Autonomic Nervous System Overview

Autonomic Nervous System

• The nervous system is divided into two main divisions: central nervous system (CNS) and peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS encompasses nerves located outside the brain and spinal cord.
• The autonomic nervous system (ANS) is largely independent (autonomous) in its functions, not under direct conscious control.
• The ANS primarily handles visceral functions such as heart rate, blood flow, and digestion.

ANS Structure

• The ANS carries nerve impulses from the CNS to effector organs via preganglionic and postganglionic neurons.
• Ganglia act as relay stations, transmitting impulses between preganglionic and postganglionic neurons.
• The sympathetic nervous system has short preganglionic and long postganglionic fibers.
• The parasympathetic nervous system has long preganglionic and short postganglionic fibers.

Functions of the Sympathetic Nervous System

• Increases heart rate and contractility, thus elevating blood pressure.
• Constricts blood vessels in skin, mucous membranes, and splanchnic area, while dilating skeletal muscle vessels.
• Dilates pupils.
• Bronchodilation (widening of the bronchial tubes).
• Inhibits salivation.
• Decreases gastrointestinal (GI) motility.
• Stimulates ejaculation.
• Inhibits bladder contraction.
• Stimulates glucose production and release.
• Contributes to the "fight-or-flight" response during emergencies.

Functions of the Parasympathetic Nervous System

• Maintains homeostasis within the body.
• Primarily involved in functions such as digestion and waste elimination.
• Generally opposes the actions of the sympathetic system in "rest-and-digest" situations.
• Pupil contraction (miosis).
• Bronchoconstriction (narrowing of bronchial tubes).
• Stimulating digestion.
• Stimulating the gallbladder and bladder.
• Relaxing the rectum.
• Causing vaginal lubrication and facilitating erection.

Enteric Nervous System (ENS)

• ENS is a portion of the peripheral nervous system in the gastrointestinal tract.
• The ENS is responsible for motility, exocrine and endocrine secretions, and microcirculation in the gastrointestinal tract.

Somatic Nervous System

• The somatic nervous system carries motor and sensory information.
• The system mediates voluntary muscle movements.
• It also receives sensory information from the skin, sensory organs, and skeletal muscles.

Reflex Arc

• The ANS and the somatic nervous system work together in reflex arcs.
• Sensory input from the body travels to integrating centers in the CNS, triggering a response by the CNS.

Neurotransmitters

• The primary chemical signals in the ANS are acetylcholine and norepinephrine, involved in various functions.
• The ANS has cholinergic and adrenergic neurons based on neurotransmitter release (ACh or norepinephrine, respectively).
• Acetylcholine mediates transmission across autonomic ganglia in both sympathetic and parasympathetic systems.
• Neurotransmission involves synthesis, storage, release, receptor binding, degradation, and recycling of neurotransmitters.

Cholinergic Receptors

• Cholinergic receptors are categorized as muscarinic and nicotinic.
• Muscarinic receptors have high affinity for muscarine, while low for nicotine.
• Nicotinic receptors have high affinity for nicotine, and low affinity for muscarine.
• Cholinergic receptors are located in sympathetic and parasympathetic nervous systems.

Cholinergic Agonists

• Cholinergic agonists mimic the action of acetylcholine.
• Examples include acetylcholine, carbachol, and bethanechol.

Cholinergic Antagonists

• Cholinergic antagonists block the action of acetylcholine.
• Examples include atropine.