Nervous System, Sense Organs, Hormones, and Tropic Responses
Organization and Function of the Nervous System
Stimulus and Response Dynamics
Irritability (Sensitivity): The ability of an organism to detect and respond to a stimulus in its internal or external environment.
Stimulus (plural: stimuli): Any change in an organism's internal or external environment that causes a physiological or behavioral reaction.
Response: The specific physical or physiological reaction of an organism to a stimulus.
Receptors: Specialized sense organs or cell clusters that detect environmental stimuli and initiate electrical signals.
Effectors: Muscles or glands that execute a biological response when stimulated by motor nerve impulses.
Nerve Impulses: High-speed electrical signals transmitted along nerve cells (neurones).
Coordinator: The central nervous system, which receives, integrates, and processes sensory inputs to determine appropriate responses.
Divisions of the Nervous System
The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
Central Nervous System (CNS): Comprises the brain and the spinal cord. It acts as the central control unit for receiving, processing, and coordinating information.
Peripheral Nervous System (PNS): Consists of cranial nerves originating from the brain, spinal nerves originating from the spinal cord, and all associated sense organs. It connects the rest of the body to the CNS.
Tissue Organization in the Central Nervous System
The CNS contains two distinct tissue regions: grey matter and white matter.
Grey Matter: Consists primarily of neurone cell bodies, dendrites, and synapses. It forms the outer cerebral cortex layer of the brain and the inner H-shaped core of the spinal cord.
White Matter: Consists primarily of myelinated nerve fibres (axons). It forms the central interior regions of the brain and the outer surrounding layer of the spinal cord.
Anatomy of the Spinal Cord
The spinal cord extends downwards from the medulla oblongata to the lower end of the vertebral column.
In cross-section, the grey matter forms a central butterfly or H-shape containing a central canal, surrounded by white matter.
Spinal nerves join the spinal cord via two distinct roots:
Dorsal Root: Contains sensory neurone axons entering the spinal cord. Cell bodies of these sensory neurones aggregate in a swollen swelling called the dorsal root ganglion.
Ventral Root: Contains motor neurone axons leaving the spinal cord towards effectors.
Mixed spinal nerves contain both sensory and motor nerve fibres combined within a single tissue sheath.
Relay Neurones in the Spinal Cord: Positioned in the grey matter and running parallel along the spinal cord length. They form synapses between sensory inputs and motor outputs, or relay impulses upward to the brain and downward to effector neurones.
Major Structural Regions of the Human Brain
The Forebrain:
Cerebrum: Divided into left and right cerebral hemispheres. Responsible for higher cognitive functions including intelligence, memory, learning, reasoning, sensory perception, and conscious control of voluntary actions.
Hypothalamus: Regulates homeostatic biological processes such as body temperature, appetite, thirst, sleep cycles, and emotional reactions.
Pituitary Gland: Designated as the master endocrine gland; controls hormone secretion across multiple peripheral endocrine glands.
The Midbrain:
Contains optic lobes responsible for visual reflexes and tracking mechanisms.
The Hindbrain:
Cerebellum: Coordinates smooth muscular activity, motor control, posture, and body balance/equilibrium.
Medulla Oblongata: Controls automatic, involuntary bodily functions such as heart rate, respiratory depth, peristalsis, and blood vessel contraction/dilation.
Neurone Structure, Synaptology, and Reflex Pathways
Functional Classes of Neurones
Sensory Neurones: Transmit electrical impulses from sensory receptors towards the CNS.
Relay (Intermediate) Neurones: Located within the CNS; transmit electrical impulses between sensory neurones and motor neurones across synaptic junctions.
Motor Neurones: Transmit electrical impulses away from the CNS to target effectors (muscles or glands).
Structural Components of Neurones
Dendron: Elongated cellular extension that transmits nerve impulses toward the cell body, branching into fine dendrites to receive impulses from receptors or other neurones.
Cell Body (Soma): Contains cytoplasm and the nucleus. It exhibits an irregular shape in motor neurones and a rounded circular shape in sensory neurones.
Axon: Elongated nerve fibre that conducts electrical nerve impulses away from the cell body.
