Autonomic Nervous System Divisions

• The autonomic nervous system (ANS) regulates activities not under direct conscious control, such as cardiac muscle, gastrointestinal tract (GIT), and exocrine glands.
• The nervous system includes the peripheral and central nervous system.
• The central nervous system consists of the brain and spinal cord.
• The divisions of the nervous system include afferent and efferent divisions; autonomic and somatic systems.

Autonomic Nervous System Divisions

• The autonomic nervous system can be further divided into enteric, parasympathetic, and sympathetic systems.
• Afferent (sensory) nerves conduct signals from various organs to the central nervous system (CNS).
• Efferent (motor) nerves conduct orders from the CNS to different organs of the body.
• Anatomically, the autonomic nervous system divides into sympathetic and parasympathetic divisions.
• Sympathetic nerve fibers originate from the thoracolumbar regions (thoracic segments 1, 4, 5, 11, 12 and lumbar segments 1, 3).
• Parasympathetic nerve fibers originate from the craniosacral regions of the CNS (cranial nerve nuclei III, VII, IX, X and sacral segments 2, 3, 4).
• The sympathetic nervous system accommodates to stressful conditions.
• The parasympathetic nervous system maintains normal conditions.
• Both the sympathetic and parasympathetic systems are physiologically antagonistic but increase salivary secretion.
• All organs receive dual supply except the liver and blood vessels.
• Efferent nerve fibers of the autonomic nervous system do not travel directly from the CNS to the effector organ like somatic nerves do. Instead, they travel as two nerves separated by a synapse.
• The nerve fiber originating from the CNS is called the preganglionic nerve, and the next one is the postganglionic nerve.
• In the sympathetic system, preganglionic fibers are short, and synapses rely on paravertebral ganglia.
• In the parasympathetic system, synapses rely nearly inside the affected organ, followed by a short postsynaptic fiber.
• Somatic motor nerve endings travel directly into skeletal muscles without relying on ganglia.
• The autonomic nerves are classified as cholinergic or adrenergic based on transmitter molecules
• Cholinergic fibers synthesize and release acetylcholine
• These include all preganglionic nerve endings and somatic (nonautonomic) motor nerve endings to skeletal muscle
• Also includes all parasympathetic postganglionic fibers and a few sympathetic postganglionic fibers (supplying sweat glands)
• Most postganglionic sympathetic fibers release norepinephrine (noradrenaline), and are adrenergic fibers
• Most autonomic nerves release cotransmitters in addition to the primary transmitter

Cholinergic Transmission

• Acetylcholine is synthesized in the cytoplasm of the nerve ending from acetyl-CoA and choline via choline acetyltransferase (ChAT) enzyme.
• Acetyl-CoA is synthesized in mitochondria, which are present in large numbers in the nerve ending.
• Choline is transported from extracellular fluid into the neuron terminal by a sodium-dependent membrane carrier (carrier A), which can be blocked by hemicholinium drugs.
• Acetylcholine (ACh) moves from the cytoplasm into vesicles by an antiporter carrier (carrier B), which can be blocked by vesamicol.
• Release of ACh occurs when an action potential reaches the terminal and triggers sufficient influx of calcium ions, destabilizing storage vesicles
• Vesicular membranes fuse with the terminal membrane through VAMPs interacts with synaptosome associated proteins (SNAPs).

Adrenergic Transmission

• Synthesis of catecholamine transmitters is more complex than that of acetylcholine
• In most sympathetic postganglionic neurons, norepinephrine is the final product

Norepinephrine & Epinephrine

• In the adrenal medulla and certain brain areas, norepinephrine is converted to epinephrine
• Synthesis terminates with dopamine in dopaminergic neurons of the CNS
• Synthesis of catecholamine transmitters occurs through several steps targeted by drugs
• Tyrosine is converted to dopa, a rate-limiting step in catecholamine transmitter synthesis, which can be inhibited by the tyrosine analog metyrosine
• A high-affinity of carrier B for catecholamines can be inhibited by the reserpine alkaloids (uptake 3)
• The carrier transporting norepinephrine and similar molecules into the neuronal cytoplasm (uptake 1) may be inhibited by cocaine and tricyclic antidepressant drugs
• Release of adrenaline from adrenergic nerve endings are calcium-dependent

Neuroeffector

• Indirectly acting sympathomimetics (e.g., tyramine and amphetamines) cause a release of transmitter from noradrenergic nerve endings
• Occurs once they are up-taken by the neuronal cells (uptake 1) but not by storage vesicles of neuronal cells (uptake 3)
• Uptake 1,3 are called neuronal while up-take 2 is called extra-neuronal

Metabolic Processes

• Norepinephrine and epinephrine can be metabolized by monoamine oxidase (MAO) and COMT
• Monoamine oxidase (MAO) is present in tissues, cytoplasm, synaptic cleft and mitochondria of neuronal cells, but not in storage vesicles of neuronal cells.
• Nerve fibers classify into cholinergic nerves (releasing Ach) and adrenergic nerves (releasing norepinephrine (NEP).
• All somatic nerves are cholinergic.
• All preganglionic nerves (parasympathetic or sympathetic or even, which supply the adrenal medulla) are cholinergic.
• All parasympathetic postganglionic fibers are cholinergic.
• Most sympathetic postganglionic fibers are adrenergic.
• Few sympathetic postganglionic fibers are cholinergic (supplying sweat glands).
• NN receptors, in autonomic ganglia, facilitate NAP conduction from preganglionic to postganglionic nerve
• NM receptors, in the neuromuscular junction, facilitate NAP conduction from somatic motor nerve end to skeletal muscle.
• Actions are mediated by autonomic Receptor Types.
• Autonomic nervous system is under the control of the hypothalamus in the CNS.
• The anterior hypothalamus controls the parasympathetic portion.
• The posterior hypothalamus controls the sympathetic portion.