Autonomic Nervous System

Autonomic nervous system (ANS) is a part of the peripheral nervous system, which is associated with the involuntary functions of the body. Autonomic nervous system controls the visceral organs and endocrine glands, and thus it is associated with the maintenance of homeostasis.

Autonomic nervous system is a motor system; hence, it is also called visceral efferent motor system. The ANS regulates the activity of heart, gastrointestinal (GI) tract, respiratory system, and salivary glands. ANS is also associated with perspiration, dilation of pupil, micturition, and sexual arousal. All the organs are innervated by the ANS, except in skeletal muscle. Skeletal muscle is supplied with the somatomotor nervous system.

11.3.1 Divisions of Autonomic Nervous System

There are two major subdivisions of autonomic nervous system: the sympathetic nervous system and parasympathetic nervous system (Fig. 11.9). The functions of these two divisions are opposite to each other. Generally, these two systems work together, typically in antagonistic manner (seems complementary to each other), which helps in the maintenance of homeostasis, as happened in regulation of heart rate and in respiratory cycles. The enteric nervous system is considered as the third subdivision of the autonomic nervous system. The enteric nervous system is composed of interconnected sensory, motor, and interneurons located in the GI tract, which control the activity of the gut. However, the neurons in the GI tract can also be influenced by the CNS through input from the sympathetic and parasympathetic subdivisions. Most of the visceral organs receive innervation from both sympathetic and parasympathetic divisions, but some organs, like sweat glands (both apocrine and merocrine type), adrenal medulla, blood vessels, pancreatic islets, pineal gland, pupillary dilator muscles, vascular smooth muscle of skeletal muscle, skin, and arrector pili, are innervated only by the sympathetic division.

However, some organs like tissues of sphincter muscle of the iris, ciliary muscle, and nasopharyngeal glands receive only parasympathetic innervations. Autonomic nervous system has slow and long-lasting effect. Some visceral organs like heart and GI tract have intrinsic neuronal system, which helps in rhythmic movements. The major regulatory center for ANS is the hypothalamus; that is why it is called the captain of autonomic nervous system. Generally, autonomous functions are involuntary, but few actions can work alongside some degree of conscious control. The hypothalamic control is related with the reticular formation, which is located in the brain stem. The efferent tracts of the reticular formation reach the sympathetic and parasympathetic nuclei located in the brain stem and spinal cord. Hypothalamus releases the releasing and inhibitory hormones, which control the release of pituitary hormones. So, neuronal as well as hormonal influences are the basis for hypothalamic regulation of different major functions of the body, like heart rate, respiration rate, blood pressure, body temperature, conjugate eye movement, locomotion, swallowing, vomiting, micturition, defecation, water balance, food intake, circadian rhythms, and emotion. The cerebral cortex also has some influence on the ANS; for example, the sight of food triggers secretion of saliva and anticipation of a walk increases the heart rate of a dog.

11.3.2 Organization of the Autonomic Nervous System

In dogs, the sympathetic fibers arise from the thoracic (T1- T13) and lumbar (L1-L3) spinal cord. Hence, sympathetic division is called thoracolumbar division, whereas

Fig. 11.9 Organization of the autonomic nervous system

Fig. 11.10 Originof sympathetic and parasympathetic fibers in dogs. Sympathetic fibers arise from the thoracic (T1-T13) and lumbar (L1-L3) spinal cord.

The parasympathetic fibers originate from the brain stem and sacral cord segments (S2-S3)

parasympathetic fibers originate from the brain stem and sacral cord segments (S2-S3) (Fig.

11.10). Thus, the parasympathetic division is called craniosacral division. In the somatic nervous system, the cell body of the neurons remains in the central nervous system (CNS) and the axon fiber extends to the skeletal muscle. Neuromuscular junction is present at the junction between the nerve fiber and the skeletal muscle.

In the autonomic nervous system, both the divisions require a two-neuron chain between the nucleus, the CNS, and the peripheral target organ (Fig. 11.9). The synapse between the two neurons occurs outside the CNS in the ganglia. The presynaptic axon is called preganglionic fiber, and the postganglionic axon is called a postganglionic fiber. The cell body of preganglionic neuron remains in the CNS, whereas the cell body of the postganglionic neuron remains in the ganglion. The postganglionic fibers transmit impulse to the target tissue and organs. Chemical synapses are present both between the preganglionic and postganglionic neurons and between the postganglionic neuron and the cells of its target organ. Generally, the preganglionic fibers are myelinated, whereas the postganglionic fibers are nonmyelinated.

