CDS Biology · Human Physiology
Nerves, Senses and Hormones
The parts of the brain and what each controls, the reflex arc, the blind spot of the eye, and the main endocrine glands with their hormones.
Why this matters
Seven CDS questions from 2016 (II) to 2026 (II). Four are about the nervous system and the eye: the parts of the brain, the reflex arc, the blind spot and schizophrenia. Three are about hormones: insulin, thyroxine and the one unpaired gland.
Concept 1 of 2: The brain, reflexes and the eye
The brain has three parts. The big forebrain does the thinking, remembering and feeling. The small midbrain links it to the hindbrain, and the hindbrain runs the things you never think about: balance, heartbeat and breathing. Some actions are too urgent to wait for the brain, so the spinal cord handles them as reflexes. The eye sends its picture to the brain along the optic nerve, and the spot where that nerve leaves the eye cannot see at all.
Definition
The nervous system:
- The brain has three main parts: forebrain, midbrain and hindbrain.
- Forebrain: the cerebrum (thinking, memory, the senses, voluntary actions) and the hypothalamus.
- Hindbrain: the cerebellum (balance and posture) and the medulla (heartbeat, breathing, blood pressure, swallowing, vomiting).
- Reflex arc (NCERT): receptor → sensory neuron → relay neuron in the spinal cord → motor neuron → effector (a muscle or gland). Information also goes on to the brain, which learns of the action.
The eye:
- The retina has rods (dim light) and cones (colour and bright light).
- The blind spot is where the optic nerve leaves the eye: it has no rods or cones, so nothing is seen there.
- The fovea (yellow spot) has the most cones and gives the sharpest vision.
Schizophrenia is a mental disorder marked by hallucinations and delusions. It is not 'split' or multiple personality, which is a different disorder.
| Part | Job or fact |
|---|---|
| Cerebrum (forebrain) | Thinking, memory, the senses, voluntary actions |
| Cerebellum (hindbrain) | Balance and posture |
| Medulla (hindbrain) | Heartbeat, breathing, blood pressure |
| Spinal cord | Carries messages to and from the brain; centre of many reflexes |
| Blind spot | Where the optic nerve leaves; no rods or conesQ CDS 2025 (I): the point of the eye with no photoreceptors is the blind spot, not the fovea. |
| Fovea | Most cones; sharpest vision |
| Schizophrenia | Hallucinations and delusionsQ CDS 2026 (II): schizophrenia is associated with hallucination, not multiple personalities. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q45Easy
Example 1 · Human Physiology · Nerves, Senses and Hormones
Which of the following are the different parts of a human brain ?
Blind spot, not fovea
The fovea is the best seeing point, packed with cones. The blind spot has no photoreceptors, because the optic nerve leaves there.
Schizophrenia is not split personality
Schizophrenia means hallucinations and delusions. 'Multiple personalities' is a different disorder, and schizophrenia is influenced by surroundings and stress, not by genes alone.
Sensory in, motor out
In a reflex arc the sensory neuron comes first, from the receptor, and the motor neuron last, to the effector. An order that starts with a motor neuron is wrong.
Concept 2 of 2: Endocrine glands and their hormones
Hormones are chemical messengers poured straight into the blood by glands that have no ducts. Each one acts on particular organs far from the gland. The pituitary, a single small gland under the brain, controls many of the others. Insulin from the pancreas lets cells take in sugar, and the thyroid needs iodine from food to make its hormone.
Definition
The main endocrine glands:
- Pituitary: one single gland at the base of the brain; the master gland. Makes growth hormone and hormones that control other glands, such as TSH for the thyroid.
- Thyroid (in the neck): thyroxine, which controls the rate of metabolism. It needs iodine; too little iodine gives goitre.
- Pancreas (islets of Langerhans): insulin, which lets body cells take up glucose and so lowers blood sugar. Too little insulin causes diabetes.
- Adrenal glands (a pair, on top of the kidneys): adrenaline, for emergencies.
- Testes (a pair): testosterone. Ovaries (a pair): oestrogen.
| Gland | Hormone | What it does |
|---|---|---|
| Pituitary (single) | Growth hormone, TSH and others | Controls growth and other glandsQ CDS 2020 (I): the pituitary is the one gland that is not paired; adrenals, testes and ovaries are. |
| Thyroid | Thyroxine | Controls metabolism; needs iodineQ CDS 2023 (I): iodine is what the thyroid needs to make thyroxin. |
| Pancreas | Insulin | Lets cells take up glucose; lowers blood sugarQ CDS 2016 (II): insulin, not GH, TSH or cortisol, is essential for glucose uptake by cells. |
| Adrenals (pair) | Adrenaline | Prepares the body for an emergency |
| Testes (pair) | Testosterone | Male characters |
| Ovaries (pair) | Oestrogen | Female characters |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q40Easy
Example 2 · Human Physiology · Nerves, Senses and Hormones
Which one of the following is essential for thyroid gland to make thyroxin?
The pituitary is single
Adrenals, testes and ovaries come in pairs. The pituitary, at the base of the brain, is one gland.
Iodine, not common salt
Thyroxine is built with iodine. Plain salt (NaCl) only helps because it is iodised; KCl and cholesterol play no part.
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 (2)
The brain, reflexes and the eye7 rows
| Part | Job or fact |
|---|---|
| Cerebrum (forebrain) | Thinking, memory, the senses, voluntary actions |
| Cerebellum (hindbrain) | Balance and posture |
| Medulla (hindbrain) | Heartbeat, breathing, blood pressure |
| Spinal cord | Carries messages to and from the brain; centre of many reflexes |
| Blind spot | Where the optic nerve leaves; no rods or conesQ CDS 2025 (I): the point of the eye with no photoreceptors is the blind spot, not the fovea. |
| Fovea | Most cones; sharpest vision |
| Schizophrenia | Hallucinations and delusionsQ CDS 2026 (II): schizophrenia is associated with hallucination, not multiple personalities. |
Endocrine glands and their hormones6 rows
| Gland | Hormone | What it does |
|---|---|---|
| Pituitary (single) | Growth hormone, TSH and others | Controls growth and other glandsQ CDS 2020 (I): the pituitary is the one gland that is not paired; adrenals, testes and ovaries are. |
| Thyroid | Thyroxine | Controls metabolism; needs iodineQ CDS 2023 (I): iodine is what the thyroid needs to make thyroxin. |
| Pancreas | Insulin | Lets cells take up glucose; lowers blood sugarQ CDS 2016 (II): insulin, not GH, TSH or cortisol, is essential for glucose uptake by cells. |
| Adrenals (pair) | Adrenaline | Prepares the body for an emergency |
| Testes (pair) | Testosterone | Male characters |
| Ovaries (pair) | Oestrogen | Female characters |
Watch out for (5)
- Blind spot, not fovea→ The brain, reflexes and the eye
- Schizophrenia is not split personality→ The brain, reflexes and the eye
- Sensory in, motor out→ The brain, reflexes and the eye
- The pituitary is single→ Endocrine glands and their hormones
- Iodine, not common salt→ Endocrine glands and their hormones
Test yourself on Human Physiology
15 past CDS 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.