Video: Reflex arcs

If you've ever found yourself touching a hot surface, stepping on something sharp, or pulling away when someone grabs you by surprise, you might have noticed that all of these scenarios have something in common -- they will all elicit a quick reaction without requiring any thought, or in other words, they trigger a reflex response. These are preprogrammed responses that are essential for our survival. So let's go and take a deeper look at the physiology of reflexes.

Reflexes are rapid, automatic, involuntary responses to specific sensory outputs. Most reflexes in the body are either protective, functioning to prevent or minimize tissue damage, such as the corneal reflex which causes the eyelids to close when something approaches or touches the eye, or they function to maintain homeostasis, such as regulating respiration and heart rate.

In the earlier examples, you may have noticed that they all began with a sensory stimulus and ended with a motor response. Between these events, neural integration occurs within the central nervous system. All together, these different components of a reflex are referred to as a reflex arc.

Whether we're talking about the knee-jerk reflex or the pupillary light reflex, the basic components that make up every reflex are the same. And there are five such components to a reflex arc. It begins with a sensory receptor which detects a stimulus, like stretch or pain. Next, this information is carried by sensory neurons towards the spinal cord or brainstem. This is where we find the integration center, which may involve one or more synapses where the signal is processed. The processed signal is then sent via motor neurons to the effector, which is typically a muscle or gland.

Now that we know the basic principle of reflexes, let's have a closer look at the different types of reflexes in the human body.

Reflexes can be broadly categorized into two types -- somatic reflexes and visceral reflexes. Somatic reflexes include all reflexes that involve skeletal muscles and may involve either cranial or spinal nerves. Somatic reflexes, like all other reflexes, are involuntary, even though skeletal muscles are typically under voluntary control. These reflexes are usually fast and help protect the body from injury. Visceral reflexes, on the other hand, regulate internal organs and are related to smooth and cardiac muscle or glands; for example, reflexes which adjust your heart rate as you exercise.

In most cases, the afferent pathway for both somatic and visceral reflexes is the same, typically involving sensory neurons carrying signals from a receptor to the spinal cord. However, there is a slight difference in the efferent pathway.

In a somatic reflex, motor neurons from the spinal cord go directly to the target skeletal muscle. In a visceral reflex, however, the efferent pathway is more complex. It follows a two-step process. A preganglionic neuron projects from the spinal cord to a ganglion, where it synapses with a postganglionic neuron, which then projects to the effector organ.

Now all reflexes can also be categorized as either monosynaptic or polysynaptic, depending on the number of synapses involved in their reflex arcs.

Monosynaptic reflexes are the simplest types of reflexes and involve a single synapse between the sensory and motor neurons within the integration center. Because there is only one synapse, monosynaptic reflexes are typically very fast.

Polysynaptic reflexes, on the other hand, are more complex and involve at least two synapses. For polysynaptic somatic reflexes, they can include one or more interneurons in between the sensory and motor neurons within the integration center. This extra processing allows for more complex responses, like withdrawing your hand from a hot surface. Visceral reflexes are inherently polysynaptic and may include one or more interneurons in the integration center, but will also include synapses in ganglia outside of the integration center.

Now that we have an appreciation of the components and complexity of reflexes in general, let's take a more detailed look at some of the most common somatic reflexes in the body. Let's begin with the simple stretch reflex.

All skeletal muscles have an optimal resting length. When a force stretches a muscle beyond this length, the stretch reflex triggers that muscle to contract, restoring it to the optimal length. This helps maintain posture, muscle tone, and prevent injury from excessive stretch. A classic example? The knee-jerk reflex, also known as the patellar tendon reflex.

When you tap on the patellar tendon, it pulls on the tendon and causes the quadriceps muscles to stretch. Specialized receptors within the muscle, called muscle spindles, detect the stretch and send signals via sensory neurons to the spinal cord. Here, the sensory neurons synapse directly with motor neurons, triggering action potentials to be sent down the motor neurons back to the quadriceps muscles, causing them to contract and return to their optimal length.

Although the stretch reflex itself is monosynaptic, this reflex arc involves more than meets the eye. In the integration center, the sensory neurons also synapse with interneurons, which in turn synapse with motor neurons that send inhibitory signals to the antagonist muscles, which in this specific example are the hamstring muscles. This is called reciprocal inhibition and it allows the agonist muscle to contract with no interference from the antagonist muscle group.

Another important reflex that protects our muscles is the Golgi tendon reflex, also known as the inverse stretch reflex. Unlike the stretch reflex which triggers contraction of muscles, the Golgi tendon reflex does the opposite. It causes muscle relaxation to prevent excessive tension and potential damage. At the heart of this reflex are the tendon organs, also known as Golgi tendon organs, specialized sensory receptors located at the musculotendinous junction that monitor muscle tension.

If the tension in the muscle rises too high, the tendon organs send signals through sensory neurons to the spinal cord. In response, inhibitory signals are sent via interneurons to the motor neurons controlling the contracting muscle while excitatory signals are simultaneously sent to the antagonist muscles. The result? The overstressed muscle relaxes and its opposing muscles contract, reducing strain and protecting muscles and tendons from injury.

Now, have you ever stepped on something sharp and yanked your foot away without even thinking? That's the flexion reflex, also known as the withdrawal reflex, a built-in protective response that helps keep you safe from harm. Here's how it works:

When a painful stimulus like a sharp object activates nociceptive afferents, these sensory neurons send signals to the spinal cord. There, the signal is relayed through interneurons, which then activate motor neurons. The result? Well, in this particular example, the motor neurons signal the hamstrings of the affected limb to contract, flexing the knee to pull the foot away from danger, all without needing any conscious thought.

Now there's a second reflex, the crossed-extension reflex, which works hand-in-hand with the flexion reflex, ensuring you don't just pull away from danger, but also stay on your feet. While the flexion reflex pulls the affected limb away, some interneurons take the signal a step further. They activate motor neurons that control the extensor muscles of the opposite limb. So if you step on something sharp with your right foot, your right knee flexes to pull away, but at the same time, your left knee extends to keep you balanced.

And that brings us to the end of this tutorial.

To learn more about this topic and consolidate your knowledge, make sure to check out our related articles and quizzes. See you next time!