VPB 121: Veterinary Physiology - II (2+1)

MECHANISM OF PHOTORECEPTION

• As the light strikes the photoreceptors, transformation of photo pigment occurs with the resultant HYPERPOLARIZATION of receptor cells.
• During darkness, the Na⁺ channel are opened / remain open on rod and cone cell membrane.
• Na⁺ leak into rods and lower the membrane potential. As light strikes the rods retinal is released from rhodopsin and causes closure of many Na⁺ channels.
• This causes HYPERPOLARIZATION of the receptor cell membrane and a decrese in the transmitter released at the synapse with the bipolar cells.
• Breakage of rhodopsin to RETINAL + OPSIN by light is temporary and Rhodopsin is resynthesized afterwork.

Activity of photoreceptors

In dark

• The cell membrane of the photoreceptors contains chemical messenger-gated Na⁺ channels.  They respond to the second messenger cGMP. 
• The Na⁺ channels are open when the cGMP is bound to them.  In absence of light, the concentration of cGMP is high; therefore it is bound with the Na⁺ channels, keeping them open. 
• The inward leak of the Na⁺ ions depolarizes the photoreceptors.  This in turn keeps the Ca ²⁺ channels opened, which triggers the release of the neurotransmitter from synaptic terminal.

In Light

• There are other biochemical cascades that take place in the light. The concentration of the cGMP is reduced. 
• The reduction in cGMP is by the cascades as follows:  The retinene absorbs light and changes the conformation and activates the photopigment. 
• The rods and cones contain G-protein called transducin, activates the enzyme phosphodiesterase.  which degrades the cGMP causes the closure of Na⁺ channels.  As the Na⁺ channels closes, the photoreceptor cell gets hyperpolarized which in turn reduced the release of neurotransmitter.