Symptoms in terms of pain would be subjected to presence of vital nerves (nerves that are still in shape enough to conduct impulses of noxious stimuli) Inflammation causes damage to peripheral nerve endings, rendering them insensitive. Symptoms in terms of inflammation (and there's an overlap here as douleur is a cardinal sign of inflammation) is circumstantial to presence of inflammatory mediators. restricted blood flow (with similar pathophysiology of compartment syndrome) could decrease presence of both, oxygen and WBCs in the inflamed pulp. Hypoxic necrosis of nerve endings coupled with decreased inflammatory mediators due to lack of viable WBCs can render the pulp asymptomatic.

This is in conjunction with acute pulpitis being symptomatic and chronic being asymptomatic. However, yes, in my clinical practice, i have come across dead pulps which were never symptomatic (though, rarely) I am not sure of what could cause that. 

It is an interesting question!

Primary contributes for symptomatic pulpitis are infection, inflammation and pressure changes. And symptoms are directly proportional to matured innervation/vital nerve fibres.

1. inflammation brings about vascular changes (vasodialation, oedema) which increases local pressure, when the pressure crosses critical intrapulpal pressure (as pulp chamber is non expandable) the pressure falls back on nerves hence pain is felt. 

2. there are certain situations where this increased pressure will never crosses critical intra pulpal pressure, eg: open type of pulpitis, where the oedema fluid simply escapes through huge cavity formed due to decay. 

3. in certain conditions like chronic irreversible pulpitis, there are micro abasses forming within the infected pulp, there are instances when these microabscesses burst and coalesce. the pressure changes brought about by these events can initiate the pain.

4. These all symptoms are highly variable among individuals, individual teeth, type (primary/permanent) and age of the teeth. individual variation is primarily because of pain threshold. in primary and young permanent teeth the neural tissue is not considerably dense and it is immature. Hence the chances of asymptomatic progress of pulpitis is high in such teeth. 

5. Finally another contributing factor can be speed of progress of lesion. in cases of ECC or rampant caries the destruction is extremely rapid, which destroys nerve fibres in fraction of time, leaving very less time for symptom development.  

6. When the pulpitis progress to periradicular abscess, the same principles of "pressure changes" apply. sooner the pressure relieved (usually sinus formation) lessor the symptoms.

Pulpal pathology is one of the least explored subject in dentistry. Glad to see this question. Hope I addressed the query at least to some extent...,

Dear Vivek Verma ,

Really thanks for ur answer

Dear Prashant Battepati ,

Too much should I thanks you

Dear Essam Al-Moraissi, 

Good question and good answers.

Pulpitis, by virtue of the enclosure by non expansive hard tissue, causes pain, both by the effect of pressure and inflammatory cytokines.  

Pain can be absent under two circumstances , one is by an open cavity that ensures drainage of the inflammatory fluids and therefore no pain by pressure, as  mentioned by Prashanth Battepati. 

The second is by partial or total necrosis of the pulp, which affects continuity of the nerves and interferes with impulse conduction.

Dear Mandana Donoghue ,

Thanks for ur summarized and pointed out answer 

Dear Essam, as a matter of fact there is some scientific evidence to explain why such an innervated tissue like dental pulp might be asymptomatic in some cases. Although some issues have been approached in the answers above, part of the explanation comes from the fact that nociception can be abolished by the presence of local inhibitory mediators in the dental pulp (local opioids and cannabinoids, somatostatin, GABA, bombesin/gastrin. As is stated in Seltzer and Bender’s Dental Pulp (2004, page 167) “considering the exceptionally rich nociceptive innervation of the pulp, such asymptomatic cases (silent pulpitis) are puzzling”, and the local up-regulation of this inhibitors have an important role in this scenario.

There are links where you can find some articles to read about this issue:

http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2591.2002.00579.x/pdf

http://www.sciencedirect.com/science/article/pii/S0967586810002894

http://www.sciencedirect.com/science/article/pii/S0099239997802658