Role of sympathetic and parasympathetic nerves is summarized in Table 11.2. The apocrine sweat glands are innervated adrenergic sympathetic postganglionic fibers and secrete a viscous fluid that may contain pheromones. The merocrine (or eccrine) sweat glands are supplied by sympathetic cholinergic postganglionic fibers. Merocrine glands are located in the skin of footpads. The adrenal medulla is innervated by sympathetic preganglionic fibers that synapse with chromaffin cells, the vestigial postganglionic neurons of the adrenal medulla. Most of the chromaffin cells release epinephrine and some norepinephrine. Blood vessels are only supplied with the sympathetic division; they are not innervated by the parasympathetic division. So, constriction and dilation of the blood vessels occur due to decreasing or increasing sympathetic stimulation, respectively.

The response of different tissues and organs to sympathetic and parasympathetic stimulation varies differently. During any adverse situation, the sympathetic stimulation results in vasoconstriction, which causes increased blood pressure, increased heart rate, increased airflow through the lungs, and epinephrine release from the adrenal gland, whereas vasodilation occurs in the heart, lungs, and skeletal muscles to supply the needed oxygen. The parasympathetic effects are suppressed during the fight-or-flight response.

11.3.2.1 Sympathetic Nervous System

The sympathetic system helps the body to cope up with the adverse conditions and strengthens the body’s defense against those conditions by expenditure of energy. The sympathetic system does it either through fight with that adverse

Table 11.2 Influence of sympathetic and parasympathetic nervous system on different tissues and organs

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Organ	Tissue	Sympathetic nervous system	Type of receptor	Parasympathetic nervous system
Skin	Apocrine sweat glands	Increase in secretion	β2	-
	Merocrine sweat glands	Increase in secretion	M₃	-
	Arrector pili	Erection	“1	-
Eye	Iris: dilator muscle	Dilation of pupil	“1	-
	Sphincter muscle	-	-	Constriction of pupil
	Ciliary muscle	-	-	Contraction
Lung	Bronchiolar muscle	Relaxation	β2	Contraction
Heart	SA node	Increase in heart rate	β1	Decrease in heart rate
	AV node and Purkinje fibers	Increase in conduction velocity	β1	Decrease in conduction velocity
	Atria, ventricle	Increase in contractility	β1	Decrease in contractility
Arteries	Skin and mucosa	Constriction	α
	Salivary glands	Constriction	α	-
	Cerebral	Slight constriction	α	-
	Skeletal muscle	Dilation	β2	-
	Coronary	Dilation	β2	Slight dilation
	Pulmonary	Dilation	β2	-
	Abdominal viscera	Constriction	α	-
Veins		Constriction, dilation	“, β2	-
Gastrointestinal system	Stomach, intestinal tract	Decrease in motility	“, β2	Increase in motility
	Sphincters	Contraction	“1	Relaxation
	Gastric gland	Decrease in secretion	“2	Increase in secretion
	Gallbladder	Relaxation	β2	Contraction
	Liver	Glycogenolysis, gluconeogenesis	“1, β2	Glycogen synthesis
	Pancreatic acini	Decrease in secretion	α	Increase in secretion
	Pancreatic Islets	Decrease in secretion	“2	-
Adrenal medulla		Secretion of E and NE	N	-
Kidney		Renin secretion	β2	-
Urinary bladder	Detrusor muscle	Relaxation	β3	Contraction
Urinary bladder	Trigone and sphincter	Contraction	“1	Relaxation
Reproductive organs	Penis	Ejaculation	“1	Erection
	Uterus (pregnant)	Contraction	“1	Variable
	Uterus (nonpregnant)	Relaxation	β2	Variable
Glands	Lacrimal	Slight secretion	“	Increase in secretion
	Salivary	Slight viscous secretion	“	Increase in watery secretion
	Nasopharyngeal	-	-	Secretion
	Pineal	Melatonin synthesis	β	-

situation or through flight from that situation.

