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Reflexes

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Reflexes

A reflex is an involuntary, automatic action the body makes in response to a stimulus. This means that we can carry out actions without even thinking about it.

We have many types of reflexes that we will examine in this article, but a good example of one we use very frequently is the protective reflex that happens when we touch something too hot. For example, when we touch a hot plate, our hand automatically moves away from the plate to avoid being burnt.

The nervous system and reflexes

The nervous system has two major parts, which then divide into their own systems. One major division is the central nervous system (CNS) which contains the brain and spinal cord. The other is the peripheral nervous system (PNS) which comprises pairs of nerves that originate from either the brain or the spinal cord.

The peripheral nervous system then divides into sensory neurones and motor neurones. The job of sensory neurones is to carry electrical signals known as nerve impulses from receptors towards the central nervous system. On the other hand, motor neurones carry nerve impulses away from the central nervous system to effectors.

A receptor is an organ or cell in the body that detects a stimulus, such as a temperature change, and conveys that information to somewhere else in the body.

An effector is a body part or cell that then responds to a stimulus with the information conveyed by the receptor, such as a muscle that contracts to move a hand away from a hot plate.

The motor nervous system

The system of motor neurones (neurones that regulate muscle movement) is called the motor nervous system. It can be further split up into two categories: the voluntary nervous system and the autonomic nervous system.

The role of the voluntary nervous system is to carry nerve impulses from the CNS to muscle effectors. The ‘voluntary’ part stems from the fact that we consciously control these movements (i.e. we do them voluntarily). This nervous system is also known as the somatic nervous system, and the reflexes carried out by this system are called somatic reflexes. The effectors of somatic reflexes are muscles and include reflexes such as moving your leg away when you step on a sharp object.

The role of the autonomic nervous system is to carry nerve impulses to glands, smooth muscle, and cardiac muscle. It differs from the voluntary nervous system in that it is not under voluntary control. Everything done by the autonomic nervous system is done subconsciously. Reflexes carried out by this nervous system are called autonomic reflexes. These affect inner organs. Examples of autonomic reflexes include breathing, control of heart rate, and swallowing.

Somatic reflexes and autonomic reflexes are the two main types of reflexes.

Smooth muscle is the type of muscle in the human body that contracts slowly and automatically. It predominantly makes up the musculature in internal organs such as the uterus and the digestive system, such as the stomach.

The spinal cord

As discussed above, the CNS is made up of the brain and the spinal cord. But what actually is the spinal cord?

There is a bone structure called the vertebral column running vertically along our back. Inside this column is the spinal cord. The spinal cord is a long, thin, tubular structure of nervous tissue. Pairs of nerves emerge from intervals along the spinal cord.

Reflex arcs

A reflex arc is the pathway of neurones that are involved in a reflex. Reflex arcs only contain three neurones: the sensory, the relay, and the motor neurones. Let's take a look at a reflex action when stepping on a pin.

  1. A pain receptor in the skin detects the stimulus (in this case the pin) and generates nervous impulses.
  2. The receptor sends the nervous impulses to the spinal cord through the sensory neurone. In this scenario, the spinal cord acts as the coordinator.
  3. The nervous impulses are then transmitted to the relay neurone (aka interneuron) in the spinal cord.
  4. The relay neurone passes this information on to the motor neurone.
  5. The nervous impulses travel through the motor neurone to the muscle in the leg, which acts as the effector.
  6. Finally, the muscles contract, causing the leg to jerk up and away from the pin. This movement is known as the response.

Reflex characteristics and their importance

The reflexes’ response is rapid, short-lived, localized, and totally involuntary. In this section, we will look at what these words mean and why they make reflexes so important for our survival.

Reflexes are involuntary

Reflexes being involuntary means that the decision-making part of our brain doesn’t need to be involved in acting. This allows the brain to focus on more complex processes in the body.

That being said, some nervous impulses may be sent to the brain, meaning that the brain has the power to override them. For example, the brain may override the reflex to move your hand from a hot plate if there is food on the plate that you don’t want to drop.

Unconscious and involuntary both have very similar meanings. Both words refer to actions that you have no biological control over, such as hiccuping and shivering. The difference between them is that unconscious also has a separate meaning on top of this, which means when you are asleep or passed out. Involuntary does not have this other meaning.

