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Two nervous systems working hand in hand.

The survival of an animal depends on its nervous system. It allows him to successfully monitor, control and respond to changes in the environment. The brain is the central organ of our thinking, reasoning and identity. Together with the spinal cord it makes up the Central Nervous System (CNS). But there is another, equally important, nervous system in our body. The Peripheral Nervous System (PNS) consists of the nerves outside the brain or the spinal cord. It carries information back and forth between the brain, our senses and other parts of the body. The CNS could not function without the information supplied by the PNS.

The nervous system, like all tissues of the body, is composed of cells. There are two major types of nerve cell: the neurons and the glia. Neurons do the job of thinking, computing, remembering, organising, sensing and activating. Glial cells nurture and protect the neurons. Neurons consist of a cell body with protrusions – short dendrites and longer axons (figure 1). They transmit information around our body in the form of electrical signals passed down their axons. The axons of motor neurons carry electrical information away from the brain via the spinal cord to the muscles and organs in the periphery. Sensory neurons transmit information in the opposite direction from the periphery via the spinal cord to the brain. Based on this information the brain then decides what action to take.

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Most people are familiar with the central nervous system (CNS) which consists of the brain and the spinal cord. Another, less well known but equally important, part of the nervous system is called the peripheral nervous system (PNS).


As its name suggests, the PNS carries information back and forth between the control centre (the brain) and the periphery. It communicates to the brain what is going on in the environment outside the body via the senses: hearing, touch, taste and smell (the eyes are unusual in that they are directly part of the brain and so are classed as CNS).


The PNS also communicates to the brain what is happening in other parts of the body. So, for instance, it controls and monitors heartbeat, blood pressure and release of hormones from glands such as the thyroid, pituitary and pancreas. It does so autonomously, in other words independently of our will. This is why this part of the PNS is called the autonomic nervous system. To illustrate just how much we depend on both nervous systems let’s go through how we start the day. Several hours before we awake our PNS is monitoring body functions and communicating to the brain (CNS) that we are soon to wake up (this is called our circadian clock activated by light and dark). The interactions between the two parts of the nervous system at this stage are like starting up a big ocean liner. There’s a lot to get ready before departure. Finally we are woken by the alarm


clock (hearing, PNS). We open our eyes and register our surroundings (sight, CNS). We register the scent of coffee and toast from the kitchen (smell, PNS). We get out of bed, our feet touch the floor and we stand up (touch, balance, PNS). We eat breakfast (taste, smell, digestion, PNS). And so on. All this while, we have used the peripheral nervous system to monitor heartbeat, blood pressure and hormone levels. And we have used our brain (central nervous system) to interpret these vital signals. Now you can appreciate how indispensable for our survival is the communication between the two parts of our nervous system.