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Biology 202
2000 First Web Report
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Myasthenia Gravis: Challenge to the Brain = Behavior Paradigm?

Camille Sinclair

The Central Nervous System (CNS) consists of the brain and spinal cord. The CNS activates a cascade of nonobservable events to produce observable behaviors. For instance, normal CNS functioning propagates muscle and body movements by sending an unobservable depolarizing signal along a motor neuron until it reaches a junction in the periphery known as the neuromuscular junction. At this junction the neurotransmitter, acetylcholine, is released from the presynaptic terminal buttons into the neuromuscular synaptic cleft/terminal where it binds to the postsynaptic receptors on the muscle to create movement (1). However, abnormal communication between the motor neurons, and muscle is propagated by the immune system not the brain- resulting in a condition known as Myasthenia Gravis (MG). The present paper will challenge the validity of the 'brain generating behavior' paradigm by examining the immune system and MG.

The Immune System in Brief

The immune system produces lymphocytes or white blood cells that defend the body against "foreign" bacterial and viral invaders, as well as provide long term immunity to disease. The blood, bone marrow, and thymus are lymphoid organs that produce stem cells (immature immune cells). Some stem cells are divided into two major classes of lymphocytes known as B and T cells (2), while others develop into large cells that engulf and destroy invading particles.

Chemical substances that are foreign to the body are called antigens. Antigens trigger B and T lymphocytes resulting in a series of immune responses. Unfortunately, the body can also attack its own substances mistaking them for invaders. The immune attack on oneself is called an autoimmune response. An immune response to antigens is the production of antibodies (3). Antibodies block antigens from cells by binding to and neutralizing these substances. Further, antibodies coat antigens making them vulnerable to attack by lymphocytes.

Myasthenia Gravis in Brief

Myasthenia Gravis (MG) is a type of autoimmune disorder. MG is the result of abnormal communication between the nerve and muscle at the neuromuscular junction whereby antibodies (immune system response) attack acetylcholine neurotransmitters.

Antibodies increase the distance between the pre and postsynaptic membranes making acetylcholine more vulnerable to attack (4). Antibodies also block or damage postsynaptic acetylcholine receptors making it impossible for acetylcholine- receptor binding (5). Hence, acetylcholine antibodies minimize communication between the CNS, motor neurons, and muscle creating behaviors that were not signaled from the brain.

For instance, the symptoms of MG include muscle fatigability, as acetylcholine is not present to stimulate the muscle for long periods of time. For example, ocular MG results in eye muscle fatigue and droopy eyelids (6). Other symptoms include fatigued throat muscles, whereby the individual has difficulty swallowing and/or slurred speech. And, in severe cases Myasthenic individuals have difficulty breathing as a result of weak lung muscles.

Does Brain Always Equal Behavior?

MG shows that there are instances when the brain cannot always generate behavior. Here, the immune system is independent of the brain and intercepts brain-to muscle communication producing "unintentional" behaviors-inability to look up due to droopy eyelids, inability to swallow, speak, inability to tap the fingers etc.

If it was in fact the case that brain controls behavior, shouldn't the brain be able to regulate the immune system, production of acetylcholine neurotransmitters, and production of acetylcholine antibodies?

It can be argued that if the immune system did not interrupt messages sent from the brain to the muscle then the brain's intentional behaviors would have been produced. However, the fact still remains that the immune system is independent enough from the brain that it can override the brain's stimuli to produce various responses/behaviors. How is it possible that our bodies are producing behaviors without consenting the brain first?

Myasthenia Gravis challenges the brain = behavior paradigm because it shows that other nonobservable activities in the body, production of antibodies, can result in behaviors that the brain has absolutely no control over. In fact the only way to treat this disorder is to override the immune system with immunosuppressants or surgically remove the thymus gland to reduce the amount of antibody production. Hence, returning power to the brain.

WORLD WIDE WEB SOURCES

1)Overview of Nerve Structure and Function; Measurement Techniques, Neuron during CNS communication.

2)Understanding the Immune System: Lymphocytes, T Cell (lymphocyte) configuration.

3)Understanding the Immune System: B Cells and Antibodies, B Cell (lymphocyte) and antibody immune response .

4)Cholinergic Aspects

5)Cholinergic Aspects, Normal ocular receptor activity results in normal responses (ability to look up). Antibody Mediated Mechanism: Blockade of Acetylcholine Neurotransmitters.

6)Disorders of the Neuromuscular Junction, Normal receptor activity produces normal muscular responses. Whereas, Myasthenic receptors produce abnormal symptoms ie. floppy eyelids.


Figure (1)


Figure (2)


Figure (3)


Figure (4)


Figure (5)

Figure (6)




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