Lesson Explainer: Reflex Actions Biology

In this explainer, we will learn how to describe the structure of the reflex arc and explain the advantages of having reflex actions.

One of the key functions of our nervous system is to respond to stimuli. A stimulus is a change in the environment. Sometimes we control these responses consciously, with our thoughts. Other times, they happen automatically, or without thinking. A response to a stimulus that occurs without the need for conscious thought is called a reflex.

Definition: Reflex

A reflex is a response to a stimulus that occurs without the need for conscious thought.

Definition: Stimulus

A stimulus is any change in a living organism’s internal or external environment that can influence the organism’s activity.

Imagine that you are sitting outside and the wind suddenly begins to blow into your eyes. Your body would automatically respond by causing you to blink, like the person in the photograph below. This is an example of a simple reflex. The stimulus is the wind in your eye. Your eye blinking shut is what we call the “response.” The response is the change that occurs in reaction to a stimulus.

Definition: Response

A response is a change in the body that occurs as a reaction to a stimulus.

Closed eyes-Woman face.

Figure1

One of the main benefits of our simple reflexes is to keep us safe and to prevent injury. By reacting to a potentially harmful stimulus without having to think about it, our bodies save precious moments. This gives us an advantage that may keep us from getting hurt.

Some good examples of this are when you automatically duck to avoid a fast-moving object, or when you lose your balance and your body reacts to keep you from falling. If our nervous system waited for us to consciously control these actions, we would not respond before we had been hit by that ball or our bodies painfully struck the ground.

Since reflexes happen automatically, without the need for us to think about them, or even get the chance to stop them, they are described as involuntary reactions.

Definition: Involuntary

Involuntary refers to a response that occurs without conscious control, automatically, and without choice.

When the reflex arc involves the skeletal muscles of the body, it is described as “somatic.” Reflex arcs also bring about response in our internal organs, and in this case we describe them as “autonomic.”

Key Term: Somatic Reflex Arc

A somatic reflex arc is an involuntary response that involves the skeletal muscles of the body.

Key Term: Autonomic Reflex Arc

A autonomic reflex arc is an involuntary response that involves the internal organs of the body.

Let’s take a closer look at how a somatic reflex arc works by examining a sequence we call the “reflex arc.” The reflex arc is the pathway that a signal follows from stimulus to response during a reflex action. The typical reflex arc of a simple reflex has seven components, which are shown in Figure 2.

First, there is the stimulus. We have mentioned earlier that a stimulus is any change in the environment that causes a response in the body.

Next, our body needs a way to detect the stimulus, and this is called the “receptor.” A receptor is a part of the body that can sense the stimulus.

The receptor will then activate a series of neurons. Neurons are specialized nervous system cells that carry signals from place to place.

First, the signal from the receptor activates a sensory neuron, which passes the signal on to a relay neuron, which then passes it to a motor neuron.

As the name suggests, sensory neurons, also called afferent neurons, are specialized for receiving signals from sensory receptors in the body. Relay neurons, also called interneurons or connector neurons, transmit these signals to the appropriate places. Motor neurons, also called efferent neurons, are specialized for causing movement or other reactions.

Finally, the motor neuron will activate an effector. The effector is the part of the body that carries out the last step in the reflex arc, the response. Effectors in the human body are most commonly muscles or glands.

Definition: Effector

An effector is the part of a body that carries out a response to a stimulus.

Example 1: Identifying the Correct Process of a Reflex Arc

Which of the following shows the correct process of a reflex arc?

