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Neurons are cells that do most of the information processing, memory, and communication functions of the nervous system. Neurons are organized into circuits, also called neural pathways. These pathways are combined into structures that make up the nervous system. All the sensations, movements, thoughts, memories, and feelings come from signals passed along neural pathways. Glial cells surround the neurons and provide nutrients and structural support for them. The meninges are membranes that cover the brain and spinal cord. The cerebral spinal fluid surrounds the brain and spinal cord.

Neurons have 3 basic parts: a cell body and 2 extensions called an axon and a dendrite. Within the cell body is a nucleus, which controls the cell's activities and contains the cell's genetic material. The axon looks like a long tail and transmits messages from the cell. Dendrites look like the branches of a tree and receive messages for the cell.

The sensory neurons carry information from the sense organs (such as the eyes and ears) to the brain. Motor neurons have long axons and carry information from the central nervous system to the muscles and glands of the body. A third type of neurons is interneurons. They only communicate with other neurons.

Neurons communicate with one another and with the organs they control by passing a minute electrical current called an action potential. The action potential is caused by chemical stimulation of the neuron. Here's how it works. A small current flows along the cell membrane of a neuron to the end of the cell (the axon). The axon comes close to, but does not touch, the next neuron in the pathway. The tiny gap between neurons is called the neural synapse. When the action potential reaches the end of the axon, the axon releases a chemical (called a neurotransmitter) into the synapse space. If the neighboring cell is a neuron, it receives the neurotransmitter through receptors called dendrites. The receiving neuron's properties are changed by the neurotransmitter and a response occurs. The response can be to generate an action potential, which continues through the receiving cell's axons to the next neural synapse. The response could also be to stop an action potential.

Acetylcholine is an excitatory neurotransmitter, meaning it makes cells more excitable. It is found in the nerves in the body, where it stimulates muscles and glands. It is also found in the brain, where it helps regulate dopamine (another neurotransmitter) in the brain. Alzheimer disease is linked to a shortage of acetylcholine. GABA is short for gamma-aminobutyric acid. It is an inhibitory transmitter, meaning it makes cells less excitable, and it helps the brain maintain muscle control. Serotonin is an inhibitory transmitter that helps the brain regulate acetylcholine.

In myasthenia gravis, the immune system creates an antibody that attacks acetylcholine receptors of the muscle cells. When acetylcholine binds to the acetylcholine receptors of the muscle cells, it causes them to contract. When an antibody blocks this binding, the cell fails to contract or contracts only weakly. This can cause drooping eyelids, weak eye muscles, and fatigue of affected muscles after exercise.

Parkinson is the result of the loss of dopamine-producing brain cells. Dopamine is a chemical messenger responsible for transmitting signals within the brain. Parkinson disease occurs when certain nerve cells, or neurons, die or become impaired. Normally, these neurons produce dopamine. Loss of dopamine causes the nerve cells to fire out of control, leaving patients unable to direct or control their movement in a normal manner.

An estimated 3 in 5 to 7 in 10 people with diabetes have some form of neuropathy. But not everyone with neuropathy has symptoms. The highest rates of neuropathy are among people who have had the disease for at least 25 years. Symptoms may affect the sensory or motor nervous system, as well as the involuntary (autonomic) nervous system. Peripheral neuropathy causes either pain or loss of feeling in the toes, feet, legs, hands, or arms. Autonomic neuropathy causes changes in digestion, bowel and bladder function, sexual response, and perspiration. It can also affect the nerves that serve the heart and control blood pressure. Autonomic neuropathy can also cause hypoglycemia (low blood sugar) unawareness. With this condition, people no longer experience the warning signs of hypoglycemia.

Cerebral palsy is a term used to describe a group of long-term (chronic) disorders that affect muscle control and movement. They appear in the first few years of life and generally don't get worse over time. The disorders are caused by faulty development of or damage to areas in the brain that disrupts the brain's ability to control movement and posture. Symptoms of cerebral palsy include trouble with fine motor tasks such as writing or using scissors, trouble maintaining balance or walking, and involuntary movements. The symptoms differ from person to person.

Huntington disease (HD) is a genetically programmed degeneration of brain neurons in certain areas of the brain. This degeneration causes uncontrolled movements, loss of intellectual abilities and emotional disturbances. HD is passed from parent to child through a mutation in the normal gene. Each child of an HD parent has a 1 in 2 chance of inheriting the HD gene.