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5.1: Reflexes: Neurons in Action

Difficulty Level: At Grade Created by: CK-12
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What is a reflex?

You touch a hot dish and jerk back your hand. You smell French fries and your mouth waters. The doctor taps your knee and your foot kicks out. What's happening? In this section you will find out. You will explore how your body reacts to different kinds of situations.

Did You Know?
Have you ever jumped when you heard a sudden loud sound? When something surprises you, your brain protects you through a startle reflex. Your muscles contract, your head moves, your heart beats faster, your eyes open wider, and your body jumps before your brain knows what made you jump. Your brain can even help protect your ears from the damaging effects of loud sounds by causing tiny muscles in your ears to contract and dampen the effect of the sound.

Your nervous system has groups of neurons that work in circuits to send out and receive information. In these circuits, information goes to the brain or spinal cord, and information about what to do comes back. The simplest circuits in the nervous system are reflexes. A reflex is an automatic reaction to a stimulus. A stimulus is something that causes an action. In a simple reflex, a neuron brings information about a stimulus to the brain or spinal cord and connects with a motor neuron. The motor neuron sends out a message to a muscle. The muscle responds to that message by contracting.

You may remember that the cerebral cortex plans and controls your behavior. But reflexes do not involve the cortex. Reflexes don't require thinking. Reflexes allow you to react automatically to a stimulus. Although reflexes are the simplest way your nervous system controls your behavior, all reflexes are not simple.

Did You Know?
Babies born in a hospital are examined right after birth and given an Apgar score. The Apgar score indicates their general health and reflex activity. An Apgar score evaluates appearance (color), heart rate, grimace (reflex responsiveness), muscle activity, and respiration effort (how strong a baby's cry is).

How Reflexes Work

You have many reflexes. In fact, much of your behavior is made up of reflexes that protect you from danger in some way. For example, gravity tries to make you fall over all the time every day. Without your having to think about it, your body constantly makes adjustments to keep you on your bicycle or even just in your chair. Simple reflexes make you blink when something gets too close to your eye and make you close your eye when dust gets into it. Vomiting is a more complex reflex that protects you from dangerous things in your stomach or throat.

The Knee Jerk Reflex

Sit on a sturdy table and let your legs dangle, hanging completely relaxed. The reflex won't work properly if you are tense.

Use your fingers to tap just below your kneecap, as shown. What happens? Try squeezing your hands together and have someone else tap your knee.

Figure 4.1 Tapping the knee will set off the knee jerk reflex.

Let's look at a simple reflex. The knee jerk reflex is a good model to demonstrate how all reflexes work. You can see how this reflex works in Figure 4.1. Your doctor tests the knee jerk reflex to check muscle response to nerves in your legs, back, and spinal cord. You can test your own knee jerk reflex in the Mini Activity: The Knee Jerk Reflex.

Once Started, Reflexes Go All the Way to Completion

Remember that all reflexes need something to get them started. This is called a stimulus. Tapping your knee, touching a hot grill, or smelling something unpleasant can stimulate or trigger sensory neurons. If the stimulus is strong enough, the reflex is started. Once started, the reflex goes to completion.

Starting a reflex is something like lighting a firecracker. If the fuse gets hot enough, it burns. The fuse can be lit with a spark, a match, or a blowtorch. But once the fuse is lit, the firecracker explodes. Once triggered, a reflex completes itself in a very short time without your having to think about what to do.

Sometimes you can hold back your reflexes. For example, you may be able to stop your knee jerk reflex if you think about it when your knee is tapped. Another example is a cough. Have you ever felt the need to cough while you are taking an exam? Every time you cough, the other people taking the exam look at you. So the next time your throat tickles, you hold back the cough. You can hold back for a little while, and then it happens again. You manage to not cough again. But the next time, the tickle (the stimulus) is too strong, and you have to cough (the reflex). You just can't help it. Some reflexes are easier to control than others.

How many reflexes can you think of? Work in small groups or in pairs to list as many as you can. Group them into categories that show how the reflexes protect you. Decide which are easier to control than others. Then share your list with the class.

Every reflex, whether it is simple or complex, has five parts. These parts take place in a specific order. In order, the five parts of a reflex are sensor, sensory neuron, control center, motor neuron, and muscle. These five parts work as a relay team to take information from the sensor to the spinal cord or brain and back to the muscles. You can use your thumb and fingers to remember the sequence that is shown in Figure 4.2. The complete sequence is called a reflex arc. Now let's see how these parts work together to create the knee jerk reflex.

