Autonomic Nervous System Biology - Year 1 BMF

Autonomic Nervous System Biology

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

Learning Objectives

• Recall the divisions of the nervous system
• Contrast the anatomical features of 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 Nervous System

• Nervous system is divided into:
  • Central nervous system (CNS): brain and spinal cord
  • Peripheral nervous system (PNS):
    • Autonomic nervous system (ANS): regulates internal organs (heart, circulation, digestive and respiratory systems)
      • Sympathetic division (arousing)
      • Parasympathetic division (calming)
    • Somatic nervous system: regulates voluntary muscles and sensory input from sense organs

Divisions of Peripheral Nervous System

• Sensory (afferent) division:
  • General: touch, pain, pressure, vibration, temperature, and proprioception (skin, body wall, limbs)
  • Special: hearing, equilibrium, vision, taste, smell
• Motor (efferent) division:
  • Somatic nervous system: innervation of all skeletal muscles
  • Autonomic nervous system (ANS): innervation of smooth muscle, cardiac muscle, and glands

Divisions of Autonomic Nervous System

• Involuntary system maintaining homeostasis
• Regulates internal organ functions (coordinate manner)
• Two divisions (sympathetic & parasympathetic) that innervate most visceral organs (dual innervation)
• Mostly opposing effects, but partially active ("tonic" activity) under most circumstances

Internal Organs/Viscera Controlled by ANS

• Heart
• Lungs
• Stomach & GI tract, spleen, pancreas
• Bladder & rectum
• Kidneys & liver
• Eye (pupil)

Control of the ANS

• CNS has central control over ANS output
• Medulla and Pons in brain stem regulate cardiovascular, respiratory and digestive systems
• Hypothalamus plays a major role in regulating heart rate, blood pressure, and respiration, often through the hypothalamus-pituitary-adrenal (HPA) axis
• Spinal cord integrates autonomic reflexes not subject to higher control (e.g., urination, defecation)

Overview of Sympathetic and Parasympathetic NS

• Parasympathetic: activated in non-emergencies, promotes restorative functions ("Rest & Digest"), conserving energy, digestion and maintenance of the body
• Sympathetic: activated in emergency situations or stress ("Fight or Flight"), increases energy expenditure, routes blood to muscles, and raises blood glucose

Advantages of Dual ANS Innervation

• Most visceral organs are innervated by both sympathetic and parasympathetic nerves (dual innervation)
• Dual innervation gives the ability for fine-tuned control over organ/tissue function (as opposed to just on or off)
• Sympathetic stimulation generally elevates activity while parasympathetic stimulation decreases activity

Exceptions to Dual Innervation

• Some organs (e.g., arterioles and veins) are innervated by only the sympathetic division.
• Blood vessels, sweat glands, etc. show exceptions to the rule of dual innervation.

Arrangement of SNS & PSNS Pathways

• Pathway from CNS to organ/effector is a two-neuron chain (preganglionic neuron synapse/ganglion → postganglionic neuron → organ) for both divisions (sympathetic & parasympathetic)

• Sympathetic nerves are short & myelinated preganglionic fiber, and long & unmyelinated postganglionic fiber

• Parasympathetic nerves are long & myelinated preganglionic fiber, and short & unmyelinated postganglionic fiber

Modified Sympathetic Nervous System - Adrenal Medulla

• Two adrenal glands, each containing outer cortex and inner medulla, extend sympathetic nervous system
• Adrenal medulla is considered a modified sympathetic ganglion
• Cells of the medulla don't have axons, but release adrenaline (epinephrine) and noradrenaline (norepinephrine) directly into blood

Origins of the Parasympathetic

• Preganglionic neurons originate in cranial nerves (III, VII, IX, X) and sacral spinal nerves (S2-S4)
• Vagus nerve (X) is a primary part of the craniosacral outflow, responsible for nearly 80% of this flow
• Ganglion is typically located closer to the target organ
• Postganglionic neuron is short & unmyelinated

Sympathetic Nervous System

• Thoracolumbar outflow from thoracic and lumbar regions for preganglionic fibers
• Preganglionic fibers are short and myelinated
• Sympathetic ganglia lie in sympathetic trunk/chain along either side of spinal cord
• Postganglionic fibers are long and unmyelinated

