What do the the different arrowheads mean in a cell signalling diagram?
Arrowhead interpretation
As I said in my comment and to add to @AlwaysConfused's answer, the arrowhead is a promotion, and flathead is an inhibition.
Typically, any other notation such as dotted lines would be explained in the figure legend of the illustration.
The below definitions are generally accepted and widely used.
But don't take these definitions for granted; there are no strict usage policies for these diagrams.
Different groups may use different nomenclature and symbols.

These definitions were taken from a legend in cellsignal.com.
Upstream and downstream
The arrowheads are not necessarily linked to up and downstream signalling.
Generally, the direction of the arrow indicates upstream/downstream.
However, at a glance, this has even less standardisation than the arrowheads.
Sometimes, like in the example in the question, upstream is generally at the top, and downstream is at the bottom.
Other times it is from left to right, or right to left.
There are many examples of cyclical events which the only way to tell is by following each arrow &quot;path&quot; for a particular signal.