Neural Control and Coordination — NCERT Class 11 Biology

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This page provides complete NCERT Solutions for CBSE Class 11 Biology Chapter 21 — Neural Control and Coordination. All intext questions (organised by page number) and chapter-end exercise questions are answered in detail. Use the section headings to jump directly to the question you need help with, or read through the full page for a comprehensive revision of the chapter.

Key Concepts (Quick Recap)

• Neuron Structure: Dendrites (receive signals), cell body (soma, metabolic centre), axon (transmits signal), myelin sheath (insulating layer of Schwann cells), nodes of Ranvier (gaps in myelin)
• Resting Membrane Potential: -70 mV (inside negative); maintained by Na+/K+ ATPase pump (3 Na+ out, 2 K+ in) and selective membrane permeability
• Action Potential: Depolarisation to +30 mV (Na+ influx) followed by repolarisation (K+ efflux) and hyperpolarisation; threshold stimulus needed (-55 mV)
• Saltatory Conduction: In myelinated neurons, impulse jumps from node to node, increasing speed up to 100 m/s
• Synapse: Chemical (neurotransmitter-mediated, synaptic cleft 20–30 nm, one-way transmission) and electrical (gap junctions, fast, bidirectional)
• Neurotransmitters: Acetylcholine (excitatory at neuromuscular junction), GABA (inhibitory), glutamate (excitatory), dopamine, serotonin, noradrenaline
• Central Nervous System (CNS): Brain (forebrain, midbrain, hindbrain) and spinal cord
• Forebrain: Cerebrum (cerebral cortex with sensory, motor, and association areas), thalamus (relay centre), hypothalamus (homeostasis, neuroendocrine link)
• Midbrain: Corpora quadrigemina (visual and auditory reflexes), cerebral peduncles
• Hindbrain: Pons (respiratory rhythm), cerebellum (balance, coordination), medulla oblongata (cardiovascular, respiratory, digestive centres)
• Peripheral Nervous System (PNS): Somatic (voluntary, innervates skeletal muscles) and Autonomic (involuntary — sympathetic and parasympathetic divisions)
• Reflex Arc: Receptor → Sensory neuron → Spinal cord (interneuron) → Motor neuron → Effector; bypasses brain for faster response
• Eye: Cornea, aqueous humour, lens, vitreous humour, retina (rods for dim light, cones for colour), fovea (point of sharpest vision), blind spot (optic disc)
• Ear: Outer ear (pinna, auditory canal), middle ear (tympanic membrane, ossicles — malleus, incus, stapes, Eustachian tube), inner ear (cochlea — organ of Corti for hearing, semicircular canals and vestibule for balance)

Intext Questions

Page 327

1. Cell body (Soma or Cyton): The cell body is the metabolic centre of the neuron. It contains the nucleus, Nissl's granules (clusters of rough endoplasmic reticulum and ribosomes), neurofibrils, and other organelles. Nissl's granules are involved in protein synthesis. The cell body integrates signals received from dendrites and passes them on to the axon.

2. Dendrites: Short, branched, cytoplasmic processes that arise from the cell body. They are the receiving (input) ends of the neuron. They transmit electrical impulses toward the cell body. A neuron may have many dendrites.

3. Axon: A long, cylindrical process that arises from the axon hillock of the cell body. It transmits electrical impulses away from the cell body to the synaptic terminal. The axon is covered by a lipid-rich myelin sheath (formed by Schwann cells in the PNS and oligodendrocytes in the CNS), which insulates the axon and speeds up impulse transmission. Gaps in the myelin sheath are called nodes of Ranvier. The axon terminates in small branches called axon terminals or synaptic knobs, which contain synaptic vesicles filled with neurotransmitters.

