When a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor  protein—rhodopsin—that are embedded  in the membranes of a stack of disks in the top part, or "outer segment," of each cell. These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain  retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding  pocket in rhodopsin. But on exposure to light, it straightens out. This alters the  three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade.
    The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential.
(source:http://www.hhmi.org/senses/a/a140-lg.htm)

• RECEPTORS cover the surface of the body, and different dimensions of the world leave traces on them. Receptors convert energy into electrical activity.

(source: http://www.hhmi.org/senses/a/a130-lg.htm)

For different kinds of sensations, different kinds of receptor cells. Rod and cone cells of the eye's retina are specialized to respond to the electromagnetic radiation of light. The ear's receptor neurons are topped by hair bundles that move in response to the vibrations of sound. Olfactory neurons at the back of the nose respond to odorant chemicals that bind to them. Taste receptor cells on the tongue and back of the mouth respond to chemical substances that bind to them. Meissner's corpuscles are specialized for rapid response to touch, while free nerve endings bring sensations of pain.
(source: http://www.hhmi.org/senses/a/a130-lg.htm)

Modality	Stimulus	Receptor types	Receptors
Vision	Light	Photoreceptor	Rods, Cones
Audition	Sound	Mechanoreceptor	Hair cells (cochlear)
Balance	Head motion	Mechanoreceptor	Hair cells (semicircular canals)
Somatic	Mechanical,    thermal,   chemical	Mechanoreceptor,   thermoreceptor   chemoreceptor pain receptor	eg. Meissner   free nerves
Taste	Chemical	Chemoreceptor	Taste buds
Smell	Chemical	Chemoreceptor	Olfactory sensory neurons

(after ref 6., Table 23-1.)
 
 
 
 
 

(source: http://www.hhmi.org/senses/a/a150-lg.htm)

Each of the senses activates a separate area of the cerebral cortex, the sheet of neurons that makes up the outer layer of the brain's hemispheres. This brain is a computer reconstruction based on data from magnetic resonance imaging (MRI). Approximate locations of the primary sensory areas are shown in color. Most of the activity takes place within convolutions that cannot be seen from the surface of the brain.
(source: http://www.hhmi.org/senses/a/a150-lg.htm)

 
 
 

RECEPTIVE FIELDS(RF). Receptors transfer their stimulation to the sensory areas of the brain through several neuronal connections. Neurons monitor the world by parceling it into small regions of space, known as receptive fields. RFs are defined in terms of single neurons.

• Where is the RF? RF location is related to stimulus modality. (consult the book: p. 16, Box 1.2)
• What is the size of the RF?
• RF size is defined by convergence
• convergence, and receptor density affects sensitivity and acuity (see figs I.2/2-6 )
• What is the structure of the RF? (see 3.1)

fig I.2/1   convergence  & receptive field	fig I.2/2  convergence & acuity (ref 3, 71, 2.40) - Two spots of light can be discriminated easily by non-converging pathways (right), while they might stay fused in one even at large interspot spacing when convergence occurs (left).	fig I.2/3  RF size varies (ref 6, 375, 26-8) -  Receptive fields of  primary somatosensory cortical neurons are smallest on the fingers and become larger on the hand and forearm (see next image).
fig I.2/4  two-point threshold varies (ref 3, 472, 11.17) - Two-point thresholds correspond to RF size: best threshold at smallest RF size.	fig I.2/5  rod and cone distribution is inhomogenous (ref 3, 49, 2.12) - Cones are concentrated in the fovea: large number of receptors, little convergence, large cortical representation -> good acuity (see next image).	fig I.2/6  equally legible letters  (fixate at the center) (ref 9, 90, 3.20) - Due to the inhomogenous receptor density in the retina, only a small spot around fixation is seen sharply. Peripheral blur is counterbalanced here by enlarged letters. Consult the book.

KEYTERMS
    lecture: receptor, transduction, receptive field, convergence, RF size, retinal receptor density, two-point threshold

    book, Chapter 1: specific nerve energies; phenomenal/naturalistic approach; experimental approach; lesion technique; evoked potential technique; brain-scan techniques; single cell techniques; Marr's computational approach; subjective contours.

STUDY QUESTIONS:

• What kind of information do the senses collect about events in the world?
• How do you distinguish sight from sound? (box 1.2 in the book)

MULTIPLE CHOICE QUESTIONS:

• The receptive field of a sensory neuron is located a. on the receptor surface; b. in the subcortical structures; c. in the cortex; d. all of the above.
• The receptive field size of a sensory neuron is determined by a. the diameter of the neuron; b. the size of the receptor cells; c. convergence; d. the size of the stimulus.

Brain, Vision, Memory; Tales in the History of Neuroscience by Charles G. Gross
1998 MIT Press, ISBN 0-262-07186

FINALIZED for this semester Jan 2004