That is why the sympathetic nervous system is also known as the “fight-or- flight” system. In the sympathetic nervous system, preganglionic sympathetic neuron cell bodies are present in a small lateral horn of the spinal cord grey matter between dorsal and ventral horns. The axons leave through the ventral root, enter the spinal nerve, and then leave it just outside the intervertebral foramen to join a longitudinal chain of autonomic ganglia.

In the head and neck region, the sympathetic innervation is mediated by the cranial cervical ganglia. Preganglionic fibers from spinal cord segments T1-T5 (some fibers may even come from T6 and T7) join the vagosympathetic trunk to reach the cranial cervical ganglion.

Postganglionic fibers arise from the cranial cervical ganglion innervate salivary glands, nasal glands, smooth muscles (blood vessels, periorbital area, eyelids, pupillary dilator), carotid body, carotid sinus, and thyroid gland (Table 11.2). Postganglionic fibers may also join the cranial laryngeal branches and pharyngeal branch of the vagus nerve.

The sympathetic trunk ganglia caudal to T4 innervate the rest of the body wall and extremities. Postganglionic fibers join the spinal nerves by way of the rami communicantes to innervate blood vessels, sweat glands, and arrector pili. These structures do not receive parasympathetic innervation, so they are an exception to the dual innervation. Increased activity of the sympathetic system results in the contraction of arteriolar smooth muscles. This leads to an increase in peripheral vascular resistance and subsequent increase in blood pressure. In contrast, decreased activity of the sympathetic system decreases vascular resistance due to relaxation of the arteriolar smooth muscle. Thus, decrease in sympathetic activity lowers blood pressure.

The organs of thoracic cavity are mainly innervated by the cervicothoracic and middle cervical ganglia. The ansa subclavia may also contribute some fibers. Preganglionic fibers originate from spinal cord segments T1-T4.

They reach postganglionic neurons in the cervicothoracic ganglion by way of the rami communicantes. Preganglionic fibers also ascend to the ansa subclavia and middle cervical ganglion, where they synapse with postganglionic neurons. These fibers innervate the vasculature and smooth muscle of the respiratory airways and the lung. The sympathetic stellate cardiac nerve mediates relaxation of the smooth muscle of the respiratory airways and blood vessels, whereas the vagus nerve contracts the smooth muscle. Stimulation of the sympathetic fibers results in an increase in heart rate by increasing the pacemaker activity of the sinoatrial (SA) node cells, impulse conduction at the atrioventricular (AV) node, and contractile force of atrial and ventricular muscle fibers.

The abdominal and pelvic viscera are innervated by spinal cord segments T5-L3. To reach the celiac ganglion in the abdominal cavity, preganglionic fibers from the thoracic spinal segments caudal to T5 descend in the sympathetic trunk and emerge as the greater thoracic splanchnic nerve at the level of the T13 sympathetic trunk ganglion. From the celiac ganglion, postganglionic fibers accompany the arteries to the stomach, duodenum, pancreas, liver, gallbladder, spleen, and adrenal glands. Motility of the gastrointestinal tract is enhanced by parasympathetic fibers of the vagus nerve. However, how the sympathetic division controls the gastrointestinal tract is not clear. It has been speculated that adrenergic fibers synapse on inhibitory α-adrenergic receptors on parasympathetic postganglionic cells of the myenteric plexus and inhibitory β-adrenergic receptors on smooth muscle fibers. Thus, peristaltic action can be decreased by the sympathetic system. The adrenal medulla receives sympathetic preganglionic fibers from cord segments T4 (or T5) to L1 (or L2). The adrenal medulla is composed of chromaffin cells, which are vestigial postganglionic neurons. Chromaffin cells secrete catecholamines (mostly epinephrine and some norepinephrine) into the bloodstream in response to signals from sympathetic preganglionic neurons. Thus, the sympathetic system regulates functions of endocrine cells in the adrenal gland. Preganglionic fibers from spinal segments L1-L3 reach the abdominal and pelvic ganglia via the sympathetic trunk. They either leave the sympathetic trunk ganglia at the level they enter or descend in the sympathetic trunk before exiting. Each sympathetic trunk ganglion of the lumbar segments gives rise to a lumbar splanchnic nerve. It is named after the level from which it arises. The first five lumbar splanchnic nerves supply one or more of the following collateral ganglia: celiac, cranial mesenteric, renal, and gonadal ganglia.