Motor End Plate: Neuromuscular junction consisting of terminal axon branches contacting muscle fibers, releasing chemical transmitters to stimulate muscle contraction.
Myelin Sheath: Concentric wrapping of lipid-rich membrane insulation around axons. Speeds up impulse conduction and prevents signal dissipation.
Node of Ranvier: Unmyelinated gap along the axon between adjacent myelin sheath segments. Allows saltatory conduction, where impulses jump between nodes to increase transmission speed.
Physiology of Nerve Impulses
Nerve fibres do not transmit raw sensations (such as pain or cold); sensations are constructed only when electrical impulses arrive at specific sensory processing centers in the brain.
Nerve impulses consist of action potentials traveling down the neuronal membrane.
Impulse Duration: Each action potential pulse lasts approximately .
Conduction Velocity: Impulses propagate along nerve fibres at speeds reaching up to .
Synaptic Transmission
Synapse: Specialized microscopic junction between two neurones, or between a neurone and an effector organ, across which electrical signals are converted into chemical signals.
Transmission Mechanism: Neurotransmitter chemical molecules stored in synaptic vesicles are released from the pre-synaptic axon terminal, diffuse across the narrow synaptic gap, and bind to specific protein receptor molecules on the post-synaptic membrane.
Unidirectional Flow: Synapses enforce one-way conduction because neurotransmitter chemicals are synthesized and released exclusively on the pre-synaptic side, while target receptor proteins exist exclusively on the post-synaptic membrane.
Synaptic Delay: Chemical diffusion across the synaptic gap slightly decreases overall signal transmission speed.
Pharmacological and Toxicological Effects on Synapses
Heroin and Morphine: Bind to and stimulate specific receptor molecules in brain synapses, triggering excessive dopamine release that causes a short-lived state of euphoria.
Spider Toxins and Tetanus Toxin: Produced by bacterial infection with Clostridium tetanus. Causes massive breakdown of presynaptic vesicles, triggering uncontrolled release of transmitter chemicals and disrupting synaptic regulation.
Tetanus Clinical Symptoms: Uncontrolled muscle spasms, lock-jaw, severe physiological rigidity, and potential heart failure.
Voluntary versus Reflex Actions
Voluntary Actions: Conscious actions initiated under deliberate cerebral control. They originate in the brain and do not strictly require sensory receptor input.
Reflex Actions: Rapid, automatic, involuntary physiological responses to specific external or internal stimuli, executed without conscious brain intervention.
Reflex Centres: Integration regions in the spinal cord and brain that instantly process sensory input and trigger output responses.
Cranial Reflexes: Coordinated by the brainstem without conscious thought; primarily govern head structures (e.g., pupil constriction, blinking, salivation).
Spinal Reflexes: Coordinated directly within the grey matter of the spinal cord (e.g., knee-jerk reflex, withdrawal reflex).
Reflex Arc Mechanisms
Reflex Arc Pathway: The shortest neural circuit traversed by nerve impulses during a reflex response:
Case Study 1: Knee-Jerk Reflex (Monosynaptic Spinal Reflex)
Tapping the patellar tendon below the knee stretches muscle spindle stretch receptors.
Stretch receptors fire impulses along sensory neurones passing through the dorsal root into the spinal cord grey matter.
Sensory neurones synapse directly onto motor neurones (relay neurones are not involved).
Motor neurones convey impulses to quadriceps leg muscles, prompting contraction and causing the lower leg to kick forward.
Reflex Arc:
Case Study 2: Painful Withdrawal Reflex (Polysynaptic Spinal Reflex)
Touching a sharp or hot object stimulates thermal or nociceptive pain receptors in the skin.
Impulses travel via sensory neurones through the dorsal root into the grey matter of the spinal cord.
Impulse crosses a first synapse to a relay neurone, then a second synapse from the relay neurone to a motor neurone.
Motor neurone transmits impulses along the spinal nerve to biceps muscle effectors, inducing muscle contraction to instantly pull the hand away.
Concurrently, ascending relay neurones carry secondary signals up the spinal cord to the cerebral cortex, producing the conscious sensation of pain after the reflex action has occurred.
