Bovine biology series
Part - 27 Ears
The ears
I do not have an anecdotal story about a cows’ ears. Other that I wish they had not been present when we used to clip cows for the fair. Yet they have functions vital to the cows’ well being.
The ear in all domestic animals is remarkable the same. The primary function of ears is receiving sound, or acoustical information in the environment. The sound waves in the air space around the ear enter the outer ear. That is the visible part of the ear that contains hair follicles. Here, the auricle, this dermal tissue serves as the antenna. The auricle in a cow is much larger than in our ear. Next in line is the auditory canal, providing the entryway into the middle ear.
The middle ear is comprised of the typanic membrane, or better known as the eardrum. The sound vibrations, depending upon pitch, duration and loudness, vibrate three very small bones called the malleus, incus and stapes. The vibrations cause these bones to physically move.
Also in the middle ear is the auditory tube, also called the Eustachian tube. This tube is connected to the pharynx, that part of the throat involved with swallowing. This auditory tube serves the role of adjusting the middle ear for changes in atmospheric pressure (we experience these changes during an airplane ride.) Swallowing several times physically adjusts the bones in the middle ear to realign themselves, therefore accounting for different pressures in the environment. The objective is, of course, to make the sound waves clearer.
The typanic membrane is comprised of three layers. The first one is dermal tissue that is a continuation of the outer ear. The middle layer is connective fibrous tissue. The third layer is the membrane layer known as the manubrium, and it is this layer of tissue that supports the first of the three bones, the malleus.
One remarkable function of the middle ear is to actually magnify the sound vibrations that are entered into the outer ear and travel down the auditory canal. One way to explain this is to consider the fact that when sound waves hit water, the extremely dense medium of water negates and greatly reduces the clarity of the sound wave. That is why sound under water is not clearly distinguished.
Well, when sound arrives at the eardrum, the three bones act in concert to magnify the sound waves, for the middle ear is largely a water medium. Without the typanic membrane and these bones, the sound entering the ear would be much more difficult to interpret, again, due to the impedance of the dense water media. So these bones concentrate and magnify the sound.
The typanic membrane, or eardrum, is delicate in that extreme sound waves, measured in intensity by decibels, can actually break the eardrum. That is because the eardrum serves as a buffer by absorbing some of the shock of loud sound. Too loud, however, and the eardrum breaks.
For some of us, hearing is not as good as it once was. I find that in my own case, I have spent too many hours on a tractor seat without ear protection. My eardrums have lost some of their resiliency, and thus when sound enters my ears, the eardrum and these three bones do not magnify it or concentrate the sound waves as good.
Another consideration is dirty ears. The dust and wax buildup can impede the sound waves, for this foreign material accumulates in the auditory canal, compressing itself against the eardrum, and like water, greatly reduces the intensity of sound.
The innermost portion of the ear is located next to the brain, and it is appropriately called the inner ear. Here, there is one structure, called the cochlea. This a rather remarkable organ, in that the cochlea is a labyrinth of three channels all serving a specific function in sending sound to the brain and in the complex adjustments so that an animal can be balanced.
As sound is amplified, magnified and concentrated from the eardrum, sound waves enter the inner ear in the part of the cochlea called the oval window. Vibrations of the oval window produce pressure waves in the watery-like fluid within the spiral labyrinth. Inside these channels are hair-like projections, resembling the cilia found in other dermal tissues. The movement of fluid in these channels causes the cilia to move as well. The cilia, when moved, alter the permeability of ion flow. That is, the bending in one direction increases the flow; the bending in the other direction decreases the flow.
The changing ionic flow is interpreted by a neurotransmitter at the end of the cilia. The neurotransmitter at the end of the cilia. The neurotransmitter, a part of the central nervous system, is wired directly to the brain, and thus, is the part of the cell responsible for initiating a nerve impulse to the brain.
So, we have sound that is mechanical energy when it enters the ear, but at some point must be conducted through a fluid medium, and once in this wet environment, be transmitted to the brain as an electrical impulse via a neurotransmitter. The neurotransmitter is a part of the auditory nerve, that bundle of nerve cells connected to the brain.
The value of the cilia, then, is obvious. Like other sensory receptors, these hair-like projections in the cochlea bridge the energy spectrum of mechanical to electrical. The changing ionic concentration, back and forth, positive and negative electrical charges, occurring with rapidity that we can sit in a concert hall and hear with amazement a philharmonic orchestra, or the cow can at the appropriate time, hear the milking machine - pipeline pre-milk rinse early in the morning….and interpret it as the time of milking soon.
The ear serves another important role, that of balance. This function is part of the inner ear as well. Behind the oval window is a vestibule that contains two chambers, the utricle and saccule. In these two chambers, there are hair like projections similar to the ones found in the other chambers of the cochlea.
These cilia respond to the movement of the head with respect to gravity. This is remarkable too, in that at the base of some of these cilia is a gelatinous substance containing many small calcium carbonate particles called otoliths (commonly called "ear stones").
These particles are heavier than the water-like fluid in the utricle and saccule, thus gravity is always pulling them downward. This centers the body in relationship to standing upright or holding the head properly in relationship to the forces of gravity.
When the position of the head changes with respect to gravity, the force on the cilia changes, and the neurotransmitters send a signal to the brain that the head is moved. The brain will send a signal to various muscle groups to adjust the body so that the animal will not fall down.
Spinning very fast can cause an upset in the equilibrium. This is because the "ear stone" particles are moved out of their downward location, and we lose the benefit of gravity in centering us. Hence, we fall down.
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