Sistema Nervioso Autónomo

• These notes pertain to the autonomic nervous system.

Introducción

• The autonomic nervous system controls involuntary physiological processes.
• It coordinates the activity of smooth muscles in visceral organs, cardiac muscle, and glands.
• Functions regulated include arterial pressure, digestive motility, sweating, and body temperature.

Organización

• The autonomic nervous system has two main divisions: the sympathetic and parasympathetic nervous systems.
• The Sympathetic Nervous System relates to the response to stress.
• The Parasympathetic Nervous System relates to vegetative responses.
• Both systems interact to regulate bodily functions.

Componentes Centrales

• The central components of the autonomic nervous system include the hypothalamus, brainstem, and spinal cord.

Hipotálamo

• The hypothalamus contains various nuclei with specific functions:
  • Preoptic and medial nuclei are associated with the secretion of gonadotropin-releasing hormone.
  • Anterior nuclei regulate thermoregulation.
  • Posterior nuclei regulate thermoregulation.
  • Ventromedial nuclei are the center of satiety; inhibits eating behavior.
  • Dorsomedial nuclei are associated with alimentation.
  • Suprachiasmatic nuclei regulate circadian rhythms.
  • Supraoptic nuclei secrete antidiuretic hormone and oxytocin.
  • Paraventricular nuclei secrete ADH and oxytocin, corticotropin-releasing hormone, and thyrotropin-releasing hormone.
  • Arcuate nuclei secrete growth hormone-releasing hormone.

Componentes Periféricos

• Peripheral components of the autonomic nervous system include preganglionic and postganglionic neurons.
• Preganglionic neurons originate from the medulla.
• Postganglionic neurons have ganglia that are either sympathetic or parasympathetic.
• Motor axons reach effector organs and tissues.
• Acetylcholine (ACh) is a neurotransmitter used in the autonomic nervous system.

Neurotransmisores

• Acetylcholine (ACh)
  • Acetylcholine is widely distributed in the nervous system.
  • It is involved in memory circuits, reward systems, and both the peripheral and autonomic nervous systems.
  • Inhibition of acetylcholinesterase results in continuous activation of muscles, glands, and the nervous system.
  • Nicotine and muscarine act as agonists (activators).
  • Botulinum toxin (inhibiting release), as well as atropine and scopolamine (blocking receptors) are antagonists.
• Noradrenaline (norepinephrine)
  • Noradrenaline is synthesized from dopamine.
  • It can be released by the adrenal gland (as a hormone) or in the central and peripheral nervous systems (as a neurotransmitter).
  • It mediates stress responses.
  • It increases heart rate.
  • It facilitates glucose release.
  • It increases blood flow to skeletal muscle.
  • It increases oxygen supply to the brain.
• Adrenaline (epinephrine)
  • Adrenaline is produced by the adrenal glands.
  • It increases heart rate.
  • It increases arterial pressure (vasoconstriction).
  • It causes bronchodilation.

Organización del Sistema Nervioso Simpático

• The sympathetic nervous system releases noradrenaline (NA), except in sweat glands (where it releases ACh).
• The adrenal medulla, part of the SNS, releases adrenaline and, to a lesser extent, noradrenaline when stimulated.
• Other postganglionic NTs of the SNS include neuropeptide Y, ATP, and substance P.

Organización del Sistema Nervioso Parasimpático

• Acetylcholine (ACh) acts on muscarinic receptors.

Receptores

• Receptors couple to the cell membrane and transduce signals in effector cells.
• ACh receptors are classified into nicotinic and muscarinic types.
• NA and adrenaline receptors are classified based on their response to specific drugs.
  • Alpha-adrenergic receptors: α1, α2.
  • Beta-adrenergic receptors: β1, β2.
• The response of a tissue/organ to sympathetic stimulation depends on the type of receptor activated.

Effects of the Sympathetic Nervous System

• Eye: Mydriasis (pupil dilation).
• Lacrimal and salivary glands: Decreased secretions.
• Lungs: Bronchodilation.
• Heart: Increased heart rate and contractility.
• Liver: Increased hepatic glucose release.
• Stomach: Decreased contractility.
• Intestine: Increased sphincter contraction.
• Bladder: Relaxation.
• Genitalia: Ejaculation.

Effects of the Parasympathetic Nervous System

• Eye: miosis (pupil constriction).
• Lacrimal and salivary glands: Increased secretion.
• Lungs: Bronchoconstriction.
• Heart: Decreased heart rate.
• Intestines: Increased motility.
• Rectum: Relaxation.
• Bladder: Contraction.

Parasympathetic action by location

• SA node: decreases heart rate and uses muscarinic receptors.
• Heart muscle: decreases atrial contractility and uses muscarinic receptors.
• AV node: decreases conduction velocity and uses muscarinic receptors.
• Vascular smooth muscle: vasodilation and uses muscarinic receptors.
• Gastrointestinal smooth muscle: increase motility and uses muscarinic receptors.
• Gastric Parietal cells: acid secretion and uses muscarinic receptors.
• Pancreas: exocrine secretion; pancreatic juice and uses muscarinic receptors.
• Pulmonary and bronchial smooth muscle: contraction and uses muscarinic receptors.
• Male reproductive system: Erection and uses muscarinic receptors.
• Female reproductive system: vascular congestion, vaginal lubrication and uses muscarinic receptors.
• Pupil: Miosis (constriction) and uses muscarinic receptors.

Sympathetic action by location

• SA node: increases heart rate and uses β1 receptors.
• Heart muscle: increases contractility and uses β1 receptors.
• AV node: increases conduction velocity and uses β1 receptors.
• Vascular smooth muscle: constriction (majority of blood vessels) and uses α1 receptors.
• Vascular smooth muscle: vasodilation and uses β2 receptors.
• Gastrointestinal smooth muscle: reduces motility and uses α2, β2 receptors.
• Gastrointestinal smooth muscle: contracted sphincters and uses α1 recptors.
• Pulmonary and bronchial smooth muscle: dilation and uses β2 receptors.
• Sudoriferous Glands: Generalized secretion and uses muscarinic receptors.
• Sudoriferous Glands: Specific secretion and uses α1 receptors.
• Male reproductive system: Ejaculation and uses α receptors.
• Female reproductive system: smooth muscle contraction during orgasm and uses α receptors.
• Pupil: Mydriasis and uses α1 receptors.

Comparative Characteristics: Sympathetic vs. Parasympathetic

• Location of preganglionic nerve cells:
  • Parasympathetic: Brainstem nuclei (III, VII, IX, X) and sacral spinal cord (S2-S4).
  • Sympathetic: Intermediolateral and intermediomedial cell columns of the thoracolumbar spinal cord (T1-L2).
• Location of ganglia:
  • Parasympathetic: Adjacent target organs.
  • Sympathetic: Paravertebral and prevertebral ganglia.
• Neurotransmitter at preganglionic neurons:
  • Parasympathetic: Acetylcholine (nicotinic receptors).
  • Sympathetic: Acetylcholine (nicotinic receptors).
• Principal neurotransmitter released by postganglionic neuron:
  • Parasympathetic: Acetylcholine (muscarinic receptors).
  • Sympathetic: Noradrenaline (adrenergic alpha and beta receptors).