CBSE Class 11 Biology Chapter 21 Notes: Neural Control and Coordination

Chapter Introduction

Chapter 21 — Neural Control and Coordination — belongs to the Human Physiology unit of CBSE Class 11 Biology. Every living organism responds to changes in its environment (stimuli). The human body uses two major systems for coordination: the neural system and the endocrine system. This chapter focuses on the neural system, which provides rapid, point-to-point coordination through electrical signals (nerve impulses).

The neural system is built from specialised cells called neurons, organised into networks that detect stimuli, process information, and command responses. Together with sense organs (eye, ear, etc.), the nervous system enables the body to perceive the external world and react appropriately. This guide covers neuron structure, nerve impulse generation and conduction, synaptic transmission, the central and peripheral nervous systems, reflex actions, and the structure and working of the eye and ear.

Key Definitions

1. Neuron — The structural and functional unit of the nervous system; consists of dendrites, cell body, and axon. 2. Resting membrane potential — The electrical potential difference across the neuronal membrane at rest, approximately −70 mV (inside negative relative to outside). 3. Action potential — The rapid reversal and restoration of membrane potential during impulse conduction (from −70 mV to +30 mV and back). 4. Depolarisation — Loss of polarity; the membrane becomes less negative (or positive) due to Na+ influx. 5. Repolarisation — Restoration of the negative membrane potential due to K+ efflux after depolarisation. 6. Synapse — The functional junction between two neurons (or a neuron and an effector) where impulse transmission occurs. 7. Saltatory conduction — The jumping of the action potential between successive nodes of Ranvier in myelinated neurons, enabling faster impulse transmission. 8. Reflex action — A rapid, involuntary, and unconscious response to a stimulus, mediated through the spinal cord or brain. 9. Accommodation — The ability of the eye to focus on both near and distant objects by changing the curvature (convexity) of the lens via ciliary muscles. 10. Retina — The innermost layer of the eyeball containing photoreceptor cells (rods and cones) that convert light into neural impulses. 11. Cochlea — The coiled, snail-shaped structure of the inner ear that houses the organ of Corti, the sensory organ for hearing.

Detailed Concepts

Neuron Structure

A neuron is the basic building block of the nervous system. It has three main parts:

• Dendrites — Short, branched cytoplasmic processes that arise from the cell body. They receive signals from other neurons or receptors. Their cytoplasm contains Nissl's granules.
• Cell body (soma / cyton) — Contains the nucleus and is the metabolic centre of the neuron. Key inclusions:
• Nissl's granules — granules of rough endoplasmic reticulum (RER) with ribosomes; site of protein synthesis.
• Neurofibrils — filamentous structures that provide structural support.
• Axon — A single, long cytoplasmic process that carries impulses away from the cell body. Key features:
• Schwann cells — wrap around the axon to form the myelin sheath (in PNS).
• Myelin sheath — a fatty, insulating layer that increases the speed of impulse conduction and prevents leakage of ions.
• Nodes of Ranvier — gaps between adjacent Schwann cells where the axon is exposed; these are the sites of depolarisation during saltatory conduction.
• Axon terminals (synaptic knobs / boutons) — swollen ends of axon branches that contain synaptic vesicles filled with neurotransmitters.

Neurons are classified by the number of processes: multipolar (one axon, many dendrites — most common in CNS), bipolar (one axon, one dendrite — found in retina), and unipolar (single process — found in dorsal root ganglia).

Nerve Impulse Conduction

Resting Membrane Potential

When a neuron is not conducting an impulse, it is in the resting state:

• The inside of the membrane is −70 mV relative to the outside (polarised state).
• Maintained by the Na+/K+ ATPase pump — actively transports 3 Na+ out and 2 K+ in per molecule of ATP hydrolysed.
• Concentration gradients: high Na+ outside, high K+ inside the cell.
• The membrane is more permeable to K+ at rest, so K+ leaks out through leak channels, contributing further to the negative interior.
• Large, negatively charged intracellular proteins (fixed anions) also contribute to the negative resting potential.

Action Potential

When a neuron receives a stimulus above the threshold level, the following sequence occurs:

1. Threshold stimulus → voltage-gated Na+ channels open. 2. Depolarisation — Na+ rushes into the cell → membrane potential rises from −70 mV to +30 mV. 3. Peak — Na+ channels close (inactivated); voltage-gated K+ channels open. 4. Repolarisation — K+ flows out of the cell → membrane potential returns toward −70 mV. 5. Hyperpolarisation (overshoot) — K+ channels are slow to close → membrane briefly becomes more negative than −70 mV. 6. Restoration — the Na+/K+ pump restores the original ion distribution (this occurs during the refractory period).

Important principles:

• All-or-none law — an action potential either occurs fully or does not occur at all; there is no partial action potential.
• Refractory period — a brief time after an action potential during which a second stimulus cannot generate another action potential. Ensures unidirectional impulse flow.

