The five parts of most reflexes are:. A sensor, which is a receptor that senses or detects a change. The monosynaptic knee-jerk reflex skips the interneuron, so it involves the sensor, sensory neuron, motor neuron, and muscle fiber Figure 2.
In a monosynaptic reflex, the message travels from the sensory neuron to the motor neuron with only one synapse. If you think about a relay race with a baton, this is similar to the way a reflex works. The runner with the baton is the action potential, and it is carried down the neuron, which is part of the track. When the runner and baton get to a specific area on the track, the baton is transferred to the next runner to move along the track.
A reflex is just like a relay race, but in a reflex, it is the action potential that moves between neurons rather than a baton and runner along parts of the track Figure 3. In the above example of a monosynaptic reflex, the interneuron is not used. It is easy to add an interneuron; just add another runner and baton, but this runner travels a very short distance.
In our body, interneurons are in the spinal cord. So, when an interneuron participates in a reflex, there is more than one synapse, and these reflexes are then called complex reflexes. There are many different reflexes in the body. Some of them are complicated and involve multiple interneurons and many synapses.
These reflexes, just like the simple monosynaptic reflex, exist in living beings, especially humans, to keep us safe! Sometimes reflexes create more than one action. Imagine removing your foot from something sharp—like a Lego piece that was left on the floor. So, the withdrawal reflex to remove your foot works with a reflex on the other side of the body telling you to put your other foot down. That reflex is called the crossed extensor reflex. These actions are all done without you thinking or planning, but your brain helps to assess the situation as an afterthought.
Who left the Lego there? Did you notice how the brain was not on the five-part list of what makes up a reflex? Well, reflexes are functions of the nervous system, which coordinates our actions. The nervous system is the network of neurons that transmits the action potentials. The sensor and neurons are outside the spinal cord, in what is called the peripheral nervous system PNS. The other part of the system is called the central nervous system CNS , and it consists of the brain and spinal cord.
The CNS is used to think, plan, and learn. Remember the earlier examples when you ducked down in response to a loud noise or lifted your foot after stepping on the Lego? In those examples, the CNS is what helped you understand what the noise was or why you moved automatically.
It did not create the initial movement, but the CNS allowed you to understand why you ducked down or that you stepped on a Lego. The CNS also plans the voluntary movements that come after the initial involuntary reflex. Perhaps you will bend down and pick up the Lego—that is a voluntary movement. Do you remember the example of the doctor tapping your knee? That is a test to see how well your body is prepared to react. There are many different types of reflexes that protect your body. The doctor tapping your tendon is a test of the simple monosynaptic reflex, which consists of a sensor, a sensory neuron, a motor neuron, and a muscle.
Once the sensory neuron is activated, an action potential travels along the sensory neuron to the spinal cord. The action potential then moves across the synapse to a motor neuron and leaves the spinal cord. More advanced reflex responses will use an interneuron, and these are called polysynaptic reflexes. Reflexes do not involve the CNS at first, but after the reflex has occurred in the body to keep it safe, the brain assists in understanding what happened.
Although we do not think about reflexes, they are important parts of our nervous system. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors sincerely thank and recognize Miss. Jenna Cooper, a student at Vernon Secondary School, for her art work contributions. The authors thank Mr. Evidence for sustained cortical involvement in peripheral stretch reflex during the full long latency reflex period. Development of the monosynaptic stretch reflex circuit. Share on Facebook. Core Concept Published: April 28, It's because of your reflex. A reflex is an involuntary say: in-VAHL-un-ter-ee , or automatic, action that your body does in response to something — without you even having to think about it.
You don't decide to kick your leg, it just kicks. There are many types of reflexes and every healthy person has them. In fact, we're born with most of them. Reflexes protect your body from things that can harm it. For example, if you put your hand on a hot stove, a reflex causes you to immediately remove your hand before a "Hey, this is hot! Other protective reflexes are blinking when something flies toward your eyes or raising your arm if a ball is thrown your way.
Even coughing and sneezing are reflexes. They clear the airways of irritating things. The reflex that the doctor checks by tapping your knee is called the patellar, or knee-jerk, reflex. It is also known as a deep tendon reflex DTR because the doctor is actually tapping on a tendon called the patellar say: puh-TEL-ur tendon. This tap stretches the tendon and the muscle in the thigh that connects to it.
A message then gets sent to the spinal cord that the muscle has been stretched.
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