The Autonomic Nervous System (Geneeskunde 1 & 2) PDF

# The Autonomic Nervous System - The ANS has sympathetic, parasympathetic, and enteric divisions. - ANS output travels along two anatomically and functionally distinct pathways: the somatic motor neurons, which innervate striated skeletal muscle; and the autonomic motor neurons, which innervate smoo...

# The Autonomic Nervous System - The ANS has sympathetic, parasympathetic, and enteric divisions. - ANS output travels along two anatomically and functionally distinct pathways: the somatic motor neurons, which innervate striated skeletal muscle; and the autonomic motor neurons, which innervate smooth muscle, cardiac muscle, secretory epithelia, and glands. ## The Autonomic Nervous System Explained - The autonomic nervous system was first defined by Langley in 1898 as including the local nervous system of the gut and the efferent neurons innervating glands and involuntary muscle. - This definition of the ANS includes only efferent neurons and enteric neurons. - It has become clear that the efferent ANS cannot easily be dissociated from visceral afferents as well as from those parts of the CNS that control the output to the ANS and those that receive interoceptive input. ## The Visceral Control System - This larger visceral control system monitors afferents from the viscera and the rest of the body, compares this input with current and anticipated needs, and controls output to the body's organ systems. ## Divisions of the ANS - The ANS has three divisions: sympathetic, parasympathetic, and enteric. - The sympathetic and parasympathetic divisions of the ANS are the two major efferent pathways controlling targets other than skeletal muscle. - Each innervates target tissue by a two-synapse pathway. - The cell bodies of the first neurons lie within the CNS. These preganglionic neurons are found in columns of cells in the brainstem and spinal cord and send axons out of the CNS to make synapses with postganglionic neurons in peripheral ganglia interposed between the CNS and their target cells. - Axons from these postganglionic neurons then project their targets. - The sympathetic and parasympathetic divisions can act independently of each other. - However, in general, they work synergistically to control visceral activity and often act in opposite ways, like an accelerator and brake to regulate visceral function. | Central Nervous System | Peripheral Ganglion | Target Cell | |---|---|---| | Preganglionic neuron | Postganglionic neuron | | | | | | - An increase in output of the sympathetic division occurs under conditions such as stress, anxiety, physical activity, fear, or excitement, whereas parasympathetic output increases sedentary activity, eating, or other "vegetative" behavior. ## The Enteric Division - The enteric division of the ANS is a collection of afferent neurons, interneurons, and motor neurons that form networks of neurons called plexuses (from the Latin “to braid”) that surround the gastrointestinal (GI) tract. - It can function as a separate and independent nervous system, but it is normally controlled by the CNS through sympathetic and parasympathetic fibers. ## Sympathetic Preganglionic Neurons - Sympathetic preganglionic neurons originate from spinal segments T1 to L3 and synapse with postganglionic neurons in paravertebral or prevertebral ganglia. ### Preganglionic Neurons - The cell bodies of preganglionic sympathetic motor neurons are located in the thoracic and upper lumbar spinal cord between levels T1 and L3. - At these spinal levels, autonomic neurons lie in the intermediolateral cell column, or lateral horn, between the dorsal and ventral horns. - Axons from preganglionic sympathetic neurons exit the spinal cord through the ventral roots along with axons from somatic neurons. - After entering the spinal nerves, sympathetic efferents diverge from somatic motor axons to enter the white rami communicantes. - These rami, or branches, are white because most preganglionic sympathetic axons are myelinated. ## Paravertebral Ganglia - Axons from preganglionic neurons enter the nearest sympathetic paravertebral ganglion through a white ramus. - These ganglia lie adjacent to the vertebral column. - Although preganglionic sympathetic fibers emerge only from levels T1 to L3, the chain of sympathetic ganglia extends all the way from the upper part of the neck to the coccyx, where the left and right sympathetic chains merge in the midline and form the coccygeal ganglion. - In general, one ganglion is positioned at the level of each spinal root, but adjacent ganglia