Neural Control and Coordination

Statement I is correct: the two cerebral hemispheres are connected by a tract of nerve fibres called the corpus callosum. Statement II is incorrect: the brain stem consists of the midbrain, pons and medulla oblongata; the cerebrum is part of the forebrain and does not form the brain stem. So Statement I is correct but Statement II is incorrect.

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. The visible coloured portion of the eye is iris. The diameter of the pupil is regulated by the muscle fibres of iris. Blind spot contains no photoreceptor cells. It is also a region where the optic nerves leave the eye and the retinal blood vessels enter it. The external layer of an eyeball is composed of a dense connective tissue and is called the sclera.

The lens in the human eye is held in its place by suspensory ligaments attached to the ciliary body.

Inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus etc., form a complex structure called limbic system. Limbic system along with the hypothalamus, is involved in the regulation of sexual behaviour, expression of emotional reactions (e.g., excitement, pleasure, rage and fear), and motivation.

Neurotransmitters are chemicals released from the synaptic vesicles at the axon terminal; they carry the nerve impulse across a chemical synapse.
Acetylcholine is the neurotransmitter at neuromuscular junctions and at cholinergic synapses.
Norepinephrine (noradrenaline) is the neurotransmitter of the adrenergic (sympathetic) nerve fibres.
Epinephrine (adrenaline) also acts as a neurotransmitter in some parts of the nervous system, besides being a hormone of the adrenal medulla.
Cortisone is a steroid hormone (a glucocorticoid) of the adrenal cortex; it acts on metabolism through the blood and is not released at synapses.
So the one that does not act as a neurotransmitter is cortisone.

Reference: NCERT Class XI, page 235.

Nissl granules are present in the cyton and even extend into the dendrites, but are absent in the axon and the rest of the neuron. Nissl granules are composed of free ribosomes and RER. They are responsible for protein synthesis.

The pre-synaptic membrane is involved in the release of neurotransmitter at a chemical synapse. The receptor sites for neurotransmitters are present on the post-synaptic membrane.

Oligodendrocytes are neuroglial cells that produce the myelin sheath in the central nervous system, while Schwann cells produce the myelin sheath in the peripheral nervous system.

The retina has rods and cones. Rods contain the reddish-purple pigment rhodopsin, which is made up of the protein opsin and retinal (the aldehyde form of vitamin A).

The cerebellum maintains body equilibrium and posture.
Language comprehension is a function of the cerebrum (temporal lobe), so this pairing is incorrect.

The hypothalamus contains centres that control body temperature and the urge for eating and drinking.

The hind brain comprises the pons, cerebellum and medulla oblongata.
The hypothalamus and corpus callosum are parts of the forebrain, and the spinal cord is not part of the brain.

The hypothalamus lies at the base of the thalamus in the forebrain.
It contains a number of centres which control body temperature and the urge for eating and drinking.
It also contains groups of neurosecretory cells that secrete hypothalamic hormones.
The thalamus is the major coordinating centre for sensory and motor signalling, the pons connects different regions of the brain, and the cerebellum maintains posture and balance.
Hence these nerve centres are contained in the hypothalamus.

The hypothalamus lies at the base of the thalamus in the forebrain.
It contains a number of centres which control body temperature and the urge for eating and drinking; it acts as the body's thermostat.
The medulla oblongata controls respiration, cardiovascular reflexes and gastric secretions.
The cerebellum maintains posture and equilibrium, and the cerebrum is the seat of intelligence, memory and voluntary actions.
Hence the hypothalamus regulates body temperature.

The vestibular apparatus of the inner ear (utricle, saccule and semicircular canals) maintains balance and posture, even without visual cues.

Retinal is an aldehyde of vitamin A, not a derivative of vitamin C.

The rods contain a purplish-red protein called rhodopsin or visual purple, which is needed for twilight (scotopic) vision.
The photopigments of the eye are made of opsin (a protein) and retinal.
Retinal is an aldehyde of vitamin A, so rhodopsin contains a derivative of vitamin A.
This is why deficiency of vitamin A causes night blindness.
Hence rhodopsin is a derivative of vitamin A.

$K^+$ is the most abundant cation inside cells; $Na^+$ is the main cation of the extracellular fluid.

The vestibular apparatus (three semicircular canals and the otolith organ made of saccule and utricle) has no role in hearing.
It is responsible for maintaining the balance and posture of the body.
Ear ossicles, organ of Corti and the Eustachian tube are all concerned with hearing or with equalising pressure on the ear drum.

In the resting state the axonal membrane is comparatively more permeable to potassium ions ($K^+$) and nearly impermeable to sodium ions ($Na^+$).
It is also impermeable to the negatively charged proteins present in the axoplasm.
As a result the axoplasm has a high concentration of $K^+$ and a low concentration of $Na^+$, while the fluid outside has the reverse.
These gradients are maintained by the sodium–potassium pump, which transports 3 $Na^+$ outwards for 2 $K^+$ into the cell; the outer surface is therefore positively charged and the inner surface negatively charged (polarised membrane).
The membrane becomes freely permeable to $Na^+$ only when a stimulus is applied.
Hence at rest the membrane is more permeable to $K^+$ ions and nearly impermeable to $Na^+$ ions.

