Your Hand Moves Before Your Brain Knows It Was Burnt
Learn the structure and functions of the spinal cord and what protects it, tell the central, peripheral and autonomic systems apart, trace an impulse round a reflex arc, and sort reflexes into natural and acquired.
Why do you feel the pain only after your hand has already moved?
Because the hand was ordered to move by the spinal cord, and the message to the brain travelled separately and arrived later.
Touch something hot and the impulse races in along a sensory neuron to the spinal cord, where a relay neuron passes it straight across to a motor neuron. The arm muscle contracts and the hand is withdrawn — all without the brain being consulted at all.
A second signal does go up to the brain, and that is the one you experience as pain. By the time it arrives, the hand is already safe. This page covers the second part of the ICSE Class 8 Biology chapter on the nervous system: the spinal cord, the three divisions of the system, and reflexes.
Touch something hot and the impulse races in along a sensory neuron to the spinal cord, where a relay neuron passes it straight across to a motor neuron. The arm muscle contracts and the hand is withdrawn — all without the brain being consulted at all.
A second signal does go up to the brain, and that is the one you experience as pain. By the time it arrives, the hand is already safe. This page covers the second part of the ICSE Class 8 Biology chapter on the nervous system: the spinal cord, the three divisions of the system, and reflexes.
What does the spinal cord do, and what protects it?
The spinal cord is a long cylindrical cord of nerve tissue that continues downward from the medulla oblongata and runs inside the vertebral column.
Its structure in cross-section. There is a narrow central canal filled with cerebrospinal fluid, surrounded by grey matter in a shape like the letter H, with white matter outside it.
Notice that this is the reverse of the brain, where grey matter is on the outside and white matter within. That single contrast is a favourite one-mark question.
Its functions:
- Conducts impulses both to the brain (from the body) and from the brain (to the body). It is the main cable between the brain and everything below the neck.
- Acts as the centre of spinal reflex actions, so that a response can be produced without involving the brain.
- Gives off spinal nerves in pairs, which run out to the muscles, skin and organs of the body.
The protective coverings of the central nervous system are the same for the brain and the spinal cord, in three layers:
- Bone. The cranium, or skull, encloses the brain; the vertebral column, or backbone, encloses the spinal cord.
- Meninges — three membranes wrapped around the nerve tissue. From outside inwards they are the dura mater, the arachnoid and the pia mater.
- Cerebrospinal fluid, lying between the membranes. It cushions the brain and cord against shocks and jolts, supplies nutrients, and removes wastes.
Why so much protection. Nerve cells, once destroyed, are not replaced — unlike skin or blood cells, which the body renews constantly. Damage to the central nervous system is therefore permanent, which is why it is the most heavily armoured tissue in the body.
This is also why an injury to the backbone can cause paralysis of everything below it. The bone has been broken and the cable inside cut, so the brain's instructions can no longer reach the muscles beyond the break, even though both the brain and the muscles are perfectly healthy.
Its structure in cross-section. There is a narrow central canal filled with cerebrospinal fluid, surrounded by grey matter in a shape like the letter H, with white matter outside it.
Notice that this is the reverse of the brain, where grey matter is on the outside and white matter within. That single contrast is a favourite one-mark question.
Its functions:
- Conducts impulses both to the brain (from the body) and from the brain (to the body). It is the main cable between the brain and everything below the neck.
- Acts as the centre of spinal reflex actions, so that a response can be produced without involving the brain.
- Gives off spinal nerves in pairs, which run out to the muscles, skin and organs of the body.
The protective coverings of the central nervous system are the same for the brain and the spinal cord, in three layers:
- Bone. The cranium, or skull, encloses the brain; the vertebral column, or backbone, encloses the spinal cord.
- Meninges — three membranes wrapped around the nerve tissue. From outside inwards they are the dura mater, the arachnoid and the pia mater.
- Cerebrospinal fluid, lying between the membranes. It cushions the brain and cord against shocks and jolts, supplies nutrients, and removes wastes.
Why so much protection. Nerve cells, once destroyed, are not replaced — unlike skin or blood cells, which the body renews constantly. Damage to the central nervous system is therefore permanent, which is why it is the most heavily armoured tissue in the body.
This is also why an injury to the backbone can cause paralysis of everything below it. The bone has been broken and the cable inside cut, so the brain's instructions can no longer reach the muscles beyond the break, even though both the brain and the muscles are perfectly healthy.
