#99 Neurons (English)

A neuron is an electrically excitable cell that processes and transmits information through electrical and chemical signals. Sensory neurons carry signals from receptors to the spinal cord and brain.
Relay neurons carry messages from one part of the CNS to another. Motor neurons carry signals from the CNS to effectors.

Signals that have to travel between neurons have to travel through a space called a synapse. Neurons play a vital role in the human body. For example, if you touch something that is so hot it burns you, you normally move your hand away without thinking too much. Neurons transfer the message to the brain that something is burning you, and triggers neurons to make you jerk your hand away. Neurons are also used when something is falling and you instinctively reach out to catch it.

If we touch something hot, for example, the process of the nerves relaying the message to the brain and then moving your hand away is called a reflex arc. The first thing that happens in a reflex arc is that a receptor detects a stimulus. Then a sensory neuron sends a signal to a relay neuron. The relay neuron then sends the message to the motor neuron. After that, the motor neuron then sends a signal to an effector (which is normally a muscle or glands). The effector then produces a response, which would be flinching away from whatever hot thing you were touching.

In a simple reflex action:
stimulus → receptor → sensory neuron → relay neuron → motor neuron → effector

Synapses
Electrical impulses travel along an axon. This triggers the nerve ending if a neuron to release chemical messengers called neurotransmitters. These chemicals then diffuse across a synapse and binds with the receptor molecules of the next neuron. The receptor molecules on the second neuron only bind to the chemicals released by the first neuron. This makes the second neuron transmit the electrical impulse.

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