AUTONOMIC NERVOUS SYSTEM-PART-1

AUTONOMIC NERVOUS SYSTEM(ANS)-OVER VIEW

In general, our nervous system is divided into two branches such as 1.Central Nervous System(CNS) which is centered at the brain systems and highly protected by the blood-brain barrier and skull.

2. Peripheral Nervous System(PNS) which is centered outside the brain midbrain, and spinal cord and is not much protected as CNS. Hence PNS is highly prone to be injured by toxins and mechanical means.

PNS is acting as a mediator between the end organs with the CNS.

The PNS  is again divided into two such as 1. The Somatic nervous system supplied to the skeletal muscles to express our voluntary desired movements to the brain by sending and receiving back and forth the impulses and commands from the brain accordingly.

2. Autonomous Nervous System is an involuntary control of smooth muscles such as the heart, lungs, and liver etc.etc.

The system is connected to the CNS in two different physiological networks known as the sympathetic and parasympathetic nervous systems.

The autonomic nervous system is defined as a collection of nuclei, cell bodies, nerves ganglia, and plexuses that provides afferent and efferent nerves supplied to the smooth muscles and visceral organs that carries commands from the brain and stimuli or impulses to the brain by the end involuntary smooth muscles and organs.75% of the ANS nerve networks are linked with the CNS through the medulla oblongata.

 The autonomic nervous system is so important as it regulates involuntary functions such as blood pressure, heart rate, and the digestive system.
As in the diagram above the ANS is divided anatomically into two major components as follows:-
1.Parasympathetic Nervous System in which there is a long preganglionic fiber that snaps with a short postganglionic fiber at the ganglion situated somewhere nearby the target organ as in the diagram shown above. The preganglionic fibers originate from cranial nerve nuclei III, VII, IX, and X as well as the III and IV sacral spinal roots and this system is known as craniosacral outflow
2.Sympathetic Nervous System in which there are short preganglionic fibers that snap with the long postganglionic fibers at the ganglia to form a sympathetic ganglionic chain adjacent to the spinal cord. All preganglionic fibers of the sympathetic nervous system originate in the thoracolumbar portion of the spinal cord and it is known as thoracolumbar outflow
Pharmacologically acetylcholine is the nerve transmitter in the preganglionic fibers of both the systems and postganglionic fibers of the parasympathetic nervous system.

Norepinephrine is the major nerve transmitter at the postganglionic nerves in the sympathetic nervous system except the postganglionic sympathetic nerve supply in which acetylcholine is the nerve transmitter. 
Responses of the effector organs to the autonomic nervous system can give us an outline of predicting various drugs that mimic or antagonize the actions of these nerves.
The two divisions of the ANS are generally antagonizing each other in some sites like the heart and digestive system and synergizing in some sites such as in sex organs.

General Functions Of ANS

Generally, the combined functions of the ANS are important in regulating the activities of vital organs which are functioning involuntarily below the level of consciousness. Thus respiration, circulation, digestion, body temperature, metabolism, sweating, and endocrine secretions of hormones are all regulated and controlled in part or entirely by ANS.

Specific Functions Of ANS

The specific functions of the two divisions of the ANS can be emphasized as follows:-

1. The sympathetic nervous system is the predominant one in normal situations as the majority of the combined nerve supply to various parts of our body belongs to this anatomical division of ANS. This nervous system is active even at rest but becomes dominant during stress by its adrenergic postganglionic activity. For example, if you feel danger or angry, or any other mind provoking thoughts, your heart rate increases, blood pressure rises, eyes dilate, blood sugar rises, lungs expand with more oxygen demand, and the face and parts of the body reddened as the blood flow shifts from the skin to skeletal muscle in order to get ready for a fight or flight.

The effector organs are responding to sympathetic(adrenergic) stimulations by the following receptors such as alpha-1,alpha-2;

beta-1,beta-2;and dopamine receptors.

In contrast, the parasympathetic nervous system which is not much innervated in our body is predominant at rest, sleep, and the situation of calmness. It is a nervous system of peace and tranquil. Stimulation of this nervous system in which the postganglionic fibers are 100% cholinergic slows heart rate, lower blood pressure, increase intestinal motility, constrict the pupil(miotic) and empty the urinary bladder. In general, this is a peace and digestive system.

The effector organs respond to this system(Cholinergic) at the receptors such as nicotinic(Nm, Nn) and muscarinic(m-1 to m-5)

The two systems are working and active all the time. Their action is antagonized to each other according to the situations. For example, the heart rate is normally under the influence of cholinergic. But when the situation gets tense the heart rate goes under the influence of the adrenergic sympathetic nervous system.

Nerve Transmitters

Apart from the anatomical sympathetic division of ANS, a pharmacological division of ANS is more convenient to understand their activities by means of the autonomical transmitters as follows:-

1.Cholinergic Nervous System in which the transmitter at the target organ is Acetylcholine

2.Adrenergic Nervous System in which the transmitter at the target organ is Norepinephrine.

The calcium ion is required to release the transmitters from their storage vesicles.