Updated NCERT Solutions for Class 11 Biology Chapter 18: Neural Control and Coordination

(a) Brain

The human brain is the central information processing organ, protected by the skull and three meninges (dura mater, arachnoid mater, pia mater). It is divided into three major parts:

1. Forebrain (Prosencephalon): Includes the Cerebrum, Thalamus, and Hypothalamus.
   • Cerebrum: The largest part, divided into two hemispheres connected by the corpus callosum. The cerebral cortex controls intelligence, memory, and voluntary actions.
   • Thalamus: A major coordinating center for sensory and motor signaling.
   • Hypothalamus: Controls body temperature, eating, drinking, and secretes hormones.
2. Midbrain (Mesencephalon): Located between the forebrain and hindbrain. It contains the corpora quadrigemina and controls reflex movements of the head and neck.
3. Hindbrain (Rhombencephalon): Includes the Pons, Cerebellum, and Medulla.
   • Pons: Interconnects different regions of the brain.
   • Cerebellum: Coordinates voluntary movements and balance.
   • Medulla Oblongata: Connects to the spinal cord and controls involuntary functions like respiration and cardiovascular reflexes.

The Brain Stem is formed by the midbrain, pons, and medulla oblongata.

(b) Eye

The human eye is a spherical organ for vision. The wall has three layers:

1. Sclera (Outer): The tough, white outer layer. Its transparent front part is the cornea.
2. Choroid (Middle): Vascular layer that forms the ciliary body and the colored iris. The iris regulates the size of the pupil.
3. Retina (Inner): The light-sensitive layer containing photoreceptor cells (rods and cones). Rods are for dim light vision, and cones are for color and daylight vision.

Other key parts include the lens, which focuses light, and the fovea, the point of sharpest vision.

(c) Ear

The ear is the organ for hearing and balance, divided into three sections:

1. Outer Ear: The pinna collects sound waves and directs them to the eardrum via the ear canal.
2. Middle Ear: Contains three tiny bones (malleus, incus, stapes) that amplify vibrations. The Eustachian tube equalizes pressure.
3. Inner Ear (Labyrinth): Consists of the cochlea (for hearing) and the vestibular apparatus (for balance). The cochlea contains the organ of Corti with hair cells that act as auditory receptors.

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

(b) Resting potential and Action potential

(c) Choroid and Retina

(a) Polarisation of the membrane of a nerve fibre (Resting State)

The resting neuron membrane is polarized. This is achieved because:

1. The **Na+-K+ pump** actively transports 3 Na+ ions out for every 2 K+ ions in.
2. The membrane is more permeable to K+ ions, allowing them to leak out.
3. This results in a positive charge on the outer surface and a negative charge on the inner surface, creating the **resting potential** (approx. -70mV).

(b) Depolarisation of the membrane of a nerve fibre (Action Potential)

When a stimulus arrives:

1. Voltage-gated **Na+ channels open**, and the membrane becomes highly permeable to Na+.
2. A rapid **influx of Na+** ions occurs.
3. This reverses the membrane polarity: the inside becomes positive and the outside becomes negative. This is **depolarisation**, which generates an **action potential**.

(c) Conduction of a nerve impulse along a nerve fibre

The action potential propagates as a wave:

1. The depolarized site (Site A) creates a local current with the adjacent resting site (Site B).
2. This current flow causes Site B to depolarize and generate its own action potential.
3. Meanwhile, Site A repolarizes as K+ ions move out.
4. This process repeats along the axon. In myelinated neurons, this impulse "jumps" between Nodes of Ranvier (**saltatory conduction**), which is much faster.

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

This occurs in a series of steps:

1. An action potential arrives at the axon terminal.
2. Voltage-gated **Ca2+ channels open**, and Ca2+ ions enter the terminal.
3. The influx of Ca2+ causes synaptic vesicles to release **neurotransmitters** into the synaptic cleft.
4. Neurotransmitters diffuse and bind to **receptors** on the post-synaptic membrane.
5. This binding opens ion channels, generating a new potential in the post-synaptic neuron.

[A well-labelled diagram of a multipolar neuron should be drawn here, showing: Cell body/Cyton, Nucleus, Nissl's granules, Dendrites, Axon, Axon hillock, Myelin sheath, Schwann cell, Node of Ranvier, and Axon terminal/Synaptic knob.]

Description: A neuron has three main parts:

1. Cell Body (Cyton): Contains the nucleus and granular bodies called Nissl's granules.
2. Dendrites: Short, branched fibers that receive impulses and transmit them towards the cell body.
3. Axon: A single, long fiber that transmits impulses away from the cell body. It may be covered by a myelin sheath.

The hypothalamus is a vital part of the brain with several crucial functions:

• Regulation of body temperature (thermoregulation).
• Control of hunger, thirst, and satiety.
• Regulation of the sleep-wake cycle (circadian rhythm).
• Control of the pituitary gland, linking the nervous system to the endocrine system.
• Involvement in emotions, mood, and sexual behavior as part of the limbic system.

The mechanism of hearing involves the conversion of sound waves into nerve impulses that the brain can interpret. The process occurs in the following steps:

1. Collection of Sound Waves: The outer ear (pinna) collects sound waves and directs them into the external auditory canal.
2. Vibration of Eardrum: The sound waves strike the tympanic membrane (eardrum), causing it to vibrate.
3. Amplification by Ear Ossicles: The vibrations are transmitted to the three ear ossicles (malleus, incus, stapes). These bones amplify the vibrations and transmit them to the oval window.
4. Transmission to Cochlea: The stapes pushes on the oval window, causing pressure waves in the fluid (perilymph) of the cochlea's scala vestibuli.
5. Stimulation of Basilar Membrane: These pressure waves travel through the perilymph, causing the basilar membrane to vibrate.
6. Activation of Hair Cells: The vibration of the basilar membrane causes the sensory hair cells of the Organ of Corti to bend against the tectorial membrane.
7. Generation of Nerve Impulse: This bending of hair cells generates action potentials in the attached auditory nerve fibers.
8. Transmission to Brain: The auditory nerve transmits these impulses to the auditory cortex of the temporal lobe in the cerebrum, where the sound is interpreted.

Assertion (A): The conduction of nerve impulses is faster in myelinated nerve fibers.

Reason (R): In myelinated fibers, the action potential jumps from one Node of Ranvier to the next.

Options:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation: The myelin sheath acts as an insulator, preventing ion flow. The impulse can only be generated at the uninsulated gaps called Nodes of Ranvier. This "jumping" from node to node, called saltatory conduction, is much faster than the continuous depolarization in non-myelinated fibers.

1. (i) What is the ability of the eye to adjust its focal length called?
   The ability of the eye to adjust its focal length to see objects at different distances clearly is called accommodation.
2. (ii) Which structures are responsible for changing the curvature of the lens?
   The ciliary body (specifically the ciliary muscles) and the suspensory ligaments are responsible.
3. (iii) What is this age-related condition called?
   This age-related condition, where the lens loses its elasticity and accommodation power, is called presbyopia.