Past Entrance Papers

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(b) Parasympathetic neural signals (a component of the autonomic nervous system) decrease the rate of heartbeat, speed of conduction of action potential, and thereby the cardiac output.

Cardiac output (CO) is the amount of blood pumped by the heart in one minute. The cardiac output is regulated by the autonomic nervous system. Autonomic nervous system has two main divisions which are sympathetic and parasympathetic nervous systems. Neural signals through sympathetic nerves increase cardiac output. The parasympathetic neural signals decrease cardiac output. Adrenal medullary hormones are produced by the adrenal glands. These hormones increase cardiac output.

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(a) is correct match which is as follows. Rods and cones are photoreceptor cells of eye.

Blind spot is the area where there is absence of any photoreceptor cells in the eye.
Fovea is the area in the eye where cones are densely packed.

Iris is the visible coloured portion of the eye.

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(b) Option (b) is correct because the organ of Corti is located on the basilar membrane. The coiled portion of the labyrinth is called the cochlea. The Eustachian tube connects the middle ear cavity with the pharynx. The middle ear contains s ossicle called stapes that is attached to the oval window of the cochlea.

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(a) Snow blindness in the Antarctic region is due to inflammation of the cornea due to high dose of UV-B radiation. It is a painful, temporary loss of vision due to overexposure to the sun’s UV rays. It also called ‘photo keratitis'(photo = light, keratitis= inflammation of the comea).

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(d) Rods and cones are two types of photoreceptor in the human retina. Cones are very sensitive to operate in daylight. They are capable of colour vision and responsible for high spatial acuity. Rods are very sensitive in dim light and do not mediate colour vision and have a low spatial acuity.

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(a) Hypothalamus is the thermoregulatory centre in the brain and it maintains the constant body temperature of \(37^{\circ} \mathrm{C}\). The hypothalamus contains a number of centres, which control body temperature. Corpus callosum is the thick band of nerve fibres that divide the cerebrum into left and right hemispheres. Medulla oblongata is the component of the hindbrain. It receives and integrates signals from the spinal cord and sends them to the cerebellum. Cerebrum is the large part of the brain and consists of two hemispheres.

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(d) Crista ampullaris is a sensory organ of rotation. It maintains the dynamic balance of the body. It is found in the ampulla of each of the semicircular canals of the inner ear.

Otolith organ consists of two structures i.e. saccule and utricle. The projecting ridge of the saccule and utricle is called macula. The otolith organ maintains the static balance of the body.

The crista ampullaris refers to the sensory organ linked to the rotation and is found in the ampullae region of all the semicircular canals in the inner ear. It detects angular deceleration and acceleration. The macula refers to the path of sensory cells which helps in monitoring the position of the head during horizontal acceleration. The saccule and utricle are otolith organs present in the inner ear.

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(c) The statement that cornea consists of dense matrix of collagen and is the most sensitive portion of the eye is correct. Rest statements are incorrect. The correct information about the statements is as follows The outer layer of the wall of eyeball, sclera, consists of a dense connective tissue containing mainly collagen and some elastic fibre. Cornea is convex, transparent layer which is non-vascularised. The cornea is the clear part of eye’s protective covering.

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(a) The muscles that move the eyeball are attached to the sclera. Suspensory ligament of lens – a series of fibers that connect the ciliary body of the eye with the lens, holding it in place.

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(d) Schwann cells and oligodendrocytes form myelin sheath around the axon. Myelin sheath serves as an insulating layer, preventing the loss of energy of the nerve impulse during its passage along the fibre.

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(c) Neurotransmitter is a chemical substance responsible for the transmission of nerve impulse across synapse. It is released by synaptic vesicle into the synaptic cleft. Neurotransmitter binds with protein receptor molecule present on post synaptic membrane causing its depolarisation and generation of action potential.

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(a) Carotene is the source of retinal which is involved in the formation of rhodopsin of rod cells. Retinal, a derivative of vitamin A, is the light-absorbing part of all visual photopigments. Photopigments are occur entirely on the surface of the membrane disc.

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(c) Nissl bodies are present in the soma or cell body of a neuron. When observed under the electron microscope, they appear to be composed of rough endoplasmic reticulum (RER) and free ribosomes hence helping in protein synthesis.

