Answer: Option D. -> The brain has external white matter and internal gray matter
:
D
The major part of the brain in vertebrates consistsof two hemispheres, left and right, separated by a fissure. The hemispheres are connected by a tract of nerve fibres called corpus callosum. Corpus callosum is responsible for the relay of impulses from both the hemispheres.The layer of cells which covers each of the cerebral hemispheres is called cerebral cortex. The cerebral cortex (outer part of the cerebral hemisphere) is referred to as the grey matter due to its greyish appearance. It contains the cytons of nerve fibres. Axons of nerve fibres are covered with the myelin sheath and constitute the inner part of the cerebral hemisphere. They give an opaque white appearance to the layer and, hence, is called the white matter.

Answer: Option C. -> Assertion [A] is True and Reason [R] is True and is a correct explanation to [A].
:
C
The Neural System is specialised to perform varied functions.
Based on function they perform, we can divide the nervous system into Central Nervous System (CNS) and Peripheral Nervous System (PNS). The PNS constitutes all the nerves and ganglia that lie outside the CNS. The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs, essentially serving as a communication relay going back and forth between the brain and the extremities.

Answer: Option B. -> Optic nerve
:
B
The inner layer of the eye is the retina and it contains three layers of cells – from inside to outside – ganglion cells, bipolar cells and photoreceptor cells. There are 2 types ofphotoreceptorsin the retina: rods and cones. Photoreceptor cells contain light-sensitive proteins, photopigments. The rods are most sensitive to light and dark changes, shape and movement and contain only one type of light-sensitive pigment. The photopigments create an impulse when light falls on them. The optic nerveleaves the eyeand transmits impulses from the retina to the brain.

Answer: Option B. -> Postsynaptic membrane will not depolarize
:
B
The Refractory Period, after an action potential is the period during which the neuron resists the production of another action potential;
Two types of Refractory Periods are seen :
1. Absolute Refractory Period (1-2 m/sec) : The voltage gated sodium channels are closed by the inactivation gates, and not the voltage regulated gate, and therefore do not open even to high levels of depolarization. Therefore action potential will not fire even at a very high stimulus.
2. Relative Refractory Period : In this period the inactivation gates of voltage gated Na+ channels are open, but the membrane is hyperpolarized after an action potential and will require a higher than normal stimulus, for an action potential to fire.
The incorrect statement here is ‘Postsynapticmembrane will not depolarize’. The initial depolarization of a membrane when a stimulus arrives is due to chemical gated sodium channels. These channels open even during the refractory period, therefore a membrane will depolarize even during the refractory period.

Answer: Option B. -> Synaptic Nerve Impulse Transmission
:
B
The picture depicts the procedure of Synaptic Nerve Impulse Transmission. The impulse travels along the following order to reach the target neuron or organ :
•Depolarisation of presynaptic neuron ⇛• Opening of Calcium channels ⇛•Calcium influx ⇛• Release of neurotransmitter ⇛• Diffusion of neurotransmitter across the synaptic cleft ⇛• Neurotransmitter binds to receptors on the postsynaptic membrane.

Answer: Option D. -> All of these
:
D
The entire human body physiology is under the control of the nervous system except the heart.The neural system provides an organised network of point-to-point connections for a quick coordination. Our nervous system helps intransmission of nerve impulse, impulse conduction across a synapse and the physiology of reflex action. Our response to various stimuli is due to the coordination of the nervous system between all body parts.

Answer: Option B. -> Nervous tissue
:
B
Schwann cells, also called neurilemmacells, whichare cellsin the peripheral nervous system that produce the myelin sheath around neuronal axons.