Autonomic Nervous System

What is the difference between the autonomic and the somatic nervous system?

Although the autonomic and the somatic nervous systems are parts of the peripheral nervous system, they differ in function. The somatic nervous system is responsible for voluntary movement, whereas the autonomic nervous system is responsible for involuntary movement.

• Somatic nervous system: This part of the peripheral nervous system communicates with your senses (the Greek word for senses is "soma"). It is also responsible for the voluntary control of your muscles. The somatic nervous system uses motor neurones that have thick myelination and end in specific powerful synapses called neuromuscular junctions. Any activity that you consciously perceive or control, such as moving fingers, reading, or speaking, fall under the banner of the somatic nervous system.

• Autonomic nervous system: This is the part of the peripheral nervous system which is in charge of the involuntary and unconscious control of processes of the body, such as heart rate, blinking, digestion, relaxation, and arousal. The signal gets passed via two, less myelinated neurones that traverse a cluster of neural cell bodies called ganglions. It works independently from voluntary control and is controlled by the hypothalamus.

What is the function of the autonomic nervous system?

The autonomic nervous system controls the involuntary processes in the body. These could be ongoing or cyclical processes like the beating of the heart, or they could be responses to specific stimuli. The autonomic nervous system has three divisions, and depending on the division, they have distinct functions.

Three divisions of the autonomic nervous system

There are three divisions - sympathetic, parasympathetic, and enteric nervous systems.

Sympathetic nervous system

The sympathetic nervous system is responsible for "fight, flight or freeze". The SNS mobilises the organism to get ready for action. It responds when stimuli are perceived as dangers and readies the body to fight the threat or flee from it.

It's an acute stress response that quickly mobilises the body's resources to deal with danger through hormones and physical changes. The body prepares for the eventuality to either run away from the threat (flee) or fight the threat (fight). Once activated, it takes around 20 minutes to an hour for the body to return to its normal state.

Parasympathetic nervous system

The parasympathetic nervous system (PNS) returns the body to homeostasis (biological balance) by counteracting the sympathetic nervous system. It slows the heart rate and breathing and blocks stress hormones.

The PNS oversees the body's response when the organism knows it's safe and can now eat and sleep in peace. Its goal is to conserve energy. It also regulates digestion and urination. So when you've just had a massage or have just finished working out, this is the nervous system division responsible for that feeling of deep relaxation afterwards.

Enteric nervous system

The enteric nervous system (ENS) is a mesh-like network of neurones in the gastrointestinal tract lining. It is called the "second brain" because of its sheer amount of neurons, and its exact function isn't well understood.

The sympathetic and the parasympathetic nervous system are antagonistic, meaning they function as opposites of each other. The sympathetic nervous system mobilises resources in the body; the parasympathetic nervous system returns the body to a resting state or homeostasis.

How does the autonomic nervous system control the heart rate?

The heart isn't dependent on impulses from the nervous system like other muscles are to keep on pumping; instead, it has its pacemaker - the sinoatrial node (SAN). This keeps a steady resting heart rate (ticking along at around 70 bpm beats per minute for humans).

Depending on whether more oxygen is needed, say to flee from a terrifying clown, or whether the organism is safe and sound and can relax, the autonomic nervous system can modify the heart rate.

To understand how the autonomic system influences the heart rate, let's first look at how the heart works without influence from the autonomic nervous system.

Heart rate without influence by the autonomic nervous system

The heart rate is controlled by the sinoatrial node (SAN), a group of cells on the right side of the heart that controls the heart rhythm by sending impulses to the cardiac muscle. The heart's chambers contract in sequence in response to stimulus sent by the SAN.

The atrioventricular node (AVN) acts as a gate for the electrical impulse between the atria and ventricle.

The sequence triggered by one excitatory wave starting from the SAN goes as follows:

1. The SAN sends an electrical impulse to the atria, making them contract.

2. The electrical impulse enters the AVN; there's a short delay (PAUSE!).

3. The AVN sends an impulse to another structure called the bundle of His, using a pathway made up of specialised muscles called the Purkinje fibre.

4. The bundle of His carries the impulse to the base of the ventricles.

5. Once at the bottom, the impulse is released from the main bundle of His into all the small branches coming out of it, making all the ventricles contract upwards at once.

Fig. 2 - Heart rate without influence by the autonomic nervous system

Influence of the autonomic nervous system on the heart rate

When the autonomic nervous system is activated, the modification is controlled by the medulla oblongata (a region in the brain stem via the sinoatrial node):

• The sympathetic nervous system increases the heart rate.

• The parasympathetic nervous system decreases the heart rate.

Chemoreceptors and baroreceptors control the activation of the two systems of the autonomic nervous system.

Chemoreceptors

The chemical changes depend on the level of carbon dioxide in the blood. When the pH level in the blood is low, the frequency with which electrical impulses are sent to the medulla oblongata increases. This increases the heart rate by increasing the impulses sent by the SAN.

An increased heart rate means more blood is sent to the lungs to absorb oxygen from the air, decreasing carbon dioxide levels in the blood. Once the pH approaches normal levels, the electrical signal sent from the chemoreceptors decreases in frequency, and the heart rate is lowered.

Baroreceptors

If the blood pressure is low, the pressure receptors send impulses to the medulla oblongata, which uses the sympathetic nervous system pathways to increase the heart rate via the SAN.

If blood pressure is high, the pressure receptors send impulses to the medulla oblongata to use parasympathetic nervous system pathways to decrease heart rate via the sinoatrial node.