Emily Dickinson uses the extended metaphor of a funeral to describe the experience of mental trauma. First, the speaker describes the mourners "treading - treading" inside her brain. The repetition of the word emphasizes the repetitiveness of the action. This effect recurs in the second stanza with the drum "beating -beating" until her mind seems to be going numb. These repetitive noises in her head may be the pulsing of her blood, or a delusion, or a mixture of both. Whichever is the case, the aural imagery captures the sense that she is losing control of her mind, as an alien rhythm gains possession of it.
In the third stanza, a further loss of control becomes apparent when the sounds are no longer even regular, like a pulse. She imagines the noises inside her head to be the creaking of a coffin being lifted. Then, in a striking image, Heaven becomes a bell, and all of human existence nothing but an ear which hears it tolling. This could be a sublime idea, but the preceding stanzas have made it clear that this is a funeral bell. Heaven's message to earth is very far from being a comforting or uplifting one.
The image in the penultimate stanza breaks free of the mind, leaving the speaker wrecked and solitary in an undefined space. In the final stanza, she falls abruptly through a gap left by the breakage of "a Plank in Reason." The depth of the fall is indicated by the repetition of "down," while the fact that the poem ends with a dash rather than a period suggests that the descent into madness is endless, and the loss of sanity is permanent.