Reflex Arc:
Sensory Reception and Anatomy of the Human Eye
Overview of Sense Organs
Sense organs contain specialized groups of sensory receptor cells tailored to detect specific environmental stimuli:
Ear: Detects sound waves and body movement/orientation (balance).
Eye: Detects light stimuli.
Nose: Detects airborne chemical odorants (smell).
Tongue: Detects dissolved chemical substances (taste).
Skin: Detects temperature changes, physical pressure, tactile touch, and tissue damage (pain).
External Features and Protection of the Eye
The eyeball rests securely within the bony socket or orbit of the skull.
Position and rotation are controlled by rectus muscles anchoring the eyeball to the bony orbit.
Eyebrows: Prevent sweat dripping down the forehead from entering the eye.
Eyelashes: Trap foreign dust particles and debris.
Eyelids: Close automatically via involuntary blinking reflexes to protect the corneal surface from mechanical injury and desiccation.
Tears: Secreted by lacrimal tear glands; blinking lubricates the cornea and spreads tear fluid containing antibacterial enzymes (such as lysozyme) to destroy pathogens.
Anatomy and Functions of the Human Eye
Sclera: Tough, fibrous, white outermost coat that protects internal ocular structures and maintains eyeball shape.
Cornea: Transparent anterior continuation of the sclera; refracts incoming light rays and allows light to enter the eye.
Conjunctiva: Thin, vascularized epithelial membrane lining the outer front surface of the eye; protects the cornea.
Choroid: Dark, pigmented middle layer rich in blood capillaries; supplies oxygen and nutrients to retinal cells and absorbs excess light to prevent internal reflection.
Iris: Pigmented circular muscle diaphragm extending from the choroid; contains circular and radial muscles that contract and relax to control pupil diameter and regulate light entry.
Pupil: Central aperture inside the iris that acts as an adjustable gateway for light entry.
Lens: Transparent, elastic, biconvex crystalline structure that fine-tunes light refraction to focus clear images on the retina.
Ciliary Body / Muscles: Ring of smooth muscle extending from the choroid; alters lens curvature during accommodation.
Suspensory Ligaments: Collagenous cords connecting the ciliary body to the lens capsule; transmit muscular tension to change lens shape.
Vitreous Humour: Transparent, jelly-like substance occupying the posterior cavity behind the lens; maintains eyeball shape, absorbs shock, and helps keep the retina pressed flat.
Aqueous Humour: Clear, watery fluid filling the anterior cavity between the cornea and lens; maintains intraocular pressure, provides nutrients to avascular lens and corneal tissues, and refracts light.
Retina: Innermost light-sensitive neural coat containing photoreceptor cells that translate light energy into nerve impulses.
Yellow Spot (Fovea Centralis): Shallow depression on the retina situated directly in line with the optical axis; packed with high-density cone cells to provide maximum visual acuity and color resolution.
Blind Spot: Exit point where optic nerve fibers leave the eyeball; completely lacks photoreceptors, rendering it incapable of detecting light.
Optic Nerve: Bundle of sensory nerve fibers conveying action potentials from retinal ganglion cells to the visual cortex of the brain.
Photoreceptor Organization and Physiology
Retinal Architecture: Light passes through the inner retinal layers in the following order:
Nerve impulses generated by rods and cones are transmitted through bipolar neurones to sensory ganglion cells, whose axons aggregate to form the optic nerve.
Comparative Characteristics of Rods and Cones
Rod Cells:
Provide monochromatic black-and-white vision in low light conditions (scotopic vision).
Possess extreme sensitivity to low light intensities, enabling night vision.
Contain the light-sensitive photosynthetic pigment rhodopsin (a biochemical derivative of Vitamin A).
Bright light bleaches rhodopsin completely, rendering rod cells inactive in intense light. Rhodopsin regenerates gradually in dark environments.
Nutritional deficiency in Vitamin A prevents adequate rhodopsin synthesis, resulting in night-blindness.
Highly sensitive to detecting motion and spatial movement.
Responsible for peripheral vision; completely absent from the central fovea centralis but densely clustered across peripheral regions of the retina.
Cone Cells:
Responsible for high-resolution color vision (photopic vision).