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(b) The limbic system (emotional motor system) is responsible for the experience and expression of emotion but not the movement. It is located in the core of the brain and includes the amygdala, hippocampus, and hypothalamus.

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(c) Photoreceptors in the human eye are unique because they are the only type of sensory cells that are relatively depolarised (about \(-35 \mathrm{mV}\) ) when it is at rest (i.e., in the dark), and hyperpolarised (to about \(70 \mathrm{mV}\) ) in response to adequate light stimulus. Nociceptors respond to potentially damaging stimuli that result in pain. Meissner’s corpuscles are a type of mechanoreceptor, responsible for touch sensitivity. Receptors generally produce graded potentials called receptor potentials.

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(d) The rods contain a photosensitive pigment called the rhodopsin. Rhodopsin is composed of opsin and retinene. The opsin is a protein and is called scotopsin in rhodopsin. The retinene is an aldehyde of vitamin \(\mathrm{A}\) and is called retinal.

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(d) The anterior horns of the spinal cord contains cells with fibres that form the anterior (motor) root end and are essential for the voluntary and reflex activity of muscles they innervate. If the anterior horn motor cells are destroyed, the nerves cannot regenerate and muscles are never useful again.

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(c) A small oval, yellowish area of the retina lying exactly opposite the centre of the cornea is named the macula lutea or yellow spot which has at its middle a shallow depression, the fovea centralis. The fovea centralis has cone cells only. It is devoid of rods and blood vessels. The fovea centralis is the place of most distinct vision.

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(d) Vestibular apparatus is a part of the inner ear that is located above the cochlea. It consists of three semicircular canals, which detect movements of the head, and the utriculus and sacculus which detect the position of the head. It does not play any role in hearing but is responsible for maintaining the balance of the body and posture.

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(d) Language comprehension is a function of the cerebrum. Cerebellum coordinates and controls rapid muscular activities such as running, typing, etc. Although it does not initiate such voluntary movements, but it is an important centre for coordinating movements and for controlling posture and balance. Cerebellum’s function is almost exclusively motor, but it is also implicated in some forms of learning.

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(d) Hypothalamus lies at the base of the thalamus. It provides an anatomical connection between the nervous and endocrine systems by its relationship to the pituitary gland. Hypothalamus is thermoregulatory centre. Hence, it is called the “thermostat” of the body. It keeps body temperature at roughly \(37^{\circ} \mathrm{C}\) by means of a complex thermostat system. Any localised injury to the hypothalamus will, hence, disrupt the regulation of body temperature.

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(d) The correct option is d Retinal is a derivative of Vitamin C. Retinol pigment is an aldehyde of vitamin A. Retinol epithelium shields the retina from excess incoming light.

Retina is the innermost, light sensitive layer of the eyeball, on which images are formed. They contain Photoreceptors which are light-sensitive cells. Rods are responsible to detect light in dim light (black and white) enabling night vision. They contain a pigment called Rhodospin which is light-sensitive. Cones are responsible to detect coloured light. There are green, blue, and red cones. Red is used to respond to long wavelengths. Blue is used to respond to short wavelengths and Green is used to respond to medium lengths. Cones are closely packed to allow the perception of details but cannot work well in dim light.

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(a) The neuromuscular junction connects the nervous system to the muscular system via synapses between efferent nerve fibres and muscle fibres.

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(a) Parasympathetic neural signal reduces both heart rate and cardiac output, through the post ganglionic fibres. These fibres are very short and are cholinergic in nature.

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(c) In the given figure, A is retina which is the innermost layer, containing photoreceptors rods and cones. B is blind spot. Optic nerves pierce through the retina at blind spot. It has no visual cells. C is aqueous humor. It nourishes the cornea and lens both of which are avascular. D is sclera. It is the outermost covering and maintains the shape of the eyeball. It also protects the inner layers of the eye.

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(b)  \(\mathrm{K}^{+}\)ions predominate in the intracellular fluid whereas \(\mathrm{Na}^{+}\)ions predominate in extracellular fluid.

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(c) A – Action potential B – Neurotransmitter C – Receptor D – Synaptic vesicles

An axon terminal contains various neurotransmitters that are released at the small gap between two communicating neurons. This gap is called a synapse. The neuron that sends nerve impulses by releasing neurotransmitters via the axon terminal at the synapse is called a presynaptic neuron.
The synaptic vesicles (C ) are present inside the axon terminals and they contain the neurotransmitters. They get attached to the pre-synaptic membrane and release the neurotransmitter in the synaptic cleft (represented by B) which gets attached to the receptor (represented by A) present on the post-synaptic membrane.

