INTRODUCTION OF AUTONOMIC NERVOUS SYSTEM

In this blogpost, we come to know about Introduction of the Autonomic Nervous System.

Terminology:

Afferent (Sensory): Nerves that covey flow of impulse from peripheral to CNS.

Efferent (Motor): Nerves that convey impulses from the brain and spinal cord
(CNS) to muscles, glands and other effector organs.

Ganglion: It is an aggregation of synapses

Neuroeffector junction: The junction a post-ganglionic axonal terminal with its
effector cell is termed a neuroeffector junction.

Nerve plexus: It is a network of nerve fibres.

Autonomic nervous system (A.N.S.) is a peripheral complex of nerves, plexuses and ganglia that are organized to modulate the involuntary activity of the secretory glands, smooth muscles and visceral organs. This system functions to sustain homeostatic conditions during periods of reduced physical and emotional activity, and equally important, to assist in internal bodily reactions to stressful circumstances.

Nerves transmit their impulses across most synapses and neuroeffector junctions by means of specific chemicals called as neurohumoral transmitters or simply neurotransmitters.

The autonomic drugs exert their actions on smooth muscles, cardiac muscles, glands and visceral organs by mimicking or modifying the action of neurotransmitters released by autonomic fibres either at ganglia or at effector cells.

Difference between autonomic and somatic nervous system:

Autonomic Nervous System (A.N.S.)

Somatic Nervous System

i. Efferent nerves of A.N.S. supply all innervated structures of the body except skeletal muscles.

ii. The most distal synaptic junction in A.N.S. occurs in ganglia that are entirely outside the cerebrospinal axis.

iii. Many autonomic fibres form extensive peripheral plexuses.

iv. When cut, organ supplied generally show some level of spontaneous activity independent of intact innervations.

v. Pre-ganglionic and post-ganglionic fibers are present.

vi. Post-ganglionic autonomic nerve fibres are generally non-myelinated (slow conduction of impulse), whereas pre-ganglionic fibres are myelinated (fast conduction of nerve impulse).

i. They supply skeletal muscles.

ii. Somatic nerves contain no peripheral ganglia, and synapses are located entirely within the cerebrospinal axis.

iii. No peripheral plexus is present in somatic nerve fibres.

iv. When cut, skeletal muscles supplied become paralyzed and undergo atrophy.

v. No such nomenclature.

vi. Most fibres of skeletal muscles are myelinated.

Difference between sympathetic and parasympathetic nervous system:

Sympathetic Nervous System (Adrenergic Nervous System)

Parasympathetic Nervous System
(Cholinergic Nervous System)

i. It arises as thoraco-lumbar outflow (T1 to L3).

ii. Ganglia are nearer to the C.N.S. The ratio of pre- and post-ganglionic fibre is generally 1:20 or more. So, the post-ganglionic fibre is longer.

iii. Distributed to effector organs throughout the body.

iv. Neurotransmitters are acetylcholine - (in ganglia) and norepinephrine - (at neuroeffector junctions).

v. Function of Sympathetic Nervous System: As a generalization, it can be said that activation of the sympathetic changes functions in a direction which fits the body for a period of activity and energy expenditure. For example, blood pressure increases, blood flow is diverted from skin and gut to the CNS and muscles, bronchioles dilate and glycogenolysis & lipolysis reveal mobilization of energy reserves.

vi. Sympathetic activity increases in stress and emergency.

vii. Sympathetic nervous system is responsible for providing continuous stimulus to the organs and the parts supplied.

viii. If nerve is cut, the animal will survive with some physiological change

i. It arises as a craniosacral outflow with 3rd (oculomotor), 7th (facial), 9th (glossopharyngeal), 10th (vagus) and 11th (spinal accessory) cranial nerves along with 2nd, 3rd and 4th sacral nerves.

ii. Ganglia are away from the C.N.S. and on or close to the organs. The ratio is generally 1:1. So, the post- ganglionic fibre is shorter [Exception- In Auerbach’s plexus, the ration is 1:8,000].

iii. Distribution is much more limited.

iv. Neurotransmitter is acetylcholine in both ganglia and at neuroeffector junctions.

v. Function of Parasympathetic Nervous System:Conversely, parasympathetic activity modulates body functions towards the needs of a period of inactivity and repair of energy deficits. Vital functions are slowed, energy consumption is reduced and increased digestive function replenishes the stores and evacuates wastes.

vi. Parasympathetic activity predomina- tes during rest.

vii. The parasympathetic nervous system is endowed with the medullary functions.

viii. If taken out, the function is usually normal but due to conservation of energy, animal will not survive long.

Typical responses of effector tissues to sympathetic and parasympathetic nerve impulses:

*Effector tissues*	*Sympathetic-mediated responses*1	*Parasympathetic -mediated responses*2
Heart Sinoatrial (SA) node Atria Atrioventricular (AV) node His-Purkinje system Ventricles	General excitation β1 – increase heart rate β1 – increase contractile force, conduction velocity β1 – increase automaticity, conduction velocity β1 – increase automaticity, conduction velocity β1 – increase contractile force, conduction velocity, irritability3	General inhibition Decrease heart rate Decrease contractile force Decrease conduction velocity; AV block … Decrease contractile force4
Blood vessels Coronary Cutaneous, mucosal Cerebral Skeletal muscle Splanchnic Renal Genital Veins Endothelium	a1 – constriction; β2 – dilation5 a1 – constriction a1 – constriction; β – dilation a1 – constriction; β2 – dilation8 a1 – constriction; β2 – dilation9 a1 – constriction; β2 – dilation9 a1 – constriction a1 – constriction a2 – dilation	Dilation6; constriction6 Dilation7 Dilation7 Dilation7 Dilation7 Dilation7 Dilation10
GI tract Smooth muscle Sphincters Secretions Gall bladder & ducts	General inhibition β1 – relaxation; a – relaxation11 a – contraction Decrease (usually) Relaxation	General excitation Increase motility and tone Relaxation Increase Contraction
Bronchioles Smooth muscle Glands	β2 – relaxation Inhibition (?)	Contraction Stimulation
Eye Radial muscle, iris Sphincter muscle, iris Ciliary muscle	a1 – contraction (mydriasis) … β – relaxation; far vision	… Contraction (miosis) Contraction; near vision
Urinary bladder Fundus Trigone, sphincter	Urinary retention β1 – relaxation a – contraction	Urination Contraction Relaxation
Splenic capsule	a – contraction, β2 – relaxation	...
Sweat glands	Secretion (cholinergic); 12β2 – Secretion (horse)
Salivary glands	a1 – scant, viscous secretion	Profuse watery secretion
Piloerector muscles	a – contraction	…
Kidney rennin release	a2 – decrease; β1 – increase	…
Uterus13	a1 – contraction, β – relaxation (non-pregnant > pregnant)	Contraction14
Genitalia Male Female	a – ejaculation …	Erection15 Erection15
Adrenal medulla	Secretion of epinephrine > norepinephrine (cholinergic)	…
Autonomic ganglia	Ganglionic discharge (cholinergic)	Ganglionic discharge16
Liver	β2 – glycogenolysis and gluconeogenesis (a in some species)	…
Pancreas Islet cells Acini	a2 – decrease; β2 – increase secretion a – decrease secretion	… Increase secretions
Fat cells	β1 – lipolysis	…
Adrenergic nerve terminals	a2 – decrease release of norepinephrine β2 – increase release of norepinephrine	± Release of norepinephrine17
Platelets	a2 – aggregation	…