Figure 4.2 You can use your thumb and fingers to remember the five parts of a reflex arc.

First Part of a Reflex Arc: Sensor

Reflexes start in structures called sensors. These structures detect one kind of energy such as touch, stretch, heat, light, smell, and vibration. Some sensors are neurons, and they fire nerve impulses when stimulated. Other sensors are not neurons but can signal nearby sensory neurons when they detect their specific stimuli. All information about the world inside and outside your body has to be changed into nerve impulses before that information can be used by the circuits of your nervous system.

When you tap your knee, you stretch the tendon that connects your quadriceps muscle to your tibia, the largest bone in your lower leg. Stretching the tendon stretches the muscle, and stretch sensors in the muscle detect this change. The stretch sensors are not neurons. They are special muscle cells that stimulate the sensory nerves to fire impulses.

Second Part of a Reflex Arc: Sensory Neuron

Each reflex has a sensory neuron. The sensor may be the nerve endings of the sensory neuron, or the sensor is another kind of cell that signals the sensory neuron. The sensory neuron begins a nerve impulse that travels to the spinal cord or the brain. Sensory neurons take messages to the spinal cord or brain from sensors in the eyes, ears, muscles, skin, and other body parts. The sensory neuron in the knee jerk reflex sends its messages to the spinal cord.

Figure 4.3 in the knee jerk reflex, a light tap (the stimulus) on the front part of your knee sends messages to your nervous system. The message responsible for the reflex goes through the spinal cord, and back out to your leg. The muscle on the top of your thigh contracts and your leg kicks forward. Traveling more slowly, the message from the tap also goes to your brain, where you realize, and react to, what has happened.

Third Part of a Reflex Arc: Control Center

In the spinal cord, the sensory neuron splits into at least three branches. Each branch forms a synapse with one of three different kinds of cells. In the case of the knee jerk reflex, one branch connects to a cell called an interneuron. The interneuron sends a message up the spinal cord to let the cerebral cortex know what is happening. Another branch goes to the motor neuron for the quadriceps muscle on the front of the thigh. The third branch goes to another interneuron that makes a connection to the motor neuron going to the biceps femoris muscle on the back of the thigh.

Neurons in the brain and spinal cord control reflexes by receiving information and deciding if the stimulus is strong enough to command a response. Sometimes the neurons in the brain and spinal cord combine information from different sources. That is why you can sometimes hold back a reflex like a cough or keep your eyes open when the eye doctor asks you to, even though an instrument is close to your eye. Information from the cortex tells the controlling neuron in the reflex arc not to respond.

The cerebral cortex is an important control center. Messages come from your eyes, ears, skin, and muscles. These messages travel along sensory neurons to get to the cortex. The cortex processes all these messages in networks of interneurons that decide how to respond. In making these decisions, the cortex also uses information from memory. The cerebral cortex is not involved, however, in completing a simple reflex like the knee jerk reflex. Let's see how that happens.

Fourth Part of a Reflex Arc: Motor Neuron

The fourth part of the knee jerk reflex arc is called the output phase. Three things happen at once during the output phase.

  • The nerve impulse to the motor neuron travels out to the quadriceps.
  • The nerve impulse is carried along an interneuron a short distance to the motor neuron for the biceps femoris muscle on the back of your leg.
  • A message traveling along interneurons starts its long trip to the cerebral cortex.

You can follow these steps in Figure 4.4.

Figure 4.4 The five steps of the knee jerk reflex arc.

Identifying Parts of a Reflex

Go back to your list of reflexes from Apply Your Knowledge. Pick two reflexes and identify the five parts of those reflex arcs.

Fifth Part of a Reflex Arc: Muscle

A message from a motor neuron tells your muscles to contract. In the knee jerk reflex, the muscle contracts when the nerve impulse reaches your quadriceps muscle. This muscle contraction should move your leg forward. But your leg won't move forward if the opposing biceps femoris muscle is also contracted. An interneuron tells the biceps motor neuron not to send nerve impulses to keep the biceps muscle relaxed. As the quadriceps muscle contracts, the biceps relaxes, and your leg moves forward as shown in Figure 4.4. All these events occur before the message about what is happening arrives in your cortex.

