(CNN) -

Here's some background information about concussions in the NFL. Reports show an increasing number of retired NFL players who have suffered concussions developed memory and cognitive issues such as dementia, Alzheimer's, depression and chronic traumatic encephalopathy (CTE).

A concussion is a type of traumatic brain injury caused by a blow to the head.

Facts: Most concussions occur without losing consciousness.

Chronic traumatic encephalopathy (CTE) is a degenerative disease of the brain and is associated with repeated head traumas like concussions.

Among the plaintiffs in concussion-related lawsuits: Art Monk, Tony Dorsett, Jim McMahon, Jamal Anderson and Ray Easterling.

Common Symptoms of Concussions: (The NFL Player Concussion Pamphlet) Imbalance Headache Confusion Memory loss Loss of consciousness Vision change Hearing change Mood change Fatigue Malaise

Statistics: (NFL) 2012 - 261 diagnosed concussions during preseason and regular-season practices and games combined.

2013 - 228 diagnosed concussions during preseason and regular-season practices and games combined.

Timeline: 1994 - NFL Commissioner Paul Tagliabue creates the Mild Traumatic Brain Injury Committee. Dr. Elliot Pellman is named chairman despite not having experience with brain injuries.

2002 - Dr. Bennet Omalu, a forensic pathologist and co-founder of the Brain Injury Research Institute, identifies chronic traumatic encephalopathy (CTE) in the brain of former Pittsburgh Steelers' center Mike Webster, 50, who committed suicide. Omalu is the first to identify CTE in American football players.

January 2005 - The NFL's Mild Traumatic Brain Injury Committee finds that returning to play after sustaining a concussion "does not involve significant risk of a second injury either in the same game or during the season."

2005 and 2006 - Dr. Omalu identifies chronic traumatic encephalopathy (CTE) in the brains of former Pittsburgh Steelers players Terry Long, 45, and Andre Waters, 44. Both had committed suicide.

February 2007 - Dr. Elliot Pellman steps down as chairman of the Mild Traumatic Brain Injury Committee but remains on the committee.

June 2007 - The NFL holds a medical conference on concussions.

August 14, 2007 - The NFL formalizes new concussion guidelines which include a telephone hotline to report when a player is being forced to play contrary to medical advice.

October 28, 2009 - Part I of the House Judiciary Committee hearing on Legal Issues Relating to Football Head Injuries. NFL Commissioner Roger Goodell defends the League's policy regarding concussions.

January 4, 2010 - Part II of the House Judiciary Committee hearing on Legal Issues Relating to Football Head Injuries. Dr. Ira Casson, one of the co-chairs of the Mild Traumatic Brain Injury Committee, denies a link between repeat head impacts and long-term brain damage.

March 2010 - The NFL's Mild Traumatic Brain Injury Committee is renamed the Head, Neck and Spine Committee. Two new co-chairs are selected, and Dr. Elliot Pellman is no longer a member of the panel.

October 20, 2010 - NFL Commissioner Goodell issues a memo to all 32 teams that warns of possible suspensions for offenders that violate the "playing rules that unreasonably put the safety of another player in jeopardy have no place in the game, and that is especially true in the case of hits to the head and neck."

February 17, 2011 - Former Chicago Bears defensive back Dave Duerson, 50, commits suicide with a gunshot wound to the chest rather than his head so his brain can be researched for chronic traumatic encephalopathy (CTE). Boston University researchers find CTE in Duerson's brain, the same disease found in other deceased NFL players.

April 19, 2012 - Former Atlanta Falcons safety Ray Easterling, 62, commits suicide. An autopsy finds signs of chronic traumatic encephalopathy (CTE). Easterling had been a plaintiff in a class action lawsuit against the NFL over concussion-related injuries filed in August 2011.

May 2, 2012 - Former NFL linebacker Junior Seau, 43, is found dead with a gunshot wound to the chest, classified as a suicide. Friends and family members say the suicide was brought on by multiple concussions, but an initial autopsy report finds no apparent brain damage. Portions of Seau's brain have been sent to the National Institutes of Health for further study.

June 7, 2012 - A unified lawsuit combining more than 80 concussion-related lawsuits on behalf of more than 2,000 National Football League players is filed in federal court in Philadelphia. The players accuse the NFL of negligence and failing to notify players of the link between concussions and brain injuries, in Multi-district Litigation Case No. 2323.

August 30, 2012 - The NFL files a motion to dismiss the concussion-related lawsuits filed by former players.

September 5, 2012 - The Foundation for the National Institutes of Health announces that the NFL has committed to donating $30 million to support research on medical conditions prominent in athletes.