The  Prïmary

N e ô C ô r t e x  !
By Ther°al L. Bynum, M.D.

      WE ARE ALL AWARE OF SENSATIONS,  all of our waking hours.    Most people accept the fact that the brain is responsible for our perception of these sensations.   The brain in fact, has five sensory systems for perception:
Vision & Audition, Olfaction, Taste & Touch!

      Each sensory system has specialized non-neural cells called Receptors.   The function of these receptors is to convert (or transduce) sensory stimulation (ie, light waves) into neural impulses (through the production of either graded potentials or action potentials), and relay those impulses to a sensory neuron.   This impulse in turn is relayed to Interneurons which are usually interposed between sensory and motor neurons, allowing impulses to follow multiple routes and to affect multiple effector cells.

      The Neocortex :    encodes (records) the information found within a sensory system not once but a number of times.
   This term information, as it applies to the nervous system, can mean a variety of different things however;  such as knowledge, facts, quantities, intensity of pain, intensity of light, temperature, thus, pressure on the bottom of the feet is information, and stored memory in the brain is also information.
      Invariably, information cannot be transmitted in its original form, but only in the form of action potentials, or nerve impulses.   Take for example, the information found within the visual system (or the entire retina);  it is represented several times (in several different ways), within the Visual cortex.   The same principle of multiple representation applies in each sensory system.   Each sensory system has a Primary Area, which has one representation, and two Secondary Areas, each of which also have several representations.   Each representation is devoted to encoding only one specific aspect of that sensory modality.
      Sensory input enters the primary zones, is elaborated in the secondary zones, and is integrated in the tertiary zones of the sensory, or posterior unit of the brain.   For an action to be executed, activity from the posterior tertiary sensory zones is sent to the tertiary zone of the motor, or frontal unit of the brain, to the secondary zone, and then to the primary motor zone, where active motor execution is initiated.

      Within The Primary Sensory Zones  of the brain, the most general features of sensory stimuli are organized in an array which accurately represent the topography, intensity, and original pattern of stimulation.   The Neocortex processes for a specific sensory modality, recording chiefly its primary features such as intensity and spacial distribution;  whereas the secondary zones process the more abstract features, with the tertiary zones synthesizing the abstract features of a number of sensory modalities.

      Most of the interactions between neurons found in the layers of the Neocortex are with cells directly above or below, and with relatively less interaction with cells more than a couple of millimeters on either side.   This is especially so in the Primary cortical zones of encoding.   This vertical bias in cortical organization forms the basis for a second type of Neocortical organization:    The Column!   

      The Cerebral Cortex   is organized into narrow vertical columns which range from 0.5 to 1.0 millimeters wide.   They run from the cortical surface to the white matter, in which each neuron in such a column is functionally similar to the others.
   Afferent axons terminate on Interneurons in Layer IV (Internal Granular Layer) and these Interneurons transmit information vertically to synapse on other Interneurons or Pyramidal cell dendrites.   Little information transfer occurs laterally, the primary direction of information travel being vertical.   Physiological studies have shown that adjacent columns are maximally excited by different afferents, providing further evidence that each column functions as a "Mini Neural Network!"  

(See Figure B )

      The Visual and Auditory Systems are known as the Exteroceptive Systems, because of their sensitivity to stimuli from the external environment.   The projection areas of vision are ^«Brodmann's Area^» (17), for audition (41), and somatic sensory or bodily sensations (3, 1,and 2).  

      For visual perception;  light energy is converted into chemical energy in the receptors of the retina, and this chemical energy is in turn converted into neural activity.   The open eye establishes the receptive field for the eye.   Within the eye is the cup-shaped retina which contains millions of receptor cells called Cones and Rods.   Because of the shape of the retina, each cell points in a particular direction, like a telescope pointing at a particular part of the sky, and the little part of the world that each receptor receives light from forms its receptive field.  

      The function of receptive fields is to allow stimuli to be located in space or on the body's surface.   Each sensory system requires three or four neurons, connected in sequence, in order to get information from the receptor cells to the cortex (ie, the Visual and Somatic Sensory Systems have three neurons, while the Auditory System has four).

