Which word belongs in a different group than the group below?

brain, glands, motor neuron, movement, muscles, sensory neuron, stimuli
brain
motor neuron
glands
stimuli

The correct answer would be glands.

All other words like brain, motor neuron, sensory neuron, stimuli, movement, and muscles can be used how the nervous system controls voluntary actions such as locomotion or movement.

The gland is an odd one as it refers to the organ or a group of cells in an organism which is involved in secreting particular chemical compounds in the body or in the surrounding medium.

For example, pituitary gland, thyroid gland et cetera.

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