11.3.2.2 Parasympathetic Nervous System

Parasympathetic nervous system is associated with the conservation and restoration of energy. Hence, the parasympathetic nervous system is known as the “rest-and-digest” system. The parasympathetic nervous system helps in the conservation of energy sources of the body. Sympathetic nervous system decreases the heart and blood pressure. It also decreases the force of contraction and thus it reduces the energy expenditure in the body. It also increases the digestive function and increases the blood flow to the GI tract. It increases the GI motility and increases the secretion of digestive enzymes. The parasympathetic division also regulates the micturition process by contracting the urinary bladder.

The preganglionic fibers of parasympathetic division arise from the cranial nerves and sacral segments of the spinal cord; that is why the parasympathetic division is also known as the craniosacral division of the ANS. The fibers of the cranial portion are distributed via four cranial nerves: the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. Within these four nerves, the first three nerves supply parasympathetic fibers to smooth muscle and glands of the head, whereas the vagus nerve supplies parasympathetic fibers to the visceral organs of the thorax and neck. It also supplies nearly all the abdominal viscera.

Parasympathetic fibers, which arise from the oculomotor nerve, supply the iris and ciliary body of the eye and control the contraction of the pupillary sphincter and the ciliary muscle. The parasympathetic stimulation results in the constriction of pupil, and the lens becomes more convex, allowing greater refraction of light for near vision. The parasympathetic fibers of the facial nerve supply the submandibular and sublingual salivary glands. It also innervates the lacrimal glands.

The parasympathetic fibers of the glossopharyngeal nerve supply the parotid and zygomatic salivary glands. The parasympathetic innervation causes increase in watery secretions from these glands. The parasympathetic fibers of the vagus nerve innervate most of the thoracic and abdominal viscera.

The distal part of the digestive tract (including the transverse colon and the area caudal to it) and the pelvic viscera are innervated by parasympathetic fibers from the sacral portion of the parasympathetic nervous system originating from cord segments S2 and S3 in dogs and S1-S3 in cats. These pelvic fibers intermix with sympathetic nerves to form the pelvic plexus. The preganglionic fibers run through the ventral root of the S2 and S3 spinal nerves; then, they together form the pelvic nerve which is present on the lateral wall of the distal rectum. Then the pelvic nerve forms a plexus. The plexus also receives the sympathetic fibers of the hypogastric nerve. The preganglionic fibers of the sacral segment either terminate in the pelvic ganglia of the pelvic plexus or pass through the plexus to terminate in the terminal ganglia in the wall of the pelvic viscera. The pelvic nerve is essential for erection, ejaculation, urination, and defecation.

11.3.3 Neurotransmitters and Their Receptors in Autonomic Nervous System

Acetylcholine (ACh) is the preganglionic neurotransmitter for both sympathetic and parasympathetic systems. The ACh is also a parasympathetic postganglionic neurotransmitter. Hence, the parasympathetic division is also known as the cholinergic division. In sympathetic division, norepinephrine acts as the postganglionic neurotransmitter, and the sympathetic division is called the adrenergic division. Exceptions are found in the merocrine (or eccrine) sweat glands, which are innervated by cholinergic sympathetic postganglionic fibers. The terminal portions of the postsynaptic axons form a number of beadlike swellings known as varicosities. They are also known as boutons en passage. Varicosities contain neurotransmitters and remain close to the surface of the effector cells.

11.3.3.1 Cholinergic Receptors

Based on the selective response to nicotine or muscarine, the cholinergic receptors are of two types: the nicotinic receptor and muscarinic receptor. Nicotinic acetylcholine receptor (nAChR) is mainly found in the neuromuscular junction as well as in all autonomic ganglia. The binding of ACh to the nAChR results in opening of the ion channels, which allows the influx of both Na⁺ and K⁺ according to the electrochemical gradient. Then the neurons are depolarized, as the driving force for Na⁺ to enter the cell far exceeds that of K⁺ to leave the cell. The activation of nicotinic receptors results in the generation of excitatory postsynaptic potentials (EPSPs) of the postsynaptic neuron.

