Dog Parvo Symptoms

Autoimmune diseases occur when the immune system cannot discriminate between self and nonself. Anything that is foreign to the body is nonself. When an organism’s immune system is threatened by foreign particles such as cancer cells, viruses such as the flu virus, microorganism, and even tiny grass particles, an immune response is elicited. Anything that causes this immune response to occur is an antigen. When a immune response is triggered, the body produces antibodies to attack the antigens. Titer is the relative measure of the strength of an antiserum (serum with antibodies). If an antigen causes cancer, it is referred to as a pathogen. In a well functioning organism the immune system does not fight against its own components or self. The lack of immune response against self is termed tolerance. When the immune system loses its tolerance, it will attack its own components. When tolerance is lost, it is referred to as autoimmunity. Diseases that cause the body to act in this fashion are therefore called autoimmune diseases.

The Nerve Impulse (Action Potential)

Organisms respond to external stimuli. If the stimulus is stong enough, a nerve impulse occurs. The nerve impulse is an all-or-none event which depolarizes the membrane of the synaptic terminal of a nerve cell. The depolarization causes an increase in the calcium ion concentration. The calcium rushes into the nerve cell and causes synaptic vesicles, which contain the neurotransmitter acetylcholine, to fuse with the presynaptic membrane of the nerve and release the acetylcholine into the synaptic cleft. The synaptic cleft is the space that separates the presynaptic cell (nerve cell) from the postsynaptic cell (muscle cell) and is the location where the two cells communicate. When the synaptic cleft separates a nerve cell from a muscle cell it is referred to a neuromuscular junction. The acetylcholine then diffuses across the synaptic cleft to the acetylcholine receptors on the postsynaptic membrane of the muscle cell. The binding of acetylcholine to the acetylcholine receptors on the postsynaptic cell (muscle cell) triggers ion channels on the postsynaptic cell to open. The opening of these ion channels on the postsynaptic membrane of the muscle cell then stimulates the muscle cell to contract.