator months of postnatal life to prevent mental retardation. Starting thyroid hormone therapy after behavioral deficits have occurred cannot reverse the mental retardation (i. Thyroid hormone deficiency during infancy RNA polymerase II DNA causes both mental retardation and growth impairment, as discussed below. Fortunately, this occurs rarely today be- cause thyroid hormone deficiency is usually detected in newborn infants and hormone therapy is given at the TRE proper time. Corepressor The exact mechanism by which thyroid hormones influ- ence differentiation of the CNS is unknown. T4 is taken up by the cell and deiod- cell replication in the brain and stimulate the growth of inated to T3, which then binds to the thyroid hormone receptor nerve cell bodies, the branching of dendrites, and the rate (TR). The activated TR heterodimerizes with a second transcrip- tion factor, 9-cis retinoic acid receptor (RXR), and binds to the of myelinization of axons. The binding of mones are presumably due to their ability to regulate the TR/RXR to the TRE displaces repressors of transcription and re- expression of genes involved in nerve cell replication and cruits additional coactivators. However, the details, particularly in the hu- RNA polymerase II and the transcription of the target gene. CHAPTER 33 The Thyroid Gland 603 Thyroid Hormones Are Essential for involved; the amounts of oxygen consumed and body heat Normal Body Growth produced depend on total body activity. The thyroid hormones are important factors regulating the Thermogenic Action of the Thyroid Hormones.

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As a which an endocrine disturbance may contribute to the pro- working definition elavil 50mg on line, however, it can be said that hormones gression or severity of disease. They Feedback Regulation Is an Important are highly potent, specialized, organic molecules produced Part of Endocrine Function by endocrine cells in response to specific stimuli and exert The endocrine system, like many other physiological sys- their actions on specific target cells. The mecha- equipped with receptors that bind hormones with high nism is usually negative feedback, although a few positive affinity and specificity; when bound, they initiate charac- feedback mechanisms are known. Both types of feedback teristic biological responses by the target cells. Although many hormones travel by just its rate of hormone secretion to produce the desired this mechanism, we now realize that there are many hor- level of effect, ensuring the maintenance of homeostasis. Since negative feedback is their target cells by diffusion through the interstitial fluid. First-order feedback regulation Hormone Receptors Determine Whether a is the simplest type and forms the basis for more complex Cell Will Respond to a Hormone modes of regulation. In this example, an endocrine cell se- In the endocrine system, a hormone molecule secreted cretes a hormone that produces a specific biological effect into the blood is free to circulate and contact almost any in its target tissue. As the biological response in- that possess specific receptors for the hormone, will re- creases, the amount of hormone secreted by the endocrine spond to that hormone. More commonly, feedback biological effects characteristic of that hormone are initi- regulation in the endocrine system is complex, involving ated. Therefore, in the endocrine system, the basis for second- or third-order feedback loops. For example, multi- specificity in cell-to-cell communication rests at the level ple levels of feedback regulation may be involved in regu- of the receptor. Similar concepts apply to autocrine and lating hormone production by various endocrine glands un- paracrine mechanisms of communication. The A certain degree of specificity is ensured by the re- regulation of target gland hormone secretion, such as adre- stricted distribution of some hormones. For example, sev- nal steroids or thyroid hormones, begins with production eral hormones produced by the hypothalamus regulate of a releasing hormone by the hypothalamus.

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