NCERT Books Free Download for Class 11 Biology Chapter 21 - Neural Control and Coordination

(a) Brain

Ans: The skull protects the human brain. There are three layers of meninges in the skull, the outermost one is the dura mater, which is tough and fibrous. The next one is the middle one and it is called the arachnoid, which is very thin and delicate. The last and the third one (which is directly attached to the brain) is the pia mater, this layer is rich in blood supply and consists of a lot of veins. There are three major parts to the brain:  

• Forebrain: The forebrain consists of the cerebral cortex, hypothalamus, and thalamus.  

• Midbrain: Located between the thalamus and hypothalamus of the frontal lobes, it is the brain's intermediate portion.  

• Hindbrain: It consists of the pons, cerebellum, and medulla.  

1. Forebrain: 

It consists of three major parts known as Hypothalamus, Thalamus and Cerebrum. 

• Thalamus: It is located in the superior part of the midbrain and consists of grey matter. Its main function is to send sensory and motor impulses to the cerebrum and also control feelings like pain, joy etc. 

• Hypothalamus: This part consists of optic chiasma, a point where optic nerve fibers cross. The infundibulum is a structure present behind the Hypothalamus. It houses Pituitary Gland. Hypothalamus’s main function is to regulate body temperature, blood pressure, appetite, hemostasis. Hypothalamus also regulates sexual behaviour. The hormones released by the pituitary gland are regulated by Hypothalamus. 

• Cerebrum: It is the major part of the brain. It is divided from the middle by the septum and each side is called a cerebral hemisphere. These two hemispheres are connected by nerve fibers. The cerebral hemispheres inner hollow walls contain the inner medulla and outer cortex. 

• The cerebral cortex has convolutions and the greater are the convolutions the greater is the intelligence of the person. 

• The cerebral cortex is responsible for functions like memory, communication and intersensory associations. 

• The CerebralMedulla contains axons of nerve fibers. Amygdala and Hippocampus found in the Cerebral hemispheres are named as limbic system or the limbic lobe. 

• The role of cerebrum is the center of memory, consciousness, voluntary actions and will power. 

2. Hindbrain: 

It is made up of three main components: pons varolii, medulla oblongata and cerebellum.

• Pons Varolli: It is situated right above medulla oblongata. It is a thick bundle of white nerve fibers. It is responsible for controlling the function of cerebellum lobes and also controls breathing (pneumotaxic center).

• Medulla Oblongata: It is a cone shaped structure located at the base of the skull. It is connected to the spinal cord. Its main function is to carry nerve impulses from the spinal cord to the brain. It is also responsible for activities like breathing, heartbeat etc. 

• Cerebellum: It is a major part of the hindbrain. It consists of two layers- the inner medulla (white matter) and the outer cortex (gray matter). The cerebellum is connected to cerebrum and medulla oblongata through tracts of cerebellar medulla or white matter. 

Its main function is to control the features of balancing and muscular activity of the body. It controls voluntary actions leading to movement of muscles. 

(b) Eye

Ans: The human eye is one of the most important organs of the body responsible for vision. It is located in the bony socket of the skull. The human eyeball is almost cylindrical in shape. There are three layers to the eyeball. X-rays can reveal the existence of a dense connective tissue layer that surrounds the eyeball called the sclera. This layer includes the cornea on its anterior portion. A blue-coloured layer at the middle of the choroid is composed of blood vessels. As the eyeball approaches the middle, the choroid layer becomes thin but becomes thicker underneath. Iris, the coloured portion of the eye, is formed from the pigmented ciliary body. An eyeball is supported by a clear crystalline lens whose position is maintained by ligaments attached to the ciliary body.  Iris muscle fibers control the diameter of the pupil, the pupil's opening surrounding the lens.  

The retina is the inner layer of the eye, which is composed of three layers of neural cells beginning on the outside and continuing inwards - ganglion cells, bipolar cells, and photoreceptor cells. Both rods and cones can serve as photoreceptor cells. A cone is responsible for vision in daylight (photopic) and color vision, and a rod is responsible for vision in twilight (scotopic). Ganglion cells from the optic nerve fibers are connected to the brain by the optic nerve in each eye.  Let us study the function of each part one by one. 

1. Outer fibrous coat: It is a protective covering that helps in maintaining the shape of the eye. It consists of two parts: Cornea and Sclera. 

• Cornea: It is a transparent coating made up of stratified epithelium (conjunctiva). It also covers the inner lining of eyelids. Its main function is to converge light that enters the eye towards the lens.

