Autonomic Nervous System

Physiology

• ANS is a division of the peripheral nervous system that supplies internal organs, smooth muscle and glands.
• It is a control system that acts unconsciously and regulates bodily functions such as:
  • Heart rate and its force of contraction
  • Digestion
  • Respiratory rate
  • Pupillary response
  • Urination
  • Sexual arousal
  • Fight-or-flight response
  • Vasomotor activity (the vasomotor center)
  • Certain reflex actions such as coughing, sneezing, swallowing and vomiting.

• ANS is regulated by integrated reflexes through the brain-stem to the spinal cord and organs.
• Hypothalamus acts as an integrator for autonomic functions, receiving autonomic regulatory input from the limbic system.

ANS branches

1. Sympathetic nervous system
2. Parasympathetic nervous system
3. Visceral / Sensory nervous system
4. Enteric nervous system

Sympathetic division

• It consists of cells with bodies in the lateral grey column from T1 to L2/3.
• These cell bodies are preganglionic neurons

• These all contain Afferent (sensory) nerves as well, known as General Visceral Afferent neurons.

Parasympathetic division

• It consists of cells with bodies in one of two locations:
  1. Brainstem (Cranial Nerves III, VII, IX, X)
  2. Sacral spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:

• These ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are:
  • Postganglionic parasympathetic splanchnic (visceral) nerves
  • Vagus nerve which innervates heart, lungs, liver and stomach

Visceral / Sensory neurons

• It is composed of primary visceral sensory neurons found in the peripheral nervous system (PNS) in cranial sensory ganglia
• Visceral sensory information constantly and unconsciously modulates the activity of the motor neurons of ANS.
• They monitor the
  • Levels of carbon dioxide, oxygen and sugar in the blood
  • Arterial pressure
  • Chemical composition of the stomach and gut content.
  • Sense of taste and smell, which, unlike most functions of the ANS, is a conscious perception.
  • Input from a nearby chemosensory center: The memory that ensures that an animal that has been poisoned by a food never touches it again
  • All this
  • Pain in any internal organ is perceived as referred pain, more specifically as pain from the dermatome corresponding to the spinal segment.

Target organ/system	Parasympathetic	Sympathetic
Digestive system	Increase peristalsis Increase secretion of digestive glands	Decrease activity of digestive system
Liver	No effect	Releasing glucose to blood
Lungs	Constricts bronchioles	Dilates bronchioles
Urinary bladder/ Urethra	Relaxes sphincter	Constricts sphincter
Kidneys	No effects	Decrease urine output
Heart	Decreases rate	Increase rate
Blood vessels	No effect	Constricts blood vessels in viscera; increase BP
Salivary and Lacrimal glands	Increases saliva and tears	Dry mouth /eyes
Eye (iris)	Constrict pupils	Dilates pupils
Eye (ciliary muscles)	Increase bulging of lens for close vision	Decrease bulging of lens; prepares for distant vision
Adrenal Medulla	No effect	Increase secretion epinephrine / norepinephrine
Sweat gland of skin	No effect	Increase perspiration
Sexual organs	Erection of genital tissues Stimulating sexual arousal.

Autonomic Dysregulation