are fused in some cases. - The most rostral ganglion, the superior cervical ganglion, arises from fusion of C1 to C4 and supplies the head and neck. - The next two ganglia are the middle cervical ganglion, which arises from fusion of C5 and C6, and the inferior cervical ganglion (C7 and C8), which is usually fused with the first thoracic ganglion to form the stellate ganglion. - Together, the middle cervical and stellate ganglia, along with the upper thoracic ganglia, innervate the heart, lungs, and bronchi. - The remaining paravertebral ganglia supply organs and portions of the body wall in a segmental fashion. ## Prevertebral Ganglia - The prevertebral plexus lies in front of the aorta and along its major arterial branches and includes the prevertebral ganglia and interconnected fibers. - The major prevertebral ganglia are named according to the arteries that they are adjacent to and include the celiac, superior mesenteric, aorticorenal, and inferior mesenteric ganglia. - Portions of the prevertebral plexus extend down the major arteries and contain other named and unnamed ganglia and plexuses of nerve fibers, which altogether make up a dense and extensive network of sympathetic neuron cell bodies and nerve fibers. - Each preganglionic sympathetic fiber synapses on many postganglionic sympathetic neurons that are located within one or several nearby paravertebral or prevertebral ganglia. - It has been estimated that each preganglionic sympathetic neuron branches and synapses on as many as 200 postganglionic neurons, which enables the sympathetic output to have more widespread effects. - However, any impulse arriving at its target end organ has only crossed a single synapse between the preganglionic and postganglionic sympathetic neurons. ## Postganglionic Neurons - The cell bodies of postganglionic sympathetic neurons that are located within paravertebral ganglia send out their axons through the nearest gray rami communicantes, which rejoin the spinal nerves. - These rami are gray because most postganglionic axons are unmyelinated. - Because preganglionic sympathetic neurons are located only in the thoracic and upper lumbar spinal segments (T1 to L3), white rami are found only at these levels. - However, because each sympathetic ganglion sends out postganglionic axons, gray rami are present at all spinal levels from C2 or C3 to the cоссух. - Postganglionic sympathetic axons from paravertebral and prevertebral ganglia travel to their target organs within other nerves or by travelling along blood vessels. - Because the paravertebral and prevertebral sympathetic ganglia lie near the spinal cord and thus relatively far from their target organs, the postganglionic axons of the sympathetic division tend to be long. - On their way to reach their targets, some postganglionic sympathetic axons travel through parasympathetic terminal ganglia or cranial nerve ganglia without synapsing. ## Organization of the Sympathetic and Parasympathetic Divisions of the ANS - The left panel shows the sympathetic division. The cell bodies of sympathetic preganglionic neurons are in the intermediolateral column of the thoracic and lumbar spinal cord. - Their axons project to paravertebral ganglia (the sympathetic chain) and prevertebral ganglia. - Postganglionic neurons therefore have long projections to their targets. - The right panel shows the parasympathetic division. The cell bodies of parasympathetic preganglionic neurons are either in the brain (midbrain, pons, medulla) or in the sacral spinal cord. - Their axons project to ganglia very near (or even inside) the end organs. - Postganglionic neurons therefore have short projections to their targets. ## Tracing of Nerve Tracts Using Pseudorabies Virus - The CNS neuroanatomy of autonomic control has been difficult to define experimentally. - However, a technique developed by Arthur Loewy and his colleagues that traces nerve tracts with the pseudorabies virus has helped to define more clearly the central pathways for autonomic control. - For example, if axons of preganglionic sympathetic neurons are exposed to pseudorabies virus, the virus is transported back into the cell bodies, where it replicates. - After a delay of several days, neurons that make synapses with these preganglionic neurons (i.e., “premotor” neurons) become infected and the virus is transported to their cell bodies. - After longer periods of incubation, neurons farther upstream are also infected. - Histological staining can then be used at different time points to visualize neurons that contain the virus at each level upstream.