The postsynaptic receptors then activate secondary messenger systems in the postsynaptic cell via G protein–linked activity. The released norepinephrine binds to the pre- and postsynaptic adrenergic receptors. In general, only 1% of the total stored norepinephrine is released with each depolarization thus, there is a tremendous functional reserve. Then the vesicles merge with the cell membrane and release their contents into the synapse (Fig.5). The neurotransmitters are stored in vesicles until the postganglionic nerve is stimulated. In the adrenal medulla, norepinephrine is methylated to epinephrine. DOPA is then converted to dopamine and, once inside the storage vesicle at the nerve terminal, is β-hydroxylated to norepinephrine. The rate-limiting step is the transformation of tyrosine to dihydroxyphenylalanine (DOPA), which is catalyzed by the enzyme tyrosine hydroxylase. Sympathetic neurotransmitters are synthesized from tyrosine in the postganglionic sympathetic nerve ending (Fig.4). In contrast, the postganglionic fibers run a long course before innervating effector organs (Fig.2). The sympathetic preganglionic fibers are relatively short because sympathetic ganglia are generally close to the central nervous system (CNS). The postganglionic neurons of the SNS then travel to the target organ. A sympathetic response, therefore, is not confined to the segment from which the stimulus originates, as discharge can be amplified and diffuse. Preganglionic sympathetic fibers not only synapse at the ganglion of the level of their origin in the spinal cord but can also course up and down the paired ganglia. The nerve fibers extend to paired ganglia, creating the sympathetic chains that lie immediately lateral to the vertebral column or extend to unpaired distal plexuses (e.g., the celiac and mesenteric plexuses). The cell bodies of these neurons lie in the spinal gray matter. Ganglionic neurons have small unmyelinated axons that release either acetylcholine (ACh) or norepinephrine (NE) to either excite or inhibit an effector, depending on the type of receptors present on the effector.The preganglionic fibers of the SNS originate from the thoracolumbar region (T1 to L2 or 元) of the spinal cord (Fig.1). The cell body of the ganglionic neuron resides within the autonomic ganglion and its axon extends to an effector (cardiac muscle, smooth muscle, or gland) forming a postganglionic fiber. The second motor neuron is called a ganglionic neuron. Preganglionic neurons have small myelinated axons that release acetylcholine (ACh) to excite a second motor neuron. This fiber projects to an autonomic ganglion of the peripheral nervous system. The axon of the preganglionic neuron extends outside of the CNS through cranial or spinal nerves forming a preganglionic fiber. The cell body of the first of the two ANS motor neurons is located in the brainstem or spinal cord and is called a preganglionic neuron. In comparison, the autonomic nervous system is composed of a chain of two lower motor neurons. These somatic motor neurons have large myelinated axons that release acetylcholine (ACh) at neuromuscular junctions. In the somatic nervous system, a single lower somatic motor neuron of the brainstem or spinal cord extends from the CNS towards a skeletal muscle through a cranial or spinal nerve, respectively. Thus, the motor response of the somatic nervous system is voluntary while the one of the autonomic nervous system is involuntary.Īnother major difference between these two systems lies within the number of lower motor neurons that are involved in the response. While the somatic motor neurons innervate and cause contraction of skeletal muscles, autonomic motor neurons innervate and control cardiac and smooth muscle, as well as glandular tissue. Moreover, the motor efferent branches of these two systems innervate different target effectors. ![]() The autonomic nervous system reflexively responds to visceral sensory stimuli, such as levels of carbon dioxide concentration in the blood or stretch caused by blood pressure, that you are not consciously aware of. The somatic nervous system consciously detects sensory stimuli from the special senses, skin and proprioceptors. The major difference between these two systems is based on whether you are conscious of its process. ![]() ![]() The nervous system can be divided into two functional parts: the somatic nervous system (SNS) and the autonomic nervous system (ANS). ![]() \)Ĭomparison between the Somatic and Autonomic Nervous System
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