Autonomic Nervous System PDF

**Autonomic nervous system** contributes to **homeostasis**- The physiological process by which the internal systems of the body are maintained at equilibrium despite variations in external conditions e.g. body temp, blood temp, hydration, acid- base balance. No direct conscious control. **Sympathe...

**Autonomic nervous system** contributes to **homeostasis**- The physiological process by which the internal systems of the body are maintained at equilibrium despite variations in external conditions e.g. body temp, blood temp, hydration, acid- base balance. No direct conscious control. **Sympathetic (Active):** - **Fight or Flight**, preparing the body for stressful situation, e.g. heart rate increasing, dilation of pupils and bronchioles, **blood flow diverted away** from skin and viscera to striated and cardiac muscle. **Parasympathetic (Passive):** - Preparing the body for **maintenance activities** and **rest** e.g., decreasing heart rate, bronchiolar and pupillary constriction, **blood directed to** the viscera, increased gut motility and digestion. **Enteric:** - Embedded in the lining of the **GI tract**, e.g. regulates gut activity, secretion of mucus and digestive enzymes. **The sympathetic nervous system is a 2-neuron relay.** **Preganglionic** and **post ganglionic** neuron: - **Preganglionic** neuron axons are **short**, **lightly myelinated** and release **acetyl choline** as a neurotransmitter. - **Postganglionic** neuron axons are **long** and **non-myelinated**, **project to targets** like smooth muscle, glands or cardiac muscles. - **90% of post ganglionic** neurons release **noradrenaline** as a neurotransmitter and **10% use acetylcholine** that mainly project to sweat glands and piloerector smooth muscle in the skin. **The parasympathetic nervous system is also a 2-neuron relay:** - Much of the **parasympathetic** nervous system is associated with **cranial nerves**. - Most parasympathetic **preganglionic** neurons are located in the **visceral efferent** motor nuclei **of cranial nerves: III (Midbrain), IX (pons, medulla) and X (medulla)**. - Parasympathetic preganglionic neurons are also located in the lateral **grey matter** of the **sacral spinal cord** (S2-S4). - Preganglionic parasympathetic neurons have **long lightly myelinated axons** that release **Ach** as a neurotransmitter. - Some **postganglionic** parasympathetic neurons are in **discrete ganglia** close to their targets (Cranial III, VII, IX). - Many postganglionic parasympathetic neurons are in **loose Plexi** embedded in their target organs (CN X and Sacral outflow). - Postganglionic parasympathetic neurons have **short non- myelinated axons** that release **Ach** as a neurotransmitter. **Preganglionic sympathetic axons take multiple routes from the spinal cord to reach postganglionic neurons:**  The **intermediolateral horn (IML)** houses the cell bodies of preganglionic neurons, which send lightly myelinated axons through the **ventral root** and **white ramus communicans** to the **sympathetic chain (paravertebral ganglia)**. Here, some axons synapse with postganglionic neurons, which send non-myelinated axons via the **grey ramus communicans** to innervate structures like blood vessels, smooth muscle, and sweat glands. Alternatively, preganglionic axons bypass the sympathetic chain via **splanchnic nerves** to synapse in **prevertebral ganglia**, targeting abdominal and pelvic organs. **Visceral sensory afferents** carry sensory information from organs back to the spinal cord via the **dorsal root ganglion (DRG)**. This system coordinates the \"fight or flight\" response, regulating internal organ function and maintaining homeostasis. **The effects of adrenaline/NA on sympathetic targets are dependent upon which adrenergic receptor types of targets express:** **Adrenergic receptors** induce **slow-acting** and prolonged postsynaptic **responses** by activating different intracellular signalling **pathways** depending on the type of **G protein** they are coupled with: **Multiple receptors also mediate the actions of acetylcholine in the ANS:** Types of **acetylcholine (ACh) receptors** in the autonomic nervous system (ANS) and their mechanisms of action. These include both **nicotinic** and **muscarinic receptors**, which mediate distinct physiological effects. **Organization of the enteric nervous system (ENS):**  The ENS is made up of **two main** nerve networks, or plexuses: the **Submucosal (Meissner\'s) plexus** and the **Myenteric (Auerbach\'s) plexus**. The **Submucosal plexus** is located in the **submucosa** layer and helps regulate secretions and blood flow in the gut. The **Myenteric plexus** is found between the **circular smooth muscle layer** and the **longitudinal smooth muscle layer**, and it controls the movements of the gut, such as **peristalsis** (the wave-like contractions that move food through the digestive system). **Physiology of the ENS:** The **myenteric** and **submucosal plexus** contain **sensory neurons, interneurons** and **autonomic motor neurons**.\ **Sensory neurons** in the **myenteric plexus** measure the degree of **stretch** in smooth muscles.\ **Sensory neurons** in the **submucosal plexus** measure the **chemical** composition of the lumen.\ **Excitatory** and **inhibitory visceromotor neurons** work together to coordinate **peristaltic** movement of the gut.\ **Secretomotor** neurons promote **mucous** release into the lumen.\ **Interneurons** connect **sensory** and **motor** neurons.\ The enteric nervous system functions **largely independently** of the brain but has regulatory **input** from **parasympathetic** (Vagus nerve) and **sympathetic** (prevertebral ganglia) neurons.\ More than 30 neurotransmitters are used in the ENS (e.g. Ach, serotonin, dopamine, neuropeptides and NO. **Regulation of autonomic nervous system activity:** **Sensory feedback** from the target tissues/organs innervated by autonomic neurons plays\ a major role in regulating the activity of the autonomic nervous system.\ Visceral sensory neurons innervate the target tissues of the ANS.\ - visceral sensory DRG neurons.\ - visceral sensory neurons of the petrosal ganglion (associated with cranial nerve IX).\ - visceral sensory neurons of the nodose ganglion (associated with cranial nerve X). **Most visceral sensory information is sent to the nucleus of the solitary tract in the medulla:** - The centrally projecting axons of petrosal (CN IX) and nodose (CN X) sensory neurons (afferents) directly innervate the visceral sensory division of the nucleus of the solitary tract (NST) in the medulla. - Visceral sensory neurons associated with spinal nerves (DRG neurons) indirectly innervate the NST.  **Visceral sensory** signals from internal organs travel via the **dorsal root ganglion (DRG)** and cranial nerves IX and X to the **nucleus of the solitary tract (NST)** in the brainstem. The NST serves as a hub, processing these inputs for **two main functions**: **local reflex activity and higher brain integration.** For **reflex activity**, the **brainstem reticular formation** activates preganglionic and postganglionic neurons, which target tissues like **smooth muscle**, **cardiac muscle**, and **glands** to regulate organ-specific functions. For **higher-level regulation**, sensory information is relayed to brain areas like the **medial prefrontal cortex**, **insular cortex**, **amygdala**, and **hypothalamus**. These regions integrate visceral input with other sensory data and emotional contexts, coordinating widespread changes in ANS activity via feedback to the brainstem.