Divided into three distinct spectral subtypes: red, green, and blue cones, each containing visual pigments tuned to absorb specific wavelengths of light.
Concentrated densely inside the fovea centralis, allowing detailed visual discrimination.
Respond rapidly to light variations, enabling perception of fast-moving dynamic images.
Inactive under low light conditions, resulting in loss of color vision in dark environments.
Ocular Mechanisms: Accommodation and Pupil Reflex
Physics of Image Formation on the Retina
Light rays reflected from an object enter the eye and undergo refraction sequentially through the cornea, aqueous humour, crystalline lens, and vitreous humour.
Refracted light rays converge precisely onto photoreceptors of the retina.
Image Properties on Retina: The image focused on the retina is real, inverted, laterally inverted, and diminished relative to the physical object.
Visual Perception: Action potentials travel along optic nerve fibers to the visual cortex in the brain, where the visual signal is processed so the object is perceived upright and correctly proportioned.
Accommodation Mechanics for Distance Vision
Accommodation refers to the reflex adjustment of lens curvature to focus objects at varying distances sharply onto the retina.
Light rays from distant objects () travel virtually parallel when reaching the cornea.
Reflex Sequence for Distant Objects:
Ciliary muscles relax, expanding the ciliary ring diameter away from the lens.
Suspensory ligaments pull outward, becoming taut.
Increased ligament tension flattens the elastic lens, making it thinner and less convex.
Focal length of the lens increases, focusing parallel light rays sharply onto the retina.
Accommodation Mechanics for Near Vision
Light rays originating from near objects diverge as they approach the eye and require greater refractive power to converge.
Reflex Sequence for Near Objects:
Ciliary muscles contract, moving the ciliary ring inward toward the lens center.
Suspensory ligaments relax and slacken, releasing tension on the lens capsule.
The elastic lens bulges outward under its intrinsic elasticity, becoming thicker and more convex.
Focal length of the lens decreases, refracting divergent light rays to focus them sharply onto the retina.
The Pupil Reflex
An involuntary cranial reflex that regulates light entry into the eye, protecting retinal cells from photic damage in bright light and maximizing vision in dim light.
Controlled by two antagonistic smooth muscle groups located within the iris: circular muscles and radial muscles.
Response in Bright Light:
Circular muscles contract while radial muscles relax.
Pupil diameter decreases (constricts), restricting the amount of light entering the eye.
Response in Dim Light:
Radial muscles contract while circular muscles relax.
Pupil diameter increases (dilates), maximizing light entry to stimulate rods.
Pupil Reflex Arc:
Human Endocrine System and Hormonal Regulation
Fundamentals of Endocrine Control
The nervous system and endocrine system operate cooperatively to coordinate metabolic functions and maintain homeostatic equilibrium.
Hormone Definition: A organic chemical regulatory substance produced and secreted in minute amounts by an endocrine gland, transported via the circulatory system to target tissues, where it alters metabolic activity.
Gland Definition: An organ or cluster of specialized secretory cells that synthesizes biochemical compounds for systemic release.
Exocrine Glands: Secretory glands that release non-hormonal substances through specialized ducts onto epithelial surfaces or body cavities (e.g., sweat glands, salivary glands).
Endocrine Glands: Ductless glands that secrete hormones directly into surrounding tissue fluids and blood capillaries for systemic circulation (e.g., adrenal glands, pituitary gland).
Target Specificity: Target cells express specific surface or intracellular protein receptors that selectively bind matching hormone molecules.
Clearance: Excess or worn-out hormones are inactivated by enzymatic breakdown in the liver and excreted by the kidneys.
Overview of Major Human Endocrine Glands
Pituitary Gland: Situated beneath the brain; regulated directly by neural and humoral signals from the hypothalamus. Secretes Antidiuretic Hormone (ADH), which increases water reabsorption in renal kidney collecting ducts. Also produces Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) to control gonadal function and maturation.
Thyroid Gland: Located in the neck region. Secretes thyroxine, which regulates basal metabolic rates, oxygen consumption, cellular energy production, and physical/mental development.
