self grasping in paticca samuppaada

I am studying the paticca samuppaada and it seems that ignorance and craving are the main driving forces behind the 12 links and our ongoing ensnarement in samsaara.
The origin of craving is fairly clearly laid out in the suttas. My understanding is that when there is contact through the doors of the senses, that we are constantly having feelings or sensations of attachment, aversion or neutral feelings. We seek to hold onto or repeat the pleasant ones, and to push away the unpleasant. Thus we react to our sensations. Through repeating this process numerous times, we then produce habits, patterns, addictions, attachment, which would perhaps fall under the link of grasping in the paticca samuppaada and the khandha of samkhaarakkhanda. Samkhaarakkhanda is often translated as impulse or reaction, which carries the idea of being impelled along by our patterns into making more karma. This process is producing our habit patterns in samsara and spinning us endlessly around on the wheel.
I hope I am reasonably clear on the above. Comments or corrections additions are welcome.
When it comes to ignorance/avijjaa, I have some uncertain areas. I can understand how ignorance, especially of our true nature (the mistake of believing we have a self essence, a real "I") conditions consciousness and leads to name/form and so on. What I don't clearly understand is how this avijjaa is fueled by the 12 inderdependent links. How does it arise? What are the body/mind processes that create it, or once created, either strengthen or weaken it. This is clearly laid out in the suttas for craving/grasping, but I can't find it so laid out for avijjaa/ignorance.
Does the attachment/aversion habit pattern in the links of contact (sensation) leading to grasping (attachment) also give impetus to the avijja/ignorance link? Is this the area where our ignorance is fueled?
Clarification on this would be most helpful.
thank you,

Paticca Samuppada

In Buddhism, ignorance refers to the lack of understanding of the Four Noble Truths, in particular that life is dukkha, also the ignorance of anatman which are actually temporary creations of the skandhas. So when ignorance is present it conditions the next link that is volitional actions or karmic activities. And when it appears, the next link consciousness which makes the distinction about the manifestation of the activities arises and with the making of distinctions, trouble starts which leads to the arising of mind and body. The six senses are the inevitale consequences of mind and body. Because of the six senses contact sets in. Contact leads to feelings.Dependent on feeling arises craving. Craving results in grasping. Grasping is the cause of Kamma which in its turn, conditions future birth. Birth is the inevitable cause of old age and dealth. All these twelve factors interdependently linked in the form of a cycle without a beginning or en ending.This is how ignorant reactivity to the natural experience of life conjures up egoism and suffering.

The twelve constituent factors linked together in a chain. Putting ignorance at the beginning does not imply that it is the First Cause, or Genesis, of all things. Ignorance is put at the beginning for the sake of clarity, by intercepting the cycle and establishing a starting point where it is considered most practical. We are in fact cautioned against assuming ignorance to be a First Cause with the following description of the conditioned arising of ignorance -- Asava-samudaya avijja-samudayo, asava-nirodha avijja-nirodho -- ignorance arises with the arising of the outflows, and ceases with their cessation. [M.I.55]

The Buddha did not always describe the Dependent Origination cycle in one fixed form (from beginning to end). The extended format was used in cases where he was explaining the principle in general, but when he was addressing a particular problem, he often applied it in reverse order, thus: aging and death <= birth <= becoming <= clinging <= craving <= feeling <= contact <= six sense bases <= body and mind <= consciousness <= volitional impulses <= ignorance.

In other descriptions he may have begun at one of the intermediate factors, depending on the problem in question. For example, he might have started at birth, feeling or at consciousness, following the steps forward up to aging and death, or tracing backwards to arrive at ignorance. Or he may have begun with some factor altogether different from the twelve links, which was then worked into the Dependent Origination chain.

Paticca Sumuppada

Suan Bee,
Thank you for your well considered comments on avijja/ignorance. I understand that it is an important factor but not a first cause, and that its arising and cessation is closely linked to the asava/outflows. I will look into this close connection between the outflows and ignorance, and will look at M1.55.
Your point that Buddhas used the paticca samuppada explanation in a creative and varied way is also well taken. I think this (the paticca samuppada) is a good teaching tool, not something to get rigidly attached to by only understanding in one way. I will try and read more different instances of how it was used by Gotama Buddha in varied ways in his teaching.
Also, I found some helpful ideas on this subject in the book Principles of Buddhist Psychology by David J Kalapahana. On pages 32 and 33 he is discussing the paticca samuppada and he makes essentially the same point you make that all the 12 factors are interlinked and interdependent. He points out that when we have contact and sensation arises, that at this point our sankhara (dispositional tendencies, impulse, reaction) enters and we have an impure reaction already colored by our ignorance, grasping etc. And he brings in an analysis of the 18 dhatus here to show that mano is playing a role in every contact-feeling-grasping experience that we go through. He feels this mano has a tendency to add a solid notion of self at this point in the chain. So this would directly fuel the avijja, which you point out is closely linked to not understanding anatta.
So I guess what this means is that each perceptual moment we go through has the potency to add to our avijja ignorance, as long as we do not bring our sati/mindfulness and the other factors of the Noble 8 Fold Path to bear.
This seems to highlight the vital importance of constant mindfulness in our practice or in our daily life.
So this research into avijja/ignorance and the Dependent Arising is very helpful as a reminder to keep up constant effort so as to not dig in deeper into samsara.
Thank you for helping me to make some progress in attaining a deeper understanding of this most important teaching.
I will continue to study, contemplate and meditate on it.