Propagation of Nerve Impulse

• Unmyelinated neurons — continuous conduction: each adjacent segment of the membrane depolarises sequentially. This is slower because every membrane patch must undergo full depolarisation and repolarisation.
• Myelinated neurons — saltatory conduction: the impulse "jumps" from one node of Ranvier to the next, skipping the myelinated segments. This is faster and more energy-efficient because only the nodes undergo depolarisation, and less ion pumping is needed for recovery.

Synapse

A synapse is the junction where two neurons meet. There are two types:

Chemical Synapse

The most common type of synapse in the human body. It consists of:

• Presynaptic neuron (sending side) → synaptic cleft (20–30 nm gap) → postsynaptic neuron (receiving side).

Mechanism of transmission:

1. Action potential arrives at the synaptic knob. 2. Voltage-gated Ca2+ channels open → Ca2+ enters the synaptic knob. 3. Ca2+ triggers fusion of synaptic vesicles with the presynaptic membrane. 4. Neurotransmitter is released into the synaptic cleft (exocytosis). 5. Neurotransmitter binds to specific receptors on the postsynaptic membrane. 6. Ion channels open on the postsynaptic membrane → generates postsynaptic potential.

7. Neurotransmitter is rapidly degraded or taken up (reuptake) to terminate the signal.

• EPSP (Excitatory Postsynaptic Potential) — depolarisation; brings membrane closer to threshold.
• IPSP (Inhibitory Postsynaptic Potential) — hyperpolarisation; moves membrane further from threshold.

Key neurotransmitters:

Electrical Synapse

• Neurons are connected directly by gap junctions (protein channels called connexons).
• Cytoplasmic continuity allows direct flow of ions and small molecules.
• Faster than chemical synapse because there is no synaptic delay.
• Bidirectional transmission (impulse can flow in either direction).
• Found in cardiac muscle, retina, and some invertebrate escape circuits.
• Less common than chemical synapses in the human body.

Central Nervous System (CNS)

The CNS consists of the brain and the spinal cord. It is the command centre that processes information and coordinates responses.

Brain

The brain is protected by the cranium and the meninges (three membranes):

• Dura mater — outermost, tough, thick layer.
• Arachnoid mater — middle, thin, web-like layer.
• Pia mater — innermost, delicate layer that adheres to the brain surface.

Cerebrospinal fluid (CSF) fills the subarachnoid space and ventricles. It protects the brain from mechanical shock, provides nutrients, and removes metabolic waste.

Cerebrum is the largest part of the brain. The outer layer (cerebral cortex) is called grey matter (cell bodies), and the inner region is white matter (myelinated axons). The corpus callosum is a bundle of nerve fibres connecting the two cerebral hemispheres.

Hypothalamus is a frequently tested topic — it is the master regulator of homeostasis and controls the pituitary gland (the "master endocrine gland").

Spinal Cord

• Extends from the medulla oblongata to the L2 vertebra.
• Cross-section: outer white matter (myelinated axons), inner grey matter (neuron cell bodies) arranged in an H-shape.
• Dorsal horn — sensory (afferent) neurons.
• Ventral horn — motor (efferent) neurons.
• Lateral horn — autonomic neurons (present in thoracolumbar region only).
• Central canal — contains CSF.
• Spinal nerves: 31 pairs — 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
• Functions: conduit for nerve impulses between brain and body, and reflex centre.

Peripheral Nervous System (PNS)

The PNS includes all neural tissue outside the CNS (brain and spinal cord). It connects the CNS to limbs and organs.

The sympathetic and parasympathetic divisions generally have opposing effects on target organs (antagonistic), but they are complementary for some functions (e.g., salivation — sympathetic makes it viscous, parasympathetic makes it watery).

Cranial nerves: 12 pairs arising from the brain. Spinal nerves: 31 pairs arising from the spinal cord.

Reflex Action and Reflex Arc

A reflex action is a rapid, involuntary, and unconscious response to a stimulus. It helps the body respond quickly to potentially harmful situations without waiting for brain processing.

Reflex arc — the neural pathway that mediates a reflex action. It consists of five components:

1. Receptor — detects the stimulus (e.g., skin pain receptor). 2. Sensory (afferent) neuron — carries the impulse from the receptor to the CNS. 3. CNS (spinal cord or brain) — processes the information (may involve interneurons). 4. Motor (efferent) neuron — carries the impulse from the CNS to the effector. 5. Effector — the muscle or gland that produces the response.

Types of reflexes:

• Spinal reflex — processed in the spinal cord (e.g., knee-jerk, withdrawal reflex).
• Cranial reflex — processed in the brain (e.g., pupillary reflex, corneal reflex).
• Simple (monosynaptic) reflex — no interneuron; direct synapse between sensory and motor neuron (e.g., knee-jerk reflex).
• Complex (polysynaptic) reflex — involves one or more interneurons in the spinal cord (e.g., withdrawal reflex).