Neurons are classified by the number of processes:

• Unipolar: One process (found in embryonic stages)
• Bipolar: One axon and one dendrite (found in the retina of the eye and olfactory epithelium)
• Multipolar: One axon and multiple dendrites (most common type in the CNS)
• Pseudo-unipolar: A single process that branches into two (found in dorsal root ganglia of spinal nerves)

(a) Myelinated vs Non-myelinated axons:

(b) Dendrites vs Axons:

(c) Resting potential vs Action potential:

Page 332

1. Arrival of action potential: When an action potential (nerve impulse) reaches the pre-synaptic terminal (axon terminal), it depolarises the pre-synaptic membrane.

2. Calcium influx: The depolarisation causes voltage-gated Ca2+ channels in the pre-synaptic membrane to open. Ca2+ ions flow into the pre-synaptic terminal from the synaptic cleft.

3. Vesicle fusion and neurotransmitter release: The influx of Ca2+ triggers the synaptic vesicles (containing neurotransmitters) to move toward the pre-synaptic membrane, fuse with it (exocytosis), and release the neurotransmitter into the synaptic cleft.

4. Neurotransmitter binding: The neurotransmitter diffuses across the synaptic cleft (20–30 nm) and binds to specific receptors on the post-synaptic membrane.

5. Post-synaptic response: Depending on the type of neurotransmitter and receptor:

• Excitatory postsynaptic potential (EPSP): If the neurotransmitter is excitatory (e.g., acetylcholine, glutamate), it opens Na+ channels on the post-synaptic membrane, causing depolarisation. If EPSPs reach the threshold, an action potential is generated.
• Inhibitory postsynaptic potential (IPSP): If the neurotransmitter is inhibitory (e.g., GABA, glycine), it opens Cl− or K+ channels, causing hyperpolarisation (making the inside more negative), which makes it harder to generate an action potential.

6. Inactivation of neurotransmitter: The neurotransmitter is quickly removed from the synaptic cleft by:

• Enzymatic degradation (e.g., acetylcholinesterase breaks down acetylcholine)
• Reuptake by the pre-synaptic neuron
• Diffusion out of the cleft

This ensures the signal is brief and precisely controlled.

Key features of chemical synapses:

• One-way transmission (pre-synaptic → post-synaptic only)
• Synaptic delay (~0.5 ms) due to neurotransmitter release, diffusion, and binding
• Can be excitatory or inhibitory
• Susceptible to drugs, toxins, and fatigue

Example: Withdrawal reflex (pulling your hand away from a hot object)

Steps:

1. Stimulus: You touch a hot stove. Thermoreceptors (pain receptors) in your skin detect the heat.

2. Receptor activation: The thermoreceptors generate an electrical impulse (action potential) in response to the heat stimulus.

3. Sensory (afferent) neuron: The impulse travels along the sensory neuron through the dorsal root into the spinal cord.

4. Integration centre (spinal cord): The sensory neuron synapses with an interneuron (association neuron) in the grey matter of the spinal cord. The interneuron processes the information and synapses with a motor neuron.

5. Motor (efferent) neuron: The motor neuron carries the impulse from the spinal cord through the ventral root to the effector organ.

6. Effector response: The motor impulse reaches the biceps muscle (effector), which contracts, pulling your hand away from the hot object.

7. Parallel pathway to brain: Simultaneously, the sensory information is also transmitted to the brain via ascending tracts. You become aware of the pain after the reflex action has already occurred.

Reflex arc pathway: Receptor → Sensory neuron → Spinal cord (interneuron) → Motor neuron → Effector

Types of reflexes:

• Somatic reflexes: Involve skeletal muscles (e.g., withdrawal reflex, knee-jerk reflex)
• Autonomic (visceral) reflexes: Involve smooth muscles, cardiac muscle, or glands (e.g., salivary reflex, pupillary reflex)

Page 337

(a) Rods — Rod cells in the retina are photoreceptors that are sensitive to dim light and help us perceive the intensity (brightness) of light.

(b) Cones — Cone cells in the retina are responsible for colour vision and visual acuity. There are three types of cones, each sensitive to a different wavelength: red (long wavelength), green (medium wavelength), and blue (short wavelength).