In the resting state the axon membrane is more permeable to $K^+$ and nearly impermeable to $Na^+$; the outer surface is positively charged and the inner surface is negatively charged (polarised membrane).
The concentration of $Na^+$ is high in the extracellular fluid and low in the axoplasm.
When a stimulus is applied, the membrane at that site becomes freely permeable to $Na^+$ because the voltage-gated $Na^+$ channels open.
This leads to a rapid influx of $Na^+$ from the extracellular fluid into the intracellular fluid.
The polarity of the membrane at the site is reversed: the outer surface becomes negatively charged and the inner surface becomes positively charged. The membrane is said to be depolarised.
The electrical potential difference across the membrane at this site is called the action potential, which is in fact termed the nerve impulse.
The outward movement of $K^+$ occurs afterwards and restores the resting potential (repolarisation).
So the action potential results from the movement of $Na^+$ ions from the extracellular fluid to the intracellular fluid.

The retina has two types of photoreceptor cells, rods and cones.
Rods are responsible for twilight (scotopic) vision, i.e. vision in poor light. They contain the purplish-red pigment rhodopsin (visual purple), a derivative of vitamin A.
Cones are responsible for daylight (photopic) vision and colour vision, and they give sharp, detailed vision in bright light. They contain the pigment iodopsin (photopsin). So the overall function is correctly stated.
Distribution is wrongly stated: cones are densely packed at the centre of the retina (fovea of the macula lutea), while rods are more numerous in the peripheral parts.
Visual acuity is wrongly stated: acuity (resolution) is high for cones and low for rods.
Visual pigments are interchanged in the last statement: rods contain rhodopsin and cones contain iodopsin.
So the correct difference is the one on overall function: rods for vision in poor light, cones for colour vision and detailed vision in bright light.

A neuron has three major parts: the cell body, the dendrites and the axon.
The cell body of a neuron is called the cyton or perikaryon; the cytoplasm immediately surrounding the nucleus is loaded with protein-synthesising machinery in the form of Nissl's granules.
Dendrites are short, tapering and much-branched processes that project out of the cell body and carry impulses towards it; they may be one to several.
The perikaryon and the dendrites are found only in nerve cells, so this pair distinguishes a nerve cell from other types of cells.
A nucleus and mitochondria are present in almost all cells.
Vacuoles and fibres are not special to neurons, and a neuron has no flagellum.
So the distinguishing pair is perikaryon and dendrites.

Mature neurons are highly specialised cells that have lost the ability to divide (they remain in the $G_0$ phase and lack centrioles for division).
So when neurons are damaged or die, they are generally not replaced by new cells.
Liver cells can divide and regenerate the liver after injury.
The Malpighian (germinative) layer of the skin divides continuously and replaces the cells of the epidermis.
Bone has osteoblasts that can form new bone cells (osteocytes) during repair.
So the cells least likely to be replaced are the neurons.

The abducens nerve is the sixth cranial nerve. It is a motor nerve.
It supplies the lateral rectus muscle of the eye ball, which turns the eye outward (abduction).
If it is injured, the eye cannot be turned outward and tends to be pulled inward, causing a squint and double vision.
So the function affected is the movement of the eye ball.
Swallowing is controlled by the glossopharyngeal and vagus nerves, the neck muscles by the spinal accessory nerve, and the tongue by the hypoglossal nerve.

Nissl's granules are dark staining granules found in the cell body (cyton) and dendrites of a neuron, but not in the axon.
Electron microscopy has shown that they are masses of rough endoplasmic reticulum with attached ribosomes and free ribosomes (polysomes).
They are the site of the intense protein synthesis needed by the neuron, e.g. for making neurotransmitters and enzymes.
So Nissl's granules are now identified as ribosomes.

In a myelinated nerve fibre, the axon is covered by a myelin sheath formed by Schwann cells (in the peripheral nervous system).
The myelin sheath is not continuous; it has small gaps at regular intervals between adjacent Schwann cells. These gaps are the nodes of Ranvier.
The neurilemma (outer covering of the Schwann cells) remains continuous over the nodes.
The impulse jumps from node to node (saltatory conduction), which makes conduction much faster.
So at a node of Ranvier, the myelin sheath is discontinuous.

The cornea is the transparent front part of the sclera. It has no blood vessels (it is avascular); it gets oxygen and nutrients from the aqueous humour and tears.
This lack of blood vessels keeps it transparent, and is also why corneal transplants are rarely rejected.
Tears are secreted by the lacrimal glands, not by the cornea; cataract is the opacity of the lens, not the cornea.
So the characteristic feature is that blood circulation is absent in the cornea.

In the dark, the rods are very sensitive because rhodopsin has built up and the pupil is wide.
On entering bright light, the excess light bleaches rhodopsin and dazzles the eye. Then the pupil constricts, rhodopsin is bleached and the cones take over, and vision becomes normal after a short time.
This adjustment of the eye to a change in light intensity is called adaptation (here, light adaptation).
Accommodation is focusing on objects at different distances.
So it is an example of adaptation.