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(b) A – Cerebral hemisphere
B – Thalamus
C – Pons varolii
D – Cerebellum

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(c) Brain is the anterior-most part of the central nervous system. Human brain can be divided into three parts: forebrain, midbrain, and hindbrain. Human hindbrain comprises the pons, cerebellum, and medulla (also called the medulla oblongata).

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(c) Vestibular apparatus is a part of the inner ear that is located above the cochlea. It consists of three semicircular canals, which detect movements of the head, and the utriculus and sacculus which detect the position of the head. It does not play any role in hearing but is responsible for maintaining the balance of the body and posture, thus necessary.

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(a) Epinephrine and norepinephrine are secreted by adrenal medulla in response to stress of any kind and during emergency situations and are called emergency hormones or hormones of flight, or fight.

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(d) 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 \(\left(\mathrm{Na}^{+}\right)\).

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(c) There are two types of photoreceptor cells of retina, namely rods and cones. The rods contain a purplish red protein called the rhodopsin (visual purple), which is a derivative of vitamin-A.

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(a) Hypothalamus is the region of the forebrain in the floor of the third ventricle, linked with the thalamus above and the pituitary gland below. It contains several important centres controlling body temperature, thirst, hunger, eating, water balance, and sexual function. It is also closely connected with emotional activity and sleep and functions as a centre for the integration of hormonal and autonomic nervous activity through its control of the pituitary secretions.

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(c) The hypothalamus is located below the thalamus just above the brainstem. The hypothalamus contains the thirst centre, the hunger centre and the body’s thermostat. Thus damage to the hypothalamus frequently results in water, glucose and temperature imbalances.

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(b) Alzheimer’s disease is a neurological disease characterized by progressive loss of intellectual ability. The disease, which is named after German physician Alois Alzheimer (1864-1915), is associated with general shrinkage of the brain tissue, with deposits of \(\beta\)-amyloid protein and abnormal filaments composed of tau protein in the brain, and changes in the neurotransmitter systems within the brain that include a loss in the activity of cholinergic neurons (neurons releasing acetylcholine). Some inherited forms are associated with a genetic locus on chromosome 21.

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(c) Hypothalamus is the thermoregulatory center of the body. It keeps body temperature at \(37^{\circ} \mathrm{C}\) by means of a complex thermostat system.

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(d) Cornea is a transparent portion that forms the anterior one-sixth of the eye ball. The cornea admits and helps to focus light waves as they enter the eye. The cornea is avascular (i.e., has no blood supply). This part of the eye absorbs oxygen from the air. The cornea was one of the first organs to be successfully transplanted because it lacks blood vessels.

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(b) Action potential is the change in electrical potential that occurs across a plasma membrane during the passage of a nerve impulse. As an impulse travels in a wavelike manner along the axon of a nerve, it causes a localized and transient switch in electric potential across the membrane from \(-60 \mathrm{mV}\) (millivolts; the resting potential) to \(+45 \mathrm{mV}\). It is due to the fact that the sodium channels open and the potassium channels remain closed. As a result, sodium channels permit the influx of \(\mathrm{Na}^{+}\)by diffusion from extracellular fluid to intracellular fluid.

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(b)  A \(\rightarrow\) Perilymph
\(\mathrm{B} \rightarrow\) Tectorial membrane
\(\mathrm{C} \rightarrow\) Endolymph
\(\mathrm{D} \rightarrow\) Sensory hair cells

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(a) Rod cell is a type of light-sensitive receptor cell present in the retina of vertebrates. Rods contain the pigment rhodopsin and are essential for vision in dim light. They are not evenly distributed on the retina, being absent in the fovea and occupying all of the retinal margin. Cone cell is a type of light-sensitive receptor cell, found in the retina of all diurnal vertebrates. Cones are specialized to transmit information about colour and are responsible for the visual acuity of the eye. They function best in bright light. They contain pigment iodopsin. They are not evenly distributed on the retina.

