Directions: In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true.
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Assertion: The PNS comprises of all the nerves of the body associated with CNS.
Reason: PNS is the site of information processing and control.
(c) The CNS includes the brain and the spinal cord and is the site of information processing and control. The PNS comprises of all the nerves of the body associated with the CNS.
Assertion : Multipolar neurons have two or more axons and one dendrite.
Reason : Multipolar neurons are found usually in the cerebral cortex.
(d) Based on the number of axons and dendrites, the neurons are divided into three types, i.e., multipolar (with one axon and two or more dendrites; found in the cerebral cortex), bipolar (with one axon and one dendrite, found in the retina of eye) and unipolar (cell body with one axon only; found usually in the embryonic stage).
Assertion: Myelinated nerve fibres are present in spinal and cranial nerves.
Reason: Myelinated nerves conduct impulses more rapidly than unmyelinated nerves.
Assertion: The resting membrane of the neuron exhibits polarity of charges.
Reason: The outer surface of the axonal membrane possesses a negative charge while its inner surface becomes positively charged.
(c) When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions \(\left(\mathrm{K}^{+}\right)\)and nearly impermeable to sodium ions \(\left(\mathrm{Na}^{+}\right)\). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains high concentration of \(\mathrm{K}^{+}\)and negatively charged proteins and low concentration of \(\mathrm{Na}^{+}\). These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump which transports \(3 \mathrm{Na}^{+}\) outwards for \(2 \mathrm{~K}^{+}\)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.
Assertion: Electrical synapses are rare in our system.
Reason: Impulse transmission across an electrical synapse is slower than that across a chemical synapse.
(c) Impulse transmission across an electrical synapse is always faster than that across a chemical synapse. Electrical synapses are rare in our system.
Assertion: Association areas are neither clearly señsory nor motor in function.
Reason: Association areas are responsible for complex functions like intersensory associations, memory and communication.
(b) The cerebral cortex contains motor areas, sensory areas and large regions that are neither clearly sensory nor motor in function. These regions are called the association areas which are responsible for complex functions like intersensory associations, memory and communication. The association area is concerned with the reception of sensory impulses and start of motor impulses.
Assertion: Medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.
Reason: Medulla contains several neurosecretory cells which secrete hormones.
(c) The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions. The hypothalamus is the basal part of diencephalon (forebrain) which contains several groups of neurosecretory cells called nuclei that produce hormones.
Assertion: Reflex arc comprises of at least one afferent neuron, one efferent neuron and a part of PNS.
Reason: The afferent neuron receives the signal from a sensory organ and transmits the impulse via a dorsal nerve root into the CNS.
(d) The reflex pathway comprises at least one afferent neuron (receptor) and one efferent (effector or excitor) neuron appropriately arranged in a series. The afferent neuron receives signal from a sensory organ and transmits the impulse via a dorsal nerve root into the CNS (at the level of the spinal cord). The efferent neuron then carries signals from CNS to the effector. The stimulus and response thus form a reflex arc.
Assertion: Choroid layer is thick over the posterior two-third of the eye ball, but it becomes thin in the anterior part.
Reason: Choroid layer contains blood vessels.
(d) The middle layer, choroid, contains many blood vessels and looks bluish in colour. The choroid layer is thin over the posterior two-third of the eye ball, but it becomes thick in the anterior part to form the ciliary body.
Assertion: When all the three types of cones are stimulated equally, a mosaic of red, green and blue lights is produced.
Reason: Cones are responsible for daylight vision.
(d) In the human eye, there are three types of cones which possess their own characteristic photopigments that respond to red, green and blue lights. The sensations of different colours are produced by various combinations of these cones and their photopigments. When these cones are stimulated equally, a sensation of white light is produced. The daylight (photopic) vision and colour vision are functions of cones and the twilight (scotopic) vision is the function of the rods.
Assertion: At fovea, the visual acuity is the greatest.
Reason: The fovea is a thick area of the retina where both rods and cones are present.
(c) 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.
Assertion: The space between the cornea and the lens is called the vitreous chamber.
Reason:The space between the lens and retina is filled with vitreous humour.
(d) The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humour. The space between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called the vitreous humour.
Assertion: The inner ear contains malleus, incus and stapes which are attached to one another in a chain-like fashion.
Reason: The malleus is attached to the tympanic membrane and the stapes is attached to the oval window of the cochlea.
(d) The middle ear contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion. The malleus is attached to the tympanic membrane and the stapes is attached to the oval window of the cochlea. The ear ossicles increase the efficiency of transmission of sound waves to the inner ear.
Assertion: The Eustachian tube helps in equalising the pressures on either sides of the ear drum.
Reason: The Eustachian tube connects the middle ear cavity with the pharynx.
Assertion: Vestibular apparatus helps us in maintaining balance of body and posture.
Reason: Due to the arrangement of semicircular canals of vestibular apparatus, movement of head in any direction will stimulate sensory cells to maintain dynamic equilibrium.
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