🧠 The Nervous System
Electrical coordination — fast, precise, short-lived
What is Coordination?
Working together of different organs to perform a function smoothly.
Nervous system uses electrical impulses for rapid, short-lived responses.
Endocrine system uses chemical hormones for slow, sustained responses.
Nervous system uses electrical impulses for rapid, short-lived responses.
Endocrine system uses chemical hormones for slow, sustained responses.
CNS vs PNS
Central NS: Brain + Spinal cord. Processes information and sends commands.
Peripheral NS: All nerves. Carries signals to/from CNS.
• Somatic: Voluntary
• Autonomic: Involuntary
Peripheral NS: All nerves. Carries signals to/from CNS.
• Somatic: Voluntary
• Autonomic: Involuntary
Nerve Impulse
Electrochemical signal along a neuron.
Created by Na⁺/K⁺ ion movement across membrane (action potential).
At synapse: chemical neurotransmitters carry signal across the gap.
Created by Na⁺/K⁺ ion movement across membrane (action potential).
At synapse: chemical neurotransmitters carry signal across the gap.
⚡ Nerve Signal Transmission
| Division | Components | Function | Speed |
|---|---|---|---|
| CNSBrain | Cerebrum, Cerebellum, Medulla, Hypothalamus | Processing, voluntary actions, memory | — |
| CNSSpinal Cord | White & grey matter; 31 pairs of spinal nerves | Reflex centre, conducts signals | — |
| PNSSensory nerves | Afferent neurons | Carry signals TO CNS | Up to 120 m/s |
| PNSMotor nerves | Efferent neurons | Carry commands FROM CNS | Up to 120 m/s |
🔬 Neuron — Structural Unit
Specialised cell for transmitting electrical impulses
🔬 Neuron Diagram — hover/tap parts
Dendrites
Short, branched projections from cell body.
Receive impulses from other neurons.
Carry impulse towards the cell body (afferent).
Multiple dendrites per neuron → more input.
Receive impulses from other neurons.
Carry impulse towards the cell body (afferent).
Multiple dendrites per neuron → more input.
Cell Body (Cyton / Soma)
Contains nucleus, ribosomes, mitochondria.
Metabolic centre of the neuron.
Contains Nissl bodies (rough ER for protein synthesis).
Controls neuron's health and survival.
Metabolic centre of the neuron.
Contains Nissl bodies (rough ER for protein synthesis).
Controls neuron's health and survival.
Axon & Myelin Sheath
Long single fibre carrying impulse away from cell body.
Myelin sheath: Fatty insulation produced by Schwann cells.
Nodes of Ranvier: Unmyelinated gaps — impulse "jumps" (saltatory conduction).
Axon terminals: Release neurotransmitters.
Myelin sheath: Fatty insulation produced by Schwann cells.
Nodes of Ranvier: Unmyelinated gaps — impulse "jumps" (saltatory conduction).
Axon terminals: Release neurotransmitters.
Synapse
Junction between two neurons or neuron + effector.
Synaptic cleft: ~20 nm gap.
Neurotransmitters released from vesicles (acetylcholine, dopamine).
Bind receptors on post-synaptic membrane → new impulse.
Synaptic cleft: ~20 nm gap.
Neurotransmitters released from vesicles (acetylcholine, dopamine).
Bind receptors on post-synaptic membrane → new impulse.
💬 Types of Neurons
| Type | Also Called | Direction | Example |
|---|---|---|---|
| Sensory Neuron | Afferent | Receptor → CNS | Detects touch, pain, temperature |
| Motor Neuron | Efferent | CNS → Effector | Signals muscles to contract |
| Relay Neuron | Interneuron / Association | Within CNS | In brain and spinal cord; connects S→M |
⚡ Reflex Action & Reflex Arc
Rapid, involuntary response — bypasses the brain's conscious control
⚡ Animated Reflex Arc
Reflex Arc Pathway:
🖐 Receptor
detects stimulus
detects stimulus
→
Sensory Neuron
carries to CNS
carries to CNS
→
Relay Neuron
spinal cord
spinal cord
→
Motor Neuron
carries from CNS
carries from CNS
→
💪 Effector
muscle / gland
muscle / gland
Why Reflex is Fast
Signal only travels to spinal cord (not brain) and back.
Shorter pathway → faster response.
Brain is informed later (that's why you feel pain after withdrawal).
Crucial for survival — avoids serious injury.
Shorter pathway → faster response.
