There are 23 discs in the human spine: 6 in the neck (cervical) region, 12 in the middle back (thoracic) region, and 5 in the lower back (lumbar) region Discs are named by the vertebral body above and below. For example, the disc between the fifth and sixth cervical vertebrae is designated "C5-6". Development
During development and at birth, vertebral discs have some vascular supply to the cartilage endplates and the anulus fibrosus. These quickly deteriorate leaving almost no direct blood supply in healthy adults. Intervertebral disc space
The intervertebral disc space is typically defined on an X-ray photograph as the space between adjacent vertebrae. In healthy patients, this corresponds to the size of the intervertebral disc. The size of the space can be altered in pathological conditions such as discitis (infection of the intervertebral disc). Function
The intervertebral disc functions to separate the vertebrae from each other and provides the surface for the shock-absorbing gel of the nucleus pulposus. The nucleus pulposus of the disc functions to distribute hydraulic pressure in all directions within each intervertebral disc under compressive loads. The nucleus pulposus consists of large vacuolated notochord cells, small chondrocyte-like cells, collagen fibrils, and aggrecan, a proteoglycan that aggregates by binding to hyaluronan. Attached to each aggrecan molecule are glycosaminoglycan (GAG) chains of chondroitin sulfate and keratan sulfate. Increasing the amount of negatively charged aggrecan increases oncotic pressure, resulting in a shift of extracellular fluid from the outside to the inside of the nucleus pulposus. The amount of glycosaminoglycans (and hence water) decreases with age and degeneration.
Tachyphylaxis
Tachyphylaxis is a subcategory of drug tolerance referring to cases of sudden, short-term onset of tolerance following the administration of a drug. Pharmacodynamic tolerance
Pharmacodynamic tolerance begins when the cellular response to a substance is reduced with repeated use. A common cause of pharmacodynamic tolerance is high concentrations of a substance constantly binding with the receptor, desensitizing it through constant interaction. Other possibilities include a reduction in receptor density (usually associated with receptor agonists), or other mechanisms leading to changes in action potential firing rate. Pharmacodynamic tolerance to a receptor antagonist involves the reverse, i.e., increased receptor firing rate, an increase in receptor density, or other mechanisms. While most occurrences of pharmacodynamic tolerance occur after sustained exposure to a drug, instances of acute or instant tolerance (tachyphylaxis) can occur. Pharmacokinetic (metabolic) tolerance
Pharmacokinetics refers to the absorption, distribution, metabolism, and excretion of drugs (ADME). All psychoactive drugs are first absorbed into the bloodstream, carried in the blood to various parts of the body including the site of action (distribution), broken down in some fashion (metabolism), and ultimately removed from the body (excretion). All of these factors are very important determinants of crucial pharmacological properties of a drug, including its potency, side effects, and duration of action. Pharmacokinetic tolerance (dispositional tolerance) occurs because of a decreased quantity of the substance reaching the site it affects. This may be caused by an increase in induction of the enzymes required for degradation of the drug e.g. CYP450 enzymes. This is most commonly seen with substances such as ethanol.