A Nerve Message Can Only Ever Travel in One Direction
Learn why the body needs coordination and which two systems provide it, label a neuron and state what each part does, tell sensory from motor and relay neurons, and place the cerebrum, cerebellum and medulla oblongata.
Why can a nerve message never travel backwards?
Because of the tiny gaps between one nerve cell and the next.
Neurons do not actually touch. At the junction between them, called a synapse, there is a microscopic gap, and the impulse crosses it by a chemical released on one side only — from the end of the axon — and detected on the other.
Since only one side can release the chemical and only the other can detect it, the crossing works in one direction and no other. A wire could carry a signal either way; a chain of neurons cannot. This page covers the first part of the ICSE Class 8 Biology chapter on the nervous system: why coordination is needed, what a neuron is, the three kinds, and the parts of the brain.
Neurons do not actually touch. At the junction between them, called a synapse, there is a microscopic gap, and the impulse crosses it by a chemical released on one side only — from the end of the axon — and detected on the other.
Since only one side can release the chemical and only the other can detect it, the crossing works in one direction and no other. A wire could carry a signal either way; a chain of neurons cannot. This page covers the first part of the ICSE Class 8 Biology chapter on the nervous system: why coordination is needed, what a neuron is, the three kinds, and the parts of the brain.
Why does the body need control and coordination at all?
Because it is made of many organs that must work together, in the right order and at the right time.
Coordination is the working together of different organs so that they function smoothly as one unit. Control is the regulation of their activity.
What would go wrong without it. Consider something as ordinary as catching a ball. The eyes must judge its position, the brain must calculate where it will arrive, the arm and hand muscles must move to the right place in time, the legs must keep the body balanced, and the heart and lungs must supply the extra oxygen needed. Any one of those happening late or in the wrong order and the catch is missed.
Three jobs coordination performs:
- Responding to the surroundings. A change in the surroundings that produces a response is a stimulus — light, sound, heat, touch, smell, taste. The body must detect it and act.
- Making the organs cooperate. Digestion, circulation, breathing and movement all have to be matched to what the body is doing.
- Keeping internal conditions steady — body temperature, blood glucose and water content must be held within narrow limits whatever the surroundings do.
The two coordinating systems of the human body are the nervous system and the endocrine system.
- The nervous system works through electrical impulses carried along neurons. It is very fast and its effects are short-lived, and it delivers its message to one specific place.
- The endocrine system works through chemical hormones carried in the blood. It is slow and its effects are long-lasting, and its message reaches the whole body.
Why both are needed rather than one. The two are suited to opposite kinds of task. Pulling your hand from a hot pan must happen in a fraction of a second and then be over — a job only nerves can do. Growing taller over several years, or running a menstrual cycle over weeks, needs a slow sustained instruction — a job only hormones can do.
Neither system could take on the other's work, which is why the body has both and why they are linked through the pituitary gland at the base of the brain.
Coordination is the working together of different organs so that they function smoothly as one unit. Control is the regulation of their activity.
What would go wrong without it. Consider something as ordinary as catching a ball. The eyes must judge its position, the brain must calculate where it will arrive, the arm and hand muscles must move to the right place in time, the legs must keep the body balanced, and the heart and lungs must supply the extra oxygen needed. Any one of those happening late or in the wrong order and the catch is missed.
Three jobs coordination performs:
- Responding to the surroundings. A change in the surroundings that produces a response is a stimulus — light, sound, heat, touch, smell, taste. The body must detect it and act.
- Making the organs cooperate. Digestion, circulation, breathing and movement all have to be matched to what the body is doing.
- Keeping internal conditions steady — body temperature, blood glucose and water content must be held within narrow limits whatever the surroundings do.
The two coordinating systems of the human body are the nervous system and the endocrine system.
- The nervous system works through electrical impulses carried along neurons. It is very fast and its effects are short-lived, and it delivers its message to one specific place.
- The endocrine system works through chemical hormones carried in the blood. It is slow and its effects are long-lasting, and its message reaches the whole body.
Why both are needed rather than one. The two are suited to opposite kinds of task. Pulling your hand from a hot pan must happen in a fraction of a second and then be over — a job only nerves can do. Growing taller over several years, or running a menstrual cycle over weeks, needs a slow sustained instruction — a job only hormones can do.
