Reflex Arcs - Online Tutor, Practice Problems & Exam Prep

The stretch reflex, initiated by muscle spindles, prevents muscle strain by causing contraction in response to unexpected stretching, exemplified by the knee jerk reflex. Simultaneously, reciprocal inhibition relaxes antagonistic muscles, ensuring smooth movement. The tendon reflex, triggered by Golgi tendon organs, prevents tendon injury by relaxing the muscle under excessive tension while activating the antagonistic muscle for balance. Both reflexes are crucial for maintaining posture and coordination during physical activities, highlighting the importance of muscle tension and reflex arcs in the muscular system.

Stretch Reflex

In this video, we're going to be covering the stretch reflex. Now the purpose of our stretch Reflex is to prevent muscle strain and potential tear injuries. So basically, if you have excessive stretching in your muscle or very unexpected stretching, this reflex will kick in to hopefully keep you safe and prevent injury. So the way that it works is that it's going to be initiated by muscle spindles in response to being stretched. And you guys remember muscle spindles, their main stimulus is stretch, right? Now that is what they respond to. And this part of the stretch reflex is monosynaptic. So it's just a sensory neuron talking to a motor neuron. And what it does is it causes the muscle to contract. So if we look down here at our figure, we have here illustrated probably the most famous stretch reflex, the knee jerk or the patellar reflex, where if you're sitting at your doctor's office, for example, and they whack your knee with a little hammer, your lower leg is going to pop out. Right? That's our knee-jerk reflex.

So what's actually happening here is that when your knee gets hit with that hammer, that tendon gets hit it causes a very unexpected stretch in your quadricep, and so your muscle spindle is going to pick up on that stretch and they're going to send a signal down your sensory neuron. You can see this is monosynaptic, so our blue sensory neuron is talking right to that purple motor neuron, and then that motor neuron will send a signal to the quadricep to contract and that contraction is what makes your lower leg pop out. So that is our stretch reflex. Now you can see there's a whole bunch of stuff happening with this hamstring as well, and that is a process called reciprocal inhibition. So reciprocal inhibition will be happening simultaneously with the stretch reflex and what it does is that it causes the neurons that are controlling the antagonistic muscle to be inhibited during that stretch reflex. And I'll show you this in one second but this part of the reflex is polysynaptic. So the actual stretch reflex itself that causes the muscle contraction is monosynaptic, and then this reciprocal inhibition that's happening simultaneously is polysynaptic. And what it does is it causes the antagonistic muscle to relax.

So if we look down here at our example, here the antagonistic muscle is going to be the hamstring, and so we're getting the simultaneous feedback from our sensory neuron going through an interneuron to our motor neuron. So it's polysynaptic and it's going to send a signal to this hamstring to relax as this quadricep is contracting. And what that does is it allows for a nice smooth movement and it allows that reflex to happen, again smoothly and without any interruption. So if we did not have this reciprocal inhibition, what could happen is that both of these muscles could try contracting at the same time and that's going to really inhibit that reflexive action. And that's important because I know that your knee jerk reflex feels kind of silly sometimes if you're just sitting at, like, your doctor's office, but the purpose of this reflex is to keep you upright. So if you're standing or you're walking and your knees start to buckle underneath you, this reflex is what allows your leg to kind of pop out and catch yourself and stop yourself from falling. So we want that to be able to happen smoothly and efficiently and with no interruptions, and that's what that reciprocal inhibition allows for. Alright. So that is our stretch reflex, and I'll see you guys in our next video to cover some more reflexes in some more detail. See you there.

Reflex Arcs Example 1

Okay. So this example reads, how does the knee jerk reflex help you maintain balance if your knees start to buckle? So let's run through these and see what we have. Alright. So a reads it directly strengthens the quadricep. Now this is a reflex. It's not going to be actually directly strengthening the muscle or making it stronger. So a is out, b is that it inhibits muscle contraction in the quadricep, but that is the opposite of what a stretch reflex does. Right? They don't inhibit muscle contraction. They create them. So b would be out. C is that it generates a strong muscle contraction in the quadricep, and that sounds exactly like what a stretch reflex does. So that's looking pretty promising. And then d reads, it triggers a conscious decision to maintain balance, and that word conscious is just the nail in the coffin for letter d here because this is an unconscious process. Right? These automatic reflexes, this is going to be happening at the spinal cord level, not even getting up to the brain. So d is out, and our answer here is c. It generates a strong muscle contraction in the quadricep. So there you go.

Why is reciprocal inhibition an important part of the stretch reflex?

• A. It amplifies the contraction of the stretched muscle.
• B. It slows down the stretch reflex.
• C. It enhances proprioception as the reflex occurs.
• D. It allows for smooth movement of extensor and flexor muscles.

