Your Hand Pulls Back Before Your Brain Knows Why
Learn the functions of the cerebrum, cerebellum and medulla, how the spinal cord is protected, how to trace a reflex arc step by step, and how reflex actions differ from voluntary ones.
Why does your hand pull back before you feel the pain?
Because the spinal cord answers without waiting for the brain. The impulse takes a short cut through the cord, the arm muscle acts, and only afterwards does the message reach the brain and you feel the heat.
Saving those fractions of a second is what a reflex is for. This page covers everything in the ICSE Class 7 Biology chapter's second part: the brain and its parts, the spinal cord, reflex action and the reflex arc, and the three kinds of action.
Saving those fractions of a second is what a reflex is for. This page covers everything in the ICSE Class 7 Biology chapter's second part: the brain and its parts, the spinal cord, reflex action and the reflex arc, and the three kinds of action.
What do the cerebrum, cerebellum and medulla do?
The brain lies inside the skull, protected by bone, by three membranes called meninges, and by the cerebrospinal fluid between them, which absorbs shocks. It has three main parts.
Cerebrum — the largest part, with a deeply folded surface. It is the seat of:
- Intelligence, memory, thinking and reasoning
- Will power and all voluntary actions
- Sensations — the centres for sight, hearing, smell, taste, touch and speech
Cerebellum — lies below and behind the cerebrum. It:
- Coordinates muscular activity so movements are smooth and precise
- Maintains balance and posture
Medulla oblongata — the lowest part, continuing into the spinal cord. It controls the involuntary activities:
- Heartbeat, breathing and blood pressure
- Swallowing, coughing, sneezing and vomiting
Cycling shows two of them working together: the cerebrum decides where to go, while the cerebellum keeps you upright.
The pair most often confused is the cerebrum and cerebellum, and one word separates them. The cerebrum decides a movement; the cerebellum coordinates it. Damage to the cerebellum does not stop a person from wanting to walk — it makes the walking unsteady.
Cerebrum — the largest part, with a deeply folded surface. It is the seat of:
- Intelligence, memory, thinking and reasoning
- Will power and all voluntary actions
- Sensations — the centres for sight, hearing, smell, taste, touch and speech
Cerebellum — lies below and behind the cerebrum. It:
- Coordinates muscular activity so movements are smooth and precise
- Maintains balance and posture
Medulla oblongata — the lowest part, continuing into the spinal cord. It controls the involuntary activities:
- Heartbeat, breathing and blood pressure
- Swallowing, coughing, sneezing and vomiting
Cycling shows two of them working together: the cerebrum decides where to go, while the cerebellum keeps you upright.
The pair most often confused is the cerebrum and cerebellum, and one word separates them. The cerebrum decides a movement; the cerebellum coordinates it. Damage to the cerebellum does not stop a person from wanting to walk — it makes the walking unsteady.
How is the spinal cord built and protected?
The spinal cord is a long, cylindrical cord of nervous tissue continuing downward from the medulla oblongata, running through the vertebral column from the neck to the lower back.
Its protection has three layers, the same as the brain's:
- The bony vertebral column, whose vertebrae surround it as a tunnel
- The three meninges
- The cerebrospinal fluid, which cushions it against shock
Inside, the arrangement is the reverse of the brain's: the spinal cord has grey matter inside and white matter outside. Thirty-one pairs of spinal nerves arise from it, each carrying both sensory and motor fibres.
Its two functions are:
- Conducting impulses to and from the brain — sensory impulses travelling up, motor instructions travelling down
- Acting as the centre of reflex actions below the neck
A doctor tapping a patient's knee to watch the leg jerk is testing exactly this second function.
The injury consequence follows directly from the first function, and it is worth knowing. Damage to the spinal cord cuts the communication line, so the parts of the body below the injury lose both sensation and movement — even though the muscles and the brain themselves are unharmed.
Its protection has three layers, the same as the brain's:
- The bony vertebral column, whose vertebrae surround it as a tunnel
- The three meninges
- The cerebrospinal fluid, which cushions it against shock
Inside, the arrangement is the reverse of the brain's: the spinal cord has grey matter inside and white matter outside. Thirty-one pairs of spinal nerves arise from it, each carrying both sensory and motor fibres.
Its two functions are:
- Conducting impulses to and from the brain — sensory impulses travelling up, motor instructions travelling down
- Acting as the centre of reflex actions below the neck
A doctor tapping a patient's knee to watch the leg jerk is testing exactly this second function.