Neither system could take on the other's work, which is why the body has both and why they are linked through the pituitary gland at the base of the brain.
What are the parts of a neuron and what does each do?
A neuron, or nerve cell, is the structural and functional unit of the nervous system. It is also the longest cell in the human body, since a single neuron may run from the spinal cord to a toe.
Its parts:
- Cell body, also called the cyton — contains the nucleus and cytoplasm. It is the metabolic centre of the neuron, keeping it alive, and it receives impulses brought in by the dendrites.
- Dendrites — short, branched extensions of the cell body. They receive impulses, from a sense organ or from a neighbouring neuron, and carry them towards the cell body.
- Axon — a single, long fibre leaving the cell body. It carries the impulse away from the cell body towards the next neuron, or towards a muscle or gland.
- Myelin sheath — a fatty covering around the axon. It insulates the axon, prevents the impulse leaking into neighbouring fibres, and speeds the impulse up considerably.
- Axon terminals, or nerve endings — the fine branches at the far end of the axon, which pass the impulse on.
- Synapse — the tiny gap between the axon terminal of one neuron and the dendrite of the next, crossed by a chemical.
The direction rule, which is the whole point of the structure. Impulses always travel:
dendrites cell body axon synapse next neuron
Dendrites are always the receiving end and the axon is always the sending end. This is why a labelled diagram is enough to tell you which way an impulse is going.
Why each neuron has many dendrites but only one axon. A neuron gathers information from many sources and then sends a single decision onward. Many inputs, one output — which is why the dendrites are numerous and branched and the axon is single and long.
What the myelin sheath is really for. Compare it with the plastic insulation on an electrical wire: it stops the signal escaping sideways into the wire beside it. But it does something an insulator does not — it also makes the impulse travel faster, which is why the fastest nerves in the body are the most heavily myelinated. A nerve whose myelin is damaged conducts slowly and unreliably.
Its parts:
- Cell body, also called the cyton — contains the nucleus and cytoplasm. It is the metabolic centre of the neuron, keeping it alive, and it receives impulses brought in by the dendrites.
- Dendrites — short, branched extensions of the cell body. They receive impulses, from a sense organ or from a neighbouring neuron, and carry them towards the cell body.
- Axon — a single, long fibre leaving the cell body. It carries the impulse away from the cell body towards the next neuron, or towards a muscle or gland.
- Myelin sheath — a fatty covering around the axon. It insulates the axon, prevents the impulse leaking into neighbouring fibres, and speeds the impulse up considerably.
- Axon terminals, or nerve endings — the fine branches at the far end of the axon, which pass the impulse on.
- Synapse — the tiny gap between the axon terminal of one neuron and the dendrite of the next, crossed by a chemical.
The direction rule, which is the whole point of the structure. Impulses always travel:
dendrites cell body axon synapse next neuron
Dendrites are always the receiving end and the axon is always the sending end. This is why a labelled diagram is enough to tell you which way an impulse is going.
Why each neuron has many dendrites but only one axon. A neuron gathers information from many sources and then sends a single decision onward. Many inputs, one output — which is why the dendrites are numerous and branched and the axon is single and long.
What the myelin sheath is really for. Compare it with the plastic insulation on an electrical wire: it stops the signal escaping sideways into the wire beside it. But it does something an insulator does not — it also makes the impulse travel faster, which is why the fastest nerves in the body are the most heavily myelinated. A nerve whose myelin is damaged conducts slowly and unreliably.
How do sensory, motor and relay neurons differ?
By which way they carry the impulse relative to the brain and spinal cord.
Sensory neurons, also called afferent neurons, carry impulses from the sense organs and receptors to the brain or spinal cord.
They are the incoming line. Touching something hot, the receptors in the skin generate an impulse and a sensory neuron carries it inwards. Every sensation — sight, sound, smell, taste, touch, pain, temperature — arrives this way.
Motor neurons, also called efferent neurons, carry impulses from the brain or spinal cord to the muscles and glands.
They are the outgoing line. The muscle or gland that acts on the instruction is called an effector. A motor neuron does not sense anything; it only delivers an order.
Relay neurons, also called interneurons or association neurons, lie within the brain and spinal cord. They connect sensory neurons to motor neurons, and to each other.
They are the switchboard. A relay neuron neither collects information from outside nor acts on the body — it decides where an incoming impulse should be sent, and it is where the choosing happens.
