How do nerves control every organ and function in the body?
Category: Biology Published: September 20, 2013
Nerves do not control every tissue and function in the human body, although they do play a large role. There are three main ways that bodily organs and functions are controlled:
- Through the central nervous system
- Through the endocrine system
- Through local self-regulation (which includes intracrine, autocrine, paracrine, and immune regulation)
Nerves carry orders from the brain and spinal cord in the form of electrical signals. Nerves also help sense the state of tissues and relay this information back to the brain and spinal cord, enabling us to experience pain, pleasure, temperature, vision, hearing, and other senses. The body uses electrical signals sent along nerves to control many functions because electrical signals can travel very quickly. At the end of each nerve's axon terminals the electrical signals are converted to chemical signals which then trigger the appropriate response in the target tissue. However, the control exerted by the nervous system inevitably resides in the brain and spinal cord, an not in the nerves, which just pass along the signals. Most signals get processed in the brain, but high-risk signals are processed and responded to by the spinal cord before reaching the brain in the effect we call "reflexes". Although the central nervous system plays a large role in controlling the body, it is not the only system that exerts control.
The endocrine system is a series of endocrine glands throughout the body that excrete certain chemical signals called hormones into the blood stream. The circulating blood then takes the hormones throughout the entire body where different tissues respond in characteristic ways to the hormones. The response of an organ or system to a hormone depends on how much of that hormone is present in the blood. In this way, endocrine glands can exert control over different organs and functions of the body by varying how much hormone they emit. In contrast to the central nervous system, the pathway of control for the endocrine system is purely chemical and not electrical. For example, the thyroid gland in the neck controls how quickly the body uses energy by secreting varying levels of thyroid hormone. Too much thyroid hormone, and you become restless, jittery, and unable to sleep. Too little thyroid hormone and you become sleepy, lethargic, and enable to think straight. A healthy body constantly monitors the activity level and adjusts the thyroid hormone levels as needed.
Other examples of endocrine glands are the adrenal glands, which prepare the body for facing an emergency, and the reproductive glands, which control body mass and reproduction. Hormones in the body control functions as diverse as libido, fertility, menstruation, ovulation, pregnancy, child birth, lactation, sleep, blood volume, blood pressure, blood sugar, the immune system, vertical growth in children, muscle mass, wound healing, mineral levels, appetite, and digestion. Ultimately, much of the endocrine system is subservient to the brain via the hypothalamus, but the endocrine system does operate somewhat independently using feedback loops.
Lastly, organs and functions in the body are controlled through local self-regulation. Rather than depend on the brain to dictate every single minute task, organs and cells can accomplish a lot on their own so that the brain is freed up for more important tasks. An organ can communicate regulatory signals through its interior using localized chemical signals such as paracrine hormone signalling. Typically, such hormones do not enter the blood stream, but are transported locally by simply flowing in the space between cells. This approach works because paracrine hormones are only meant to operate on nearby cells. For example, the clotting of blood and healing of wounds are controlled locally through an exchange of paracrine hormones. The organ with the highest degree of self-regulation is probably the liver. The liver hums along nicely, performing hundreds of functions at once without much direction from the rest of the body. An organ can also communicate through its interior electrochemically. For instance, the heart does not beat because a nerve is telling it to. The heart beats on its own through a cyclic wave of electrical impulses. While it is true that the brain can tell the heart to speed up or slow down, the actual beating of the heart is controlled locally.
Also, each cell of the body has some degree of self-regulation internal to the cell itself. Some cells exert more internal control than others. For instance, white blood cells hunt down and destroy germs in a very independent fashion, as if they were autonomous organisms. Active white blood cells do not wait for the brain or a hormone to tell them to do their job. Sperm cells are so autonomous that they can continue to survive and function properly even after completely leaving the male's body.
In reality, the central nervous system, the endocrine system, and the local regulation systems are not independent, but exert control over each other in a complicated manner.