Sometimes our body can react in a split second, faster than it could take to send the information to the brain for processing. These reflexes are the subject of this lesson. We’ll cover what a reflex arc is, as well as the cells involved and why they are needed.
What Is a Reflex Arc?
Have you ever been cooking and accidentally bumped your hand against a hot pan? Likely, before you could even register what happened, you jerked your hand away, maybe even clutching your hot skin.
When something like this happens, it feels like you simply react to the situation automatically, without thinking. Although biologically this might seem impossible, it’s exactly what really happens in your nervous system.Although we think of the brain as being the boss of all of our actions and thoughts, some actions actually take place without the brain’s input.
These reactions are called reflexes. However, very few reactions are actually true reflexes. People usually think catching an object flying at their head, like a baseball, is a reflex, but it is not.
The information goes to your brain for processing before you actually respond. Thus, some of us are much better at catching the baseball than others.A true reflex arc involves only a few neurons, or cells of the nervous system, and the information goes only from your body to your spinal cord, not your brain. Let’s look at the cells that make up the reflex arc and how they work.
Reflex Arc Components
Most reflex arcs have five main components: receptors, sensory neurons, interneurons, motor neurons and muscles. However, not all reflexes use interneurons. Some connect sensory neurons directly to motor neurons and do not use interneurons.
Let’s go through each of these components.Throughout your body, neurons have special proteins in their membrane called receptors. Receptors respond to signals in the environment. Some receptors respond to pressure. When the cell is compressed, the receptors are activated, letting your brain know something is pressing on your skin or organs. Other receptors respond to pain or to chemical stimuli, like smells or tastes.
Sensory receptors in your ears respond to vibrations in the air that we interpret as sound, and receptors in your eyes respond to light.Sensory neurons are the cells that contain sensory receptors. They send information from the body to the central nervous system, the brain and spinal cord. These cells are activated when the receptor gets a signal from the environment. The activated sensory neuron extends into the spinal cord, sending an electrical signal all the way to another neuron, the interneuron.
Interneurons are like the middleman of the nervous system. They connect sensory input to other cells that are required for action. In a reflex arc, the sensory neuron sends a signal to the interneuron and activates it. The interneuron then relays that signal to the next neuron, a motor neuron.
Motor neurons connect with interneurons in the spinal cord. They send messages from the central nervous system to the body. The motor neurons run out of the spinal cord and connect with a muscle. Motor neurons, like sensory neurons, can be quite long, connecting the spinal cord with our most distant appendages.Muscles are what cause us to take action. When the motor neurons are activated, they send a signal to the muscle to contract.
When the muscle contracts, it shortens, pulling on the bones it is connected to, which causes us to jerk our hand or leg away from the stimuli.
Picture It All Together
All of these different components work together to make up a reflex arc. It all starts with the sensory receptors. The cells containing the receptors are the sensory neurons. These neurons then travel to the spinal cord.
In the spinal cord, the sensory neuron will connect with an interneuron. The interneuron is only housed in the spinal cord; it has no parts in the rest of the body. That interneuron will connect to another neuron, which is the motor neuron. The motor neuron extends from the spinal cord and connects to a muscle, the last component of the reflex arc.
Function of a Reflex Arc
To understand why reflexes are important, let’s look at an example. The stretch reflex is a reflex that protects your muscles from over extending, and thus tearing. Imagine holding a full coffee cup.
When you initially put the coffee cup in your hand, it’s heavy. Your muscles in your forearm stretch and your arm extends downward. This stretch activates the sensory neurons in your muscles. They send a signal to the spinal cord, which is relayed to the motor neurons. This type of reflex only involves two neurons, no interneurons.The motor neurons then send a signal to your muscle to contract, raising your arm so you don’t drop your coffee. This all happens in a split second.
You probably don’t even notice it. Stretch reflexes are important in keeping our posture and allow us to remain stable and balanced despite changing forces on our body.Reflexes are also important for keeping us safe. The withdrawal reflex allows us to quickly remove parts of our body from painful stimuli. Again, imagine grabbing a hot pot on the stove. The heat triggers receptors in your skin that activate sensory neurons.
The sensory neurons send a signal to the interneurons in your spinal cord and then to motor neurons. The motor neurons connect with muscles in your arms, causing them to contract and quickly withdraw your hand from the painful stimuli.
Reflex arcs are neural circuits that do not involve the brain. Receptors are activated by environmental stimuli and activate sensory neurons that connect to interneurons in your spinal cord. The interneurons connect to motor neurons that cause your muscles to contract, although sometimes sensory neurons connect directly to motor neurons.
Reflex arcs serve to protect your muscles from over extending, such as during stretch reflexes and to prevent damage to the body, such as during withdrawal reflexes.