How do the central, peripheral and autonomic nervous systems differ?
By where they are and what they control.
The central nervous system consists of the brain and the spinal cord. It is the control and coordination centre — it receives information, processes it, decides, and issues instructions. Nothing in it runs out into the body.
The peripheral nervous system consists of all the nerves arising from the central nervous system:
- Cranial nerves, in pairs arising from the brain
- Spinal nerves, in pairs arising from the spinal cord
Its job is to connect the central nervous system to every part of the body, carrying sensory impulses inwards and motor impulses outwards. It is the wiring; the central nervous system is the controller.
The autonomic nervous system controls the involuntary activities of the internal organs — heartbeat, breathing, digestion, the movement of food along the gut, and secretion by glands — all without any conscious control.
It has two divisions that work in opposition to each other:
- The sympathetic division prepares the body for action — it speeds the heart, quickens breathing, raises blood pressure and slows digestion. This is the division that acts along with adrenaline.
- The parasympathetic division calms the body down — it slows the heart, slows breathing and promotes digestion.
Why two opposing divisions rather than one. A single system could only turn an organ up or leave it alone. With two pulling in opposite directions, the heart rate can be raised and lowered continuously to whatever the moment requires, which is far finer control. It is the same design as insulin and glucagon in the endocrine chapter — two opposing agents holding a value steady.
A point of overlap that confuses students. The autonomic nervous system is not a fourth system alongside the other two — it is a part of the nervous system that runs through both. Its nerves belong to the peripheral nervous system and its control centres lie in the central nervous system. The classification is by function, not by location, which is why the three categories are not neatly separate.
The central nervous system consists of the brain and the spinal cord. It is the control and coordination centre — it receives information, processes it, decides, and issues instructions. Nothing in it runs out into the body.
The peripheral nervous system consists of all the nerves arising from the central nervous system:
- Cranial nerves, in pairs arising from the brain
- Spinal nerves, in pairs arising from the spinal cord
Its job is to connect the central nervous system to every part of the body, carrying sensory impulses inwards and motor impulses outwards. It is the wiring; the central nervous system is the controller.
The autonomic nervous system controls the involuntary activities of the internal organs — heartbeat, breathing, digestion, the movement of food along the gut, and secretion by glands — all without any conscious control.
It has two divisions that work in opposition to each other:
- The sympathetic division prepares the body for action — it speeds the heart, quickens breathing, raises blood pressure and slows digestion. This is the division that acts along with adrenaline.
- The parasympathetic division calms the body down — it slows the heart, slows breathing and promotes digestion.
Why two opposing divisions rather than one. A single system could only turn an organ up or leave it alone. With two pulling in opposite directions, the heart rate can be raised and lowered continuously to whatever the moment requires, which is far finer control. It is the same design as insulin and glucagon in the endocrine chapter — two opposing agents holding a value steady.
A point of overlap that confuses students. The autonomic nervous system is not a fourth system alongside the other two — it is a part of the nervous system that runs through both. Its nerves belong to the peripheral nervous system and its control centres lie in the central nervous system. The classification is by function, not by location, which is why the three categories are not neatly separate.
What is a reflex action, and what path does the impulse take?
A reflex action is a sudden, automatic, involuntary response to a stimulus, produced without the will of the brain.
The path the impulse follows is the reflex arc, and it has five parts in a fixed order:
receptor sensory neuron relay neuron in the spinal cord motor neuron effector
Traced through, for touching a hot pan:
- The receptors in the skin of the finger detect the heat — the stimulus — and generate an impulse.
- A sensory neuron carries the impulse from the finger to the spinal cord.
- A relay neuron inside the spinal cord passes it directly across to a motor neuron.
- The motor neuron carries the instruction out to the arm muscle.
- The effector — the arm muscle — contracts, and the hand is pulled away.
The muscle or gland that acts is the effector, and the action itself is the response.
Why the spinal cord and not the brain. Speed. Going to the brain and back would take noticeably longer, and in the case of a burn or a falling object that delay matters. Routing the impulse through the spinal cord gives the shortest possible path from stimulus to response.
The brain is informed, just not consulted. A separate impulse does travel up the spinal cord to the brain, where it is felt as pain. So the sequence is: hand moves first, pain felt afterwards — which is exactly what happens, and which is the strongest everyday evidence that the brain was not part of the decision.
