Is the Graphics card part of the logic board?

After waking up from a sleep by tapping the spacer bar. My LCD screen went all funny. Little colour wiggly lines from top to bottom on the LCD screen. once I used my mouse the screen changed to about 3cm square up where you find the pointer after start up or rebooting. I could move that 3cm square any where on the LCD screen with my mouse.

Also when I do start up I do receive the chime and in the back ground you can see the Apple logo and the wheel as you would see with a normal start up, but it is behind the colour lines. Then the LCD screen will go blue with 3cm square up in the left hand corner.

My question is. If it is the Graphics card, can it be replaced or is it part of the logic board??

My iMac is a mid 2010 EMC 2389.

Thanks for all you help here.

John