Adrenal Medulla: Located atop each kidney. Secretes adrenaline to prepare the body for immediate emergency "fight or flight" responses.
Pancreas (Islets of Langerhans): Dual endocrine/exocrine organ. Endocrine islet tissue secretes insulin and glucagon to regulate systemic blood glucose levels.
Testes (Male Gonads): Secrete testosterone, which stimulates spermatogenesis and promotes secondary male sexual characteristics during puberty.
Ovaries (Female Gonads): Secrete oestrogen and progesterone. Oestrogen stimulates uterine mucosal repair and endometrial thickening; progesterone maintains endometrial integrity for pregnancy. Both regulate oocyte maturation and female secondary sexual traits.
Homeostatic Regulation of Blood Glucose and Pathophysiology of Diabetes
Glycemic Homeostasis Dynamics
Normal baseline blood glucose concentration in healthy humans is maintained within a strict physiological range between and
Endocrine control is mediated by microscopic endocrine cell clusters in the pancreas known as the Islets of Langerhans:
Beta () Cells: Synthesize and secrete insulin when blood glucose rises above set limits.
Alpha () Cells: Synthesize and secrete glucagon when blood glucose drops below normal limits.
Actions of Insulin and Glucagon
Insulin Action (Hypoglycemic Hormone):
Increases glucose uptake across muscle and peripheral cell membranes.
Promotes glycogenesis (conversion of excess blood glucose into storage glycogen in liver and skeletal muscle cells).
Converts excess glucose into triglycerides for fat storage within adipose tissue.
Accelerates cellular respiration rates to oxidise glucose into and .
Glucagon Action (Hyperglycemic Hormone):
Stimulates glycogenolysis (breakdown of liver glycogen stores back into glucose released into the blood).
Promotes gluconeogenesis (synthesis of new glucose molecules from fats, glycerol, and amino acid substrates).
Physiological Consequences of Hormone Imbalance
Insulin Deficiency: Causes persistent hyperglycemia, glucosuria, tissue wasting, and rapid fat breakdown leading to excessive production of acidic ketone bodies (ketoacidosis), which lowers blood pH to potentially fatal levels.
Insulin Excess: Triggers severe hypoglycemia, causing profound neurological impairment, insulin shock, loss of consciousness, and potentially death.
Glucagon Deficiency: Leads to chronic hypoglycemia, muscle weakness, and lethargy.
Glucagon Excess: Causes persistent metabolic overactivity and elevated blood glucose levels.
Pathophysiology of Diabetes Mellitus
A chronic metabolic disorder characterized by systemic inability to maintain normal blood glucose balance.
Glucosuria occurs when blood glucose levels exceed the reabsorptive capacity of renal kidney tubules, causing glucose to overflow into excreted urine.
Clinical Manifestations: Chronic hyperglycemia, frequent urination (polyuria), persistent thirst (polydipsia), delayed wound healing, fatigue, and unexplained weight loss.
Classification and Management of Diabetes Mellitus
Type 1 Diabetes (Juvenile-Onset Diabetes):
Typically develops during childhood or early adolescence.
Autoimmune destruction of pancreatic islet cells leads to absolute insulin deficiency.
Can be triggered by environmental factors or viral infections damaging pancreatic tissue.
Required Treatment: Requires lifelong daily insulin injections combined with continuous blood glucose monitoring and dietary management.
Type 2 Diabetes (Late-Onset Diabetes):
Commonly develops in adults aged .
Circulating insulin levels may be normal or elevated, but peripheral target cells exhibit insulin resistance (loss of cellular sensitivity to insulin).
Strongly linked to obesity, physical inactivity, and genetic predisposition.
Required Treatment: Managed through dietary carbohydrate restriction, regular physical exercise, weight reduction, oral hypoglycemic medications, and supplemental insulin therapy if necessary.
Adrenaline Physiology and Comparative Neuro-Endocrine Analysis
Adrenaline Action and Emergency Stress Response
Adrenaline (epinephrine) is synthesized and secreted by the adrenal medulla in response to acute physical or psychological stress, fear, excitement, or danger.
Secretion is directly triggered by sympathetic neural stimulation from the hypothalamus.
