A reflex is something that is stereotyped in form, meaning that all members of a certain species will have that same reflex; reflexes can also be recognized in all normal members of a species. As reflexes are built in an organism, they do not need to be learned. Snakes, for example, are born with all of the reflexes they need for survival. They emerge from an egg and slither away to hunt without having to learn any of these skills. When looking at definitions, reflexes are defined as “fast, automatic, protective biological control systems that link a stimulus to a response”. A stimulus is a change in the internal or external environment of an organism. 

Reflexes are crucial to survival because they are involuntary reactions, meaning that they happen without you having to think about them. Reflexes happen through the Central Nervous System sending electrical signals to the muscles before the brain has the opportunity to pick up and process the messages. So, this entire response will occur without your brain having to process it, and instead lesser control centers of the nervous system, such as the spinal cord, process them instead. Many reflexes such as sneezing and focusing your eyes occur naturally, but there are other reflexes that can be learned, and these are known as conditioned responses.  We will cover conditioned responses in greater depth in future blog posts. 

Reflexes govern much of how the simplest animals behave and help to ensure their survival. For example, reflex reactions help simple animals find shelter and protection from predators and also find food for themselves to survive. In fact, humans also demonstrate these very simple reflexes. These include: newborn reflexes, such as sucking and grasping, pupil reflexes, and knee-jerk reflexes, which most of you are probably familiar with.  

Reflexes, once again, play a major role in our body’s normal function. Autonomic reflexes help to maintain various aspects of homeostasis, such as heart rate, breathing rate, blood pressure, and even digestion. Reflexes also help us carry out automatic actions of swallowing, sneezing, coughing, and vomiting. Another role that reflexes play is the maintenance of balance and posture. For instance, spinal reflexes control the trunk and limb muscles of humans, ensuring that we are balanced and upright. There are also brain reflexes that involve the reflex center in the brainstem, which typically controls things like eye movement. 

The reason that reflexes are involuntary is that they need to be fast in order to protect the body. If you pick up a hot plate, the reflex reaction will make you drop it almost immediately to protect your hand from being burnt. The quicker you drop the plate, the less damage will be done to your hands. However, most of us try to consciously override reflexes, and this is very possible. Referring back to the hot plate example, when you are carrying your dinner on the plate, you wouldn’t want to drop the plate and spill your dinner all over the floor. Instead, you might try to resist that reflex so you can instead safely put the plate down, even if your hand gets a bit burned. Now, the point is that a reflex is typically something that will promote your survival and help you in a situation as fast as possible, so the brain itself doesn’t process the stimulation in order to save time and protect your body faster. However, the brain does still have the ability to override reflexes if we command it to. 

There are three neurons behind the speed and coordination of reflexes, and they are sensory, relay(interneurons), and motor neurons. Besides the three neurons, nervous coordination also requires sensory receptors that detect a stimulus and effectors which produce the reaction.  All reflexes have a stimulus component and a response component corresponding to sensory and motor neural circuits respectively. The simplest examples of such reflexes are spinal motor reflexes such as the knee-jerk reflex, also known as the patellar reflex, and the finger withdrawal reflex. 

In a knee-jerk reflex, sensory neurons send signals to the spinal cord when the knee is tapped with a rubber hammer. Almost everyone should be familiar with the knee-jerk reflex because doctors regularly test for it during an annual check-up. In the spinal cord, the sensory neuron synapses on the dendrites of a motor neuron, which sends nerve impulses back out to the muscle fibers, making the leg jerk. Similarly, in a finger withdrawal reflex, pain, such as the heat from a candle, stimulates the sensory receptors of the hand. These receptors then send a nerve impulse through sensory neurons, also known as afferent neurons, to the spinal cord. In the spinal cord, the nerve impulse is quickly directed to the motor neurons, also known as efferent neurons, which tell the muscle fibers of the finger, the effectors, to move away from the candle immediately. Make sure to notice that all of these reflex reactions were really quick and there was no brain involvement whatsoever. 

Typically when talking about these reflexes, we have what is called a reflex pathway or reflex arc, which is what governs the operation of all reflexes. In a reflex pathway, the receptors respond to a stimulus and generate an impulse which is transmitted to the Central Nervous System, specifically to the spinal cord, through sensory neurons. In the spinal cord, the sensory neuron synapses onto an interneuron, which then synapses onto a motor neuron. The nerve impulse/signal is integrated in the spinal cord and is transmitted from the sensory neuron to a motor neuron, which carries the impulse all the way to an effector. An effector is an organ, gland, or muscle that responds to the stimulus of a nerve impulse. For example, in the knee-jerk reflex, the effector is the hamstring muscle of the leg, which contracts and moves as a result of the stimulus. 

Simplified Reflex Pathway/Arc: Receptor → Sensory Neurons → Spinal Cord (involvement of interneurons) → Motor Neurons → Effector

Now let’s talk about some common reflexes that you have probably heard of. The gag reflex typically happens as a result of a throat obstruction, and the response would be gagging and vomiting to try and get the lodged piece of food out of your throat. The tear reflex is due to eye irritation, and the reaction is the formation of tears to try and get the irritant out of your eye. The startle reflex is caused by a loud noise or surprise action and it usually results in the jumping or jarring of the head, arms, body, and eyes. Basically, it is the movement of the entire body from the stimulus. The salivary reflex is caused by the stimulus of food in the mouth and it results in the production of saliva so that you can chew and digest the food. There is also the pupillary reflex. So when too much light is entering the eyes, your pupils contract to reduce the amount of light. These are just some of the common examples of reflexes that use the simple reflex pathway that we previously discussed. 

A common question that usually pops up is whether or not a reaction and a reflex are the same thing. Reactions are voluntarily controlled actions, which means that we have to think and tell our hand to pick up a book. On the other hand, reflexes are involuntary actions, so they happen without us having to think about them. Reactions take place through the sensory nerves sending signals to the brain. In reactions, we do not bypass the brain like reflexes. Instead, the signals are directed to various parts of the brain, such as the thalamus, sensory area, and motor area for them to be processed. From the motor area, motor neurons will transmit the signal back to the specific muscle that you want to move, and this completes the reaction. In this case, we can see that reactions take considerably longer than reflexes to occur because they involve higher brain processing, while reflexes do not. 

It is so important to realize that reactions and reflexes are very different from one another, and they each involve various aspects of the central and peripheral nervous system to function. Nevertheless, both are crucial in living our everyday lives. In our next biopsychology blog, we will be covering a fascinating topic: Maslow’s hierarchy of human needs.