Discuss about photoreceptors.

Photorecepors : The receptor cells of eye which are light sensitive and have the photo-pigments are called photoreceptors or visual cells.

They are of two types 1. rod cells 2. cone cells.

1. Rod cells : The rod cells contain a purplish pigment called rhodopsin. They funcion in dim light and at night. They are responsible for the Scotopic vision. 

Bleaching : Bright light splits rhodopsin into a scotopsin and a carotenoid pigment retinene or retinal and this process is called bleaching. 

Dark adaptation : In the dark, rhodopsin is resynthesized from scotopsin and retinal. This process is called “dark adaptation.” It makes the rods functional.

2. Cone cells : The cones have a pigment called iodopsin. They function in daylight and artificial bright light produces images and give colour vision.

The cone cells conain three different wavelength.One pigment (erythrolabe) is sensitive o red light another (chlorolabe) to green and the third (cyanolabe) to blue. 

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Explain the Mechanism of vision.

The mechanism of vision:

i. The light rays in visible wavelength is focussed on the retina through the cornea and lens generate potentials (impulses) in rods and cones.

ii. The photopigments which is composed of opsin and retinal is dissociated into opsin and retinal because of light. .

 iii. The change in the structure of the opsin causes changes in the membrane permeability.

iv. A potential differences is generated in the photoreceptor cells which produces signal that generating action potentials in the ganglion cells through the bipolar cells.

v.  The action potential is transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analysed and the image is formed on the retina is recognised. 

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Explain resting membrane potential.

1. Resting membrane potential : Plasmamembrane of axon of neuron has +ve charge on outer surface and –ve charge on interior side, this electrical potential across the membrane is called resting membrane potential.

When a neuron is not conducting any impulse i.e. when it is resting, the membrane i s more permeable to K+ and nearly impermeable to Na+.

Similarly, the membrane is impermeable to the negatively charged proteins. 

Thus the axoplasm acquires high concentration of Kand negatively charged proteins and low concentration of Na+.

However the fluid outside the axon contains a low concentration of K+ and high concentration of Na+ . This forms a concentration gradient. These ionic gradients across the resting membrane are maintained by the active
transport of ions by the sodium-potassium pump which transports 3 Na+ outwards for 2 K+ into the cell. As a result, the outer surface of the
axonal membrane possesses a positive charge while its inner surface has a negative charge and is said to be polarised. This electrical potential across the resting membrane is called the Resting Potential. 

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The protective ridge present in the saccule and utricle?


Macula
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Discuss the mechanism of reflex action.


The reflex action follows a definite path which is in the form of an arc and this path is called reflex path.

The mechansim of reflex action can be described in following steps :

1. Reception of stimulus : The receptor organs (sensory organs) receive the stimulus and they transfer it to afferent nerve fibres.

2. Conduction of stimulus : The afferent nerve fibres conduct the stimulus to grey matter of spinal cord.

3. Conversion of stimulus into order : In grey matter of spinal cord, stimulus is converted into order by neurons.

4. Conduction of order : The order is carried by efferent nerve fibres to effectors (muscles).

5. Action of muscles : The muscles or effectors after receiving the order act accordingly.


Discuss the mechanism of reflex action.

 

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