A reflex is involuntary, sudden response by a muscle or gland to stimuli.

Reflexes are how the nervous system can take quick actions without involving the brain; they are done without thinking or consciousness.

Let’s follow this diagram:

A pain/pressure receptor in your finger detects something dangerous.

The sensory neuron (red) sends a signal to a relay neuron (green)

The relay neuron sends a signal to the motor neuron (blue)

This sends the signal to the effector (in this case, a muscle.)

The muscle contracts to pull your hand away from the danger,

Reflex arc intro

Here we can see pain receptors in your hand sense high temperature. The signal is sent up to a relay neuron in a vertebrae (in your spinal cord) Then …[tbd]

Reflex arc GIF

Is the brain involved at all?

Not in the reflex, not at all!

But as we see from this more complete diagram, information from this process does make it from the nerve in your vertebrae, up to your brain. That way you know about what happened.

Reflex arc complete



Moving away your hand after touching a hot object. This is a reflex action and is immediate. You don’t think twice before removing your hand from the hot object. You feel the burn and immediately the action is taken.

The act of breathing. You don’t breathe and think. Breathing comes naturally. Controlling the breathing would need a lot of effort. Otherwise, it’s involuntary and done unconsciously.

When a doctor taps the knee with a small mallet, the knee jerks upwards. The knee jerk reaction is very natural. The patient doesn’t think before doing it.

Reflexes can be like lifting up your foot automatically as soon as something gets stuck on your foot. We don’t think whether or not we should lift up our foot. It’s quick, sudden and immediate.

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-LS1-2. Develop and use a model to illustrate the key functions of animal body systems: Emphasis is on the primary function of the following body systems (and structures): digestive (mouth, stomach, small intestine [villi], large intestine, pancreas), respiratory (lungs, alveoli, diaphragm), circulatory (heart, veins, arteries, capillaries), excretory (kidneys, liver, skin), and nervous (neurons, brain, spinal cord).

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