Ch. 15 Autonomic Nervous System

brainstem, spinal cord and hypothalamus

3 regions that regulate the functioning of the ANS

prevertebral ganglia

AKA collateral celiac, superior mesenteric, inferior mesenteric

postganglionic axon

AKA ganglionic neuron an axon of the neuron that extends to the organ itself have unmyelinated axons that are even smaller in diameter than preganglionic axons. The neurotransmitter released from the ganglionic neuron in response to a nerve signal is either ACh or norepinephrine (NE). Both neurotransmitters can either excite or inhibit an effector depending upon the type of receptors present within the effector small and mostly unmyelinated, propagation of nerve signals is relatively slow in comparison to nerve signal propagation along somatic motor axons.

CNS control of autonomic nervous system

ANS is regulated by parts of brain and spinal cord Hypothalamus is crucial (contains nuclei controlling both divisions of the ANS) • Essential for emotional ANS processes such as fight-or-flight • Can be influenced by areas of cortex, thalamus, limbic system Brainstem nuclei mediate visceral reflexes • E.g., maintenance of blood pressure Spinal cord governs some ANS reflexes • E.g., defecation and urination as controlled by the parasympathetic nervous system

differences between sympathetic and parasympathetic

Length of preganglionic and postganglionic axons. Parasympathetic preganglionic axons are longer, and postganglionic axons are shorter, when compared to their counterparts in the sympathetic division. In the sympathetic division, preganglionic axons are shorter and postganglionic axons are longer. Number of preganglionic axon branches. Parasympathetic preganglionic axons tend to have few (less than 4) branches, whereas sympathetic preganglionic axons tend to have many branches (more than 20). Location of ganglia. Parasympathetic autonomic ganglia are either close to or within the effector (terminal ganglia and intramural ganglia, respectively). In comparison, sympathetic autonomic ganglia are relatively close to the spinal cord, and are on either side of the spinal cord or anterior to the spinal cord (sympathetic trunk ganglia and prevertebral ganglia, respectively).

parasympathetic division

The branch of the autonomic nervous system that generally conserves bodily resources. craniosacral division REST & DIGEST

LEARNING STRATEGY The analogy of a corporation can help you understand the hierarchy of control of the ANS:

The hypothalamus is the president of the Autonomic Nervous System corporation. It oversees all activity in this system. The autonomic reflex centers in the brainstem and spinal cord are the vice presidents of the corporation. They have a lot of control and power, but ultimately they must answer to the president (hypothalamus). Preganglionic and ganglionic neurons are the workers in the corporation. They are ultimately under the control of both the president and the vice presidents.

sympathetic division

The part of the autonomic nervous system that arouses the body to deal with perceived threats. thoracolumbar division. FIGHT OR FLIGHT

preganglionic axon

an axon that leaves the CNS to synapse with a second motor neuron outside the CNS extends from this cell body and exits the CNS in either a cranial nerve or a spinal nerve. This axon projects to an autonomic ganglion in the peripheral nervous system. Preganglionic neurons have myelinated axons that typically are small in diameter, and nerve signals always result in the release of acetylcholine to excite the second neuron.

LEARNING STRATEGY A good way to understand the two-neuron ANS chain

compare it to the U.S. airline system, which uses connecting flights and airport hubs to transport the maximum number of people in the most cost-effective way. Imagine that you are flying from Indianapolis to Miami for spring break: Your first flight from Indianapolis to Chicago is the preganglionic axon. Although flying north to Chicago is out of your way, the airline sends you to an airport hub because it is more efficient to send all Indianapolis passengers to this main location before they take different flights throughout the United States. The airport hub in Chicago is the autonomic ganglion, the point where preganglionic and postganglionic flights meet up. Other preganglionic flights are arriving at the airport hub, and here all these passengers will connect with other flights. Your connecting flight from Chicago to Miami is the ganglionic neuron. This flight will take you to your final destination, just as a postganglionic axon sends a nerve signal to an effector organ. On the plane with you are people from other preganglionic flights who all want to go to Miami as well. Is using two different flights the most direct way for you to get from Indianapolis to Florida? Of course not. But it is the most cost-efficient (or energy-efficient) way for the airlines to transport and disperse many passengers with a limited number of planes. The connecting-flight arrangement also allows for the extensive convergence of passengers to an airport hub, and for the extensive divergence of passengers from the hubs to the final destinations. Thus, the two motor neurons used in the ANS allow for neuronal convergence and divergence. The autonomic nervous system is similar to connecting airline flights and airport hubs in that both try to group and disperse many different items (nerve signals or passengers) with a limited number of neurons or flights.

Spinal nerve branches

posterior ramus and anterior ramus

somatic nervous system

processes that are perceived or controlled consciously A subdivision of the peripheral nervous system. Enables voluntary actions to be undertaken due to its control of skeletal muscles

Hypothalamus

the integration and command center for autonomic functions the central brain structure involved in emotions and physiologic processes, which are regulated through the ANS. For example, the sympathetic division's fight-or-flight response originates in the sympathetic nucleus in this brain region.

Function of ANS

visceral motor neurons act on visceral effectors to maintain homeostasis without our conscious awareness. You can survive in an unconscious state because of the ANS

Lower Motor Neurons of Autonomic Nervous System

• ANS uses a chain of two motor neurons to reach effector • First is the preganglionic neuron • Cell body within brainstem or spinal cord • Thin, myelinated axon projects to autonomic ganglion in peripheral nervous system • Releases ACh from synaptic knob to excite second motor neuron • Second is the ganglionic (postganglionic) neuron • Cell body within autonomic ganglion • Very thin, unmyelinated axon projects to cardiac muscle, smooth muscle, or gland • Releases ACh or norepinephrine (NE) from synaptic knob to either excite or inhibit the effector (effectors have a variety of receptor types)

autonomic nervous system (ANS)

• Also called autonomic motor or visceral motor system • Processes regulated below conscious level • Transmits signals from CNS to heart, smooth muscle, glands • Responds to visceral sensory inputs (e.g., from blood vessels) • Those sensory neurons are not considered part of ANS • Functions to maintain homeostasis • Keeps conditions inside body within optimal ranges