(c) Inner ear (cochlea) — The cochlea, specifically the organ of Corti on the basilar membrane, contains hair cells that convert sound vibrations into nerve impulses. The intensity (loudness) of sound is determined by the amplitude of vibrations, which activates different numbers of hair cells and different firing rates of auditory neurons.

Chapter-End Exercise

1. Forebrain (Prosencephalon):

• Cerebrum: The largest part of the brain, divided into two cerebral hemispheres by a longitudinal fissure. The outer layer is the cerebral cortex (grey matter) with prominent folds (gyri) and grooves (sulci). The cortex is divided into four lobes: frontal (motor areas, speech, thinking), parietal (sensory areas, touch, temperature), temporal (auditory areas, memory), and occipital (visual areas). The inner white matter contains myelinated axons, and deep within are grey matter masses called basal ganglia (control voluntary movements). The corpus callosum connects the two hemispheres.

• Thalamus: A paired structure located below the cerebrum. It is the major relay centre for sensory and motor signals going to and from the cerebral cortex. It also regulates consciousness, sleep, and alertness.

• Hypothalamus: Located below the thalamus. It is the master control centre for homeostasis and the neuroendocrine link. It controls body temperature, hunger, thirst, sleep, emotional behaviour (rage, pleasure), and regulates the pituitary gland through releasing/inhibiting hormones. It also contains the suprachiasmatic nucleus (biological clock).

2. Midbrain (Mesencephalon):

• Located between the forebrain and hindbrain.
• Contains the corpora quadrigemina (four rounded prominences): two superior colliculi (involved in visual reflexes) and two inferior colliculi (involved in auditory reflexes).
• Contains cerebral peduncles (bundles of fibres connecting the cerebrum to the brainstem and spinal cord).
• Contains the substantia nigra (involved in movement regulation; degeneration causes Parkinson's disease).

3. Hindbrain (Rhombencephalon):

• Pons Varolii: A bridge of fibres that connects the two halves of the cerebellum and links the medulla to the midbrain. It contains respiratory centres that help regulate breathing rhythm.

• Cerebellum: Located posterior to the pons and medulla. It is the second largest part of the brain, with two hemispheres and a central vermis. The cerebellum coordinates voluntary movements, maintains posture and balance, and enables smooth, precise motor activity. Damage to the cerebellum causes ataxia (loss of coordination).

• Medulla oblongata: The lowest part of the brain that connects to the spinal cord. It contains vital centres that control: (a) respiration (breathing rate), (b) cardiovascular functions (heart rate, blood pressure), (c) reflexes such as swallowing, coughing, sneezing, vomiting, and salivation. Damage to the medulla can be fatal.

The brain is protected by three meninges (dura mater, arachnoid mater, pia mater), cerebrospinal fluid (CSF), and the blood-brain barrier.

(a) Central Nervous System (CNS) vs Peripheral Nervous System (PNS):

(b) Resting potential vs Action potential:

(Covered in detail in the intext questions section above. Brief comparison:)

(c) Sympathetic vs Parasympathetic Nervous System:

1. Collection of sound waves: The pinna (external ear) collects sound waves and directs them into the external auditory canal.

2. Amplification: Sound waves strike the tympanic membrane (eardrum), causing it to vibrate. These vibrations are transmitted through the three ear ossicles in the middle ear — malleus (hammer), incus (anvil), and stapes (stirrup). The ossicles amplify the vibrations about 20 times and transmit them to the oval window of the cochlea.

3. Transmission to inner ear: The stapes fits into the oval window (fenestra vestibuli) of the cochlea. As the stapes vibrates, it pushes the oval window inward, creating pressure waves in the perilymph of the scala vestibuli.

4. Fluid vibration: The pressure waves in the perilymph travel through the scala vestibuli, around the helicotrema (at the apex of the cochlea), and into the scala tympani. The vibrations of the perilymph cause vibrations in the basilar membrane.

5. Hair cell stimulation: The vibration of the basilar membrane causes the hair cells (stereocilia) of the organ of Corti to bend against the tectorial membrane. This bending opens mechanically-gated ion channels, causing K+ influx and depolarisation of the hair cells.