A reflex you can test on yourself. Have someone tap gently just below the kneecap while your leg hangs freely, and the lower leg jerks forward. This is the knee-jerk reflex, and you cannot prevent it by deciding not to — which is what involuntary means. Trying and failing to suppress it is a better demonstration than any diagram.
A boundary case worth knowing. Not every reflex goes through the spinal cord. Blinking, sneezing and the narrowing of the pupil in bright light are reflexes controlled by the brain — specifically the medulla and midbrain. So a reflex is defined by being automatic, not by which part of the central nervous system handles it.
The path the impulse follows is the reflex arc, and it has five parts in a fixed order:
receptor sensory neuron relay neuron in the spinal cord motor neuron effector
Traced through, for touching a hot pan:
- The receptors in the skin of the finger detect the heat — the stimulus — and generate an impulse.
- A sensory neuron carries the impulse from the finger to the spinal cord.
- A relay neuron inside the spinal cord passes it directly across to a motor neuron.
- The motor neuron carries the instruction out to the arm muscle.
- The effector — the arm muscle — contracts, and the hand is pulled away.
The muscle or gland that acts is the effector, and the action itself is the response.
Why the spinal cord and not the brain. Speed. Going to the brain and back would take noticeably longer, and in the case of a burn or a falling object that delay matters. Routing the impulse through the spinal cord gives the shortest possible path from stimulus to response.
The brain is informed, just not consulted. A separate impulse does travel up the spinal cord to the brain, where it is felt as pain. So the sequence is: hand moves first, pain felt afterwards — which is exactly what happens, and which is the strongest everyday evidence that the brain was not part of the decision.
A reflex you can test on yourself. Have someone tap gently just below the kneecap while your leg hangs freely, and the lower leg jerks forward. This is the knee-jerk reflex, and you cannot prevent it by deciding not to — which is what involuntary means. Trying and failing to suppress it is a better demonstration than any diagram.
A boundary case worth knowing. Not every reflex goes through the spinal cord. Blinking, sneezing and the narrowing of the pupil in bright light are reflexes controlled by the brain — specifically the medulla and midbrain. So a reflex is defined by being automatic, not by which part of the central nervous system handles it.
How do natural and acquired reflexes differ, and what makes an action voluntary?
Natural reflexes are present from birth; acquired reflexes have to be learnt.
Natural reflexes, also called inborn or unconditioned reflexes, need no learning or experience. Every healthy person has them and always has:
- Blinking when something approaches the eye
- Sneezing and coughing
- Withdrawing the hand from something hot or sharp
- The knee-jerk reflex
- Salivating at the sight or smell of food
- The pupil narrowing in bright light
- Sucking in a newborn baby
Acquired reflexes, also called conditioned or learnt reflexes, are developed by repeated practice and experience:
- Cycling without thinking about balance
- Swimming
- Writing, typing and playing an instrument
- Driving a vehicle
- Applying the brakes on seeing a red light
The interesting thing about acquired reflexes. Every one of them began as a voluntary action needing full attention. A child learning to cycle thinks about every movement. After enough practice the same movements happen automatically, and the rider can hold a conversation while doing them. So an acquired reflex is a voluntary action that has been practised into an automatic one — which is what learning a skill physically means.
Voluntary and involuntary actions:
- Voluntary actions are under the control of the will, and are controlled by the cerebrum. Walking, writing, talking, eating, lifting an object. You can start them, stop them and change them at any moment.
- Involuntary actions are not under the control of the will, and are controlled by the medulla oblongata and the autonomic nervous system. Heartbeat, breathing, digestion, the movement of food along the gut, and the beating of the heart. They continue while you sleep and cannot be stopped by deciding to.
Where reflexes sit in that division. A reflex action is involuntary, since it cannot be prevented by choosing. But it differs from ordinary involuntary actions such as heartbeat in two ways: it is a response to a specific stimulus, and it is very rapid. Heartbeat has no stimulus and never stops; a reflex fires once, when something happens.
So the three categories run: voluntary actions you choose, involuntary actions that run continuously on their own, and reflex actions that fire automatically in response to a stimulus.
