The Autonomic Nervous System Overview

The Autonomic Nervous System

• The autonomic nervous system controls subconscious bodily functions.
• It's divided into two branches: sympathetic and parasympathetic.

Sympathetic Nervous System

• Also known as "fight-or-flight"
• Associated with survival in emergency situations
• Responsible for energy expenditure.
• Causes an increase in heart rate, force of heart contraction, and dilation of bronchioles and pupils.
• Decreases GI motility and function.
• Increases blood flow to muscles.
• Decreases blood flow to other organs.

Parasympathetic Nervous System

• Also known as "rest-and-digest" or "rest-and-restore."
• Associated with processes that occur in the normal resting state.
• Responsible for energy conservation.
• Causes decreased heart rate, force of heart contraction and constriction of bronchioles and pupils.
• Increases in GI motility and function.

Structure of the Autonomic Nervous System

• Sympathetic nervous system: arises from thoracic and lumbar regions of the spinal cord, often referred to as the thoracolumbar system.
• Parasympathetic nervous system: arises from the brain and sacral region of the spinal cord, often referred to as the craniosacral system.
• The autonomic nervous system involves two neurons:
  • First neuron (preganglionic): originates in the central nervous system (CNS) and extends to a cluster of nerve cells called an autonomic ganglion.
  • Second neuron (postganglionic): nerve cell bodies located in the autonomic ganglion. Its axon connects to the target organ (muscle or gland).

Neurotransmitters

• Sympathetic:
  • Primary neurotransmitter is norepinephrine.
  • Related neurotransmitters are norepinephrine, epinephrine (adrenaline), and dopamine.
  • Neurons that release norepinephrine are called adrenergic.
  • Norepinephrine/epinephrine is released into bloodstream directly from the adrenal medulla.
• Parasympathetic:
  • Acetylcholine is the primary neurotransmitter.
  • Neurons that release acetylcholine are referred to as cholinergic neurons.
  • All preganglionic neurons are cholinergic. The postganglionic neuron determines if the effect is sympathetic or parasympathetic.

Receptors involved (examples)

• Sympathetic:

  • Alpha 1, 2 adrenergic receptors, Beta 1, 2 adrenergic receptors

• Parasympathetic:

  • Nicotinic, Muscarinic cholinergic receptors

• Alpha 1 adrenergic: cause vasoconstriction in certain blood vessels.

• Beta 1 adrenergic: increases heart rate and force of contraction

• Beta 2 adrenergic: causes bronchodilation (opening of bronchioles)

• Nicotinic receptors in sympathetic/parasympathetic: Found on the second neuron (postganglionic) in both systems, but also found in the somatic (voluntary) system between motor neurons and muscle.

• Muscarinic receptors in parasympathetic: Found on target organs and tissues supplied by postganglionic neurons of the parasympathetic nervous system (PSNS).

Summary of impulses in the Autonomic Nervous System

• Presynaptic neurons extend to autonomic ganglia.
• Presynaptic neurons release acetylcholine into the synaptic cleft.
• Acetylcholine binds to nicotinic ganglionic receptors on postganglionic neurons.
• Postsynaptic neurons transmit impulses to the end organs, releasing either norepinephrine (sympathetic) or acetylcholine (parasympathetic).

Drugs affecting the Autonomic Nervous System

• Some drugs enhance neurotransmission (receptor agonists).
• Some drugs induce neurotransmitter release.
• Some drugs prevent neurotransmitter breakdown.
• Other drugs suppress neurotransmission (presynaptic blockers, receptor antagonists, ganglion blockers).

Clinical Relevance of the Autonomic Nervous System

• Clinical conditions—Epinephrine (adrenaline) is used to treat conditions like anaphylaxis, cardiac arrest or shock
• Treatments—Atropine is to treat conditions like bradycardia or respiratory distress, and blocks "rest and digest."

Explore the intricacies of the autonomic nervous system, including its two main branches: sympathetic and parasympathetic. Understand their functions, effects on the body, and how they help maintain homeostasis during different situations, from emergencies to restful states.