![]() Parasympathetic and sympathetic neurons are defined anatomically, with parasympathetic neurons arising from the sacral region of the spinal cord and from the brainstem and sympathetic neurons arising from thoracic and lumbar regions of the spinal cord. However, parasympathetic postganglionic neurons release ACh to relay their information at the neuroeffector junction, whereas most sympathetic postganglionic fibers release norepinephrine (NE, also called noradrenaline). Both sympathetic and parasympathetic preganglionic neurons release acetylcholine (ACh) as the neurotransmitter at ganglia. Due to the different ganglionic locations, the lengths of the preganglionic fibers relative to the postganglionic fibers also differ. However, the location of the ganglia differs for the two systems, with parasympathetic ganglia located close to the organ innervated and most sympathetic ganglia located near the spinal cord. Thus the preganglionic fibers of both the sympathetic and parasympathetic systems synapse with postganglionic fibers at autonomic ganglia. Postganglionic neurons send their axons directly to effector organs (heart, blood vessels, visceral organs, and glands), where they relay information to these cells these synapses are often referred to as neuroeffector junctions. Preganglionic neurons have their cell bodies in the spinal cord and the brainstem and their nerve terminals at autonomic ganglia, where they relay information to cell bodies of postganglionic neurons. ![]() The outflow of both divisions from the CNS consists of two neuron relays named after their anatomical location relative to the autonomic ganglia, or relay centers. In contrast, stimulation of the parasympathetic system conserves energy (“rest and digest”) and leads to responses characterized by decreased heart rate, blood pressure, and respiration increased secretions and constriction of the pupil ( miosis).Īlthough the parasympathetic and sympathetic systems differ both anatomically and functionally, they also share some features. Stimulation of the sympathetic system expends energy and leads to “flight, fright, or fight” responses characterized by increased heart rate, blood pressure, and respiration increased blood flow to skeletal muscles andĭilation of the pupil ( mydriasis). The two major divisions of the ANS, the parasympathetic and sympathetic systems, function in parallel to maintain homeostasis by regulating bodily functions. Somatosensory nerves are located throughout the body and convey touch, temperature, position in space, and pain.ĭIVISIONS OF THE AUTONOMIC NERVOUS SYSTEM Sensory nerves are divided into somatosensory and special senses such as hearing and vision. Nerves that control skeletal muscles are called somatic motor nerves and are functionally and anatomically different from autonomic nerves. Although the ANS innervates most bodily structures, it does not innervate skeletal muscle fibers, which are under the voluntary control of higher centers in the CNS. Regulation of the functions of these organs is generally not under conscious control, which is why the ANS is often referred to as the “involuntary” nervous system. The ANS controls key visceral processes, including cardiac output, blood flow to specific organs, glandular secretions, waste removal, and activities related to reproduction. Smooth muscle function is also controlled by chemical substances released locally or systemically. The ANS innervates the heart, blood vessels, visceral organs, exocrine glands, and virtually all other organs that contain smooth muscle. All these systems are interconnected and work together.įIGURE 9–1 Divisions of the nervous system. ![]() The CNS is comprised of the brain and spinal cord and integrates and controls all bodily functions as well as thought processes. The PNS is subdivided into the autonomic nervous system (ANS), which controls automatic functioning, like breathing and heart rate, and the somatic nervous system, which receives sensory information and sends information to the CNS and from the CNS to skeletal muscles. The nervous system is divided into the peripheral nervous system (PNS) and the central nervous system (CNS) (Fig. The human nervous system is the most complex of all systems in the body and is responsible for perceiving, processing, and transmitting information throughout the organism and generating responses to the information. Chapter 9 Introduction to the Autonomic Nervous System
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