Reflex arcs need an interneuron to turn off one reflex to make room for another. An interneuron that turns off, or inhibits, a reflex is called an inhibitory interneuron. In the vomiting reflex, inhibitory interneurons keep you from breathing at the wrong time. Other inhibitory interneurons keep your hand from squeezing a hot grill, as muscles on the back of your hand contract to pull the hand away. Reflexes protect your body. Inhibitory interneurons help reflexes work the right way every time.

So the reflex arc consists of these five steps in order-sensor, sensory neuron, control center, motor neuron, and muscle. These five parts work as a relay team to take information up from the sensor to the spinal cord or brain and back down to the muscles.

Figure 4.5 The knee jerk reflex can be summarized in a diagram that shows the neural connections. The numbers refer to the 5 parts of a reflex are, a “+” means the synapse is excitatory and a “-” means the synapse is inhibitory. Because there is an inhibitory synapse in the path to the biceps muscle, it relaxes in response to the knee jerk stimulus.

React First, Think Later A nerve impulse from your big toe can travel to your spinal cord at a speed of 448 kilometers per hour (280 miles per hour). How long will it take for your spinal cord to get the message that you stubbed your toe? You can figure it out by dividing the distance the nerve impulse must travel by the speed it moves. Measure the distance from the end of your big toe, around your heel, and up your back until even with your belly button.

Now you have a good idea of what a reflex arc is and how it works. A conscious action works like a reflex but is slightly more complicated because your brain plays a larger role. Think about running a race. You use your cerebral cortex to make plans for the race. Just before the race starts, your cortex reminds you to check your shoelaces and get into the right starting position. Your cortex also anticipates the race and makes your heart beat faster even though you are standing still. Everyone is in line and you're feeling nervous. Then the starter's gun goes off. That sound is carried to the hearing part of your cortex. The hearing part of the cortex interprets the sound and sends messages to the motor region of the cortex. Motor commands from your cortex go down your spinal cord and out to the muscles you use to run. While you run, many reflex arcs coordinate the use of your muscles, control your breathing and heartbeat, and even make you sweat as your body temperature rises.

Figure 4.6 shows the five steps involved in any action your body takes. You can see that for you to perform any activity, your brain must constantly process information. Section 5 of the text will explore the eyes and ears as examples of how the body and cortex gather information.

Figure 4.6 Any responsive action your body takes uses a 5-step reaction.

  1. Sensor hears starting gun.
  2. Auditory nerve transmits information to brain.
  3. Auditory cortex in temporal lobe processes information and sends it to motor cortex in frontal lobe.
  4. Motor neurons carry messages to muscles.
  5. Muscles react and the runner is off!

Sometimes in addition to tapping your knee to make sure you have healthy reflexes, the doctor tickles the bottom of your foot. Why is that?

Activity 4-1: How Fast Is Your Reaction lime?


Your reaction time is the amount of time it takes for your brain to receive a signal from one of your senses and make your body do something in response. You can use a 20-centimeter metric ruler to measure your reaction time.


  • 20cm ruler
  • Activity Report


Step 1 Place your arm on a tabletop with your hand extended over the table's edge. Have a friend or family member hold the ruler so that the zero end is exactly between your thumb and forefinger. Do not grasp the ruler, but be ready to catch it when your partner drops it.

Step 2 Concentrate on your partner's fingers. When he or she lets go, catch the ruler as fast as you can. Repeat this five times.

Figure 4.7 Be ready to catch the ruler as it is dropped.

Each time, record on the Activity Report the reading on the ruler.

Step 3 Switch roles with your partner and repeat the procedure.

Step 4 On the Activity Report, graph your results, showing mean values and range of values.

What happens when someone throws a ball at you and you blink? Trace the path the message takes through your nervous system.

Review Questions

  • Sample answers to these questions will be provided upon request. Please send an email to teachers-requests@ck12.org to request sample answers.
  1. What is a reflex response to a stimulus? Give three examples, describing both the stimulus and the reflex.
  2. Name the five parts of a reflex in order.
  3. What is the difference between a sensory neuron and a motor neuron?
  4. If you touch a hot stove, why do you pull your hand away before you feel pain?
  5. What role does an inhibitory interneuron play in a reflex?

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Date Created:
Feb 23, 2012
Last Modified:
Jan 30, 2016
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