The Autonomic Nervous System: Controlling Your Body From Within

Learning Objectives:

• Understand the basic structure of the autonomic nervous system (ANS).
• Analyze the effects of sympathetic and parasympathetic divisions on different organs.
• Grasp the neurotransmitters and receptors involved in the ANS.
• Explore the structures that regulate ANS activity.

Introduction:

The autonomic nervous system, also known as the visceral nervous system, governs involuntary bodily functions. This system controls fundamental processes such as heart rate, blood pressure, respiration, digestion, excretion, and temperature regulation.

Distinction from the Somatic Nervous System:

The somatic nervous system is responsible for controlling voluntary actions, such as moving your limbs, talking, and so on.

Structure of the ANS:

The ANS comprises two primary branches: the sympathetic and parasympathetic divisions. Both share a similar structure, consisting of:

• Central Region: Preganglionic neurons residing in the lateral horns of the spinal cord (C8-L2 for sympathetic, S2-S4 for parasympathetic) and specific brain regions (for parasympathetic).
• Preganglionic Neuron: This neuron sends its axon to an autonomic ganglion.
• Autonomic Ganglion: The site of synapse between the preganglionic and postganglionic neurons.
• Postganglionic Neuron: This neuron transmits its axon to the target organ.
• Responding Organ: Smooth muscle, cardiac muscle, glands, blood vessels, etc., are controlled by the ANS.

Transmission Pathway:

The efferent pathway of the ANS involves:

1. Central Nervous System
2. Preganglionic Fiber
3. Ganglion
4. Postganglionic Fiber
5. Target Organ

Features of Autonomic Nerve Fibers:

• Run along the cell membrane of muscle, forming grooves on the muscle and creating bulbous nodes containing neurotransmitters.
• Neurotransmitters are released upon stimulation.
• One neuron can stimulate multiple cardiac and smooth muscle cells.

Termination of the ANS:

There is no clear point of entry.

Sympathetic and Parasympathetic Divisions:

These two divisions work in a balanced manner to ensure harmonious regulation of bodily functions.

Sympathetic Nervous System:

• Central Region: Neurons located in the cervical spinal cord segments C8-T1 and extending down to the lumbar segments L1-L2.
• Function: Prepares the body for immediate responses, such as fight or flight.
• Effects: Increases heart rate, elevates blood pressure, enhances respiration, dilates pupils, inhibits digestion.
• Alternate Name: Thoracolumbar system.

Parasympathetic Nervous System:

• Central Region: Located in the pons, medulla oblongata, and sacral spinal cord segments.
• Function: Associated with rest and digest activities.
• Effects: Decreases heart rate, lowers blood pressure, constricts pupils, stimulates digestion.
• Alternate Name: Craniosacral system.

Structure and Function of Ganglia:

Sympathetic Nervous System:

• Paravertebral Ganglia: Arranged in two chains alongside the vertebral column, including the superior cervical ganglion, middle cervical ganglion, stellate ganglion, thoracic ganglia, and lumbar ganglia.
• Prevertebral Ganglia: Examples include the celiac plexus, inferior mesenteric ganglion.

Parasympathetic Nervous System:

• Ganglia Located Near or at the Target Organ: Examples include the ciliary ganglion, pterygopalatine ganglion, submandibular ganglion, sublingual ganglion.

Axon Characteristics:

• Sympathetic: Short preganglionic fibers, long postganglionic fibers (due to ganglia being farther from the target organ).
• Parasympathetic: Long preganglionic fibers, short postganglionic fibers (due to ganglia being close to the target organ).

Neurotransmitters and Receptors:

• Acetylcholine: Neurotransmitter of sympathetic preganglionic fibers, parasympathetic preganglionic fibers, parasympathetic postganglionic neurons, sweat glands, arrector pili muscles, and some blood vessels.
• Norepinephrine: Neurotransmitter of sympathetic postganglionic fibers (except for sweat glands, arrector pili muscles, and some blood vessels).

Acetylcholine Receptors:

• Cholinergic Receptors:
  • Nicotinic: Found in ganglia, skeletal muscle.
  • Muscarinic: Located in target organs of the parasympathetic system.

Norepinephrine Receptors:

• Adrenergic Receptors:
  • Alpha 1: Smooth muscle of blood vessels, sweat glands.
  • Alpha 2: Smooth muscle of blood vessels, pancreatic cells.
  • Beta 1: Cardiac muscle, sinoatrial node, AV node.
  • Beta 2: Smooth muscle of blood vessels, walls of hollow organs.
  • Beta 3: Adipose tissue.

Effects of the ANS on Different Organs:

Organ	Sympathetic System	Parasympathetic System
Eye	Dilates pupils	Constricts pupils
Glands	Reduces secretion, vasoconstriction	Increases secretion
Sweat Glands	Sweating	Sweating in palms
Blood Vessels	Vasoconstriction	Vasodilation or no effect
Heart	Increases heart rate, increased contractility	Decreases heart rate, decreased contractility
Lungs	Dilates bronchioles	Constricts bronchioles
Gut	Decreases motility	Increases motility
Liver	Releases glucose	Increases glycogen synthesis
Gallbladder	Relaxes	Contracts
Kidneys	Decreases urine output	–
Bladder	Relaxes	Contracts
Adrenal Medulla	Releases epinephrine and norepinephrine	–

ANS Tone:

• The activities of both divisions occur continuously, and the balance of this activity is called tone.
• Tone is established by epinephrine, norepinephrine secretion, and partly by direct sympathetic stimulation.
• ANS tone plays a critical role in maintaining normal organ function.

Regulation of ANS Activity:

• Hypothalamus: The highest central region of the ANS.
• Reticular Formation: Regulates respiration, cardiovascular functions.
• Cerebral Cortex: Influences ANS activity based on emotions and mood.
• Hormones: Thyroxine, adrenaline, noradrenaline stimulate the sympathetic system.

Note:

• Compensation: If one division is removed, the remaining division compensates to maintain normal organ function.
• Sensitivity: Organs become more sensitive to the neurotransmitter of the removed division due to increased receptor numbers.
• Effect of Epinephrine and Norepinephrine: Epinephrine exerts a stronger effect on the cardiovascular system than norepinephrine.
• Selective Action of Neurotransmitters and Drugs: Medications can be designed to target specific receptors selectively.

Conclusion:

The autonomic nervous system is a complex system that plays a critical role in controlling involuntary bodily functions. Understanding the structure and function of this system provides insight into how organs work and how these functions can be regulated.