Lesson 12

Efferent Architecture of the ANS

  • The autonomic nervous system (ANS) has two main divisions: sympathetic nervous system (SNS) and parasympathetic nervous system (PNS).

    • SNS: "fight or flight"
    • PNS: "feed and breed" or "resting and digesting"

  • Key Characteristics of ANS Efferent Architecture:

    • 2 divisions: sympathetic and parasympathetic
    • 2 neurons involved: preganglionic and postganglionic
    • 2 message types: electrical (action potential) and chemical (neurotransmitters)
    • 2 neurotransmitter release sites: ganglia and effectors
    • 2 synapse types: neuron-to-neuron and neuron-to-non-neuron
    • 2 major neurotransmitters: acetylcholine (ACh) and norepinephrine (NE)
    • 2 receptor families: cholinergic and adrenergic

  • Important Notes:

    • Exception: Adrenal medulla lacks postganglionic neuron; it secretes epinephrine (Epi) and norepinephrine (NE) into the bloodstream.

Comparative Anatomy & Physiology

  • Origin of Neurons:

    • SNS: Thoracolumbar region (T1 - L3)
    • PNS: Craniosacral (CN 3, 7, 9, 10 and S2-S4)

  • Ganglia Location:

    • SNS: Near spinal cord (sympathetic chain)
    • PNS: Near or inside effector tissue/organ

  • Post-to-Preganglionic Fiber Ratio:

    • SNS: 1:1 to 3:1 (precise control)
    • PNS: 30:1 (providing mass response)

  • Fiber Characteristics:

    • SNS: Short, myelinated B fibers (preganglionic), Long, unmyelinated C fibers (postganglionic)
    • PNS: Long, myelinated B fibers (preganglionic), Short, unmyelinated C fibers (postganglionic)

  • Neurotransmitters:

    • ACh at ganglia (nicotinic type N receptors)
    • NE from postganglionic fibers (adrenergic receptors: alpha, beta, dopamine)
    • ACh (muscarinic receptors) at sweat glands and some blood vessels

  • Receptors:

    • Ganglia: Nicotinic type N (NN)
    • Effector organs: Adrenergic receptors (a- and ß-), Muscarinic receptors