### Organisation of the Autonomic Nervous System How is the organidation of the autonomic nervous system different from that of the somatic nervous system? **Autonomic Nervous System (ANS)** operates without conscious instruction. Coordinates systems functions: Cardiovascular, Respiratory, Digestive, Urinary and Reproductive. (All these functions are to a large extent under ANS control). **Organization Similarities of SNS and ANS** Are efferent divisions. Carry motor commands: - SNS controls skeletal muscles - ANS controls visceral effectors (Smooth musculature that is operating most of the organs. The heart is not made of smooth muscles but striate, however it still is under strong influence from ANS). **The ANS** Motor neurons synapse on visceral motor neurons in autonomic ganglia (small populations of neurons). Ganglionic neurons control visceral effectors. **Organization Similarities of SNS and ANS** ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image102.png]] **Preganglionic Fibers** Axons of preganglionic neurons leave CNS and synapse on ganglionic neurons. **Autonomic Ganglia** It is a peripheral Ganglia that contains many ganglionic neurons. Such ganglionic neurons innervate visceral effectors: cardiac muscle, smooth muscle, glands, adipose tissues. **Postganglionic Fibers** Axons of ganglionic neurons begin at autonomic Ganglia and extend to peripheral target organs. **Somatic or Visceral Sensory Information** Trigger visceral reflexes (Set of sensors that sense things like blood oxygenation levels, the ANS would increase breath rate in case of oxygen lacks). Motor commands of reflexes distributed by ANS. **What are the divisions and functions of the ANS?** All of the organs receive commands by both the innervations, which indeed work in a complementary manner. - **Sympathetic Division**: "kicks in" only during exertion, stress, or emergency. It is the part of the nervous system that needs to be activated when your level of awareness needs to be increased. - **Parasympathetic Division**: controls during resting conditions. It is the one involved when you are in a resting situation. For example allows for digestion, which it may be stopped by the sympathetic division when we suddenly face a stressing situation (e.g., catching a bus). **Divisions of the ANS** - 2 divisions may work independently: some structures innervated by only one of the two divisions. - 2 divisions may work together: each controlling one stage of a complex process. ### Sympathetic ANS ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image103.png]] - Preganglionic fibers (thoracic and superior lumbar) synapse in ganglia near spinal cord. - Preganglionic fibers are short (they go from CNS to ganglia). - Postganglionic fibers are long. (contained in the sympathetic ganglia). **Fight or Flight** Sympathetic division readies body for crisis. Increase in sympathetic activity: - Stimulates tissue metabolism - Increases alertness **Structure of the Sympathetic Division** - Preganglionic neurons located between segments T~1~ and L~2~ of spinal cord. - Ganglionic neurons in ganglia near vertebral column. - Cell bodies of preganglionic neurons in lateral gray horns. - Axons enter ventral roots of segments. **SANS Ganglionic Neurons** Occur in three locations: - Sympathetic Chain Ganglia - Collateral Ganglia (innervate organs) - Adrenal Medullae **Organization of Sympathetic Division** Ventral roots of spinal segments T~1~ - L~2~ contain sympathetic preganglionic fibers. **Sympathetic Innervation** The Sympathetic innervation is generally widespread, while the Parasympathetic one is generally much more local. ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image105.png]] **Adrenal Medulla** - Preganglionic fibers entering adrenal gland proceed to center (adrenal medulla). - Modified sympathetic ganglion. - Preganglionic fibers synapse on neuroendocrine cells. - Specialized neurons secrete hormones into bloodstream. **Generalized Sympathetic Activation** - Release of Epinephrine (Adrenaline) by Adrenal Medulla. - Affect alpha and beta receptors throughout body (these work through G Proteins). ### Parasympathetic ANS - Preganglionic fibers originate in brain stem and sacral segments of spinal cord. - Synapse in ganglia close to (or within) target organs. - Preganglionic fibers are long (they go all the way to the organs). - Postganglionic fibers are short. **Rest and Repose** Parasympathetic division stimulates visceral activity. It conserves energy and promotes sedentary activities. **What are the structures and functions of the parasympathetic division of the ANS?** ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image106.png]] **The Parasympathetic Division** **Autonomic Nuclei** - Are contained in the mesencephalon, pons, and medulla oblongata: in lateral gray horns of spinal segments S~2~ -- S~4~. **Ganglionic Neurons in Peripheral Ganglia** Preganglionic fiber synapses on 6-8 ganglionic neurons: - Terminal ganglion (near target organ) - Intramural ganglion (embedded in tissues of target organ). **Parasympathetic Innervation** ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image107.png]] **Vagus Nerve** Often mentioned because of the vagus reflex, when you have a long sustained activation of the sympathetic division, then the vagus nerve is activated to bring back the body to a state of rest, reduce heartbeat (e.g., runners at the end of the marathon collapse because blood misses in the brain due to the vagus nerve reflex). - It provides 75% of all parasympathetic outflow. - Branches intermingle with fibers of sympathetic division. **What are the effects of parasympathetic neurotransmitters on target organs and tissues?** **Parasympathetic Activation** - Centers on relaxation, food processing and energy absorption. - Localized effects last a few seconds at most. **Parasympathetic Neurons** - All release ACh as neurotransmitter. - Effects vary widely. ### Enteric Nervous System (ENS) - It is considered to be the third division of ANS. - Extensive network in digestive tract walls. - Complex visceral reflexes coordinated locally. - Roughly 100 million neurons (It's very complex, yet it doesn't give rise to any conscious perception of its operations). - All neurotransmitters are found in the brain. ### Comparing Sympathetic and Parasympathetic Divisions - **Sympathetic: - Widespread impact. - Reaches organs and tissues throughout body. - **Parasympathetic:** - Innervates only specific visceral structures. **Sympathetic and Parasympathetic Divisions** ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image109.png]] **What is the relationship between the two divisions of the ANS, and the significance of dual innervation?** **Dual Innervation** - Most vital organs receive instructions from both sympathetic and parasympathetic divisions. - The two divisions commonly have opposing effects. **Autonomic Tone** The fact that we have this constant influx of para and sympathetic innervations in the organs. It is an important aspect of ANS function: - If nerve is inactive under normal conditions, can only increase activity. - If nerve maintains background level of activity, can increase or decrease activity. **Somatic and Visceral Motor Pathways** - Many parallels in organization and function. - Integration at brain stem. - Both systems under control of higher centers.