Biology Reference
In-Depth Information
substance of the receptors. Furthermore, he explained the steady
discharge of impulses recorded after light adaptation had stabilized
on the assumption that the rate of decomposition and regeneration of
the photosensitive substance had reached a state of equilibrium.
7.3 The electrical responses in single optic
nerve fibres of the frog
Hartline also made use of cold-blooded vertebrate frog retina in his
investigation. In accord with his results on
Limulus
, the frequency
of discharge in different fibres was found to be greater the higher the
intensity of the retinal illumination, and it could be reduced and
increased by light and dark adaptation. However, while the response
pattern in nerve fibres of
Limulus
was found to be essentially similar
in character, Hartline found three quite different response patterns
in the frog retina:
1.
In some fibres (so-called 'on' fibres) the discharge was similar to that
of
Limulus
; the impulses were discharged regularly as long as the light
was on.
2.
Other fibres ('on-off' fibres) discharged impulses briefly when the light was
turned on and again when it was turned off with no activity in between.
3.
A third category of fibres ('off' fibres) discharged impulses only when
the light was turned off.
This diversity of response was ascribed to the complex nervous
structure interposed between the receptor elements and the ganglion
cells of the frog retina.
An interesting feature of the impulses in the 'off' fibres was
their strong suppression by re-illumination. This phenomenon was
interpreted to mean that excitation of nerve cells may inhibit nerve
activity. Hartline also found the 'on-off' fibres of the frog retina to
be extremely sensitive to movements of retinal images: the more
rapid and more extensive the movement, the greater the number of
impulses at 'on' and 'off'.
Moreover, exploring the retina of the frog using a small spot
of light, Hartline found that the receptive field of a ganglion cell