The complete pathway, which every question on this topic is built on:
receptor sensory neuron relay neuron motor neuron effector
For the hot pan: skin receptors detect the heat, a sensory neuron carries it to the spinal cord, a relay neuron passes it straight across, a motor neuron carries the instruction to the arm muscle, and the muscle contracts and pulls the hand away.
How to identify the type in a question. Ask where the impulse is going. Towards the brain or spinal cord means sensory; away from it means motor; entirely inside it means relay. The names afferent and efferent say the same thing, and the initial letters help — a for arriving, e for exiting.
Why three types rather than two. A direct sensory-to-motor connection would allow only one fixed response to each stimulus. The relay neuron is what makes a choice possible — the same touch can be ignored, investigated or fled from, depending on what the relay neurons in between decide to do with it.
Sensory neurons, also called afferent neurons, carry impulses from the sense organs and receptors to the brain or spinal cord.
They are the incoming line. Touching something hot, the receptors in the skin generate an impulse and a sensory neuron carries it inwards. Every sensation — sight, sound, smell, taste, touch, pain, temperature — arrives this way.
Motor neurons, also called efferent neurons, carry impulses from the brain or spinal cord to the muscles and glands.
They are the outgoing line. The muscle or gland that acts on the instruction is called an effector. A motor neuron does not sense anything; it only delivers an order.
Relay neurons, also called interneurons or association neurons, lie within the brain and spinal cord. They connect sensory neurons to motor neurons, and to each other.
They are the switchboard. A relay neuron neither collects information from outside nor acts on the body — it decides where an incoming impulse should be sent, and it is where the choosing happens.
The complete pathway, which every question on this topic is built on:
receptor sensory neuron relay neuron motor neuron effector
For the hot pan: skin receptors detect the heat, a sensory neuron carries it to the spinal cord, a relay neuron passes it straight across, a motor neuron carries the instruction to the arm muscle, and the muscle contracts and pulls the hand away.
How to identify the type in a question. Ask where the impulse is going. Towards the brain or spinal cord means sensory; away from it means motor; entirely inside it means relay. The names afferent and efferent say the same thing, and the initial letters help — a for arriving, e for exiting.
Why three types rather than two. A direct sensory-to-motor connection would allow only one fixed response to each stimulus. The relay neuron is what makes a choice possible — the same touch can be ignored, investigated or fled from, depending on what the relay neurons in between decide to do with it.
What do the cerebrum, cerebellum and medulla oblongata do?
The brain lies inside the skull, wrapped in three protective membranes called the meninges with cerebrospinal fluid between them to cushion it against shocks. Its three main parts have quite different jobs.
Cerebrum — the largest part, forming the upper and greater portion of the brain, with a deeply folded surface.
Its functions:
- The seat of intelligence, memory, thinking, reasoning and will
- The centre for all voluntary actions — everything you decide to do
- Receives and interprets sensations — sight, hearing, touch, taste, smell, pain and temperature
- The seat of emotions
So the cerebrum is where you think and where you decide. Its folds exist to pack a large surface area into a small skull.
Cerebellum — lying below and behind the cerebrum, and much smaller.
Its functions:
- Maintains balance and posture of the body
- Coordinates muscular activity, so that movements are smooth and precise rather than jerky
The cerebrum decides to pick up a glass; the cerebellum ensures the hand arrives smoothly and closes with the right force. A person with a damaged cerebellum can still decide to walk and still move their legs, but the movement becomes unsteady — showing the two jobs are genuinely separate.
Medulla oblongata — the lowest part, continuous with the spinal cord below it.
Its functions — it controls the involuntary activities that never reach your awareness:
- Heartbeat and blood pressure
- Breathing
- Swallowing, coughing, sneezing, vomiting and salivation
None of these needs a decision, and none of them stops while you sleep.
The dividing line to remember. Cerebrum for voluntary and for thinking, cerebellum for balance and coordination, medulla for involuntary. Every function asked about falls into one of those three, and matching the function to the wrong part is the commonest error in this chapter.
Where the rest of the system fits. The brain and the spinal cord together make up the central nervous system, and the nerves running out from them make up the peripheral nervous system. The relay neurons of the previous section lie in the central nervous system; the sensory and motor neurons run out through the peripheral one.
