Nervous System and Sense Organs

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The Nervous System controls and coordinates body activities by transmitting electrical impulses (nerve impulses) rapidly throughout the body. It works alongside the endocrine system but provides faster responses.

Two Main Divisions:

1. Central Nervous System (CNS):

   • Brain: Receives, processes, and stores information
   • Spinal cord: Reflex actions and nerve pathway

2. Peripheral Nervous System (PNS):

   • Somatic (voluntary): Controls conscious activities — moving your arm, walking
   • Autonomic (involuntary): Controls unconscious functions
     • Sympathetic: “Fight or flight” — increases heart rate, dilates pupils
     • Parasympathetic: “Rest and digest” — decreases heart rate, stimulates digestion

Neuron Structure:

• Cell body (soma): Contains nucleus and most organelles
• Dendrites: Receive impulses from other neurons
• Axon: Carries impulse away from cell body
• Myelin sheath: Fatty insulation that speeds up impulse transmission
• Nodes of Ranvier: Gaps in myelin for saltatory conduction

Synapse: Junction between two neurons where neurotransmitters diffuse across the synaptic cleft. Acetylcholine and noradrenaline are common neurotransmitters.

Reflex Arc: Stimulus → Receptor → Sensory neuron → Relay neuron → Motor neuron → Effector → Response

⚡ WAEC Tip: In reflex arc questions, trace the path carefully. The relay neuron is in the spinal cord. Reflexes are automatic — you don’t think about them, so they bypass the brain for speed.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of the nervous system.

How a Nerve Impulse Works:

Resting Potential (≈ −70 mV):

• More Na⁺ outside the neuron, more K⁺ inside
• Na⁺/K⁺ pump actively transports 3 Na⁺ out and 2 K⁺ in per ATP
• Neuron membrane is polarised (electrical gradient)

Action Potential:

1. Depolarisation: Stimulus opens Na⁺ channels → Na⁺ rushes in → membrane potential becomes positive (+40 mV)
2. Repolarisation: Na⁺ channels close, K⁺ channels open → K⁺ rushes out → membrane returns to negative
3. Refractory period: Na⁺/K⁺ pump restores original ion distribution

Impulse Conduction:

• In myelinated axons: Saltatory conduction — impulse “jumps” between Nodes of Ranvier
• Speed: Up to 120 m/s in humans
• All-or-none principle: If threshold is reached, impulse is always the same strength

The Brain:

Part	Function
Cerebrum	Conscious thought, memory, language, vision
Cerebellum	Balance, coordination, posture
Medulla oblongata	Autonomic functions: heart rate, breathing
Hypothalamus	Temperature regulation, hunger, thirst, hormones
Thalamus	Relay station for sensory information

Sense Organs:

Eye:

• Sclera: Tough outer coat
• Cornea: Transparent front, refracts light
• Iris: Coloured part, controls pupil diameter
• Lens: Biconvex, focuses light onto retina
• Retina: Contains rods (black/white, low light) and cones (colour, bright light)
• Fovea: Area of sharpest vision
• Optic nerve: Carries impulses to brain

Ear:

• Outer ear: Pinna collects sound waves; auditory canal directs them
• Middle ear: Tympanic membrane (eardrum) vibrates; ossicles (malleus, incus, stapes) amplify and transmit vibrations
• Inner ear: Cochlea (hearing) — hair cells convert vibrations to nerve impulses; Semicircular canals (balance)

Common Eye Defects:

• Myopia (short-sightedness): Eyeball too long or lens too convex → image focuses in front of retina → corrected with concave lens
• Hypermetropia (long-sightedness): Eyeball too short or lens too flat → image focuses behind retina → corrected with convex lens
• Astigmatism: Uneven curvature of cornea → blurred vision → corrected with cylindrical lens

⚡ Common Student Mistakes: Confusing the roles of rods and cones. Rods work in dim light but don’t detect colour. Cones need bright light and detect colour. Also confuse sympathetic and parasympathetic — remember: sympathetic = stress response (like seeing a lion), parasympathetic = calm state (like after eating).

