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Neural Control and Coordination Class 11 Notes Biology Chapter 21
Topics and Subtopics in Class 11 Biology Chapter 21 Neural Control and Coordination:
|Section Name||Topic Name|
|21||Neural Control and Coordination|
|21.2||Human Neural System|
|21.3||Neuron as Structural and Functional Unit of Neural System|
|21.4||Central Neural System|
|21.5||Reflex Action and Reflex Arc|
|21.6||Sensory Reception and Processing|
The human body has several organs. These organs cannot perform their functions independently. In order to maintain homeostasis for the normal physiology of the human body, functions of these organs/organ systems in our body must be coordinated, so that they can work in proper manner.
Topic 1 Nervous System : An Overview
Coordination is the process through which two or more organs interact and complement the functions of one another. On the other hand, integration is a process, which makes two or more organs to work as a functional unit in harmony.
For instance, when we do exercise, we observe significant increase in the rate of respiration, heart beat, blood flow, sweating, etc., to meet enhanced need of nutrients and energy for increased activities of lungs, heart, muscles and many other body organs, when we stop exercising, we witness that the increased activities of lungs, heart, nerves, kidneys, muscles, etc., gradually return to normal. Thus, during exercise, functions of various organs of the body are coordinated and integrated.
In higher animals (including human), two types of systems have been developed for the control, coordination and integration, i.e., nervous system and endocrine system. The nervous system provides an organised network of point to point connections for quick neural coordination. The endocrine system provides chemical integration through hormones.
The neural system is the control system of the body which consists of highly specialized cells called neurons. The sensory neurons detect and receive information from different sense organs (receptors) in the form of stimuli and transmit the stimuli to the Central Neural System (CNS) through sensory nerve fibers. In CNS the processing of information is done and a conclusion is drawn.
The conclusion is sent to different organs (effectors) through motor nerves. These effectors then show the response accordingly.
The neural or nervous system is present in most of the multicellular animals. Its complexity increases from lower to higher animals.
Invertebrates have relatively simpler nervous system than the vertebrates.
Human Neural System
The whole nervous system of human being is derived from embryonic ectoderm.
The human neural system is divided into two parts
(i) the Central Neural System (CNS)
(ii) the Peripheral Neural System (PNS)
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 (brain and spinal cord).
The nerve fibres of the PNS are of two types
(a) Afferent Fibres They transmit impulses from tissues/organs to the CNS.
(b) Efferent Fibres They transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.
The PNS is divided into two divisions i.e., somatic neural system and autonomic neural system.
The somatic neural system relays impulses from the CNS to skeletal muscles while, the autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.
The autonomic neural system is further classified into sympathetic neural system and parasympathetic neural system.
Neuron (Structural and Functional Unit of Neural System)
Neurons are the longest cells in the body. Human neural system has about 100 billion neurons. Majority of the neurons occur in the brain. Fully formed neurons never divide and remain in interphase throughout life.
A neuron is a microscopic structure composed of three major parts
1. Cell Body (Cyton or Soma)
Like a typical cell it consists of cytoplasm, nucleus and cell membrane. The cytoplasm has typical cell organelles like mitochondria, Golgi apparatus, rough endoplasmic reticulum, ribosomes, lysosomes, certain granular bodies, neurofibrils, neurotubules and Nissl’s granules.
Presence of neurofibrils and Nissl’s granules is the characteristic to all neurons. Neurofibrils play a role in the transmission of impulses.
2. Dendrites (Dendrons)
Dendrites are usually shorter, tapering and much branched processes that project out of the cell body. They also contain Nissl’s granules and may be one to several in number.
They conduct nerve impulses towards the cell body and are called afferent processes (receiving processes).
Axon is a single, usually very long process of uniform thickness. The part of cyton from where the axon arises is called axon hillock (most sensitive part of neuron).
The axon contains neurofibrils and neurotubules but does not have Nissl’s granules, cell organelles and granular bodies. The axon ends (distal end) in a group of branches, the terminal arborization (axon terminals).
When terminal arborisations of the axon meet the dendrites of another neuron to form a synapse, each branch terminates as a bulb-like structure called synaptic knobs, which possess mitochondria and secretory vesicles (containing chemicals called neurotransmitters). The axons transmit nerve impulses away from the cell body to a synapse or to a neuromuscular junction.
There are two types of axon
In myelinated nerve fibres Schwann cells form myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated nerve fibres are found in cranial and spinal nerves.
In non-myelinated nerve fibres Schwann cell does not form myelin sheath around the axon and are without nodes of Ranvier. They are commonly found in autonomous and somatic neural systems.
Types of Neurons on the Basis of Structure
Based on the number of axon and dendrites, the neurons are divided into three types
(i) Multipolar neurons These neurons have several dendrites and an axon. They are found in cerebral cortex.
(ii) Bipolar neurons These neurons have one dendrite and one axon. They are present in the retina of eye.
(iii) Unipolar neurons These neurons have cell body with one axon only. These are found usually in the embryonic stage.
Main Properties of Neural Tissue
The neural tissue has two outsandingproperties
(a) Excitability It is the ability of nerve cells to generate an electrical impulse in response to a stimulus by altering the normal potential difference across their plasma membrane.
(b) Conductivity It is the ability of nerve cells to rapidly transmit the electrical impulse as a wave from the site of its origin along their length in a particular direction.
Functions of Neural System
The nervous system serves the following important functions
(i) Control and coordination Nervous system controls and coordinates the working of all parts of the body so that it functions as an integrated unit. This is achieved by three overlapping processes, i.e., sensory input, integration and motor output.
(ii) Memory Nervous system stores the impressions of previous stimuli and retrieves (recalls) these impressions in future. These impressions are referred to as the experiences or memory.
(iii) Homeostasis Nervous system helps in the maintenance of the body’s internal environment, i.e., homeostasis.
Generation and Conduction of Nerve Impulse
Nerve impulse is a wave of bioelectric/electrochemical disturbance that passes along a neuron during conduction of an excitation.
Impulse conduction depends upon
(i) Permeability of axon membrane (axolemma).
(ii) Osmotic equilibrium (electrical equivalence) between the axoplasm and Extracellular Fluid (ECF) present outside the axon.
The generation of a nerve impulse is the temporary reversal of the resting potential in the neuron.
It occurs in following three steps
Polarisation (Resting Potential)
In a resting nerve fibre (a nerve fibre that is not conducting an impulse), the axoplasm (neuroplasm of axon) inside the axon contains high concentration of K+ and negatively charged proteins and low concentration of Na+.
