Long Answer Type Questions

Q.1. Explain the mechanism of conduction of the nerve impulse through synapse. (KVS
2015)
Ans. Conduction of nerve impulse through synapse:
(i) Most impulse transmission across the synapse between neurons takes place at the
chemical synapses.
(ii) At a chemical synapse, the membranes of pre and post synaptic neurons are separated by
a fluid filled space called synaptic cleft.
(iii) In the chemical synapse, the axon terminal called synaptic knobs contains numerous
synaptic vesicles in its axoplasm.
(iv) The vesicles contain neurotransmitter responsible for the transmission of the nerve
impulse across the synapse.
(v) When an impulse (action potential) arrives at the axon terminal it stimulates the movement
of the synaptic vesicles towards the membrane, where they fuse to discharge their contents
(neurotransmitter) into the synaptic cleft.
(vi) The released neurotransmitter binds with the protein receptor molecules on the post
synaptic membrane.
(vii) This binding action changes the membrane potential of the post synaptic membrane,
opening ion channels in the membrane and allowing the entry of ions; this causes the
depolarization and generation of action potential in the post-synaptic membrane.

Q.2. Describe the microscopic structure of a neuron with the help of a diagram.
Ans. The neurons system is made up of highly specialized cells called as neurons. A neuron is
composed of three main parts, namely cell body or cyton, dendrite and axon.
(i) Cyton It contains a well-defined nucleus, surrounded by granular cytoplasm called
neuroplasm and all the cell organelles along with, Nissl's granules. Only centrosome is absent
because nerve cells have lost the ability to divide.
(ii) Dendrites These are branched cytoplasmic projections of cyton known as the dendrons
which receives nerve impulses.

(iii) Axon: It is a very long process and projects from a part of cyton axon hillock. The lower
end of the axon is branched and each branch terminates as a bulb like structures called nerve
endings which have swollen synaptic knobs. There knobs possess synaptic vesicles, which
store neurotransmitters. The axons transmit nerve impulses away from the cell body to a
synapse or to a neuromuscular junction.
Q. 3. Explain the following processes:
(i) Polarisation of the membrane of a nerve fibre.
(ii) Depolarisation of the membrane of a nerve fibre.
(iii) Conduction of a nerve impulse along a nerve fibre.
(iv) Transmission of a nerve impulse across a chemical synapse.
Ans. When a neuron is not conducting an impulse, i.e., resting, the axonal membrane is more
permeable to K+ and nearly impermeable to Na +. Similarly, the membrane is impermeable to
negatively charged proteins present in the axoplasm. Consequently, the axoplasm contains
high concentration of K + and negatively charged proteins and low concentration of Na +. In
contrast, the fluid outside contains a low concentration of K+, a high concentration of Na and
thus forms a concentration gradient. These ionic gradients across the resting membrane are
maintained by the sodium-potassium pump. 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.

(ii) Depolarization of the membrane of a nerve fibre:

(a) Stimulation of an axon immediately enhances manifold its membrane permeability to Na +.
As a result, Na+ ions diffuse across the membrane from the extracellular fluid (ECF) where
their concentration is higher, to the interior of the fibre where the concentration is much lower.
But the membrane permeability to K + starts rising somewhat later only, so there is
simultaneous rise in the outward diffusion of K+ from the cell interior having a higher K +
concentration.
(b) These effects lower the overall cation concentration outside and enhance its concentration
inside the membrane.
(c) The membrane is, thus deposited, with its interior becoming electropositive to the exterior.
(d) The depolarization spreads a local current. It induces nearby passive Na+ channels to
open and so as to depolarize the nearby site.
(e) Hence the initial depolarization passes outward over the membrane and spreads out in all
directions from the site of stimulation.

(iii) Conduction of nerve impulse along a nerve fibre:

(a) It is a property of nerve fibre to become excited by a stimulus and then conduct that
stimulus for the required and appropriate response.
(b) In conducting a stimulus, the nerve axon has to pass through resting phase to active
phase and then the recovery phase.

(iv) Transmission of a Nerve impulse across a chemical Synapse:

(a) The physiological junction between two neurons across which nerve impulses can be
transmitted is known as synapse.
(b) Synapse occur between the knob like axon endings of one neuron and the dendrites of cell
body of another.
(c) At the junction of two neurons a narrow fluid filled space called synaptic cleft is present.
(d) Knob like endings of one neuron form many membrane bound vesicles called synaptic
vesicles.
(e) Since they help in the transmission of nerve impulse, they are also called as
neurotransmitters.
(f) When a nerve impulse reaches the axon terminal the synaptic vesicles get stimulated and
release their stored chemicals in the synaptic cleft. These chemicals then diffuse through
these clefts to reach the membrane of the next neurons and stimulate the next neurons.
Q. 4. Explain what are simple sensory receptors and special sensory receptors.
Ans. There are five senses: touch, vision, hearing, smell and taste. While touch is a complex
general sense, the other four are special senses. The general sensory receptors are simple
receptors that are mostly modified dendritic ends of sensory neurons. Such receptors are
present throughout the body: in the skin, mucous membranes, connective tissues and
muscles. These monitor most of the types of general sensory information such as tactile
sensation (a mix of touch, pressure, stretch and vibration), heat, cold, pain and muscle sense
(perception). In contrast, special sensory receptors are distinct receptor cells that are actually
confined to the head region and are highly localized within complex sensory organs like eyes
and ears and tissues of the taste buds and olfactory epithelium. These sensory organs and
tissues are collection of cells of many different types (receptor and non-receptor cells),
working together to accomplish a specific receptive process.
Q. 5. Explain the sense of smell (olfaction).