Tendon Reflex

In this video, we're going to be learning about the tendon reflex. Now the purpose of our tendon reflex is to prevent tendon injury. So it's basically going to help us avoid very strong muscle contractions that could potentially cause our tendons to tear or even separate from our muscle and bone. Right? We don't want that. And this is a polysynaptic reflex. So both our tendon reflex and the simultaneous reciprocal activation are both going to be polysynaptic. So the way that this reflex works is that it's initiated by those Golgi tendon organs in response to tension. You guys remember how those work, right? We have these collagen fibers in our tendon and these Golgi tendon organs have nerve endings kind of coiled or woven between those fibers. And when our muscle is contracting, that causes tension in our tendon and that causes those fibers to kind of squeeze together and that squeezing motion makes those nerve endings fire. So that's what's happening here and this reflex causes the muscle to relax. So you can see in our example down here, we have a person doing leg extensions. They have a weight, kind of placed over here around their ankle and they're trying to lift that leg up and doing that motion is going to engage their quadriceps. So we're having some contraction in the quad. So let's say you're at the gym and you're doing your leg extensions and you put a little bit too much weight on there and got a little too ambitious. Right? That's going to cause too much tension, in the tendon of the knee in this example. And so what's going to happen is those Golgi tendon organs will send a signal up the sensory neuron. We see we have an interneuron here. It's polysynaptic and then a signal will get sent down this motor neuron telling the quadricep to relax. Okay? So that is our tendon reflex, and then simultaneously, with that we have reciprocal activation. So in reciprocal activation, the neurons controlling the antagonistic muscle are going to become activated during that tendon reflex and that is going to cause the antagonistic muscle to contract. So in our example here that would be our hamstring once again and so you can see the signal is getting sent from the sensory neuron to an interneuron to a motor neuron right to our hamstring and that would make that hamstring contract. And just like that reciprocal inhibition we learned about, the purpose of this is just to ensure smooth movement of the reflex. So as this muscle gets to relax, this one gets engaged and that allows you to maintain balance, coordination, posture, and it's going to prevent overextension of the knee here. Okay, so it just kind of helps again help the reflex happen smoothly and helps you stay safe during that reflexive action. Alright. So that is our tendon reflex, and I'll see you guys in our next video to finish up our section on reflex arcs. See you there.

Reflex Arcs Example 2

Okay. So in this example it says, during a strength training session, Wilfred says his tendon reflex is helping him enhance his muscular endurance. Is he correct? Why or why not? So let's run through these and see what they say. Alright. So a says, yes. He is correct. The tendon reflex allows for muscles to reserve energy. So our tendon reflex is not really in charge of energy expenditure. Right? That's not really its job. So a is out. B reads, no the tendon reflex is increasing sensory perception in the muscles. So this is definitely part of sensory perception in our muscles but it's not increasing it necessarily. Right? It's just kind of part of that system. So b is out. C reads, no. The tendon reflex is preventing excessive muscle tension. That's looking very promising. That is the purpose of our reflex, and it responds to excessive tension. Let's just go through d really quick just in case. So d reads, no, the tendon reflex is stimulating muscle contraction, relaxation. Right? It does the opposite of that. So our answer is indeed c, The tendon reflex is preventing excessive muscle tension. And there you have it.

Which of these mechanisms causes golgi tendon organs to start firing action potentials?

• A. As the muscle contracts, collagen fibers 'squeeze' the nerve endings of the golgi tendon organ.
• B. As the muscle relaxes, collagen fibers 'squeeze' the nerve endings of the golgi tendon organ.
• C. As the muscle contracts, free nerve endings in the golgi tendon organ are compressed by muscle fibers.
• D. As the muscle stretches, free nerve endings in the golgi tendon organ are compressed by muscle fibers.

Flexor & Crossed-Extensor Reflexes

In this video, we're going to be learning about 2 reflexes: the flexor reflex and the crossed extensor reflex. The flexor or the withdrawal reflex occurs when we have rapid withdrawal of a body part from a harmful or painful stimulus. What's happening here is that the flexor muscles in that limb are contracting. So, if I were to reach out and touch a hot pot, I would withdraw my hand. Right? That's a flexor reflex. And this is an ipsilateral reflex, which means that the motor activity will occur on the same side of the body as the painful or harmful stimulus. So if I were to step on a tack with my left foot, my left leg would exhibit that flexor reflex and it would withdraw.

Now, if the flexor reflex happens in a load or a weight-bearing limb, for example, in your legs, it's going to simultaneously happen with a crossed extensor reflex. In a load-bearing or weight-bearing limb, these will happen simultaneously, and this crossed extensor reflex will cause an extension of the opposite limb. This helps you maintain balance during the flexor reflex. This crossed extensor reflex is contralateral, which means it's going to occur on the opposite side of the body as the painful or harmful stimulus.

