The nervous system, the intricate paths

Delimitation cycle

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When he gave his « I have a dream » speech in 1963, Martin Luther King was not to imagine that 45 years later, a black man would be President of the United States. This has not, however, changed the discrimination that some people have towards people who are different from themselves. Yet the strength of humanity is in the diversity of human beings that compose it.

Life will never be anything other than what we do with our lives.
« Oh my God! » by Tanya Wexler

The nervous system compiles information of a different nature in order to generate the best action that will maintain the integrity of the human body:

Always in threes, information travels

The structure of neurons -nerve cells- reflects its functional characteristics.

The neuron carries efferent information through an action potential:

  • Dendrites capture information from the external environment and propagate it to the cell body;
  • The cell body or pericaryon, soma, contains the nucleus, a single axon leaves it;
  • The axon carries information to target cells or another neuron, where it will pass it through specialized junctions called synapses.

Although the human nervous system has over 10 billion neurons (a more than rough estimate), they can nevertheless be classified into one of three functional types:

  • Motor neurons: carry efferent impulse from the central nervous system (CNS) or ganglia (a group of cell bodies of neurons outside of the CNS) to target cells (effectors).
    – Somatic efferent axons target skeletal muscle,
    – Vegetative efferent axons (visceral) target smooth muscle, heart muscle and glands.
  • Sensory neurons: carry related information from receptors to the CNS.
    – Afferent somatic axons carry pain, temperature, tactile sensations, pressure and proprioception (unconscious),
    – Vegetative (visceral) axons carry pain and other sensitivities (such as nausea) from the viscera, glands and smooth muscle to the CNS.
  • Interneurons: transmit information between sensory and motor neurons, thus forming an integrated network between cells. They probably make up over 99% of all neurons in the body.

In the CNS, axons are myelinated by an oligodendrocyte (a glial cell or neuroglia) while axons in the peripheral nervous system (PNS) are surrounded by a Schwann cell (a glial cell), without all being myelinated, but when they are, it is the Schwann cell that myelinates them.

Two signals for orientation

The nervous system is made up of two anatomical parts with specific roles:

  • The central nervous system (CNS): the brain and the spinal cord, it is the center of regulation and integration;
  • The Peripheral Nervous System (PNS): Cranial nerves, spinal nerves, and ganglia, these are the lines of communication between the CNS and the body. It splits into two ways:
    – The sensitive pathway (afferent) which contains somatic and vegetative neurofibers (axons and dendrites) and propagates nerve impulses from receptors to the CNS,
    – The motor pathway (efferent) which contains somatic and vegetative neurofibers and propagates nerve impulses from the CNS to the effectors (muscles and glands).
    The motor pathway will then divide into two with the somatic nervous system and the autonomic nervous system
    which in turn will split into two with the sympathetic nervous system and the parasympathetic nervous system.

Within the CNS, there is a fundamental binary distinction, concerning the distribution of synapses.

The gray matter:

  • It is a region where synapses (interneuronal connections) are established;
  • It contains all cell bodies, all CNS synapses, and capillaries;
  • The cells of the neuroglia (the supporting and protective cells of neurons) are: astrocytes, oligodendrocytes, microglia.

The white matter:

  • It is a region devoid of synapses;
  • It consists mainly of bundles of myelinated axons (grouping of bundles),
  • The cells of the neuroglia are the oligodendrocytes for the CNS; and Schwann cells for SNP.

The distribution of gray matter and white matter differs between the brain and the spinal cord:

  • In the brain, the gray matter is peripheral, it defines a cortex around the central white matter.
  • In the spinal cord, the gray matter and the cell bodies of the neurons that correspond to it are located in the central part of the spinal cord, where they describe a butterfly or H-shape that can be distinguished from the white matter around.

The SNP is made up of 31 spinal nerves that emerge from the spinal cord at the level of the vertebrae.

There are 8 cervical pairs, 12 thoracic pairs, 5 lumbar pairs and a coccygeal pair.

Each spinal nerve is formed:

  • From a ventral root: for the motor skills of the limbs,
  • From a dorsal root: for the sensitivity of the limbs.

The ventral branches converge to form plexuses.

Crosses to balance

A plexus is not much different from a large network of different rail tracks that connect into a main track such as a rail yard.

The nerve plexus is a mixture of nerve fibers originating from spinal nerves of adjacent levels, which ultimately distributes itself into several « terminal » nerve branches, which reach the peripheral regions and innervate its skeletal muscles, joints, and skin.

Nerve fibers are of three types:

  • Somatic motor nerve fibers to innervate skeletal muscles;
  • Sympathetic postganglionic nerve fibers intended to innervate the smooth muscle of hair follicles, vessels and sweat glands;
  • Sensitive nerve fibers to give the central nervous system information from the outside world, captured at the level of the skin.

Here is an example of the description of the brachial plexus formed by the ventral branches of the C5-T1 spinal nerves:

  • The roots: The five ventral branches of C5-T1 constitute the “roots” of the plexus;
  • The trunks: The five roots reorganize into three upper, middle and lower trunks which engage under the collarbone in front of the first rib;
  • The divisions: Each trunk gives two divisions: an anterior division and a posterior division, and therefore six divisions in total;
  • The cords: the three posterior divisions combine to form the posterior cord, the medial and lateral cord are made up of the combinations of the anterior divisions;
  • Terminal branches: The plexus gives rise to five large branches that innervate the shoulder, arm, forearm and hand.

A number of small nerves arise from this organization (5-3-2-3-5) to innervate some muscles of the back, and of the anterior and lateral region of the rib cage.

Let’s use the foundation offered by a triangular base and the amplitude of binary complementarity in order to experience common happiness:

If the human body is able to coordinate all the information
that these sensors send him to take actions, without losing his balance,
by alternating a base containing 3 types of data and binary limits;

We can define scales at three levels for observing the world
{Global – National – Individual} {Private – Professional – Citizen};
and define binary signals to guide our common choices
{war – peace} {freedom – comfort};

Let’s try to keep the world in balance,
let’s decide together the vital signals to live in harmony.

Bibliographic references:
The information on the nervous system ies taken from:
– the book « The Netter’s Anatomy Coloring Book », John T. HANSE, Elsevier Masson SAS, 2017 (the french version),
– the first year medical course.
The picture of the neuron is taken from the book « The Netter’s Anatomy Coloring Book », John T. HANSE, Independently published, 2019 (the french version).
The picture of the brachial plexus is taken from the book « Atlas of Human Anatomy », Seventh edition, Franck H. NETTER, Elsevier Masson SAS, eBook.

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