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Biology 202
2004 First Web Paper
On Serendip

You couldn't catch it if I threw it at you: A neurobiological explanation of Cerebral Palsy

Erin Okazaki

I was 15 and volunteering at my brother's school for children with learning and developmental disabilities when I met Blake Matheson. Unlike most of the other kids, Blake had cerebral palsy (CP) and was confined to a wheelchair. At the time, I didn't know anything about CP and remember nothing but sweaty palms and racing thoughts as I approached him. After an awkward moment of introductory silence, he started asking me questions. Not only did we become friends, we introduced the other to aspects of our worlds they would have never otherwise known. Though my family's move the following summer ended my time at the school, my humbling experience with Blake remains a constant echo of people's tendency to make assumptions (however innocent) about others based on shallow observations of outward physical appearance and behavioral differences. And in the end, walking away with nothing.

The goal of the following discourse is to provide a useful way of thinking about cerebral palsy in the context of the nervous system. I hope that such an examination will enhance our understanding of behavior associated with CP and ultimately demystify common misconceptions. First, I will explore why box models of the nervous system are useful in explaining CP. Next, I plan to investigate how recognized differences in the nervous system provide useful ways of thinking about specific CP mechanisms and treatment. Finally, I will close with an evaluation of the nervous system's limitations in defining behavior. In effect, this discussion will demonstrate support that cerebral palsy is yet another condition consistent with observations characterizing the notion of brain = behavior.

CP occurs as a result of irreversible damage, before, during or after birth, to the networks of brain cells (neurons) and connecting "cables" (white matter) that control movement. In effect, it is not a disease that can be "caught," but a medical condition dealing with muscle control that affects posture and movement (1). CP is a generic term that covers four distinct cerebral palsies - spastic, athetoid, ataxic, and mixed. In addition, further classification of CP is characterized by body location: quadriplegia (all four limbs), hemiplegia (one side of the body) or diplegia (either in both legs or both arms) (3).

Thinking about behavioral outcomes in terms of boxes is especially helpful in the case of CP. Such analysis offers an explanation of the occurrence of different CP's all within the realm of motor disability. Behaviors associated with various types of CP's will change in relation to the severity and location of brain damage. Athetoid cerebral palsy, caused by damage to the basal ganglia, is characterized by the lack of coordinated smooth movements; ataxic cerebral palsy, evidenced when there is damage to the cerebellum, hinders depth perception and balance; spastic cerebral palsy, mainly caused by damage in the cerebellum, results in stiff difficult movement; children with mixed cerebral palsy may display a combination of two or more of the above types (2). This suggests that behavior is highly dependent on brain organization. Damage to the white matter of the brain does not result in a random expression of behaviors; instead damage to the white matter severs connections between specific internal interconnections linking "boxes" producing a very specified behavior expression consistent with damage only that compartmentalized region.

Further developing the function of "boxes" in explaining the CP is the notion that physical and behavioral differences of the nervous system do not necessary imply mental retardation or a learning disability. In the case of Blake, I noticed that with the help of communication devices, he was able to relay and communicate ideas which were often far more developed than those of his "normal" looking peers who, unlike Blake struggled with learning disabilities. The above discussion about specific "boxes" generating specific outcomes implies that intellectual outcomes are independent of motor outcomes. Thus, causality cannot be used to assume that an individual with behavior differences automatically has cognitive disabilities. Only one-forth to one-half of children with CP experience some type of learning problem such as a learning disability (1). It is important to note that individuals with learning disabilities are usually within the normal range of intelligence as opposed to those with severe learning problems such as mental retardation -- were average intelligence is below normal (4). It is also important to note that many "tracts" run between different "boxes." This suggests the existence of many pathways to achieve the same outcome. In effect, it may be possible for other interconnections (i.e. axon bundles) to take over in the event that damage occurs in one "tract" to still produce the same result. Only if the severity and location of the same motor-hindering brain injury also affects the internal interconnections between "boxes" of the brain specific to the facilitation of intellectual outcomes, and other non-affected white matter interconnections cannot compensate to recreate the output, might causality be determined (5).