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(d) Nerve is a strand of tissue comprising many nerve fibres plus supporting tissue enclosed in a connective tissue sheath. The signal that travels along the length of a nerve fibre and is the means by which information is transmitted through the nervous system is called nerve impulse. It is marked by the flow of ions across the membrane of the axon caused by changes in the permeability of the membrane, producing a reduction in the potential difference that can be detected as the action potential. The strength of the impulse produced in any nerve fibre is constant.

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(c) Bowman’s gland, also called the olfactory gland is any of the branched tubuloalveolar glands situated in the mucous membrane of the olfactory region of the nasal cavity that produce mucus to moisten the olfactory epithelium and dissolve odour-containing gases.

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(d) The cytoplasm immediately surrounding the nucleus is loaded with protein synthetic machinery and is called perikaryon, dendrites are usually shorter, tapering and much-branched processes which may be one to several. These two are only present in nerve cells.

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(d) When the set point of the hypothalamus is disturbed by high temperature, it stimulates vasodilation and sweating while at low temperature there is vasoconstriction and shivering.

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(b) Bowman’s gland is a type of tubular mucous glands that occurs in the olfactory epithelium. Their ducts open on the olfactory epithelial surface. These gland secret watery mucus to protect and keep the epithelium moist. So, the correct option is b

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(a) Cortisone is one of the main hormones released by the adrenal gland in response to stress. Cortisone suppresses the immune system, thus reducing inflammation and attendant pain and swelling at the site of the injury. Neurotransmitters are endogenous chemicals, that transmit signals across a synapse from one neuron (nerve cell) to another neuron. Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by receptors on other synapses. Whereas norepinephrine, acetylcholine, and epinephrine are neurotransmitters. 

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(a) The autonomous nervous system regulates the secretion of glands whereas, the glands do not regulate the nervous system.

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(c) Options (a), (b) and (d) are reflex actions. Autonomic nervous system is involved in the peristalsis of the intestine which is effected through mysentric plexus.

Sympathetic fibres decrease peristaltic movements while parasympathetic fibres increase these movements.

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(a) Abducens is the sixth cranial nerve that innervates the external rectus muscle of the eyeball. It is responsible for turning the eye outwards. The movement of the tongue is controlled by the hypoglossal nerve. Neck movements is controlled by the facial nerve. Swallowing is by glossopharyngeal.

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(c) Parkinsonism is caused by degenerations of neurons in the Substantia nigra tract which are essentially dopaminergic. This striatum controls muscle tones and coordinates movements. An imbalance is caused by deficiency of dopamine (an inhibitory neurotransmitter) vis-a-vis.

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(b) The neurons are cells specialised to conduct an electrochemical current. Neuron cells do not have the capability of division.

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(c) In the resting nerve fibre, in the external medium (tissue fluid), sodium ions \(\left(\mathrm{Na}^{+}\right)\) predominate, whereas within the fibre (intracellular fluid) potassium ions \(\left(\mathrm{K}^{+}\right)\)predominate. Due to different concentrations of ions on the two sides of the membrane, sodium ions tend to passively diffuse into the nerve fibre and potassium ions tend to diffuse out of the nerve fibre down their electrochemical gradients. The membrane of a resting nerve fibre is, however, more permeable to potassium than to sodium. Because of this selective permeability of the membrane, potassium leaves the nerve fibre faster than sodium enters it. This makes the membrane of the resting nerve fibre polarized, extracellular fluid outside it being electropositive (positively charged) with respect to the cell contents inside it.

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(a) Vagus nerve arises from the side of the medulla oblongata. It innervates the larynx, trachea, oesophagus, stomach, lungs, heart, and intestines. It is a mixed nerve. It controls the visceral sensations and visceral movements, i.e., heartbeat, respiratory movements, peristalsis, sound production, etc. The movement of the tongue is controlled by the hypoglossal nerve as it innervates the muscles of the tongue.

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(c) Cell body of a nerve cell contains basophilic granules called Nissl’s granules. These granules appear to be cisternae of rough endoplasmic reticulum with numerous attached and free ribosomes. They probably synthesize proteins for the cell.

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(b) : At the level of node of Ranvier the myelin sheath is discontinuous but not the neurilemma lining. Actually, myelin sheath is an integral part of Schwann cell – which form a continuous neurilemmal covering. Each Schwann cell wrap around the neurite to form concentric layers of the plasma membrane. But at the level of the junction between two Schwann cells myelin cannot be formed and thus a gap appears.

