A. Sensory experiences are mediated by different sense organs.
In 1830's Johannes Muller published the Handbook of Human Physiology - Law of specific nerve energies: we are aware of our world through the activation of nerves that are specific for certain energies.  He divided these nerves/energies into the classes that were originally set forth by Aristotle: seeing, hearing, touch, smell, and taste.  These 5 senses are refered to as  modalities. Therefore if the nerve that carries visual information is stimulated the sensation we have will be of light.  Within a given modality there may be submodalities or qualities ( taste can be sweet, sour, bitter, and light has color, and intensity.  Submodalities are generally served by specific receptors.
 
 
 

Energy	Disturbance of physical media			Chemicals / Molecules			Photons	Electric/Magnetic Fields
Modality	Hearing	Touch	Kinesthesia/Proprioception	Taste	Smell	Interoreceptors	Vision	Electro/Magnetoreception
Conversion of Energy to electrical signal (Table 10.2)	1. Transduction  2. Receptor potential  3. Electrotonic spread  4. Impulse Generation
perception/sensation	.	.	.	.	.	.	.	.

 

(Table 10.1) 'In modern terms, we recognize that there are specific receptor cells tuned to be sensitive to different forms of energy in the environment. The forms of energy serve as stimuli for the receptor cells. Receptors can be sensitive to signals from within the body (interoceptors or  visceroceptors) and from outside the body (exteroceptors or teleceptors).  The activation of some of these receptors will cause a conscious sensation while others produce a unconscious reaction (muscle reflexes, stretch receptors in the vasculature - associated with blood pressure and heart rate, and in skeletal muscle - associated with muscle reflexes). Some sensory modalities produce both conscious and unconscious sensations eg. visual system.

B. Sensory systems or modalities or sensation in general can be discussed in many different ways.  Here we will first discuss  general aspects of sensory processing and then focus on the sensory receptors, and sensory circuits and pathways involved with particular modalities.  When appropriate or possible  perception  of those stimuli will be discussed.

C. Generatl aspects:

1. (Fig 10.1) Sensory receptors: these organs transduce stimuli in the environment into electrical signals that can be interpreted by the central nervous system. They can be found in different concentrations in different organs producing various sized receptive fields and contributing to the phenomenon of the homunculus (i.e. the fovea has a high concentration of photoreceptors while the non-foveal regions do not have as high a concentration of photoreceptors). The  process of converting an energy into a useable signal has 4 parts to it: Transduction, receptor potential generation, electortonic spread and impulse generation.

1. Transduction - Conformational change in proteins found in specialized  area of receptor cell. Also refers to the whold process from conversion to electrical signal to perception (see Table 10.3)
2. Receptor Potential generation (Table 10.2):  The activation of these proteins,  irrespective of the type of stimuli being used to stimulate them,  generates  a receptor or generator potential (Ch 7- stretch receptor).  In most cases this potential occurs either as a  result of the activation of a basic G-protein linked second messenger systems or the direct gating of membrane channels. This potential increases in response to longer duration/intensity of  stimulation but generally adapts to a constant stimuli (Adaptation).
3. Electrotonic Spread - spread of receptor potential. Discussion of electrophysiology
4. Impulse Generation: The receptor potential in and of itself is not able to transmit information to the CNS.  This process must be done by action potentials. Therefore, at some point, receptor potentials must be transformed into impulse discharges or action potentials.  This process can be thought of as a conversion of an analog signal into an all or none digital signal. Discussion of action potentials

• Basic Principles:
• DivergenceCoivergence - Innervation of many cells by an axon from a given cell. Innervation of a given cell by many cells.
• Parallel/Serial processing
• Unimodal sensory pathways - specific/Multimodal sensory pathways - nonspecific
• Microcircuits
•    Lateral inhibition
• feedback
• feedforward -similar to

There is a mistake on pg 232 col 1 where it reads "cilia (taste & vision)" it should read "cilia (taste and hearing)"