Neurodegenerative Disease

Introduction:

The nervous system is a maze of nerve cells, electrical impulses and molecules. It is divided in two major areas: The central and the peripheral nervous systems. The central nervous system is composed of the brain and the spinal chord. The peripheral nervous system connects the brain and spinal chord to the rest of the body.

Cells of the nervous system, called nerve cells or neurons, are specialized to carry "messages" through an electrochemical process. The human brain has about 100 billion neurons. Neurons are the oldest and longest cells in the body! You have many of the same neurons for your whole life. Although other cells die and are replaced, many neurons are never replaced when they die. In fact, you have fewer neurons when you are old compared to when you are young.

Neurons come in many different shapes and sizes. Some of the smallest neurons have cell bodies that are only 4 microns wide. Some of the biggest neurons have cell bodies that are 100 microns wide. (Remember that 1 micron is equal to one thousandth of a millimeter!!).

Each neuron that makes up the communication line, itself, resembles broccoli. Incoming messages are received in the dendrites, which mirror the branching buds of the veggie. The chemical messages pass down these sprouts into the cell body and then through one main long stalk or axon of the neuron. Finally the signal heads out to connected neurons.

Neurons vary in the amount of information they receive and transmit. Some have an extremely large social circle and receive and transmit thousands of messages. Others have only a few connections.

All messages, however, are passed to connected neurons in the form of chemicals called neurotransmitters. They flow from a message-sending neuron across a gap called a synapse and onto target neurons. The chemicals attach to a slot on the surface of the receiving neuron -- a protein called a receptor site. Many scientists compare the union to a key fitting in a lock. Once attached, different neurotransmitters either trigger "go" signals that allow the message to be passed to the next neuron in the communication line or produce "stop" signals that prevent the message from being forwarded. The signals are in the form of charged particles or ions. A large concentration of positively-charged particles entering a receiving neuron tells it to pass on the message. On the other hand, a large concentration of negatively-charged particles entering the neuron will inhibit it from passing on the message.

image:http://faculty.washington.edu/chudler/gif/spiback1.gif

The nervous system is divided into two major areas. These are the central and peripheral nervous systems. The former is composed of the brain and spinal chord. The latter is what connects the central nervous system to muscles and organs. Disruption of nerve impulses in the central and peripheral nervous systems has many causes. Apart from the physical severment of nerves due to physical injury, diseases of nerve cells are responsible for many a debilitating condition of humans. In the next section will concentrate on these diseases, in particular on Alzheimer's disease.