The Basic of Autonomic Nervous System

Dr. Siti Hajar Adam

Faculty of Medicine and Defence Health

National Defence University of Malaysia (UPNM)

Learning Outcomes (TLO)

1. Describe the functional organization of ANS
2. Describe the components of in parasympathetic and sympathetic nervous systems
3. Describe the functions of parasympathetic and sympathetic systems

Introduction

1. The ANS is sometimes called the involuntary nervous system because it carries out its functions without requiring a conscious effort.
2. It influences a wide array of physiological processes via its innervation of smooth muscle, cardiac muscle, and pacemaker cells, exocrine and endocrine glands, adipose tissue, liver cells, and lymphatic tissue.
3. The ultimate responsibility of the ANS is to maintain homeostasis despite perturbations exerted by the external and internal environments.

The Nervous System Division

Anatomic Organization Of Autonomic Outflow

1. Sympathetic and parasympathetic neurons serve as the final common pathway from the CNS to visceral targets.
2. However, contrast to the somatomotor nervous system, the peripheral motor portions of the ANS are made up of two neurons: preganglionic and postganglionic neurons.
3. The cell bodies of the preganglionic neurons are located in the intermediolateral column of the spinal cord and in motor nuclei of some cranial nerves

Organization of the ANS

1. Synapses between neurons are made in the autonomic ganglia.
1. Parasympathetic ganglia are located in or near the effector organs.
2. Sympathetic ganglia are located in the paravertebral chain.
2. Preganglionic neurons have their cell bodies in the CNS and synapse in autonomic ganglia.
1. Preganglionic neurons of the sympathetic nervous system originate in spinal cord segments T1–L3, or the thoracolumbar region.
2. Preganglionic neurons of the parasympathetic nervous system originate in the nuclei of cranial nerves and in spinal cord segments S2–S4, or the craniosacral region.
3. Postganglionic neurons of both divisions have their cell bodies in the autonomic ganglia and synapse on effector organs (e.g., heart, blood vessels, sweat glands).

Pre and Postganglionic Neurotransmitter

What is the similar features with motor/somatic neuron?

• One similar feature of autonomic preganglionic neurons and α- motor neurons is that acetylcholine is released at their nerve terminals
• This is the neurotransmitter released by all neurons whose axons exit the CNS, including all motor neurons, preganglionic sympathetic neurons, and preganglionic parasympathetic neuron

Neurotransmitter of the ANS

• Adrenergic neurons release norepinephrine as the neurotransmitter.
• Cholinergic neurons, whether in the sympathetic or parasympathetic nervous system, release acetylcholine (ACh) as the neurotransmitter.
• Nonadrenergic, noncholinergic neurons include some postganglionic parasympathetic neu-rons of the gastrointestinal tract, which release substance P, vasoactive intestinal peptide (VIP), or nitric oxide (NO).

Neurotransmitter of ANS: Acetylcholine (Ach) & Norepinephrine

• Acetylcholine and norepinephrine are the principal
• The cholinergic autonomic neurons (ie, release acetylcholine) are
• all preganglionic neurons
• all parasympathetic postganglionic neurons
• sympathetic postganglionic neurons that innervate sweat glands
• sympathetic postganglionic neurons that end on blood vessels in some skeletal muscles and produce vasodilation when stimulated vasodilator nerves
• The remaining sympathetic postganglionic neurons are noradrenergic (ie, release norepinephrine).

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Receptor for sympathetic & parasympathetic

Adrenergic receptors (Alpha receptors)

• a. Alpha 1 Receptors
• are located on vascular smooth muscle of the skin and splanchnic regions, the GI and bladder sphincters, and the radial muscle of the iris.
• produce excitation (e.g., contraction or constriction).
• are equally sensitive to norepinephrine and epinephrine.
• b. Alpha 2 Receptors
• are located on sympathetic postganglionic nerve terminals (autoreceptors), platelets, fat cells, and the walls of the GI tract (heteroreceptors).
• often produce inhibition (e.g., relaxation or dilation).

Adrenergic receptors (Beta receptor)

• Beta 1 Receptors
• are located in the sinoatrial (SA) node, atrioventricular (AV) node, and ventricular muscle of the heart.
• produce excitation (e.g., increased heart rate, increased conduction velocity, increased contractility).
• are sensitive to both norepinephrine and epinephrine, and are more sensitive than the α1 receptors.
• Beta 2 Receptors
• are located on vascular smooth muscle of skeletal muscle, bronchial smooth muscle, and in the walls of the GI tract and bladder.
• produce relaxation
• *are more sensitive to epinephrine than to norepinephrine.
• *are more sensitive to epinephrine than the α1 receptors.

Summary of sympathetic and parasympathetic receptors

2. Cholinergic receptors (Nicotinic)

• a. Nicotinic receptors
• are located in the autonomic ganglia (NN) of the sympathetic and parasympathetic nervous systems, at the neuromuscular junction (NM), and in the adrenal medulla (NN). The receptors at these locations are similar, but not identical.
• are activated by ACh or nicotine.
• produce excitation.
• are blocked by ganglionic blockers (e.g., hexamethonium) in the autonomic ganglia, but not at the neuromuscular junction.

2. Cholinergic receptors (Muscarinic)

• B. Muscarinic receptors
• are located in the heart (M2), smooth muscle (M3), and

glands (M3).

• are inhibitory in the heart (e.g., decreased heart rate, decreased conduction velocity in AV node).
• are excitatory in smooth muscle and glands (e.g., increased GI motility, increased secretion).
• are activated by ACh and muscarine.
• are blocked by atropine.

Effect of ANS on the organ system

• Go to padlet and share your thoughts on what is the effect of ANS in the organ system: https://padlet.com/drhajaradam/sk9wguot0p2awt3y

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Photo from Camilo Jimenez, Unsplash

Autonomic centers—brainstem and hypothalamus

• 1. Medulla
• Vasomotor center
• Respiratory center
• Swallowing, coughing, and vomiting centers
• 2. Pons
• ■ Pneumotaxic center
• 3. Midbrain
• ■ Micturition center
• 4. Hypothalamus
• Temperature regulation center
• Thirst and food intake regulatory centers