Regulation of autonomic nervous system activity

The efferent nervous activity of the ANS is regulated by several regions in the central nervous system (CNS):

• Hypothalamus and brainstem

• Cerebral cortex and limbic system

Many homeostatic control centers are located in the hypothalamus and the brainstem. Through their effects on the ANS, these regions of the brain work together to control cardiovascular, respiratory, and digestive activity. The cerebral cortex and the limbic system also influence ANS activities associated with emotional responses by way of hypothalamic-brainstem pathways. For example, blushing during an embarrassing moment, a response most likely originating in the frontal association cortex, involves vasodilation of blood vessels to the face. Finally, many autonomic reflexes, such as the micturition reflex (urination), are mediated at the level of the spinal cord. Although these reflexes are subject to influence from higher nervous centers, they may occur without input from the brain.

The significance of the contribution to homeostasis by autonomic reflexes warrants further discussion. As described in Chapter 1, maintenance of homeostasis requires continuous sensory input through afferent pathways to the CNS regarding the internal state of the body. The CNS then integrates this information and initiates various negative feedback responses through motor pathways that alter organ activity. The result is the maintenance of the appropriate balance or equilibrium of variables in the internal environment needed for optimal body function. When considering the ANS, much of this sensory input from the thoracic and abdominal viscera is transmitted by afferent fibers of cranial nerve X, the vagus nerve. Other cranial nerves also contribute sensory input to the brain. The sensory signals are transmitted to control centers in the hypothalamus and brainstem. These centers then initiate the proper reflex responses from the visceral organs and tissues by way of the ANS.

An example of this type of reflex is the baroreceptor reflex (see Figure 1.2). Baroreceptors located in some of the major systemic arteries are sensory receptors that monitor blood pressure. If blood pressure decreases, the number of sensory impulses sent from the baroreceptors to the cardiovascular control center in the brainstem also decreases. As a result of this change in baroreceptor stimulation and sensory input to the brainstem, ANS discharge to the heart and blood vessels is adjusted to increase heart rate and vascular resistance so that blood pressure increases to its normal value.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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