Systems of the brain

Systems of the brain is part of cognitive processes and structures of the brain.

The nervous system encompasses a large range of sub-systems, as shown below (click for larger image).

nervous system

Central nervous system (CNS)

The central nervous system comprises of the brain and the spinal cord. The spinal cord is used as a messenger between the brain and the rest of the body, sending messages between the brain and the peripheral nervous system.

Peripheral nervous system (PNS)

Conversely, the peripheral nervous system sends messages from the body (organs, muscles etc.) to the central nervous system in the brain. It also sends messages from the brain to the body, allowing us to move. There are two subsystems of the peripheral nervous system: the somatic nervous system and the autonomic nervous system.


The somatic nervous system is used to control (voluntary) movement of the body. As suggested in the diagram above, the somatic nervous system makes use of both sensory and motor neurons. Sensory neurons send sensory information (that is, stimuli relating to senses such as touch, smell, taste etc.) toward the central nervous system. This information is then processed by the basal ganglia – part of the forebrain – before motor neurons are sent to particular parts of the body required to respond to the sensory information.

For example, imagine that you accidentally touch a hot stove. Sensory neurons in, let’s say, your fingers, would be sent via the peripheral nervous system to the brain, whereby they would indicate that the stove was painfully hot. In response, motor neurons would be sent back to the fingers to allow them to move away from the stove.


Conversely, the autonomic nervous system is responsible for keeping the automatic processes of internal organs and glands (such as our heartbeat, blinking and breathing, although some of these can be influenced voluntarily) running smoothly. It also partly controls emotion. The autonomic nervous system includes two branches which work exclusively (not at the same time): the sympathetic nervous system and the parasympathetic nervous system.

Sympathetic nervous system

You may have heard of the ‘fight or fight’ (sometimes ‘fight, flight or freeze’) response. This is a big part of the sympathetic nervous system, which activates when an organism perceives itself to be in threat (whether or not they actually are in threat is immaterial). When this response is activated, numerous physiological changes in the body occur. These may include:

  • Increased heart rate
  • Increased perspiration
  • Increased respiration rate
  • Inhibited digestion
  • Loosening of bladder
  • Dilation of pupils
  • Dry mouth
  • Adrenaline released

Parasympathetic nervous system

Conversely, the parasympathetic nervous system is in action when an individual is relaxed and does not think that they are in immediate threat. Following the fight or flight response of the sympathetic nervous system, the parasympathetic nervous system takes hold to return the body’s processes to regular functioning (homeostasis). The physiological consequences of the parasympathetic nervous system are inverse to those of the sympathetic nervous system:

  • Decreased heart rate
  • Decreased perspiration
  • Decreased respiration rate
  • Improved digestion
  • Tightening of bladder
  • Smaller pupils
  • Increased salivation
  • Adrenaline withheld