NDA Biology · Human Physiology
Nervous System and Sense Organs
Nerve cells carry electrical signals using sodium and potassium ions; the reflex arc and brain coordinate responses; and the eye converts light into images.
Why this matters
8 PYQs — the joint-largest cluster in the chapter. Three blocks: the nerve impulse (sodium and potassium ions), the reflex arc plus the brain's hindbrain role, and eye anatomy (cornea, retina, rods vs cones). The eye is heavily tested.
Concept 1 of 3: The nerve impulse — sodium and potassium
Definition
How the electrical signal travels:
- At rest, the neuron is more negative inside, with a sodium–potassium pump holding the gradient.
- Depolarisation — sodium (Na⁺) ions flow IN, flipping the charge: this is the impulse.
- Repolarisation — potassium (K⁺) ions flow OUT, restoring the resting state.
- So nerve signal transmission needs both sodium and potassium.
Worked example
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Human Physiology · Nervous System and Sense Organs
Both ions — not just sodium
Concept 2 of 3: The reflex arc and brain regions
Definition
The reflex pathway and brain division:
- Reflex arc: receptor → sensory neuron → spinal cord → motor neuron → effector. The spinal cord (not the brain) processes the reflex.
- Forebrain (cerebrum) — thinking, voluntary movement, sensation.
- Hindbrain (medulla + cerebellum) — involuntary actions: blood pressure, heartbeat, salivation, vomiting, balance.
Worked example
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 2 · Human Physiology · Nervous System and Sense Organs
A reflex does not go through the brain
Concept 3 of 3: The eye — parts and photoreceptors
Definition
Eye parts in the path of light, plus the photoreceptors:
- Cornea — transparent front membrane where light enters; avascular (no blood vessels); made of proteins (collagen) and cells; NOT light-sensitive.
- Iris / pupil — the iris (coloured) controls the pupil (the aperture letting light in).
- Lens — focuses light onto the retina.
- Retina — the light-sensitive screen at the back where the image forms.
- Rods (dim light) and cones (colour vision) — the retina's photoreceptors.
| Part | Role |
|---|---|
| Cornea | Transparent front; light ENTERS here; avascular, proteins + cells NDA 2021 — the cornea is composed of proteins and cells; it is NOT light-sensitive and has NO blood vessels. |
| Iris / Pupil | Iris controls the pupil — the light aperture |
| Lens | Focuses light onto the retina |
| Retina | Light-sensitive screen; IMAGE forms here |
| Cones / Rods | Cones = colour vision; rods = dim-light vision |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 3 · Human Physiology · Nervous System and Sense Organs
Cornea vs retina — entry vs image
Rods vs cones
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (1)
The eye — parts and photoreceptors5 rows
| Part | Role |
|---|---|
| Cornea | Transparent front; light ENTERS here; avascular, proteins + cells NDA 2021 — the cornea is composed of proteins and cells; it is NOT light-sensitive and has NO blood vessels. |
| Iris / Pupil | Iris controls the pupil — the light aperture |
| Lens | Focuses light onto the retina |
| Retina | Light-sensitive screen; IMAGE forms here |
| Cones / Rods | Cones = colour vision; rods = dim-light vision |
Watch out for (4)
- Both ions — not just sodium→ The nerve impulse — sodium and potassium
- A reflex does not go through the brain→ The reflex arc and brain regions
- Cornea vs retina — entry vs image→ The eye — parts and photoreceptors
- Rods vs cones→ The eye — parts and photoreceptors
Test yourself on Human Physiology
15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.