ANS

Overview of Autonomic Nervous System (ANS)

  • The Autonomic Nervous System (ANS) is part of the Peripheral Nervous System (PNS).

    • The PNS includes other elements beyond the ANS.

    • Important to pay attention to acronyms in their specific context.

Nervous System Division

  • The ANS is divided into two primary pathways:

    • Sympathetic Nervous System (SNS)

    • Parasympathetic Nervous System (PNS)

  • The pathways typically have opposite effects on the same organs.

    • Sympathetic Pathway:

    • Causes dilation of airways (relaxation).

    • Parasympathetic Pathway:

    • Causes constriction of airways.

Functions of the Autonomic Nervous System

  • The PNS is often described as responsible for the "rest and digest" response.

    • Stimulates digestion and other restorative processes.

  • The SNS is involved in the "fight or flight" response.

    • Inhibits digestion, rerouting energy to muscle function in stressful situations (e.g., fleeing an attacker).

Specific Organs and Innervation

  • Some organs exhibit specific types of innervation:

    • Sympathetic Exceptions:

    • Adrenal gland

    • Sweat glands

    • Piloerector muscles

    • The cardiovascular system, particularly arteries, is under only sympathetic control, with no parasympathetic innervation.

Interaction of the Sympathetic and Parasympathetic Systems

  • Generally, the two systems work against each other, creating balance (homeostasis).

  • Exception to this is the sexual organs:

    • Both systems work together during processes like ejaculation, erection, and orgasm.

Signals in ANS Pathways

  • Both systems can send excitatory and inhibitory signals depending on the body context.

  • The state of the body determines the nature of the signals from these pathways.

Regulation of Internal Functions

  • The ANS regulates numerous internal functions:

    • Blood pressure

    • Body temperature

    • Gastrointestinal motility and nutrient absorption

    • Glandular secretion (e.g., sweat and saliva)

    • Hormonal release

    • Blood glucose levels

    • Sexual function

Mechanisms of Control

  • Homeostatic control requires three elements:

    1. Sensory Receptors:

    • Found throughout the body (somatic and visceral receptors).

    1. Decision Center:

    • Integrates sensory input and compares it to the set point (e.g., hypothalamus).

    1. Effectors:

    • Specific organs that execute the response.

  • The integration center is primarily the hypothalamus.

    • It monitors parameters and directs the response through sympathetic or parasympathetic pathways.

  • In cases of high body temperature, the hypothalamus triggers increased sweating to eliminate excess heat.

Pathway Composition

  • Information flows from the Central Nervous System (CNS) to effectors through nerves with the following structure:

    • Preganglionic Fibers:

    • Originate from the CNS and synapse at autonomic ganglia.

    • Postganglionic Neurons:

    • Relay the signals to the target organs,

    • Can innervate multiple tissue types.

Varicosities

  • Postganglionic fibers exhibit varicosities, which release neurotransmitters at various points along the fibers, influencing smooth muscles and other tissues.

Types of Neurons in ANS

  • Somatic Motor Neurons:

    • Typically one neuron innervates multiple skeletal muscle fibers.

  • Autonomic Motor System:

    • A single preganglionic fiber may innervate multiple different tissues.

Neurochemistry of the Pathways

  • Sympathetic Pathway:

    • Preganglionic generally releases acetylcholine (cholinergic),

    • Postganglionic primarily releases norepinephrine (adrenergic).

    • The adrenal medulla directly releases epinephrine (adrenaline) into the bloodstream.

  • Parasympathetic Pathway:

    • Entirely cholinergic, utilizing acetylcholine at both the ganglion and the target tissues (binding muscarinic receptors).

Summary of Neurotransmitters

  • Epinephrine (adrenaline):

    • Predominant in adrenal gland, acts on blood-borne targets.

  • Norepinephrine (noradrenaline):

    • More common neurotransmitter outside adrenal input.

Recap of Key Points

  • The two systems' structures lead to different actions on organs based on their neurotransmitter activity and receptor binding.

  • Homeostasis is maintained through the balance of sympathetic and parasympathetic signals acting on various systems of the body.

Additional Considerations

  • Familiarity with alpha and beta adrenergic receptors is critical for understanding cardiovascular function.

  • Differences in locations of autonomic ganglia (sympathetic closer to CNS, parasympathetic closer to target tissues) signify differing functional dynamics of these pathways.

  • Receptor characteristics:

    • Cholinergic Pathways: Acetylcholine is the neurotransmitter here.

    • Adrenergic Pathways: Norepinephrine is utilized for signaling in the sympathetic nervous system, with a complex interaction with epinephrine from the adrenal medulla.