Natural reflexes, also called inborn or unconditioned reflexes, need no learning or experience. Every healthy person has them and always has:
- Blinking when something approaches the eye
- Sneezing and coughing
- Withdrawing the hand from something hot or sharp
- The knee-jerk reflex
- Salivating at the sight or smell of food
- The pupil narrowing in bright light
- Sucking in a newborn baby
Acquired reflexes, also called conditioned or learnt reflexes, are developed by repeated practice and experience:
- Cycling without thinking about balance
- Swimming
- Writing, typing and playing an instrument
- Driving a vehicle
- Applying the brakes on seeing a red light
The interesting thing about acquired reflexes. Every one of them began as a voluntary action needing full attention. A child learning to cycle thinks about every movement. After enough practice the same movements happen automatically, and the rider can hold a conversation while doing them. So an acquired reflex is a voluntary action that has been practised into an automatic one — which is what learning a skill physically means.
Voluntary and involuntary actions:
- Voluntary actions are under the control of the will, and are controlled by the cerebrum. Walking, writing, talking, eating, lifting an object. You can start them, stop them and change them at any moment.
- Involuntary actions are not under the control of the will, and are controlled by the medulla oblongata and the autonomic nervous system. Heartbeat, breathing, digestion, the movement of food along the gut, and the beating of the heart. They continue while you sleep and cannot be stopped by deciding to.
Where reflexes sit in that division. A reflex action is involuntary, since it cannot be prevented by choosing. But it differs from ordinary involuntary actions such as heartbeat in two ways: it is a response to a specific stimulus, and it is very rapid. Heartbeat has no stimulus and never stops; a reflex fires once, when something happens.
So the three categories run: voluntary actions you choose, involuntary actions that run continuously on their own, and reflex actions that fire automatically in response to a stimulus.
Exam tip
Exam tip: name all five parts of the reflex arc in order
Write the reflex arc as a chain and include every link: **receptor sensory neuron relay neuron (in the spinal cord) motor neuron effector. Leaving out the relay neuron or the effector is the commonest way to lose marks here.
Use the correct words — stimulus for what starts it, effector for the muscle or gland, response for what happens.
Say why a reflex goes through the spinal cord: it saves time, giving the shortest path. And add that the brain is informed afterwards, which is why pain is felt after the hand has moved.
Remember the spinal cord has grey matter inside and white matter outside — the reverse of the brain.
Name the three coverings of the central nervous system: bone (cranium and vertebral column), meninges (dura mater, arachnoid, pia mater), and cerebrospinal fluid.
Define the three divisions by what they are: CNS is brain and spinal cord; PNS is the nerves; ANS controls involuntary activity of internal organs through its sympathetic and parasympathetic divisions.
Give examples with every reflex type — natural reflexes such as blinking and the knee-jerk, acquired reflexes such as cycling and writing.
And distinguish carefully: a reflex is involuntary and a response to a stimulus and** rapid. Heartbeat is involuntary but is none of the other two.
Use the correct words — stimulus for what starts it, effector for the muscle or gland, response for what happens.
Say why a reflex goes through the spinal cord: it saves time, giving the shortest path. And add that the brain is informed afterwards, which is why pain is felt after the hand has moved.
Remember the spinal cord has grey matter inside and white matter outside — the reverse of the brain.
Name the three coverings of the central nervous system: bone (cranium and vertebral column), meninges (dura mater, arachnoid, pia mater), and cerebrospinal fluid.
Define the three divisions by what they are: CNS is brain and spinal cord; PNS is the nerves; ANS controls involuntary activity of internal organs through its sympathetic and parasympathetic divisions.
Give examples with every reflex type — natural reflexes such as blinking and the knee-jerk, acquired reflexes such as cycling and writing.
And distinguish carefully: a reflex is involuntary and a response to a stimulus and** rapid. Heartbeat is involuntary but is none of the other two.
Did you know
Why can you not tickle yourself?
Someone else running a finger along your sole produces helpless laughter. Do exactly the same thing yourself and almost nothing happens.
The difference is prediction. When you move your own hand, the brain issues the instruction and therefore knows in advance precisely where the touch will land and when. It compares the sensation that arrives with the one it expected, finds them matching, and treats the touch as unremarkable.
A touch from someone else cannot be predicted. The sensation arrives without any matching expectation, and the reflex response is triggered in full.
The same mechanism explains a great deal more than tickling. It is why you do not notice the feeling of your own clothes all day, and why you can hold your own eyelid open but flinch when someone else reaches towards your eye. The nervous system is continuously filtering out what it caused itself — and a reflex fires hardest at whatever it did not see coming.
The difference is prediction. When you move your own hand, the brain issues the instruction and therefore knows in advance precisely where the touch will land and when. It compares the sensation that arrives with the one it expected, finds them matching, and treats the touch as unremarkable.
