Biology · Chapter 06

🧠 Nervous System

Brain, nerves, reflex action.

💡 The Body's Electrical Network

The nervous system controls everything — thoughts, movement, senses, organs — by sending electrical signals through specialized cells called neurons.

3 parts of a neuron:

• Dendrites — receive signals from other neurons
• Cell body — processes the signal
• Axon — long fibre that sends signal to the next neuron

Signals jump between neurons across tiny gaps called synapses using chemicals called neurotransmitters.

Two divisions:

• Central Nervous System (CNS) — brain + spinal cord
• Peripheral Nervous System (PNS) — all nerves outside CNS

⚡ Reflex action — bypassing the brain

When you touch a hot pan, your hand jerks back BEFORE you feel the pain. Why? Because the signal takes a shortcut through the spinal cord.

Reflex arc path: Receptor (skin) → Sensory neuron → Spinal cord → Motor neuron → Muscle

The brain gets the message after the action is already taken. This saves precious milliseconds in emergencies.

Neuron structure — dendrites, axon, myelin sheathWikimedia Commons / CC BY-SA 3.0

Human brain — inferior view showing lobesWikimedia Commons / CC BY-SA 3.0

🧠 Brain parts

• Cerebrum — biggest, controls thinking, memory, voluntary movement
• Cerebellum — coordination, balance, fine movement
• Medulla oblongata — automatic functions (breathing, heartbeat, BP)
• Hypothalamus — controls body temperature, hunger, thirst
• Pituitary — master gland (under hypothalamus)

💡 Nerve speed

Nerve signals travel up to 120 metres per second in myelinated neurons (faster than a car!). Without myelin, signals only travel ~1 m/s.

That's why a giraffe (with its long neck) needs heavily myelinated neurons — a signal from brain to foot must travel quickly.

🎬

Reflex Arc in Action

Animation

The signal goes hand → spinal cord → back to hand. Brain hears about it later. Hence the "ouch!" delay.

🔬

Voluntary vs Involuntary Actions

Interactive

Pick an action — see what controls it.

Action typeVoluntary

Controlled byCerebrum (conscious brain)

You choose to walk. Cerebrum sends signals through motor neurons to leg muscles.

Practice (NCERT): Differentiate between reflex action and voluntary action with examples.

Reflex action:
• Automatic, very fast (milliseconds)
• Controlled by spinal cord, brain involved later
• Example: jerking hand back from hot object, knee-jerk, sneezing

Voluntary action:
• Conscious, planned
• Controlled by cerebrum (front part of brain)
• Example: writing, walking, talking, dancing

Reflexes evolved to protect the body in dangerous situations without waiting for slow conscious thought.

Practice (NEET): Which part of brain controls balance and coordination?

The cerebellum (Latin for "little brain") controls balance, posture, and coordination of voluntary movements. It's located at the back of the head, below the cerebrum. If your cerebellum is damaged, you can still move — but movements become jerky and uncoordinated (called ataxia). When you ride a bicycle, dance, or do yoga, your cerebellum is doing the precise calculations.