Sense Organs

The body has specialised sense organs to detect external stimuli. The two most important for this chapter are the eye and the ear.

Eye

The human eye is a spherical structure (~2.5 cm diameter). Its wall has three layers:

1. Sclera — outermost, tough, white layer; maintains eyeball shape. The transparent anterior portion is the cornea, which refracts light entering the eye. 2. Choroid — middle, vascular layer; dark pigment prevents internal scattering of light. Anterior modifications:

3. Retina — innermost layer containing photoreceptors (rods and cones) and neurons.

• Ciliary body — contains ciliary muscles that control lens curvature for accommodation.
• Iris — muscular diaphragm that controls pupil size:
• Radial muscles — contract → pupil dilates (more light).
• Circular muscles — contract → pupil constricts (less light).

Lens — biconvex, transparent, lies behind the iris and pupil. Suspended by suspensory ligaments attached to the ciliary body. Changes shape for accommodation.

Humours:

• Aqueous humour — watery fluid in the anterior chamber (between cornea and lens); maintains intraocular pressure; nourishes the cornea and lens.
• Vitreous humour — gel-like substance in the posterior chamber (between lens and retina); maintains eyeball shape.

Rods vs Cones:

Fovea centralis — a small depression in the retina at the visual axis; region of sharpest vision; contains only cones (no rods). Images are focused here during bright light conditions.

Blind spot (optic disc) — the point where the optic nerve exits the eye; contains no photoreceptors — no image is formed here.

Eye defects:

Ear

The ear performs two functions: hearing and balance (equilibrium). It has three regions:

Outer ear:

• Pinna — visible external flap; collects and directs sound waves.
• Auditory canal (external auditory meatus) — tube leading to the eardrum; contains wax glands (ceruminous glands) that protect the canal.
• Tympanic membrane (eardrum) — thin membrane at the boundary of outer and middle ear; vibrates when sound waves hit it.

Middle ear:

• Air-filled chamber containing three tiny bones called ossicles — the smallest bones in the body:
• Malleus (hammer) — attached to the tympanic membrane.
• Incus (anvil) — connects malleus to stapes.
• Stapes (stirrup) — attached to the oval window of the inner ear.
• The ossicles amplify the sound vibrations approximately 20 times as they transmit from the tympanic membrane to the oval window.
• Eustachian tube (auditory tube) — connects the middle ear to the nasopharynx; equalises air pressure on both sides of the eardrum (important during altitude changes, e.g., swallowing during take-off).

Inner ear:

• Consists of the bony labyrinth (filled with perilymph) and the membranous labyrinth (filled with endolymph).
• Cochlea — coiled, snail-shaped structure for hearing:
• Contains the basilar membrane with the organ of Corti (spiral organ).
• Organ of Corti — contains hair cells (the actual auditory receptors) resting on the basilar membrane, with their stereocilia embedded in the tectorial membrane.
• Helicotrema — the opening at the apex where the scala vestibuli and scala tympani connect.
• Vestibular apparatus — for balance and equilibrium:
• Semicircular canals (3 canals) — detect angular acceleration (rotational movement of the head).
• Utricle and saccule — detect linear acceleration and gravity (posture and orientation); contain macula with otoliths (calcium carbonate crystals).

Hearing mechanism (pathway of sound):

Sound waves → Pinna (collects) → Auditory canal → Tympanic membrane (vibrates) → Ossicles amplify (malleus → incus → stapes) → Oval window → Perilymph vibrates → Basilar membrane vibrates → Hair cells in organ of Corti depolarise → Impulses via auditory nerve → Temporal lobe of cerebrum (auditory cortex).

Important Diagrams

These diagrams are frequently asked in CBSE exams. Practise drawing and labelling them:

1. Neuron structure — Label: dendrites, cell body (cyton), Nissl's granules, axon, myelin sheath, Schwann cells, nodes of Ranvier, axon terminals (synaptic knobs). 2. Action potential graph — Plot membrane potential (mV) vs time (ms). Label: resting potential (−70 mV), depolarisation phase (+30 mV peak), repolarisation, hyperpolarisation, threshold level, refractory period. 3. Chemical synapse — Label: presynaptic neuron, synaptic knob, synaptic vesicles, neurotransmitter, synaptic cleft, postsynaptic neuron, receptors, Ca2+ channels. 4. Human brain (sagittal section) — Label: cerebrum, thalamus, hypothalamus, midbrain, cerebellum, pons, medulla oblongata, corpus callosum. 5. Cross-section of the eye — Label: sclera, cornea, choroid, retina, lens, iris, pupil, aqueous humour, vitreous humour, optic nerve, fovea, blind spot. 6. Structure of the ear — Label: pinna, auditory canal, tympanic membrane, malleus, incus, stapes, oval window, cochlea, semicircular canals, utricle, saccule, Eustachian tube.

