Autonomic Nervous System (ANS)

Overview and Organization

The autonomic nervous system (ANS) is a major component of the peripheral nervous system responsible for regulating involuntary bodily functions. It consists of motor neurons that innervate smooth muscle, cardiac muscle, and glands, operating largely via subconscious control. The ANS is also known as the involuntary nervous system or visceral motor system.

• Functions: Adjusts heart rate, blood pressure, digestive processes, and shunts blood to areas of need.

• Effectors: Smooth muscle, cardiac muscle, glands.

• Control: Operates without conscious input.

ANS vs. Somatic Nervous System

The ANS and somatic nervous system both contain motor fibers but differ in their effectors, neurotransmitters, and pathways. The somatic system controls voluntary movements of skeletal muscles, while the ANS regulates involuntary functions.

• Somatic Nervous System: Releases acetylcholine (ACh); always excitatory; single neuron pathway.

• ANS: Preganglionic fibers release ACh; postganglionic fibers release ACh or norepinephrine (NE); can be excitatory or inhibitory; two-neuron pathway.

• Effectors: Somatic - skeletal muscle; ANS - cardiac muscle, smooth muscle, glands.

Integration of Somatic and Autonomic Systems

Higher brain centers regulate and coordinate both somatic and autonomic systems. Most spinal and many cranial nerves contain both types of fibers, allowing adaptations that involve both skeletal muscles and visceral organs.

• Example: During exercise, skeletal muscles require more oxygen and glucose, so the ANS increases heart rate and opens airways.

Divisions of the Autonomic Nervous System

Parasympathetic Division

The parasympathetic division promotes maintenance functions and conserves body energy. It dominates during rest and relaxation, keeping body energy use low while carrying out maintenance activities.

• Effects: Low blood pressure, heart rate, and respiratory rate; high gastrointestinal activity; pupils constricted for close vision.

• Example: Relaxing and reading after a meal.

Sympathetic Division

The sympathetic division mobilizes the body during activity, especially in situations of exercise, excitement, emergency, or embarrassment. It prepares the body for "fight or flight" responses.

• Effects: Increased heart rate, dry mouth, cold and sweaty skin, dilated pupils, shunting blood to skeletal muscles and heart, dilating bronchioles, and causing the liver to release glucose.

• Example: Vigorous physical activity.

Dual Innervation and Homeostasis

Most visceral organs receive input from both sympathetic and parasympathetic divisions, which often have opposing effects. This dynamic antagonism maintains homeostasis.

• Antagonistic Effects: Sympathetic accelerates heart rate; parasympathetic slows it down.

• Example: Iris of the eye - sympathetic dilates pupil, parasympathetic constricts it.

• Cooperative Effects: Saliva production - sympathetic stimulates mucus, parasympathetic stimulates enzyme secretion.

Neurotransmitters and Receptors

Key Neurotransmitters

The contrasting effects of the ANS divisions are largely due to the neurotransmitters they use and the receptors present in target cells.

• Acetylcholine (ACh): Used by all preganglionic neurons and parasympathetic postganglionic neurons.

• Norepinephrine (NE): Used by sympathetic postganglionic neurons.

Cholinergic and Adrenergic Neurons

• Cholinergic: Neurons that secrete ACh; cells with receptors for ACh.

• Adrenergic: Neurons that secrete NE; cells with receptors for NE.

Anatomical Differences Between Sympathetic and Parasympathetic Divisions

Key Anatomical Features

• Sites of Origin: Parasympathetic fibers are craniosacral (brain and sacral spinal cord); sympathetic fibers are thoracolumbar (thoracic and lumbar spinal cord).

• Relative Lengths of Fibers: Parasympathetic has long preganglionic and short postganglionic fibers; sympathetic has short preganglionic and long postganglionic fibers.

• Location of Ganglia: Parasympathetic ganglia are near or in the visceral effectors; sympathetic ganglia are close to the spinal cord.

Visceral Reflexes

Components and Differences

Visceral reflex arcs are similar to somatic reflex arcs but have key differences. They regulate involuntary functions such as emptying the rectum and bladder, and blood pressure control.

• Components: Receptor, afferent neuron, interneuron, efferent neuron, effector.

• Differences: Visceral reflex arc has two consecutive neurons in the motor pathway; afferent fibers are visceral sensory neurons; effectors are smooth muscle, cardiac muscle, and glands.

• Example: Baroreflex for blood pressure regulation is a negative feedback loop.

Neural Pathways

Somatic vs. Autonomic Pathways

• Somatic: Motor neuron axons from CNS reach directly to skeletal muscle.

• Autonomic: Signal travels across two neurons (preganglionic and postganglionic) to reach the effector, synapsing in an autonomic ganglion.

Clinical Relevance and Disorders of the ANS

Control of ANS Activity

• CNS Centers: Brainstem, spinal cord, hypothalamus, and cerebral cortex.

• Main Integrative Center: Hypothalamus.

• Cerebral Input: May modify ANS activity subconsciously via limbic system structures.

Disorders of the ANS

• Autonomic Neuropathy: Damage to autonomic nerves, often seen in diabetes mellitus. Symptoms include sexual dysfunction, dizziness, urinary incontinence, sluggish pupil reactions, and impaired sweating. Prevention involves maintaining good blood glucose levels.

• Hypertension: Overactive sympathetic vasoconstrictor response to stress; treated with adrenergic receptor-blocking drugs (beta blockers).

• Raynaud’s Disease: Painful, exaggerated vasoconstriction in fingers and toes; treated with vasodilators.

• Autonomic Dysreflexia: Life-threatening uncontrolled activation of autonomic neurons in individuals with spinal cord injuries; can cause dangerously high blood pressure and increased risk of stroke.

Summary Table: Comparison of Somatic and Autonomic Motor Neurons