The internal organs of our body (like the heart, stomach, intestines) are controlled by part of the peripheral nervous system, known as the autonomic nervous system (ANS). In most situations, we do not realize how the ANS functions, this happens in an involuntary way. For example, we cannot see the work of blood vessels in the same way as we have an effect on heart rate. Although most autonomous functions are reflective, some of them can be controlled consciously by a person, but to a certain extent. This is swallowing, breathing and sexual arousal.
Providing homeostasis, the autonomous (or autonomic) nervous system is very important in the selection of behavior, actions controlled by the brain. This happens in emergency situations that provoke stress and require us to concentrate internal forces in the fight against the current situation as well as under relaxing circumstances that contribute to recovery and rest.
ANS consists of three departments:
- sympathetic nervous system (SNS);
- parasympathetic nervous system (PNS);
- Enteral nervous system (the reticular structure of nerve fibers, which is found in several organs and which can be considered as the third component of the autonomic nervous system).
The sympathetic nervous system acts as a mediator in reactions associated with tense situations, by increasing the heart rate and increasing blood pressure. It ensures that the body is ready to act immediately in stressful situations or when in danger. This corresponds to the classic “fight or flight” response, mediated by the two main chemical messengers - epinephrine (adrenaline) and norepinephrine. For this reason, the SNA is called the "working nerve."
The parasympathetic nervous system, in contrast, is the “calm” part of the ANS. It is also known as the "nerve of calm." While the sympathetic nervous system prepares the body for stressful situations, PNS serves as a “refill” of energy and recovery. It stimulates the actions that occur when the body is at rest, especially during eating, nap, sexual arousal.
But the sympathetic and parasympathetic sections of the ANS, although they function against each other, are not opposites. Rather, it is an interconnected complex that creates balance within our body. Between these departments there are dynamic interactions that are regulated by secondary intermediaries (cyclic adenosine monophosphate and cyclic guanosine monophosphate). For example, when the heart receives neural stimulation from the PNS, the heartbeat slows down, and vice versa, when the heart receives neural stimulation by SNS neurons, heart rate increases.
Sympathetic activation can inhibit parasympathetic activation presynaptically. Similarly, the parasympathetic nervous system is involved in the presynaptic inhibition of the movement of the sympathetic nerves.
The functions of a balanced autonomic nervous system are vital. If the interaction between the "working nerve" and the "nerve of calm" is violated, some restrictions arise, thereby jeopardizing the quality of life.
So, overstimulation of the SNS can lead to problems such as anxiety, arterial hypertension and digestive disorders. Overstimulation of PNS can result in reduced pressure and a feeling of fatigue.
The parasympathetic nervous system, as well as the sympathetic, is not concentrated in one area, but distributed over a large area. The autonomic centers of the PNS are located in the region of the brain stem and in the region of the sacral region of the spinal cord. In the medulla oblongata, cranial nerves, VII pair, IX pair and X pair form preganglionic parasympathetic fibers. From the medulla oblongata or spinal cord, the preganglionic fiber (long) is carried towards the ganglia, which are very close to the target organ, and makes a synapse. The synapse uses a neurotransmitter called acetylcholine. In this area from the ganglion, the postganglionic fiber (short) is projected directly onto the target organ, using also acetylcholine.
Acetylcholine acts on two types of cholinergic receptors: muscarinic and nicotinic (or acetylcholine receptors). Although the parasympathetic nervous system uses acetylcholine (as a neurotransmitter), petids (cholecystokinin) can also perform this function.