The injury consequence follows directly from the first function, and it is worth knowing. Damage to the spinal cord cuts the communication line, so the parts of the body below the injury lose both sensation and movement — even though the muscles and the brain themselves are unharmed.
How do you trace the path of a reflex arc?
A reflex action is a sudden, automatic, involuntary response to a stimulus, which does not involve the will of the brain. The path the impulse travels is the reflex arc, and it has five steps in fixed order:
- Receptor — the sense organ that detects the stimulus.
- Sensory neuron — carries the impulse to the spinal cord.
- Spinal cord — where a relay neuron passes the impulse straight across to a motor neuron.
- Motor neuron — carries the instruction out to the muscle.
- Effector — the muscle or gland that responds.
Traced through a real example — touching a hot vessel:
Heat receptors in the skin of the fingers detect the heat. A sensory neuron carries the impulse to the spinal cord. A relay neuron there passes it to a motor neuron. The motor neuron carries it to the arm muscle. The muscle contracts and the hand is pulled away.
Other reflexes to name: blinking when something approaches the eye, sneezing at dust, the knee jerk, salivating at the smell of food, and coughing.
The reason this route is used is speed, and the explanation is what earns marks. Sending the message up to the brain and back would take noticeably longer, and in that time the hand would be burnt. The brain is informed afterwards, which is why you feel the pain a moment after your hand has already moved.
- Receptor — the sense organ that detects the stimulus.
- Sensory neuron — carries the impulse to the spinal cord.
- Spinal cord — where a relay neuron passes the impulse straight across to a motor neuron.
- Motor neuron — carries the instruction out to the muscle.
- Effector — the muscle or gland that responds.
Traced through a real example — touching a hot vessel:
Heat receptors in the skin of the fingers detect the heat. A sensory neuron carries the impulse to the spinal cord. A relay neuron there passes it to a motor neuron. The motor neuron carries it to the arm muscle. The muscle contracts and the hand is pulled away.
Other reflexes to name: blinking when something approaches the eye, sneezing at dust, the knee jerk, salivating at the smell of food, and coughing.
The reason this route is used is speed, and the explanation is what earns marks. Sending the message up to the brain and back would take noticeably longer, and in that time the hand would be burnt. The brain is informed afterwards, which is why you feel the pain a moment after your hand has already moved.
What is the difference between voluntary, involuntary and reflex actions?
All three are responses, but they differ in who controls them.
Voluntary actions are performed consciously, under the control of the will, and are directed by the cerebrum. Walking, writing, speaking, eating and reading are voluntary.
Involuntary actions go on continuously without any conscious control, and are directed by the medulla oblongata and the autonomic nervous system. The beating of the heart, breathing, digestion and the movement of the intestines are involuntary.
Reflex actions are sudden and automatic responses to a stimulus, controlled mainly by the spinal cord. Withdrawing the hand from a hot object, blinking and sneezing are reflexes.
So the distinction between involuntary and reflex is worth stating carefully: involuntary actions are continuous and need no stimulus, while a reflex is a single sudden response to a particular stimulus.
To keep the nervous system healthy:
- Eat a balanced diet, especially foods with vitamin B, since nerves need it
- Get enough sleep and rest, which allow the brain to recover
- Exercise regularly to improve blood supply to the brain
- Avoid alcohol, tobacco and drugs, which damage nerve cells
- Wear a helmet while riding, protecting the skull and brain from injury
- Avoid prolonged stress, and maintain good posture to protect the spine
One group sits between the categories, and examiners like it. Breathing is normally involuntary, yet you can consciously hold your breath — so it can be brought under voluntary control for a short while, unlike the heartbeat, which cannot.
Voluntary actions are performed consciously, under the control of the will, and are directed by the cerebrum. Walking, writing, speaking, eating and reading are voluntary.
Involuntary actions go on continuously without any conscious control, and are directed by the medulla oblongata and the autonomic nervous system. The beating of the heart, breathing, digestion and the movement of the intestines are involuntary.
Reflex actions are sudden and automatic responses to a stimulus, controlled mainly by the spinal cord. Withdrawing the hand from a hot object, blinking and sneezing are reflexes.
So the distinction between involuntary and reflex is worth stating carefully: involuntary actions are continuous and need no stimulus, while a reflex is a single sudden response to a particular stimulus.