• Sclera: It is made of dense white fibrous tissues and forms the white part of the eye. It is responsible for maintaining and supporting the structure of the eyeball.

2. Middle Vascular coat -This cost consists of three parts - Iris, Ciliary Body and Choroid

• Iris: It is found at the junction of the cornea and sclera. It is pigmented in nature and the color of the iris is imparted by pigmented cells of the choroid. The colors can be black, brown, green or blue. Iris is composed of two muscles important for its function - Radial and Circular Muscles. It is responsible for the amount of light entering into the eye and adjusting the size of the eye according to it. Circular muscles cause contraction of the eye under bright light whereas Radial muscles cause relaxation of the eye under dim light. 

• Ciliary Body: It is present behind the iris and contains muscles responsible for changing the shape of lens when focussed on near objects. It is also responsible for the formation of aqueous humor. 

• Choroid: It is a continuation of the inner part of sclera, made up of fibrous connective tissues. It’s main function is to supply the retina with oxygen. 

3. Inner Neurosensory Coat - The retina makes the innermost layer of the eyeball. The retina is in contact with choroid from outside and vitreous humor on inside. 

4. The external surface consists of four layers: 

• Pigmented layer - It is made up of cells bearing dark brown pigment. 

• Photoreceptors layer - They are of two types - Rods and Cones.

• Rods - They are shaped like a rod and hold purplish-red pigment called Rhodopsin. They are sensitive to dim light and provide vision only in the dark (scotopic vision). 

• Cones - These are conical in shape and contain pigment called Iodopsin. These are sensitive to bright lights therefore are responsible for daylight vision. These cones respond to green, red and blue light. 

5. Blind spot - The point where optic nerves exit towards the brain does not contain any photoreceptor cells, this point is called as blind spot.

6. Lens - The lens present in the eye is biconvex in nature. Its structure is elastic and transparent. Lens is present right behind the iris and it is protected by a thin membrane known as lens capsule. The lens is held in place with the help of ligaments. Lens along with ligaments separate the eyeball into two chambers known as - Aqueous chamber and Vitreous chamber. 

• Aqueous Chamber: The space between the lens and cornea contains a fluid which is watery and thin in nature. 

• Vitreous Chamber: The space between the retina and the lens contains a thick transparent gel called vitreous humor. 

(c) Ear

Ans: It is a sensory organ that protects the body from danger, enables hearing and also helps in maintaining the balance of the body. It can be divided into three parts- 

1. Outer ear - The outer ear is made up of the pinna and external auditory meatus (canal). The middle ear consists of the eustachian tube and the otoliths. As sounds are created in the air, the pinna collects its vibrations. An external auditory meatus runs from the tympanic membrane (eardrum) inward. In the pinna and meatus skins, hairs are very fine and wax glands are present. Connective tissues cover the outside of the tympanic membrane and mucous membrane covers the inside.  

2. Middle ear -  The middle ear is divided into three ossicles—the malleus, the incus and the stapes. The incus and the stapes are linked together in a chain-like arrangement. This complex of bones is attached to the tympanic membrane and the oval window of the cochlea by the malleus and stapes. It increases the efficiency of sound waves being transmitted to the inner ear through ear ossicles. Middle ear cavities connect to the pharynx through a Eustachian tube. As the pressure is equalized on both sides of the eardrum, the Eustachian tube works.  

3. Inner ear -  Also referred to as a labyrinth. Bony labyrinths and membranous labyrinths exist within the labyrinth. The bony labyrinth is filled with perilymph, whereas the membrane labyrinth is filled with endolymph. The vestibular apparatus and cochlea are the two parts of the labyrinthine membrane. There are three semicircular canals and an otolith in the vestibular apparatus (the macular nerve forms the sensory pathway between the utricle and saccule). Several semicircular canals lie at right angles to one another in a different plane. Perilymph surrounds the bony canals, permitting the membranous canals to travel. Canals are swollen at the bottom and have thick ridges called ampullae, which attach to a projection called crista ampullaris. Maculas are ridges that project from the saccule and utricle. In addition, the vestibular apparatus maintains posture and balance when the crista and macula are activated.  

An extension of Sacculus is the Cochlea. Besides being the main organ for hearing, it also controls balance. Three membranes make up the cochlea. A hearing organ called the organ of Corti is found on the basilar membrane, which is covered in hair cells. 

