The Longest Cell in Your Body Runs From Hip to Toe
Learn why the body needs coordination, label every part of a neuron and its job, tell sensory from motor and relay neurons, and map the central, peripheral and autonomic nervous systems.
How long can a single nerve cell be?
A single neuron can stretch from the base of your spine right down to your toe — a metre or more of one cell. Its long axon carries an impulse that whole distance in a fraction of a second.
No other cell in the body comes close. This page covers everything in the ICSE Class 7 Biology chapter's first part: the need for coordination, the structure of a neuron, the three kinds of neuron, and the organisation of the nervous system.
No other cell in the body comes close. This page covers everything in the ICSE Class 7 Biology chapter's first part: the need for coordination, the structure of a neuron, the three kinds of neuron, and the organisation of the nervous system.
Why does the body need control and coordination?
Because the body is made of many organs that must work together, at the right moment, as a single unit.
Without coordination, the stomach would produce digestive juices when there was no food, the legs would move without the eyes guiding them, and the heart would not speed up when you began to run.
The nervous system has three main functions:
- Receiving stimuli — the sense organs detect changes inside and outside the body: light, sound, touch, heat, smell and taste.
- Interpreting them — the brain and spinal cord analyse the information and decide what to do.
- Bringing about a response — messages are sent to muscles or glands, which act.
So touching a hot vessel involves all three in order: the skin detects heat, the spinal cord decides, and the arm muscles pull your hand away.
The body actually has two coordinating systems, and the difference matters. The nervous system works through electrical impulses and acts in a fraction of a second; the endocrine system works through hormones in the blood and acts far more slowly but for longer. Speed is exactly why the nervous system exists.
Without coordination, the stomach would produce digestive juices when there was no food, the legs would move without the eyes guiding them, and the heart would not speed up when you began to run.
The nervous system has three main functions:
- Receiving stimuli — the sense organs detect changes inside and outside the body: light, sound, touch, heat, smell and taste.
- Interpreting them — the brain and spinal cord analyse the information and decide what to do.
- Bringing about a response — messages are sent to muscles or glands, which act.
So touching a hot vessel involves all three in order: the skin detects heat, the spinal cord decides, and the arm muscles pull your hand away.
The body actually has two coordinating systems, and the difference matters. The nervous system works through electrical impulses and acts in a fraction of a second; the endocrine system works through hormones in the blood and acts far more slowly but for longer. Speed is exactly why the nervous system exists.
What are the parts of a neuron and what does each do?
A neuron is the structural and functional unit of the nervous system. It has these parts:
- Cyton (cell body) — the broad central part containing the nucleus and cytoplasm. It receives impulses from the dendrites and controls the cell's activities.
- Dendrites — short, branched fibres arising from the cyton. They receive impulses and carry them towards the cell body.
- Axon — the single long fibre leaving the cyton. It carries impulses away from the cell body.
- Myelin sheath — a fatty white covering around the axon. It insulates the axon and greatly speeds up the impulse.
- Node of Ranvier — the small gaps in the myelin sheath at regular intervals. The impulse jumps from node to node, which is what makes conduction so fast.
- Nerve endings (axon terminals) — the fine branches at the axon's tip, which pass the impulse on to the next neuron or to a muscle.
The tiny gap between two neurons is called a synapse, and the impulse crosses it with the help of a chemical.
An insulated electrical wire is the closest everyday comparison — the myelin sheath does the same job as the plastic on a copper wire.
The nodes are the detail most often left out, and they carry a mark because of what they explain. An impulse does not crawl along the whole axon; it leaps between the gaps, which is why a myelinated nerve conducts many times faster than an unmyelinated one.
- Cyton (cell body) — the broad central part containing the nucleus and cytoplasm. It receives impulses from the dendrites and controls the cell's activities.
- Dendrites — short, branched fibres arising from the cyton. They receive impulses and carry them towards the cell body.
- Axon — the single long fibre leaving the cyton. It carries impulses away from the cell body.
- Myelin sheath — a fatty white covering around the axon. It insulates the axon and greatly speeds up the impulse.
- Node of Ranvier — the small gaps in the myelin sheath at regular intervals. The impulse jumps from node to node, which is what makes conduction so fast.
