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.