Reflexes are protective

Reflexes are not a learnt behaviour. We are simply born with them, so the same reflexes are present in all healthy humans. Some reflexes allow us to move away from danger, protecting ourselves and ensuring our survival.

A protective reflex is blinking when something such as a ball comes flying towards your eyes. This reflex protects your eyes from being damaged by the object, keeping your vision safe.

Reflexes are rapid

Reflexes cause action to happen very quickly. This is because of the very short reflex arc pathway. This means that nervous impulses can travel from the receptor to the effector almost instantly. Furthermore, the fact that we do not have to decide about whether to act saves time, too. This is beneficial as it helps us respond to danger as quickly as possible to keep ourselves safe.

Reflexes are short-lived and localized

‘Localized’ means that the response only affects the parts of the body involved, and ‘short-lived’ means that the response does not last for a long time. This is beneficial for the situations where reflexes are used, as it allows us to resume normal behaviour as soon as the danger has passed. In the example above, the blinking response only affects your eyes and is over once the object has stopped coming towards you.

Examples of reflexes

Many types of reflexes are in our body. Four key examples of reflexes include:

  • The stretch reflex.
  • The flexor (withdrawal) reflex.
  • The crossed-extensor reflex.
  • The Golgi tendon reflex.

The stretch reflex

When a muscle is stretched, muscle spindle receptors will send nervous impulses to the spinal cord, causing the same muscle to contract. At the same time, the antagonistic muscle is relaxed.

Some muscles work in ‘antagonistic pairs.’ As one muscle contracts, the other muscle relaxes or lengthens. An example of an antagonistic pair is the biceps and triceps. When our biceps contract, our triceps relax and vice versa.

Another common example of this reflex is the knee-jerk reflex. Doctors do this test on patients to test that specific spinal cord segments are working. It involves striking a tendon near the knee (called the patellar tendon) with a rubber hammer. This will cause the patellar tendon to pull on the leg’s quadriceps muscle (the effector). The stretch reflex will then counteract this stretch as the muscle ‘thinks’ it is stretching very rapidly. The response to this will be that the knee will jerk, which is why the reflex is called the ‘knee-jerk reflex’!

Flexor (withdrawal) reflex

When a body part comes into something dangerous, such as touching a hot plate as mentioned above, the body part will reflexively pull away. This occurs because sensory receptors in the skin send nervous impulses to the spinal cord, which will cause the body part to contract and move away.

The sensory information for this type of reflex will also travel to the brain, so conscious decision-making can occur and override the reflex from happening.

If you were having a vaccine injected into your arm, you might reflexively move it away due to the pain. However, you can override this reflex by keeping your arm still.

Crossed-extensor reflex

As we saw above, stepping on a sharp object causes a withdrawal reflex of the leg. You might wonder why this doesn’t cause us to topple over, and the answer to this is the crossed-extensor reflex. When the withdrawal reflex of the injured leg happens, the opposite leg extends at the same time so that the person can appropriately shift their balance to the opposite foot during the reflex.

Golgi tendon reflex

This is a reflex that happens due to the Golgi tendon organs. Golgi tendon organs are types of sensory receptors that sense changes in muscle tension. They lie in between a muscle and its tendon.

When a muscle is stimulated by a stretch that is too much or too long, the Golgi tendon organs will detect this and send nervous impulses to the spinal cord that cause the stretched muscle to relax.

As a result, the Golgi tendon reflex is also referred to as the ‘inverse stretch reflex’ as it has the opposite effect to the stretch reflex. In the stretch reflex, the antagonistic (opposite) muscle is relaxed, whereas in the inverse stretch reflex, it is the muscle that is stretching that is relaxed.

Reflexes - Key takeaways

  • A reflex is an involuntary, automatic action that the body makes in response to a stimulus.
  • Our nervous system has different divisions. The two major ones are the central nervous system and peripheral nervous system.
  • A reflex arc is the pathway of neurones that are involved in a reflex. The three major neurones involved in reflex arcs are sensory neurones, relay neurones, and motor neurones.
  • Reflexes are rapid, protective, involuntary, short-lived, and localized.
  • We have different types of reflexes in the body. Four key examples are the stretch reflex, the flexor reflex, the crossed-extensor reflex, and the Golgi tendon reflex.