- Nerve endings (axon terminals) — the fine branches at the axon's tip, which pass the impulse on to the next neuron or to a muscle.
The tiny gap between two neurons is called a synapse, and the impulse crosses it with the help of a chemical.
An insulated electrical wire is the closest everyday comparison — the myelin sheath does the same job as the plastic on a copper wire.
The nodes are the detail most often left out, and they carry a mark because of what they explain. An impulse does not crawl along the whole axon; it leaps between the gaps, which is why a myelinated nerve conducts many times faster than an unmyelinated one.
What is the difference between sensory, motor and relay neurons?
They differ by the direction in which they carry impulses.
Sensory (afferent) neurons carry impulses from the sense organs and receptors to the brain or spinal cord. The nerve from your skin to the spinal cord is sensory.
Motor (efferent) neurons carry impulses from the brain or spinal cord to the muscles or glands, which then act. The nerve from the spinal cord to your arm muscle is motor.
Association (relay or connector) neurons lie within the brain and spinal cord, and connect a sensory neuron to a motor neuron.
The complete path in order is therefore: receptor, sensory neuron, relay neuron in the central nervous system, motor neuron, effector.
A hand pulling back from a hot tawa uses all three in that sequence within a fraction of a second.
The way to remember the pair is to read the names literally. Sensory neurons bring sensations in; motor neurons carry orders out to move. A nerve that carries both kinds of fibre is called a mixed nerve, and most of the body's nerves are mixed.
Sensory (afferent) neurons carry impulses from the sense organs and receptors to the brain or spinal cord. The nerve from your skin to the spinal cord is sensory.
Motor (efferent) neurons carry impulses from the brain or spinal cord to the muscles or glands, which then act. The nerve from the spinal cord to your arm muscle is motor.
Association (relay or connector) neurons lie within the brain and spinal cord, and connect a sensory neuron to a motor neuron.
The complete path in order is therefore: receptor, sensory neuron, relay neuron in the central nervous system, motor neuron, effector.
A hand pulling back from a hot tawa uses all three in that sequence within a fraction of a second.
The way to remember the pair is to read the names literally. Sensory neurons bring sensations in; motor neurons carry orders out to move. A nerve that carries both kinds of fibre is called a mixed nerve, and most of the body's nerves are mixed.
How is the nervous system organised?
It is divided into three parts.
Central nervous system (CNS) — the brain and the spinal cord. It is the control centre, receiving information, interpreting it and issuing instructions. Both are protected by bone: the brain by the skull, the spinal cord by the vertebral column.
Peripheral nervous system (PNS) — all the nerves arising from the CNS and spreading through the body. It has two sets:
- Cranial nerves — 12 pairs, arising from the brain, serving the head, neck, face and internal organs.
- Spinal nerves — 31 pairs, arising from the spinal cord, serving the rest of the body.
Autonomic nervous system (ANS) — controls the involuntary organs such as the heart, stomach, intestine, blood vessels and glands, without conscious effort. It has two divisions that work in opposition:
- Sympathetic — prepares the body for action in an emergency. It speeds up the heart, widens the pupils, and slows digestion.
- Parasympathetic — restores the body to a calm state. It slows the heart, narrows the pupils, and promotes digestion.
The balance shows itself whenever you are startled — a sudden fright makes your heart pound, and once the danger passes it settles again.
The two divisions do not take turns switching off. They act antagonistically and continuously, each pulling the other way, and the body's actual state is the balance between them — which is why recovery after a scare is as active a process as the fright itself.
Central nervous system (CNS) — the brain and the spinal cord. It is the control centre, receiving information, interpreting it and issuing instructions. Both are protected by bone: the brain by the skull, the spinal cord by the vertebral column.
Peripheral nervous system (PNS) — all the nerves arising from the CNS and spreading through the body. It has two sets:
- Cranial nerves — 12 pairs, arising from the brain, serving the head, neck, face and internal organs.
- Spinal nerves — 31 pairs, arising from the spinal cord, serving the rest of the body.
Autonomic nervous system (ANS) — controls the involuntary organs such as the heart, stomach, intestine, blood vessels and glands, without conscious effort. It has two divisions that work in opposition:
- Sympathetic — prepares the body for action in an emergency. It speeds up the heart, widens the pupils, and slows digestion.
- Parasympathetic — restores the body to a calm state. It slows the heart, narrows the pupils, and promotes digestion.
The balance shows itself whenever you are startled — a sudden fright makes your heart pound, and once the danger passes it settles again.
The two divisions do not take turns switching off. They act antagonistically and continuously, each pulling the other way, and the body's actual state is the balance between them — which is why recovery after a scare is as active a process as the fright itself.
Exam tip
Exam tip: labelling the neuron and naming the direction
Nervous-system marks come from precise names and correct directions.
In a neuron diagram, label all six parts — cyton, dendrites, axon, myelin sheath, node of Ranvier and nerve endings — and add the function beside each when asked. The node of Ranvier is the label most often missed.
For neuron types, answer with the direction: sensory carries impulses from receptors to the CNS; motor carries them from the CNS to effectors. Simply naming them earns little.
Quote the numbers for the peripheral system: 12 pairs of cranial nerves and 31 pairs of spinal nerves.
For the autonomic system, give a paired effect to show the opposition: sympathetic speeds the heart, parasympathetic slows it.
And use the word synapse for the gap between two neurons, not "joint" or "junction".
In a neuron diagram, label all six parts — cyton, dendrites, axon, myelin sheath, node of Ranvier and nerve endings — and add the function beside each when asked. The node of Ranvier is the label most often missed.
For neuron types, answer with the direction: sensory carries impulses from receptors to the CNS; motor carries them from the CNS to effectors. Simply naming them earns little.
Quote the numbers for the peripheral system: 12 pairs of cranial nerves and 31 pairs of spinal nerves.
For the autonomic system, give a paired effect to show the opposition: sympathetic speeds the heart, parasympathetic slows it.
And use the word synapse for the gap between two neurons, not "joint" or "junction".
Did you know
Why does an impulse jump along a nerve instead of sliding?
Because the myelin sheath insulates most of the axon, leaving only the small gaps exposed.
An impulse can be generated only where the axon's membrane meets its surroundings — and that happens only at the nodes of Ranvier. So instead of travelling the whole length step by step, the signal is regenerated at one node and leaps straight to the next.
Skipping the insulated stretches is what makes a myelinated nerve so much faster than a bare one, and it is why a signal can cover a metre from spine to toe in a small fraction of a second.
An impulse can be generated only where the axon's membrane meets its surroundings — and that happens only at the nodes of Ranvier. So instead of travelling the whole length step by step, the signal is regenerated at one node and leaps straight to the next.
Skipping the insulated stretches is what makes a myelinated nerve so much faster than a bare one, and it is why a signal can cover a metre from spine to toe in a small fraction of a second.
Key takeaways
Neurons and the nervous system: quick revision
- The nervous system receives stimuli, interprets them and brings about a response, acting in a fraction of a second — far faster than the hormonal system.
- A neuron has a cyton with the nucleus, dendrites that receive impulses, a long axon that carries them away, an insulating myelin sheath, nodes of Ranvier and nerve endings.
- The gap between two neurons is a synapse, crossed with the help of a chemical.
- Sensory neurons carry impulses to the CNS, motor neurons carry them to muscles and glands, and relay neurons connect the two inside the CNS.
- The CNS is the brain and spinal cord; the PNS has 12 pairs of cranial and 31 pairs of spinal nerves.
- The autonomic system controls involuntary organs, with the sympathetic division preparing for action and the parasympathetic restoring calm.
You will remember all of this far better after answering five questions on it than after reading it twice.
- A neuron has a cyton with the nucleus, dendrites that receive impulses, a long axon that carries them away, an insulating myelin sheath, nodes of Ranvier and nerve endings.
- The gap between two neurons is a synapse, crossed with the help of a chemical.
- Sensory neurons carry impulses to the CNS, motor neurons carry them to muscles and glands, and relay neurons connect the two inside the CNS.
- The CNS is the brain and spinal cord; the PNS has 12 pairs of cranial and 31 pairs of spinal nerves.
- The autonomic system controls involuntary organs, with the sympathetic division preparing for action and the parasympathetic restoring calm.
You will remember all of this far better after answering five questions on it than after reading it twice.