(i) In contrast, the fluid outside axon contains a low concentration of K+ and a high concentration of Na+ and thus form a concentration gradient.
(ii) These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump, which transports 3Na+ out wards and 2K+ inwards (into the cell).
(iii) 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.
(iv) The electrical potential difference across the resting plasma membrane is called as the resting potential. The state of the resting membrane is called polarised state.
Depolarisation (Action Potential)
When a stimulus of adequate strength (threshold stimulus) is applied to a polarised membrane, the permeability of the membrane to Na+ ions is greatly increased at the point of stimulation (site A).
(i) This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. The polarity of the membrane at the site A is thus, reversed and said to be depolarised.
(ii) The electrical potential difference across the plasma membrane at the site A is called the action potential, another name of nerve impulse.
(iii) At adjacent sites, e.g., site B, the membrane (axon) has positive charge (still polarised) on the outer surface and a negative charge on its inner surface.
(iv) The stimulated negatively charged point on the outside of the membrane sends out an electrical current to the positive point next to it. As a result, a current flows on the outer surface from site B to site A, while on the inner surface current flows from site A to site B.
This process (reversal) repeats itself over and over again and a nerve impulse is conducted through the length of the neuron.
(i) The rise in the stimulus-induced permeability to Na+ is extremely short-lived. It is quickly followed by a rise in permeability to K+.
(ii) Within a fraction of a second, Na+ influx stops and K+outflow begins until the original resting state of ionic concentration is achieved. Thus, resting potential is restored at the site of excitation, which is called repolarisation of the membrane. This makes the fibre once more responsive to further stimulation.
(iii) In fact until repolarisation occurs neuron cannot conduct another impulse. The time taken for this restoration is called refractory period.
* When an impulse travels along a myelinated neuron, depolarisation occurs only at the nodes of Ranvier. It leaps over the myelin sheath from one node to the next. This process, is called saltatory conduction.
* This process accounts for the greater speed of an impulse travelling along a myelinated neuron than along a non-myelinated one. It is upto 50 times faster than the non-myelinated nerve fibre.
A nerve impulses is transmitted from one neuron to another through junctions called synapses. It is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron.
There are mainly two types of synapses
(i) The membranes of pre and post-synaptic neurons are in very close proximity (i.e., in continuity). The continuity is provided by the gap junction (small protein tubular structures) between the two neurons.
(ii) In electrical synapse, there is minimal synaptic delay because of the direct flow of electrical current from one neuron into the other across these synapses.
Thus, impulse transmission across an electrical synapses is always faster than that across a chemical synapse. In such synapses, transmission of impulse is very similar to impulse conduction along a single axon.
(iii) Electrical synapses are rarely found in our system. It is found in cardiac muscle fibres, smooth muscle fibres of intestine and the epithelial cells of lens.
The membranes of pre and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft.
A brief description of the mechanism of synaptic transmission is given below
(i) When an impulse (action potential) arrives at a pre-synaptic knob, calcium ions from the synaptic cleft enter the cytoplasm of the pre-synaptic knob.
(it) The calcium ions cause the movement of the synaptic vesicles to the surface of the knob.
The synaptic vesicles are fused with the pre-synaptic (plasma membrane and get ruptured (exocytosis) to discharge their contents (neurotransmitter) into the synaptic cleft.
(iii) The neurotransmitter of the synaptic cleft binds with specific protein receptor molecules, present on the post-synaptic membrane.
(iv) This binding action changes the membrane potential of the post-synaptic membrane, opening channels in the membrane and sodium ions to enter the cell. This causes the depolarisation and generation of action potential in the post-synaptic membrane. Thus, the impulse is transferred to the next neuron.
(v) The new potential developed may be either excitatory or inhibitory.
Topic 2 Human Nervous System
The human neural system can be categorised to
(a) Central Nervous System (CNS)
(b) Peripheral Nervous System (PNS)
Central Nervous System (CNS)
It is the integrating and command centre of the nervous system which consists of the brain and spinal cord (as discussed earlier).
The brain is the central information processing organ of our body and acts as the ‘command and control system’.
It controls the following activities
(i) The voluntary movements and balance of the body.
(ii) Functioning of vital involuntary organs, e.g., Lungs, heart, kidneys, etc.
(iii) Thermoregulation, hunger and thirst.
(iv) Circardian (24 hrs) rhythms of our body.
(u) Activities of several endocrine glands and human behaviour.
(vi) It is also the site for processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.
The brain is the anterior most part of the central neural system, which is located in the cranium
(cranial cavity) of the skull.
Protective Coverings of the Brain
It is covered by three membranes or meninges (cranial meninges)
(i) The outermost membrane, the duramater is the tough fibrous membrane adhering close to the inner side of the skull.
(ii) The middle very thin layer called arachnoid membrane (arachnoid mater).
(iii) The innermost membrane, the piamater is thin, very
delicate, which is in contact with the brain tissue.
The human brain weights from 1200-1400 g. The human neural system has about 100 billion neurons, majority of them occur in the brain.
The human brain is divisible into three parts
(i) Forebrain (ii) Midbrain (iii) Hindbrain
i. The forebrain
It consists of Olfactory lobes The anterior part of the brain is formed by a pair of short club-shaped structures, the olfactory lobes. These are concerned with the sense of smell.
Cerebrum It is the largest and most complex of all the parts of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres connected by a large bundle of myelinated fibres the corpus callosum.
* The outer cover of cerebral hemisphere is called cerebral cortex. The cerebral cortex is referred to as the grey matter due to its greyish appearance (as neuron cell bodies are concentrated here).
The cerebral cortex is greatly folded. The upward folds, gyri, alternate with the downward grooves or sulci. Beneath the grey matter there are millions of medullated nerve fibers, which constitute the inner part of the cerebral hemisphere. The large concentration of medullated nerve fibres gives this tissue an opaque white appearance. Hence, it is called the white matter.
* Lobes A very deep and a longitudinal fissure, separates the two cerebral hemispheres. Each cerebral hemisphere of the cerebrum is divided into four lobes, i.e., frontal, parietal, temporal and occipital lobes.
In each cerebral hemisphere, there are three types of junctional areas
* Sensory areas receive impulses from the receptors and motor areas transmit impulses to the effectors.
* Association areas are large regions that are neither clearly sensory nor motor in junction. They interpret the input, store the input and initiate a response in light of similar past experience. Thus, these areas are responsible for complex functions like memory, learning, reasoning and other intersensory associations.
Distction the posterioventral part of forebrain.
