Autonomic Nervous System

Our next topic in Veterinary Physiology 1 is about the Autonomic Nervous System (ANS). This post will be discussing the the main divisions of the nervous system and the major structural and functional differences between the somatic and autonomic nervous systems. I'll describe the functions of the ANS in an organism as well as explain the concept of dual innervation. I'll finish off with comparing the two branches of the ANS. Enjoy!

The Main Divisions of the Nervous System. 

The divisions of the Nervous system can be summarised in the diagram below.

 The Somatic and Autonomic Nervous Systems

There are several differences between the somatic nervous system (SNS) and the autonomic nervous system (ANS). The primary function of the ANS is to maintain homeostasis in the body. Somatic responses are faster than those of the ANS, however, responses from the ANS can be more diverse and are able to coordinate the activity of many body systems. In addition, the ANS has 2 neurons between the central nervous system and the target organ while the SNS has only one neuron. Also, the ANS uses acetylcholine and noradrenaline as a neurotransmitter while the SNS only uses acetylcholine. The ANS can also be inhibitory or stimulatory while the SNS can only be stimulatory. This can be summarised in the table below: 

	Somatic NS	Autonomic NS
Function	Voluntary control of movement	Control of involuntary functions
Action	stimulatory	Inhibitory/stimulatory
Effector Organ	Skeletal muscle	Smooth muscle, heart, glands
No. of neurons between CNS and target	1	2
Transmitter	acetylcholine	Acetylcholine/noradrenaline
Transmitter released from	Axon terminals	Axon terminals and varicosities
Receptor type	ionotropic	Metabotropic  and ionotropic

 Roles of the ANS

The main role of the ANS is to maintain homeostasis and to respond to stress. The ANS is able to direct the body's resources when in times of need. An example of this is the redirection of blood flow from the digestive system to the skeletal muscles during strenuous exercise.

Dual Innervation

The ANS consists of two branches: the Sympathetic Nervous System and the Parasympathetic NS. These two divisions usually exist as separate nerves each innervating the same organ. The sympathetic nervous system is activated when the body prepares for stress (a flight or fight response). The parasympathetic nervous system is activated in order to maintain normal body function during times of rest (the rest-and-digest response). These two divisions are constantly at work but at different proportions depending on the situation. Dual innervation is essential to maintain homeostasis. 

Sympathetic and Parasympathetic Nervous Systems

As I said earlier, there are two divisions of the ANS, the sympathetic and parasympathetic nervous systems. Their anatomy is compared in the diagram below:

The Sympathetic (red) and Parasympathetic (blue) Nervous Systems

Their differences can be summarised in a table form:

	Sympathetic	Parasympathetic
Spinal origin of preganglionic cell	Thoracolumbar	Brain stem and sacral region
Ganglia	Sympathetic chain ganglia or collateral ganglia or adrenal medulla	Parasympathetic ganglia
Location of ganglia	Near spinal column	Near target organ
Preganglionic nerve length	short	Long
Focus point	Adrenal gland	Parasympathetic nerves

And that's it for this post, if you have any question please feel free to ask :)