It coordinates rapid multi-organ responses to prepare the body for immediate "fight or flight" action.
Systemic Physiological Effects of Adrenaline
Heart: Increases heart rate and stroke volume, boosting blood pressure to speed delivery of oxygen and glucose to active skeletal muscles (may produce transient hypertension or palpitations).
Respiratory Centre in Brain: Increases breathing rate and depth, expanding pulmonary gas exchange to rapidly oxygenate blood and remove excess carbon dioxide (causes hyperpnea or panting).
Liver: Accelerates glycogen breakdown into glucose, elevating blood glucose concentration to provide metabolic fuel for exercising tissue.
Skeletal Muscles: Increases muscle tone and readies muscular tissue for explosive force output (produces feelings of physical tension and anxiety).
Pupils: Stimulates radial iris muscle contraction, dilating the pupils to maximize light capture and broaden the visual field.
Skin and Digestive Arterioles: Constricts peripheral arterioles supplying cutaneous skin layers and digestive organs, diverting blood flow to skeletal muscles (causes skin pallor and dry mouth).
Alimentary Canal Muscles: Relaxes gastrointestinal smooth muscle wall tissue, inhibiting digestion and peristaltic contractions to conserve metabolic energy (causes a empty "hollow" sensation in the stomach).
Comprehensive Comparison: Nervous System vs. Endocrine System
Signal Type: Nervous system uses electrical impulses (action potentials); Endocrine system uses chemical hormones.
Conduction Pathway: Nervous impulses travel along specialized neuronal membranes; Hormones travel globally through the vascular bloodstream.
Response Speed: Nervous actions are extremely rapid (milliseconds); Hormonal responses develop more slowly (seconds to hours).
Duration of Effect: Nervous responses are brief and short-lived; Hormonal actions are prolonged and long-lasting.
Control Nature: Nervous impulses can be voluntary or involuntary; Endocrine secretion is strictly involuntary.
Target Specificity: Nervous effects are localized to precise muscle groups or glands; Hormonal effects are widespread, influencing multiple target organs simultaneously.
Signal Circuitry:
Nervous Circuit:
Endocrine Circuit:
Plant Tropisms and Hormonal Control Mechanisms
Plant Tropisms and Growth Dynamics
Tropism: Directional growth movement of a plant organ in response to an directional environmental stimulus.
Positive Tropism: Growth movement directed towards the incoming stimulus source.
Negative Tropism: Growth movement directed away from the incoming stimulus source.
Phototropism: Directional plant growth in response to unilateral light.
Gravitropism (Geotropism): Directional plant growth in response to the force of gravity.
Experimental Verification of Tropisms
Phototropism in Shoots: A potted shoot exposed to light coming from one side bends and grows towards the light source within (positive phototropism). Control shoots illuminated uniformly grow straight upward.
Gravitropism in Roots (Radicles): Germinating seedlings placed horizontally inside a dark chamber bend their radicles downward towards gravity (positive gravitropism). Control seedlings mounted on a continuously rotating clinostat grow straight horizontally because gravity acts equally across all sides.
Localization of Growth Region: Marking equidistant ink lines along a straight root tip demonstrates that bending curvature occurs specifically within the region of elongation, where cells stretch lengthwise.
Adaptive Significance of Plant Tropisms
Positive Phototropism in Shoots: Positions leaves for maximum light capture needed for photosynthesis; elevates flowers into open air to enhance pollination by flying insects.
Negative Gravitropism in Shoots: Drives shoots to grow vertically upward, lifting photosynthetic leaves above shaded ground levels to compete for light and carbon dioxide.
Positive Gravitropism in Roots: Drives main taproots deep into soil layers to securely anchor the plant and access underground water and dissolved mineral nutrients.
Lateral Root Behavior: Secondary lateral roots grow horizontally or slightly downward, expanding the soil area explored for nutrient uptake.
Auxins and Cellular Elongation Mechanics
Auxin: Primary plant hormone class; chemically known as indoleacetic acid (IAA).
Production Site: Synthesized continuously by actively dividing cells in apical meristems (shoot tips and root tips).
Transport: Transported downward away from the tip via active transport into the cell elongation zone, where it softens cell walls to promote cell extension.
