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

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(c)

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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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(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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(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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(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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(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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(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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(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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(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) 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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(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 \(200Å\) 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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(c)

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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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(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) ‘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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(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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(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.