Heat oven to 400°F. Lightly grease a 12 cup muffin pan.
Whisk together 1 cup whole wheat flour, 3/4 cup wheat bran, 3/4 cup granulated sugar, 1 1/2 teaspoon ground cinnamon, 1 teaspoon baking powder, 1 teaspoon baking soda, 1/2 teaspoon salt, 1/4 teaspoon ground ginger, and 1/8 teaspoon freshly grated nutmeg in a large bowl. Set aside.
In a medium bowl, whisk together 1 cup fresh or canned pumpkin puree, 2 large eggs, 2/3 cup buttermilk, and 1/4 cup canola oil.
Gently stir the pumpkin mixture into the flour mixture until just combined (it's okay if some lumps remain) and fold in 1/4 cup raisins, if using. Divide batter evenly among muffin cups.
Bake for 20-25 minutes, until a toothpick inserted in the center of a muffin comes out clean. Cool slightly in pan, then remove muffins from pan and cool on a wire rack.