To keep the nervous system healthy:
- Eat a balanced diet, especially foods with vitamin B, since nerves need it
- Get enough sleep and rest, which allow the brain to recover
- Exercise regularly to improve blood supply to the brain
- Avoid alcohol, tobacco and drugs, which damage nerve cells
- Wear a helmet while riding, protecting the skull and brain from injury
- Avoid prolonged stress, and maintain good posture to protect the spine
One group sits between the categories, and examiners like it. Breathing is normally involuntary, yet you can consciously hold your breath — so it can be brought under voluntary control for a short while, unlike the heartbeat, which cannot.
Exam tip
Exam tip: writing all five steps of the reflex arc
The reflex arc is the most examined item in this chapter, and it is marked step by step.
Write all five in order — receptor, sensory neuron, spinal cord, motor neuron, effector — and then apply them to the named example the question gives. Missing the relay neuron inside the spinal cord is the commonest omission.
Add the reason: the impulse takes a short cut through the spinal cord to save time, and the brain is informed only afterwards.
For brain parts, match each to its own function and do not mix them: cerebrum thinks and decides, cerebellum coordinates and balances, medulla controls heartbeat and breathing.
For the spinal cord, mention all three protective layers — vertebral column, meninges, cerebrospinal fluid.
And when comparing involuntary with reflex actions, use the words continuous and sudden response to a stimulus.
Write all five in order — receptor, sensory neuron, spinal cord, motor neuron, effector — and then apply them to the named example the question gives. Missing the relay neuron inside the spinal cord is the commonest omission.
Add the reason: the impulse takes a short cut through the spinal cord to save time, and the brain is informed only afterwards.
For brain parts, match each to its own function and do not mix them: cerebrum thinks and decides, cerebellum coordinates and balances, medulla controls heartbeat and breathing.
For the spinal cord, mention all three protective layers — vertebral column, meninges, cerebrospinal fluid.
And when comparing involuntary with reflex actions, use the words continuous and sudden response to a stimulus.
Did you know
Why can you hold your breath but not stop your heartbeat?
Because breathing has a voluntary override and the heartbeat does not.
Breathing is normally run by the medulla oblongata without any thought, but the muscles it uses — the diaphragm and rib muscles — are skeletal muscles, the kind the cerebrum can also command. So you can take charge of them for a while.
The heart is made of cardiac muscle, which has no voluntary connection at all. No amount of willing changes it, which is exactly why it has kept beating through every night you have slept.
Breathing is normally run by the medulla oblongata without any thought, but the muscles it uses — the diaphragm and rib muscles — are skeletal muscles, the kind the cerebrum can also command. So you can take charge of them for a while.
The heart is made of cardiac muscle, which has no voluntary connection at all. No amount of willing changes it, which is exactly why it has kept beating through every night you have slept.
Key takeaways
The brain and reflex action: quick revision
- The brain is protected by the skull, three meninges and cerebrospinal fluid.
- Cerebrum: thinking, memory, will and sensations. Cerebellum: coordination and balance. Medulla oblongata: heartbeat, breathing and swallowing.
- The spinal cord runs from the medulla through the vertebral column, has grey matter inside and white outside, and gives off 31 pairs of spinal nerves.
- Its two jobs are conducting impulses to and from the brain, and acting as the centre of reflexes.
- A reflex arc runs receptor, sensory neuron, spinal cord, motor neuron, effector — a short cut that saves time, with the brain informed afterwards.
- Voluntary actions are willed and run by the cerebrum; involuntary ones are continuous and run by the medulla; reflexes are sudden automatic responses run by the spinal cord.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Cerebrum: thinking, memory, will and sensations. Cerebellum: coordination and balance. Medulla oblongata: heartbeat, breathing and swallowing.
- The spinal cord runs from the medulla through the vertebral column, has grey matter inside and white outside, and gives off 31 pairs of spinal nerves.
- Its two jobs are conducting impulses to and from the brain, and acting as the centre of reflexes.
- A reflex arc runs receptor, sensory neuron, spinal cord, motor neuron, effector — a short cut that saves time, with the brain informed afterwards.
- Voluntary actions are willed and run by the cerebrum; involuntary ones are continuous and run by the medulla; reflexes are sudden automatic responses run by the spinal cord.
You will remember all of this far better after answering five questions on it than after reading it twice.