2. Compare the following:  

(a) Central neural system (CNS) and Peripheral neural system (PNS) 

Ans: The difference between the central neural system (CNS) and Peripheral Nervous System is given below:-

(b) Resting potential and action potential  

Ans: The difference between resting potential and action potential is given below:-

(c) Choroid and Retina  

Ans: The difference between choroid and retina is given below:-

3. Explain the following processes:  

(a) Polarization of the membrane of a nerve fiber   

Ans: The membrane becomes polarized when its resting potential changes. The concentration inside the axon when in resting condition. Thus, potassium ions move faster from the inside to the exterior than sodium ions. A positively charged membrane becomes negatively charged inside and a positively charged membrane becomes negative. An example of this would be polarized nerves or polarized membranes.  Here are the steps that take place during polarization of the membrane of a nerve fiber: 

• When the depolarised region of the nerve becomes polarized, the amount of K+ ions outside the nerve fibers is more and the membrane of the axon has more number of Na+ ions. 

• As the membrane gets polarized, it becomes permeable to K+ ions and impermeable to Na+ ions. 

• 3 Na+ ions are passed outside the axon while the 2 K+ ions are passed inside the axon with the help of Sodium-Potassium Pump through active transport. 

• The inner side of the axon becomes electronegative and the outer side of the axon becomes electropositive due to the transfer of K+ and Na+ ions. 

(b) Depolarization of the membrane of a nerve fiber  

Ans: An action potential occurs when a nerve fiber receives an electrical stimulus. As sodium ions pass through the membrane, potassium ions are less permeable. Consequently, the nerve fiber becomes positively charged inside, and negatively charged outside. This depolarization of the membrane is referred to as depolarization.  

Depolarisation of the membrane of a nerve fiber takes place in the following steps: 

• Axon has more concentration of K+ ions while Na+ has more concentration outside the Axon. 

• When the nerve fiber is triggered due to stimulus the permeability of Na+ ions and K+ ions is reversed. 

• Na+ ions permeability increases across the membrane. 

• The outer side of the membrane acquires a negative charge while the inner side of the membrane acquires a positive charge. 

• This creates action potential due to the depolarization of the membrane of the nerve fiber. 

(d) Conduction of a nerve impulse along with a nerve fiber 

Ans: Nerve fibers are divided into two types - myelinated and unmyelinated. Since Schwann cells surround the axon of a myelinated nerve fiber and form the myelin sheath, the impulse travels back and forth rapidly in the myelinated nerve fiber. Ions cannot pass through the myelin sheath. The nerve fibers do not exchange ions and depolarise efficiently along their entire length as a result. Ranvier's nodes occur only at some points. A normal unmyelinated nerve fiber experiences ionic exchange along its full length, which then causes repolarization of depolarized areas and depolarization of other areas.  Conduction of a nerve impulse along a nerve fiber takes place in the following steps: 

• Consider a site A on the depolarised region, here, the outer part of the membrane is negatively charged and the inner part of the membrane is positively charged. 

• Consider site B, the charges are reversed, here, the outer part of the membrane is positively charged and the inner part of the membrane is negatively charged. 

• Therefore, the flow of current at site A is on the inner surface of the membrane from A to B and at site B, the flow of current is on the outer surface from B to A. This completes the circuit of current flow.

• This results in the conduction of impulse at site B and causes site B to depolarize. 

• And results in the conduction of impulse at site B and causes site A to depolarize. 

• Site C is adjacent to site B which will be polarized when site B is in a depolarized state.

(d) Transmission of a nerve impulse across a chemical synapse  

Ans: On chemical synapses, there is a fluid-filled space between pre-and postsynaptic neurons, called a synaptic cleft. After receiving an impulse, synaptic vesicles move toward the plasma membrane and fuse with the plasma membrane in the synaptic cleft, where they release their neurotransmitters. A number of receptors are present on the postsynaptic membrane, which binds to released neurotransmitters. Postsynaptic neurons form new potentials in response to ion channels opened by this binding. An excitatory or inhibitory potential can be developed.  

4. Draw labeled diagrams of the following:  

Ans:

(a) Neuron  

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(b) Brain 

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(c) Eye

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 (d) Ear  

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5. Write short notes on the following:  

(a) Neural coordination   

Ans: A neural system facilitates the interaction and complementing of the activities of two or more organs. Interconnected and interdependent are all of the body's physiological functions. Coordinating and integrating all the organ's functions, the brain and endocrine system work together. Brain systems provide fast coordination by organizing a network of interconnected points. Hormones enable chemical integration by the endocrine system.

