Study Notes: Autonomic Nervous System (Chapter 14)

Divisions of the Nervous System and Peripheral Architecture

The nervous system is segmented into the Central Nervous System ($CNS$), comprising the brain and spinal cord, and the Peripheral Nervous System ($PNS$), which encompasses all neural tissue outside of the $CNS$. The $PNS$ is further subdivided into the Afferent (Sensory) Division and the Efferent (Motor) Division. The Afferent Division provides input to the $CNS$ through Somatic Afferent receptors located in the skin, skeletal muscles, and joints, as well as Visceral Afferent receptors located in organs, tissues, and smooth muscle. Somatic Sensory Neurons monitor the external environment and physical positions, while Visceral Sensory Neurons monitor the internal environment and the status of various organ systems. Special sense organs also contribute to this input. The Efferent Division handles output and is divided into the Somatic Nervous System ($SNS$) and the Autonomic Nervous System ($ANS$). The $SNS$ is considered the voluntary nervous system, primarily controlling skeletal muscle contractions through somatic motor neurons and including reflex arcs. The $ANS$ is the involuntary nervous system, which utilizes visceral motor neurons to regulate the Sympathetic Division (responsible for the "fight or flight" response) and the Parasympathetic Division (responsible for "rest and digest" activities).

Functional Comparison of Somatic and Autonomic Nervous Systems

The Somatic Nervous System operates under conscious control and is responsible for the innervation of skeletal muscles. The primary integrative center for $SNS$ activity is located within the Cerebral Cortex. In contrast, the Autonomic Nervous System operates without conscious instruction, innervating visceral effectors such as glands, smooth muscle, cardiac muscle, and fat (adipose) tissue. The $ANS$ is essential for coordinating the functions of major organ systems, including the cardiovascular, respiratory, digestive, urinary, and reproductive systems. The primary integrative center for $ANS$ activity is the Hypothalamus. While the $SNS$ focuses exclusively on skeletal muscle, the $ANS$ regulates heart rate, gland secretion, and the contraction or relaxation of smooth muscle across all other organ structures.

The Enteric Nervous System (ENS): The Brain of the Gut

The Enteric Nervous System, often referred to as the "Brains of the Gut," extends throughout most of the gastrointestinal ($GI$) tract and is found within the walls of the $GI$ tract, pancreas, and gallbladder. It consists of an extensive network of approximately $100 \text{ million}$ neurons, including enteric sensory neurons, enteric interneurons, and enteric motor neurons. These neurons form groups known as ganglia that constitute Enteric Plexuses. Enteric sensory neurons utilize chemoreceptors to monitor chemical changes within the $GI$ tract and mechanoreceptors to monitor the stretching of tract walls. Enteric motor neurons control the contraction of smooth muscles to move food (motility), regulate the secretion of organs (such as acid secretion in the stomach), and manage the activity of $GI$ tract endocrine cells. A critical feature of the $ENS$ is the "short reflex," which allows complex visceral reflexes to be initiated and coordinated locally, functioning independently of the $CNS$. This remains true even if the connection between the brain and the gut is severed. However, the $CNS$ can influence the $ENS$ through "long reflexes." An example of a long reflex is the smell or thought of food, which prompts the $CNS$ to signal the $ENS$ to increase the production of gastric secretions.

Neurotransmission and Receptor Types in the ANS and SNS

Neurons in the peripheral nervous system are classified by the neurotransmitters they release at the synaptic cleft. Cholinergic neurons exocytose Acetylcholine ($\text{ACh}$) and are exclusive to that transmitter. Adrenergic neurons exocytose Norepinephrine ($\text{NE}$) and are dedicated to its secretion. Receptors on the post-synaptic membrane or effectors are categorized based on the molecule they bind. Cholinergic receptors include Nicotinic and Muscarinic types; only $\text{ACh}$ can bind to these. Nicotinic receptors are always excitatory and are found at the neuromuscular junction ($NMJ$) of the $SNS$, where $\text{ACh}$ binding causes skeletal muscle contraction. Adrenergic receptors bind $\text{NE}$ or Epinephrine ($\text{Epi}$) and are categorized into alpha ($\alpha$) and beta ($\beta$) types. Alpha receptors include $\alpha_1$ (generally excitatory, causing blood vessel constriction and pupil dilation) and $\alpha_2$ (generally inhibitory, such as decreasing digestive gland secretion). Beta receptors include $\beta_1$ (increasing metabolic activity and heart contraction force), $\beta_2$ (triggering relaxation of respiratory smooth muscles and dilating airways), and $\beta_3$ (involved in lipolysis).

Anatomical Organization of the Autonomic Nervous System

The organization of the $ANS$ involves a two-neuron relay from the $CNS$ to the effector. Visceral motor neurons located in the brain stem and spinal cord are called preganglionic neurons. Their axons, known as preganglionic fibers (Type B fibers), leave the $CNS$ and synapse on postganglionic neurons within autonomic ganglia. The axons of postganglionic neurons are called postganglionic fibers (Type C fibers) and extend to innervate visceral effectors such as cardiac muscle, smooth muscle, glands, and adipose tissue. The Sympathetic Division is also known as the Thoracolumbar Division because its preganglionic cell bodies are located in the lateral gray horns of spinal cord segments $T1 - L2$. The Parasympathetic Division is known as the Craniosacral Division because its preganglionic fibers originate in the brain stem (cranial nerves) and the sacral segments ($S2 - S4$) of the spinal cord. Most visceral organs receive dual innervation, meaning they are innervated by both divisions, often resulting in opposing effects, such as the sympathetic division increasing heart rate while the parasympathetic division decreases it.