6. Neurotransmitter release: Depolarised hair cells release glutamate (neurotransmitter) at their basal surface, which stimulates the cochlear nerve fibres (part of the VIII cranial nerve — vestibulocochlear nerve).

7. Signal transmission to brain: The auditory nerve carries the electrical signals to the cochlear nucleus in the medulla, then to the superior olivary complex, lateral lemniscus, inferior colliculus, medial geniculate body (thalamus), and finally to the auditory cortex in the temporal lobe of the cerebrum, where sound is perceived and interpreted.

8. Pressure equalisation: The round window (fenestra cochleae) bulges outward to relieve the pressure created by the inward movement of the oval window, allowing the fluid in the cochlea to vibrate. The Eustachian tube connects the middle ear to the pharynx and helps equalise pressure on both sides of the tympanic membrane.

(a) The cochlea (specifically the basilar membrane within the cochlea) determines the pitch of sound. Different regions of the basilar membrane vibrate maximally at different frequencies: the base of the cochlea (near the oval window) responds to high frequencies, and the apex (near the helicotrema) responds to low frequencies. Hair cells at each location are tuned to specific frequencies, allowing pitch discrimination.

(b) The vestibular apparatus in the inner ear maintains the balance of the body. It consists of three semicircular canals (anterior, posterior, and lateral) that detect rotational movements, and the otolith organs (utricle and saccule) that detect linear acceleration and the position of the head relative to gravity. The semicircular canals contain endolymph and hair cells in the ampulla (crista ampullaris) that respond to angular acceleration. The utricle and saccule contain otoliths (calcium carbonate crystals) that shift with head position, stimulating hair cells in the macula.

(c) Cone cells (photoreceptor cones) of the retina are responsible for colour vision. There are three types of cone cells, each containing a different photopigment (photopsin/iodopsin):

• Red cones (L-cones): Sensitive to long wavelengths (~560 nm, red)
• Green cones (M-cones): Sensitive to medium wavelengths (~530 nm, green)
• Blue cones (S-cones): Sensitive to short wavelengths (~420 nm, blue)

All colours we perceive result from the varying stimulation of these three cone types. Cone cells are concentrated in the fovea centralis and function best in bright light.

Outer layer (Fibrous tunic):

• Sclera: The tough, white, opaque outer layer that maintains the shape of the eyeball and protects the inner structures. It is made of dense connective tissue.
• Cornea: The transparent, anterior portion of the sclera that allows light to enter the eye. It is avascular (no blood vessels) and receives nutrition from aqueous humour. It provides the initial refraction of light.

Middle layer (Vascular tunic / Uvea):

• Choroid: The dark, vascular, pigmented layer beneath the sclera. It prevents internal reflection of light (due to melanin) and nourishes the retina.
• Ciliary body: An extension of the choroid that contains ciliary muscles and secretes aqueous humour. The ciliary muscles control the shape of the lens for accommodation.
• Iris: The coloured, visible part of the eye (anterior extension of the choroid). It has two muscles — the sphincter pupillae (circular, constricts pupil) and dilator pupillae (radial, dilates pupil) — that regulate the size of the pupil to control the amount of light entering the eye.
• Pupil: The central opening in the iris through which light passes to the lens.

Inner layer (Neural tunic / Retina):

• The retina contains three layers of neural cells: photoreceptors (rods and cones) → bipolar neurons → ganglion cells.
• Rods (~120 million): Contain rhodopsin, function in dim light, provide black and white vision.
• Cones (~6 million): Contain photopsin (three types), function in bright light, provide colour vision and sharp acuity.
• Fovea centralis: A small depression in the macula lutea where only cones are present — the point of sharpest vision.
• Blind spot (optic disc): The point where the optic nerve exits the retina; has no photoreceptors (no vision).

Lens system:

• Lens: A transparent, biconvex structure held in place by the suspensory ligaments (zonules) attached to the ciliary body. It provides fine focusing (accommodation).
• Aqueous humour: Clear, watery fluid in the anterior chamber (between cornea and lens) that nourishes the cornea and lens, and maintains intraocular pressure.
• Vitreous humour: Clear, gelatinous substance in the posterior chamber (between lens and retina) that maintains the shape of the eyeball and supports the retina.

