Autonomic Nervous System: Structure, Function, and Divisions

Autonomic Nervous System (ANS) and Somatic Motor System

Overview of the Nervous System

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all neural tissue outside the CNS. The PNS is further subdivided into afferent (sensory) and efferent (motor) divisions.

• Afferent Division: Brings sensory information to the CNS.

• Efferent Division: Transmits motor commands from the CNS to muscles and glands.

The efferent division is split into the Somatic Nervous System (controls skeletal muscle contractions) and the Autonomic Nervous System (regulates smooth muscle, cardiac muscle, and glandular activity).

The Autonomic Nervous System (ANS)

The ANS is responsible for involuntary regulation of internal body functions. It innervates smooth muscle, cardiac muscle, and glands, and regulates visceral functions such as heart rate, blood pressure, digestion, and urination.

• Visceral Motor Division: The ANS is the general visceral motor division of the PNS.

• Somatic Motor Division: Controls voluntary skeletal muscle activity.

Comparison of Autonomic and Somatic Motor Systems

Somatic Motor System

The somatic motor system controls voluntary responses to external stimuli and consists of a single, well-myelinated motor neuron extending from the CNS to the skeletal muscle, allowing rapid impulse conduction.

• One motor neuron: Direct connection from CNS to skeletal muscle.

• Myelinated axons: Fast conduction of impulses.

Autonomic Motor System

The autonomic motor system regulates involuntary internal body functions and consists of a chain of two motor neurons: a myelinated preganglionic neuron and a nonmyelinated postganglionic neuron.

• Preganglionic neuron: Myelinated, originates in CNS.

• Postganglionic neuron: Nonmyelinated, extends to target organ.

• Neurotransmitters: Acetylcholine (ACh) and norepinephrine (NE) are commonly involved.

Example: The autonomic system controls heart rate and digestion, while the somatic system controls voluntary muscle movement.

Divisions of the Autonomic Nervous System

Sympathetic and Parasympathetic Divisions

Both divisions innervate many of the same structures but generally produce opposite effects.

• Sympathetic Division: "Fight, flight, or fright"—activated during exercise, excitement, and emergencies.

• Parasympathetic Division: "Rest and digest"—concerned with conserving energy and routine maintenance.

Anatomical Differences

• Length of Preganglionic Fibers:

  • Sympathetic: Short

  • Parasympathetic: Long

• Length of Postganglionic Fibers:

  • Sympathetic: Long

  • Parasympathetic: Short

• Branching of Axons:

  • Sympathetic: Highly branched, influences many organs

  • Parasympathetic: Few branches, localized effect

Origin of Divisions

• Sympathetic Division: Thoracolumbar outflow (T1–L2)

• Parasympathetic Division: Craniosacral outflow (brainstem and S2–S4)

Sympathetic Pathways

Structure and Function

Sympathetic pathways originate from the thoracolumbar region of the spinal cord and involve short preganglionic fibers and long postganglionic fibers. The neurotransmitters involved are acetylcholine (ACh) at the ganglion and norepinephrine (NE) at the effector organ.

• Paravertebral Ganglia: Also called sympathetic trunk ganglia, paired and run alongside the vertebral column.

• Prevertebral Ganglia: Unpaired, located anterior to the vertebral column, found only in the abdomen and pelvis.

Ganglion Type	Location	Major Ganglia	Innervated Organs
Paravertebral	Along vertebral column	Superior, middle, inferior cervical ganglia	Skin, blood vessels, eyes, salivary glands, respiratory tract, heart
Prevertebral	Anterior to vertebral column	Celiac, superior/inferior mesenteric, aorticorenal, renal	Stomach, intestines, liver, spleen, kidneys, bladder, reproductive organs

Parasympathetic Pathways

Structure and Function

Parasympathetic pathways originate from the brainstem and sacral spinal cord (S2–S4). They feature long preganglionic fibers and short postganglionic fibers, with acetylcholine (ACh) as the neurotransmitter at both synapses.

• Cranial Outflow: Innervates organs of the head, neck, thorax, and abdomen.

• Sacral Outflow: Supplies remaining abdominal and pelvic organs.

Ganglion	Target Organs	Function
Ciliary	Eye	Pupil constriction, lens accommodation
Sphenopalatine	Nasal mucosa, lacrimal gland	Secretion, tear production
Submandibular	Submandibular/sublingual glands	Saliva secretion
Otic	Parotid gland	Saliva secretion

• Nerve Plexuses: Cardiac, pulmonary, aortic, esophageal, celiac, superior/inferior mesenteric plexuses supply heart, lungs, digestive organs, and more.

• Sacral Outflow: Pelvic splanchnic nerves innervate distal large intestine, bladder, ureters, and reproductive organs.

Interactions and Tone of Autonomic Divisions

Dual Innervation and Functional Balance

Most organs receive dual innervation from both sympathetic and parasympathetic divisions, which often exert opposite effects. The sympathetic division acts as the "accelerator," while the parasympathetic division serves as the "brakes."

• Sympathetic Tone: Maintains blood pressure by keeping blood vessels in a state of partial contraction.

• Parasympathetic Tone: Dominates heart, digestive, and urinary tract organs under resting conditions, but can be overridden by sympathetic activity during stress.

Summary Table: Sympathetic vs. Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1–L2)	Craniosacral (Brainstem, S2–S4)
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Location of Ganglia	Near CNS (paravertebral, prevertebral)	Near or within target organ (terminal)
Main Neurotransmitters	ACh (preganglionic), NE (postganglionic)	ACh (both)
General Function	"Fight or flight" (emergencies, stress)	"Rest and digest" (maintenance, energy conservation)

Key Terms and Definitions

• Autonomic Nervous System (ANS): The part of the PNS that controls involuntary functions by innervating smooth muscle, cardiac muscle, and glands.

• Somatic Nervous System: The part of the PNS that controls voluntary movements of skeletal muscles.

• Preganglionic Neuron: The first neuron in the autonomic pathway, originating in the CNS.

• Postganglionic Neuron: The second neuron in the autonomic pathway, extending to the effector organ.

• Ganglion: A cluster of neuronal cell bodies outside the CNS.

• Sympathetic Trunk: A chain of paravertebral ganglia running alongside the vertebral column.

• Neurotransmitter: Chemical messenger released by neurons to communicate with other cells (e.g., acetylcholine, norepinephrine).

Relevant Equations

• Nerve Impulse Conduction Velocity: $v = \frac{d}{t}$, where $v$ is velocity, $d$ is distance, and $t$ is time.

Example Application

• Sympathetic Activation: During a stressful event, the sympathetic division increases heart rate, dilates pupils, and redirects blood flow to skeletal muscles.

• Parasympathetic Activation: After a meal, the parasympathetic division stimulates digestion and slows heart rate.

Additional info: The notes include diagrams and tables that clarify the anatomical pathways and functional differences between the sympathetic and parasympathetic divisions. These details are essential for understanding the physiological regulation of organ systems by the ANS.