Its main parts are as follows
* Epithalamus is a thin membrane of non-nervous tissue. It is the posterior segment of the diencephalon.
* The cerebrum, wraps around a structure called thalamus, which is a major coordinating center for sensory and motor signalling.
The hypothalamus, that lies at the base of thalamus contains a number of centres, which control body temperature, urge for eating and drinking. It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones.
The inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus, etc., form a complex structure (limbic lobe or limbic system) that are involved in the regulation of sexual behaviour,expression of emotional reactions, e.g„ excitement, pleasure,rage and fear and motivation,
The midbrain is located between the thalamus hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passes through, the midbrain.
The dorsal portion of the midbrain mainly consists of two pairs (i.e., four) of rounded swellings (lobes) called corpora quadrigemina.
The hindbrain consists of
(a) Pons consists of fibre tracts that interconnect different regions of the brain.
(b) Cerebellum is the second largest part of the human brain (means litde cerebrum). It has very
convoluted surface in order to provide the additional space for many more neurons.
(c) Medulla (oblongata) is connected to the spinal cord and contains centres, which control respiration, cardiovascular reflexes and gastric secretions.
* Midbrain and hindbrain form the brain stem. It is the posterior part of the brain that continues with the spinal cord.
* Out of the twelve pairs of cranial nerves (in higher vertebrates), ten pairs come from the brain stem.
(i) It forms the posterior part of the CNS, running mid-dorsally in the neural canal of the vertebral column. In an adult, the spinal cord is about 42-45 cm long. Its diameter varies at different levels.
(ii) The spinal cord is formed of two types of nervous tissue, i.e., grey matter and white matter.
(iii) The grey matter is surrounded by white matter, which consists of groups of myelinated axons.
(iv) The spinal nerve tracts are divisible into two, ascending (conducting sensory impulses towards brain) and descending (conducting motor impulses from brain).
(v) Spinal cord conducts impulses to and from the brain and controls most of the reflex activities and provides a means of communication between spinal nerves and the brain.
Reflex Action and Reflex Arc
The entire process of response to a peripheral nervous stimulation, that occurs involuntarily, i.e., without conscious effort or thought and requires the involvement of a part of the central nervous system is called a reflex action. The nervous pathway taken by nerve impulses in a reflex action is called reflex arc.
Types of Reflexes
Reflexes are categorised into two
(i) Unconditioned (inborn reflexes and transmitted through heredity) breast feeding and swallowing.
(ii) Conditioned (acquired after birth, i.e., adopted during the course of life time.) e.g., Withdrawl of a body part (like limb) which comes in contact with objects that are extremly hot, cold, pointed or animals that are scary or poisonous.
Mechanism of Reflex Action
(i) The reflex pathway comprises atleast, one afferent (receptor) neuron and one efferent (effector) neuron arranged in a series.
(ii) 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 spinal cord).
(iii) The efferent neuron then carries signals from CNS to the effector. The stimulus and response in this way forms a reflex arc, e.g., Knee jerk reflex as shown above in the diagram.
Peripheral Nervous System (PNS)
The peripheral nervous system consists of
1. Somatic Neural System (SNS)
2. Autonomic Neural System (SNS)
1. Somatic Neural System
The somatic neural system contains nerves which relay impulses from CNS to skeletal muscles. These can be further categorised into cranial (from brain) and spinal nerves on the basis of their origin.
These nerves emerge specifically from the forebrain and brain stem.
* Trochlear is smallest and thinnest nerve and possess difficulty in surgical operations.
* Trigeminal is also called dentist nerve. It is the largest cranial nerve. At its origin it is associated with ‘Gasserian Ganglion’.
* Facial nerve is associated with geniculate ganglion at its origin.
Their functions in comparative manner in a nut shell are given below
ii. Spinal Nerves
All spinal nerves are mixed, having sensory and motor fibres in approximately equal numbers. In humans, 31 pairs of spinal nerves are present as Cervical (8 pairs), Thoracic (12 pairs), Lumber (5 pairs), Sacral (5 pairs), Coccygeal (1 pair).
There are 10 pairs of cranial nerves in fishes and amphibians and 12 pairs in rest of the higher chordates.
There are 10 pairs of spinal nerves found in fishes and amphibians and 31 pairs in humans.
Based on their functions, the nerve fibres of PNS are divided into two groups, i.e., afferent fibres and efferent fibres.
The afferent nerve fibres transmit sensory impulses from tissues/organs to the CNS and form the sensory or afferent pathway. The efferent nerve fibres transmit motor impulses from CNS to the concerned tissues/organs and form the motor or efferent pathways.
2. The Autonomic Neural System (ANS)
The autonomic neural system consists of the sympathetic and parasympathetic nervous system. The former is called thoraco-lumber outflow and the latter is called craniosacral outflow depending upon their origin.
Topic 3 Sensory Reception and Processing
The sensory organs (receptors) enable us to detect all types of changes in the environment and send appropriate signals to the CNS, where all the inputs are processed and analysed. Signals are then sent to different centres of the brain.
The most complex sensory receptors consist of numerous sense cells, sensory neurons and associated accessory structures. For example, eye (sensory organ for vision) and the ear (sensory organ for hearing).
The organ of sight are a pair of eyes in human.
The eyes are situated in the deep protective bony cavities, called the orbits or eye sockets of the skull.
Parts of an Eye
The adult human eye ball is nearly spherical in structure. It consists of tissues present in three concentric layers
(i) Outermost fibrous layer composed of sclera and cornea.
(ii) Middle layer consists of choroid, ciliary body and iris.
(iii) Innermost layer consists df retina.
(i) Sclera is an opaque outermost covering, composed of dense connective tissue that maintains the shape of the eyeball and protects all the inner layers of the eye.
(ii) Cornea is a thin transparent, front part of sclera, which lacks blood vessels but is rich in nerve endings.
(i) Choroid is a pigmented layer (bluish) present beneath the sclera. It contains numerous blood vessels and nourishes the retina. The choroid layer is thin over the posterior two-thirds of the eye ball, but it becomes, thick in the anterior part to form the ciliary body.
(ii) The eye ball contains a transparent crystalline structure called lens. Ciliary body holds the lens in position, stretching and relaxation of ciliary body changes the focal length of the lens for accomodation.
(iii) Iris forms a pigmented circle of muscular diaphragm attached to the ciliary body in front of the lens. Its pigment gives eye its colour.
The movement of muscle fibres of iris controls the size (diameter) of pupil.
(iv) Pupil is the aperture surrounded by the iris. It contains two types of smooth muscles, circular muscles (sphincters) and radial muscles (dilators) of ectodermal origin.