      Two important events can occur at the synapses between the neurons in the relays.   First- some kind of motor response can be produced.   And second- at a synapse, the code carrying the message can be modified in a variety of two ways:   descending impulses from the cortex can block or amplify it, or the code can simply be made more elaborate or more precise.

      In some of the sensory systems it takes one or more cells to transduce the stimulus energy into neural energy (ie, for vision, a light-sensitive or photoreceptive cell and then a bipolar cell), but once transduced, all sensory information from all sensory systems is coded by action potentials.   The sensory information is conducted into the brain by bundles of axons, called nerves, until they enter the brain or spinal cord and tracts thereafter, and every nerve carries the same kind of signal.   The presence of a stimulus can be coded by an increase or decrease in the discharge rate of a neuron, and the amount of change can code the intensity.   Qualitative visual changes, such as changes from red to green, can be coded by activity in different neurons or even different levels of discharge in the same neuron (ie, more activity = "redder"; less activity = "greener").  

(See Figure F )

      The human retina contains two types of Photoreceptive cells:   Cones and Rods, both of which function to transduce light energy into action potentials.   Rods, sensitive to dim light (as little as one photon), are utilized mainly for night vision.   Cones are able to transduce bright light and are used for daytime and color vision.   There are three types on Cones, maximally responsive to the wavelengths producing Hues of Red, Blue, or Yellow.   These three primary Hues are the basic building blocks of all color vision!   All other colors perceived are derived from these three!   Second generation Hues (3) are:   Orange, Violet, and Green.   Third generation Hues (6) derived from the secondary hues are:   Yellow- Orange, Red-Orange, Red-Violet, Blue-Violet, Blue-Green, and Yellow-Green.  

   The Photoreceptive cells of the retina (cones and rods), are connected to bipolar cells, in which the receptor cells induce graded potentials.   The bipolar cells in turn induce action potentials in the ganglion cells that actually send axons into the brain.   Axons of these ganglion cells leave the retina and form the ^«Optic Nerve.^»   Just before entering the brain, the two optic nerves partly cross (forming the optic chiasm).   From there, half of the respective nerve fibers (from each eye), cross such that each visual half-field will be distributed to the opposite hemisphere of the brain.   Having entered the brain, the ganglion cells divide, forming a number of seperate pathways.   The largest of these is the Geniculostriate System.   The projection that goes to the Lateral Geniculate Body (LGB) or nucleus of the Thalamus, then projects to the Primary and Secondary Visual Cortex, Brodmann's areas (17, 18).

      Ganglion cells of the Geniculostriate System synapse in the LGB.   The LGB has six well-defined layers:   layers 2, 3, and 5 receive fibers from the ipsilateral eye and layers 1, 4, and 6 receive fibers from the contralateral eye.
   The six layers of the LGB can be subdivided into two groups on the basis of cell size and the kind of information they signal.   Layers 5 and 6, are known as the Magnocellular Layer, which are large cells that code information about luminance contrast.   Layers 1-4, known as the Parvocellular Layer, are small cells that code information about color.
      The inputs to these two zones are from distinctly different populations of ganglion cells and the outputs to the Visual cortex are segmented as well.   There is a zone in layer IV of the cortex that receives input only from the Parvocellular Layer, one that receives input from the Magnocellular Layer, and one that receives input from both.   This segregation continues in the Secondary Visual Cortex, Layer V_II(area 18), as well.
      It is believed that these distinct pathways form the basis of three distinct cortical-visual systems, specialized for shape, motion and depth (i.e., space) and color, respectively.  

      Most of Brodmann's area (17), the Primary Visual Cortex, is devoid of interhemispheric connections except that portion of the cortex representing the visual meridian.   This cortex represents the visual world topographically.   The motor and sensory areas for distal portions of the limbs (mostly hands and feet), also have no connection; since their essential function is to work independently of each other, connections are not necessary.