Anodea Judith, author of Eastern Body, Western Mind, says:
A chakra is a center of activity that receives, assimilates, and expresses life force energy. The word chakra literally translates as wheel or disk and refers to a spinning sphere of bioenergetic activity emanating from the major nerve ganglia branching forward from the spinal column. There are six of these wheels stacked in a column of energy that spans from the base of the spine to the middle of the forehead. And the seventh which is beyond the physical region. It is the six major chakras that correlate with basic states of consciousness...
Chakras are commonly described, as above, as energy centers in the spine located at major branchings of the human nervous system, beginning at the base of the spinal column and moving upward to the top of the skull. Chakras are considered to be a point or nexus of metaphysical and/or biophysical energy of the human body. Each chakra is also associated with a specific color, gem, and mantra/sound. Through asana practice, pranayama, meditation, chanting, massage techniques, and color therapy we are able to open the chakras and increase flow of energy up the sushumna (similar to the spinal column).
Sahasrara or the crown chakra is generally considered to be the chakra of consciousness. Its role may be envisioned somewhat similarly to that of the pituitary gland, which secretes hormones to communicate to the rest of the endocrine system and also connects to the central nervous system via the hypothalamus. The thalamus is thought to have a key role in the physical basis of consciousness. Symbolised by a lotus with one thousand petals, it is located on the crown of the head. Color Violet
Ajna (along with Bindu, either or both are considered to correspond to the third eye) is linked to the pineal gland which may inform a model of its envisioning. Ajna is held as the chakra of time, awareness and of light. The pineal gland is a light sensitive gland that produces the hormone melatonin which regulates sleep and awakening. Symbolised by a lotus with two petals. Color Indigo
Vishuddha (also Vishuddhi) or the throat chakra may be envisioned as relating to communication and growth, growth being a form of expression. This chakra is paralleled to the thyroid, a gland that is also in the throat and which produces thyroid hormone, responsible for growth and maturation. Symbolised by a lotus with sixteen petals. Color Blue
Anahata or the heart chakra is related to complex emotion, compassion, love, equilibrium and well-being. It is related to the thymus, located in the chest. The thymus is an element of the immune system as well as being part of the endocrine system. It produces T cells responsible for fending off disease and may be adversely affected by stress. Symbolised by a lotus with twelve petals. Color Green
Manipura or the solar plexus chakra is related to the transition from simple or base to complex emotion, energy, assimilation and digestion, and is held to correspond to the roles played by the pancreas and the outer adrenal glands, the adrenal cortex. These play a valuable role in digestion, the conversion of food matter into energy for the body. Symbolised by a lotus with ten petals. Color Yellow
Swadhisthana or the sacral chakra is located in the sacrum (hence the name) and is related to base emotion, sexuality and creativity. This chakra is considered to correspond to the testicles or the ovaries that produce the various sex hormones involved in the reproductive cycle which may cause dramatic mood swings. Symbolized by a lotus with six petals. Color Orange
Muladhara or the base or root chakra is related to instinct, security, survival and also to basic human potentiality. This centre is located in the region between the genitals and the anus. Although no endocrine organ is placed here, it is said to relate to the inner adrenal glands, the adrenal medulla, responsible for the fight and flight response when survival is under threat. In this region is located a muscle that controls ejaculation in the sexual act in the human male. A parallel is charted between the sperm cell and the ovum where the genetic code lies coiled and the kundalini. Symbolised by a lotus with four petals. Color Red