Our perception of color is facilitated by specialized cells in the retina of our eyes called cones. Cones are sensitive to different wavelengths of light and enable us to perceive colors. There are three types of cones, each sensitive to specific ranges of wavelengths corresponding roughly to red, green, and blue light. When light enters the eye, it is absorbed by these cones, and the information is processed by the brain to generate our perception of color.

The part of our body responsible for maintaining balance is the vestibular system, which is located in the inner ear. Specifically, the semicircular canals and otolith organs within the inner ear detect changes in head position and movement. This information is sent to the brain, where it is processed to help us maintain our balance and sense our orientation in space.

The eye regulates the amount of light that falls on the retina primarily through the action of the iris and the pupil. The iris is a circular muscle surrounding the pupil, and it can contract or relax to adjust the size of the pupil. In bright conditions, the iris contracts, making the pupil smaller to reduce the amount of light entering the eye. In dim conditions, the iris relaxes, allowing the pupil to dilate and let in more light. This process, known as pupillary reflex, helps regulate the amount of light reaching the retina to optimize visual sensitivity in different lighting conditions.

(a) Role of Na+ in the generation of action potential: Sodium ions (Na+) play a crucial role in the generation of action potentials, which are brief electrical signals that propagate along the membrane of neurons. During the resting state of a neuron, the membrane is polarized, with a negative charge inside and a positive charge outside. When a stimulus depolarizes the membrane, voltage-gated sodium channels open, allowing Na+ ions to rush into the neuron. This influx of positive charge depolarizes the membrane further, leading to the generation of an action potential. The rapid influx of Na+ ions initiates the rising phase of the action potential, creating an electrical impulse that travels along the neuron. Subsequently, voltage-gated potassium channels open, allowing potassium ions (K+) to leave the neuron and repolarize the membrane, restoring its negative charge. Thus, the influx of Na+ ions is essential for triggering and propagating action potentials in neurons.

(b) Mechanism of generation of light-induced impulse in the retina: In the retina, photoreceptor cells called rods and cones convert light energy into electrical signals that can be interpreted by the brain as visual information. When light enters the eye and strikes the retina, it is absorbed by photopigments located in the outer segments of rods and cones. This absorption of light causes a change in the conformation of the photopigment molecule, leading to the activation of a signaling cascade within the photoreceptor cell. Specifically, the activation of photopigments triggers a decrease in the concentration of cyclic guanosine monophosphate (cGMP) within the photoreceptor cell, which results in the closure of cGMP-gated sodium channels in the cell membrane. This closure of sodium channels leads to hyperpolarization of the photoreceptor cell, reducing its release of neurotransmitter (glutamate) onto bipolar cells. The change in neurotransmitter release from photoreceptor cells alters the activity of bipolar cells, which in turn transmit the visual signal to retinal ganglion cells and eventually to the brain via the optic nerve. Thus, the generation of light-induced impulses in the retina involves a series of biochemical and electrical events initiated by the absorption of light by photopigments in rods and cones.