The second area of examination investigates the extent to which we can use observations about the nervous system to explain why people with cerebral palsy behave differently. To conduct a thorough analysis, the focus will be placed on spastic CP, as it is prevalent in 80% of all CP cases (2). Spastic CP occurs as a result of abnormal motoneuron excitability (8). Under normal circumstances, muscles usually have enough tone to facilitate movement and maintain posture while adjusting for speed, gravity, and varying flexibility. This movement occurs as sensory nerve fibers communicate how much muscle tone the muscle has as it relays the information "to tense" to the spinal cord which then carries the message to the brain (7). The command to reduce muscle tone follows the opposite path of direction from nerves in the brain via the spinal cord. These two processes work in tandem to coordinate smooth muscle movement and strength. On the other hand, an individual with spastic cerebral palsy cannot control the muscle's amount of flexibility. In effect, the relay from the muscle floods the spinal cord and creates a muscle that is too tense (spastic) (6). The inability of the nervous system to facilitate coordination between the stretch receptors, sensory neurons and interneurons in the spinal cord creates stiff muscles, limits stretching, and hinders muscle range. Over time, spasticity becomes the major cause of physical deformities in limbs (1).

Knowledge of abnormal motoneuron excitability in the nervous system is used to create CP management techniques specific to various types of spasticity. The first technique, selective dorsal rhizotomy (SDR) is currently the only permanent procedure that reduces spasticity and is favored in young children with velocity-dependent spasticity (10). SDR involves cutting hyperactive sensory nerve fibers that originate from the muscle and enter the spinal cord rootlets so as to reduce message flow to the muscle (7). In effect, nerve cells in the spinal cord receive less information from the muscle sensory neuron resulting in a more even distribution of nerve cell traffic in the spinal cord. Another relatively new method is the intrathecal baclofen pump used for patients with diffuse spasticity. It works off the nervous system's failure to release gamma amino butyric acid (GABA), a chemical neurotransmitter that signals the relaxation of the lower back and leg muscles producing an inhibitory affect on the thalamus (4). When baclofen is injected into the spinal cord, it mimics the functions of GABA - blocking abnormal nerve signals and allowing for greater muscle control (7). In the end, both treatments address the muscle neuron's inability to send controlled messages along an interneuronal mechanism, resulting in improvements in standing, sitting, walking and balance control. Though both methods clearly use different mechanisms, both techniques have gained positive responses.

Despite the fact that neurobiological advancements have enhanced our current understanding of cerebral palsy, there are limitations to which the aspects of behavior can be explained by the nervous system. Currently, all treatment for cerebral palsy focuses on symptom maintenance. Little is known about the exact nervous system interactions that cause the death of white matter tissue or why CP primarily affects motor function (7). Prevention of cerebral palsy can only be addressed once researchers understand the process of normal brain development and what mechanisms go awry during development causing nervous system anomalies that are observed as behavior differences (9). Once understood, comparisons might then be made between the brain and nervous system functions in CP and non-CP development to investigate the exact mechanisms leading to brain damage, and possibility of prevention prescriptions. The key to understanding brain development lies within fetal development. We can apply our observations about the box model's usefulness in characterizing cerebral palsy behavior to ask questions about what happens during this time of rapid cell division. At what levels do brain cells specialize into different types? How do they know where to assemble in their respective parts of the brain? We can further the depth of developmental questions by asking about the process by which white matter develops and the nature of connective branches that form crucial connections with other brain and nervous system cells.

CP presents us with yet another example of how the "brain" generates sets of behaviors unique to its construction and organization. People with CP lack the ability to control their motor faculties due to neurodevelopmental impairments caused by damage to specific areas in the white matter of their brain. This behavior is consistent with the severity and location of the damage as generalized by the four major types of palsies. The CP brain accounts for the differences caused by brain damage and produces a slightly different set of behaviors depending on the extent of damage. Though the nervous system is useful in explaining CP behavior, it does not account for all aspects of behavior. This leaves us with suggestions about what we should look for in the nervous system - particularly in the area of developmental neurobiology and the implications such research might have on CP prevention. Cerebral palsy offers a unique look at the neurological triumphs of medicine while simultaneously presenting a humbling manifestation that we are all at the mercy of our own misunderstandings. Though scientific shortcomings are reconciled through trial and error, education is the only way by which clarification and personal understanding is achieved. Maybe this discussion about CP has in a small way continued where Blake and I left off 7 years ago.


1) Miller-Dwan's Regional Rehabilitation Medical Center , specifically devoted to providing information about spastic CP
2) University of Virginia Medical School , Children's Center, tutorial for cerebral palsy
3)About Cerebral Palsy , Information focused on specific types of cerebral palsy
4) American Association on Mental Retardation , provides distinguishing characteristics between mental retardation and learning disabilities
5) Cerebral Palsy Resource Center , information and links about treatment, diagnosis, care, ect.
6) University of Alabama , defines the mechanisms of spastic CP
7) St. Louis Children's Hospital , surgical treatment options for spastic CP
8)Kennedy Krieger Institute , general overview of CP and current research initiatives
9) National Institute of Health , general overview of CP
10) Ontario Federation for Cerebral Palsy , information about spastic CP

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