Responding to the Environment (Human) - Grade 12

Humans respond to stimuli (changes in the environment) using the nervous system and sense organs (like the eye and ear).

🧠 1. THE NERVOUS SYSTEM

➤ What is the Nervous System?

A fast-acting system that uses electrical impulses to coordinate the body's responses to internal and external stimuli.

✨ Main Functions:

• Detects stimuli through receptors
• Processes information
• Sends out responses to effectors (muscles/glands)

🧩 Divisions of the Nervous System:

Part	Description
Central Nervous System (CNS)	Brain + Spinal cord – processes and interprets information
Peripheral Nervous System (PNS)	Nerves outside the CNS – carries messages to/from CNS

🌐 Types of Neurons:

Type	Function
Sensory Neuron	Carries messages from receptors to CNS
Interneuron (Relay)	Found in the CNS, connects sensory and motor neurons
Motor Neuron	Carries messages from CNS to effectors (muscles/glands)

🌀 Reflex Arc:

A rapid, automatic response to a stimulus.

Example: Touching a hot surface

• Receptor detects heat
• Sensory neuron → spinal cord
• Interneuron processes
• Motor neuron → muscle
• Hand pulls away (response)

Diagram: Reflex arc flowchart showing receptor → sensory neuron → spinal cord → motor neuron → effector.

👁️ 2. THE HUMAN EYE

➤ Function of the Eye:

To detect light stimuli and convert them into electrical impulses sent to the brain via the optic nerve.

🧱 Structure and Functions of Eye Parts:

Part	Function
Cornea	Bends (refracts) light into the eye
Iris	Controls amount of light entering eye
Pupil	Opening in iris where light enters
Lens	Focuses light on the retina
Ciliary muscles	Change lens shape for focusing
Retina	Contains photoreceptors (rods and cones)
Optic nerve	Sends impulses to the brain
Aqueous & Vitreous humour	Maintain eye shape and refract light

👀 How Vision Works:

• Light enters eye through cornea → pupil
• Lens focuses image on retina
• Retina’s photoreceptors (rods = dim light, cones = color)
• Impulse sent to brain via optic nerve
• Brain interprets image

🔁 Accommodation:

The ability of the lens to change shape to focus on near or distant objects.

Distance	Lens Shape	Ciliary Muscle
Near	Rounder/thicker	Contracts
Far	Flatter/thinner	Relaxes

☀️ Pupillary Mechanism:

Adjusts the pupil size depending on light.

Light Condition	Pupil	Iris Muscles
Bright Light	Contracts (smaller)	Circular muscles contract
Dim Light	Dilates (larger)	Radial muscles contract

⚠️ Common Vision Defects:

Defect	Cause	Correction
Short-sightedness (Myopia)	Lens too strong / eyeball too long	Concave lens
Long-sightedness (Hyperopia)	Lens too weak / eyeball too short	Convex lens
Astigmatism	Irregular cornea shape	Special lens
Cataracts	Cloudy lens	Lens replacement surgery

👂 3. THE HUMAN EAR

➤ Function of the Ear:

1. Hearing (detecting sound)
2. Balance

🧱 Structure and Functions of the Ear:

Section	Main Structures	Function
Outer Ear	Pinna, Auditory canal	Collects sound and channels it to middle ear
Middle Ear	Eardrum (tympanic membrane), Ossicles (hammer, anvil, stirrup)	Transfers sound to inner ear
Inner Ear	Cochlea, Semicircular canals, Auditory nerve	Cochlea: hearing; Semicircular canals: balance

👂 How Hearing Works:

• Sound waves enter auditory canal
• Eardrum vibrates
• Ossicles amplify vibrations
• Vibrations → cochlea (fluid-filled)
• Hair cells in cochlea detect sound
• Signal sent to brain via auditory nerve

⚖️ Balance:

• Controlled by semicircular canals (detect head movement)
• Vestibular nerve sends signals about position to the brain

📌 Summary Table: Nervous System vs Eye vs Ear

System/Organ	Stimulus Detected	Receptor	Response
Nervous System	All types	Nerve endings	Sends impulse to CNS
Eye	Light	Rods & cones (retina)	Vision
Ear	Sound & movement	Hair cells (cochlea and canals)	Hearing & balance