ANS Neurotransmitters and Receptors

• Sympathetic & parasympathetic preganglionic neurons release acetylcholine (ACh)

• Most postganglionic sympathetic neurons release noradrenaline (norepinephrine)

• Most postganglionic parasympathetic neurons release acetylcholine (ACh)

• Receptors activated by NTs can be ionotropic or metabotropic

• Ionotropic receptors are typically ligand-gated ion channels

• Metabotropic receptors typically involve second messengers and require G proteins

Additional Complexity and Exceptions

• Several exceptions to general rules about dual innervation
• Some organs are solely or predominantly innervated by one division (e.g., salivary and sweat glands, blood vessels) and may have both

Cholinergic Receptors

• Nicotinic receptors: found on all postganglionic autonomic nerve cell bodies and are ionotropic (fast response)
• Muscarinic receptors: found on smooth muscle, glands, and cardiac muscle, are metabotropic (slower) and have 5 subtypes

Nicotinic Receptors

• ACh binding opens intrinsic Na+/K+ channels
• Ionotropic
• Leads to depolarization of the postsynaptic cell

Muscarinic Receptors

• Metabotropic
• M2: inhibitory, found on cardiac tissue, increases K+ conductance, inhibits calcium channels (decreases heart contraction)
• M3: excitatory, found in digestive system, increases glandular secretions and smooth muscle contraction

NT receptors & Parasympathetic division

• Preganglionic fibers release acetylcholine (ACh) which activates nicotinic receptors
• Postganglionic fibers release acetylcholine (ACh) which activates muscarinic receptors

Adrenergic Receptors

• Only found at effector organ synapses
• After postganglionic sympathetic nerve activation
• Two major classes (α and β) are coupled to G proteins.

• α1: excitatory, located on most sympathetic target cells, increases contraction of arterioles raising blood pressure • α2: inhibitory, located in digestive system, decreases smooth muscle contraction, reducing GIT motility • β1: excitatory, located in the heart causing contraction of cardiac muscle increasing heart rate and force • β2: inhibitory, located on skeletal muscle, vessels and organs, causing relaxation of smooth muscle in bronchioles, increasing dilation.

Termination of NT Effects

• Acetylcholine: destroyed by acetylcholinesterase at synapses
• Noradrenaline: re-uptake by pre- and postsynaptic cells, then metabolized/recycled

STEPS OF NEUROCHEMICAL TRANSMISSION

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

ANS Drugs

• Drugs can mimic or inhibit the effects of ANS neurotransmitter/receptor interactions
• Muscarinic antagonists (e.g., atropine) reduce parasympathetic activity
• Adrenergic agonists (e.g., salbutamol) stimulate sympathetic activity on certain organs (only effects certain organs e.g. bronchioles but not other sites)
• Adrenergic antagonists (e.g., atenolol) inhibit sympathetic activity (lowers blood pressure)

Autonomic Dysfunction

• Many forms (e.g., orthostatic hypotension, neurocardiogenic syncope, chronic stress disorders)
• Often caused by issues with the hypothalamus-pituitary-adrenal axis (HPA)

Comparison of Autonomic and Somatic NS

• Autonomic (involuntary, branched): uses two neuron-chains (pre/post ganglionic fibers), and regulates smooth, cardiac and glandular tissues, may be acetylcholine or norepinephrine at effector organs and exhibit either stimulatory or inhibitory effects
• Somatic (voluntary, single-neuron): innervates skeletal muscles, only releases acetylcholine and has solely stimulatory effect

Books for further study

• Neuroscience: exploring the brain, 3rd ed. by Bear, Connors, Paradiso
• Boron, Boulpaep, Medical Physiology, by Boron and Boulpaep
• Rhoades, Medical Physiology, Principles for Clinical Medicine

This quiz focuses on the autonomic nervous system as part of the Body: Movement and Function course for Year 1 students. It covers the divisions, functions, and anatomical features of the sympathetic and parasympathetic systems, along with neurotransmitters involved. Test your understanding of these essential biological concepts.