(b) Forebrain  

Ans: Among the three parts of the cortex are the cerebrum, thalamus, and hypothalamus.

• The brain's main structure is the cerebrum. The Left and right cerebral hemispheres are separated by a fissure in the cerebrum. Connecting the hemispheres is the corpus callosum. Cells that cover the cerebral hemisphere make up the cerebral cortex, which is a layer of pronounced folds. It is referred to as grey matter because of its greyish colouration. Several portions of the cerebral cortex have no obvious sensory or motor function. A variety of complex activities are performed by association areas, including intercessory associations, memory, and communication. In the cerebral hemisphere-interior section, the fibers of the tract are protected by the myelin sheath. White matter is named as a result of its impenetrable appearance. Intercessory associations, memory, and communication are all tasks that the association areas are accountable for. The myelin sheath, which makes up the interior section of the cerebral hemisphere, protects the tract fibers. They give the layer an impenetrable white appearance, thus the name "white matter."

• Thalamus: There is a region within the cerebrum wrapped around the middle of the forebrain named the Thalamus. Sensory and motor signaling are coordinated at this center.

• Hypothalamus: In the hypothalamus, there are numerous centers that regulate body temperature, urges for eating, and thirst. In addition to controlling growth and sexual behaviour, it is connected with the pituitary gland. 

(c) Midbrain

Ans: The midbrain: From the forebrain to the hindbrain, the midbrain lies between the thalamus and the hypothalamus. This section of the brain passes through a canal known as the cerebral aqueduct. 

(d) Hindbrain

Ans: In the hindbrain, you will find pons, cerebellum, and medulla.

There are many neurons in the cerebellum, so its surface is very convoluted in order to accommodate those extra neurons. 

In the brain, the medulla and spinal cord are connected.

The medulla is home to centres that regulate respiratory functions, cardiovascular reflexes, and gastric secretions. 

(e) Retina

Ans: Retina, also known as the photoreceptor, is a layer of neural cells located at the back of the eye. It includes ganglion cells, bipolar cells and astrocytes. They act as photoreceptors. Rods and cones make up photoreceptor cells. A cone is a sensory organ that is responsible for the perception of daylight and color whereas the rods are responsible for twilight vision. Images of objects are formed on the retina by the light entering through the cornea and the lens.

(f) Ear ossicles   

Ans: Malleus, incus, and stapes are three ossicles located in the middle ear, which are connected one to another through a chainlike arrangement. Among the three components of the ear, malleus is connected to the tympanic membrane, incus is attached to the stapes, and stapes is attached to the cochlea's oval window.  By transmitting sound waves effectively into the inner ear, ear ossicles increase hearing efficiency.

(g) Cochlea 

Ans: The cochlea is the coiled section of the labyrinth. There is an upper and a lower scala tympani in the cochlea, composed of reissner's and basilar membranes. Scala media, which fills the cochlea, contains endolymph. A rectangular window open to the middle ear accompanies the oval window at the base of the cochlea, and around the window opens to it at the base of the cochlea.  

(h) Organ of Corti

Ans: In the organ of Corti, hair cells function as auditory receptors located on the basilar membrane. On the inside surface of the Corti organ, hair cells are arranged in rows.

(i) Synapse

Ans: The synaptic cleft is a gap between postsynaptic cells and presynaptic cells that divides the synaptic membranes. Synapses are of two different types: chemical and electrical. 

6. Give a brief account of:  

(a) Mechanism of synaptic transmission  

Ans: A synapse is a point where two neurons meet. It exists between one neuron's axon terminal and the dendrite of the next neuron, divided by a cleft.

Synaptic transmission occurs in two ways.

• Chemical Transmission- A neurotransmitter (acetylcholine) is released across the synaptic cleft when a nerve impulse reaches the endplate of an axon. This substance is produced in the neuron's cell body and delivered to the axon terminal. Acetylcholine diffuses over the cleft and attaches to receptors on the surface of the next neuron's membrane. This results in membrane depolarization and the initiation of an action potential.

• Electrical Transmission- An electric current is created in the neuron in this sort of transmission. This electric current causes an action potential, which results in nerve impulse transmission across the nerve fiber. This technique of nerve conduction is quicker than the chemical method of transmission.