Brain is informed later (that's why you feel pain after withdrawal).
Crucial for survival — avoids serious injury.
Types of Reflexes
Spinal reflex: Controlled by spinal cord.
Ex: Touching hot object, knee-jerk
Cranial reflex: Controlled by brain.
Ex: Pupil dilation, blinking
Conditioned reflex: Learned.
Ex: Salivation at smell of food (Pavlov)
Ex: Touching hot object, knee-jerk
Cranial reflex: Controlled by brain.
Ex: Pupil dilation, blinking
Conditioned reflex: Learned.
Ex: Salivation at smell of food (Pavlov)
Examples in Daily Life
• Withdrawing hand from hot object
• Knee-jerk reflex (patellar reflex)
• Sneezing, coughing
• Pupil constriction in bright light
• Blinking when something approaches eye
• Yawning, swallowing
• Knee-jerk reflex (patellar reflex)
• Sneezing, coughing
• Pupil constriction in bright light
• Blinking when something approaches eye
• Yawning, swallowing
🧬 Human Brain
Protected by cranium and cerebrospinal fluid (CSF)
🧬 Brain Structure Diagram — click a part
Cerebrum (Forebrain)
Largest part (~85% of brain weight).
Functions: Voluntary actions, thought, memory, speech, reasoning, emotions, interpretation of sense data.
Divided into 2 hemispheres by corpus callosum.
Outer layer: Cerebral cortex (grey matter).
Functions: Voluntary actions, thought, memory, speech, reasoning, emotions, interpretation of sense data.
Divided into 2 hemispheres by corpus callosum.
Outer layer: Cerebral cortex (grey matter).
Cerebellum (Hindbrain)
Second largest part, at back of brain.
Functions: Balance, posture, precision of voluntary movements, coordination.
"Coordinates muscles" — damage causes unsteady gait.
Surface folded → more surface area.
Functions: Balance, posture, precision of voluntary movements, coordination.
"Coordinates muscles" — damage causes unsteady gait.
Surface folded → more surface area.
Medulla Oblongata (Hindbrain)
Part of brain stem connecting brain to spinal cord.
Functions: Heartbeat, breathing, blood pressure, swallowing, vomiting, sneezing.
Controls ALL involuntary actions. Vital centre — even slight damage is life-threatening.
Functions: Heartbeat, breathing, blood pressure, swallowing, vomiting, sneezing.
Controls ALL involuntary actions. Vital centre — even slight damage is life-threatening.
Hypothalamus & Thalamus
Hypothalamus: Thermoregulation (body temp), hunger, thirst, circadian rhythm, emotional responses. Controls pituitary gland — links nervous and endocrine systems.
Thalamus: Relay station — routes sensory signals to correct part of cerebrum.
Thalamus: Relay station — routes sensory signals to correct part of cerebrum.
🛡️ Protection of Brain
| Layer | Name | Function |
|---|---|---|
| Outer bone | Cranium (skull) | Hard protective covering |
| 3 membranes | Meninges (Dura, Arachnoid, Pia) | Shock-absorbing connective tissue layers |
| Fluid | Cerebrospinal Fluid (CSF) | Cushions brain; buoyancy; removes waste |
| Blood-brain barrier | Tight junctions in brain capillaries | Keeps toxins out of brain tissue |
💊 Endocrine System
Chemical coordination — slow, broad, long-lasting
🔆 Endocrine Glands — animated
🧠 Pituitary
Location: Base of brain
Hormones: GH (growth), TSH, ADH, LH, FSH, Oxytocin
Role: Master gland — controls all other glands
⚠️ Excess GH in childhood → Gigantism
Hormones: GH (growth), TSH, ADH, LH, FSH, Oxytocin
Role: Master gland — controls all other glands
⚠️ Excess GH in childhood → Gigantism
🦋 Thyroid
Location: Front of neck
Hormone: Thyroxine (iodine needed)
Role: Metabolism, growth, development
⚠️ Deficiency → Goitre (swollen neck)
⚠️ Excess → Hyperthyroidism
Hormone: Thyroxine (iodine needed)
Role: Metabolism, growth, development
⚠️ Deficiency → Goitre (swollen neck)
⚠️ Excess → Hyperthyroidism
⚡ Adrenal
Location: On top of each kidney
Hormone: Adrenaline (epinephrine)
Role: Fight-or-flight: ↑HR, ↑BP, dilates pupils, diverts blood to muscles
Called "hormone of emergency"
Hormone: Adrenaline (epinephrine)
Role: Fight-or-flight: ↑HR, ↑BP, dilates pupils, diverts blood to muscles
Called "hormone of emergency"
🫁 Pancreas
Location: Behind stomach
Hormones: Insulin (↓glucose), Glucagon (↑glucose)
Islets of Langerhans: Beta (insulin), Alpha (glucagon)
⚠️ Insulin deficiency → Diabetes mellitus
Hormones: Insulin (↓glucose), Glucagon (↑glucose)
Islets of Langerhans: Beta (insulin), Alpha (glucagon)
⚠️ Insulin deficiency → Diabetes mellitus
♂️ Testes
Hormone: Testosterone
Role: Male sex characteristics — voice deepening, facial hair, muscle growth, sperm production
Also stimulates growth of reproductive organs.
