Atharvan Upanishad or the Mundaka Upanishad belongs to Atharvaveda, and is addressed to the one who is keen to be wise in Wisdom of, the resplendent Prime Existence. The Wisdom contained in the Upanishad being mystical needs some preparatory foundation before it can be communicated as Knowledge, since only he who has renounced the pleasures of senses and have offered their hair as a mark o submission and becoming receptive with clarity of mind to the Knowledge reflects and meditates on the Knowledge communicated to become well-established in the Wisdom of Brahman
Brihad Aranyaka Upanishad (I.4.17) clarifies the importance of each organs as centers of energy. The Individual Self is verily what the Mind makes it to be; speech is not what is only spoken but also the words which came to be formed ere they come to be expressed; breath is what gives expression to the words; perception, verily, is the true wealth of a person because what he perceived is what he gains. The hearing the resonance of his perception is what makes him luminous; his (individual) self is his Karma, for the self, verily, is what performs actions.