(b) Mechanism of Vision  

Ans: The retina is the eye's innermost layer. Inner ganglion cells, middle bipolar cells, and outermost photoreceptor cells make up the three layers. A photoreceptor cell is made up of a protein called opsin and a vitamin A aldehyde known as retinal. The separation of the retinal from opsin protein occurs when light rays are focused on the retina through the cornea. Opsin's structure is altered as a result of this. The permeability of the membrane changes as the structure of opsin changes, resulting in a potential differential in the cells. This causes an action potential in the ganglionic cells, which is then communicated to the brain's visual cortex via optic nerves. The impulses are analyzed in the cortex portion of the brain, and an image is generated on the retina.

(c) Mechanism of Hearing  

Ans: Sound waves are collected by the pinna of the external area and sent to the eardrum or external auditory canal. Vibrations are formed when these waves hit the tympanic membrane. The vibrations are then conveyed to the oval window, fenestra ovalis, via the malleus, incus, and stapes, three ear ossicles. These ossicles in the ear operate as a lever, transmitting sound waves to the inner ear. The vibrations of the fenestra ovalis are conveyed to the cochlea. The lymph produces sound waves as a result of this. A ripple in the basilar membrane is caused by the creation of waves. The sensory hair cells on the organ of Corti bend against the tectorial membrane as a result of this action. Sound waves are turned into nerve impulses as a consequence of this. Auditory nerves then carry these signals to the auditory cortex of the brain. The impulses are analyzed and sound is identified in the cerebral cortex of the brain.

7. Answer briefly:  

(a) How do you perceive the color of an object? 

Ans: Color vision is mediated by cones. Cones respond to different types of light depending on their characteristics, such as green, blue and red. Light from different sources stimulates these cells in different ways. We see different colors due to the combination of signals generated.

(b) Which part of our body helps us in maintaining body balance?  

Ans: Cochlear canals are formed by three semicircles in the inner ear. Keeping the body in balance is the job of the Cochlea.  

(c) How does the eye regulate the amount of light that falls on the retina?  

Ans: The pupil is a small opening between the iris and the lens of the eye that regulates light entering the eye. In the dim light, they expand to let more light fall on the retina, whereas in the presence of intense light they contract. 

8. Explain the following: 

(a) Role of Na+ in the generation of the action potential.  

Ans: Ionization of Na+ is responsible for the action potential. By diffusion into the inside of the axoplasm, the Na+channels, which are normally closed, become opened and allow the inflow of Na+ions.  After the membrane has depolarized, its electrical potential moves from 70 mV toward zero.  

(b) Mechanism of generation of light-induced impulse in the retina.  

Ans: Photopigments composed of both retinal and opsin in the eye are photosensitive and pigmented substances. Retinal dissociates from opsin when exposed to light, altering the structure of opsin. The bipolar neurons generate action potentials. In order to recognize the rect image, the brain analyzes the neural signals and action potentials presented by the optic nerves.  

(c) Mechanism through which a sound produces a nerve impulse in the inner ear.  

Ans: Acoustic energy is transmitted to the inner ear when it falls on the eardrums. Vibrations are transferred to the lymphatic fluid from the oval window in the cochlea. When the waves are accompanied by ripples in the basilar membrane, the cells of the hair are bent, forcing them against the tectorial membrane. The result of this is the generation of nerve impulses in the associated afferent neurons and their transmission to the auditory cortex of the brain, where they are analyzed and recognized as sound.  

9.  Differentiate between:  

(a) Myelinated and unmyelinated axons  

Ans: Differences between Myelinated and unmyelinated axons:-  

(b) Dendrites and axons  

Ans: Differences between Dendrites and axons:-

(c) Rods and cones  

Ans: Differences between Rods and Cones:-

(d) Thalamus and Hypothalamus  

Ans: Differences between Thalamus and Hypothalamus:-

(e) Cerebrum and Cerebellum  

Ans: Differences between Cerebrum and Cerebellum:-

10. Answer the following:  

(a) Which part of the ear determines the pitch of a sound?  

Ans: Cochlea.

(b) Which part of the human brain is the most developed?  

Ans: Cerebrum.

(c) Which part of our central neural system acts as a master clock?  

Ans: Hypothalamus.

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 charisma  

Ans : (c) blind spot

12. Distinguish between  

a) Afferent neurons and Efferent neurons  

Ans:   

b) Impulse conduction in a myelinated nerve fiber and an unmyelinated nerve fiber

Ans:  

 c) Aqueous humor and Vitreous humor  

Ans:

d) Blindspot and Yellow spot  

Ans: 

e) Cranial nerves and Spinal nerves  

Ans: