Why your hand jumps away before you feel the burn

How to use this simulation

1. Leave the type on Withdrawal and press Trigger stimulus. A red pulse leaves the receptor in the skin, runs along the sensory neuron, crosses two synapses in the grey matter of the spinal cord and reaches the muscle, which contracts and pulls the hand down and away.
2. Watch the amber branch. At the same moment a second, slower signal climbs to the brain. The race chart at the bottom right shows the reflex finishing first and the pain being felt afterwards.
3. Press Step through to advance one stage at a time. The Current step reading names each stage, so you can say exactly what is happening at the dorsal root, at each synapse and at the ventral root.
4. Switch the type to Knee-jerk. The relay neuron disappears: the sensory neuron synapses directly on the motor neuron, so the arc has only two neurons and one synapse in the cord.
5. Drag the conduction speed slider, or choose Myelinated or Unmyelinated, and change the total path length. Read the reflex time in the panel: it is path length divided by speed, plus the synaptic delays.
6. Switch Show labels off and try to name every part from memory, then switch it back on to check yourself.

The hot pan problem

You grab a pan handle that someone left on the stove. Before you even think the word hot, your hand has already flown off it. Only afterwards does the sharp, stinging pain arrive, and by then the handle is no longer in your fingers. It feels as if your hand made the decision without asking you, and in a way it did.

That order, move first and feel later, is not a trick of your imagination. It is built into your body. The movement was handled by a short circuit through the spinal cord that never waited for your brain. The pain was handled by a longer route that did go up to the brain, and that longer route is simply slower.

This page lets you run that circuit yourself. You will trigger the stimulus, follow one glowing impulse along the real pathway, change the nerve fibres and the path length, and read the time the reflex takes. The numbers in the readings panel are not decoration: they are the same arithmetic you will use in the solved problems below.

Why does the body bother with a shortcut at all? Because thinking is slow. A decision that passes through the cerebral cortex needs a couple of hundred milliseconds. Burning skin cannot wait that long, so evolution moved the most urgent protective responses into the spinal cord.

What a reflex action really is

Your body does two broad kinds of things. Some actions you choose: writing a word, kicking a ball, picking up a cup. Others simply happen: your pupils shrink in bright light, you blink when something flies toward your eye, you jerk your hand from a flame. The second kind are reflex actions.

The textbook definition is short. A reflex action is a sudden, automatic and involuntary response of the body to a stimulus. Involuntary means you do not will it, and you usually cannot stop it. Automatic means it needs no learning in most cases. Rapid means it is over in a fraction of a second.

Every reflex has a stimulus (the trigger, like heat, a tap or a speck of dust) and a response (the muscle or gland that reacts). Between the two runs a fixed route of neurons. That route is the reflex arc, and it is the whole subject of this page.

Four features separate a reflex from other actions, and examiners love to ask for them.

• It is fast, because the route is short and often passes through only one or two synapses.
• It is automatic and involuntary, so it does not need conscious decision.
• It is usually protective, keeping the body away from damage or keeping an organ working.
• It follows a fixed path, the reflex arc, and the same stimulus produces nearly the same response each time.

Words you need first

Definition questions are the easiest marks in this chapter, and almost all of them come from this list. Learn each term together with its partner term, because the exam often asks for both.

The five parts of a reflex arc

Every reflex arc is a relay race with five runners. The baton is the nerve impulse. Follow it in the simulation and you will see it handed over at each step.

How long does a reflex take?

The nerve impulse is not instant. It runs along fibres at a finite speed and waits briefly at each synapse. That makes the reflex time easy to compute, and the simulation does exactly this arithmetic.

First, the conduction time. The total path is the sensory fibre plus the motor fibre. On the default setting that is 1.4 m, so each half is 0.7 m. At 60 m/s a half takes 11.67 ms.

Second, the synaptic delay. Every synapse in the cord adds about 0.5 ms. A withdrawal reflex with a relay neuron has two such synapses; a knee-jerk has one.

• Fast myelinated fibres conduct at roughly 60 m/s in this model; slow unmyelinated fibres at about 1 m/s.
• Pain is only registered after the signal climbs about 0.6 m at 30 m/s (20.0 ms), crosses one more synapse and is processed for about 100 ms. These three values are illustrative round numbers.
• The model counts conduction and the cord synapses only. It leaves out the neuromuscular junction and the time the muscle needs to build force, which add a few tens of milliseconds in real life.

Reflex action versus voluntary action

This comparison table appears, in some form, in nearly every exam on this chapter. Notice the pattern: the reflex trades flexibility for speed.

Why pain arrives late

Look at the amber branch in the simulation. When the sensory neuron reaches the spinal cord, its signal does two things at once. One copy goes straight into the reflex circuit and out to the muscle. Another copy is passed up the white matter of the cord to the brain.

The reflex copy has a short job: cross one or two synapses and run down the motor fibre. The brain copy has a longer job: climb about 0.6 m, cross another synapse, and then the brain must process it into the conscious experience of pain, which the model sets at 100 ms.

On the default drop the reflex is done after 24.33 ms, while pain is felt at 132.17 ms. The hand is already moving 107.83 ms before you feel anything. That is the head start the shortcut buys you.

