Friday, 25 September 2015

All About ... the Ears.

In last Friday's blog I covered the Central and Peripheral Nervous systems . These systems interpret and act on information received from the Sensory Systems. The five senses are hearing, sight, smell, taste and touch.  Today I thought I would find out more about hearing and a horses'  ears. 

The ear is not only important for hearing but also for balance and enabling us to detect body movements.  For horses the ears also demonstrate their mood!  Sound is a sensation that is felt when the ear structures are acted upon by vibrations (sound waves) in the air.

The ear is divided into 3 parts; the outer ear which is air filled, the middle ear which is also air filled and the inner ear which is fluid filled.

The outer ear is the part which is mobile.  The ears of a horse are always on the move and they have far better hearing than humans.  By moving their ears towards a sound they are able to much more accurately pinpoint the direction the sound is coming from.  The cone shaped ears help to direct and concentrate the sound waves into the ear canal.  The outer ear consists of a flap (pinna) which the horse is able to move towards the source of a sound.  The sound waves then travel down the auditory canal until they hit the ear drum which is a tough membrane. 

The middle ear is the cavity behind the ear drum (tympanic membrane).  When the sound waves hit the ear drum it vibrates and transmits the vibrations to the 3 little bones (ossicles) called the hammer (malleus), the anvil (incus) and the stirrup (stapes).  These ossicles transfer the vibrations to the inner ear.  Equal pressure with the outside air is maintained due to the connection to the back of the throat by the Eustachian Tubes. There is one of these on each side.  

The inner ear is filled with fluid called the endolymph and is made up of the cochlea and the vestibular apparatus.  The cochlea is responsible for our hearing.  It is a long 3 chambered coiled tube which looks like a snail's shell.  The stirrup transmits the vibrations to the cochlea through the oval window (fenestra ovalis).  When this oval window vibrates it sets up pressure waves in the fluid.  These pressure waves in turn stimulate the hair cells (mechanoreceptors) which vibrate.  The vibrations generate nerve impulses which pass along the auditory nerve to the brain.  The pressure waves travel until they meet the round window (fenestra rotundis) when they dissipate.  The round window bulges into the middle ear.  

The vestibular apparatus help the horse to maintain balance and produce smooth coordinated movements.  This apparatus consists of semi-circular canals which contain cristae and sacs containing maculae.  There are 3 semi-circular canals placed so that changes of movement in any direction can be detected.  They are filled with fluid.  The cristae is the sense organ, each of them (there is 1 in each canal) have hair cells which move in response to movement of the structure in which they are embedded (the cupola).  These hair cells in turn elicit the nerve impulses which are then sent to the brain.

The 2 sacs found in the vestibular apparatus contain maculaue (a sense organ) these contain a jelly like fluid and are lined with hair cells.  There are granules which float in the fluid and the movement of these in response to changes in body position affect the hair cells.  As before the hair cells affect the nerve impulses sent to the brain.

Information from the vestibular apparatus is sent to the spinal cord where the body position can be adjusted.  Also, the information is sent to the brain where it is processed and interpreted before motor impulses are sent around the body or the information is sent on to other areas of the brain.

The ears are amazing.  Horse's rely on them in the wild and in domestic life to keep them safe.  They can hear things much further away than we can which certainly gives them an advantage.  Horse's ears come in all shapes and sizes and we must look after them!

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Until next time!


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