1. Light entry: Light from an object enters the eye through the cornea, which refracts the light rays.

2. Pupil regulation: The iris regulates the amount of light entering the eye by adjusting the pupil size. In bright light, the sphincter pupillae constricts the pupil; in dim light, the dilator pupillae dilates the pupil.

3. Accommodation: The lens changes its curvature to focus light from objects at different distances:

• Distant objects (>6 m): Ciliary muscles relax, suspensory ligaments become taut, and the lens becomes thin (flat), decreasing its refractive power.
• Near objects: Ciliary muscles contract, suspensory ligaments relax, and the lens becomes thick (more convex), increasing its refractive power.

4. Image formation: The cornea and lens together focus the light rays on the retina, forming a real, inverted image. The brain processes this inverted image and interprets it as upright.

5. Phototransduction (in rods):

• Rhodopsin (visual purple) in rod cells consists of opsin (a protein) and retinal (a derivative of vitamin A).
• In the dark, retinal exists in the 11-cis form bound to opsin. Na+ channels are open, and the rod is depolarised, releasing glutamate (inhibitory).
• When light hits rhodopsin, 11-cis retinal is isomerised to all-trans retinal, which detaches from opsin. This triggers a cascade: transducin is activated → phosphodiesterase (PDE) is activated → cGMP is broken down → Na+ channels close → the rod hyperpolarises → less glutamate is released → bipolar cells depolarise → ganglion cells fire action potentials.

6. Phototransduction (in cones): Similar process, but cones use photopsin (iodopsin) instead of rhodopsin. There are three types of cones (red, green, blue), each with a different photopigment sensitive to different wavelengths. Colour vision results from the combined stimulation of these three cone types.

7. Signal transmission: The photoreceptor signals are transmitted through bipolar neurons to ganglion cells. Horizontal cells and amacrine cells provide lateral interactions (lateral inhibition) that enhance contrast and visual acuity.

8. Visual pathway: Axons of ganglion cells form the optic nerve (II cranial nerve). At the optic chiasm, fibres from the nasal (medial) halves of each retina cross over to the opposite side. The fibres then continue through the optic tract to the lateral geniculate nucleus (LGN) of the thalamus, and then to the primary visual cortex (V1) in the occipital lobe, where the visual image is perceived and interpreted.

9. Binocular vision: Since both eyes view an object from slightly different angles, the brain receives two slightly different images. This disparity (binocular disparity) allows depth perception (stereopsis).

(a) Myopia vs Hypermetropia:

(b) Aqueous humour vs Vitreous humour:

(c) Blind spot vs Yellow spot (Macula lutea):

Significance:

1. Homeostasis: The ANS maintains internal balance by continuously adjusting organ functions in response to changing internal and external conditions. It regulates heart rate, blood pressure, body temperature, digestion, and many other vital functions.

2. Dual control: The ANS has two opposing divisions that work together to fine-tune organ function:

• Sympathetic nervous system ("fight or flight"): Prepares the body for emergency situations — increases heart rate, dilates pupils, inhibits digestion, increases blood glucose, dilates bronchioles.
• Parasympathetic nervous system ("rest and digest"): Promotes routine, energy-conserving activities — decreases heart rate, constricts pupils, stimulates digestion, promotes salivation.

3. Smooth and coordinated organ function: The sympathetic and parasympathetic systems work antagonistically (e.g., sympathetic increases heart rate while parasympathetic decreases it), providing fine-tuned control rather than simple on/off regulation.

4. Rapid response to stress: The sympathetic division activates instantly in response to threats, mobilising energy reserves and preparing the body for action (increased heart rate, bronchodilation, glycogenolysis).

5. Energy conservation and restoration: The parasympathetic division promotes activities that conserve and restore energy (digestion, absorption, defecation, urination).

