9.3 Secrets of the Brain tissues - A Starter

Physician's Notebooks 9 - See Homepage - http://physiciansnotebook.blogspot.com

Update 03 Septr 2021
3. The Tissue of the Central Nervous System (The following column shows the important headings in order of appearance in the text. Use search & find  or scroll down to find.)

A main Function of the nervous system

The non-neural tissue of the CNS

The nerve cells, neurons,

Nerve transmission

Myelin cover of Nerve Fibers

Important diseases of Myelin Cover, are demyelinating diseases
Final Summary of Secrets of the Brain Tissues

A main Function of the nervous system is signaling for the purposes of action, sensation and thinking. The signals run between cells in brain & spinal cord and the epithelial sensory cells in the periphery and also to the cells of muscles and glands. And the signaling results in motor output & action, sensory input, and consciousness & thinking. This is controlled by neural tissue whose cells are the neurons all of which are interconnected with other neurons by connecting fibers -- axons -- that carry the signals forward from the transmitting neuron and with more distal fibers -- dendrites  -- that carry signals into the receiving neuron.

The basic cell of the nervous system is the neuron. It is an offshoot of the ectoderm (surface layer skin) under the stimulus of developmental chemicals in the early embryo.
The Neuron in Schematic View (Below): This shows a neuron whose axon stimulates a skeletal muscle unit (a motoneuron) and it has the typical structure of all neurons. Note at the bottom part of the Figure the single axon that carries signals or commands from the neuron and is sheathed in segments by myelin and in the upper part you can see the terminal segments of four dendrites that receive signals and transmit them into their neuron cell body, or soma.

 Neurons, are often found in groups that may be called "ganglia" (singular, ganglion), e.g., basal ganglia; or also may be called nuclei (singular, nucleus, e.g., basal nuclei) or “body” or by a distinctive name (e.g., the thalamus). 

   Like all cells, a neuron has an outer cell membrane that encloses its cytoplasm contents and a nucleus (at the cell's center) that contains the DNA that is controlling the cell. A neuron grows out its own fibers, which are extensions of the outer cell membrane, and the cell fluid content continues into each fiber core. A neuron may have many nerve fiber extensions. The purpose of the fiber is to transmit the signals from neuron to other neurons or, in opposite direction, via peripheral receptors, to receive and/or transmit the data it receives from the periphery. (The periphery is the outside world and the rest of the body) Nerve fibers may be long or short. A neuronal fiber that transmits a signal outwardly away from its neuron is called an axon. A neuron has only one axon but that axon may form branches. A neuron's receiving fibers are called dendrites. The dendrites are many per neuron and thinner than axons and extend shorter distances out from its neuron cell body than its axon. The dendrites receive signals from axons or cell bodies of other neurons into their neuron. 

In most cases of nerve transmission, the signals are passed out from a neuron cell body through its axon (or, for reception, into and through its dendrite) to another neuron cell body that relays the signal forward (feed forward) or backward (feedback) to one of its dendrites and the dendrite passes the signal back to its neuron cell body. Note a basic rule of transmission is that signals are sent out by the neuron cell body into its axon and then connected by synapse to other neurons via dendrites or directly on to the cell body. In axons there is normally only one direction, which is forward out of the cell body. The passage-relays of the signals from axon to dendrite or directly to neuron cell body are through the synapse, a switchboard-like connection. (Shown in a later chapter)

                          

Diagram Examples of Neurons

A.    Neuron in cerebral cortex. The cell body has incoming signal dendrites at up-, down- and reader's  left-side apexes and an outgoing signal axon fiber on reader's right.

B.    Pseudo-unipolar neuron (most neurons are bi- or multi-polar, meaning 2 or more nerve fibers exit or enter; a few are unipolar; the "pseudo" means the neuron was basically bipolar but developed its unique T-shape unipolar function later) in a sensory spinal dorsal root ganglion. The cell body has a single main fiber coming out of it and the fiber has T-shaped limbs, one limb transmitting signal in from periphery and the other limb passing the signal toward the center. This pseudounipolar structure of transmission avoids using a synapse, and only occurs in neurons of spinal cord dorsal root ganglia.

