Study Notes on the Autonomic Nervous System (Chapter 14)

Understanding the ANS: Knowledge of the autonomic nervous system (ANS) is crucial for anticipating the effects and side effects of medications on patients.

Definition: The autonomic nervous system (ANS) consists of motor neurons that:
- Innervate smooth muscles, cardiac muscle, and glands.
- Make adjustments to ensure optimal support for body activities, adjusting blood flow, heart rate, blood pressure, digestive processes, etc.
- Operate under subconscious control, also known as the involuntary nervous system or visceral (autonomic) motor system.

Structural Variations: The ANS and somatic nervous system (SNS) both have motor fibers but differ in:
- Effectors:
  - SNS innervates skeletal muscles.
  - ANS innervates cardiac muscle, smooth muscle, and glands.
- Number of Neurons:
  - SNS uses one neuron from CNS to effectors.
  - ANS employs a two-neuron chain:
  - Preganglionic Neuron: Cell body in CNS; extends to a ganglion.
  - Postganglionic Neuron: Cell body in ganglion, extends to effector.
- Neurotransmitter Actions:
  - SNS: All motor neurons release acetylcholine (ACh), which is always excitatory.
  - ANS: Preganglionic fibers release ACh; postganglionic fibers release norepinephrine (NE) at most sympathetic fibers and ACh at most parasympathetic fibers. Effects can be excitatory or inhibitory depending on receptor types.

Somatic Nervous System: Cell body is in the CNS, single thick myelinated Group A axon extends to skeletal muscle.
Autonomic Nervous System: Uses a two-neuron chain:
- Preganglionic neuron with a thin, lightly myelinated axon extending to ganglion.
- Postganglionic neuron with a non-myelinated axon extending to effectors.

Somatic Nervous System: All somatic motor neurons release ACh, always producing excitatory effects.
ANS Effects:
- Preganglionic fibers release ACh.
- Postganglionic fibers:
  - Most sympathetic fibers release NE.
  - Most parasympathetic fibers release ACh.
- Effects vary based on receptor type, either excitatory or inhibitory.

Higher brain centers regulate and coordinate both SNS and ANS.
Spinal and cranial nerves contain both fibers, with adaptations typically involving both muscular and visceral responses.
Example: Active muscles demand more oxygen and glucose, leading to increased heart rate and opened airways via ANS interventions.

Two Main Divisions:
- Parasympathetic Division: Promotes maintenance functions, conserves energy, referred to as the “rest-and-digest” system.
  - Functions: Directs digestion, diuresis, and defecation.
  - Examples: Post-meal relaxation leads to low blood pressure, heart rate, increased gastrointestinal activity, and pupil constriction for close vision.
- Sympathetic Division: Mobilizes the body during activity, referred to as the “fight-or-flight” system.
  - Functions and Examples: Activation during exercise or stress leads to increased heart rate, decreased glandular activity, and blood shunting to muscles.

Key Differences:
- Sites of Origin:
  - Parasympathetic fibers are craniosacral, originating from the brain and sacral spinal cord.
  - Sympathetic fibers are thoracolumbar, originating from thoracic and lumbar regions.
- Relative Lengths of Fibers:
  - Parasympathetic division features long preganglionic and short postganglionic fibers.
  - Sympathetic division features short preganglionic and long postganglionic fibers.
- Location of Ganglia:
  - Parasympathetic ganglia are in or near effectors.
  - Sympathetic ganglia lie close to the spinal cord.

Preganglionic fibers originate from brain stem and sacral spinal cord, with long axons synapsing close to target organs.
Cranial Part:
- Fibers run in cranial nerves (e.g., oculomotor, facial, glossopharyngeal, vagus), controlling various glands and muscles.
Sacral Part: Originates from neurons in the sacral spinal cord, serving pelvic organs.

More complex than the parasympathetic division, innervating many more organs.
Sympathetic Trunks:
- Preganglionic fibers enter sympathetic trunk ganglia via white rami communicantes, with 23 ganglia typically organized in the trunk.
- Sympathetic pathways can synapse at the same level, a higher or lower level, or synapse in collateral ganglia.

Major Neurotransmitters: ACh and norepinephrine (NE).
- Cholinergic Fibers: Release ACh; all autonomic preganglionic and most parasympathetic postganglionic axons branch.
- Adrenergic Fibers: Release NE; most sympathetic fibers.
- Receptors: Types include nicotinic and muscarinic for ACh, alpha and beta for NE.

Reflex arcs begin with sensory input from visceral sensory neurons responding to changes in viscera.
Three neuron reflex arcs exist in gastrointestinal tract, involving enteric nervous systems.
Visceral reflex arcs consist of:
- Receptor
- Sensory neuron
- Integration center (often involving preganglionic neurons)
- Motor neuron (two-neuron pathway)
- Effector (smooth muscle, cardiac muscle, and glands).

Cholinergic Receptors
- Nicotinic: Excitatory, found at all postganglionic neurons and neuromuscular junctions.
- Muscarinic: May be excitatory or inhibitory based on organ type.
Adrenergic Receptors:
- Alpha: Typically excitatory; primarily involved in smooth muscle contraction.
- Beta: Can have various effects, including inhibition of smooth muscles and increased heart rate.

Controlled by several CNS centers, with the hypothalamus acting as the main integrative center.
Influences include cortical input via the limbic system and direct modulation by brainstem centers regulating bodily functions
Voluntary Control: Possible in some cases through biofeedback mechanisms.

Common disorders include autonomic neuropathy, hypertension, and Raynaud’s disease.
Associated symptoms of autonomic dysregulation can lead to complications such as dizziness upon standing, urinary incontinence, and difficulties with sweat regulation.

ANS structures develop from the neural tube and neural crest. Efficiency declines with age, leading to various conditions like constipation and orthostatic hypotension.