Electrical tuning
Encyclopedia
Electrical tuning is a mechanism by which vertebrate
s such as frog
s and reptile
s, which lack a long cochlea
, discriminate sound
. Mammal
s have long cochleae, and are able to distinguish different sounds by mechanisms such as mechanical tuning, in which the stiffness and length of hair cell
s’ stereocilia
makes a given cell best suited to respond to a certain type of stimulus. The basilar membrane
of animals with long cochleae also vibrates at different locations along the membrane in response to different frequency sounds. Since the reptilian ear lacks a long cochlea, electrical tuning provides an alternative mechanism for perceiving differences in sound.
With a combination of voltage-gated calcium channels and calcium sensitive K+ channel
s, the cell
s set up an oscillation
in voltage
and oscillate in response to a depolarizing
stimulus
. Ca++
enters the cell and depolarizes it, opening K+
channels which allow K+ to leave and hyperpolarize the cell. Since the exit of K+ from the cell is delayed, the voltage increases, but then with the exit of the K+ the cell overshoots the membrane potential and hyperpolarizes. Different cells have different delays for K+ leaving and thus the voltage oscillates at different frequencies. The delays can be as short as 0.7 ms
or as long as 150 ms, whereas the Ca++ entry always occurs within about 1 ms (Fettiplace, 1987). Thus by varying the length of delay for K+ to leave, cells' ion concentration
s can oscillate at specific frequencies
. Cells with higher oscillating frequencies respond best to higher-frequency sounds, while those with lower frequencies respond best to lower frequency ones. Which cells resonate best with a given sound tells the brain
what the frequency of the sound is. Hair cells that respond to high frequency stimuli send information to specific neurons, and the information remains segregated in the brain. Thus, information about sound frequency is preserved, rather than being lost as it would if all information from different hair cells converged on the same neuron or group of neurons.
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s such as frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...
s and reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, which lack a long cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
, discriminate sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
. Mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s have long cochleae, and are able to distinguish different sounds by mechanisms such as mechanical tuning, in which the stiffness and length of hair cell
Hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...
s’ stereocilia
Stereocilia
In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...
makes a given cell best suited to respond to a certain type of stimulus. The basilar membrane
Basilar membrane
The basilar membrane within the cochlea of the inner ear is a stiff structural element that separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani .-Function:...
of animals with long cochleae also vibrates at different locations along the membrane in response to different frequency sounds. Since the reptilian ear lacks a long cochlea, electrical tuning provides an alternative mechanism for perceiving differences in sound.
With a combination of voltage-gated calcium channels and calcium sensitive K+ channel
Potassium channel
In the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...
s, the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s set up an oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
in voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
and oscillate in response to a depolarizing
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
stimulus
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
. Ca++
Calcium in biology
Calcium plays a pivotal role in the physiology and biochemistry of organisms and the cell. It plays an important role in signal transduction pathways, where it acts as a second messenger, in neurotransmitter release from neurons, contraction of all muscle cell types, and fertilization...
enters the cell and depolarizes it, opening K+
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
channels which allow K+ to leave and hyperpolarize the cell. Since the exit of K+ from the cell is delayed, the voltage increases, but then with the exit of the K+ the cell overshoots the membrane potential and hyperpolarizes. Different cells have different delays for K+ leaving and thus the voltage oscillates at different frequencies. The delays can be as short as 0.7 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
or as long as 150 ms, whereas the Ca++ entry always occurs within about 1 ms (Fettiplace, 1987). Thus by varying the length of delay for K+ to leave, cells' ion concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
s can oscillate at specific frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. Cells with higher oscillating frequencies respond best to higher-frequency sounds, while those with lower frequencies respond best to lower frequency ones. Which cells resonate best with a given sound tells the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
what the frequency of the sound is. Hair cells that respond to high frequency stimuli send information to specific neurons, and the information remains segregated in the brain. Thus, information about sound frequency is preserved, rather than being lost as it would if all information from different hair cells converged on the same neuron or group of neurons.