In To decrease the heartbeat, or even

In this lesson, you will learn about reflexes in the human body. Different types of reflexes will be discussed and examples will be provided when appropriate. Finally you will be able to test your knowledge of the content by taking the quiz.

Definition of Reflex

Have you ever burnt yourself on a stove? What happened when you put your hand on the stove when it was hot? Did you have to say, ‘Wow that is hot; I need to remove my hand!’? No! You moved your hand immediately because of a reflex. Reflex, or reflex action, is the involuntary movement of any organ or body part that has received a stimulus. It happens without any consciousness and is immediate. Reflexes protect the body from harm.

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So, how did that reflex happen after you touched the stove? In your hand are nerve endings that pick up a stimulus and carry it to the spinal column and brain. The nerve ending (in this case of burning your hand) is called a sensory neuron (also known as an afferent neuron) because it takes the stimulus – heat to the hand – to the spinal column and brain for interpretation. The brain then interprets the stimulus and sends a message back to the muscles in the hand by a motor neuron (also called an efferent neuron).

The process of the sensory neuron carrying the stimulus to the spinal column and brain, and then the brain sending a message to the motor neuron is called a reflex arc.

Types of Reflexes

There are several ways to classify the types of reflexes in the body.First, let’s talk about the type or function of reflex. Reflexes can be classified according to the type and function of the muscles or organs that move or function because of the reflex. Some reflexes that move skeletal muscles are called: flexor, extensor, locomotor, and statokinetic.

Reflexes that involve functions of internal organs include digestive, cardiovascular, excretory, and secretory.Next, we can talk about the degree of complexity of the reflex. Reflexes are classified according to the degree of complexity of the neuron (nerve) organization within the reflex arcs. In this classification, there are monosynaptic (or monosegmental) reflexes, which involve only one segment of the central nervous system, and multisynaptic (or intersegmental) reflexes, which involve more than one segment of the central nervous system.

So, let’s dive into this topic a bit more. You probably already know that mono means one and multi means more than one. But, what does that mean in relation to synapses? Monosynaptic means there is only one neuron involved each way on the path to the spinal cord (called an afferent or sensory neuron) and one away from the spinal cord (called an efferent or motor neuron). An example of a monosynaptic reflex is the patellar (knee jerk) reflex.

A multisynaptic reflex, as you can guess, is more complex. In the simplest form, a multisynaptic reflex has more than one neuron (called interneurons) within the reflex arc process. It also involves more than one area of the central nervous system – usually the spinal cord and the brain. An example of this type is the flexor reflex.Finally, let’s look at the influence on muscle or organ of the reflex. Reflexes can have an excitatory (often known as facilitating) action or an inhibitory (weakening and suppressing) activity. For example, we can examine the reflex actions on the heartbeat.

The excitatory reflex of the sympathetic nerve will increase the heartbeat. To decrease the heartbeat, or even stop the heartbeat, the vagus nerve is the inhibitory reflex.

Common Reflexes and Examples

The stretch reflex is one of the simplest reflexes and is monosynaptic. It contracts a muscle that is being stretched and contributes to balance and coordination.

A deep tendon reflex is an example of a stretch reflex. How does this work? When you’re standing up, your knees bend slightly, and without the deep tendon reflex located around your knees, you could fall. That reflex will straighten your knees and keep you standing upright if you lose your balance. Aside from the knees, deep tendon reflexes are located along the outside of your elbows, in the crooks of your arms, and at your wrists and ankles. This is the same reflex that the doctor checks around your knee.When the knee is tapped with the hammer, it takes about 50 milliseconds before the leg kicks.

That is really fast! When the doctor tapped the hammer, it caused the thigh muscle to stretch. That stretch information is then sent to the spinal cord via the sensory neuron. After one synapse in the spinal cord, the information is sent back out to the thigh muscle via the motor neuron that then makes the thigh contract. A few more examples of this type of reflex include blinking your eye when something gets close to it or raising your hands to your face when a ball is thrown at you. Shivering when you are cold is also an example of the stretch reflex.

A flexor reflex, which is also called a withdrawal reflex, is a multisynaptic reflex. It is a type of protective reflex where a painful stimulus triggers the contraction of flexor muscles. These reflexes have a slower onset and a longer response than the stretch (extensor) reflexes. A flexor reflex requires the contraction of a flexor muscle and relaxation of the extensor muscle in that limb. A good example of a flexor reflex is when you touch your forearm to the side of a hot oven. You withdraw your forearm from the oven or the burning source.

The crossed extensor reflex is also a multisynaptic reflex that is usually associated with a same painful stimulus, like what triggers a flexor or withdrawal reflex. In the crossed extensor reflex, the contralateral extensor muscles contract (and the ipsilateral flexor muscles are inhibited). This tends to adjust for any shift in the center of gravity so that balance is maintained and body weight is properly supported. An example of this reflex would be if you stepped on a bee. The sting of the bee makes you pull away your foot. Your other leg stiffens to keep you from falling or losing your balance.

Lesson Summary

Reflex, or reflex action, is the involuntary movement of any organ or body part that has received a stimulus. It happens without any consciousness and is immediate. Reflexes protect the body from harm.The process of the sensory neuron carrying the stimulus to the spinal column and brain, and then the brain sending a message to the motor neuron is called a reflex arc.

There are many ways to classify reflexes, including by type and function, number of segments utilized in the central nervous system, and reflex action on muscles and organs. These classifications are useful when you trace the actions of the reflexes. Common reflexes included stretch, flexor, and crossed extensor reflexes.

The Types of Reflexes

Our reflexes, like the withdrawal reflex, protect our bodies from harm.
Woman pulling away from heat source
Monosynaptic Multisynaptic
Only one neuron involved in the reflex arc Multiple neurons and more than one area of the central nervous system involved in the reflex arc
Example: stretch reflex Example: withdrawal (flexor) reflex

Learning Outcomes

This lesson on reflexes is designed to prepare you to reach the following goals:

  • Remember the definition and purpose of a reflex
  • Classify the body’s reflexes
  • Name the two common reflexes and give examples of ways in which they might be used
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