Mechanism of Shoot Phototropism:
Unilateral light exposure causes auxins to migrate laterally from the illuminated side to the shaded side of the shoot tip.
Higher auxin concentration on the shaded side causes those cells to absorb water and elongate faster than cells on the illuminated side.
Differential growth causes the shoot stem to curve toward the light source.
Mechanism of Shoot Gravitropism:
When a shoot is oriented horizontally, gravity causes auxins to accumulate along the lower side of the stem.
Elevated auxin concentration on the lower side stimulates faster cell elongation, bending the shoot upward (negative gravitropism).
Mechanism of Root Gravitropism:
Unlike shoots, high auxin concentrations inhibit cell elongation in root tissue.
When a root is oriented horizontally, auxin accumulates along the lower side of the root tip.
Inhibited growth on the lower side allows cells on the upper side to elongate normally, causing the root tip to curve downward toward gravity (positive gravitropism).
Classic Coleoptile Tropism Experiments
Experiment A (Decapitated Tip): Tip removed; coleoptile shows no growth and no phototropic bending.
Experiment B (Untreated Control): Intact tip; coleoptile elongates normally and bends towards unilateral light.
Experiment C (Opaque Cap): Tip covered with an opaque black paper cap; coleoptile grows vertically upward but does not bend towards unilateral light, proving the tip is the photoreceptive site.
Experiment D (Agar Block Transfer): Decapitated tip placed on an agar block for to absorb auxin. Placing the loaded agar block offset onto one side of a decapitated coleoptile in total darkness causes cell elongation on that side, bending the stem away from the agar block.
Agricultural Applications of Synthetic Growth Regulators
Synthetic Auxins (e.g., / 2,4-dichlorophenoxyacetic acid): Manufactured chemical compounds mimicking natural auxins, widely used as selective broadleaf weedkillers.
Mode of Action: Absorbed selectively by broad-leaved dicot weeds, causing rapid, uncontrolled growth and abnormally accelerated metabolic respiration. Weeds rapidly exhaust their stored carbohydrate reserves and die, leaving narrow-leaved monocot crops unharmed.
Review Questions and Conceptual Discussion
Nervous System Questions
Question 1: What is the difference between a nerve and a nerve fibre?
Answer: A nerve fibre is an individual elongated cellular extension (axon or dendron) growing out from a single neurone cell body. A nerve is a macroscopically visible structure consisting of multiple bundles of nerve fibres bound together within protective connective tissue sheaths.
Question 2: In what ways are sensory neurones and motor neurones similar in (i) structure, (ii) function?
Answer: (i) Structurally, both possess cell bodies containing nuclei and cytoplasm, elongated axon/dendron processes, node-interrupted myelin sheaths, and form synaptic junctions with other cells. (ii) Functionally, both conduct unidirectional electrical action potentials using membrane depolarization.
Question 3: Can a nerve fibre and a nerve carry both sensory and motor impulses? Explain your answers.
Answer: An individual nerve fibre cannot; it is a single extension that conducts impulses in one direction only. However, a whole nerve can carry both sensory and motor impulses if it is a mixed spinal nerve containing separate bundles of sensory and motor nerve fibres inside a shared connective tissue sheath.
Question 4: Which receptors and effectors are involved in the reflex actions of sneezing, blinking, and contraction of the iris?
Answer:
Sneezing: Receptors are olfactory/nasal mucosal epithelial irritant receptors; effectors are intercostal respiratory muscles, diaphragm, and laryngeal muscles.
Blinking: Receptors are corneal tactile/pain receptors; effectors are orbicularis oculi eyelid muscles.
Iris Contraction: Receptors are retinal photoreceptors (rods and cones); effectors are circular iris smooth muscles.
Question 5: Explain why the tongue may be considered to be both a receptor and an effector organ.
Answer: The tongue acts as a receptor organ because its surface contains taste buds containing gustatory receptor cells. It acts as an effector organ because it contains skeletal muscle tissue that contracts in response to motor nerve signals during chewing and swallowing.
Question 6: Discuss whether coughing is a voluntary or reflex action.
