Explore topic-wise InterviewSolutions in .

(a) Polarisation of the membrane of a nerve fibre: The fluid inside the membrane contains high concentration of K⁺ and negatively charged proteins and low concentration of Na⁺. 

In contrast, the fluid outside the axon contains a low concentration of K⁺, a high concentration of Na⁺ and thus 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 becomes negatively charged and therefore is polarised.

(b) Depolarisation of the membrane of a nerve fibre: When a stimulus is applied at a site on the polarised membrane, the membrane at the site A becomes freely permeable to Na⁺. This leads to a rapid influx of Na⁺ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. The polarity of the membrane at the site is thus reversed and hence depolarised.

Dendrites: Short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites. These fibres transmit impulses towards the cell body. 

Axon: The axon is a long fibre, the distal end of which is branched. Each branch terminates as a bulblike structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters. The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.

(a) Mechanism of Vision:

•  The light rays in visible wavelength focussed on the retina through the ornea and lens generate potentials (impulses) in rods and cones.
• Light induces dissociation of the retinal from opsin resulting in changes in the structure of the opsin. This causes membrane permeability changes. As a result, potential differences are generated in he photoreceptor cells. This produces a signal that generates action potentials in the ganglion cells through the bipolar cells.
• These action potentials (impulses) are transmitted by the optic nerves to the visual cortex area of the brain, where the nervous impulses are analysed and the image formed on the retina is recognised based on earlier memory and experience.

(b) Mechanism of Hearing: The outer part of the ear collects sound. That sound pressure is amplified through the middle portion of the ear and, in land animals, passed from the medium of air into a liquid medium. The change from air to liquid occurs because air surrounds the head and is contained in the ear canal and middle ear, but not in the inner ear. The inner ear is hollow, embedded in the temporal bone, the densest bone of the body. The hollow channels of the inner ear are filled with liquid, and contain a sensory epithelium that is studded with hair cells. The microscopic "hairs" of these cells are structural protein filaments that project out into the fluid. The hair cells are mechanoreceptors that release a chemical neurotransmitter when stimulated. Sound waves moving through fluid push the filaments; if the filaments bend over enough it causes the hair cells to fire. In this way sound waves are transformed into nerve impulses.

Cones are responsible for color vision. They require brighter light to function than rods require. There are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colors). The color seen is the combined effect of stimuli to, and responses from, these three types of cone cells.

The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eye ball. Photoreceptor cells are not present in that region and hence it is called he blind spot. At the posterior pole of the eye lateral to the blind spot, here is a yellowish pigmented spot called macula lutea with a central pit called the fovea. The fovea is a thinned-out portion of the retina where only the cones are densely packed. It is the point where the visual acuity (resolution) is the greatest.

 Resting Potential and Action Potential: Neurons are excitable cells because their membranes are in a polarized state. Different types of ion channels are present on the nervous membrane. These ion channels are selectively permeable to different ions. When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions (K⁺) and nearly impermeable to sodium ions (Na⁺). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains high concentration of K⁺ and negatively charged proteins and low concentration of Na⁺

In contrast, the fluid outside the axon contains a low concentration of K⁺, a high concentration of Na⁺ and thus 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 becomes negatively charged and therefore is polarised. The electrical potential difference across the resting plasma membrane is called as the resting potential. 

When a stimulus is applied at a site on the polarised membrane, the membrane at the site becomes freely permeable to Na⁺. This leads to a rapid influx of Na⁺ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. The polarity of the membrane at the site is thus reversed and hence depolarised. The electrical potential difference across the plasma membrane at the site A is called the action potential, which is in fact termed as a nerve impulse.

The pupil in the eye functions like an aperture. This dilates in case of low light and constricts in case of intense light thereby regulating the amount of light falling on the retina. 

The Inner ear has three semi-circular canals forming cochlea. Cochlea is responsible for maintaining the body balance.

The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humor. The pace between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called vitreous humor.