The verb relate denotes several types of connection between multiple entities:
[WITH OBJECT] 1.0 Make or show a connection between:
the study examines social change within the city and relates it to developments
in the country as a whole
a supercomputer could relate all those factors
1.1 (be related) Be causally connected:
high unemployment is related to high crime rates
1.2 (be related) Be connected by blood or marriage:
he was related to my mother
people who are distantly related
1.3 [NO OBJECT] (relate to) Have reference to; concern:
the new legislation related to corporate activities
2.0 [NO OBJECT] (relate to) Feel sympathy for or identify with:
kids related to him because he was so rebellious
3.0 Give an account of; narrate:
various versions of the story have been related by the locals
The history of relate and its constituent parts offers some insight into the nature of the relationship relate describes:
1520s, "to recount, tell,"
from Middle French relater "refer, report" (14c.)
and directly from Latin relatus, used as past participle of referre "bring back, bear back" (see refer),
from re- "back, again" + latus (see oblate (n.)).
Meaning "stand in some relation; have reference or respect" is from 1640s;
transitive sense of "bring (something) into relation with (something else)" is from 1690s.
Meaning "to establish a relation between" is from 1771.
Sense of "to feel connected or sympathetic to" is attested from 1950, originally in psychology jargon.
word-forming element meaning "back to the original place; again, anew, once more," also with a sense of "undoing," c. 1200,
from Old French and directly from Latin re- "again, back, anew, against,"
"Latin combining form conceivably from Indo-European *wret-, metathetical variant of *wert- "to turn" [Watkins].
"person devoted to religious work," 1756,
from Medieval Latin oblatus, noun use of Latin oblatus, variant past participle of offerre "to offer, to bring before,"
from ob- (see ob-) + latus "carried, borne" (used as suppletive past participle of ferre "to bear"),
from *tlatos, from PIE root *tele- "to bear, carry" (see extol).
etymonline.com emphasis added
It doesn't always happen this way, because meanings change arbitrarily over time, but all of the ODO definitions continue the carry back to the original theme established by the origin of relate. Consider the intuitive interpretation of the ODO example sentences:
the study examines social change within the city and carries it back to the original developments in the country as a whole
1.1 high crime rates are carried back to [the original cause] high unemployment
1.2 people who are distantly carried back to [the original ancestor]
1.3 the new legislation carried back to [the original issue]corporate activities
2.0 kids were carried back to [the original rebel] him because he was so rebellious
3.0 the locals have carried back various versions of the [original] story
Examples sentences 1 and 2 of the OP relate precisely to the carry back theme of ODO definition 1.3. Relate connects the noun phrase my question conceptually to the noun phrase your earlier work, revealing that the question carries back to the original earlier work.
Example sentences 3 and 4 of the OP seems to relate weakly to ODO definition 1.1. The noun phrase nerve cells is connected to the noun phrase one another. Since neither actually carries back to the other as the original, it sounds slightly off to the native ear.
Example sentences 3 and 4 could be a weak metaphoric application of ODO definition 2.0, since the nerve cells could be seen as "related in the same fashion as people are related*, but then the preposition by would need to replace with to maintain the people blood analogy between nerve cells and fibers.
Example sentences 3 and 4 could also be a weak metaphoric application of ODO definition 3.0, since the nerve cells could be seen as relating by means of fibers in the same fashion as people relate with stories. This metaphor is weak because the people story analogy would be fulfilled more precisely with nerve cell impulse.
Examples 3 and 4 are not wrong necessarily, but they do stretch the meaning of relate beyond its established core.
Although the present form and the participial form are almost universally interchangeable in meaning, the present form establishes a subtle emphasis on the activity of carrying back. In contrast, the participial form establishes a subtle emphasis on the status. The participle form is not a passive construction per se, but the focus on the status gives the reader a static adjectival sense of the meaning, because of its similarity with a predicate adjective construction:
Activity focus: My question relates to your earlier work.
Status focus: My question is related to you earlier work.
Activity focus: Nerve cells relate to one another through thin fibers.
Status focus: Nerve cells are related to one another through thin fibers.
The difference between the present form relate[s] and the participial form is/are related creates a subtle difference in focus, that will have a slight influence on the interpretation of the sentences. Spot on usage of relate:
- My question connects to your earlier work.
- [I am] connecting my question to your original earlier work.
Slightly stretched usage of relate:
- Nerve cells connect to one another through thin fibers.
- Thin fibers are connecting nerve cells to one another.