To give you an example, we have this situation here where this guy is stepping on some Legos. It's like the story of my life. Shout out to my children. But you can see he's stepping on the Legos here with his right foot. And so what's happening is that he got this sensory information coming in. This signal is traveling up the sensory neuron into the spinal cord. You can see we have a polysynaptic reflex. We have 2 interneurons there, and then the spinal cord sends this message via that motor neuron to have that flexor reflex withdraw the leg and pick up that leg. This one here in green is our flexor reflex, and then you can see simultaneously what's happening is this crossed extensor reflex where you can see the left leg is now extending and going down to kind of shift the weight so that he doesn't just fall over as he's withdrawing that limb. You can see that this is also a polysynaptic reflex. You've got 2 interneurons involved in this one as well.

Alright. So those are flexor and crossed extensor reflexes, and I will see you guys in our next video. Bye-bye.

Reflex Arcs Example 3

Okay. So this one asks us, which of the following reflexes is contralateral? And as a reminder, contralateral means it's happening on the opposite side of the body as the stimulus. So the only reflex here that is contralateral is the cross extensor reflex. So as a reminder, if we have a person and he's walking around and he steps on a painful stimulus with his right foot, what's going to happen is we're going to see a flexor reflex in that same limb. That's an ipsilateral reflex, and that will lift his leg. Then, we'll see a cross extensor reflex happen in the other limb that extends his left leg down and helps him maintain his balance. So you can see here we're on the opposite side of the body as that stimulus, making it contralateral. Just to quickly kind of look through these other ones, the knee jerk and pupillary light reflexes are both also ipsilateral. So if the doctor whacked your right knee with the little hammer, your right leg is going to pop up, and if someone were to shine a light in my left eye, my left eye would exhibit that pupillary light reflex. Right? So our answer here is going to be b), the crossed extensor reflex is contralateral and there you have it.

Here’s what students ask on this topic:

What is the stretch reflex and how does it work?

The stretch reflex is a muscle contraction response to unexpected stretching, aimed at preventing muscle strain and injury. It is initiated by muscle spindles, which detect the stretch and send a signal via a sensory neuron to the spinal cord. This reflex is monosynaptic, meaning the sensory neuron directly communicates with a motor neuron. The motor neuron then sends a signal back to the muscle, causing it to contract. A classic example is the knee jerk reflex, where tapping the patellar tendon causes the quadriceps to contract, making the lower leg kick out. Simultaneously, reciprocal inhibition occurs, relaxing the antagonistic muscle (hamstring) to ensure smooth movement.

What is the tendon reflex and what role do Golgi tendon organs play in it?

The tendon reflex prevents tendon injury by causing muscle relaxation in response to excessive tension. It is initiated by Golgi tendon organs, which are sensory receptors located within tendons. When a muscle contracts strongly, it increases tension in the tendon, compressing the Golgi tendon organs and triggering them to send a signal via a sensory neuron to the spinal cord. This reflex is polysynaptic, involving interneurons that relay the signal to motor neurons. The motor neurons then send a signal to the muscle to relax, preventing potential tendon damage. Simultaneously, reciprocal activation occurs, causing the antagonistic muscle to contract, ensuring balance and coordination.

What is reciprocal inhibition and how does it function during the stretch reflex?

Reciprocal inhibition is a process that occurs simultaneously with the stretch reflex to ensure smooth and efficient movement. During the stretch reflex, when a muscle contracts in response to being stretched, reciprocal inhibition causes the antagonistic muscle to relax. This is achieved through a polysynaptic pathway involving interneurons. For example, in the knee jerk reflex, while the quadriceps contract, reciprocal inhibition ensures that the hamstrings relax. This prevents both muscles from contracting simultaneously, which would hinder the reflex action and disrupt smooth movement. Reciprocal inhibition is crucial for maintaining posture and coordination during reflexive actions.

What is the flexor reflex and how does it differ from the crossed extensor reflex?

The flexor reflex, also known as the withdrawal reflex, involves the rapid withdrawal of a body part from a harmful stimulus, such as pulling your hand away from a hot object. This reflex is ipsilateral, meaning the motor response occurs on the same side as the stimulus. In contrast, the crossed extensor reflex occurs simultaneously with the flexor reflex in weight-bearing limbs. It involves the extension of the opposite limb to maintain balance. For example, if you step on a sharp object with your right foot, the right leg will withdraw (flexor reflex), while the left leg will extend to support your weight (crossed extensor reflex). The crossed extensor reflex is contralateral, occurring on the opposite side of the stimulus.

How do reflex arcs contribute to maintaining posture and coordination?

Reflex arcs are crucial for maintaining posture and coordination by providing rapid, automatic responses to changes in muscle length and tension. The stretch reflex, initiated by muscle spindles, prevents muscle strain by causing immediate muscle contraction in response to unexpected stretching. Simultaneously, reciprocal inhibition relaxes the antagonistic muscle, ensuring smooth movement. The tendon reflex, triggered by Golgi tendon organs, prevents tendon injury by relaxing the muscle under excessive tension while activating the antagonistic muscle for balance. These reflexes work together to maintain posture, prevent falls, and ensure coordinated movements during physical activities, highlighting the importance of reflex arcs in the muscular system.