Muscarinic acetylcholine receptors (mAChRs) are present in effector tissues innervated by parasympathetic postganglionic fibers. They are also present in the merocrine sweat glands innervated by cholinergic sympathetic fibers. Different types of muscarinic receptors are present, like M1, M2, and M3. All of these receptors are coupled to G proteins linked to second messenger systems. The binding of ACh to mAChR results in generation of either excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs). The postsyn- aptic response depends on the receptor subtype activated by ACh and subsequent opening of ligand-gated channels for specific ions. So, the action of ACh at a synapse depends on the muscarinic receptor subtypes present in the tissue. For example, binding of ACh to M2 receptors in the heart results in decreasing heart rate and contraction force, whereas binding of ACh to M3 receptors in bronchioles and urinary bladder causes contraction of bronchiolar and bladder smooth muscles. Atropine is an anticholinergic drug, which blocks parasympathetic effects. Atropine is used to dilate the pupil or to suppress salivation and respiratory secretions.

11.3.3.2 Adrenergic Receptors

There are two types of adrenergic receptors, the alpha (α) and beta (β). The α-adrenergic receptors are mainly excitatory, which cause vasoconstriction of blood vessels, raising blood pressure, constricting sphincters of the gastrointestinal tract, contracting urethral smooth muscle, and dilating the pupils. The β-adrenergic receptors are of different types, like β 1, β2, and β3. The β1-adrenergic receptors in the heart (cardiac muscle, pacemakers) increase heart rate and contraction force. Beta-blockers that act on the β1 receptors of the heart reduce heart rate and prevent arrhythmias. The β2-adrenergic receptor is present in smooth muscle of the gastrointestinal tract. These β2 receptors relax gastrointestinal smooth muscle. The β2-adrenergic receptor is also present in the vascular smooth muscle of the heart and skeletal muscle, as well as bronchiolar smooth muscle. The β3-adrenergic receptors are present in the urinary bladder and relax detrusor muscle in response to norepinephrine released from the sympathetic hypogastric nerve.

11.3.4 EntericNervousSystem

The enteric nervous system (ENS) is considered as the third division of the ANS. Enteric nervous system is the network of motor and sensory neurons located within the walls of the gastrointestinal tract and its accessory glands (viz. pancreas, liver). It is influenced by both the parasympathetic and sympathetic divisions of the ANS, but the system is functional without input from outside the viscera. It is comprised of two well-organized neural plexuses. The submucosal plexus is present just deep to the inner lining of the gut. The another is myenteric plexus, found within the muscular layer, involved in the control of digestive tract motility. It senses the environment of the lumen and regulates gastrointestinal blood flow and epithelial cell function.

The enteric nervous system contains large number of neurons almost equal to the number of neurons present in the entire spinal cord. Enteric nervous system contains all the elements of the nervous system; hence, it is also known as the “mini brain.” It contains the sensory neurons, interneurons, and motor neurons. The sympathetic as well as parasympathetic nerves connect the CNS to the ENS or directly innervate the GI tract. Though the ENS can act autonomously, normal function of GI tract often requires communication between the central nervous system and the enteric nervous system.

Learning Outcomes

• Nervous system is a special system which coordinates to collect the information from the external as well as the internal environment and sends to the respective center(s) to generate responses through the motor system. It is morphologically and functionally divided into two components, central nervous system (CNS) and peripheral nervous system (PNS). The central nervous system consists of the brain and spinal cord. The peripheral nervous system comprises sensory and motor nerves that run throughout the body.

• The nervous system is made up of a large number of cells (over 100 billion). The cells are mainly of two types, the neurons and neuroglia or glial cells. Neurons are the basic unit of the nervous system as the functions of the nervous system are carried out by neurons. Neurons are specialized types of conducting cells.

• The communication of neurons between each other and with other cells of the body, like muscle and glandular cells, occurs very fast, at specialized junctions called synapse. Neurotransmitters are the chemical transmitters or chemical messenger substances liberated at the nerve endings and help to transfer the nerve impulses in the presynaptic neuron to an adjacent cell (neighboring postsynaptic neurons or muscle or gland cells).

• Autonomic nervous system (ANS) is a part of the peripheral nervous system, which is associated with the involuntary functions of the body. Autonomic nervous system controls the visceral organs and endocrine glands, and thus it is associated with the maintenance of homeostasis. There are two major subdivisions of autonomic nervous system: the sympathetic nervous system and parasympathetic nervous system.