  1. Stimulus, receptor, motor neuron, relay neuron, sensory neuron, effector, response
  2. Stimulus, receptor, sensory neuron, relay neuron, motor neuron, effector, response
  3. Receptor, stimulus, sensory neuron, relay neuron, motor neuron, effector, response
  4. Receptor, stimulus, relay neuron, sensory neuron, motor neuron, effector, response
  5. Stimulus, receptor, sensory neuron, relay neuron, motor neuron, response, effector

Answer

A reflex is an involuntary response to a stimulus. Every reflex begins with a stimulus, or a change in the environment. Take for example the corneal, or blinking, reflex. The stimulus in this reflex action is an irritation or touch to the surface of the eye. The stimulus is detected by the receptor. The receptor is a part of the body that senses a stimulus, in this example, it is touch and pressure receptors in the eye. The receptor activates a signal in a sensory neuron, which transmits the signal to a relay neuron, which passes it to a motor neuron. As the name suggests, sensory neurons are specialized for receiving signals from sensory receptors in the body. Relay neurons, also called interneurons, transmit these signals to the appropriate places. Relay neurons are located in the central nervous system, the brain or the spinal cord. Motor neurons are specialized for causing movement or other reactions. The motor neuron will activate a reaction in the effector. The effector is the part of the body that carries out the response. In the case of this example, the effectors are the muscles that close the eyes, causing a person to blink. The response is the action carried out that was caused by the stimulus. In this example, the response is blinking.

Therefore, we can conclude that the correct process of a reflex arc is B: stimulus, receptor, sensory neuron, relay neuron, motor neuron, effector, response.

Example 2: Describing the Role of Receptors in the Human Body

Which of the following best describes the role of receptors in the human body?

  1. They bring about a response.
  2. They transmit signals across the central nervous system.
  3. They detect changes in the external environment.
  4. They transmit electrical impulses across synapses.
  5. They release hormones.

Answer

Receptors are parts of the body that are able to detect a stimulus. We have receptors all over our bodies. There are touch, temperature, and pressure sensors in our skin. Receptors in our eye detect light, color, and motion. The receptors in our ears are specialized for detecting sounds. The receptors on our tongues are specialized for detecting different tastes. Within the reflex arc, specific receptors trigger a response that occurs without the need for conscious thought. The receptor works together with the nervous system, which carries the signal to an effector. The effector carries out the response to the stimulus.

Using this information, we can conclude that the statement that best describes the role of receptors is that they detect changes in the external environment.

Let’s illustrate the reflex arc using an example.

Imagine that your hand accidentally touches something very hot. Without thinking, your arm jerks your hand away from the hot object. What does our reflex arc look like?

  1. Stimulus: the hot object
  2. Receptor: heat and pain sensors in your skin
  3. Sensory neuron: carries the signal to a relay neuron
  4. Relay neuron: transmits the signal to other neurons including a motor neuron
  5. Motor neuron: carries the signal to the muscles of your arm
  6. Effector: the muscles of your arm
  7. Response: the muscles of the arm reacting to pull your hand away from the hot object

This example reflex arc is illustrated in Figure 3.

Example 3: Identifying the Effector and the Response in a Reflex Action

A bright light is pointed into your eye. Your pupil contracts to restrict the light entering.

  1. In this reflex action, what is the effector?
    1. The eye
    2. The light
    3. The muscles around the pupil
    4. Your eyelids
  2. In this reflex action, what is the response?
    1. The muscles around the pupil
    2. Light receptors in eyes
    3. The pupil contracting
    4. Blinking

Answer

This question describes a reflex action of a part of the eye and asks us to determine both the effector and the response in this scenario. First, let’s recall that a reflex action is a response to a stimulus that occurs without the need for conscious thought. The pupillary reflex described in this question is an example of a reflex action. When your eye is exposed to a bright light, the pupil constricts and gets smaller. Small muscles in the iris contract, pulling the opening of the pupil more closed. This allows less light to enter the eye and prevents damage to the delicate tissues inside of the eye. In a reflex arc, the response is the action that occurs because of the stimulus. The response is carried out by the effector. In this case, the stimulus is the light shining into the eye since this is the change in the environment that activates the reflex action.

Part 1

Using the information above, we can conclude that the effector in this reflex action is C: the muscles around the pupil.

Part 2

Using the information above, we can conclude that the response in this reflex action is C: the pupil contracting.

Before we examine another example, let’s take a moment to review some information about the nervous system. Our nervous system is composed of two major parts: the peripheral nervous system and the central nervous system. An outline of this is given in Figure 4.

