NCERT Solutions for Class 11 Biology Chapter 18 Neural Control and Coordination
NCERT Solutions for Class 11 Biology Chapter 18 Neural Control and Coordination provides revision notes and step-by-step textbook solutions detailing neural system organization, structure of neurons, conduction of nerve impulses, synaptic transmission, central nervous system (human brain), reflex action, and sensory organs (eye and ear).
Overview of Topics & Subtopics (Click to Jump)
1. Neural System & Structure of Neuron
Short Note:
• Human Neural System Organization:
- Central Nervous System (CNS): Includes Brain and Spinal Cord (site of information processing and control).
- Peripheral Nervous System (PNS): Comprises all nerves connecting body parts to CNS. Divided into Somatic Neural System (relays impulses from CNS to skeletal muscles) and Autonomic Neural System (relays impulses from CNS to involuntary organs/smooth muscles; split into Sympathetic and Parasympathetic systems).
• Structure of Neuron: Functional and structural unit composed of three major parts:
- Cell Body (Cyton): Contains cytoplasm, cell organelles, and Nissl's granules.
- Dendrites: Short, branched fibers radiating out from cyton; transmit impulses towards cell body.
- Axon: Long fiber carrying impulses away from cell body. Terminated by synaptic knobs containing synaptic vesicles with neurotransmitters.
• Types of Neurons: Unipolar (embryonic stage), Bipolar (retina of eye), Multipolar (cerebral cortex).
2. Generation & Conduction of Nerve Impulse & Synapse
Short Note:
• Resting Potential (Polarized State): Membrane of resting axon is more permeable to K⁺ and almost impermeable to Na⁺. Maintained by Na⁺/K⁺ ATPase pump (pumps 3 Na⁺ outwards for 2 K⁺ indoors). Inside becomes negatively charged relative to outside (-70 mV).
• Action Potential (Depolarization): Application of stimulus increases membrane permeability to Na⁺ → rapid influx of Na⁺ → reversal of polarity (inside becomes positive). This wave of depolarization forms the nerve impulse.
• Repolarization: Na⁺ permeability drops quickly; permeability to K⁺ increases → K⁺ flows outward restoring resting potential.
• Synaptic Transmission:
- Electrical Synapse: Membranes of pre- and post-synaptic neurons are in very close proximity; impulse jumps directly and rapidly.
- Chemical Synapse: Separated by fluid-filled synaptic cleft. Action potential at presynaptic terminal triggers release of neurotransmitters (e.g., Acetylcholine) from vesicles into cleft, binding to receptors on post-synaptic membrane.
3. Central Nervous System (Human Brain Structure)
Short Note:
• Brain is protected inside cranial cavity and covered by cranial meninges (outer Dura mater, middle Arachnoid, inner Pia mater).
• Forebrain:
- Cerebrum: Largest part, divided into right and left cerebral hemispheres connected by Corpus Callosum. Outer layer (Cerebral cortex/grey matter) controls memory, sensory processing, and voluntary actions.
- Thalamus: Major relay station for sensory and motor signaling.
- Hypothalamus: Regulates body temperature, urge for eating and drinking, and controls pituitary hormone secretions.
• Midbrain: Located between thalamus/hypothalamus and pons. Contains four round swellings called Corpora Quadrigemina (visual and auditory reflexes).
• Hindbrain: Consists of Pons (fiber tracts interconnecting regions), Cerebellum (convoluted surface for motor coordination & balance), and Medulla Oblongata (controls respiration, cardiovascular reflexes, gastric secretions).
4. Reflex Action, Reflex Arc & Sensory Organs (Eye & Ear)
Short Note:
• Reflex Action: Entirely involuntary response to a peripheral nervous stimulus without conscious thought. Path followed is Reflex Arc: Receptor → Afferent neuron → Interneuron (CNS) → Efferent neuron → Effector (muscle/gland).
• Human Eye (Organ of Vision):
- Wall consists of Sclera (outer, anterior forms cornea), Choroid (middle vascular layer forming ciliary body & iris), and Retina (innermost layer containing photoreceptors: Rods for twilight vision, Cones for daylight/color vision).
- Fovea: Point of highest visual acuity (contains closely packed cones only).
• Human Ear (Organ of Hearing & Balance):
- External & Middle Ear: Pinna, auditory meatus, tympanic membrane, and 3 ear ossicles (Malleus, Incus, Stapes).
- Inner Ear (Labyrinth): Contains coiled Cochlea (hearing unit with Organ of Corti containing hair cells) and Vestibular Apparatus (saccule, utricle, and 3 semicircular canals for body balance).
Intext & Conceptual Questions
1. Polarization (Resting State): In a resting neuron, the axon membrane is more permeable to K⁺ and impermeable to Na⁺. The active Na⁺/K⁺ pump pumps 3 Na⁺ out for every 2 K⁺ in, maintaining a negative charge inside (-70 mV).
2. Depolarization (Action Potential): When a threshold stimulus is applied, voltage-gated Na⁺ channels open, causing a rapid influx of Na⁺. The inner surface becomes positively charged (+30 mV), generating an action potential.
3. Propagation: The action potential creates a current flow to the adjacent resting region, depolarizing it and propagating the impulse down the length of the axon.
4. Repolarization: Na⁺ channels close and voltage-gated K⁺ channels open, allowing K⁺ to flow out of the cell, restoring the negative resting potential inside.
The Hypothalamus plays a critical role in maintaining homeostasis. Major functions include:
1. Thermoregulation: Acts as the body's thermostat to regulate body temperature.
2. Control of Autonomic Functions: Regulates hunger, thirst, satiety, and sleep-wake cycles.
3. Endocrine Control: Produces hypothalamic releasing and inhibiting hormones that control the anterior pituitary gland, as well as synthesizing Oxytocin and Vasopressin (ADH).
Textbook Chapter End Exercise Questions
| Feature | Chemical Synapse | Electrical Synapse |
|---|---|---|
| Synaptic Cleft | Fluid-filled synaptic cleft is present between pre- and post-synaptic membranes. | Pre- and post-synaptic membranes are in direct physical contact (gap junctions). |
| Mediator | Transmission occurs via chemical messengers called neurotransmitters. | Transmission occurs directly via electrical current flow. |
| Speed of Transmission | Relatively slower due to synaptic delay in chemical release. | Extremely rapid, instantaneous signal conduction. |
| Occurrence | Most common type of synapse in human nervous system. | Rare in human system (found in specific cardiac & nerve circuits). |
| Feature | Rods | Cones |
|---|---|---|
| Vision Type | Responsible for twilight / dim-light vision (scotopic vision). | Responsible for daylight / bright-light and color vision (photopic vision). |
| Visual Pigment | Contain purple-red protein pigment called Rhodopsin (Vitamin A derivative). | Contain iodopsin / photopsin pigments sensitive to Red, Green, and Blue light. |
| Distribution on Retina | Distributed abundantly throughout peripheral retina; absent at fovea. | Densely concentrated in the central region (macula lutea and fovea centralis). |
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.