(a) Central nervous system :
It is a hollow dorsally placed structure lying along the mid dorsal axis of the body.
(ii) It includes the brain and spinal cord.
(iii) It is the site of information processing and control.
Peripheral nervous system:
(i) The nerves arising from the CNS consititute the peripheral nervous system.
(ii) The nerves originate from the brain and spinal
cord are known as cranial nerves and spinal nerves respectively.
(b) Resting potential and action potential
Resting potential : The electrical potential difference across the resting plasma mambrane is called as resting potential.
Action potential : When a stimulus is applied at any site on the polarised membrane, the electrical potential difference across the plasma membrane at that site is called the action potential. It is termed as nerve impulse.
(c) Choroid and retina
Choroid : Choroid is the middle layer of the eye ball. It contains many blood vessels and looks bluish in colours.
Retina: Retina is the inner layer of eye ball. It contains ganglion cell, bipolar cells and photoreceptor cells.