The brain is the most complex organ in the human body, and one of its most mysterious. Its internal structures are hidden from view, and the best evidence most of us have that it is functioning properly is the collection of abilities we so often take for granted. We wake, we walk and run, our hearts beat, we taste, we breathe, and we think, all as a result of the highly coordinated electrical and chemical activity within and between the cells that make up our brains.
Occasionally, however, these cells function abnormally—they misfire. In some people, brain cells misfire repeatedly and in unison, causing changes in behavior, sensation, or motor function. These occurrences are called seizures, and a person who has experienced two or more seizures is said to have epilepsy. Unfortunately, for 65 to 70 percent of people with epilepsy, the underlying cause or causes of seizures remain a mystery. This is complicated by the fact that seizure types vary so widely.
In general, the physical manifestations of a seizure depend on where in the brain the seizure occurs and how much of the brain it involves. Therefore, developing an understanding of the basic structure of the brain and how it functions normally can be an important first step in demystifying epilepsy.
The brain is made up of three main structures: the cerebrum or cerebral hemispheres, the brain stem, and the cerebellum. The cerebrum is the largest and most recognizable of the three structures and is the one most often involved in epilepsy. The cerebral cortex is the highly folded, neuron-rich outer layer of the cerebrum that is referred to as gray matter. Underlying the cerebral cortex is a layer composed of white matter, which is rich in nerve fibers that are important in transmitting signals from neurons in the cortex to cells in other parts of the body. The cerebrum is divided into right and left halves, or hemispheres, which are connected near the center of the brain by a bundle of fibers called the corpus callosum. Axons, the fibers that make up the corpus callosum, have a fatty, protein-rich covering, called the myelin sheath, that aids in the transmission of electrical signals from one neuron to another and from one side of the brain to the other.
Each hemisphere can be further divided into four functionally distinct lobes: frontal, parietal, occipital, and temporal. The frontal lobe is most often associated with personality, motor function, and a type of cognitive function called executive functioning; the parietal lobe is involved in sensory interpretations and in creating associations among experiences; the occipital lobe processes visual information; and the temporal lobe is involved in memory, speech, and auditory and olfactory functions. Some complex functions are associated with more than one lobe. Interestingly, each hemisphere of the brain processes information from and controls movement on the opposite side of the body. This means, for example, that neurons in the left frontal lobe send signals to and receive signals from the right side of the body.
Two of the most clearly defined areas of the brain in terms of their function are the motor and sensory strips. These areas form the border between the frontal and parietal lobes of each hemisphere. Scientists have identified particular sections along these strips that are responsible for either movement or sensory perception in parts of the body, such as the face, hands, arms, and legs. This knowledge has been helpful in identifying the origin of certain types of seizures.
Where Seizures Occur
Seizures can occur anywhere in the brain, but in children they frequently occur in the temporal and frontal lobes, affecting the functions that these regions control. A region of particular importance in adults with epilepsy, but less so in children, is the mesial, or middle, part of the temporal lobe. This region is comprised of such structures as the hippocampus and amygdala, which control emotion, and the uncus, which is responsible for processing smells. As a result, seizures that arise in these areas commonly produce strong emotions, such as fear, or the sensation of an acrid odor, such as burning rubber.
Seizures that begin in the temporal lobe often are not restricted to that lobe. They can propagate through the vast networks of neurons that connect the temporal and frontal lobes, producing more widespread seizure activity. Sometimes they spread widely enough to become generalized seizures, which involve the entire brain and alter consciousness.
To learn more about seizure types and where in the brain they arise, see the Seizure Types page.
The human brain is made up of about 100 billion neurons, and many more support cells, called glial cells. Neurons are found in all sections of the brain and throughout the nervous system in the rest of the body. The neurons function by sending electrical and chemical, or electrochemical, signals to other neurons or directly to tissues such as muscle fibers through highly organized networks. Neurons that have similar functions are generally aggregated into the networks that form the brain's internal structure such as lobes and functionally distinct regions of the brain such as the motor and sensory strips that control specific actions, and senses. Complex functions may be controlled by multiple regions of the brain.
Neurons use electrical impulses to quickly transmit signals that control the release of chemicals, called neurotransmitters. When an electrical impulse arrives, it stimulates the release of a neurotransmitter that is then passed across the synapse to the dendrites of other neurons or to the cells of a target tissue. This electrochemical communication between one neuron and another or between a neuron and a cell of a target tissue can occur over great distances in a very short period of time. For example, in just a fraction of a second, an impulse that originates in the motor strip of the brain may travel down the neurons' communication lines, the axons, to the region of the spinal cord responsible for limb movement, and on to the muscles of the designated limb.
Excitation and Inhibition
Neurotransmitters are produced in the cell bodies of neurons and are a means of transmitting and regulating the signals in the nervous system. One way they do this is by having an excitatory or an inhibitory effect on target cells, that is, by promoting or damping activity. When neurotransmitters excite a cell, the likelihood that the cell will be induced to send its own electrochemical signals increases. When they inhibit a cell, the likelihood that the cell will send these signals decreases.
Epileptic seizures result when a sufficient number of neurons fire signals abnormally, leading to an alteration of sensation, behavior, or consciousness. This requires that abnormal cells be able to recruit other normal cells to fire at the same time. Neurologists think that this recruitment is made possible when there is an excess of excitation and/or a lack of inhibition. This activity may happen in any part of the brain, or throughout the entire brain.
To learn more about the importance of threshold in determining seizure risk, see the Causes section.
Neurotransmitters are the focus of intensive research and offer hope for people with epilepsy. Many of the drugs being developed to treat epilepsy act by either increasing activity in the inhibitory systems or decreasing activity in the excitatory systems. There is ongoing research into how neurotransmitters are made, how packets of neurotransmitters release their contents, and how receptors on target cells alter the permeability of cell membranes.
Epilepsy and Development
Types and causes of seizures in children vary with age, and differ dramatically from those of adults. These differences are likely due to structural changes that occur during development. Genetic disorders such as tuberous sclerosis complex, Sturge-Weber syndrome, neurofibromatosis, and Angelman syndrome can also cause brain abnormalities that increase the likelihood of abnormal neuronal activity.
Childhood brain development is also important to consider, given that epileptic seizures can so profoundly impact cognitive function and learning. Seizures alter brain functions by overactivating, interrupting, or destroying vital networks of brain activity. Often the developing brain can compensate for this impact by creating new functional neuronal networks. This is what is referred to as the brain's plasticity. Unfortunately, the potential for plasticity diminishes over time. This explains why chronic frequent seizures can compromise cognitive function and makes clear the need for early identification and control of seizures.
It is clear that epilepsy has profound effects on the developing child's brain. About half of children with epilepsy experience learning difficulties, especially those involving problems with attention and memory. Additionally, one in four children with epilepsy has some form of mental health problem, which may result from anxiety about how they are perceived by friends and family or may be a more direct result of the disorder.
As with other manifestations of epilepsy, the location and type of seizures dramatically affect the role those seizures will play in development. In general, partial seizures, which affect relatively isolated regions of the brain, may cause learning disabilities and language problems, but seldom cause severe mental retardation. In contrast, generalized seizures, which affect large regions of the brain, may lead to a deterioration of inherent intelligence potential.
Lastly, the hemisphere in which seizures occur may affect development. For example, seizures that occur in an individual's dominant hemisphere (left for right- handed people, right for left-handed people) are much more likely to result in problems with language processing, while seizures in the nondominant hemisphere are more likely to affect nonverbal functions such as perceptual and motor skills.