(v) Sympathetic stimulation causes the radial muscles to contract and the pupil to dilate or get larger. Parasympathetic stimulation causes the circular muscles to contract and the pupil to constrict.
The inner layer is the retina and it contains three layers of cells from inside to outside, i.e., ganglion cells, bipolar cells and photoreceptor cells.
The photoreceptors or visual cells are of two types, i.e., rods (rod cells) and cones (cone cells). Both of these cells contain light sensitive proteins called the photopigments.
The twilight (scotopic) vision is the function of the rods. These cells contain a purplish-red protein called the rhodopsin (visual purple), which contains a derivative of vitamin-A.
The daylight (photopic) vision and colour vision are functions of cones. There are three types of cones, which possesses characteristic photopigments that respond to red, green and blue lights.
The sensation of different colours are produced by various combinations of these cones and their photopigments. In case of equal stimulation of these cones, a sensation of white light is produced.
The optic nerves are connected with the brain. These nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eye-ball. Photoreceptor cells (rods and cones) are not present in that region and hence, it is called blind spot, as no image is formed at this spot.
Macula Lutea and Fovea Centralis
At the posterior pole of the eye lateral to the blind spot, there is a small oval, yellowish area of the retina called the macula lutea or yellow spot, which has at its middle a shallow depression, the fovea centralis (fovea).
The fovea is a thinned out portion of the retina where only the cones are densly packed. It is the point where the visual acuity (resolution) is the greatest.
Contents of the Eye
(i) Aqueous Humour The space between the cornea and lens is called the aqueous chamber, which contains a thin watery fluid called aqueous humour.
(ii) Vitreous Humour The space between the lens and retina is called the vitreous chamber, which is filled with a transparent get called the vitreous humour.
Mechanism of Vision
In human eyes, the vision is called binocular vision (i.e., both the eyes can be focused on a common object).
(i) Retina receives light rays (in visible wavelength) through the cornea and lens generate impulses in rods and cones.
(ii) The photosensitive compounds (photopigments) in the human eye are composed of opsin (a protein) and retinal (an aldehyde of vitamin-A).
(iii) The received light induces dissociation of the retinal from opsin resulting in changes in the structures of the opsin. This causes the changes in the permeability of membrane.
As a result, the potential differences are generated in the photoreceptor cells. This produces a signal that generates action potential in the ganglion cells through the bipolar cells.
(iv) These impulses (action potentials) are transmitted by the optic nerves to the visual cortex of the brain.
(v) In brain, neural impulses are analysed and the image formed on the retina is recognised (based on earlier
memory and experience).
(i) Cataract This is a eye disease generally occur in older people (above 60 years). Lens becomes opaque due to disease or ageing. It leads to blindness. It can be corrected by wearing suitable glasses or by replacing the defective lens with a normal lens from a donor.
(ii) Myopia (near or short sightedness) It occurs due to convexity of lens or longer eye ball, which results in an image of distant objects being formed in front of the retina, and can be corrected by wearing spectables or concave lenses.
(iii) Hypermetropia (far or long sightedness.) The image of nearer object becomes blurred. It is due to image being formed beyond the retina due to eye ball being short or lens being flattened. It can be corrected by wearing convex or convergent lenses.
(iv) Presbiopia It generally occurs after 40 years. The loss of elasticity in the eye lens occurs so that near objects (written or printed words) are not correcdy visible. It can be correct’d by convex/bifocal lenses.
Ears are a pair of statiocoustic organs meant for both sensory functions, i.e., hearing and maintenance of body balance.
The ears are located on the sides of the head.
In most mammals, the ear is a flap of tissue also called pinna. It is a part of auditory system.
The mammalian ear can be anatomically divided into three major sections
1. External Ear
The external ear consists of pinna and the auditory canal (external auditory meatus), which collect sound waves and channel them to tympanic membrane (ear drum) separating the outer ear from the middle ear.
The auditory canal leads inwards and extends upto the tympanic membrane (the ear drum).
There are very fine hairs and wax-secreting sebaceous glands in the skin of the pinna and the meatus. The tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside.
2. Middle Ear
The middle ear contains three ossicles called malleus (hammer), incus (anvil) and stapes (stirr-up), 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 (a membrane beneath the stapes) of cochlea.
These ossicles increase the efficiency of transmission of sound waves to the inner ear.
The middle ear also opens into the Eustachian tube, which connects with the pharynx and maintains the pressure on either sides of the ear drum. It also enables you to ‘pop’ your ears when you change altitude.
3. Inner Ear
The inner ear consist of a labyrinth of fluid-filled chambers within the temporal bone of the skull. The labyrinth consists of two parts the bony and membranous labyrinths. The bony labyrinth is a series of channels. Inside the channels, membranous labyrinth lies, which is surrounded by a fluid called perilymph.
The membranous labyrinth is filled with a fluid called endolymph. The coiled portion of the labyrinth is called cochlea.
The membranes constituting cochlea (the Reissner’s and basilar), divide the bony labyrinth into two large canals, i.e., an upper vestibular canal (scala vestibuli) and a lower tympanic canal (scala tympani).
These (both) canals are separated by a small cochlear duct called scala media. The vestibular and tympanic canals contain and the cochlear duct is filled with endolymph.
At the base of the cochlea, the scala vestibuli ends at the oval window while, the scala tympani terminates at the round window, which opens to the middle ear.
Organ of Corti
The floor of the cochlear duct, the basilar membrane bears the organ of Corti. It contains the mechanoreceptors of the ear. The hair cells are present in rows on the internal side of the organ of Corti, that act as auditory receptors. The basal end of the hair cell is in close contact with the afferent nerve fibres.
A large number of processes called stereo cilia are projected from the apical part of each hair cell. Above the rows of hair cells is a thin elastic membrane called tectorial membrane.
(i) The inner ear also contains a complex system called vestibular apparatus (located above the cochlea). It is composed of three semicircular canals and the otolith organ consisting of the saccule and utricle.
(ii) Each semicircular canal lies in a different plane at right angles to each other. The membranous canals are suspended in the perilymph of the bony canals. The base of canals is swollen and is called ampulla, which contains a projecting ridge called crista ampullaris, which has hair cells.
(iii) The saccule and utricle contain a projecting ridge called macula. The crista and macula are the specific receptors of the vestibular apparatus responsible for the maintenance of balance of the body and posture.
Mechanisms of Hearing
(i) Sound waves from the environment are received by the external ear and it directs them to the ear drum.