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(d) A primitive type of nervous system, without brain is found in the form of intra epidermal nerve net. However, there is no brain in Hydra to coordinate the response.

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(b) The rod cells of the eye contain a purplish pigment called visual purple, or rhodopsin. They function in dim light and at night. Bright light splits rhodopsin into a lipoprotein scotopsin and a carotenoid pigment retinene. The splitting of rhodopsin depolarizes the rod cell. In the dark, rhodopsin is resynthesized from scotopsin and retinene. This process is called “dark adaptation.” It makes the rods functional. It takes some time for rhodopsin to be reformed. This is why on entering a dark room during the daytime or on coming out of a well-lighted room at night, we feel blind for a while. When we go from darkness into bright light, we feel difficulty in seeing properly for a moment till rhodopsin is bleached and cones become functional.

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(c) Cornea forms the anterior one-sixth of the fibrous coat. It is transparent, circular, and fully visible from in front. It is composed of a peculiar variety of connective tissue covered externally by stratified nonkeratinized squamous epithelium and internally by simple squamous epithelium. It lacks blood vessels. It is nourished by lymph from an adjacent area.

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(b) The word noise is taken from the Latin word nausea and is defined as an unwanted or unpleasant sound that causes discomfort.
\(\begin{array}{ll}\text { Source } & \text { Intensity (dB) } \\ \text { Breathing } & 10 \\ \text { Broadcasting studio } & 20 \\ \text { Trickling clock } & 30 \\ \text { Library } & 30-35 \\ \text { Normal conversation } & 35-60 \\ \text { Telephone office Noise } & 60-80 \\ \text { Alarm clock } & 70-80 \\ \text { Traffic } & 50-90 \\ \text { Motorcycle } & 105 \\ \text { Jet fly over } & 100-110 \\ \text { Train whistle } & 110 \\ \text { Air craft } & 110-120\end{array}\)

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(c) Only nerve cells do not form layers, they also remain structurally separate from each other (though communicate with each other through synapse). Nerve cells or neurons are the cells specialised to conduct an electrochemical current, nerve tissue is made up of these cells and supporting cells.

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(b) Depolarization of a nerve cell membrane occurs during the passage of an action potential along the axon where the nerve is transmitting an impulse. During depolarization, the activation gates of \(\mathrm{Na}^{+}\) channels open, and the \(\mathrm{K}^{+}\)channels remain closed. \(\mathrm{Na}^{+}\)rush into the axon. Entry of sodium ions leads to depolarization (reversal of polarity) of the nerve membrane so that the nerve fibre contents become electropositive with respect to the extracellular fluid.

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(b) When a nerve fibre is stimulated, its membrane becomes more permeable to sodium ions. Hence, more sodium ions enter the axon than potassium ions leaving it. As a result, the positive and negative charges on the outside and inside of the membrane are reversed. The membrane with reversed polarity is called depolarised.

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(c) Carrot and ripe papaya contain carotene from which vitamin-A is synthesised.
Vitamin-A is necessary for the formation of rhodopsin. The visual pigments in vertebrate eyes are located in the tips of specialised sensory cells called rod cell and cone cells. Rod cells contain rhodopsin and are responsible for black and white vision.

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(b) Trigeminal nerve is the largest \(5^{\text {th }}\) cranial nerve. It has 3 branches –
(i) Ophthalamic, a sensory branch from the skin of the nose, eyelids, forehead, and scalp, and from the conjunctiva and lacrimal glands.
(ii) Maxillary, also sensory branch from the skin and mucous membrane of cheeks and upper lip, and from lower eyelids.
(iii) Mandibular, a mixed branch innervating the lower jaw, lower lip, pinna, and tongue.

Vagus nerve is the \(10^{\text {th }}\) cranial nerve and innervates larynx, trachea, oesophagus, stomach, lungs, heart, and intestines. Facial nerve is the \(7^{\text {th }}\) cranial nerve and innervates muscles of the face and back, taste buds, and salivary glands.

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(b) Sympathetic nervous system is a component of autonomic nervous system consisting of a pair of sympathetic trunks, preganglionic sympathetic fibres, postganglionic sympathetic fibres, and collateral ganglia. It quickens the rate and force of heartbeat while it inhibits secretion of saliva and gastric juice.

