Autonomic Nervous System: Structure, Function, and Comparison

Autonomic Nervous System

Somatic vs. Autonomic Nervous Systems

The nervous system is divided into the somatic and autonomic branches, each responsible for different types of control and responses in the body.

• Somatic Nervous System: Controls voluntary movements by innervating skeletal muscles. It consists of a single motor neuron extending from the central nervous system (CNS) to the target muscle.

• Autonomic Nervous System (ANS): Regulates involuntary functions such as heart rate, digestion, and respiratory rate. It uses a two-neuron chain: preganglionic and postganglionic fibers.

• Preganglionic Fiber: The first neuron in the ANS pathway, originating in the CNS and synapsing in a ganglion.

• Postganglionic Fiber: The second neuron, extending from the ganglion to the target organ.

• Enteric Nervous System: A specialized network within the gastrointestinal tract, capable of autonomous function but influenced by the ANS.

Feature	Somatic NS	Autonomic NS
Control	Voluntary	Involuntary
Effectors	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Neural Pathway	Single neuron	Two-neuron chain
Neurotransmitter	Acetylcholine	Acetylcholine, norepinephrine

Sympathetic Division of the ANS

The sympathetic division prepares the body for 'fight or flight' responses, increasing alertness and metabolic activity.

• Anatomy: Preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord (T1-L2).

• Ganglionic Neurons: Located in sympathetic chain ganglia (paravertebral), collateral ganglia, and the adrenal medulla.

• Target Organs: Heart, lungs, blood vessels, sweat glands, digestive organs, adrenal medulla.

• Adrenal Medulla: Acts as a modified sympathetic ganglion, releasing epinephrine and norepinephrine directly into the bloodstream.

• Effects of Sympathetic Stimulation:

  • Increased heart rate and blood pressure

  • Dilation of bronchioles

  • Inhibition of digestive activity

  • Pupil dilation

• Neurotransmitters:

  • Preganglionic neurons release acetylcholine (ACh)

  • Postganglionic neurons release norepinephrine (NE) (most), some release ACh

Parasympathetic Division of the ANS

The parasympathetic division promotes 'rest and digest' activities, conserving energy and maintaining homeostasis.

• Anatomy: Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2-S4).

• Ganglionic Neurons: Located in or near the walls of target organs (terminal ganglia).

• Target Organs: Heart, lungs, digestive tract, urinary bladder, reproductive organs.

• Effects of Parasympathetic Stimulation:

  • Decreased heart rate

  • Constriction of bronchioles

  • Stimulation of digestive activity

  • Pupil constriction

• Neurotransmitters:

  • Preganglionic neurons release acetylcholine (ACh)

  • Postganglionic neurons release acetylcholine (ACh)

Dual Innervation

Most organs receive input from both sympathetic and parasympathetic divisions, allowing precise regulation of physiological functions.

• Concept: Dual innervation means that both divisions can stimulate or inhibit the same organ, often with opposing effects.

• Example: The heart receives sympathetic input to increase rate and parasympathetic input to decrease rate.

Visceral Reflexes

Visceral reflexes are automatic responses to stimuli involving internal organs, mediated by the ANS.

• Components: Sensory receptor, afferent neuron, integration center, efferent neuron, effector organ.

• Example: Baroreceptor reflex regulates blood pressure by adjusting heart rate and vessel diameter.

Additional info: The enteric nervous system is sometimes called the "second brain" due to its complexity and autonomy in controlling gastrointestinal function.