(ii) The ear drum vibrates due to sound waves and the vibrations are send to oval window through the ear ossicles (malleus, incus and stapes).
(iii) The vibrations are passed through the oval window on to the fluid of the cochlea, where they generate waves in the lymph.
(iv) The waves in the lymph induce a ripple in the basilar membrane.
(v) These movements of the basilar membrane bend the hair cells, pressing them against the tectorial membrane. Due to this, the nerve impulses are
generated in the associated afferent neurons. These impulses are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognised.
(i) Meniere’s Syndrome It is a hearing loss due to pathological distension of membranous labyrinth.
(ii) Eustachitis It occurs due to inflammation of Eustachian tube.
(iii) Tympanitis It is due to inflammation of ear drum.
(iv) Otalgia Pain occurs in ear.
(v) Otitis media Acute infection in middle ear.
CBSE Class 11 Biology Chapter-21 Important Questions
1 Marks Questions
1. How does an impulse travel across a synapse?
Ans. The impulse travel across a synapse from the axons to the cell body and dendrites to the next neuron.
2.How many pairs of cranial nerves are present in man?
Ans. 12 pairs.
3.What is saltatory conduction?
Ans. Saltatory conduction refers to the type of conduction of nerve impulse by a myelinated nerve fibre, where the action potential jumps form one node of Ranvier to the other.
4.Name the band of nerve fibers that joins the two cerebral hemisphere in mammals.
Ans. Corpus callosum.
5.What is threshold stimulus for nerve cell?
Ans. The minimum intensity / strength of a stimulus required to initiate depolarization of neuron is called threshold stimulus.
6.What is a compound eye?
Ans. In insects the eye is composed of many independent visual elements called commatidia such an eye is called compound eye.
7.What types of neurons are found in dorsal root of spinal nerve?
Ans. Sensory neurons.
8.What is the basic unit of neural system?
9.Why is blind spot devoid of the ability for vision?
Ans. Blind spot has no photoreceptor cells – rods or cones.
10.Name the fluid present in membranous labyrinth.
11.Name the area of ratina where only cones are densly packed.
12.Name the inner most meanings of the brain.
13.To which part of the brain communication and memory arc associated?
14.Name the bundle of fibers that connect two cerebral hemisphere in human being.
Ans. Corpus callosum.
15.Name the photo pigment present in the rod cells.
16.Why can impulses flow only in one direction?
Ans. Because each synoapse allows impulse to cross it in a single direction.
17.Where is hypothalamus located in the brain?
Ans. At the base of thalamus.
2 Marks Questions
1.What is a reflex?
Ans. Reflex is an involuntary action performed by muscle under the direction of spinal cord in response to the stimulus. It is an automatic response to a stimulus which is not under conscious control. A large number of activities of animals are conducted by reflexes e. g. Respiration, peristalsis, watering of the mouth, secretion of saliva in the mouth, etc.
2.What happens when the membrane of a nerve cell carries out a sodium pump?
Ans. When the membrane carries a sodium pump, it carries three sodium ions from the axoplasm to the cell exterior:
– It transfers two potassium ions exchange from the ECF to the cell interior.
– The exterior is positively charged.
3.What are the events that take place at the point of stimulation of axon?
Ans. At the point of stimulation the membrane permeability changes; it becomes freely permeable to Na+ ions.
There is a rapid inflow of Na+ ions and the interior / axoplasm becomes positively charged and the exterior becomes negatively charged.
This condition is known as depolarized state and the potential difference across the membrane is known as action potential.
Now the current flows through the axoplasm from the depolarized region to due next polarised region and through the ECF from the polarised region to the depolarised region.
4.Give parts of neuron.
Ans. Neuron is a microscopic structure made up of 3 parts-
a) Cell body – In contains cytoplasm with typical cell organelles and some granular bodies called Nissl’s granules.
b) Dendrites – The short fibers that branch repeatedly and project out of the cell body. They transmit impulse towards the cell body or cyton.
c) Axon – It is a long fibre. Its distal end is branched. Each branch terminates into bulblike structure called as synoptic knob.
5.Describe the role & location of ciliary body in human eye.
Ans. Location:- The choroid becomes thick where the cornea & sclera meet; It is called ciliary body.
Function: The ciliary body continues in front of the lens to form an opaque structure called iris.
6.What is mosaic vision?
Ans. This type of vision is found insects due to compound eye. A complete image of the object as seen by the compound eye is formed by a number of small lineages each of which is contributed by an ommatidium. Such an image formed by many bits of images is called a mosaic image and the vision as the mosaic image vision.
7.Where does cerebrospinal fluid occur in our body? Mention two if its function.
Ans. Cerebrospinal fluid is found in the subarachnoid space between arachnoids and parameter of the menings around the brain and spinal cord and also in the cavities of the brain.
1) It protects brain and spinal card by acting as a cushion to absorb shocks.
2) It helps in removing harmful metabolites drugs etc. away from the brain.
8.What is the chemical and difference between rods & cones?
|1.||These are more secretive to light and are meant for vision in dim light.||These are meant for vision in bright light.|
|2.||They do not have the ability to make colored image||They have ability to make colored image.|
|3.||They contain the visual pigment rhodopsin.||These contain the pigment iodopsin.|
9.Why are gray matter and white matter contained in human nervous system named so?
Ans. Gray matter contains spindle, pyramidal, cell bodies with grayish brown appearance and hence called as gray matter.
White matter contains millions of myelinated axons; the large amount of myelin gives this tissue an opaque white appearance and hence called white matter.
10. Fill in the blanks in the different columns A to D:
Equalise the pressure on either sided of ear drum.
Regulate amo9un of light to pass into the eye.
Ans. (a) To collect sound waves
(b) Eustachian tube
(c) Colour vision
3 Marks Questions
1.Differentiate between dorsal spinal roots and ventral spinal roots.
|Dorsal spinal Roots||Ventral spinal Roots|
|1.||They are made of sensory (afferent) nerves.||They are made of motor (efferent) nerves.|
|2.||They have dorsal root ganglia.||They have no ganglia.|
|3.||Their cell bodies are located in dorsal root ganglia.||The cell bodies of ventral spinal nerve root is located in ventrolateral horn of grey matter.|
2.Describe human neural system.
Ans. It is divided into two parts-
1) Central Neural system (CNS) – CNS includes brain and spinal cord. This is the site of information processing and control.
2) Peripheral neural system (PNS) – PNS consists of all nerves of the body associated with the CNS. Nerve fibers of PNS are of two types i.e. afferent fibers and efferent fiber.
