The Autonomic Nervous System: Controlling the Body Automatically

The autonomic nervous system, also known as the involuntary nervous system, is a complex network that controls automatic bodily functions such as heart rate, breathing, digestion, and excretion. This system operates independently of our conscious awareness.

The autonomic nervous system consists of two main branches: the sympathetic nervous system and the parasympathetic nervous system, which work antagonistically to maintain the body’s internal balance (homeostasis).

The Sympathetic Nervous System: “Fight or Flight”

The sympathetic nervous system is activated when the body encounters danger or stress. It prepares the body for action by:

• Increasing heart rate and contractile force of the heart: delivering more blood and oxygen to the muscles.

• Dilating pupils: enhancing vision in low light conditions.

• Dilating airways: increasing oxygen intake.

• Increasing glycogen and lipid breakdown: providing energy for muscle activity.

• Decreasing digestive and urinary system activity: focusing energy on organs essential for the response.

The central portion of the sympathetic system lies within the gray matter of the lateral horn of the spinal cord from the first thoracic (T1) to the third lumbar (L3) segments. Sympathetic nerve fibers travel from here to the paravertebral or prevertebral ganglia, then to the target organs.

The Parasympathetic Nervous System: “Rest and Digest”

The parasympathetic nervous system works in opposition to the sympathetic system, helping the body relax and recover from activity. It:

• Decreases heart rate: slowing down the heart rate and reducing oxygen consumption.

• Constricting pupils: narrowing the field of vision, allowing the eye to focus better in bright light conditions.

• Constricting airways: decreasing oxygen intake.

• Increasing digestive and urinary system activity: promoting digestion and excretion.

The central portion of the parasympathetic system resides in two locations: the nuclei of cranial nerves III, VII, IX, X and the gray matter of the sacral segments S2 to S4 of the spinal cord. Parasympathetic nerve fibers travel from here to terminal ganglia located near or within the walls of the target organs.

Balancing the Two Systems

The sympathetic and parasympathetic nervous systems operate in tandem and constantly adjust each other to maintain the body’s internal balance. This balance is crucial for the health and proper functioning of all organs in the body.

Any disruptions in the functioning of the autonomic nervous system can lead to various health issues such as high blood pressure, digestive disorders, anxiety disorders, and depression.