Detailed Sympathetic Division (Thoracolumbar) Pathways

In the Sympathetic Division, preganglionic fibers are generally short, while postganglionic fibers are long. The exception to this rule is the pathway to the Adrenal Medulla, where the preganglionic fiber is long. Preganglionic neurons are always cholinergic, releasing $\text{ACh}$ onto nicotinic receptors of postganglionic neurons. Most sympathetic postganglionic neurons are adrenergic, releasing $\text{NE}$ onto $\alpha$ or $\beta$ receptors on effectors. Five distinct scenarios describe sympathetic innervation. Scenario 1 involves postganglionic axons innervating the skin, sweat glands, and arrector pili muscles via spinal nerves after synapsing in the sympathetic chain ganglia (paravertebral ganglia). Scenario 2 involves innervation of the heart and lungs in the thoracic cavity via sympathetic nerves. Scenario 3 describes the innervation of structures in the head, such as the dilator pupillae of the iris and salivary glands, via cephalic periarterial nerves. Scenario 4 involves preganglionic axons that pass through the chain to form splanchnic nerves (Greater, Lesser, Lumbar, and Sacral), which synapse at collateral ganglia (Prevertebral: Celiac, Superior Mesenteric, Inferior Mesenteric, and Hypogastric) to innervate abdominopelvic organs. Scenario 5 describes the direct innervation of the Adrenal Medulla by the Greater Splanchnic Nerve. Here, the preganglionic neuron synapses with Chromaffin cells (specialized postganglionic neurons without axons), which secrete $75-80\%$ Epinephrine and $15-20\%$ Norepinephrine directly into the bloodstream as hormones.

Detailed Parasympathetic Division (Craniosacral) Pathways

In the Parasympathetic Division, preganglionic fibers are long and postganglionic fibers are short. Preganglionic neurons originate in the brain stem and exit via four cranial nerves: Oculomotor ($III$), Facial ($VII$), Glossopharyngeal ($IX$), and Vagus ($X$). The Vagus nerve ($X$) is particularly significant, carrying nearly $80\%$ of the total craniosacral outflow and innervating the heart, lungs, and most abdominal organs. Preganglionic fibers also originate from sacral segments $S2 - S4$, forming Pelvic Splanchnic Nerves that innervate the distal large intestine, rectum, urinary bladder, and genitalia. Synapses occur in terminal ganglia (near the target organ) or intramural ganglia (embedded in the organ wall). All parasympathetic neurons (both pre- and postganglionic) are cholinergic. Pre-ganglionic neurons release $\text{ACh}$ onto nicotinic receptors, while post-ganglionic neurons release $\text{ACh}$ onto muscarinic receptors at the visceral effectors. Stimulation of the parasympathetic division leads to reduced metabolic rate, increased digestive motility, and constriction of the pupils.

Physiological Responses and Autonomic Tone

The Sympathetic Division prepares the body for physical activity through glycogenolysis (breakdown of glycogen to glucose in the liver), lipolysis (breakdown of triglycerides in adipocytes), and proteolysis (breakdown of proteins into amino acids). It increases heart rate, blood pressure, and mental alertness. The Parasympathetic Division promotes energy storage through glycogenesis (creating glycogen) and lipogenesis. Autonomic tone refers to the resting state of an organ regulated by a constant level of background activity from both divisions. Vagal Tone describes the parasympathetic dominance over the heart at rest; decreasing vagal tone or increasing sympathetic stimulation will increase the heart rate. Vasomotor Tone refers to the sympathetic division's continuous partial contraction of blood vessels. An increase in vasomotor tone causes vasoconstriction, while a decrease causes vasodilation. The overall regulation of autonomic tone is governed by the Hypothalamus, which issues instructions to visceral motor neurons to maintain physiological balance.

Autonomic Plexuses and Reflexes

Autonomic plexuses are specialized nerve networks in the thoracic and abdominopelvic cavities formed by a mix of sympathetic postganglionic fibers, parasympathetic preganglionic fibers, and visceral sensory fibers. These include the Cardiac, Pulmonary, Esophageal, Celiac, Superior Mesenteric, Inferior Mesenteric, and Hypogastric plexuses. Visceral reflexes are functionally similar to somatic reflexes but involve visceral motor responses. A long reflex involves the $CNS$ and sensory information traveling to the brain or spinal cord before a motor command is sent. A short reflex bypasses the $CNS$ entirely, with sensory stimuli (chemical, pressure, or stretch) being processed within autonomic ganglia or the $ENS$ to produce a local response in the visceral effector.

Numerical and Structural Summary Data

• Sympathetic outflow: Spinal segments $T1 - L2$.
• Parasympathetic outflow: Cranial nerves $III, VII, IX, X$ and spinal segments $S2 - S4$.
• Vagus Nerve ($X$) Outflow: Approximately $80\%$ of all parasympathetic output.
• Enteric Neuron Count: Approximately $100 \text{ million}$.
• Adrenal Medulla Secretion: $75-80\%$ Epinephrine ($\text{Epi}$) and $15-20\%$ Norepinephrine ($\text{NE}$).
• Preganglionic Fibers: Type B (myelinated).
• Postganglionic Fibers: Type C (unmyelinated).
• Sympathetic Ganglia: Includes chain ganglia (paravertebral) and collateral ganglia (Celiac, Superior Mesenteric, Inferior Mesenteric, Hypogastric).
• Parasympathetic Ganglia: Includes terminal ganglia (Ciliary, Pterygopalatine, Submandibular, Otic) and intramural ganglia.