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

Neurotransmission at Synapses:

1. Action potential arrives at synaptic knob
2. Voltage-gated Ca²⁺ channels open → Ca²⁺ enters
3. Vesicles fuse with presynaptic membrane
4. Neurotransmitter (e.g., acetylcholine) released into synaptic cleft
5. Neurotransmitter binds to receptors on postsynaptic membrane
6. Ion channels open → postsynaptic potential (excitatory or inhibitory)
7. Neurotransmitter is broken down by enzyme (e.g., acetylcholinesterase) to prevent continuous stimulation

Excitatory vs Inhibitory Synapses:

• Excitatory: Depolarises postsynaptic membrane (e.g., acetylcholine at neuromuscular junction)
• Inhibitory: Hyperpolarises postsynaptic membrane (e.g., GABA in brain)

Synaptic Transmission Facts:

• Synapses are SLOWER than axon conduction (chemical vs electrical)
• Synapses allow signal amplification, integration, and modulation
• Drugs affect synapses (e.g., nicotine mimics acetylcholine)
• Some toxins block synaptic transmission (e.g., botulinum toxin blocks acetylcholine release)

Conditioned Reflexes:

• Learned reflexes (Pavlov’s dogs — bell + food = salivation to bell alone)
• Important for survival — organisms learn to avoid harmful stimuli
• Unlike simple reflexes, involve the cerebral cortex

Hypothalamus — The Body’s Control Centre:

• Osmoregulation: Controls water balance via ADH (antidiuretic hormone)
• Thermoregulation: Shivering, sweating, vasodilation/vasoconstriction
• Hunger and thirst: Controls feeding behaviour
• Endocrine control: Produces releasing and inhibiting hormones that act on the pituitary gland

Hormonal vs Nervous Control:

Feature	Nervous	Endocrine
Speed	Fast (milliseconds)	Slow (seconds to hours)
Duration	Brief	Long-lasting
Effect	Specific target	Wide-ranging
Pathway	Electrical impulses	Blood-borne hormones

Taste and Smell:

• Taste buds (on tongue, soft palate): Detect sweet, salty, sour, bitter, umami
• Olfactory receptors (in nose): Detect volatile chemicals
• Both work together — most “taste” is actually smell (why food tastes bland when you have a cold)

Hearing Range:

• Human hearing: 20 Hz to 20,000 Hz
• Ultrasound: >20,000 Hz (used in sonar, cleaning)
• Infrasound: <20 Hz (used by elephants for long-distance communication)

Structure of the Retina:

• Rods: 120 million, contain rhodopsin (visual purple), work in dim light, no colour, low acuity
• Cones: 6 million, 3 types (red, green, blue), need bright light, high acuity
• Fovea: Only cones, sharpest vision
• Blind spot: Where optic nerve exits — no photoreceptors

Accommodation:

• Near objects: Ciliary muscles contract → suspensory ligaments relax → lens becomes thicker (more convex) → increased refractive power
• Distant objects: Ciliary muscles relax → suspensory ligaments pull taut → lens becomes thinner → reduced refractive power

Common Eye Diseases:

• Cataract: Clouding of lens (common in elderly; surgery replaces lens)
• Glaucoma: Increased pressure in eyeball damaging optic nerve
• Conjunctivitis: Inflammation of conjunctiva (“pink eye”)
• Night blindness: Vitamin A deficiency affects rhodopsin production

Nerve Disorders:

• Meningitis: Inflammation of meninges (protective membranes around CNS)
• Poliomyelitis: Viral infection of motor neurons → muscle weakness/paralysis
• Multiple sclerosis: Autoimmune destruction of myelin sheath

Structure of a Nerve: A nerve is a bundle of neuron axons wrapped in connective tissue:

• Endoneurium: Wraps individual axon
• Perineurium: Wraps fascicle (bundle of axons)
• Epineurium: Outer connective tissue layer

⚡ WAEC Examination Patterns: Draw and label the structure of a neuron and a synapse. Explain the mechanism of nerve impulse transmission. Differentiate between CNS and PNS. Explain reflex arcs with a specific example (e.g., touching a hot object). Describe how the eye focuses on near vs distant objects.