Our most important sense organs
One of the more complex and fascinating examples of human biomechanical engineering resides in our ears. These intricate sensory organs capture and transmit sound waves and maintain our sense of equilibrium, spatial orientation, and balance. The exquisite anatomy and form of our most important sense organs (please excuse the author bias) correlates with the precise function of each part.
Unfortunately, many things can go wrong with such delicate and finely tuned apparatus. To develop a better understanding of the problems that can affect hearing and balance, some basic knowledge of the inner workings of the ear should be applied.
How we hear
The ear is made up of three distinct parts: The outer (external) ear, the middle ear, and the inner ear or labyrinth. (Figure 1)
Sound waves enter the auricle (the part you can tug on) and are funneled down the ear canal to vibrate the eardrum (tympanic membrane). From here, the three middle ear bones (ossicles) vibrate in succession and pass the sound waves to the inner fluids of the cochlea. Up to this point, all of the sound energy is vibratory and is in the form of mechanical energy. Any problem along this part of the pathway (for example, a perforated ear drum, fluid inside the middle ear, or fixation of the ear bones) will reduce transmission of those sound waves and result in what is termed a conductive hearing loss.
The cochlea consists of a hollow, seashell-like structure with approximately 2-3/4 turns arranged with low tones located at the point (apex) and high tones at the base. When the fluid inside the cochlea vibrates, a wonder takes place. The mechanical vibratory energy is converted into nerve impulses-electrical energy that then travel up to your brain where they are perceived and understood (hopefully) as sound.
The part of the cochlea that converts vibrations into electricity is the organ of Corti. (fig 2)
These cells are not made of hair; rather, when viewed under a microscope, they have a hairy appearance due to their many cilia. Any problem in this part of the pathway will result in a neurosensory hearing loss. For example, the hair cells can be damaged by excessive noise, toxic drugs, infections, and the effects of growing older. The acoustic nerve that connects the cochlea to the brain can also be damaged by a rare tumor (acoustic neuroma) or by reduced blood flow (mini-stroke). If the inner ear fluid (endolymph) becomes imbalanced, hearing loss, tinnitus, and dizziness can result in a syndrome called Meniere’s Disease (fig 3).
Any condition that results in a loss of hearing can cause tinnitus: ringing or any abnormal sound in the ears or head. This is usually described as high pitched ringing but it can take on other forms. The exact mechanism for tinnitus is still poorly understood. Remember, the inner ear is comprised of two parts — the cochlea for hearing and the vestibular portion for balance and equilibrium. When the vestibular apparatus malfunctions, our worlds can be turned upside down — literally! It turns out that the vestibule also contains hair cells. These hair cells have tiny cilia (hairs) that bend from side to side when the fluid that surrounds them moves. Every time we turn our heads or bend over, the fluid inside the vestibule and semi-circular canals (fig 4) moves opposite to the direction of head movement. The hair cells sense the fluid motion and convert the mechanical energy to electrical (nerve) impulses that transmit to the brain. This way, the brain can instruct the rest of the body (the eyes, neck muscles, trunk, and legs) to take corrective action to maintain balance and avoid falling.
Dizziness results when the signals from the vestibule to the brain are abnormal or not symmetrical. A common form of dizziness is termed benign paroxysmal positional vertigo (BPPV). This occurs when tiny salt crystals (fig 4) break loose and start floating around the inner ear whenever the patient lies on their back or looks up or down. These crystals disturb the fluid inside the inner ear and send abnormal signals to the brain. The result is dizziness or vertigo. Viruses can also attack the inner ear and alter the signals transmitted to the brain (labyrinthitis/vestibular neuritis) and cause dizziness. Lastly, what’s the difference between the terms vertigo and dizziness? Vertigo is a particular kind of dizziness that has a spinning component, usually caused by inner ear problems. Dizziness is a less specific term and can have many causes.
As with most things in nature, form and function are linked. The design of our hearing and balance system is an evolutionary wonder allowing us to capture the auditory world and navigate our space — capabilities that are vital to life as we have come to know it.