First, there’s a sensory nerve cell attached to a sea hare’s water pipe.
If there’s a big bang from the sea, the nerve cell senses it and converts the environmental stimulus into a bioelectrical signal.
The second is a motor nerve cell attached to the gills that controls the contraction of the muscles around the gills.
There is a direct connection between this sensory nerve cell and the motor nerve cell.The output of the former, the axon, and the receiving end of the latter, the dendrites, can transmit electrical signals directly.
In this way, the sea hare’s shriveled gill reflex is understood very simply.
When the water fluctuates dramatically, the sensory nerve cell in the front senses the impact of the water, creating a bioelectrical signal that is sent to the motor nerve cell in the back.This motor nerve cell, in turn, produces electrical signals that drive muscle contractions and complete the parochial reflex.
And as you can see from this very classic case, it’s actually not that difficult to develop a stimulus-reflex response model in an animal.
At a minimum, two neurons need to be connected, one for sensation and one for motor output, to do this.
In fact, even in humans, similar coping patterns may not require very sophisticated biology.Such as:
You probably know that there’s a test called the knee jerk reflex, and a lot of times it’s done in a medical exam.
Lift one leg and tap it with a small hammer under the knee, and the calf will bounce off.
Such a reflex in the human body essentially requires the participation of only two nerve cells.One senses the knock, and one contracts the muscles and raises the calves.
The stimulus-reflex model has two big flaws
By this point, you already know the benefits of the stimulus-reflection model and how to implement it.It sounds effective, universal, and simple.
So, to return to the question at the beginning of lecture: Why do we need to learn?Don’t you just respond to the environment with stimulus reflexes?
No, because despite these benefits, the stimulus reflex model still has two big flaws.
The first defect is its limitation.
Because such a stimulus reflex pattern requires an animal to carry all the genetic information needed to create it in the brain before birth, in its genetic material DNA.Such as:
In the case of the sea hare, it requires a gene in its DNA that tells it where to grow a sensory nerve cell, which senses the impact of the water, where to grow a motor nerve cell, which controls the contraction of its gills, and how to get the two nerve cells to find each other and make a connection.