Biological Olfaction - The Electronic Nose Artificial Olfaction Technology - page 80

Table 3.2 Human sensory system
Sensory system Modality Stimulus energy Receptor class

Visual Vision Light Photoreceptor

Auditory Hearing Sound Mechanoreceptor

Vestibular Balance Gravity Mechanoreceptor

Somato sensory Somatic sensors:

Touch Pressure Mechanoreceptor

Proprioception Displacement Mechanoreceptor

Temperature sense Thermal Thermoreceptor

Pain Chemical, thermal or mechanical Chemoreceptor

Thermoreceptor

Gustatory Taste Chemical Chemoreceptor

Olfactory Smell Chemical Chemoreceptor

Fig. 3.11 Diagram summarizing the events that take place in transduction

The receptor cells must be activated. The activation process involves opening
of ion channels to cause a change in the cell's membrane potential. In different
sensory systems, different mechanisms cause ion channels to open in response to a
stimulus.
Figure 3.11 summarizes the events that take place during the transduction in
sensory system. The opening of ion channels creates a receptor potential. Like an
EPSP in a neuron, the receptor potential is graded; its size reflects in some way the
properties of the stimulus. In general, larger the magnitude of the stimulus, larger
is the receptor potential. The receptor potential causes release of neurotransmitter
onto the dendrite of the primary afferent (nerve fiber projecting to the central
nervous system). The larger the receptor potential, the greater the quantity of
neurotransmitter released. If enough excitatory neurotransmitter is released to
bring the primary afferent neuron to threshold, it will fire an action potential.