The central nervous system consists of the brain and the spinal cord. The peripheral nervous system consists of all of the nerves that are not in the brain or the spinal cord but are instead distributed throughout the body.

Nerves are the main organs of the peripheral nervous system. A nerve is a bundle of many related neurons. There are 12 pairs of cranial nerves, which are nerves that arise directly from the brain. There are 31 pairs of spinal nerves, which arise from the spinal cord: 8 pairs of cervical nerves, 12 pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves. Cranial nerves are usually either sensory nerves or motor nerves, but spinal nerves are usually made up of mixed sensory and motor nerves.

Within the reflex arc, the sensory neuron, the motor neuron, and sometimes the receptor are components of the peripheral nervous system. However, relay neurons are only found in the central nervous system. The relay neuron of a reflex arc is almost always found in the spinal cord. This is because your reflexes do not need conscious input from the brain.

The fact that reflexes are processed within the spinal cord is the reason that some simple reflexes, like the knee jerk reflex, are used to test the health of the spinal cord. In the knee jerk reflex, also called the patellar reflex, a doctor strikes a part of your knee joint with a small rubber hammer causing the lower leg to kick out in response. If a doctor performs this test and there is no response, they can infer that there is an issue at a particular location in the spinal cord! You can see a simple summary of this test in Figure 5.

Another example of a simple reflex that you may be familiar with is your cough reflex. If you are drinking a glass of water and some fluid accidentally enters the trachea, the tube leading from your throat to your lungs, it will initiate this reflex.

In this example, the stimulus is water in the trachea. The receptors are some of the special cells within the trachea that recognize foreign substances. The receptors will activate a sensory neuron, which passes the signal to a relay neuron in the spinal cord within your neck. The relay neuron activates a set of motor neurons, which cause you to cough. The effectors are the muscles of your diaphragm, neck, and face that are involved in coughing, and the response is the coughing action.

It is important to keep in mind that each of our simple reflexes is an adaptation specialized for serving a purpose, usually protecting the body from injury. The blinking reflex we described first is meant to keep particles from touching your delicate eye. The heat response reflex keeps you from burning your hand. The patellar reflex is a part of the complex set of balance reflexes that help prevent you from falling. The cough reflex protects your lungs from foreign substances that could cause choking.

By responding almost instantaneously to a potentially harmful stimulus, our reflexes are a part of how our nervous system responds to the world around us to help to keep us safe and alive!

Example 4: Explaining the Benefit of Reflex Actions

Why is it beneficial that reflex actions do not require thinking?

  1. Signals are not passed to the brain, allowing the body to respond more quickly.
  2. Signals bypass the entire central nervous system, so reactions are very slow.
  3. Signals are not passed to the brain, allowing the body to respond slowly.
  4. Signals are passed to the brain, allowing the body to respond more quickly.
  5. Signals bypass the entire central nervous system, so reactions are very quick.

Answer

One of the main functions of our reflex actions is to prevent injury to the body. Take the pain reflex for example. If you were to touch something hot, your hand would immediately jerk away without you even thinking about it. By moving your hand away from the hot object as quickly as possible, your body avoids being badly burned. If these types of actions required thinking, they could not occur as quickly. Your hand would still be in contact with the hot surface while your brain processed the information and made a decision. For this reason, most reflex arcs initially bypass the brain and instead are quickly processed through the spinal cord within the central nervous system. By reacting to a potentially harmful stimulus without having to think about it, our bodies save precious moments, which could be the difference between mild and quite serious injury.

It is beneficial that reflex actions do not require thinking because signals are not passed to the brain, allowing the body to respond more quickly.

Let’s review what we have learned about reflex arcs in this explainer.

Key Points

  • The human nervous system comprises the central nervous system and the peripheral nervous system.
  • A stimulus is a change in the environment of an organism, which will bring about a response.
  • Reflex actions largely involve the peripheral nervous system.
  • The 7 components of a reflex are as follows: stimulus receptor sensory neuron relay neuron (interneuron) motor neuron effector response.
  • Reflexes are involuntary reactions specialized for protecting the body from injury.

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