(a) Afferent nerve fibers transmit impulses from tissues / organs to CNS.
(b) Efferent nerve fibers transmit impulses from CNS to concerned peripherel tissues / organs.
PNS is further divided into –
(1) Somatic neural system – It relays impulse from CNS to skeletal muscles.
(2) Autonomic neural system – ANS transmits impulses from CNS to involuntary organs as well as the smooth muscles of body It is again divided into two parts –
a) sympathetic neural system
b) Para sympathetic neural system.
3.Why do giant squids have very thick nerve fiber?
Ans. The velocity of a nerve impulse in a nerve fiber depends on two factors i. e. on its myelinated and also on the thickness of the fibers. The impulses travel faster in thicker nerve fibers since giant squids are very large sized aquatic animals they have thick nerve fibers.
4.Where are synaptic vesicles found? Name their chemical contents? What is the function of these contents?
Ans. Synaptic vesicles are found in the bulbous expansion called synaptic knob, at the nerve terminal-
Each synaptic vesicle contains as many as 10,000molecules of a neurotransmitter substance that is responsive for transmission of nerve impulse across the synapse.
When a wave of depolarization reaches the presynaptic membrane, the voltage gated calcium channels concentrated at the synapse open & Ca++ ions diffuse into the terminal form the surrounding fluid.
– The Ca++ = ions stimulate the synaptic vesicles to move to the terminal membrane, fuse with it and then and then rupture by exocytosis into the cleft.
– This neurotrarmitter diffuses across the synapse and stimulates the membrane of the next neuron.
5.Give the location and function in the human eye, of the following –
(i) cornea (ii) Iris (iii) Vitreous humor
Ans. 1) Cornea – It is the dome – shaped part of sclera that is transparent and more curved.
Function – It refract light towards retina.
2) Iris – It is the colored (pigmented) at front and formed by choroid.
Functions :- (i) It encloses pupil.
(ii) Iris contains cilliary muscles which regulate the size of pupil and controls the amount of light.
3) Vitreous humor – It is present in posterior chamber of eye.
Functions:- (i) It helps in shape to the eye & supports retina & lens.
(ii) It refracts the light rays.
6.Why are nerve impulses conducted more rapidly in myelinated nerve fiber than in a non – myelinated one? Explain.
Ans. In a myelinated nerve fiber, the lipid rich myelin acts as an insulator and depolarization occurs in the nodes of Ranvier where myelin sheath is absent. Since the action potential jumps from one node of Ravines to another, the conduction becomes faster and such a type of conduction is called saltatory conduction.
In a non–myelinated fiber, the depolarization occurs all along its length and hence conduction becomes slower.
7. Observe the diagram given right and answer the following questions :
(i) Label the parts A and B
(ii) Give the function of C and D.
(iii) Name the layers which wrap this organ.
Ans. (i) A : Cerbrum
B : Corpus callosum
(ii) C : Balancing of body and maintain posture
D : Vomiting, coughing, breathing, salivation or any other correct answer (any one).
(iii) Piameter, arachnoid and duramater.
5 Marks Questions
1.Draw a labeled diagram to show the structural view of human ear in the sectional view.
2.What is meant by the resting membrane potential of neuron. How do ion channels & sodium – potassium pumps contribute to the resting potential?
Ans. Resting membrane potential.
– The electrical potential difference across the membranes of a resting neuron is called resting membrane potential.
– The membrane is polarized, with a negative interior and positively charged exterior.
– The permeability of membrane to K+ ions is greater than its permeability to Na+ ions.
– The negatively charged protein molecules can cross the membrane.
– The sodium pump transports 3 Na+ ions to the exterior, while in exchange only 2K+ ions comes inside.
– Hence the surface carries a positively charge, which the interior negatively charged.
3.reflex arc. Taking one example, describe the functioning of the various components of a spinal.
Ans. A Reflex arc is the specific neural pathway from stimulus to reflex. Components of Reflex arc are –
(1) Receptors – These are the organs / tissues which receive the stimulus and send it as an impulse.
(2) Sensory or afferent nerves – These are neurons which conduct the impulse from the receptor to the central Nervous system (spinal cord)
(3) Relay or intermediate neurons – They are neurons which conduct the impulse from the afferent neurons to the efferent neurons.
(4) Effectors / motor neurons – These neurons conduct the impulse from the spinal cord/ relay neurons to the effectors organ concerned.
(5) Effectors – It is the organ / tissue or gland that functions accordingly.
NCERT TEXTBOOK QUESTIONS FROM SOLVED
1. Briefly describe the structure of the following:
(a) Brain (b) Eye (c) Ear
Solution: (a) Brain: The brain acts as control and command system of the body. It is protected by skull and is covered by three meninges. It is divisible into three main regions: forebrain, midbrain and hindbrain.
(i) Forebrain – It consists of three regions:
(a) Olfactory lobes: These are a pair of very small, solid club-shaped bodies which are widely separated from each
other. They are fully covered by cerebral hemispheres.
(b) Cerebrum – It is the largest and most complex of all the parts of human brain. A deep cleft divides the cerebrum into right and left cerebral hemispheres, connected by myelinated fibres, the corpus callosum.
(c) Diencephalon – It encloses a slit-like cavity, the third ventricle. The thin roof of this cavity is known as the epithalamus, the thick right and left sides as the thalami, and floor as the hypothalamus.
(ii) Midbrain – It is located between thalamus/ hypothalamus of forebrain and pons of hindbrain. Its upper surface has two pairs of rounded protrusious called corpora quadrigemina and two bundles of fibres called crura cerebri.
(iii) Hindbrain – It consists of:
(a) Cerebellum – The second largest part of the human brain is the cerebellum. It consists of two lateral cerebellar hemispheres and central worm-shaped part, the vermis. The cerebellum has its grey matter on the outside, comprising three layers of cells and fibres. It also has Golgi cells, basket cells and granule cells.
(b) Pons varolii – An oval mass, called the pons varolii, lies above the medulla oblongata. It consists mainly of nerve fibres which interconnect different regions of the brain.
(c) Medulla oblongata – It extends from the pons varolii above and is continuous with the spinal cord below. The mid brain, pons varolii and medulla oblongata are collectively called brain stem.
(b) Eye: Eye is a hollow spherical structure composed of three coats:
– Outer fibrous coat
– Middle vascular coat
– Inner nervous coat
(i) Fibrous coat: It is thick and protects the eyeball. It has two distinct regions – sclera and cornea. Sclera covers most of the eye ball. The sclera or white of the eye contains many collagen fibres. Cornea is a transparent portion that forms the anterior one – sixth of the eyeball. The cornea is avascular (i.e., lacks blood supply).