Ans. Sense of smell (olfaction): Nose contains the receptors of smell, in the mucous coated
thin, yellowish patch of olfactory epithelium. It is located way up at the roof of the nasal cavity
on either sides of the nasal septum.

Fig. Human nose showing olfactory bulb and magnified view of olfactory epithelium

The olfactory epithelium contains three types of cells: (i) millions of olfactory receptor cells; (ii)
columnar supportive cells; (iii) short basal cells. Olfactory receptors are unusual bipolar
sensory neurons. The thin dendrites of each of these neurons run to the surface of the
epithelium where these bear a cluster of about 20 modified cilia which function as receptor
sites. These cilia extend from the olfactory epithelium into the thin coat of nasal mucous
secreted by the supportive cells and olfactory glands. This mucous is a solvent that captures
and dissolves the air borne odour molecules. Once dissolved, the chemicals bind to the
specific receptors on the cilia stimulating he receptor cells. This causes depolarization and
ultimately action potential in the receptor cell. The axons of the olfactory receptors unite to
form the olfactory nerve which transmits the information directly to the overlying olfactory bulb,
a relay station in the brain. Unlike receptor ends of other senses, the axons of the olfactory
receptors directly extend from the outside environment (the nasal cavity) into the olfactory
bulb, a part of the brain. The number of receptors stimulated indicates the strength of the
stimulus.
As with taste, some of the smell, can be really painful. The nasal cavity contains pain
receptors that respond to irritants such as ammonia, vinegar or hot chilly pepper. Impulses
from these pain receptors reach the brain. The brain combines these sensations with those of
smell to identify the odours. Although humans do have a good sense of smell - we can detect
about 10,000 different odours.
Q. 6. Explain the sense of touch.

Ans. Sense of touch: Skin is the sensory organ for touch and is also the largest sense organ.
Our sense of touch allows us to feel light sensation like the touch of a feather as well as a
heavy sensation like a stone falling on the toe. These sensations come from millions of
microscopic simple sensory receptors located all over the skin and associated with the
general sensations of contact or pressure, heat, cold, and pain. The receptors are located at
different levels within the skin and distributed unevenly. Some parts of the body have a large
number of these such as the finger tips, making them more sensitive.
Structurally, these touch receptors are either free dendritic endings or encapsulated dendritic
endings present in the skin (and other parts of the body). When stimulated, these transmit the
sensation to the brain. Given below is a list of some of these receptors present in the
skin.
(i) Free or bare dendritic nerve endings are present throughout the epidermis taking an
extensive branching or "zigzag" form. These respond chiefly to pain and temperature but
some respond to pressure as well. The root hair plexuses, network of free nerve endings that
surround hair follicles, are light touch receptors that detect bending of hairs.
(ii) Meissner's corpuscles are located in the papillary layer of the dermis just below the
epidermis, which respond to touch our skin.
(iii) Pacinian corpuscles are scattered deep in the dermis and in the subcutaneous tissue of
the skin. These are stimulated by deep pressure. These receptors are best suited to monitor
vibrations, on-off pressure.
(iv) Ruffini's corpuscles respond to heat.
(v) Krause's corpuscles are excited by cold and are found in large numbers in the face and
hands.
Q. 7. Explain the sense of taste (gustation).

Ans. Sense of taste (gustation): The sense of taste and smell work closely together. If we
cannot smell something we cannot taste it either. One reason why we cannot taste (or smell)
food well with a common cold is that with the nasal passages inflamed and coated with thick
mucus layer the smell receptors are practically nonfunctional. The receptor cells for taste are
located in taste buds. Humans have about 10,000 taste buds. The majority of taste buds are
located in pockets around the papillae (peg-like projections of the mucous membrane) on the
surface and sides of the tongue, but there are some on the surface of the pharynx and the
larynx. Each taste bud contains about 40 specialized receptor cells or gustatory cells, many
more supporting cells and some basal cells that replace the worn out cells of the taste buds.
Unlike the receptors for smell, that are modified sensory neurons, the receptor cells for taste
are not neurons, but rather specialized cells with slender microvilli on their outer ends. The
microvilli protrude into the surrounding fluids through a narrow opening called the taste pore.
Dissolved chemicals contacting the microvilli bind to specific receptor present on the microvilli,
thereby depolarizing the cell. The dendrites of the associated sensory neurons coil intimately
around the receptor cells and synapse with them so that, when a receptor cell is stimulated
and depolarized, it releases neurotransmitter which leads to the generation of an action
potential in the associated sensory neuron. Each dendrite receives signals from several
receptor cells within the taste bud. Nerve fibers emerging from the taste buds pass to the
brain stem. From here the nerve impulse is relayed to the taste centre in the cerebral cortex of
the brain that perceives the taste sensation.

Normally our taste sensations are complicated mixture of qualities. In humans, there are f our
basic taste senses: sweet, sour, salt, and bitter. The receptors for these four basic tastes have
their areas of greatest concentration on different parts of the tongue: sweet and salty on the
front bitter on the back, and sour on the sides. A few substances stimulate only one of the four
types of receptors, but most stimulate two, three, or all four types to varying degrees. The
sensation and flavour of the food we experience are thus produced by a combination of these
four basic sensations, modified by accompanying sensations of smell, texture and
temperature.