8. THE LATERAL LINE ORGAN MECHANORECEPTOFS
253
These findings intimately relate to ultrastructural findings on the basis
of which the observed receptor potential characteristics have been predicted earlier (Flock and Wersall, 1962). As seen in Fig. 4 each hair cell
is asymmetrically polarized by the presence of a kinocilium in one end of
the bundle and by the stepwise increasing length of the stereocilia toward
the kinocilium. Neighboring cells face opposite directions, toward the
head or toward the tail alternately. There is strong indication that this
morphological polarization underlies the directionality of the receptor
potential in such a way that bending of the sensory hairs in the direction
in which the kinocilium is leading causes depolarization of the hair cell
whereas opposite bending causes hyperpolarization.
It is a task for future researchers to investigate the mechanism of
generation and the possible role of the receptor potential; at the present
time some possible mechanisms might be considered.
C. Origin of the Receptor Potential
The appearance of a receptor potential is an early sign of excitation
of receptor elements in sense organs, this applies to the eye, the organs
of olfaction, and to the inner ear and other mechanoreceptors throughout
the animal kingdom. It is likely to play a causative role as an early step
in the sensory process by which hair cells detect mechanical motion of
molecular dimensions. It is of importance to inquire what is the underlying mechanism capable of such extraordinary performance. At least
two possible mechanisms suggest themselves; one relates to the mechanical sensitivity of the cell membrane in mechanoreceptors such as the
Pacinian corpuscle and one to the mechanical sensitivity of cilia.
The receptor potential can be recorded extracellularly as well as intracellularly. Electric current thus flows in a path that engages the hair cell;
when the hair cell is depolarized it enters the apical end and leaves
through the cell wall over the rest of the cell. The current thus passes
the sensory hair bundle and the apical cell membrane where the mechanical excitatory energy acts; this is the likely location of a sensitive
mechanoelectric transducer. The plasma membrane that covers the
stereocilia is a triple-layered membrane (Fig. 12) which comprises a
sizable increase of area exposed to mechanical energy. Perhaps the membrane accommodates gates for electric current or ion flow. Gating may
be governed by mechanical interaction directly in the membrane, or it
may be controlled by a transducer at some other site.
The hair cell bears a single kinocilium characterized by an internal
253
These findings intimately relate to ultrastructural findings on the basis
of which the observed receptor potential characteristics have been predicted earlier (Flock and Wersall, 1962). As seen in Fig. 4 each hair cell
is asymmetrically polarized by the presence of a kinocilium in one end of
the bundle and by the stepwise increasing length of the stereocilia toward
the kinocilium. Neighboring cells face opposite directions, toward the
head or toward the tail alternately. There is strong indication that this
morphological polarization underlies the directionality of the receptor
potential in such a way that bending of the sensory hairs in the direction
in which the kinocilium is leading causes depolarization of the hair cell
whereas opposite bending causes hyperpolarization.
It is a task for future researchers to investigate the mechanism of
generation and the possible role of the receptor potential; at the present
time some possible mechanisms might be considered.
C. Origin of the Receptor Potential
The appearance of a receptor potential is an early sign of excitation
of receptor elements in sense organs, this applies to the eye, the organs
of olfaction, and to the inner ear and other mechanoreceptors throughout
the animal kingdom. It is likely to play a causative role as an early step
in the sensory process by which hair cells detect mechanical motion of
molecular dimensions. It is of importance to inquire what is the underlying mechanism capable of such extraordinary performance. At least
two possible mechanisms suggest themselves; one relates to the mechanical sensitivity of the cell membrane in mechanoreceptors such as the
Pacinian corpuscle and one to the mechanical sensitivity of cilia.
The receptor potential can be recorded extracellularly as well as intracellularly. Electric current thus flows in a path that engages the hair cell;
when the hair cell is depolarized it enters the apical end and leaves
through the cell wall over the rest of the cell. The current thus passes
the sensory hair bundle and the apical cell membrane where the mechanical excitatory energy acts; this is the likely location of a sensitive
mechanoelectric transducer. The plasma membrane that covers the
stereocilia is a triple-layered membrane (Fig. 12) which comprises a
sizable increase of area exposed to mechanical energy. Perhaps the membrane accommodates gates for electric current or ion flow. Gating may
be governed by mechanical interaction directly in the membrane, or it
may be controlled by a transducer at some other site.
The hair cell bears a single kinocilium characterized by an internal