Question

Choose a special sense process (e.g. Vision or Hearing, or another one of your choosing) and explain how the special sense organ acts as a transducer, changing one form of stimulus into another (an action potential). Be sure to describe the detailed physiology, including important anatomical components where necessary

Answer 1

Let us take the example of the eye as the special sense organ which helps in vision. The eye acts as a transducer by converting light to a neural signal (action potential). The main anatomical components of the eye are the

• cornea (the outer transparent layer that helps focus light into the eye)
• iris (colored part of the eye that regulates the amount of light entering the eye)
• pupil (aperture in the pupil that determines how much light enters the eye)
• retina (nerve layer that lines the back of the eye, senses the light that enters and converts this light into nerve impulse that travels through the optic nerve to the brain)
• lens (helps to alter the wavelength of light that enters)
• macula ( a small region in the centre of the retina that contains special light sensitive cells that enable to see finer details)
• vitreous (fluid that fills the middle of the eye)

PHYSIOLOGY OF VISION - The first process that is involved in the process of vision is the refraction of light that enters the eye. The light waves that travel in parallel to each other bend or get refracted as they pass through the cornea, iris, pupil, lens and the vitreous humor before it reaches the retina. This light then gets focused on the retina. For the light to fall on the retina, there has to be a process called accommodation. This process involves either the shortening or lengthening of the lens to focus the objects correctly. The retina contains two types of photoreceptors - rods and cones. Rods help in dark vision and cones help in color vision. Rods contain a light sensitive pigment called rhodposin. Rhodopsin is made of two components - Scotopsin and a light sensitive molecule called retinal. Retinal exists in two forms - cis and trans. When light falls on the rod cells, it is absorbed by rhodposin and this then gets broken down as scotopsin and 11 cis- retinal. This process is known as bleaching.

11 cis- retinal absorbs the photon of light and gets converted to all trans- retinal. This in turn causes the activation of scotopsin to an enzyme thus producing large amounts of transducin. Transducin activates another enzyme called phosphodiesterase. This then hydrolyses cyclic GMP which causes a stop in the flow of Na⁺ ions inside the rod cells. The inside of the rod cells then become more negative and so reaches a hyperpolarised state. These hyperpolarised cells then send a neural signal to the adjacently lying bipolar cells. From here the impulse moves to the amacrine cells followed by the ganglion cells thus causing an action potential. Action potentials from different rod cells join together and are sent to the brain via the optic nerve. The brain then processes this information and enables us to perceive what we see.

A similar process happens in the cones. However the difference is that there are three types of cone cells - those that are sensitive to red, green and blue. The perception of color depends on which cone is stimulated. The cone cells contain iodopsin (similar to rhodopsin in rod cells) which is composed of 11 cis- retinal and photopsin. The final perceived color is the combination of all three colors.

The impulse carried by the optic nerve reaches the thalamus and synapses with the lateral geniculate body. This projects into the primary visual cortex in the occipital lobe of the cerebrum and thus the image is perceived.