Video: Reflex Actions

In this video, we will learn how to describe the structure of the reflex arc, and explain the advantages of having reflex actions.

17:48

Video Transcript

In this video, we will learn about reflexes, their purposes, and their mechanisms. Then, we will answer some practice questions. And finally, we’ll review what we’ve learned. So, let’s go ahead and get started.

What do we mean by reflex action? Well, a reflex is an action that doesn’t require conscious control. Many of your body’s normal functions fall under the umbrella of reflexes, from swallowing to breathing, to blinking your eyes, staying balanced and upright, and walking around. All of these are reflexes because you can do them without thinking. Some reflexes you’re born with, like swallowing, breathing, and blinking. These are called simple reflexes. Some reflexes you develop over time with practice, things like balancing, walking, or riding a bike. These are called conditioned reflexes. For the purposes of this lesson, we’re only interested in simple reflexes. Let’s try looking at an example.

Well, if you’ve ever tried to put in a contact lens or gotten a little dust in your eye, you have experienced a reflex action which causes you to blink. Well, we know that reflexes occur without conscious thought. They also serve the purpose of protecting you from injury. They’re able to happen very quickly because you don’t have to think about them, but how does this occur?

Well, first, something touches your eye; we call this the stimulus. A special nerve cell called a sensory neuron detects the touch and carries the signal to your brain stem, where the signal is passed to a relay neuron. That, in turn, carries the signal to a motor neuron, which themselves carry the signal to the muscles of your eyelid. In this reflex, those muscles are the effector, causing your eye to close in response to the stimulus. This series of events, from stimulus to response, is called a reflex arc, and it happens in the blink of an eye.

At this point, we’re ready to further define a reflex. Beyond just emotion that you don’t control, a reflex is a response to a stimulus that is processed in the central nervous system, CNS, for short, without the need for conscious thought. The diagram to the left represents your nervous system. Your central nervous system consists of your brain and spinal cord. It’s responsible for the processing and integration of information. The relay neurons of the reflex arc are located here, in the central nervous system. In contrast, the peripheral nervous system is responsible for transmitting information to and from the central nervous system. It consists of sensory neurons that detect changes inside and outside the body and motor neurons that cause movement.

Before we look at some more examples of reflex arcs, let’s take a moment to briefly investigate the form and function of a neuron. Here, we have a simplified diagram of a neuron or nerve cell. Neurons are more specifically nerve cells that are responsible for transmitting signals from place to place, and they have some important parts that support this function. The main part of the cell is the cell body. The cell body contains the nucleus and other organelles. It’s responsible for handling the basic life functions of the neuron.

Next are the dendrites. These tree-like branches receive signals from other cells. This long part is the axon. Like a power cable, it carries signals from place to place. At the end of the axon, we find the axon terminals. These are the parts of the neuron that connects to other cells. Your reflexes rely on neurons to transmit and integrate the signal between the receptor and the effector. Let’s look at another example and investigate the reflex arc in a little more detail.

If you touch a hot object, you know you’ll instantly pull your hand away. So, this is another example of a reflex action. But how does it work? First, the hot object touches your skin. This is the stimulus or the change in the environment that triggers the chain of events in the reflex arc. Next, pain receptors in the skin recognize the stimulus. The receptors trigger a sensory neuron, which senses the change and carries the signal to the central nervous system, in this case, the spinal cord.

Next, a relay neuron within your spinal cord receives the signal and relays or passes it to another neuron that will trigger the response to the stimulus. The neuron that will trigger that response is a motor neuron. Motor neurons control movement. The motor neuron carries the signal to the muscles of your arm. In this reflex arc, those muscles are the effector. The effector is the part of your body that causes the action that’s the response to the stimulus.

The last step in this process is the response. The response is the reaction to the stimulus, in this case, the effect your muscles contract, causing you to pull your hand away from the hot flame. This entire reflex arc, from stimulus to response, occurs in just a fraction of a second. And it’s processed into your spinal cord, so you don’t even have to think about it.

