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


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 responses1

Parasympathetic -mediated responses2


Sinoatrial (SA) node Atria

Atrioventricular (AV) node His-Purkinje system Ventricles

General excitation

β– increase heart rate

β– increase contractile force, conduction velocity β– increase automaticity, conduction velocity


β– increase automaticity, conduction velocity

       β– 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


a– constriction; β– dilation5

a– constriction

a– constriction; β – dilation

a– constriction; β– dilation8

a– constriction; β– dilation9

a– constriction; β– dilation9

a– constriction

a– constriction

a– dilation


Dilation6; constrictionDilation7


GI tract

Smooth muscle Sphincters Secretions

Gall bladder & ducts

General inhibition

β– relaxation; a – relaxation11

a – contraction Decrease (usually) Relaxation

General excitation Increase motility and tone Relaxation

Increase Contraction

    Bronchioles Smooth muscle Glands


β– relaxation Inhibition (?)


Contraction Stimulation


Radial muscle, iris Sphincter muscle, iris Ciliary muscle


a– contraction (mydriasis)

β – relaxation; far vision


Contraction (miosis) Contraction; near vision

Urinary bladder


Trigone, sphincter

Urinary retention β– relaxation

a – contraction

Urination Contraction Relaxation

Splenic capsule

a – contraction, β– relaxation


Sweat glands

Secretion (cholinergic); 12β– Secretion (horse)


Salivary glands

a– scant, viscous secretion

Profuse watery secretion

Piloerector muscles

a – contraction

Kidney rennin release

a– decrease; β– increase


a– contraction, β – relaxation (non-pregnant > pregnant)


    Genitalia Male Female

a – ejaculation


Erection15 Erection15

Adrenal medulla

Secretion of epinephrine > norepinephrine (cholinergic)

Autonomic ganglia

Ganglionic discharge (cholinergic)

Ganglionic discharge16


β– glycogenolysis and gluconeogenesis (a in some species)


Islet cells Acini


a– decrease; β– increase secretion

a – decrease secretion


Increase secretions

Fat cells

β– lipolysis

Adrenergic nerve terminals

a– decrease release of norepinephrine β– increase release of norepinephrine

± Release of norepinephrine17


a– aggregation

Previous Post Next Post