6. Emotional expression: The ANS mediates many physical manifestations of emotions — sweating, blushing, increased heart rate during anxiety, tearing during sadness.

1. Tympanic membrane (Eardrum):

• A thin, oval, semi-transparent membrane that separates the external auditory canal from the middle ear.
• It vibrates when sound waves strike it and transmits these vibrations to the ear ossicles.
• It is concave on the outer surface and convex on the inner surface.

2. Ear ossicles: Three tiny bones connected in series — the smallest bones in the body:

• Malleus (Hammer): Attached to the tympanic membrane on its inner side. The handle (manubrium) of the malleus is embedded in the tympanic membrane. It transmits vibrations from the eardrum to the incus.
• Incus (Anvil): The middle bone, articulating with the malleus on one side and the stapes on the other. It acts as a bridge between the malleus and stapes.
• Stapes (Stirrup): The smallest bone in the human body (~3 mm). Its footplate fits into the oval window (fenestra vestibuli) of the cochlea. It transmits vibrations from the incus to the inner ear fluid.

The ossicles amplify sound vibrations approximately 20 times as they transmit them from the large tympanic membrane to the much smaller oval window (hydraulic lever principle and lever action of the ossicles).

3. Eustachian tube (Auditory tube):

• A canal (~3.5 cm long) that connects the middle ear to the nasopharynx.
• It helps equalise air pressure on both sides of the tympanic membrane, ensuring proper vibration.
• Normally closed, but opens during yawning, swallowing, or chewing.
• Also drains any fluid from the middle ear into the pharynx.

4. Oval window and Round window:

• Oval window (Fenestra vestibuli): An opening covered by the footplate of the stapes. It transmits vibrations to the perilymph of the inner ear.
• Round window (Fenestra cochleae): An opening covered by a flexible membrane that bulges outward when the stapes pushes the oval window inward, relieving pressure in the inner ear fluid.

Structure of the organ of Corti:

• It rests on the basilar membrane and consists of hair cells (sensory receptor cells) and supporting cells (pillar cells, Deiters' cells, Hensen's cells).
• There are two rows of hair cells: inner hair cells (one row, ~3500) and outer hair cells (three rows, ~12,000). Inner hair cells are the primary sensory receptors; outer hair cells amplify the response.
• Each hair cell has ~50–100 stereocilia (microvilli-like projections) on its apical surface. The stereocilia are arranged in rows of increasing height and are connected by tip links.
• Above the hair cells lies the tectorial membrane, a gelatinous structure that the tips of the outer hair cell stereocilia are embedded in.

Role in hearing:

1. When sound vibrations enter the cochlea through the oval window, they create pressure waves in the perilymph of the scala vestibuli. 2. These pressure waves cause the basilar membrane to vibrate. 3. The vibration of the basilar membrane causes the hair cells to move relative to the tectorial membrane. The stereocilia bend against the tectorial membrane. 4. The bending of stereocilia opens mechanically-gated cation channels (via tip links), allowing K+ from the endolymph (which has a high K+ concentration) to flow into the hair cells. 5. The influx of K+ depolarises the hair cells, triggering the release of the neurotransmitter glutamate at the basal surface. 6. Glutamate stimulates the dendrites of the cochlear nerve (part of the VIII cranial nerve — vestibulocochlear nerve). 7. The auditory nerve carries these signals to the brain, where they are interpreted as sound.

Pitch discrimination: Different frequencies of sound cause different regions of the basilar membrane to vibrate maximally (tonotopic organisation). High frequencies vibrate the base of the cochlea; low frequencies vibrate the apex. This is how we perceive different pitches.

Loudness discrimination: Louder sounds cause greater amplitude of basilar membrane vibration, bending more stereocilia and increasing the firing rate of auditory neurons.

Internal Links

• [Chapter 21 Guide — Neural Control and Coordination](/school/cbse/class/11/biology/cbse-neural-control-and-coordination)
• [All Class 11 Biology Solutions](/school/cbse/class/11/biology/solutions)
• [Class 11 Biology Subject Guide](/school/cbse/class/11/biology)