Before we move on to our questions, let’s take a look at just two more examples. When studying reflexes, it’s important to not just memorize the steps of the reflex arc, but to also remember that each reflex is an adaptation that serves a specific purpose. Our reflexes allow us to respond rapidly to stimuli without the need for conscious thought. The reflexes that we’ve already looked at, like your corneal reflex and your pain reflex, serve the purpose of preventing injury to your body. Other reflex actions like trying to dodge a fast-moving object may be more complex than the reflexes we’ve learned about already. But they still follow the general pattern of the reflex arc and help you to avoid danger. For these examples, we’ll just focus on the stimulus, the receptor, the effector, the response, and the purpose of the reflex.

Now, imagine that you’re sitting in a room alone studying biology of course. When unexpectedly you hear a loud noise from the next room, it causes you to jump a little and maybe make a little sound. This is an example of a startle reflex. The stimulus is the sudden noise. The receptors are special structures found inside your ears. The effectors are muscles of your face and body that cause you to react. The response is you jumping a little in your seat, which has to do with your body’s straightening and tensing. You may even vocalize a bit. The purpose of the startle reflex is to protect the vital parts of your body and to begin to prepare your body for potential escape from danger.

Let’s look at another example. The last reflex we’ll look at before we move on to some practice questions is the pupillary reflex. The pupil of your eye is the opening in the center that looks like a black dot. The iris is the part that gives your eye its color. It opens and closes, changing the size of your pupil. The stimulus for the pupillary reflex is bright light shining into your eye. The receptor is sensory cells in the back of the eyeball that detect light. The effectors are the muscles of the iris which contract. This causes the response, which is the pupil shrinking and getting smaller, causing less light to be able to enter the eye. The purpose of the pupillary reflex is to prevent damage to the delicate tissues inside of the eye, which can be caused by too much exposure to bright light.

Now that we’ve learned about reflexes, their mechanisms, and their purposes, we’re ready to try some practice questions together.

Which of the following shows the correct process of a reflex arc?

This question is asking us to review the answer choices and to pick the one where the steps of the reflex arc are in the correct order. In order to answer this question, we’ll look at an example reflex and map out its reflex arc. Then, we’ll be able to easily choose the correct order. But what is a reflex? A reflex can be defined as an automatic response to a stimulus that occurs without the need for conscious thought. Reflexes serve an adaptive purpose, usually to prevent injury or as a response to danger.

An example of one such reflexe is called your corneal reflex. Your corneal reflex occurs when a foreign object, such as some dust, comes in contact with the surface of your eye, causing you to blink. The steps that outlined the mechanism of a reflex is known as a reflex arc. Let’s take a closer look at the reflex arc for your corneal reflex.

First, there’s the dust touching the surface of your eye. This is called the stimulus, or the change in the environment, that triggers the reflexive action. Next, the foreign object is detected by special sensory cells on the surface of your eye. In the reflex arc, these are known as the receptor. The receptor triggers a sensory neuron, which senses the change and carries a signal to your central nervous system, where it’s passed onto a relay neuron which, in turn, passes or relays the information to a motor neuron.

The motor neuron’s job is to cause motion. It does this by activating the muscles in your eyelids, which are the effector in this relay arc. The effector is the part of your body that causes the response to the stimulus. Finally, your eyelids come together, causing you to blink. This is the last step in the reflex arc, and it’s called the response. Now, we’re able to choose the correct answer choice. The correct process of a reflex arc is stimulus, receptor, sensory neuron, relay neuron, motor neuron, effector, response.

Let’s try another practice question together.

You reach over a hot oven and accidentally touch a hot baking tray. You immediately pull your arm away. In this reflex action, what is the stimulus? In this reflex action, what is the receptor? In this reflex action, what is the effector? In this reflex action, what is the response?

