Autonomic Nervous System - DEM Year 1

Autonomic Nervous System Biology Course Notes

• Course: The Body: Movement and Function (BMF)
• Lecturer: Dr. Ebrahim Rajab ([email protected])
• Date: 10/11/2024
• Class: DEM Year 1

Learning Objectives

• Recall the divisions of the nervous system
• Contrast the anatomical features of the sympathetic and parasympathetic systems
• Identify the functions of the sympathetic and parasympathetic nervous systems
• Describe the neurotransmitters and receptors located in the ANS

Divisions of the Nervous System

• Nervous System:

  • Central Nervous System (CNS): Brain and spinal cord
  • Peripheral Nervous System (PNS):
    • Autonomic Nervous System (ANS): Communicates with internal organs and glands
    • Somatic Nervous System: Communicates with sense organs and voluntary muscles
      • Sensory (afferent) nervous system: Sensory input
      • Motor (efferent) nervous system: Motor output

• ANS Divisions:

  • Sympathetic: Arousal ("fight or flight")
  • Parasympathetic: Calming ("rest and digest")

Divisions of the Autonomic Nervous System (ANS)

• The ANS is involuntary and maintains homeostasis.
• Regulates internal organs (heart, circulation, digestion, respiratory system).
• Has two divisions (sympathetic and parasympathetic).
• Most visceral organs have dual innervation by both divisions.
• Divisions exert mostly opposing effects.
• Partial activation (tonic activity) is common under most circumstances.

Internal Organs/Viscera Controlled by ANS

• Specific organs, such as the heart, lungs, stomach and GIT, spleen, pancreas, bladder, rectum, kidneys, liver, and pupils, are controlled by the ANS

Control of the ANS

• CNS: Central control of ANS output.
• Brain Stem: Medulla and Pons control cardiovascular, respiratory and digestive systems.
• Hypothalamus: Major role in controlling heart rate, blood pressure, and respiration (via medulla).
• Spinal Cord: Integrates autonomic reflexes not subject to higher control (e.g., urination, defecation).

Overview of Sympathetic and Parasympathetic NS Roles

• Parasympathetic: Active in non-emergencies, promotes restorative/maintenance functions ("rest and digest"), conserves energy
• Sympathetic: Active in emergencies or stress, promotes "fight or flight" response, increases cardiac output

Advantages of Dual ANS Innervation

• Most visceral organs are dually innervated by both sympathetic and parasympathetic nerves.
• These divisions usually exert opposite effects, allowing for precise control and fast transitions between rest/digest and fight/flight states.
• Heart rate is increased by sympathetic stimulation and decreased by parasympathetic stimulation.

Advantages of Dual ANS Innervation

• The two ANS divisions often control each other reciprocally; increased activity in one division generally causes a decrease in the other.
• Exceptions exist, such as blood vessels, sweat glands, and the liver

Arrangement of SNS & PSNS Pathways

• ANS Pathways: Each pathway from the CNS to the organ/effector is a two-neuron chain (preganglionic neuron → synapse/ganglion → postganglionic neuron).
• Sympathetic: Preganglionic fibers are short, myelinated, and postganglionic fibers are long and unmyelinated.
• Parasympathetic: Preganglionic fibers are long and myelinated, and postganglionic fibers are short and unmyelinated; ganglia are near the target organ.

Modified Sympathetic Nervous System – Adrenal Medulla

• Adrenal glands are beside the kidneys; consist of a cortex and a medulla.
• The medulla is part of the sympathetic nervous system and considered a modified sympathetic ganglion.
• Preganglionic fibers directly innervate the adrenal medulla cells.
• These cells release neurochemicals (adrenaline/epinephrine primarily, with some noradrenaline) directly into the bloodstream.

Origins of the Parasympathetic “Craniosacral Outflow”

• Originates from cranial nerves (III, VII, IX, X) and sacral spinal nerves (S2-S4).
• The vagus nerve (X) is a major pathway for parasympathetic outflow.
• Preganglionic neurons are long and myelinated while postganglionic neurons are short and unmyelinated.
• Ganglia are located close to the target organ.

Sympathetic Nervous System “Thoracolumbar Outflow”

• Originates from the thoracic and lumbar spinal cord segments (T1-L3).
• Preganglionic fibers are short, myelinated and postganglionic fibers are long, unmyelinated.
• Ganglia are located along the sympathetic trunk/chain on either side of the spinal cord.

ANS Neurotransmitters and Receptors

• Parasympathetic: Preganglionic and postganglionic neurons release acetylcholine (ACh)
• Sympathetic: Preganglionic neurons release ACh, and most postganglionic neurons release norepinephrine (noradrenaline).
• Most postganglionic parasympathetic neurons release ACh.

Cholinergic Receptors - nicotinic and muscarinic

• Nicotinic: Found on all postganglionic autonomic cell bodies; ionotropic; fast response.
• Muscarinic: Found on effector cell membranes; metabotropic; diverse effects.

Nicotinic Receptors

• ACh binding opens intrinsic Na+/K+ channels resulting in depolarization of postsynaptic cell. Response is rapid.

Muscarinic Receptors

• Different subtypes (M1-M5) have different effects, responding to changes in K+ conductance and calcium channels, leading to excitatory or inhibitory responses.

NT receptors: Parasympathetic Division

• Parasympathetic stimulation involves release of ACh into effector tissues, resulting in a cellular response via the Muscarinic receptors.

Adrenergic Receptors

• Located at effector synapses; postganglionic sympathetic nerves; respond to norepinephrine/epinephrine.
• Subtypes (α1, α2, β1, β2, β3) have diverse effects on target cells, leading to different physiological outcomes.

Adrenergic Receptors (α1, α2, β1, β2, β3)

• α1: Excitatory response in most sympathetic target cells
• increased contraction of arterioles
• α2: Inhibitory response in the digestive system
• decrease in smooth muscle contraction.
• β1: Excitatory response mainly in the heart
• increase force of cardiac contraction, heart rate.
• β2: Inhibitory response in skeletal muscles and organs.
• relaxation of smooth muscle, dilation of arterioles, bronchioles.
• β3: Excitatory response in adipose tissues
• lipolysis.

Termination of NT Effects

• Acetylcholine (ACh): Degraded by acetylcholinesterase at synapses
• Norepinephrine (NE): Re-uptake by pre- and post-synaptic cells, then metabolized/recycled.

Steps of Neurochemical Transmission/Potential Targets for Pharmacologial Intervention

• Nerve Terminal: Neurotransmitter release
• Post Synaptic Membrane: Neurotransmitter-receptor interaction
• Neurotransmitter Effect Termination: Neurotransmitter degradation

ANS Drugs

• Drugs can mimic (agonists) or inhibit (antagonists).
• Muscarinic antagonist (example: Atropine): Blocks muscarinic receptors, reducing parasympathetic actions (e.g., in surgery).
• Adrenergic agonist (example: Salbutamol): Activates β2 receptors, dilating bronchioles (e.g., in asthma/COPD).
• Adrenergic antagonist (example: Atenolol): Blocks β1 receptors, lowering blood pressure (e.g., in hypertension).

Autonomic Dysfunction

• Many forms include orthostatic hypotension, neurocardiogenic syncope, and chronic stress disorders.
• Common causes include trauma, inflammation, drugs, and neurodegenerative diseases.

Comparison of Autonomic and Somatic NS

• Autonomic: Two neuron chain (pre/post-ganglionic), innervates cardiac, smooth muscle, exocrine and some endocrine glands; dual innervation (mostly opposing effects)
• Somatic: Single neuron, innervates skeletal muscle, exclusively stimulation affects, under voluntary control.

Further Reading (Books)

• Neuroscience: Bear, Connors, Paradiso (3rd edition, chapters 5, 6, and 15)
• Medical Physiology: Boron, Boulpaep
• Medical Physiology: Rhoades

Test your knowledge on the Autonomic Nervous System as covered in the Body: Movement and Function course. This quiz will challenge you on the divisions, anatomical features, functions, and neurotransmitters of the sympathetic and parasympathetic systems.