Answer: Coughing is primarily an involuntary spinal/cranial reflex triggered by airway irritants, but it can also be voluntarily initiated or suppressed by conscious signals sent from the cerebral cortex.
Question 7: What sensation would you expect to feel if a warm pin-head was pressed onto a touch receptor in your skin? Explain your answer.
Answer: Touch, because sensory receptors transmit specific sensory signals to dedicated visual, tactile, or thermal processing areas in the brain regardless of the mechanical or thermal stimulus applied.
Question 8: If you could intercept and 'listen to' the nerve impulses travelling in the spinal cord, could you tell which ones came from pain receptors and which from temperature receptors? Explain your answer.
Answer: No, because all action potentials consist of identical electrical impulses. The specific sensation depends entirely on which regional tract in the spinal cord the impulses travel through and which brain region receives them.
Question 9: Would you expect synapses to occur in grey matter or in white matter? Explain your answer.
Answer: In grey matter, because grey matter contains neurone cell bodies, dendrites, and terminal axon arborizations where synaptic connections are formed. White matter consists almost exclusively of myelinated axons.
Question 10: If the spinal cord were damaged at a point about one-third of the way up the vertebral column, what effect would you expect this to have on bodily functions?
Answer: Paralysis and loss of sensation across all lower body regions below the injury level, because ascending sensory impulses and descending motor impulses are blocked at the damaged site.
Sense Organ (Eye) Questions
Question 1: Apart from the cells that detect chemicals, what other types of receptor must be present in the tongue?
Answer: Mechanoreceptors (detecting tactile touch and texture), thermoreceptors (detecting temperature), and nociceptors (detecting pain from spicy or damaging compounds).
Question 2: Describe and explain how the eye changes its focus from a distant object to a near object.
Answer: Ciliary muscles contract, moving inward toward the lens; suspensory ligaments slacken, releasing tension on the elastic lens; the lens becomes thicker and more convex, decreasing its focal length to focus divergent light rays onto the retina.
Hormone Questions
Question 1: The pancreas has a dual function in producing digestive enzymes as well as hormones. Which other endocrine glands have a dual function and what are their other functions?
Answer: The testes (produce testosterone and sperm cells) and the ovaries (produce oestrogen/progesterone and egg cells).
Question 2: What are the effects on body functions of (a) too much insulin, (b) too little insulin?
Answer: (a) Too much insulin causes severe hypoglycemia, leading to muscle tremors, dizziness, insulin shock, and potential fatality. (b) Too little insulin causes hyperglycemia, glucosuria, ketoacidosis, dehydration, progressive tissue wasting, and diabetic coma.
Question 3: Why do you think urine tests are carried out to see if a woman is pregnant?
Answer: Because during pregnancy, the developing placenta secretes human chorionic gonadotropin (hCG) hormone into the bloodstream, which is filtered by the kidneys and excreted in urine.
Plant Tropism Questions
Question 1: Why is it incorrect to say: (a) 'Plants grow towards the light,' (b) 'If a root is placed horizontally, it will bend towards gravity'?
Answer: (a) It is inaccurate because only specific plant parts (shoots) exhibit positive phototropism, whereas roots are non-phototropic or negatively phototropic. (b) Roots bend downward in response to gravity, not 'towards' gravity itself.
Question 2: Explain why a clinostat is used for the controls in tropism experiments.
Answer: A rotating clinostat exposes all sides of a plant evenly to gravity or light, eliminating any single-sided stimulus while keeping other environmental conditions identical.
Question 3: What do you think might happen if a potted plant were placed on its side and the shoot illuminated from below (i.e. light and gravity are acting from the same direction)?
Answer: Auxin accumulates on the lower side due to gravity, promoting cell elongation on the lower side and causing the shoot to curve upward away from gravity (negative gravitropism), even though this moves it away from the light source below.
Question 4: It is suggested that it is the very tip of the radicle that detects the one-sided pull of gravity even though it is the region of extension that responds. How could you test this hypothesis?
Answer: Decapitate the extreme root tip of horizontal radicles while leaving intact root tips as controls. If decapitated roots fail to bend downward while intact roots do, it confirms that gravity detection occurs in the root tip.