The brain is split into two halves, the left and the right hemispheres. At the base of the brain is the cerebellum and leading from the brain down into the spinal cord is the brainstem. All the information that is detected by the nerve endings in the body is passed up the spinal cord and brainstem to one of the cerebral hemispheres. There the brain decides what it needs to do and sends the relevant instructions back down the same route to activate the muscles. For example, if you touch something very hot, your brain receives the message and tells your hand to move away.
The left hemisphere largely controls the right-hand side of the body, and the right hemisphere the left side. The brain has specific parts devoted to specific functions. For example, the language areas are usually in the left-hand side of the brain, except in a small proportion of left-handed people in whom the language area is on the right. The areas processing information about vision are at the back of the brain. Control of muscle and sensory function is situated near the front of the brain in the frontal and parietal lobes, and co-ordination is controlled by the cerebellum.
The blood to the brain travels from the heart through the aorta (the main artery from the heart) and then into one of four arteries that lead to the brain. At the front are the two carotid arteries that you can feel pulsating in your neck, either side of the windpipe, and at the back, running alongside the vertebral column (the spine), are the two vertebral arteries. Once the four arteries have entered the skull, they are linked together in the circle of Willis this is a very important anatomical feature, because, if one artery is blocked, sometimes there is enough blood provided from the other arteries joining the circle to prevent major damage being done. Leading off the circle of Willis are the six major cerebral arteries – anterior, middle and posterior (one of each on either side), supplying respectively the front, middle and back parts of the brain. Any of these can be blocked, causing a stroke, but the commonest to be affected is the middle cerebral artery.