Nervous System Divisions

Nervous System Divisions Diagram

The nervous system is comprised of the central nervous system (CNS) and the peripheral nervous system. The CNS consists of the brain and spinal cord, and the peripheral nervous system is made up of the autonomic nervous system and the somatic nervous system.

The autonomic nervous system is divided into the sympathetic nervous system and the parasympathetic nervous system.

Fig. 2 - The human nervous system is comprised of multiple systems.

We can explore each division of the nervous system to find out what it is they specialise in doing.

Central Nervous System Divisions and Peripheral Nervous System Divisions

The nervous system can be placed into divisions, and the two main divisions are the central nervous system and the peripheral nervous system.

• Central nervous system (CNS) - The central nervous system includes the brain and spinal cord. The CNS is the centre of control for the entire organism. It’s responsible for conscious decisions as well as automatic reactions (reflexes) to stimuli.

• Peripheral nervous system (PNS) -The peripheral nervous system connects the CNS to the body, the nervous system sends impulses from the peripherals to and from the CNS. The peripheral nervous system then is subdivided by function into the autonomic nervous system and the somatic nervous system. The autonomic nervous system can either be aroused or calmed. Depending on the response, it is overseen by the sympathetic nervous system (fight-or-flight response) and the parasympathetic nervous system (rest and digest response).

Central Nervous System Divisions

The central nervous system includes the brain and the spinal cord. This subsystem has physiological measures in place that prevent harmful toxins from entering the central nervous system. A specific, plasma-like fluid circulates in and around the central nervous system called cerebrospinal fluid (CSF).

It has several molecular structures and membranes functioning as security gates, preventing toxins from entering the brain even if they’re already circulating in the body in substances such as blood.

The Brain

If you compare the size of other mammals' brains to human brains, the human brain-to-body ratio is the same as that of a mouse or monkey. Therefore, if a rat or mouse were as tall as a human, their brains would be the same size as the human brain. Brains are very different from organism to organism - some animals don’t have a brain - such as a jellyfish. On the other hand, some animals, such as octopuses, have much larger brain-to-body ratios than humans.

However, the primary structural difference between humans and other animals is that the brain’s surface area, called the cerebral cortex, is much larger than that of other mammals.

Conscious and unconscious decisions are made in the brain. The brain stem connects the brain to the spinal cord.

The Spinal Cord

The spinal cord is a tubular structure of nerves that extend from the brain into the peripheral nervous system. It reaches from the base of the brain called the hindbrain to the second lumbar vertebra in the lower back, about five centimetres above the pelvis.

To enable the body to react quickly, specialised neurons, called relay neurons, carry out unconscious reactions to stimuli known as reflexes.

The spinal cord includes the nerve endings that serve as connectors to the peripheral nervous system.

Peripheral Nervous System Divisions

In the peripheral nervous system, information gets passed to the CNS and from the CNS to muscles and organs, known as effectors. Information taken in by the senses (smell, taste, sight) and receptors (touch, heat, pain) is passed to the CNS for integration.

The peripheral nervous system is subdivided into the somatic nervous system and the autonomic nervous system. These two divisions of the nervous system run parallel to each other (they are not divided by location).

• Somatic nervous system: This part of the peripheral nervous system communicates with your senses (“soma”). It also is responsible for the voluntary control of your muscles. Any activity that you consciously control, such as moving fingers or speaking, falls under the banner of the somatic nervous system.

• Autonomic nervous system: This is the part of the peripheral nervous system in charge of the involuntary and unconscious control of processes of the body such as heart rate, blinking, digestion, relaxation and arousal. It works autonomously and is influenced by a specific part of the brain called the hypothalamus. The autonomic nervous system can again be divided into two functional units.

Autonomic Nervous System Divisions

The autonomic nervous system, as we discussed briefly before, controls the unconscious decisions your body makes.

Fig. 3 - The parasympathetic and sympathetic nervous systems have different effects on the body.

The sympathetic and parasympathetic nervous systems are functional divisions of the autonomic nervous system that are automatically activated in response to stimuli.

Sympathetic Nervous System Division

The sympathetic nervous system is known more commonly as the fight-or-flight division of the nervous system and prepares the body to move if necessary.

• The sympathetic nervous system (responsible for “fight, flight or freeze”): the part of the autonomic nervous system that is also called the fight-or-flight response (or in more modern textbooks, fight, flight or freeze response). It mobilises the organism in response to stimuli perceived as dangerous to be able to fight the danger or flee from it.

  • When activated, the sympathetic nervous system causes pupils to dilate, allowing for a better perception of light. It makes the body release stress hormones into the bloodstream, which mobilises carbohydrates in the body for energy. The heart rate increases to get more energy to all parts of the body quickly to carry out fast movements.

    So if you hear a bump in the night and your heart starts racing, and your breathing is rapid, the sympathetic nervous system is the nervous system division responsible.

Parasympathetic Nervous System Division

The parasympathetic nervous system calms the system down and is commonly referred to as the rest-and-digest division of the nervous system.

• The parasympathetic nervous system (responsible for “rest and digest”): the part of the autonomic nervous system that returns the body to its homeostasis (biological balance) by counteracting the sympathetic nervous system.

  • It slows the heart rate and breathing and blocks stress hormones. This is the body's response when the organism knows it’s safe and can now eat and sleep in peace and safety without danger. 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 you feel afterwards.