Role: Male sex characteristics — voice deepening, facial hair, muscle growth, sperm production
Also stimulates growth of reproductive organs.
♀️ Ovaries
Hormones: Oestrogen, Progesterone
Role: Female sex characteristics — breast development, menstrual cycle, pregnancy maintenance.
Oestrogen at puberty; Progesterone during pregnancy.
Role: Female sex characteristics — breast development, menstrual cycle, pregnancy maintenance.
Oestrogen at puberty; Progesterone during pregnancy.
🔄 Feedback Mechanism
Hormonal secretion is controlled by negative feedback.
Example: Blood sugar rises → Pancreas secretes insulin → Glucose absorbed into cells → Blood sugar falls → Insulin secretion stops.
This maintains homeostasis — constant internal environment.
Example: Blood sugar rises → Pancreas secretes insulin → Glucose absorbed into cells → Blood sugar falls → Insulin secretion stops.
This maintains homeostasis — constant internal environment.
High Blood Sugar
→
Pancreas → Insulin
→
Glucose→Glycogen
→
Blood Sugar ↓
→
Insulin Stops
🌿 Plant Coordination
Plants coordinate via hormones and tropism movements — no nervous system
🌱 Tropism Animations
Phototropism
Response to light
Shoot: +ve (bends toward)
Root: −ve (bends away)
Auxin causes uneven growth
Shoot: +ve (bends toward)
Root: −ve (bends away)
Auxin causes uneven growth
Geotropism
Response to gravity
Root: +ve (grows downward)
Shoot: −ve (grows upward)
Also called gravitropism
Root: +ve (grows downward)
Shoot: −ve (grows upward)
Also called gravitropism
Hydrotropism
Response to water
Root: +ve (grows toward water)
Helps roots find water in soil
Root: +ve (grows toward water)
Helps roots find water in soil
Thigmotropism
Response to touch
Tendrils: +ve (coil around support)
Eg: Pea plants, cucumber
Tendrils: +ve (coil around support)
Eg: Pea plants, cucumber
🌱 Plant Hormones (Phytohormones)
| Hormone | Type | Main Functions | Example Effect |
|---|---|---|---|
| Auxin (IAA) | Promoter | Cell elongation; phototropism; apical dominance | Shoot bends toward light; inhibits lateral buds |
| Gibberellin | Promoter | Stem elongation; seed germination; fruit growth | Dwarf plants grow tall; seedless grapes |
| Cytokinin | Promoter | Cell division; delays leaf senescence | Keeps cut flowers fresh longer |
| Abscisic Acid | Inhibitor | Stomatal closure; seed dormancy; wilting response | Stomata close in drought; seeds don't germinate in winter |
🔬 Auxin & Phototropism Mechanism
1. Light falls on one side of shoot tip.
2. Auxin produced at tip migrates to the shaded side.
3. Higher auxin concentration on shaded side → more cell elongation.
4. Shaded side grows longer → shoot bends toward light.
5. In roots, high auxin inhibits elongation (roots are more sensitive to auxin).
6. Root tip shaded side: less growth → root bends away from light.
Experiments: Darwin (1880) — coleoptile bending; Went (1926) — named auxin, proved tip produces it.
2. Auxin produced at tip migrates to the shaded side.
3. Higher auxin concentration on shaded side → more cell elongation.
4. Shaded side grows longer → shoot bends toward light.
5. In roots, high auxin inhibits elongation (roots are more sensitive to auxin).
6. Root tip shaded side: less growth → root bends away from light.
Experiments: Darwin (1880) — coleoptile bending; Went (1926) — named auxin, proved tip produces it.