8. THE LATERAL LINE ORGAN MECHANORECEPTORS
255
structure of nine peripheral double-barreled tubules surrounding a central pair of simple tubules (Fig. 13), the peripheral tubules have an
intracellular extension which constitute the basal body. From the wall
of the basal body a basal foot projects in a direction away from the stereocilia or rather away from the cuticular plate in which the rootlets of the
stereocilia are planted. This structure is quite similar to that of other types
of cilia. The sensitivity of cilia to mechanical deformation is well known
and has been particularly studied by Thurm (1968). He concludes from
experiments on mussel cilia that the basal portion of the cilium, particularly the shaft and perhaps the basal body, are the principal mechanically sensitive portions. The eff eclive stimulus is a shearing displacement
parallel to the cell surface. Intracellular potential changes have been
recorded in ciliated cells during mechanical stimulation. However,
Thurm concluded on the basis of his experiments that mechanical sensitivity and beat induction in the cilium are not dependent on a change
in membrane potential: This is rather a secondary effect perhaps produced by some output from the basal body. To summarize, it seems that
mechanically sensitive cilia have some effector output, chemical or electrical, which is generated by a sensitive transducer, which may then be
directed to induce a ciliary beat or to cause a membrane potential change
depending on the requirements. One possibility is that the receptor potential is generated in this way by the action of some agent produced by
perhaps the basal body on the apical cell membrane. Resulting changes
in membrane permeability may allow the movement of ions along their
electrochemical potential gradients and thus cause potential changes recorded as electrical events by the microelectrode.
IV. TRANSMISSION AT THE SENSORY SYNAPSE
In sense organs containing primary receptor cells the sensitive site
is an integral part of the sensory neuron (Pacinian corpuscle, etc.). Here
the receptor potential acts to directly trigger nerve action potentials. In
secondary receptor cells such as hair cells, spike initiation is probably
not caused directly by the receptor potential but via synaptic transmission
between the neuroepithelial sensory cell and the innervating afferent nerve
endings. In this case the role of the receptor potential has been a more
controversial question.
The hair cells are innervated by nerve endings from bipolar sensory
neurons which contact the bottom of the cell (Fig. 14). This synapse is
characterized by the presence, inside the hair cell, of a dense synaptic
255
structure of nine peripheral double-barreled tubules surrounding a central pair of simple tubules (Fig. 13), the peripheral tubules have an
intracellular extension which constitute the basal body. From the wall
of the basal body a basal foot projects in a direction away from the stereocilia or rather away from the cuticular plate in which the rootlets of the
stereocilia are planted. This structure is quite similar to that of other types
of cilia. The sensitivity of cilia to mechanical deformation is well known
and has been particularly studied by Thurm (1968). He concludes from
experiments on mussel cilia that the basal portion of the cilium, particularly the shaft and perhaps the basal body, are the principal mechanically sensitive portions. The eff eclive stimulus is a shearing displacement
parallel to the cell surface. Intracellular potential changes have been
recorded in ciliated cells during mechanical stimulation. However,
Thurm concluded on the basis of his experiments that mechanical sensitivity and beat induction in the cilium are not dependent on a change
in membrane potential: This is rather a secondary effect perhaps produced by some output from the basal body. To summarize, it seems that
mechanically sensitive cilia have some effector output, chemical or electrical, which is generated by a sensitive transducer, which may then be
directed to induce a ciliary beat or to cause a membrane potential change
depending on the requirements. One possibility is that the receptor potential is generated in this way by the action of some agent produced by
perhaps the basal body on the apical cell membrane. Resulting changes
in membrane permeability may allow the movement of ions along their
electrochemical potential gradients and thus cause potential changes recorded as electrical events by the microelectrode.
IV. TRANSMISSION AT THE SENSORY SYNAPSE
In sense organs containing primary receptor cells the sensitive site
is an integral part of the sensory neuron (Pacinian corpuscle, etc.). Here
the receptor potential acts to directly trigger nerve action potentials. In
secondary receptor cells such as hair cells, spike initiation is probably
not caused directly by the receptor potential but via synaptic transmission
between the neuroepithelial sensory cell and the innervating afferent nerve
endings. In this case the role of the receptor potential has been a more
controversial question.
The hair cells are innervated by nerve endings from bipolar sensory
neurons which contact the bottom of the cell (Fig. 14). This synapse is
characterized by the presence, inside the hair cell, of a dense synaptic