Endocrine and nervous systems are the major control systems in higher animals. The nervous system depends on sensors in the skin or sensory organs to receive stimuli and transmit a message to the spinal cord or brain. Sensory input is processed and a signal is sent to an effector system, such as muscles or glands, that effects the response to the stimulus. The endocrine system is the second type of extrinsic control, and involves a chemical component to the reflex. PTH is released into the blood when blood calcium levels are low.
Some reflexes have a combination of nervous and endocrine response.
TSH acts on the thyroid, causing it to increase production of thyroxin. Intrinsic Local, or intrinsic, controls usually involve only one organ or tissue. When muscles use more oxygen, and also produce more carbon dioxide, intrinsic controls cause dilation of the blood vessels allowing more blood into those active areas of the muscles. Positive feedback control is used in some cases. Input increases or accelerates the response.