Nervous System in human is broadly classified into
three board categories
a) The Central Nervous System comprising of the brain and spinal
cord
b) The Autonomic Nervous System
c) The Peripheral Nervous System
Here, we discuss the Autonomic Nervous System
Autonomic Nervous System controls the action of the organs at the
subconscious level. It is the part of system not under voluntary control.
This is why it is named as ‘autonomic.‘
Autonomic Nervous System shares a relationship with the Somatic
Nervous System and has functional links with it. Although the
Autonomic fibers share either sympathetic or parasympathetic nerves up
to CNS, it has a common pathway with somatic nerves inside CNS. So,
there may be some interconnections.
Autonomic Nervous System has a great relationship with Endocrine
System. Like examples are given.
a) Cortisol is a catecholamine hormone secreted from Adrenal
Cortex, and from sympathetic nerve endings.
b) Adrenal Medulla is itself a modified sympathetic ganglion.
Hormones secreted from Adrenal Medulla is actually secreted from
sympathetic nerve endings.
c) Thyroxine or the T4 hormone, secreted from the Thyroid gland
increases the number of beta-adrenergic receptors in the heart.
Classifications: Autonomic Nervous System is composed of two
types of Nervous System:
1. Sympathetic Nervous System
2. Parasympathetic Nervous System
Sympathetic Vs Parasympathetic Nervous System
| Sympathetic Nervous System | Parasympathetic Nervous System | |
| Sympathetic Nervous System efferents or in short the sympathetic efferents arise from the spinal cord as thoracolumbar outflow. It extends from a level of T1 to 3 rd or 4 th lumber, i.e. L3 or L4. | The parasympathetic nervous system has a craniosacral outflow, i.e. it originates from only brain and sacral part of spinal cord. The Parasympathetic parts of cranial nerve include the 3 rd nerve, i.e. Occulomotor7 th nerve, i.e. Facial, 9 th nerve, i.e. Glossopharyngeal and 10 th nerve, i.e. Vagus nerve. The sacral counterpart of Parasympathetic nerve system has root value of S2 to S4. | |
| The preganglionic nerve efferents come out through efferent root of the spinal segment. It then passes through the white rami, and it communicates the white rami which connect the ventral root to the paravertebral sympathetic chain. In this point or region, most of the neural ends of preganglionic fiber send and relay with postganglionic neurons. | The preganglionic nerve efferents come out through the lateral horn of the spinal cord, the preganglionic nerve fiber arises, traveling long up to the target organ where there is a ganglion, there they terminate. | |
| Two or more types of neurotransmitters are seen in the Sympathetic Nervous System. Acetylcholine:The preganglionic nerve fibers that originate from the spinal cord and ends up in the sympathetic ganglion secrete acetylcholine inside the ganglion. This acetylcholine acts on nicotinic receptors. b) Noradrenaline: The postganglionic nerve fibers that originate from the sympathetic ganglion and ends up in viscera secrets noradrenaline inside the viscera. This secreted noradrenaline acts on adrenergic receptors, which are 5 in numbers- α1, α2, β1, β2, and β3. | The neurotransmitter of the Parasympathetic Nervous System is only acetylcholine. | |
| Receptors in the Sympathetic Nervous System include adrenergic receptors, which are 5 in numbers- α1, α2, β1, β2, and β3. | Muscarinic receptors or simply M receptors are of 5 types a) M1 – present in the brain b) M2 – present in the heart c) M3 – present in the stomach d) M4 – present in the pancreas e) M5 – located in smooth muscles. | |
| Postganglionic fiber is the supplier of the sympathetic system. It is the fiber which connects the sympathetic ganglion with different visceral organs. There are two other possibilities – either they may return to the spinal cord through gray rami communicantes, or they may spread along spinal nerves to supply to autonomic effectors like sweat glands or vascular smooth muscles. | The preganglionic nerve fibers originate from inside the brain and spinal cord, and here, they are longer unlike that of sympathetic nervous system where the postganglionic nerve fiber was longer. The postganglionic nerve fibers are very short; originate from the Parasympathetic ganglia situated very close to the target organ. | |
| Through α receptors, they constrict the soft tissue dilator- pupillae, which results in dilation of pupils of the eye. | They tend to constrict the pupil, stimulating the tissue constrictor- pupillae. | |
| Through α receptors, they decrease peristaltic movements by decreasing motility and tone of stomach and intestine . | They increase peristalsis as well as increases gastric emptying. | |
| Through α receptors, Facilitates adrenergic sweating, by contraction of pilomotor muscles, which causes sweating in hands and palms. | They increase salivation and lacrimation. | |
| Β1 receptors are exclusively present in heart and kidneys. So stimulating them to increase heart rate. Through β1 receptors, It exerts positive chronotropic, dromotropic, ionotropic, and bathmotropic effects on heart. | Effects on the heart include all opposite to that of sympathetic nervous system. It has negative ionotropic, chronotropic, bathmotropic effects. All these effects are carried by vagus nerve. | |
| Through β3 receptors, Being exclusively present in adipose tissue, it initiates the lipolysis. | Parasympathetic Nervous System has no such effects. | |
| It causes vasodilation of skeletal muscles and liver. It causes dilation of bronchial smooth muscles. This property is used in inhalers, giving β2 agonists. | They increase gland secretion, e.g. of gastric, pancreatic, etc. | |
| The sympathetic nervous system is the actual seat of triple F response. The 3 Fs stand for Fright Fight Flight. It prepares the body to tackle dangerous situations. | Parasympathetic Nervous System has no such effects. |