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(d) Synapse is the close proximity of the axon of one neuron and the dendrite or cyton of another neuron with a gap of just about \(200 \AA\) in between. A nerve impulse is transmitted across the synapse by the release from the presynaptic membrane of neurotransmitter, which diffuses across the synaptic cleft to the post synaptic membrane. This triggers the propagation of the impulse from the dendrite along the length of the post synaptic neuron.

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(a) Neurons or nerve cells are the structural and functional unit of nervous system. These have a special structure but vary greatly in size and shape. Each neuron has a cell body which encloses cytoplasm and has a nucleus. A number of processes arise from the cell body. There is usually a single axon and a variable number of dendrites. The medullated nerve fibres is composed of a shining, white, fatty substance called myelin.

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(d) A reflex action is a nerve mediated spontaneous, automatic and involuntary response to a stimulus acting through specific receptors without consulting the will of the animal. When a reflex arc consists of only two neurons in an animal (one sensory neuron, and one motor neuron), is defined as monosynaptic.

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(b) Stretching and relaxation of suspensory ligament changes the focal length of lens for accommodation.

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(d) Anesthetic chemicals block the voltage gate sodium and potassium channel.

Today, the most common modern general anesthetics are mixtures of inhalable gases, which include nitrous oxide (laughing gas) and various derivatives of ether, such as Isoflurane, Sevoflurane, and desflurane.

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(c) A Nissl granule is a large granular body found in neurons. These granules are rough endoplasmic reticulum (RER) with rosettes of free ribosomes and are the site of protein synthesis.

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(a) Photosensitive means sensitive to light. The rod cells of retina contain a purplish pigment called rhodopsin. They function in dim light and at night. Rhodopsin consists of a protein component, opsin, linked to a nonprotein chromophore, retinal (or retinene), a derivative of vitamin A. Light falling on the rod is absorbed by the retinal, which changes its form and separates from the opsin component. This initiates the transmission of a nerve impulse to the brain.

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(c) Vagus nerve is the tenth cranial nerve. It arises from the side of medulla oblongata. It innervates the larynx, trachea, oesophagus, stomach, lungs, heart and intestines. It is a mixed nerve. It controls the visceral sensations and visceral movements, i.e., heart beat, respiratory movements, peristalsis, sound production, etc.

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(c) Dorsal part of the cranium is formed of two large and flat frontoparietals which are articulated together by a mid dorsal sagittal suture together and are collectively called frontoparietal. Endo-frontoparietal consists of a frontal bone (in front) and a parietal bone (behind). But now it has been proved that it is only the frontal bone, the parietals are not present in frog due to the absence of neck.

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(a) ‘Fenestra ovalis’ is an oval aperture through which tympanic cavity is connected with auditory capsule which houses the internal ear.

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(a) The blood is supplied into visceral organs by both SNS (sympathetic nervous system) and PNS (parasympathetic nervous system) involuntarily. The sympathetic fibres increase the rate and force of heart beat, constrict most blood vessels and raise the arterial blood pressure. The parasympathetic fibres decrease the rate and force of heart beat, dilate many blood vessels and lower the arterial blood pressure.

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(d) Both sympathetic and parasympathetic nerve fibres innervate visceral organs and coordinate their activity antagonistically, but this is not under body’s conscious control.

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(b) Cornea is non-vascular, i.e. no blood supply so, its transplantation is
outstandingly successful.

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(c) Sympathetic nerves arise from thoracic and lumbar spinal segments.

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(b) Normally, the breathing process (inspiration and expiration) is controlled involuntarily by a breathing centre located in the medulla oblongata. The ventral portion of the breathing centre (inspiratory centre) increases the rate and depth of inspiration while the dorsal and lateral portions of the centre (expiratory centre) inhibit inspiration and stimulate expiration.

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(b) By training, a particular response can be obtained to a stimulus other than the one which normally evokes that response. Such a reflex is known as the conditioned reflex.
The conditioned reflexes were first demonstrated in $1920^{\prime}$ ‘s by the Russian physiologist I.P. Pavlov. He found that the sight and smell of food reflexly cause flow of saliva in hungry animals. He rang a bell every time he offered food to a dog. The bell did not induce salivation by itself in the beginning of the experiment. Gradually, the dog learnt to associate the bell with food. Eventually, mere ringing of bell, without presenting food, induced salivation in the dog. Thus, ringing of bell can substitute sight of food to cause salivation. Pavlov called sound of the bell as conditioned stimulus, salivation in response to bell a conditioned response, food itself as unconditioned stimulus, and salivation in response to food an unconditioned response. A conditioned reflex is established when a new sensory clue (the bell) becomes associated with an inborn reflex (salivation).