Common Mistakes

1. Confusing depolarisation with repolarisation — Depolarisation is Na+ influx (membrane becomes positive); repolarisation is K+ efflux (membrane returns to negative). Do not mix up the ion and direction.

2. Saying myelin sheath is made by neurons — The myelin sheath is formed by Schwann cells in the PNS and oligodendrocytes in the CNS. Neurons themselves do not produce myelin.

3. Confusing cerebrum with cerebellum — The cerebrum controls intelligence, memory, learning, and voluntary actions. The cerebellum controls balance, posture, and coordination of voluntary movements. They sound similar but have very different functions.

4. Writing that rods detect colour — Only cones detect colour (trichromatic — red, green, blue). Rods detect dim light and are responsible for scotopic (night) vision; they cannot distinguish colours.

5. Forgetting that the blind spot has no photoreceptors — The blind spot (optic disc) is where the optic nerve exits the eye. There are no rods and no cones at this point, so no image is formed here.

6. Confusing Eustachian tube function — The Eustachian tube equalises air pressure between the middle ear and the atmosphere. It does not transmit sound. Sound is transmitted by the ossicles.

7. Saying sympathetic and parasympathetic are antagonistic for ALL functions — While they are generally antagonistic (opposing), some functions are complementary. For example, both stimulate salivation — sympathetic makes it viscous, parasympathetic makes it watery.

Exam Tips

1. Draw the neuron diagram with all parts — This is one of the most commonly asked diagrams. Label every part: dendrites, cell body, Nissl's granules, axon, myelin sheath, Schwann cells, nodes of Ranvier, and synaptic knobs.

2. Action potential graph — Draw and label the graph with depolarisation, repolarisation, hyperpolarisation, and threshold level. Mention the values (−70 mV, +30 mV).

3. Chemical synapse — Always mention the role of Ca2+ in triggering vesicle fusion and neurotransmitter release. This is a specific detail examiners look for.

4. Brain parts and functions — Create a table with forebrain, midbrain, and hindbrain divisions. The hypothalamus is frequently tested — remember it controls homeostasis and the pituitary gland.

5. Eye accommodation — Explain the role of ciliary muscles and how lens curvature changes. For distant objects: muscles relax → lens flat. For near objects: muscles contract → lens convex.

6. Reflex arc — Always draw the 5-component pathway: Receptor → Sensory neuron → CNS → Motor neuron → Effector. Label each part clearly.

7. Ear ossicles — Know the names in order: malleus → incus → stapes. Remember they amplify sound approximately 20 times. The Eustachian tube equalises pressure, not transmits sound.

Quick Revision

• Neuron: dendrites (receive signals), cell body (Nissl's granules, nucleus), axon (carries impulse), myelin sheath (Schwann cells), nodes of Ranvier (gaps for saltatory conduction).
• Resting potential: −70 mV maintained by Na+/K+ pump (3 Na+ out, 2 K+ in per ATP).
• Action potential: −70 mV → +30 mV (depolarisation) → −70 mV (repolarisation); all-or-none law.
• Depolarisation: Na+ influx; Repolarisation: K+ efflux; Na+/K+ pump restores ionic balance.
• Saltatory conduction: faster impulse transmission in myelinated neurons (jumps between nodes of Ranvier).
• Chemical synapse: Ca2+ entry → vesicle fusion → neurotransmitter release → binds receptor → postsynaptic potential (EPSP or IPSP).
• Key neurotransmitters: ACh (excitatory), GABA (inhibitory), dopamine (motor/reward), glutamate (excitatory), serotonin (mood).
• Brain: cerebrum (intelligence), thalamus (relay), hypothalamus (homeostasis), cerebellum (balance), medulla (vital centres — respiration, cardiovascular).
• PNS: somatic (voluntary) + autonomic (sympathetic = fight/flight, parasympathetic = rest/digest).
• Reflex arc: receptor → sensory neuron → CNS → motor neuron → effector; rapid, involuntary.
• Eye: cornea (refracts light), lens (accommodation), retina (rods = dim light, cones = colour), fovea centralis (sharpest vision, only cones), blind spot (no photoreceptors).
• Eye defects: myopia (concave lens), hypermetropia (convex lens), astigmatism (cylindrical lens), presbyopia (bifocal), cataract (surgery).
• Ear: pinna, ossicles (malleus-incus-stapes, amplify ~20x), cochlea (organ of Corti — hair cells for hearing), semicircular canals (angular acceleration/balance), utricle/saccule (linear acceleration, posture), Eustachian tube (pressure equalisation).