Its said that the outside world and the inner world are identical and the same.
“piNDabrahmaaNDayoraikya.n li~Ngasuutraatmanorapi |svaapaavyaakR^itayoraikya.n svaprakaashachidaatmanoH |” (Yoga Kundalini Upanishad 1:81)
“The microcosm and the macrocosm are one and the same; so also the Linga and Sutratman, Svabhava (substance) and form and the self-resplendent light and Chidatma”.
“shariire sakala.n vishvaM pashyatyaatmaavibhedataH | brahmaaNDo.ayaM mahaamaargo raajadantordhvakuNDalii |” (Yoga Kundalini Upanishad 2:49)
“Then he sees the whole universe in his body as not being different from Atman. This path of the Urdhva-Kundalini (higher Kundalini), O chief of Kings, conquers the macrocosm”.
In Uttara Gita, Lord Krishna said to Arjuna.:
“tasya madhyagatAH sUryasomAgniparameshvarAH | bhUtalokA dishaH kShetrasamudrAH parvatAH shilAH | dvIpAshcha nimnagA vedAH shAstravidyAkalAkSharAH | svaramantrapurANAni guNAshchaite cha sarvashaH | bIjaM bIjAtmakAsteShAM kShetraj~nAH prANavAyavaH | suShumnAntargataM vishvaM tasminsarvaM pratiShThitam.h |” (Uttara Gita 2:15-17)
“The Sun, the Moon, and the other Devatas, the fourteen Lokas of Bhur, Bhuvar, etc., the ten directions,East, West, etc., the sacred places, the seven oceans,the Himalaya and other mountains, the seven Islands of Jambu, etc., the seven sacred rivers, Ganga, etc., the four Vedas, all the sacred philosophies, the sixteen vowels and twenty-four consonants,the Gayatri and other sacred Mantras, the eighteen Puranas and all the Upa-Puranas included, the three Gunas, Mahat itself, the root of the Jivas, the Jivas and their Atman, the ten breaths, the whole world, in fact, consisting of all these, exists in the Susumna”.
And the backbone through which runs the Sushumna nerve is called as the Meru Mountain in scriptures. Uttara Gita clarifies this out as follows.
“gudasya pR^iShThabhAge.asminvINAdaNDasya dehabhR^it.h | dIrghAsti mUrdhniparyantaM brahmadaNDIti kathyate | tasyAnte suShiraM sUkShmaM brahmanADIti sUribhiH |” (Uttara Gita 2:13)
“Like the backbone of a Veena, or harp, the long tract of bone with many joints that stretches from the seat right up to the head of a human being is called the Meru-Danda (spinal cord). There is a minute aperture or hole that passes right through this Meru-Danda from the Muladhara to the head; it is through this hole that there passes a Nadi which the yogis called the Brahma-Nadi or Susumna”.
Within this Mahameru (spine) also called as Brahmadanda or Merudanda are seated all the 33 categories of Gods and inside the Sushumna nadi are situated all the worlds. When a Yogi sits in Yogic posture making his backbone straight and while doing Pranayama, he agitates the backbone (Mahameru) with his Yogic posture.
At first the Yogi kindles the fire called Kundalini by Pranayama where expiration and inspiration of breaths which are the libations (havis) given to the sacrifice of Kundalini awakening.
After the awekening of Kundalini, Maa Shakti rises upwards through Sushuma naadi and after piercing 3 knots of Brahma, Vishnu and Rudra she reaches Sahasrara Chakra and get united with her husband Lord Shiva. This union makes the yogi immortal and gives them moksha. As i have discussed in this answer.
I hope this clarifies your queries.