Cerebrum — the largest part, forming the upper and greater portion of the brain, with a deeply folded surface.
Its functions:
- The seat of intelligence, memory, thinking, reasoning and will
- The centre for all voluntary actions — everything you decide to do
- Receives and interprets sensations — sight, hearing, touch, taste, smell, pain and temperature
- The seat of emotions
So the cerebrum is where you think and where you decide. Its folds exist to pack a large surface area into a small skull.
Cerebellum — lying below and behind the cerebrum, and much smaller.
Its functions:
- Maintains balance and posture of the body
- Coordinates muscular activity, so that movements are smooth and precise rather than jerky
The cerebrum decides to pick up a glass; the cerebellum ensures the hand arrives smoothly and closes with the right force. A person with a damaged cerebellum can still decide to walk and still move their legs, but the movement becomes unsteady — showing the two jobs are genuinely separate.
Medulla oblongata — the lowest part, continuous with the spinal cord below it.
Its functions — it controls the involuntary activities that never reach your awareness:
- Heartbeat and blood pressure
- Breathing
- Swallowing, coughing, sneezing, vomiting and salivation
None of these needs a decision, and none of them stops while you sleep.
The dividing line to remember. Cerebrum for voluntary and for thinking, cerebellum for balance and coordination, medulla for involuntary. Every function asked about falls into one of those three, and matching the function to the wrong part is the commonest error in this chapter.
Where the rest of the system fits. The brain and the spinal cord together make up the central nervous system, and the nerves running out from them make up the peripheral nervous system. The relay neurons of the previous section lie in the central nervous system; the sensory and motor neurons run out through the peripheral one.
Exam tip
Exam tip: dendrites receive, the axon sends
In every neuron answer, state the direction: dendrites carry impulses towards the cell body and the axon carries them away. That one sentence settles half the questions on this topic.
Give the myelin sheath both its functions — it insulates the axon and speeds up the impulse. Naming only insulation is half the mark.
Identify a neuron's type by where the impulse is going: towards the brain or spinal cord is sensory, away is motor, within is relay.
Learn the pathway as a chain and write it in order: **receptor sensory neuron relay neuron motor neuron effector. Call the responding muscle or gland the effector.
Match the brain parts carefully — cerebrum for thinking, memory, reasoning and voluntary actions, cerebellum for balance and coordination, medulla oblongata for involuntary activities such as heartbeat and breathing.
Name the synapse as the gap between neurons, and say the impulse crosses it by a chemical, which is why transmission is one-way.
When comparing the two coordinating systems, answer in pairs: nerves are fast, brief and specific; hormones are slow, long-lasting and body-wide.
And call the neuron the structural and functional unit** of the nervous system.
Give the myelin sheath both its functions — it insulates the axon and speeds up the impulse. Naming only insulation is half the mark.
Identify a neuron's type by where the impulse is going: towards the brain or spinal cord is sensory, away is motor, within is relay.
Learn the pathway as a chain and write it in order: **receptor sensory neuron relay neuron motor neuron effector. Call the responding muscle or gland the effector.
Match the brain parts carefully — cerebrum for thinking, memory, reasoning and voluntary actions, cerebellum for balance and coordination, medulla oblongata for involuntary activities such as heartbeat and breathing.
Name the synapse as the gap between neurons, and say the impulse crosses it by a chemical, which is why transmission is one-way.
When comparing the two coordinating systems, answer in pairs: nerves are fast, brief and specific; hormones are slow, long-lasting and body-wide.
And call the neuron the structural and functional unit** of the nervous system.
Did you know
Why is the smallest of the three brain parts the one you cannot live without?
The cerebrum is by far the largest part of the brain and holds everything that makes a person themselves — memory, reasoning, language, decisions. The medulla oblongata is a small stalk at the very bottom.
Yet serious damage to parts of the cerebrum, though devastating, can leave a person alive. Damage to the medulla is almost immediately fatal.
The reason is what each controls. The cerebrum runs the activities you choose, and choices can be lost without the body stopping. The medulla runs heartbeat and breathing — activities that have continued without interruption since before birth and cannot pause even for a minute.
This is also why the medulla sits where it does, at the very base, protected behind the thickest part of the skull and shielded by everything above it. The brain's most sophisticated functions are on the outside, where a blow reaches them first, and the handful of functions that must never stop are buried deepest.
