Question

Define the following
(i) Nerve impulse
(ii) Axon
(iii) Cyton
(iv) Action potential
(v) Reflex action
(vi) Yellow spot
(vii) Blind spot
(viii) power of accommodation

Answer 1

(i) The information that is transmitted along the neurons in the form of electric impulses is called nerve impulses.

(ii) An axon is a long fibre that transmits impulses away from the neuron's cell body.

(iii) The cyton is the cell body of the neuron that includes the nucleus and other structures.

(iv) Action potential is the electrical potential difference that develops across a plasma membrane at a site where a stimulus is applied. This action potential is also termed a nerve impulse.

(v) A sudden movement or response to a stimulus that occurs in a very short duration of time (within a second, usually) and does not involve will or any thinking is called a reflex action.

(vi) The yellow spot is a small oval-shaped spot present in the human eye situated close to the centre of the retina. It helps us to see objects with the highest clarity.

(vii) A blind spot is an obscuration in the visual field. It is the point of entry of the optic nerve on the retina and is insensitive to light. It is an area where a person's view is obstructed.

(viii) One of the remarkable properties of the eye is its ability to distinctly observe objects situated at widely different distances from it. This property is called the accommodating power of the eye. It is achieved by changing the curvature of the crystalline lens and its focal length by changing the tension in the ciliary muscles holding the lens.