After a subdued start, the FTSE 100 ended Thursday's session in a volatile fashion as traders digested an underwhelming interest-rate cut from the European Central Bank (ECB), comments from Mario Draghi and better-than-expected economic data from the States.The ECB did what was expected at midday by cutting its main refinancing rate from an already-record-low 0.75% to 0.5%. This move was thought to have already been priced into markets over the past week or so given that recent Eurozone data has missed expectations and inflation has eased.One thing that did surprise traders though was the ECB's decision to cut the marginal lending rate by 50 basis points to 1.0%.Market Analyst Craig Erlam from Alpari said: "This is clearly an effort by the ECB to encourage banks to lend, however I don't see this having any impact on the real economy as it doesn't address the fact that banks don't want to lend when the economy in stuck in recession and businesses don't want to borrow when the outlook is so grim."In the following press conference, ECB President Mario Draghi also caught many off-guard after saying that the bank would consider a negative deposit rate (it is currently at zero). The move would effectively charge banks to park funds at the ECB and is thought to be a incentive to boost lending.Erlam said: "Obviously, as we've seen both in the Eurozone and the UK, incentives don't guarantee anything. However, this would be a bold step from the ECB and has understandably been met with approval in the markets."Meanwhile, giving stocks a lift late on was the news that initial jobless claims in the US fell by 18,000 to 324,000 last week, the lowest number since January 2008. The consensus estimate was for a slight rise.FTSE 100: Glencore leaps after merger; Shire slumpsCommodities group Glencore International surged in afternoon trade after finally completing its multibillion-pound merger with resources firm Xstrata to create the world's fourth-largest mining company and the world's biggest commodities trader. Oil and gas firm BG Group was a strong riser after it cheered investors with a one per cent rise in revenue and other operating income. Sector peer Shell was also higher as investors shrugged off the news of its CEO quitting and focused on forecast-beating profits in the first quarter. Financial services group Legal & General was higher after hailing record assets, sales and cash generation in the first quarter of 2013 as its UK and US businesses exceeded management's expectations. Medical technology group Smith & Nephew rose after it said it plans to give back $300m to shareholders in a buyback plan. The group, which makes artificial hips, said the buyback is part of its new capital allocation framework for further investments and acquisitions. Meanwhile, drug maker Shire lifted first-quarter earnings 10% but revenues missed consensus estimates by 5.0%, sending shares sharply lower late on. Product sales in the three months to end-March 2013 were up 1.0% to $1.1bn against a strong set of comparatives from the equivalent period in 2012, but this was less than analysts were expecting. FTSE 250: Imagination Technologies hit by profit warningChip maker Imagination Tech plummeted after warning that profits would fall below expectations after delays to several deals hit licensing revenues. Multinational energy and environmental consultancy company RPS Group also fell sharply after saying that a slowdown in investment in resource projects in the Australia-Asia Pacific region has "reappeared". Liberum Capital downgraded the stock to 'hold'. Meanwhile, shares in NMC Health were rebounding one has after being hit with a downgrade from Numis Securities, which changed its rating on the stock from 'buy' to 'hold'. FTSE 100 - RisersGlencore International (GLEN) 331.15p +5.36%BG Group (BG.) 1,136.00p +4.27%Polymetal International (POLY) 704.00p +3.53%Imperial Tobacco Group (IMT) 2,370.00p +2.78%Whitbread (WTB) 2,620.00p +2.75%Legal & General Group (LGEN) 174.80p +2.46%Standard Life (SL.) 386.50p +2.14%Antofagasta (ANTO) 912.50p +1.96%Smith & Nephew (SN.) 749.00p +1.70%WPP (WPP) 1,082.00p +1.69%FTSE 100 - FallersShire Plc (SHP) 1,885.00p -6.68%Melrose Industries (MRO) 244.20p -2.32%BT Group (BT.A) 279.10p -2.31%IMI (IMI) 1,220.00p -2.09%Tullow Oil (TLW) 977.00p -1.81%Weir Group (WEIR) 2,118.00p -1.76%Aggreko (AGK) 1,750.00p -1.63%Sage Group (SGE) 335.80p -1.55%Wood Group (John) (WG.) 766.50p -1.48%Experian (EXPN) 1,136.00p -1.47%FTSE 250 - RisersTelecity Group (TCY) 969.50p +4.81%NMC Health (NMC) 286.00p +3.36%Hochschild Mining (HOC) 254.00p +2.83%Britvic (BVIC) 456.10p +2.73%Ocado Group (OCDO) 171.90p +2.32%Filtrona PLC (FLTR) 728.00p +2.10%Carillion (CLLN) 277.70p +2.10%CSR (CSR) 508.50p +2.05%New World Resources A Shares (NWR) 153.70p +1.79%Inchcape (INCH) 511.50p +1.79%FTSE 250 - FallersImagination Technologies Group (IMG) 315.40p -25.70%RPS Group (RPS) 218.70p -12.87%Inmarsat (ISAT) 691.00p -7.74%Heritage Oil (HOIL) 144.20p -5.01%Homeserve (HSV) 206.20p -4.58%Computacenter (CCC) 440.00p -4.33%Dunelm Group (DNLM) 821.50p -3.64%Millennium & Copthorne Hotels (MLC) 535.00p -3.60%Bodycote (BOY) 502.00p -3.55%Spectris (SXS) 2,064.00p -3.10%BC