Module 1 Lesson 5 - 4

Reflex Arcs

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In some sports, you need to be fast. In hockey. To score, for example, the forward must be faster than the defender. The goalie has to react quickly to stop the puck. Did the goalie think about it, or did the goalie just do it? 

Some neurons are organized into special neural pathways that allow a person to react before becoming consciously aware of what is happening. These unlearned, unconscious, and very rapid pathways are called reflex arcs. The resulting behaviour is called a reflex. It occurs extremely fast to protect the body from potentially harmful injury.

Reflex arcs involve very few neurons and can occur very rapidly in about 50 milliseconds. Reflex arc does not involve the conscious sensation, decision-making, and voluntary movement part of the brain. Withdrawal reflex involves only three neurons: sensory neuron, interneuron, and motor neuron. With fewer neurons, the reflex can happen more rapidly because the neurotransmitters in the synapses send signals more slowly than the myelinated axons do.

When the sensory neuron transmits the impulse to the interneuron and the withdrawal reflex occurs, the impulse also travels through the spinal cord to the sensory cortex area of the cerebrum. Because this involves many more neurons, it takes longer than the withdrawal reflex. That is why, when you place your hand on a hot stove, you do not feel the pain or cry out until after you withdraw your hand.

Parts of a reflex arc

1.    Receptor
2.    Sensory neuron
3.    Interneuron
4.    Motor neuron
5.    Effector

Watch and Listen

Watch the following video to learn more about the reflex arc.

 

You can also check an interesting applet about reflexes from the BBC by clicking on the image below.

Self-Check

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To review your understanding of the reflex arc, write a step-by-step description of the process illustrated below. Use all terms you have learned, and then name the type of reflex.