Introduction to Reflex Arcs: Study with Video Lessons, Practice Problems & Examples

Reflex arcs are rapid, automatic responses to stimuli, involving a sensory receptor, sensory neuron, integration center (brain or spinal cord), motor neuron, and effector (muscle or gland). Reflexes can be classified developmentally as innate or acquired, by response type as somatic or autonomic, and by complexity as monosynaptic or polysynaptic. Most reflexes are polysynaptic, with examples including the knee jerk (monosynaptic) and conditioned taste aversion (polysynaptic). Understanding these classifications aids in grasping the nervous system's functionality and reflexive actions.

Introduction to Reflex Arcs

So in this video, we're going to be introducing reflex arcs. Now reflexes are rapid, automatic responses to a stimulus. This is a motor response that does not require any conscious perception. It's going to happen automatically. Before we go any further, I want to remind you of a term that we first introduced quite a few videos ago when we first talked about the nervous system, and that is interneuron. As a reminder, an interneuron acts, basically it's going to be positioned between a sensory and a motor neuron and it's going to transmit impulses from that sensory to that motor neuron. It will have synapses with each of them. Okay. So that's what an interneuron is. That's going to come up a little later in our lesson today. Reflexes don't just happen all willy-nilly. They follow specific paths and those paths are called reflex arcs. A reflex arc is a neuronal pathway that controls a reflex action, and these arcs have five steps to them. So we're going to go over those steps now.

The first step just involves a sensory receptor detecting, okay? You can see here in our little illustration that would be like a little receptor in your finger detecting heat or maybe even pain because the heat is getting way too close to the finger. In the second step, a sensory neuron, which we have depicted here for you in blue, is going to transmit the impulse from the receptor to the central nervous system. The sensory receptor fired an action potential and now that action potential is traveling up that sensory neuron. Next, we're going to enter the integration center. Remember, we're in the central nervous system now, so the integration center can either be the brain or the spinal cord, and that will depend on the complexity of the reflex. Simpler reflexes can integrate in the spinal cord, more complex ones will integrate in the brain. In our example here, we are in the brain, of course. The integration center consists of interneurons, which again are neurons that can be found between sensory and motor neurons and they allow them to communicate. This integration center is basically where the sensory information will get processed in some way by the central nervous system and then the CNS will send out some kind of motor impulse as a response to that stimulus.

You can see here that we also noted or a single synapse. This can look one of two ways. We can have a situation where we have a sensory neuron, an interneuron, and a motor neuron, or like we have depicted here for you, you can just have the sensory neuron and the motor neuron. Both of those are possible and again that will depend on the complexity of the reflex and how fast it needs to be. Once the brain has figured out what kind of impulse to send, it's going to send that impulse via a motor neuron which we have depicted here for you in purple. The motor neuron is going to conduct impulses from the integration center, whether that is the brain or spinal cord, down to the effector. Finally, we're at the effector, which is either going to be a gland cell or a muscle fiber that will respond to the stimulus in some way. If it's a gland cell, it's going to secrete something. If it's a muscle, it's going to contract. In this case, it's going to be a muscle contracting and we're going to be dropping that match. We don't want that fire touching our hand. Right? So those are the steps of a reflex arc and I will see you in our next video to talk about reflexes in a bit more detail. I'll see you there.

Introduction to Reflex Arcs Example 1

Okay. So this example asks us, which of the following is the first step in a reflex arc? So thinking about this just conceptually, we know that before anything can happen, right, before any signals can get sent, before our brain can process it and send more information down to our motor system, we have to first detect a stimulus. Right? That is always going to be the first thing. So our answer here is c. To give you a little more information, motor neuron activation would have to happen after the brain or the spinal cord has received the sensory information and can then send out a signal, so that's happening quite a bit later. Integration is once that signal gets sent to either the brain or the spinal cord, so that information has already been detected and has to get sent somewhere. And then the effector response is the very last step, right? That's when the information we've gone from stimulus up that sensory neuron, it's been integrated, we have that impulse go down the motor neuron and then the effector actually gets to respond. So that's the very last step. So our answer here is c. The stimulus detection is the first step, and there you have it.

Which stage of a reflex arc would be significantly different in a reflex that is initiated by the brain compared to a reflex that is initiated by the spinal cord?

• A. Stage 1- Receptor.
• B. Stage 2- Sensory neuron.
• C. Stage 3- Integration center.
• D. Stage 4- Motor Neuron.
• E. Stage 5- Effector.

Types of Reflex Arcs

In this video, we're going to be learning how to classify reflexes. So there are actually many ways to do this, but we're going to cover the 3 most common. So first and foremost, we can classify reflexes developmentally. Basically thinking about whether this is something we are born with or something that we acquire over time. So we can have innate reflexes, which are genetically programmed during natural development. For example, if you were to startle a newborn baby, they're going to throw their arms out in the air. That is called a startle reflex and it is innate. We are born with that reflex, but we can also have acquired reflexes, and these are complex learned motor patterns. For example, if you were driving your car and you saw an accident happen right in front of you, you probably hit the brake before you even had to think about it. That is an acquired or learned reflex.

Now, we can also classify reflexes based on their response type. So we can have somatic reflexes which use our somatic nervous system and these are rapid involuntary motor responses to a stimulus and those will involve our musculoskeletal system. And, we can also have autonomic or visceral reflexes and these use our autonomic nervous system And these will be non-skeletal responses that get carried out in internal organs. So smooth muscle, cardiac muscle, glands, all of that stuff is covered under visceral reflexes or autonomic reflexes.

