The Vertebrate Ear-Control Of High And Low-Frequency Sounds

The ear plays a vital role in hearing sounds and also helps us recognize speech that comes from the environment around us. In humans, hearing starts at birth and continues throughout life.

Humans hear sound through the ears. Sound waves enter through the outer ear canal or auditory meatus and travel into the middle ear and inner ear. There they stimulate tiny hairs called cilia, which vibrate and activate nerves that send signals to the brain. These nerve impulses allow us to perceive sound.

Functional Units of Vertebrate Ear

The ear in vertebrates has two functional units:

1. Auditory apparatus: It is concerned with hearing.

2. Vestibular apparatus: It is concerned with posture and equilibrium.

1. Ear Of Amphibians (Frog)

Pressure waves transmit energy through some medium. These waves produce sound. The medium may be air or water. The middle ear senses sound pressure. But the air has very small sound pressure. The sound pressure of air is less than 0.1% of the pressure in water. Therefore, hearing in the air poses serious problems for vertebrates. Thus adaptation in the air took place.

Structure of Amphibian Ear

The land vertebrates have developed an acoustic transformers called the eardrum, tympanic membrane, or tympanum. It is composed of a thin stretched membrane. The tympanum first evolved in the amphibians. The ears of anurans (frogs) consist of a tympanum, a middle ear, and an inner ear.

The tympanum is a modified integument. It is stretched over a cartilaginous ring. It vibrates in response to sounds and transmits these movements to the middle ear. The middle ear is a chamber behind the tympanum. The ear of the frog is adapted to detect low and high frequencies:

1. High Frequency Sound (1000 To 5000 HZ)

Columella or stapes is small ossicle (bone). It touches the tympanum. The opposite end of the columella touches the membrane of the oval window. This membrane stretches between the middle and inner ears. High-frequency (1,000 to 5,000 Hz) sounds strike the tympanum.

These waves are transmitted through the middle ear through the columella. Columella strikes the oval window. It produces pressure waves in the fluid of the semicircular canals of the inner ear. These pressure waves in the inner ear fluid stimulate receptor cells.

2. Low-Frequency Sound (100 To 1000 HZ)

The operculum is the second small ossicle. It also touches the oval window. Some vibrations are produced by the substrate (land). They are transmitted through the front appendages and the pectoral girdle. They cause the operculum to vibrate. It produces pressure waves in the inner ear. It stimulates the second patch of sensory receptor cells. These receptor cells are sensitive to low frequency (100 to 1,000 Hz) sounds.

Control Of High And Low-Frequency Sounds

Muscles are attached to the operculum and columella. These muscles can lock these ossicles. It allows a frog to screen out high or low frequency sounds. This mechanism is adaptive. It allows frogs to use low and high-frequency sounds in different situations.

(a) Mating calls is a high-frequency sounds. These calls have primary importance only during the breeding season.

(b) At other times, low-frequency sounds are important. It warns the animals about predators.

Salamanders lack a tympanum and middle ear. They live in streams, ponds, caves, and beneath leaf litter. They have no mating calls. They hear only substratum sounds. These sound waves are transmitted through the skull into the inner ear.

Balancing Organ In The Amphibians

The amphibians have developed semicircular canals for the sense of equilibrium and balance. These canals detect rotational movements and gravity. Since the semicircular canals have a similar function in all vertebrates.