The non-neural tissue of the CNS: are the Glia, made of glue-like connecting cells. They make the myelin covers of nerve fibers, assist nerve cells during development and healing, help feed the neurons and transmission & reception of signals by the neurons. In contrast to neurons, they can reproduce and are the cause of the brain tumors. The glia and neurons have the same cell origin in the embryo - the primordial ectoderm under a chemical stimulation to develop in the embryo. 

Myelin cover of Nerve Fibers in Peripheral Nerve is made by glia called the Schwann Cells. In the Central Nervous System (CNS), other type glia called Oligodrendroglia perform this function.

In the above diagram of Myelin-covered Peripheral Nerve Fiber (Axon), note the single Schwann Cell and its layered wrapping and the segment constrictions.

These glia cells produce the wrappings around nerve fibers that form the myelin cover that gives white color to a nerve fiber; they make the “white matter” of the CNS. The myelin wrap not only insulates but also by its structure it speeds nerve transmission by electronic "jump" between successive nodes. 

   The myelination of nerve fibers is not completed until age 20s and that is a reason teenagers and adolescents do not have the advantage of the full brain power of a young adult.

   Important diseases of Myelin Cover, are demyelinating diseases, in which the problem is destruction of the myelin cover after it has formed. Most important are multiple sclerosis in CNS and the Guillain-Barre syndrome in the peripheral nervous system, the disease of the late novelist of Catch-22 fame, Joseph Heller.

Final Summary of Secrets of the Brain Tissues
Visualize the nervous system in a human body with the brain hemispheres inside the head, the mid-brain a little above the neck and the spinal cord hanging down in the mid-line back. And further visualize the nervous system as being blown up like a somewhat expanded balloon in the above-given actual shapes with the nervous system central cavities, the ventricles as its core.
   With that in mind, the CNS tissues can be looked at from a outside-to-inside, rostral-caudal (head to tail) aspect. Starting at the top of the head, in the brain, from outside to inside we have the cerebral cortex, a 6-cell layer, several millimeter thin tissue, like a bark of a tree trunk, that covers the surface of the brain hemispheres. Then, getting inside just deep to the cerebral cortex is the subcortex that looks mostly white sprinkled with with gray neuronal nuclei. The neuronal nuclei of the subcortex has important neuron groups like basal ganglia, thalamus and hypothalamus, and amygdala surrounded by the sea of white matter nerve fibers that are running between the cortex and lower brain and CNS structures. Then, the deepest level tissue is the ventricles, the core of the CNS producing fluid that cushions and provides nourishment to the brain and, having in its lining, the potential source of new nervous tissue stem cells in lower animals and, maybe in humans too. Then as we continue downward in rostro-caudal direction comes the brain stem composed mostly of the pons and medulla with the cerebellum piggy-backed on and behind the lower pons, and then the spinal cord.
   Functionally, the reader will come across the terms decorticate, decerebrate, mid-brain and spinal preparations. These are based on lower-animal experiment where, first, the cerebral cortex has been literally peeled off the living brain like peeling bark from the tree trunk (decorticate preparation) in one group, and, second, in another group, the main brain globes (cerebrum) have been separated from the below CNS (decerebrate preparation). A 3rd group that only consists of animals without cerebral cortex and without subcortex, the unctions from the brain stem or spinal cord down. The way such animals are kept alive and how each group functions has told neuroscientists much about the levels of the nervous system.

Decorticate

Decorticate posturing, with elbows, wrists and fingers flexed, and legs extended and rotated inward

Decerebrate rigidity or abnormal extensor posturing.

Finally to mention, the nervous system (brain and spinal cord are the CNS, or central nervous system; and the PNS, or peripheral nervous system of body nerves outside the spinal cord and brain) can be considered in an hierarchical way starting chemically with the molecules of neurotransmitters; then subcellularly with the inside-cell structures like mitochondria, golgi apparatus, etc.; then cells like neurons and glia, then as connective tissue of groups of cells like the synapses and nerve fibers; then as tissues that are cells grouped into nuclei and ganglia like the Thalamus, and also like subcortex; and then, finally, as parts - the brain, the spinal cord, the peripheral nerves.  
  In the chapters to follow in Notebooks 9, keeping these above concepts in mind, and occasionally re-reading here, without trying to memorize them, will be helpful, with the use of Wikipedia (or if you are attending courses), to help the reader's understanding.

                            END OF CHAPTER
To read next click 9.4 Central Nervous System Overview