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.

All Human Physiology notes NDA Biology strategy

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

Intuition

A neuron sends a signal as a wave of electrical change along its membrane. Two ions do the work: sodium (Na⁺) rushes IN to fire the signal (depolarisation), and potassium (K⁺) flows OUT to reset it (repolarisation). Both are essential — it is not one ion alone.

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

A nerve fibre is bathed in a fluid with no potassium ions. Even if sodium is plentiful, why does signalling fail?

Sodium influx can still depolarise the membrane and start an impulse.
But potassium outflow is what repolarises (resets) the membrane afterwards.
Without potassium, the neuron cannot return to its resting state to fire again.
So reliable signalling needs both ions.

Answer:Both Na⁺ and K⁺ are needed — Na⁺ fires the impulse, K⁺ resets the membrane.

Practice this conceptself-check · 3 quick reps

Try it yourself

Which single element from this list is essential for nerve signalling: lithium, sodium, rubidium, caesium?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which two ions enable the nerve impulse?
2.
Which ion flows in during depolarisation?
3.
Which ion flows out during repolarisation?

From the bank · past-year question

Example 1Human PhysiologyMODERATE

The transfer of electrical signals by nerve cells in human body is enabled by

[Q73 · Sep · 2022]

Both ions — not just sodium

A question may offer 'sodium' alone and 'sodium and potassium' as separate options. The complete answer for impulse transmission is sodium AND potassium — sodium fires it, potassium resets it.

Drill 1 more on the nerve impulse — sodium and potassium

Concept 2 of 3

The reflex arc and brain regions

Intuition

A reflex (like jerking your hand off a hot plate) is fast because it does not wait for the brain — the signal loops through the spinal cord. The brain itself is divided into regions; the hindbrain quietly runs the involuntary jobs that keep you alive.

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

Put the reflex arc in order: spinal cord, receptor, effector, motor neuron, sensory neuron.

A stimulus is detected by a receptor.
A sensory neuron carries the signal to the spinal cord.
The spinal cord relays it to a motor neuron.
The motor neuron triggers the effector (muscle/gland) — all without the brain.

Answer:Receptor → sensory neuron → spinal cord → motor neuron → effector.

Practice this conceptself-check · 3 quick reps

Try it yourself

Which brain region controls involuntary actions such as blood pressure, salivation and vomiting?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Give the reflex arc sequence.
2.
Which part processes a reflex — brain or spinal cord?
3.
Which brain region controls heartbeat and blood pressure?

From the bank · past-year question

Example 2Human PhysiologyEASY

Which one of the following depicts the correct circuit of a reflex arc?

[Q86 · Sep · 2018]

A reflex does not go through the brain

The correct reflex arc passes through the spinal cord, not the brain — that is why reflexes are fast. An option routing the reflex 'receptor → sensory → brain → motor → effector' is wrong for a spinal reflex.

Drill 1 more on the reflex arc and brain regions

Concept 3 of 3

The eye — parts and photoreceptors

Intuition

Light enters the eye through the clear cornea, passes the pupil and lens, and forms an image on the retina at the back. The retina holds two photoreceptors: rods for dim light and cones for colour. The cornea is a recall trap — it is transparent, has NO blood vessels, and is made of proteins and cells.

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

Try it yourself

Two facts: where does light first enter the eye, and which photoreceptor gives colour vision?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Through which part does light enter the eye?
2.
On which part is the image formed?
3.
Which photoreceptor is responsible for colour vision?
4.
Does the cornea have blood vessels?

From the bank · past-year question

Example 3Human PhysiologyMODERATE

Cornea in human eye

[Q131 · Sep · 2021]

Cornea vs retina — entry vs image

Light enters through the cornea; the image forms on the retina. The cornea is transparent and avascular (no blood vessels) and is NOT the light-sensitive part — that is the retina.

Rods vs cones

Cones = colour vision, work in bright light. Rods = dim-light / night vision, no colour. The bank tests this pair directly.

Drill 3 more on the eye — parts and photoreceptors

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

Mastery check — 5 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Human PhysiologyEASY

Which one of the following elements is needed in the human body to transfer electrical signals by nerve cells?

[Q89 · Apr · 2018]

Example 2Human PhysiologyEASY

Identify the functions that are controlled by the hind brain of human being

[Q136 · Sep · 2025]

Example 3Human PhysiologyEASY

The part of the human eye on which the image is formed is

[Q131 · Apr · 2020]

Example 4Human PhysiologyEASY

Light enters the eye through a thin membrane called

[Q88 · Apr · 2020]

Example 5Human PhysiologyEASY

Colour vision in human eyes is the function of photoreceptor cells named

[Q95 · Apr · 2017]

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