Symbols in biology
I've been trying to find a good example that isn't either trivial (human
language), complex (protein interactions) or with highly contested
implications (DNA-protein translation). The best I've come up with is
the knee-jerk reflex-arc.
Knee Jerk Reflex
Source: Toole & Toole (1987) "Understanding biology for advanced level",
- 1. Stimulus. A blow to a tendon in the knee stretches the
- 2. Receptor. Muscle spindles detect the stretching and
stimulate a nerve signal.
- 3. Sensory (afferent) neurone. This neurone transmits the
signal to the spinal cord.
- 4. Motor (efferent) neurone. A synapse passes the signal
from the sensory neurone to the motor neurone. The motor neurone
transmits the signal back to the muscle.
- 5. Effector. The quadriceps muscle receives the signal
from the motor neurone and contracts. (This occurs at a "motor end
- 6. Response. "The lower leg jerks upwards as a
consequence of the muscle contraction." (Summarised from Toole &
In this example, the stimulus is symbolised in forms that are embodied in
chemical and physical processes. Some of the signifying processes are as
At each point in this chain, the physical embodiment of the signal (the
concentration wave, the acetylcholine transmission, etc.) signifies the
stimulus that caused the initial muscle stretch. These physical entities can
have other meanings in other cellular contexts.
- a. The neurone signals are embodied as travelling waves of
change in sodium, potassium, and chlorine ion concentrations across
a highly extended cell membrane.
- b. When stimulated, the synapse and motor end plate junctions
pass acetylcholine across a narrow gap.
- c. The motor end plate causes calcium ions to be released within
each cell of the muscle.
- d. Calcium ions bind to "troponin C" - a protein that interacts
with "myosin" and "actin" filments to contract each muscle cell.
From this, I would say that acetylcholine and calcium act as symbols in
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