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(a) Pupil is the opening which controls the amount of light entering in eye. When light intensity is high, it decrease in size and when light intensity is low it dilates to allow more light in the eye to make eye enable to see the object.

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(d) A small area of the optical part of the retina lying exactly opposite to the centre of the cornea is called the macula lutea, or yellow spot which has a yellow pigment (xanthophyll). The macula lutea has at its middle a shallow depression, the fovea centralis. The fovea has cone cells only, and is the place of most distinct vision. Away form the fovea, the rod and cone cells occur in equal numbers, and at the periphery of the retina, the rods are more numerous than the cones. This is why we see better in dimlight by looking out of the corner of the eye. The point on the retina from where the optic nerve starts is called the blind spot, or optic disc, as it lacks the receptor cells and is insensitive to light.

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(d) At the junction of the sclera and the cornea, the vascular coat sharply bends into the cavity of the eyeball to form a thin, coloured partition. This partition is called iris. It is perforated at the middle by an aperture called pupil. The iris contains two sets of smooth muscles: sphincters and dilators. These muscles regulate the amount of light entering the eyeball by varying the size of the pupil. The sphincter muscles are arranged in rings. Their contraction makes the pupil smaller in bright light so that less light enters the eye. The dilator muscles are arranged in a radial manner. Their contraction widens the pupil in dim light to let in more light. Iris, by regulating the size of the pupil, allows light to pass only through the centre of the lens, which is optically the most effective part.

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(d) Sclera is the external layer of eye that maintains its shape and also protects it. Choroid is the middle layer of eye. Uvea is the pigmented layer of eye that lies beneath sclera and cornea. It comprises of choroid, iris and ciliary body. Retina is the inner layer of eye that contain photoreceptor cells.

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(c) Central nervous system is mostly made up of association neurons.

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(b) Afferent nerve fibres carry impulses from the receptors to the central nervous system. Efferent nerve fibres conduct nerve impulses from the central nervous system to the effector organs such as muscles and glands.

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(a) Vagus nerve is Xth cranial nerve. It is mixed in nature having both sensory and motor fibres.

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(a) Ectoderm forms: Eye (conjunctiva, cornea, lens, retina, iris, ciliary muscles) Mesoderm forms: Sclera & choroid of the eye.

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(d) The ventricles consist of four hollow, fluid-filled spaces inside the brain. The third ventricle is also known as diacoel. The third ventricle consists of a narrow channel between the hemispheres through the area of the thalamus. It is connected by the cerebral aqueduct or aqueduct of Sylvius or iter in the midbrain portion of the brainstem to the fourth ventricle in the pons and medulla. Metacoel is the IV ventricle, rhinocoel is the I ventricle and paracoel is the II ventricles.

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(d) The action of the parasympathetic nervous system is opposite to that of the sympathetic nervous system. If the sympathetic nervous system accelerates an action, the parasympathetic nervous system slows it. However, neither system is exclusively exitatory or inhibitory. The parasympathetic fibres constrict the pupil, decrease the rate and force of heartbeat, dilate many blood vessels, lower the arterial blood pressure, quicken the peristaltic movements, and contract the urinary bladder.

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(a) X nerve i.e. the vagus nerve (mixed) that arises from the side of medulla controls the visceral sensations and movements of the larynx, lungs, heart, stomach, and intestines. IX nerve innervates pharynx and tongue. VIII nerve innervates the internal ear and VII nerve innervates the face, neck, taste buds, and salivary glands.

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(b) The retina consists of both pigmented layer and the sensory layer. The pigment cells reinforce the light absorbing property of choroid in reducing the scattering of light in the eye. The sensory layer consists of rods and cones required for vision.

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(a) Vulture, some other birds, and man have an acute vision.

Acute bird vision derives from the retina’s fovea – the center of sharpest vision – that’s densely packed with about 400,000 photoreceptor cones per square millimeter compared to about 200,000 in humans.