Chakras, Nadis and Meridians
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Reiki – the chakras, nadis, and meridians are very important to reiki, yoga, and acupuncture among others. Both the nadis and meridians work with the "Chi" energy.
Reiki is responsible for restoring both the functioning of the chakras, nadis, and meridians. Reiki and yoga are good ways to cleanse and filter out the meridians and chakras.
In terms of yoga, this can be achieved by putting into practice some of the following techniques:

Pranayama (breathing techniques);
Samanu (purification of the nadis);
Asanas (exercises to correct body postures);
Meditation with Mudras and Bandhas (hand positions and body postures)

Below: from the book “Touch for Health A Practical Guide to Natural Health with Acupressure Touch and Massage
by John Thie, D.C.

Meridians
Meridians are perceived as a net of energy channels that cross the human body, charging it with the "Chi" energy. Acupressure vessels, or meridians, are located throughout the body. They are the pathways of the positive “yang” and negative “yin” energy power which carries on some of the communication between parts of the body.
Meridians are classified as yin and yang on the basis of the direction in which they flow on the surface of the body. They inter-connect deep within the torso, but you work with the part that is on the surface and is accessible to touch techniques.
    Yang energy and yang meridians flow from the sun, and yang meridians run from the fingers to the face or from the face to the feet.
    Yin energy and yin meridians flow from the Mother Earth, flow from the feet to the torso, and from the torso along the inside (yinside” of the arms to the fingertips. Since the meridian flow is actually one continuous, unbroken flow, the energy flows in one definite direction, and from one meridian to another in a well-determined order.
You can view this order of the meridians as a wheel.

As you go around this wheel following the meridian lines, the flow follows this order on the body:
From torso to fingertip (alongside of arm – yin)
From fingertip to face (alongside/back of arm – yang)
From face to feet(along outside of leg-yang)
From feet to the torso (along the inside of the leg-yin)

You go through this four-step process three times to cover the twelve meridians. Running the meridians with the hand can be a quick and energizing massage. Complete the massage by running the central and governing meridians, which run directly up the back and front of the body to the upper and lower lip.
    According to acupuncture, there are 12 major meridians ruling specific body organs:
stomach “yang”, spleen “yin”, heart “yin”, small intestine “yang”, bladder “yang”, kidneys “yin”, gallbladder “yang”, lungs “yin”, liver “yin”, large intestine “yang”. 
Such techniques are said to achieve their effects by manipulation and, ideally, balancing of the energy running through a network of complex bodily patterns.

They all start from the central channel “Sushumnâ” then to the periphery where they gradually become thinner.

Meridians all start below the base Chakra and work their way up the body.
They are not nerves but rather channels for the flow of consciousness.)