http://www.wretch.cc/album/show.php?i=lin440315&b=20&f=1556292331&p=161

Source of image: http://www.planetary.org/news/2011/0930_Mars_Exploration_Rover_Update.html

Nerve cells are unique in their shape and size. No other things resemble them. Nerve cells are shaped like stars with dendrites and range from 4 microns to 100 microns in diameter. (Ref.1:http://www.enchantedlearning.com/subjects/anatomy/brain/Neuron.shtml) 

(Ref.2: http://en.wikipedia.org/wiki/Neuron)

The star/pyramidal shapes could not be depressions or mineral fibers, etc. The reasons include:

1. Nerve cell remains exist not only in the black spots but also in the adjacent white spots, as evidenced in the second figure above.  

2. Mineral fibers are shaped like straight needles, except white asbestos (chrysotile). White asbestos do not grow into star/pyramidal shapes, so they could not be the dendrites shown in the figures above.

3.  They are not shadows, as there is nothing near that can project shadows of star/pyramidal shapes.    

4. They are not cracks, as no cracks/fractures take the shapes of stars/pyramids of 100 microns across.

5. They are not burrows created by bacteria, as no burrows resemble the shapes of stars/pyramids of 100 microns across. 

Anyway, who says Martian neurons must be smaller than Earthly ones?