(ii)Vascular coat: It comprises of 3 regions : choroid, iris, ciliary body.
(a) Choroid : It lies adjacent to sclera and contains numerous blood vessels and pigmented cells.
(b) Iris: The iris is a circular muscular diaphragm containing the pigment giving eye its colour. It extends from the ciliary body across the eyeball in front of the lens. It 2. has an opening in the centre called the pupil.
It contains two types of smooth muscles, circular muscles (sphincters) and radial muscles (dilators), of ectodermal origin.
(c) Ciliary body: Behind the peripheral margin of the iris, the vascular coat is thickened to form the ciliary body. It is composed of the ciliary muscles and the ciliary processes.
(iii) Nervous coat: It consists of retina which is neural and sensory layer of an eye ball. It consists of three layers; ganglion cells, bipolar cells and photoreceptor cells (rods and cones).
Lens: It is a transparent, biconvex, elastic structure that bends light waves as they pass through its surface. It is composed of epithelial cells that have large amounts of clear cytoplasm in the form of fibres.
Chambers of eyeball: The lens, suspensory ligament and ciliary body divide the eye into an anterior aqueous chamber and a posterior vitreous chamber which are filled with aqueous humour and vitreous humour respectively.
(c) Ear: There are three portions in an ear:
(i) External ear: It further has 2 regions: pinna and external auditory canal or meatus.
(a) Pinna: The pinna is a projecting elastic cartilage covered with skin. Its most prominent outer ridge is called the helix. The lobule is the soft pliable part at its lower end composed of fibrous and adipose tissue richly supplied with blood capillaries. It is sensitive as well as effective in collecting sound waves.
(b) External auditory canal: It is an S-shaped tube leading inward from the pinna. It is a tubular passage supported by cartilage in its exterior part and by bone in its interior part.
(ii) Middle ear: It consists of 3 small bones called ear ossicles – malleus, incus and stapes, which are attached to one another and increase efficiency of transmission of sound waves to inner ear.
(iii) Internal ear: It consists of bony and
2. Compare the following:
(a) Central neural system (CNS) and Peripheral neural system (PNS).
(b) Resting potential and action potential.
(c) Choroid and retina.
Solution: (a) CNS: It lies along the mid-dorsal axis of the body. It is a hollow, dorsally placed structure and comprises of brain and spinal cord. It is a centre of information processing and control.
PNS: Nerves arising from the central nervous system constitute the peripheral nervous system. It carries information to and from the CNS. It includes spinal nerves and cranial nerves.
(b) Resting potential: Outside the plasma membrane of a nerve fibre is the extracellular fluid which is positively charged with respect to the cell contents inside the plasma membrane. A resting nerve fibre shows a potential difference between inside and outside of this plasma membrane. This difference in the electrical charges across the plasma membrane is called the ‘resting potential’. A membrane with resting potential across it, is said to be electrically polarized. Action potential : Action potential is another name of nerve impulse. The contents inside a cell at the excited state becomes positively charged with respect to extracellular fluid outside it. This change in polarity across the plasma membrane is known as an action potential. The membrane with reversed polarity across it is said to be depolarized.
(c) Choroid: Choroid lies adjacent to the sclera and contains numerous blood vessels that supply nutrients and oxygen to the other tissues especially of retina. It contains abundant pigment cells and is dark brown in colour.
Retina: It is the neural and sensory layer of the eye ball. It is a very delicate coat and lines the whole of the vascular coat. Its external surface is in contact with the choroid and its internal surface with vitreous humour. It contains ganglion cells, bipolar cells and photoreceptor cells. membranous labyrinth. Membranous labyrinth consists of three semicircular ducts, utricle, saccule and cochlea.
3. Explain the following processes:
(a) Polarisation of the membrane of a nerve fibre.
(b) Depolarisation of the membrane of a nerve fibre.
(c) Conduction of a nerve impulse along a nerve fibre.
(d) Transmission of a nerve impulse across a chemical synapse.
Solution: (a) Polarisation of the membrane of a nerve fibre : In the resting (not conducting impulse) nerve fibre the plasma membrane separates two solution of different chemical composition but having approximately the same total number of ions. In the external medium (tissue fluid), sodium ions (Na+) and Cl– ions predominate, whereas within the fibre (intracellular fluid) potassium ions (K+) predominate. The differential flow of the positively charged ions and the inability of the negatively charged organic (protein) ions within the nerve fibre to pass out cause an increasing positive charge on the outside of the membrane and negative charge on the inside of the membrane. This makes the membrane of the resting nerve fibre polarized, extracellular fluid outside being electropositive (positively charged) with respect to the cell contents inside it.
(b) Depolarisation of the membrane of a nerve fibre: During depolarisation, the activation gates of Na channels open, and the K channels remain closed. Na+ rush into the axon. Entry of sodium ions leads to depolarisation (reversal of polarity) of the nerve membrane, so that the nerve fibre contents become electropositive with respect to the extracellular fluid.
(c) Conduction of a nerve impulse along a nerve fibre: Nervous system transmits information as a series of nerve impulses. A nerve impulse is the movement of an action potential as a wave through a nerve fibre. Action potentials are propagated, that is, self-generated along the axon. The events that set up an action potential at one spot on the nerve fibre also transmit it along the entire length of the nerve fibre. The action potential then moves to the neighbouring region of the nerve fibre till it covers the whole length of the fibre.
(d) Transmission of a nerve impulse across a chemical synapse: At a chemical synapse, the membranes of the pre- and post- synaptic neurons are separated by a fluid- filled space called synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and burst to release their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors, present on the post- synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.
4. Draw labelled diagrams of the following:
(a) Neuron (b) Brain
(c) Eye (d) Ear
5. Write short notes on the following:
(a) Neural coordination (b) Forebrain
(c) Midbrain (d) Hindbrain
(e) Retina (f) Ear ossicles
(g) Cochlea (h) Organ of Corti
Solution: (a) Neural coordination : When higher animals respond to various stimuli, each response to a specific stimulus generally involves many organs (parts) of their bodies. Therefore, it is necessary that all the concerned organs (parts) of the body should work in a systematic manner to produce the response. The working together of various organs (parts) of the body of multicelullar organism in a proper manner to complement the functions of each other is called coordination. This is achieved by three overlapping processes of nervous system-sensory input, integration and motor output.