Frequently Asked Questions about Reflexes

The two main types of reflexes are somatic reflexes, which are performed consciously, and autonomic reflexes, which are performed subconsciously. The effectors for somatic reflexes are muscles, whilst the effectors for autonomic reflexes are inner organs.

The four types of reflex are the stretch reflex, the flexor (withdrawal) reflex, the crossed-extensor reflex, and the Golgi Tendon reflex

Common somatic reflexes include moving your hand away when you touch a hot object or moving your leg away when you step on a sharp object. 


Common autonomic reflexes include breathing and swallowing.

A reflex, or reflex action, is an involuntary, automatic action that the body makes in response to a stimulus. 

Reflexes are important for our protection from danger and survival. This is firstly because they are involuntary, so the decision-making part of our brain doesn’t need to be involved in carrying out the action. Secondly, they are rapid, meaning that we can respond to danger quickly. Thirdly, they are short-lived and localised meaning that they only affect the body part susceptible to danger and allow us to resume normal behaviour once the danger has passed.

Final Reflexes Quiz

Question

What is a reflex?

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Answer

An involuntary, automatic action the body makes in response to stimuli.

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Question

What are the two major divisions of the nervous system?

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Answer

1) The central nervous system (CNS) which contains the brain and spinal cord. 


2) The peripheral nervous system (PNS) which is made up of pairs of nerves that originate from either the brain or the spinal cord.

Show question

Question

What are the two divisions of the peripheral nervous system?

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Answer

The sensory nervous system and the motor nervous system.

Show question

Question

What is the role of sensory neurones?

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Answer

To carry electrical signals known as nerve impulses from receptors towards the central nervous system.

Show question

Question

What is the role of motor neurones?

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Answer

To carry nerve impulses away from the central nervous system to effectors.

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Question

What are the two divisions of the motor nervous system?

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Answer

The voluntary nervous system and the autonomic nervous system.

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Question

What is the role of the voluntary nervous system?

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Answer

To carry nerve impulses from the central nervous system to muscle effectors.

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Question

What is the role of the autonomic nervous system?

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Answer

To carry impulses to glands, smooth muscle, and cardiac muscle.

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Question

What is the main difference between the voluntary and the autonomic nervous system?

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Answer

All the movements done by the voluntary nervous system are controlled consciously whereas everything done by the autonomic nervous system is done subconsciously.

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Question

What is the bone structure that the spinal cord is located in?

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Answer

The vertebral column.

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Question

What is the general structure of the spinal cord?

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Answer

The spinal cord is a long, thin, tubular structure of nervous tissue. Pairs of nerves emerge from intervals along the spinal cord.

Show question

Question

What does ‘reflex arc’ mean?

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Answer

The pathway of neurones that are involved in a reflex.

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Question

What is the correct order of neurones that nerve impulses pass through in a reflex arc?

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Answer

1) Sensory neurone, 2) Relay neurone, 3) Motor neurone

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Question

What does it mean when reflexes are described as involuntary?

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Answer

This means that the decision-making part of our brain doesn’t need to be involved in carrying out the action.

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Question

How is blinking when an object comes flying towards your eyes a protective reflex?

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Answer

Reflexes are protective because they help to protect us from danger and ensure our survival. The reflex of blinking when an object comes flying towards you protects your eyes from being damaged.

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Question

What are the four types of reflexes?

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Answer

  • The Stretch Reflex
  • The Flexor (Withdrawal) Reflex
  • The Crossed-Extensor Reflex
  • The Golgi tendon reflex

Show question

Question

Why does striking the patellar tendon with a rubber hammer cause the knee jerk reflex?

Show answer

Answer

This causes the patellar tendon to pull on the leg’s quadriceps muscle (the effector). The stretch reflex will then counteract this stretch as the muscle ‘thinks’ it is stretching very rapidly. The response to this will be that the knee will jerk.

Show question

Question

Why does your hand reflexively move away when you touch a hot plate?

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Answer

Due to the flexor (withdrawal) reflex. Sensory receptors in the skin send nervous impulses to the spinal cord. Nerve impulses are then sent to the motor neurones and then to the hand (the effector). This causes the hand to contract and move away.

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Question

What is the alternative name for the Golgi tendon reflex?

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Answer

The inverse stretch reflex.

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