The term consciousness is one of those things that seems obvious until you actually try to define it. Many people use the term differently. Psychologists don't necessarily mean the same thing when they say "consciousness" that a Vedic yogi means. Partially because of the difficulty agreeing on what we mean by the word, consciousness is an elusive phenomena to understand.
Like so many other things in life, consciousness is perhaps best understood when it is lost. By studying various causes of diminished consciousness, neurologists can determine what structures and chemicals of the brain are important in maintaining alert and aware of your surroundings.
Neurologists sometimes find it helpful to divide consciousness into two different components. For example, the so-called content of consciousness consists of neurological networks that manage sensations, movement, memory and emotion. Arousal or level of consciousness, on the other hand, consists more of how alert you are (e.g. are you awake or not), your ability to pay attention to things (e.g. whether you are still reading this), and how aware you are of your surroundings (e.g. whether you know where and when you are).
The Reticular Activating System
The brainstem is only about as big around as your thumb, but it contains many essential structures for breathing, moving, and for staying awake and alert. Neurotransmitters, such as norepinephrine, are released by the brainstem to almost every other part of the brain, encouraging increased brain activity.
One of the most important regions for staying awake is the reticular activating system (RAS) in the brainstem. The reticular activating system receives signals from many other parts of the brain, including sensation pathways from the spinal cord (so you wake up if you're uncomfortable). The RAS also sends stimulating signals to many areas in the rest of the brain. For example, the RAS "talks with" the basal forebrain, which then relays stimulating signals throughout the cerebral cortex. The basal forebrain is active both during wakefulness and when you're dreaming.
The RAS also sends signals to the the hypothalamus, which controls functions such as heart rate and also helps keep you awake by releasing histamine. Most histamine is outside the brain, where it plays a role in allergic reactions and immune responses. Relatively recently, neurons containing histamine were also found in the brain, which help maintain alertness. This is why taking anti-histamines such as Benadryl can lead to drowsiness.
The RAS also communicates with the thalamus. In addition to playing a role in maintaining normal alertness, the thalamus serves as a relay station for the sensory information approaching the cerebral cortex, and commands to movement back from the brainstem to the body. If the thalamus does not relay sensory information to the cortex, such as a pinprick to the foot, the person is unaware that anything happened at all.
The Cerebral Cortex
The cerebral cortex covers the surface of the brain, and is where a great amount of information is processed. Different areas of the cerebral cortex control language, memory, and even your personality. While just damaging part of the cerebral cortex may not lead to a loss of consciousness, it can lead to a loss of awareness of part of your surroundings.
For example, many people who have a stroke or other lesion in their right parietal lobe lose awareness of the left side of their external surroundings, a symptom known as neglect. If asked to draw a clock or room, they will only draw the part on the right. In extreme cases, people with neglect don't even recognize their left hand, and if the hand is held up in front of them, they may claim that it belongs to someone else. This is a kind of loss of content of consciousness, although the person remains completely awake.
If many different regions of the cerebral cortex are injured, or if a person suffers injury to their thalamus or brainstem, the person may slip into a coma. A coma is an extreme loss of consciousness from which it is impossible to arouse someone. Many different things can cause a coma, and the likelihood of recovery varies.
The study of consciousness is very complex. Consciousness depends on the integration of several interconnected networks in the nervous system. A full understanding of consciousness is elusive at best. Furthermore, while we've talked about a few parts of the brain involved with someone's level of consciousness or alertness, it must also be recognized that consciousness is less of an "off-on" switch than a "dimmer" switch with many different shades. Healthy people pass through these shades of consciousness every day and night. Understanding the level of consciousness of those who cannot respond due to a neurological illness may be especially challenging.
Jerome B. Posner and Fred Plum. Plum and Posner's Diagnosis of Stupor and Coma. New York: Oxford University Press, 2007.
Hal Blumenfeld, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002