Question

Q.What property of the auditory system confers it the ability to distinguish sound frequency – pitch?
a. Hair cells with long cilia respond to low frequency sound; hair cells with short cilia respond to high frequency sound.
b. Oscillation of the tectorial membrane follows precisely the frequency of sound.
c. The graded thickness and width along the length of the basilar membrane allow different parts to vibrate in response to different sound frequency.
d. The brain compares the relative input from the two ears and compute the sound frequency.
e. High frequency sound gives rise to a larger depolarization of hair cells than low frequency sound.

Q. Cochlear hair cells are unusual among neurons in that when stimulated, K+ channels open and K+ enters the cell. Why might this be so?
a. K+ concentration is higher on the outside and lower in the inside.
b. Na+ concentration is low on the outside.

c. At rest, the potential across the membrane is positive in the inside.

d. Cochlear hair cells hyperpolarize when stimulated.
e. The lack of Na+ channels prevents the influx of Na+.

Q. The sodium channel in the plasma membrane of photoreceptors is:

a. a ligand-gated channel.
b. a voltage-gated channel.
c. a metabotropic receptor.

d. a transmitter-sensitive channel.

e. a photo-activated channel.

Q. In the photo-transduction pathway, cGMP:
a. absorbs and transduces light energy into electrical energy.
b. activates transducin by replacing GDP in the nucleotide binding site.

c. inhibits phosphodiesterase in the presence of light.
d. is released into the extracellular space in response to light.
e. acts as a ligand that opens membrane channels.

Answer 1

Ans.

1).

• From a high pitched whine of a mosquito to a low rumbles of distant thunder, a wide range of frequencies can be detected by human ear.
• Sensory cells known as hair cells are responsible for detecting such sounds.
• Hair cells are spread around a flat surface known as basilar membrane.
• This basilar membrane is fold in a step wise manner along with hair cells and detect sounds of different frequencies according to the thickness and width of membrane.
• The graded thickness and width along the length of the basilar membrane allow different parts to vibrate in response to different sound frequency.

2).

• Mechanoreceptors of cochlear hair cells depends upon the ionic gradient with a unique arrangement in the inner ear.
• This permit the flow of potassium ions in a passive manner.
• An unusual high K+ concentration and positive potential of scala media fluid helps in achieveing such electrochemical gradient.
• So, becuase of the higher K+ concentration on the outer side compare to inner side causes entering of K+ when K+ channels open upon stimulation of cochlear hair cells neurons.

3).

• Visual phototransduction is carried out by photoreceptor, a specialized type of neuroepithelial cell found in retina.
• Photoreceptors convert light into signals capable of stimulating biological processes.
• The sodium channel in the plasma membrane of photoreceptors is a metabotropic receptor.
• This sodium channel is regulated by a Na/Ca K exchanger in photoreceptors.

4).

• In the process of photo-transduction, Opsin is converted to metarhodopsin 2.
• Metarhodopsin 2 activates transducin (a G protein) by replacing GDP in the nucleotide binding site.
• After dissociating from it's bound GDP, transducin bind GTP and this complex activates phosphodiesterase (PDE6), stimulating it's activity.
• PDE6 then hydrolyzes cGMP, thus reducing it's cytosolic level which results in closing of cyclic nucleotide gated channels thereby preventing further entry of any ions.