• The sympathetic system helps the body to cope up with the adverse conditions and strengthens the body’s defense against those conditions by expenditure of energy. The sympathetic system does it either through fight with that adverse situation or through flight from that situation. Parasympathetic nervous system is associated with the conservation and restoration of energy. Hence, the parasympathetic nervous system is known as the “rest-and- digest” system.

• The enteric nervous system (ENS) is considered as the third division of the ANS. Enteric nervous system is the network of motor and sensory neurons located within the walls of the gastrointestinal tract and its accessory glands (viz. pancreas, liver).

Exercises

Objective Questions

Q1. Which was the first neurotransmitter discovered?

Q2. Unipolar neurons are mainly found in which part of the nervous system?

Q3. Which structure produces cerebrospinal fluid in the ventricles of brain?

Q4. Vomiting center is located in which part of the brain?

Q5. What is the major function of mammillary bodies?

Q6. Thermoregulatory center is located in which part of the brain?

Q7. Almost all of the cerebral cortex has direct two-way communication with which subcortical structure?

Q8. What is ganglion?

Q9. The summation of the excitatory as well as inhibitory activity occurs in which part of the neuron?

Q10. Free nerve endings are sensory receptors for which sensations?

Subjective Questions

Q1. Describe the different phases of synaptic transmission.

Q2. Write the basic characteristics of neurotransmitter.

Q3. Write the functions of different types of glial cells.

Q4. Write the fate of neurotransmitter.

Q5. Describe the different layers of meninges.

Q6. Write the formation and absorption of CSF.

Q7. Classify sensory receptors on the basis of structure.

Q8. Write the functions of cerebellum.

Q9. Describe the components of reflex arc.

Q10. Write the difference between sympathetic and parasympathetic nervous system.

Answer to Objective Questions

A1. Acetylcholine

A2. The afferent division of the PNS

A3. Choroid plexus

A4. Medulla oblongata

A5. Act as a relay center for olfaction and are associated with memory

A6. Hypothalamus

A7. Thalamus

A8. A ganglion is the collection of neuron cell bodies in the PNS

A9. Axon hillock

A10. Temperature and pain

Keywords for Answer to Subjective Questions

A1. (1) Axon potential arrives at the axon terminal, (2) voltage-gated Ca²+ channels open, (3) Ca²+ enters the presynaptic neuron, (4) Ca²⁺ signals to neurotransmitter vesicles, (5) vesicles move to the membrane and dock, (6) neurotransmitters released via exocytosis, (7) neurotransmitters bind to receptors, (8) signal initiated in postsynaptic cell

A2. (1) Must be synthesized in the neuron, (2) should be stored in the presynaptic terminal, (3) should be released at the synapse in amounts sufficient to exert a defined action, (4) should have its specific receptors on postsyn- aptic membrane, (5) should be removed quickly by the specific mechanism as soon its action is over.

A3. Astrocytes: form support in CNS, form the blood-brain barrier, secrete neurotrophic factors, take up K⁺, neurotransmitters. Oligodendrocytes: form myelin sheaths in CNS. Microglia: phagocytic in function. Ependymal cells: create barriers between compartments. Schwann cells: form myelin sheaths in PNS. Satellite cells: support cell bodies.

A4. There are two processes for removal of neurotransmitters after their action. These are (1) enzymatic inactivation in the synaptic cleft and (2) diffusion away from the synaptic cleft.

A5. Meninges have three layers, viz. dura mater, arachnoid mater, and pia mater (from outside to inside).

A6. CSF is secreted by the ependymal cells located in the choroid plexus. CSF is absorbed and returns to the venous system mainly into dural venous sinuses, which are present intracranially between the endosteal layer and meningeal layers of the dura mater.

A7. (1) Free nerve endings, (2) encapsulated nerve endings, (3) specialized receptors.

A8. Postural adjustments in order to maintain balance.

A9. (1) Receptors, (2) sensory neuron, (3) interneuron, (4) motor neuron, (5) effectors.

A10. The sympathetic system helps the body to cope up with the adverse conditions and strengthens the body’s defense against those conditions by expenditure of energy. The sympathetic system does it either through fight with that adverse situation or through flight from that situation. That is why the sympathetic nervous system is also known as the “fight-or-flight” system. Parasympathetic nervous system is associated with the conservation and restoration of energy. Hence, the parasympathetic nervous system is known as the “rest-and-digest” system.

Autonomic Nervous System

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