The term Central Nervous System (CNS) refers, strictly speaking, to the brain and the spinal cord, though it is commonly extended to include the coverings of those organs (the meninges) and the bony structures which surround them (the skull and spinal column).
The CNS is the very core of our existence. The brain controls our thoughts, emotions, understanding, memory and speech. It interprets and explores our external environment and regulates the functioning of the body as an organism. The spinal cord is the pathway through which information is passed from the brain to the body and back.
Like all living organisms, the CNS is composed of billions of cells of various different kinds. The next section deals with the most important of these cells.
There are two major types of cell in the central nervous system - neurons (nerve cells) and glial cells.
Neurons (of which each of us has about 100 billion) are the cells that do all the clever stuff. By passing electrical signals to each other, they are responsible for every movement we make, every decision we take, every word we speak or emotion we feel.
Glial cells are the backroom boys of the CNS, supporting the neurons. There are several types of glial cells, each with its own specialised function:
- Astrocytes - pass nutrients from the blood vessels to the neurons which consume large quantities of glucose to produce the energy they need. They then carry away the by-products of this digestion back to the blood vessels.
- Oligodendrocytes - form the substance called myelin which insulates the axon of a neuron. Myelin is essential for the rapid transmission of electrical impulses.
- Ependymal - cells form the lining of the ventricles of the brain where water (ventricular fluid) is stored.
Before we look at the brain itself, we need to consider where it lives. The brain is such an important and delicate organ that it needs special protection from harm. This is provided by the coverings of the brain.
The Skull Base
The skull base is a complex and irregular surface of bone on which the brain rests. Contained within the skull base are the eye orbits, ear canals, two carotid arteries, two vertebral arteries, 12 cranial nerves and the venous system (the veins) of the brain.
The skull base is made up of three basic anatomical regions, the anterior (front) fossa, the middle fossa and the posterior (back) fossa. The anterior compartment is the region above a person's eyes. The middle compartment is the region behind the eyes and is centred around the pituitary gland. The posterior compartment houses the brainstem and cerebellum.
The brainstem connects the brain and spinal cord, and contains the origin of nerves involved in the control of breathing, blood pressure, eye movements, swallowing, etc. This connection occurs through the large hole, known as the foramen magnum. The cerebellum, which lies above the foramen magnum, is involved with coordination and balance. The roof of the skull base is composed of the brain itself and an area of dura, called the tentorium, on which the brain.
The cerebrum is the largest area of the brain. The surface of the cerebrum is covered in a greyish layer called the cortex (from the Latin word for "bark"). It is split into two halves (hemispheres) by a deep fissure, but the two sides are joined at the bottom by the corpus callosum which connects the two halves and delivers messages from one half to the other. For reasons which, even in the 21st century, remain unclear, nearly all the signals from the brain to the body and vice versa cross over on their way to and from the brain. This effectively means that the right cerebral hemisphere controls the left side of the body and the left hemisphere controls the right. Where damage occurs to one side of the brain, it is the opposite side of the body where the effect is seen.
Each cerebral hemisphere is responsible for different thought processes. The left hemisphere controls the more analytical processes (particularly language), whereas the right is involved in the more creative areas such as art, mathematics and music.
Each hemisphere is further divided into four lobes by fissures (also known as grooves or sulci) and gyri (the "bumps" which can be seen on the surface of the brain).
The cerebellum (literally "little brain") lies at the base of the skull, separated from the cerebrum by a fold of dura known as the tentorium. The cerebellum coordinates the brain's control of skilled, delicate and repetitive movements such as those involved in playing the piano or sending a text message. It also maintains our balance and posture by regulating muscle tone and sensing the position of our limbs.
The Inner Brain
Deep inside the brain, hidden underneath the cerebral hemispheres are a number of small but vitally important structures.
The hypothalamus controls many important functions. It helps control behaviours such as eating, sex and sleeping as well as regulating body temperature and emotions. Extending from the hypothalamus, and partly controlled by it, is the pituitary gland which secretes several important hormones including prolactin, corticotropin and growth hormone.
The thalamus acts as a relay station between the spinal cord and the cerebrum. A tract of nerves leads from the hypothalamus and thalamus to the hippocampus, which functions as a kind of memory index, sending memories out to the appropriate part of the cerebral hemispheres for long-term storage and retrieving them when required.
The Brain Stem
The brain stem, consisting of the mid brain, the pons and the medulla oblongata, connects the brain and the spinal cord and acts as a pathway for information being transmitted between the brain and the body. It is also the control centre for the most basic of bodily functions such as blood pressure, heart rate and breathing. All but two of the cranial nerves arise in the brain stem. The anatomical area where the pons meets the cerebellum is known as the cerebellopontine angle.
There are twelve pairs of cranial nerves, traditionally referred to both by name and by Roman numeral. Some are involved solely with sensation, some solely with movement and some with both.
Blood Supply to the Brain
Nutrients and oxygen are carried to the brain by many blood vessels. These are found both on the surface of the brain and deep within it. As well as bringing in chemicals which the brain needs to function properly, the blood removes materials from the brain, either because they are unwanted or because they are required elsewhere in the body.
The brain gets its blood supply from two pairs of arteries, the internal carotid arteries and the vertebral arteries. At the base of the brain, the vertebral arteries (right and left) join together to form the basilar artery. The basilar artery joins up with the internal carotid arteries to form a ring known as the Circle of Willis (named after Thomas Willis, the great 17th century neuroanatomist), from which the cerebral arteries (anterior, middle and posterior) branch off into the brain. The Circle of Willis acts as a protective mechanism - even if one of the arteries gets blocked, blood will continue to flow around the circle and maintain the supply.
Although the brain accounts for only about 2% of our body weight, it receives 15-20% of the body's blood supply. Because the brain is so vital to our survival, and brain cells die so quickly if deprived of oxygen, the body automatically gives the brain top priority when it comes to blood. Even when other organs are crying out for blood, the body will always try to maintain the brain's supply.