A touch from someone else cannot be predicted. The sensation arrives without any matching expectation, and the reflex response is triggered in full.
The same mechanism explains a great deal more than tickling. It is why you do not notice the feeling of your own clothes all day, and why you can hold your own eyelid open but flinch when someone else reaches towards your eye. The nervous system is continuously filtering out what it caused itself — and a reflex fires hardest at whatever it did not see coming.
Key takeaways
The spinal cord, nervous divisions and reflexes: quick revision
- The spinal cord continues downward from the medulla oblongata inside the vertebral column, with a central canal, grey matter inside and white matter outside — the reverse of the brain.
- Its functions: conducts impulses to and from the brain, acts as the centre of spinal reflexes, and gives off spinal nerves.
- Protective coverings: bone (cranium for the brain, vertebral column for the cord), the three meninges (dura mater, arachnoid, pia mater), and cerebrospinal fluid which cushions, nourishes and removes waste.
- Nerve cells are not replaced, which is why the central nervous system is so heavily protected and why a spinal injury can cause permanent paralysis below it.
- Central nervous system — brain and spinal cord, the control and coordination centre.
- Peripheral nervous system — the cranial and spinal nerves, connecting the centre to the body and carrying sensory impulses in and motor impulses out.
- Autonomic nervous system — controls the involuntary activity of internal organs, with a sympathetic division that prepares for action and a parasympathetic division that calms. It is a functional part of the system, not a fourth system.
- A reflex action is a sudden, automatic, involuntary response to a stimulus, without the will of the brain.
- Reflex arc: **receptor sensory neuron relay neuron in the spinal cord motor neuron effector.
- It uses the spinal cord to save time; the brain is informed afterwards, which is why pain follows the movement. Some reflexes — blinking, sneezing, pupil narrowing — are handled by the brain instead.
- Natural (inborn) reflexes need no learning: blinking, sneezing, coughing, withdrawal from pain, knee-jerk, salivating, pupil narrowing.
- Acquired (conditioned) reflexes come from practice: cycling, swimming, writing, typing, driving. Each began as a voluntary action.
- Voluntary actions are controlled by the cerebrum and can be started or stopped at will. Involuntary actions are controlled by the medulla and the autonomic system and continue during sleep.
- A reflex is involuntary, but unlike heartbeat it is a response to a stimulus and is very rapid**.
Try drawing the reflex arc from memory with all five parts labelled, then sorting ten actions into voluntary, involuntary and reflex — those two tasks cover most of what this chapter asks.
- Its functions: conducts impulses to and from the brain, acts as the centre of spinal reflexes, and gives off spinal nerves.
- Protective coverings: bone (cranium for the brain, vertebral column for the cord), the three meninges (dura mater, arachnoid, pia mater), and cerebrospinal fluid which cushions, nourishes and removes waste.
- Nerve cells are not replaced, which is why the central nervous system is so heavily protected and why a spinal injury can cause permanent paralysis below it.
- Central nervous system — brain and spinal cord, the control and coordination centre.
- Peripheral nervous system — the cranial and spinal nerves, connecting the centre to the body and carrying sensory impulses in and motor impulses out.
- Autonomic nervous system — controls the involuntary activity of internal organs, with a sympathetic division that prepares for action and a parasympathetic division that calms. It is a functional part of the system, not a fourth system.
- A reflex action is a sudden, automatic, involuntary response to a stimulus, without the will of the brain.
- Reflex arc: **receptor sensory neuron relay neuron in the spinal cord motor neuron effector.
- It uses the spinal cord to save time; the brain is informed afterwards, which is why pain follows the movement. Some reflexes — blinking, sneezing, pupil narrowing — are handled by the brain instead.
- Natural (inborn) reflexes need no learning: blinking, sneezing, coughing, withdrawal from pain, knee-jerk, salivating, pupil narrowing.
- Acquired (conditioned) reflexes come from practice: cycling, swimming, writing, typing, driving. Each began as a voluntary action.
- Voluntary actions are controlled by the cerebrum and can be started or stopped at will. Involuntary actions are controlled by the medulla and the autonomic system and continue during sleep.
- A reflex is involuntary, but unlike heartbeat it is a response to a stimulus and is very rapid**.
Try drawing the reflex arc from memory with all five parts labelled, then sorting ten actions into voluntary, involuntary and reflex — those two tasks cover most of what this chapter asks.