Run the experiment with Unmyelinated fibres and the order can flip: the reflex then takes 1,401.0 ms and the pain arrives at 820.5 ms, so in this toy model you would feel the pain first. It is a useful warning. A reflex is only protective if its motor fibres are fast, and that is exactly why motor neurons to muscles are heavily myelinated.

Withdrawal reflex and knee-jerk compared

The two reflexes in the simulation are the two that textbooks use, and they teach different things. The withdrawal reflex is the classic protective one. The knee-jerk is the textbook example of the simplest possible arc.

In the withdrawal reflex a relay neuron sits between the sensory and motor neurons, so the signal crosses two synapses. In the knee-jerk the sensory neuron synapses directly on the motor neuron. This is called a monosynaptic reflex, from mono, one, and synapse.

Unconditioned and conditioned reflexes

Not every reflex is inborn. The Russian physiologist Ivan Pavlov showed that a dog salivates when food touches its tongue, and that after he rang a bell again and again just before feeding, the dog learned to salivate to the bell alone. He split reflexes into two groups.

Try it in the simulation

Spend ten minutes on these experiments. Each one is a question an examiner could ask, answered by what you see.

• Press Trigger stimulus on the defaults and watch the race chart. Which bar finishes first, and by how many milliseconds?
• Press Step through repeatedly and name each stage aloud. Count the synapses you cross.
• Switch to Knee-jerk. How many neurons and how many synapses are now in the arc, and what happened to the relay neuron?
• Choose Unmyelinated and compare the reflex time with the myelinated value of 24.33 ms. By what factor did it grow?
• Drag the conduction speed slider down slowly. At what speed does the order flip so that pain is felt before the movement?
• Shorten the path to its minimum, then lengthen it to its maximum. How much of the reflex time is conduction, and how much is synapses?
• Hide the labels and try to name all the parts from memory, then show them to check.

Solved problems

Each solution states the formula first, then substitutes the numbers. Write it in the same order in an exam. Every figure is computed by the same code that drives the simulation.

Common mistakes

These are the slips that cost marks every year. Read them before the exam, not after.

• Saying the brain controls a reflex. The spinal cord is the control centre; the brain is told afterwards.
• Mixing up sensory and motor. Sensory carries information in to the cord, motor carries the command out to the muscle.
• Calling the relay neuron a motor neuron. It is short, stays inside the cord and connects the other two.
• Forgetting the effector and ending the arc at the motor neuron. The arc ends at the muscle or gland.
• Believing that a reflex never involves the brain. The brain is informed in parallel, which is why you feel pain.
• Using minutes or seconds for reflex times. They are milliseconds.
• Drawing the arrow of a synapse backwards. Impulses cross a synapse in one direction only.

Reflexes in real life

Once you know the arc you will see it everywhere. Doctors use reflexes as a quick health check. A tap on the patellar tendon should make the leg kick; if it does not, or kicks too hard, the problem may lie in a nerve, in the spinal cord or higher up. Because the arc is so short, a missing knee-jerk points to a break somewhere along the arc.

• Blinking protects the eye from dust and sudden objects. It is a brainstem reflex, not a spinal one.
• Sneezing and coughing clear the airway of irritants.
• Pupil narrowing in bright light protects the retina and widens again in dim light.
• Salivation when food reaches the mouth begins digestion.
• A newborn baby grasps a finger placed in the palm and sucks when a nipple touches the lips.
• A sudden loss of balance triggers instant corrections in your leg muscles before you decide to step.

Exam corner (CBSE class 10 and board exams)

Typical questions with the answers an examiner expects. Practise saying each in two or three sentences.

• Define reflex action. A rapid, automatic, involuntary response to a stimulus, controlled by the spinal cord.
• Define reflex arc. The pathway from receptor through sensory neuron, relay neuron and motor neuron to the effector.
• Why are reflex actions fast? The route is short and passes through the spinal cord without waiting for the brain.
• Name the parts of a reflex arc in order. Receptor, sensory neuron, relay neuron, motor neuron, effector.
• Where does the sensory neuron enter the cord and the motor neuron leave? Through the dorsal and ventral roots respectively.
• Give two differences between a reflex and a voluntary action. Control centre (spinal cord against cerebrum) and speed (fast against slow).
• Draw and label a reflex arc. Label receptor, sensory neuron, dorsal root ganglion, relay neuron, grey and white matter, motor neuron, effector.

Revision summary

Read this the night before. If a line surprises you, go back to its section.

• A reflex action is rapid, automatic and involuntary.
• Its pathway is the reflex arc: receptor, sensory neuron, relay neuron, motor neuron, effector.
• The spinal cord is the reflex centre; the brain is informed but does not decide.
• Sensory fibres enter by the dorsal root; motor fibres leave by the ventral root.
• Grey matter holds the connections; white matter carries signals up and down.
• Reflex time = L ÷ v × 1000 + synapses × 0.5 ms.
• Myelin raises speed, so fast reflexes use myelinated fibres.
• The knee-jerk is monosynaptic: two neurons, one synapse.
• Pain is felt after the movement because it travels the longer route to the brain.
• Unconditioned reflexes are inborn; conditioned reflexes are learned.

Frequently asked questions