Neurons from the lateral horn of the spinal cord (preganglionic nerve fibers – solid lines)) project to the chain ganglia on either side of the vertebral column or to collateral (prevertebral) ganglia that are anterior to the vertebral column in the abdominal cavity. In, the “circuits” of the sympathetic system are intentionally simplified. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots, but through ascending or descending connections through the bridges within the chain.Ī diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. There are typically 23 ganglia in the chain on either side of the spinal column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia that runs alongside the vertebral column. A central neuron in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. It is referred to as the thoracolumbar system to reflect this anatomical basis. The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide array of ganglia that project to the many effector organs simultaneously. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. More oxygen needs to be inhaled and delivered to skeletal muscle. To respond to a threat-to fight or to run away-the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. Sympathetic Division of the Autonomic Nervous System One causes heart rate to increase, whereas the other causes heart rate to decrease. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. At each target effector, dual innervation determines activity. Homeostasis is the balance between the two systems. The sympathetic system is associated with the fight-or-flight response, and parasympathetic activity is referred to by the epithet of rest and digest. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system causes contraction of skeletal muscles. The major differences between the two systems are evident in the responses that each produces. The nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. Describe the signaling molecules and receptor proteins involved in communication within the two divisions of the autonomic nervous system.Explain the differences in output connections within the two divisions of the autonomic nervous system.Name the components that generate the sympathetic and parasympathetic responses of the autonomic nervous system.By the end of this section, you will be able to:
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