What are the mechanics of balance in our bodies?

Q: What are the body parts, or systems, that help us balance?

A: You're asking a very interesting question. I never even thought about it until I went to medical school. When I learned what I'm about to tell you, I thought it was interesting. However, I didn't appreciate how important problems with balance would be for my patients.

A lot of people begin to notice subtle problems with balance when they enter their 60s. Over the next two decades, their balance can become problematic enough that it affects their health. Poor balance can cause falls that can lead to disability, and sometimes death. Good balance, by contrast, builds confidence and fosters independence.

Our daily balancing acts require intricate coordination between body systems. These include the central nervous system (brain and spinal cord), the vestibular system (brain and inner ear), the visual system (brain and eye), and a vast web of position-sensing nerves called proprioceptors.

• A part of the brain called the cerebellum oversees balance and movement. It receives sensory information, such as sight, sound and touch, from our nerves. Another part of the brain oversees other aspects of balance, such as attention, planning and movement. It also supplies memories important to balance. For example, when you face a balance challenge like a slippery sidewalk or rocky path, you have a memory of how you moved your body in the past to deal with such situations.

• The spinal cord serves as a bridge between brain and body. Nerves along its length receive feedback from the peripheral nervous system, a lacework of nerve fibers that branch out from the central nervous system to the far reaches of the body. The spinal cord also initiates reflexes, such as the quick-stepping response to an unexpected push. It delivers commands to the muscles too, telling them to make voluntary movements.

• The body's balance mechanism is called the vestibular system. It is housed in the inner ear structure and is made up of three semicircular tubes. These tubes consist of bone-encased membranes filled with fluid and lined with hair cells. As you move, fluid in the canals shifts and bends the hair cells. This prods them to fire messages to the brain telling it how much you moved and in which direction.

• In the visual system, the eyes send visual information to the brain, continually logging where you are in relation to surrounding objects.

• The position-sensing nerves called proprioceptors are responsible for proprioception - the ability to perceive where your body is in space. Proprioception helps you stay balanced and move through your environment without stumbling or bumping into things. Found primarily in muscles, tendons and joints, proprioceptors stream information to the brain. The brain instructs muscles to contract in large or small ways as conditions change (when you're on uneven ground, for example).

I've put descriptions and photos of two exercises to help improve your balance on my website, AskDoctorK.com. As I have grown older, I've come to appreciate how important balance is, and how helpful the exercises are. I think you will, too.

• Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.