This question describes a scenario and then asks us to pick out the different parts of the reflex action. The scenario starts with accidentally touching a hot baking tray, which leads to the reflexive action of pulling your arm away. Let’s recall that a reflex action is a response to a stimulus that occurs without the need for conscious thought. The mechanism of this reflexive action is a pattern of events known as a reflex arc. And the steps in this pattern are stimulus, receptor, sensory neuron, relay neuron, motor neuron, effector, and response. For the purposes of this question, we’re specifically interested in the stimulus, the receptor, the effector, and the response.

Let’s recall that the stimulus is the change in the environment. The receptor is the part of the body that detects the stimulus or the change. The effector is the part of the body that carries out the response to the stimulus and the response is the reaction that we observe. The scenario in the question provides our stimulus and our response for us. The stimulus occurs when you touch the hot baking tray, and the response occurs when you pull your arm away. So, now, we’re just left to determine what the receptor and the effector are.

Touching a hot tray hurts. That’s because the heat activates special structures in your skin called pain receptors. Well, we know you eventually pull your arm away, but how? The muscles in your arm are the effector that carry out the response of pulling your arm away. So, now, we have the answers to our questions. In this reflex action, the stimulus is the hot baking tray. The receptor is pain receptors in the skin. In this reflex action, the effector is the muscles in your arm, and the response is the movement of your arm.

Let’s try one last practice question.

Which of the following is not an advantage of having reflex actions? (A) Reflex actions prevent a person from feeling pain. (B) Reflex actions take care of basic bodily functions like breathing and heart rate. (C) Reflex actions are involuntary. (D) Reflex actions do not require the brain to think about responses before acting. (E) Reflex actions allow a quick response to danger.

This question is asking us to read the answer choices and choose the one that is not an advantage of having reflex actions. So, we can expect for four of these choices to represent things that are advantages of having reflex actions. And we’re looking for the one choice that is not an advantage. In order to answer this question, first, we’ll review what we know about reflex actions.

A reflex can be defined as an action that does not require conscious control. This means that reflexes are involuntary or that they happen automatically. It also means that you don’t have to think about them. Our reflexes are adaptations. That means that they each have a purpose in helping us to survive. Reflex actions can happen very quickly, since you don’t have to think about them before they’re carried out.

Many reflex actions, such as pulling your hand away quickly from a hot stove, are meant to prevent injury to your body. Other reflexes, such as ducking and covering your head when you spot a fast-moving object, are reactions that help you to avoid danger. With this information in mind, let’s read through our answer choices again. We’ll mark down which of these represent advantages and which do not.

Reflex actions prevent a person from feeling pain. Well, when you touch something hot and reflexively pull your hand away, it’s true it may hurt more if you held it there longer. But the reflex action doesn’t prevent you from eventually feeling the pain of the burn. This one is not an advantage of having reflex actions.

Reflex actions take care of basic bodily functions, like breathing and heart rate. Well, your breathing and your heart rate are involuntary, and they generally occur without any need for your conscious thought. Imagine if you constantly had to concentrate on keeping your heart pumping. Taking care of bodily functions is an advantage of reflex actions.

Reflex actions are involuntary. Well, the fact they happen automatically is part of how they happen so quickly. This is another advantage of having reflex actions. Reflex actions do not require the brain to think about responses before acting. Once again, the fact that reflex actions do not require conscious thought is part of the reason that they can occur so rapidly. This is an advantage of having reflex actions.

Reflex actions allow a quick response to danger. One of the main advantages of reflex actions is that they occur rapidly and allow you to avoid danger. So, the choice that is not an advantage of having reflex actions is reflex actions prevent a person from feeling pain.

Finally, let’s take a moment to review what it is we’ve learned. In this lesson, we learned that a reflex is a response to a stimulus that is processed in the central nervous system without the need for conscious thought. We also investigated the reflex arc, which is the mechanism or pattern of events that occurr during a reflex.

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