Lectures for Neuroethology

Lecture 14: General Sensory Transduction and transformation

Dr. Paul A. Moore
office: LSC 226
phone: 2-8556
Email: pmoore@bgnet.bgsu.edu

Stimulus energies are not in the same form as neural information. Therefore, one of the main properties of the sensory system is to detect and transform stimulus energies into meaningful information that can travel along a nerve cell. This is called transduction.

Many steps involved in the general transformation of stimulus energies into nervous system.

Movement of stimuli through the environment (Transmission)
- Movement of sound through air, water, etc.
Filtering done by a mechanical filter (Physical filtering)
- Light traveling through the lens of the eye, sound in the ear canal
Selective sensitivity by sensory receptor cell (Selectivity)
- Photoreceptors only respond to light and to selective colors
Transduction into neural energy (Transformation)
- Opening of channels, activation of receptor cells
Amplification
- Second messenger systems

Details on the transformation of stimulus energies

Transmission (look at table from previous lecture)
Physical filters
Examples of physical filtering:
- Lens of the eye captures light from the front and focuses energy onto fovea
- Breathing increases the volume of air sampled
Selective sensitivity by sensory receptor cells
Either done by physical construction of the receptor cell (hair example)
Or done by differential receptor protein expression (vision or chemoreception)
Transduction of the stimulus energy and amplification
Stimulus impinges upon receptor cell
Receptor protein activated (at this point two possible pathways)

*Pathway 1*	*Pathway 2*
Channels opened directly by stimulus energy (example: hair cells, mechanoreceptors, some taste cells)	Enzyme cascade started (either Cyclic AMP pathway or IP3) (Examples: chemoreception and vision) Channels open

Back to a common pathway:

Na+ flux into cell
Receptor current
Receptor potential (graded response)
Modulated response of the receptor cell either in the form of an all-or-nothing Action Potential (Chemoreception) or modulated release of neurotransmitter onto secondary neuron (Vision, Taste)

It is important to note that receptor potentials are graded. This means that the amplitude of the receptor potential is in direct proportion to the intensity of the stimulus. Conversely, the amplitude of the action potential is not graded and is independent of the intensity of the stimulus. So how does a sensory system encode intensity?

What needs to be encoded?

Intensity
Information on type of stimulus (Selectivity again)
Changes in intensity over time

1.) Intensity Coding

Receptor potentials are graded.

Receptor potentials are graded. For example, when stimulus energy increases the amplitude of the receptor potential increases.

Action potentials are not graded

Conversely, action potentials are not graded. Intensity is encoded as the rate of firing. As intensity increases, the neuron fires at a higher rate. In addition, sensory systems encode intensity in a logarithmic fashion. For example, a sensory neuron will fire 10 action potentials for a stimulus of intensity X. If the intensity increases 10 fold then the sensory neuron doubles its firing rate

2.) Selectivity
Selectivity for a certain stimulus energy can be generated mechanically by the sensory system (as in mechanoreception), by biochemical properties of the cell, or by anatomical structure

Mechanical Selectivity
Below is a simplified drawing of a hair cell from the side and the top.

Hair cells are selectively sensitive to displacement in the line of movement that is in the direction of the kino and stereocilia. Displacement from the kinocilia toward the stereocilia causes a positive response from the hair cell. Displacement in the direction from the stereocilia toward the kinocilia causes a hyperpolarization or negative response. Movement in the direction of the double-headed arrow does not elicit a response. This is an example of mechanical selectivity.

Biochemical selectivity: Some sensory systems have differential expression of receptor proteins into receptor cells that make them selective for certain stimuli. For example, some cone cells have visual pigments that are sensitive only to red light, others have different pigments. Chemoreceptor cells express receptor proteins that are sensitive to certain chemicals.

Anatomical structure: Receptor cells feed their information into anatomically distinct areas in the brain or may have different anatomically sensitive areas. For example, sweet reception on your tongue is perceived in a specific area. This information is carried to specific taste centers in the CNS and NOT to other sensory areas.

3.) Rate of change of stimulus
Rate of change is an important feature to encode. It can provide information on movement of stimulus toward or away from an individual, information on size of the stimulus, direction of the stimulus and a number of other important features. Two main ways to encode this are:

# of neurons activated
For example, as a visual stimulus approaches an organism it activates a larger number of neurons in the retina.
Range fractionation of neurons
Range fractionation (covered nicely in the handout) is simply having a number of sensory neurons with different sensitivities. As the stimulus approaches, then neurons will be activation in sequential fashion. For example, imagine a sensory system with 3 neurons, A, B, and C. Neuron A is sensitive to stimuli with intensities from X to 5X, B is sensitive to stimuli from 3X to 8X, and C is sensitive to stimuli from 5X to 10X. As a stimuli approaches our model sensory system, the intensity will change from X to 10X. Far away, only neuron A is firing. As the stimulus approaches, the intensity will now change to 3X and A and B will fire. As the stimulus continues to approach, A, B, and C will fire. And so on.

Last modified: 00/02/25