And then finally, we can organize them based on their the complexity of the neural pathways that they follow. So we can have monosynaptic reflexes, which as the name implies, involves a single synapse during integration. So, we will have our sensory neuron and our motor neuron and just one synapse there, and we can also have polysynaptic reflexes which again as the name implies involve multiple synapses during integration. Here we would have our sensory neuron, at least one interneuron, and a motor neuron, and you can see we have multiple synapses there. Generally speaking, most reflexes are polysynaptic. There are actually relatively few examples of monosynaptic reflexes in the human body, so just keep that in mind as we move forward.

Now, we're going to go through just a couple commonly known reflexes and just kind of show you how we can categorize them developmentally, based on their response type, and based on their complexity. And just a quick heads up, I don't expect you to know if any of these are monosynaptic or polysynaptic, so I'll just be giving you those answers. But we can kind of work through the development and response type together. Alright, so without further ado, we're going to start with our knee jerk reflex, one of our most well-known reflexes. This is when you're sitting at your doctor's office, they whack your knee with the little hammer, and your leg pops up. That is our knee jerk, and developmentally speaking, this is not something that you have to learn. Right? This is innate. Your doctor's probably been doing this to you since before you can even remember. Right? And in terms of the response type, this is involving our musculoskeletal system, so it must be a somatic reflex. And in terms of the complexity, these are actually monosynaptic. So this is one of the best examples of a monosynaptic reflex in the human body and we're actually going to have a whole video on these coming up so stay tuned for that. Now next, we have our Babinski reflex, which if you've never heard of it, is when you take a newborn baby's foot and you stroke the bottom of their foot and it makes them fan out their little toes. It is the cutest thing in the world. But if it's happening in a newborn baby developmentally, it's probably innate. Right? So these are innate reflexes, and in terms of the response type these are also using our musculoskeletal system so they're going to be somatic. And these are polysynaptic. Again, most reflexes are polysynaptic just in general.

And finally, we have conditioned taste aversion. So this is when, if you've ever had a stomach bug, you know how sometimes whatever food you ate right before you got sick, even if it's not what made you sick, afterward, the idea of that food is, like, repulsive to you. Just thinking about it can make you feel nauseous or make you want to vomit. That is conditioned taste aversion. And so developmentally, this is something that you have learned. Right? Your brain has learned to associate that food with the feeling of being sick. So this is an acquired reflex. And in terms of the response type, this is not involving our musculoskeletal system. Right? This is involving a feeling of nausea in our internal organs. So this is autonomic, and these are also polysynaptic.

Alright, so those are some examples of reflexes and that is how we can categorize different types of reflexes. So now that you have a good handle on reflexes more generally, we're going to start going over some types of reflexes in a bit more detail in our upcoming videos. So I'll see you there.

Introduction to Reflex Arcs Example 2

Okay. So this question asks how would you classify the pupillary light reflex, which causes your pupils to constrict when the eye is exposed to bright light? So right away, just thinking through this, this reflex is innate. Right? We do not have to learn how to do this. So this is going to be an innate reflex, which limits us to options a and b. And then, thinking about this, we know that this reflex does not involve the musculoskeletal system. Right? It involves the pupil, which is under the control of the autonomic nervous system, which would make it option b: this reflex is autonomic, innate, and it is polysynaptic. So there you have it.

Here’s what students ask on this topic:

What are the five steps of a reflex arc?

The five steps of a reflex arc are:

1. Sensory Receptor: Detects a stimulus, such as heat or pain.

2. Sensory Neuron: Transmits the impulse from the receptor to the central nervous system (CNS).

3. Integration Center: Located in the CNS (brain or spinal cord), where the sensory information is processed. This can involve interneurons or a single synapse.

4. Motor Neuron: Conducts impulses from the integration center to an effector.

5. Effector: A muscle or gland that responds to the stimulus, such as contracting or secreting a substance.

How are reflexes classified developmentally?

Reflexes are classified developmentally into two categories:

1. Innate Reflexes: These are genetically programmed and present at birth. An example is the startle reflex in newborns.

2. Acquired Reflexes: These are learned over time through experience. An example is hitting the brake when you see an accident while driving.

What is the difference between somatic and autonomic reflexes?

Somatic reflexes involve the somatic nervous system and result in rapid, involuntary motor responses involving the musculoskeletal system. An example is the knee jerk reflex. Autonomic reflexes involve the autonomic nervous system and result in responses in internal organs, such as smooth muscle, cardiac muscle, or glands. An example is the reflex that causes nausea in response to a stomach bug.

What is the role of interneurons in a reflex arc?

Interneurons act as a bridge between sensory and motor neurons within the central nervous system (CNS). They transmit impulses from sensory neurons to motor neurons and are involved in the integration center of the reflex arc. Interneurons allow for more complex processing of sensory information and can facilitate multiple synapses, making them crucial for polysynaptic reflexes.

What is a monosynaptic reflex, and can you provide an example?

A monosynaptic reflex involves a single synapse between a sensory neuron and a motor neuron during the integration process. This type of reflex is typically faster due to the direct communication between the two neurons. An example of a monosynaptic reflex is the knee jerk reflex, where tapping the patellar tendon causes the leg to extend without involving interneurons.