(b) Forebrain: It consists of: Olfactory lobes, the paired structures concerned with the sense of smell. Cerebrum which is the largest and most complex of all the parts of the human brain. It is divided by a cleft into left and right cerebral hemispheres which are connected by a large bundle of myelinated fibres the. corpus callosum. The outer cover of cerebral hemisphere is called cerebral cortex. It consists of sensory and motor areas. Hypothalamus region of forebrain contains centres which control body temperature, hunger and also contains group of neurosecretory cells.
(c) Midbrain: The midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passess through the midbrain. The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina. Midbrain and hindbrain form the brain stem.
(d) Hindbrain: The hindbrain comprises pons, cerebellum and medulla. Pons consists of fibre tracts that interconnect different regions of the brain. Cerebellum has very convoluted surface in order to provide the additional space of many more neurons. The medulla of the brain is connected to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.
(e) Retina: Retina is the inner layer of an eye and it contains three layers of cells-from inside to outside – ganglion cells, bipolar cells and photoreceptor cells. There are two types of photoreceptor cells, namely, rods and cones. These cells contain the light-sensitive proteins called the photopigments. The daylight (photopic) vision and colour vision are functions of cones and the twilight (scotopic) vision is the function of the rods. The rods contain a purplish-red protein called the rhodopsin or visual purple, which contains a derivative of Vitamin A. 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.
(f) Ear ossicles : There is a small flexible chain of three small bones called as ear ossicles – the malleus (hammer shaped), the incus (anvil shaped) and the stapes (stirrup shaped) in the middle ear. Malleus is attached to the tympanic membrane on one side and incus on the other side. Incus in turn is connected with the stapes. Malleus is the largest ossicle, however stapes is the smallest ossicle.
(g) Cochlea : It is the main hearing organ which is connected with saccule. It is a spirally coiled tube that resembles a snail shell in appearance. It tapers from a broad base to an almost pointed apex.
(h) Organ of Corti: It is a structure located on the basilar membrane which contains hair cells that act as auditory receptors. The hair cells are present in rows on the internal side of the organ of Corti.
(i) Synapse : It is the junction between the axon of one neuron and the dendrite or cyton of another neuron for transmission of nerve impulse.
6. Give a brief account of
(a) Mechanism of synaptic transmission.
(b) Mechanism of vision.
(c) Mechanism of hearing.
Solution: (a) Mechanism of synaptic transmission: Refer answer 3 (d)
(a) Mechanism of vision: The light rays in visible wavelength focused on the retina through the cornea and lens generate potentials (impulses) in rods and cones. Light induces
dissociation of the retinal from opsin resulting in changes in the structure of the opsin. This causes membrane permeability changes. As a result, potential differences are generated in the photoreceptor cells. This produces a signal that generates action potentials in the ganglion cells through the bipolar cells. These action potentials (impulses) are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analysed and the image formed on the retina is recognised based on earlier memory and experience.
(b) Mechanism of hearing : The external ear receives sound waves and directs them to the ear drum. The ear drum vibrates in response to the sound waves and these vibrations are transmitted through the ear ossicles (malleus, incus and stapes) to the oval window. The vibrations are passed through the oval window on to the fluid of the cochlea, where they generate waves in the lymphs. The waves in the lymphs induce a ripple in the basilar membrane. These movements of the basilar membrane bend the hair cells, pressing them against the tectorial membrane. As a result, nerve impulses are generated in the associated afferent neurons. These impulses are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognised.
7. Answer briefly.
(a) How do you perceive the colour of an object?
(b) Which part of our body helps us in maintaining the body balance?
(c) How does the eye regulate the amount of light that falls on the retina?
Solution: (a)In humans, colour vision results from the activity of cone cells, a type of photoreceptor cells. 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, sensation of white light is produced. Yellow light, for instance, stimulates green’and red cones approximately to equal extent, and this is interpreted by the brain as yellow colour.
(b) Ears (cristae and maculae present in internal ears).
(c) 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 pupil. Contraction of sphincter muscles makes the pupil smaller in bright light so that less light enters the eye. Contraction of dilator muscles widens the pupil in dim light so that more light goes in eye to fall on retina.
8. Explain the following.
(a) Role of Na+ in the generation of action potential.
(b) Mechanism of generation of light-induced impulse in the retina.
(c) Mechanism through which a sound produces a nerve impulse in the inner ear.
Solution: (a) The action potential is largely determined by Na+ ions. The action potential results from the following sequential events
(i) Disturbance caused to the membrane of a nerve fibre by a stimulus results in leakage of Na+ into the nerve fibre.
(ii) Entry of Na+ lowers the trans-membrane potential difference.
(iii) Decrease in potential difference makes the membrane more permeable to Na+ than to K+ ions so that more Na+ enter the fibre than K+ leave it.
(iv) Accumulation of Na+ in the nerve fibre initiates depolarisation (action potential), making the axonic contents positively charged relative to the extracellular fluid.
(v) With continued addition of Na+ the potential reaches zero and then plus 40-50 millivolts. This is the peak of action potential.
(vi) Permeability of a depolarised membrane to Na+ then rapidly drops, there are now as many Na+ on the inside of the membrane as on the outside.
(b) Refer answer 6 (b)
(c) Refer answer 6 (c)
9. Differentiate between
(a) Myelinated and non-myelinated axons
(b) Dendrites and axons
(c) Rods and cones
(d) Thalamus and Hypothalamus
(e) Cerebrum and Cerebellum
Solution: (a) Differences between myelinated and non-myelinated axons are as follows:
(b) Axon and dendrites can be differentiated as follows:
(c) The differences between rods and cones are as follows:
(d) Thalamus and hypothalamus can be differentiated as follows:
(e) Cerebrum and cerebellum can be differentiated as follows:
10. Answer the following.
(a) Which part of the ear determines the pitch ofa sound?
(b) Which part of the human brain is the most developed?
(c) Which part of our central neural system acts as a master clock?
Solution: (a) The receptor cells in the organ of Corti (Internal ear).
(b) Cerebrum (cerebral hemispheres).
(c) Pineal gland present in diencephalon of forebrain acts as a master clock, which maintains biological rhythm.
11. The region of the vertebrate eye, where the optic nerve passes out of the retina, is called the
(a) fovea (b) iris
(c) blind spot (d) optic chiasma
Solution: (c) blind spot
12. Distinguish between
(a) Afferent neurons and efferent neurons
(b) Impulse conduction in myelinated nerve fibre and unmyelinated nerve fibre
(c) Aqueous humour and vitreous humour
(d) Blind spot and yellow spot
(e) Cranial nerves and spinal nerves
(b) Refer answer 9(a)