Autonomic Nervous System (ANS)
The Autonomic Nervous System, or ANS, is the involuntary part of the nervous system. It controls automatic functions of the body such as heartbeat, breathing, digestion, and glandular activities. Although these actions are not under voluntary control, a person can sometimes sense their effects — for example, a rapid heartbeat during stress.
Divisions of Autonomic Nervous System
The ANS is divided into two major parts:
- Sympathetic Nervous System (Thoracolumbar Outflow)
- Parasympathetic Nervous System (Craniosacral Outflow)
Both systems work together to maintain homeostasis and regulate vital body functions like blood pressure, digestion, temperature, and metabolic rate.
General Role of the ANS
The Sympathetic Nervous System becomes more active during stress, danger, or physical activity — preparing the body for “fight or flight.” The Parasympathetic Nervous System dominates during rest and digestion, promoting “rest and repair” functions.
Effector Organs of the ANS
- Smooth muscles (airways, blood vessels)
- Cardiac muscles (heart rate and force)
- Glands (digestive and other secretions)
Structure of Autonomic Pathways
Each pathway includes two neurons:
- Preganglionic neuron: Originates in the brain or spinal cord.
- Postganglionic neuron: Connects from ganglion to the target organ.
Sympathetic Nervous System
The sympathetic division originates in the thoracic and lumbar regions of the spinal cord, hence called the thoracolumbar outflow.
Preganglionic Neurons
These neurons have their cell bodies in the spinal cord (T1 to L2/L3 levels). They release acetylcholine as the neurotransmitter at sympathetic ganglia.
Postganglionic Neurons
These neurons extend from the ganglia to the target organs, releasing noradrenaline (norepinephrine) as the neurotransmitter. However, those that supply sweat glands and skeletal muscle blood vessels release acetylcholine.
Sympathetic Ganglia
There are two types of sympathetic ganglia:
- Lateral Chain (Paravertebral) Ganglia: Extend from the neck to the sacrum on both sides of the vertebral column. This chain allows rapid and widespread activation of the sympathetic response.
- Prevertebral Ganglia: Located in the abdominal cavity near arteries. The main ones are:
- Coeliac Ganglion
- Superior Mesenteric Ganglion
- Inferior Mesenteric Ganglion
Parasympathetic Nervous System
The parasympathetic system originates in the brain (cranial outflow) and sacral region of the spinal cord (sacral outflow). It uses acetylcholine at both synapses.
Preganglionic Neurons
These neurons are long and originate from the brain (cranial nerves III, VII, IX, X) or sacral spinal cord segments (S2–S4). They synapse close to or within the effector organs.
Postganglionic Neurons
These neurons are short and located in or near the organ they control, ensuring specific and localized responses.
Functions of the Autonomic Nervous System
The ANS regulates involuntary reflexes like heartbeat, blood vessel contraction, digestion, and glandular secretions. It ensures smooth coordination between body organs to maintain internal balance.
Sympathetic Functions
- Prepares body for stress (“fight or flight”)
- Increases heart rate and blood pressure
- Dilates pupils and airways
- Stimulates adrenal glands to release adrenaline and noradrenaline
- Boosts metabolic rate and energy production
Parasympathetic Functions
- Promotes relaxation and energy conservation
- Slows heart rate and breathing
- Enhances digestion and nutrient absorption
- Stimulates urination and defecation
Together, the two systems maintain the body’s balance — ensuring that energy is available during stress and that recovery and repair occur during rest.
Detailed Notes:
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