Thursday, September 23, 2010

9.6b Cerebral Cortex 2: Microstructure and Architectonics

Physician's Notebooks 9 - - See Homepage

6b. Micro-structure of Cerebral Cortex and Underlying Brain
Update 09 May 2019. The following descending column are headings in this chapter that you may use search & find or scroll down to.
The brain when sliced shows surface gray matter cortex,
Micro-structure of the Cerebral Cortex
Internal Structure and Deeper Parts of the Cerebral Hemispheres
Functional activities of the Cortex
White Matter of Brain
Wiring System involving the Cortex
Keep in mind that 2 more chapters on the cerebral cortex follow this one

The brain when sliced shows surface gray matter cortex, like the bark of a tree, covering an underlying white matter.
These are figures of, left to right, vertical (coronal slice) full-brain slices. (Visualize yourself behind a standing person and looking with x-ray vision into the back of his head to see coronal slices of this brain) The surface cover cerebral cortex gray matter (deep brown in this figure) is here exaggerated in thickness and the white matter is the lighter brown beneath the deep brown cortex. The other colored structures deep in the brain (subcortical) are the basal ganglia,; the right & left thalami (colored deep brown like cortex) form inward bulges in the floor of cerebral 3rd ventricle. (The white knobby down-pointing cavity)  
 The surface cortex is normally 1.5 to 4 mm (6 to 16 one hundredths of an inch) thick depending on area sampled. (The cortical thickness can be measured by MRI; but an abnormally thin cortex may be seen in Alzheimer disease, schizophrenia, or clinical depression while abnormally thick cortex is seen in autism) Note from the above figure the islands of various colors, which are the paired basal ganglia and thalami.
  On microscopic examination of brain slices - the cerebral cortex is seen as vertical columns of neurons connected by their fibers in a typical wiring circuit further described in the below section.
Micro-structure of the Cerebral Cortex (Use magnifying glass for inspection)

A micro view of vertical columns from the cerebral cortex surface (top) to deepest cerebral cortex. In the white space on your left you see Roman numbering of the horizontal layers. Below, under the lowest horizontal line, you see A, B, C, D for the vertical columns that are drawn to show a particular type of neuron. Columns A & B are drawn to show large and small cortical neurons. In column C, along its right vertical, are Arabic numeral subcategory horizontal layers from 1at top to 6bat bottom of cortex which is stained black for the connecting fibers and note the high numbers of horizontal-running fibers in layers 1a, 4, 5b, 6a and lower. The vertical columns are the basic units of cortex I call modules. All neurons in one such tall thin column are connected in a single micro function. Incoming signals enter in the fibers at topmost horizontal layer I and in each column pass downward via fibers to individual neurons and exit from bottom layer, VI, as output. Input from and to the Thalamus comes directly from it to layer IV. Much input and output to the cerebral cortex comes from the same-side thalamus and much of the output-input passes signals to and back via the thalamus from and to the basal ganglia and cerebellum.  In column D on your most right side, stained for neuron and fiber connections, are neuron-to-neuron networks.
Internal Structure and Deeper Parts of the Cerebral Hemispheres: The cerebral cortex is the seat of thought. The micro architectonics are vertical columns (the modules) of neurons connected with other neurons in the same column, and each column receiving input and output from and to other parts of cortex, mainly via the thalami, from and to subcortical nuclei and from and to the cerebellum. These columns are functional units. Examples: within the sensory cortex of the parietal lobe, the neurons in a column are activated by a single type of sensory receptor from a point surface on the body in the skin. Or in the visual (occipital lobe) cortex all the neurons within a column receive input from the same visual cell in the eye retina and thus transmit the same part of the external visual world. Still in visual cortex, some columns transmit color only, others form, others motion; and all the data get synthesized into the image in your conscious mind. On microscopy each column is structured in horizontal layers, from layer I on the surface to deepest layer VI bordering the underlying white matter. The flow of information signals is input into Layer I or Layer IV and output from Layer VI with much back and forth signaling in the in-between layers. Certain layers receive only from the thalami.
Functional activities of the Cortex- such as vision or motor control or perception of sensation are localized to cortex surface areas on macroscopic (naked eye) level, and the qualities of these functions are further differentiated on microscopic level within functional segments based on micro modules of vertical cortex. Neurons in the cortical units intercommunicate, and receive signals that are parsed and passed through the thalami. The current of flow in the signals comes in through fibers running tangentially along the top of the cortex (Layer I) or from deep in the hemisphere (to Layer IV). Then the cortical neurons transmit impulses downward (efferent signals to effect actions) on output fibers into the white matter to connect with subcortical, spinal and periphery.