Yet serious damage to parts of the cerebrum, though devastating, can leave a person alive. Damage to the medulla is almost immediately fatal.
The reason is what each controls. The cerebrum runs the activities you choose, and choices can be lost without the body stopping. The medulla runs heartbeat and breathing — activities that have continued without interruption since before birth and cannot pause even for a minute.
This is also why the medulla sits where it does, at the very base, protected behind the thickest part of the skull and shielded by everything above it. The brain's most sophisticated functions are on the outside, where a blow reaches them first, and the handful of functions that must never stop are buried deepest.
Key takeaways
Coordination, neurons and the brain: quick revision
- Coordination makes different organs work together smoothly; control regulates their activity. It is needed to respond to stimuli, to make organs cooperate, and to keep internal conditions steady.
- A stimulus is a change in the surroundings that produces a response.
- The two coordinating systems: the nervous system — electrical impulses along neurons, fast, brief, and specific; and the endocrine system — hormones in the blood, slow, long-lasting, and body-wide.
- A neuron is the structural and functional unit of the nervous system and the longest cell in the body.
- Cell body (cyton) — holds the nucleus, the metabolic centre. Dendrites — short and branched, carry impulses towards the cell body. Axon — single and long, carries impulses away. Myelin sheath — insulates the axon and speeds up the impulse. Axon terminals pass it on.
- A synapse is the gap between neurons, crossed by a chemical released from the axon side only — which is why transmission is one-way.
- Many dendrites but one axon: many inputs, one output.
- Sensory (afferent) neurons carry impulses from receptors to the brain or spinal cord. Motor (efferent) neurons carry them from the brain or spinal cord to muscles and glands, called effectors. Relay neurons lie within the brain and spinal cord and connect the other two.
- The pathway: **receptor sensory neuron relay neuron motor neuron effector.
- The relay neuron is what makes a choice of response possible.
- The brain is protected by the skull, the meninges and cerebrospinal fluid.
- Cerebrum — largest; intelligence, memory, thinking, reasoning, will, all voluntary actions, and interpreting sensations.
- Cerebellum — below and behind; balance, posture and coordination of muscular activity so movements are smooth.
- Medulla oblongata — lowest, continuous with the spinal cord; involuntary activities — heartbeat, breathing, blood pressure, swallowing, coughing, sneezing, vomiting.
- Brain + spinal cord = central nervous system; the nerves form the peripheral nervous system**.
Try labelling a neuron with a function against each part, then matching a dozen body functions to the correct brain region — those two drills cover nearly everything this chapter asks.
- A stimulus is a change in the surroundings that produces a response.
- The two coordinating systems: the nervous system — electrical impulses along neurons, fast, brief, and specific; and the endocrine system — hormones in the blood, slow, long-lasting, and body-wide.
- A neuron is the structural and functional unit of the nervous system and the longest cell in the body.
- Cell body (cyton) — holds the nucleus, the metabolic centre. Dendrites — short and branched, carry impulses towards the cell body. Axon — single and long, carries impulses away. Myelin sheath — insulates the axon and speeds up the impulse. Axon terminals pass it on.
- A synapse is the gap between neurons, crossed by a chemical released from the axon side only — which is why transmission is one-way.
- Many dendrites but one axon: many inputs, one output.
- Sensory (afferent) neurons carry impulses from receptors to the brain or spinal cord. Motor (efferent) neurons carry them from the brain or spinal cord to muscles and glands, called effectors. Relay neurons lie within the brain and spinal cord and connect the other two.
- The pathway: **receptor sensory neuron relay neuron motor neuron effector.
- The relay neuron is what makes a choice of response possible.
- The brain is protected by the skull, the meninges and cerebrospinal fluid.
- Cerebrum — largest; intelligence, memory, thinking, reasoning, will, all voluntary actions, and interpreting sensations.
- Cerebellum — below and behind; balance, posture and coordination of muscular activity so movements are smooth.
- Medulla oblongata — lowest, continuous with the spinal cord; involuntary activities — heartbeat, breathing, blood pressure, swallowing, coughing, sneezing, vomiting.
- Brain + spinal cord = central nervous system; the nerves form the peripheral nervous system**.
Try labelling a neuron with a function against each part, then matching a dozen body functions to the correct brain region — those two drills cover nearly everything this chapter asks.