White Matter of Brain: If we peel away the cortex, we see beneath it the white matter in subcortex that makes up much of Brain. Under microscope the white color is explained by an absence of neurons and by the myelin sheaths of the nerve fibers. Recall that every neuron in the nervous system is in communication with other neurons so where there are concentrations of many neurons, as in cerebral cortex and other gray matter, there must be concentrations of their fibers as in the sub-cortical white matter.
By use of special dyes and other micro technique the white matter nerve fiber mass can be seen to be separate bundles of single neuron fibers united as fiber tracts. Three types of fiber tracts are classified by distance covered and direction of communication. Shortest tracts connect neurons from nearby cerebral cortical columns to each other. These are thinnest. Then the long tracts: One type is transverse (sideways between right and left brain) tracts known as commissures (e.g., the corpus callosum that communicates between left and right cerebral cortex brain). Another type fiber bundle runs up and down, signaling either up or down between brain and peripheral parts of nervous system and body, and is called long tract. As the fibers descend, they form long tracts easily identified by dye staining. Most famous is the Pyramidal Corticospinal Tract from motor cortex in left or right rear frontal lobe cerebral cortex sending signals down and crossing over to opposite side spinal cord levels and then, with 1 or 2 spinal neuron synapse connections, to voluntary muscles. Some other long up-down tracts are sensory (sending signals up) and cerebellar (sending signals both up and down) tracts. These tracts bring together similar function nerve impulse fibers from particular areas or of particular function in Brain and can be traced through mid brain, lower brain and spinal cord where individual fibers are parceled at all levels and then run in peripheral nerves that get ever smaller until they reach destinations.
It is important to appreciate the significance of the gatherings of neuron transmission fibers (axons) in long tracts because it explains symptoms of brain strokes. Especially in brain stem and upper spinal cord, the long tracts become concentrated together and a small area of damage from hemorrhage or tumor compression can, just like an electrical interruption in your car, disable your body, as in brain stroke. Also this knowledge is important to neurosurgeon treating chronic pain by making cuts in sensory tracts. Finally, as will be considered, is the question of how the neurons send out these millions of microfibers to connect with distal targets during embryo, fetal and childhood development and the affects on intelligence and psychological make up when particular neuron fibers miss or get misdirected to targets . 
Wiring System involving the Cortex: The neurons in the cerebral cortex are at the top of the CNS hierarchy. They receive input of the sensory systems (touch, vision, hearing, balance, etc.) by a connection that starts at the peripheral sensory organs or cells, passes the signal up the spinal cord and also into the cranial nerves and eventually arrives at the thalamus. These sensory signals pass through several neurons as electrical impulses in neuron fibers. For example with touch, the signal starts in the skin, runs in the sensory nerve to the spinal cord at the particular level of the body and passes to the 2nd neuron in the cord and then the signal runs up the cord, usually crossing to the opposite side and connects with a 3rd neuron in the lower brain and then with a 4th neuron in the thalamus. The thalamus then parcels out the sensory signal to cortex in the particular body locations. Each sense (touch, vision, hearing) has its own separate pathway and there is no integration until the level of the thalamus. This parallel system is called labeled lines in that you may visualize one line of impulse carrying a labeled body sense (touch, heat) from the surface skin to the thalamus just below the cerebral cortex. The thalamus acts as a computerized switchboard and with sensory signals integrates them as needed. For example, if you decide to touch the back of your right hand with your left index finger, your thalamus passes the vision of the back of the right hand you will touch to the occipital lobe cortex neurons (from the eye retina optic nerve branch) to make a conscious image of the location, sends signals to the motor and parietal cortices to activate your left hand to touch the back of right hand, sends other signals to the cerebellum giving it the measures of what you plan to do, and then, as you touch back of your right hand with the finger, the thalamus passes the touch sensation to the neurons of consciousness. This is just one of many examples and a great simplification but gives some idea of thalamus integration. Also it means that feelings on the surface of our body can as well come from electrical stimulation of a labeled line nerve as from the actually touch or other stimulus the feeling originates from.
Internal body state input also sends its labeled lines from the viscera and internal tissues via the autonomic-nerves system in 3 or 4 neuron synapse relays up the spinal cord and into mid brain and then into thalamus where the signals are parceled out to cerebral cortex areas as necessary like in the sensory system. 
 And the third part in the wiring system, cognition, originates in the pre-frontal lobe cerebral cortex where executive decisions are made based sensory input and memories of past acts and sent to the motor cortex in the rear frontal lobe.
 The motor system is the main output in the wiring system - motor programs are activated integrated into our cognition that cause us to behave in a way that promotes our best survival and reproduction. This recapitulates the wiring system that involves 3 main parts - sensory, internal body state, and cognition - and feeds forward (or feeds back in case of reflex response) to the main output, the motor cortex and nerves. In this simplified description I have not mentioned connections to other subcortical areas like the basal ganglia and the amygdala, which are much involved in modifying our behavior and, separately, emotion to it. These are dealt with in other sections.
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