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#20-01

Physiology Of The Human Body

The Nervous System
Organization & Properties

Membrane Potentials

ch7
p.75

All living Cell Membranes may be demonstrated to have Electrical Potentials across them. Measurements may be made by placing a fine electrode inside a Cell and in the fluid around the Cell and connecting the two through a Voltmeter.

An electrical difference or Potential can be recorded, which is usually referred to as a Resting Potential (one that exists when the Membrane is NOT stimulated or active in transmitting a DePolarization Wave).

In its resting state, the Membrane may also be said to be Polarized. The value of the resting Potential varies in different Cells, from 5 to 100 mV, and the inside of the Cell is electrically Negative to its environment.

What follows, applies to all Cells, but to Nerve and Muscle Cells in particular.


Origin Of The Resting Potential

Animal tissue and fluids have no significant supply of free Electrons, electrical charges being carried by Ions of dissolved dissociable substances.

It makes sense, then, to suggest that an unequal distribution of charges and development of an electrical difference on the two sides of a Cell Membrane is the result of an unequal distribution of Ions on the two sides of the Membrane.

If one looks at the distribution of major Ions in the fluids outside the Cell and inside the Cell, the necessary concentrations are present.

Comparison of the species of Ions between ECF and ICF shows a marked difference and show that in both compartments, Anions (negatively charged) and Cations (positively charged) balance.

There also is an obvious Diffusion Gradient for Ions that have large differences in ECF and ICF. Movement is largely prevented, by the electrical attraction of positive for negative Ions, in spite of diffusion tendencies. Thus, the diffusion gradient is balanced by a Voltage Gradient.

p.76

Measured Potentials are about 90 mV- to the exterior. The match between calculated Potential and actual value, suggests that Cloride- (CL-) is passively distributed and needs no other mechanism than diffusion and Voltage Gradients to explain its distribution.

The close agreement between calculated and actual values for K+ suggest that it may be responsible for the Resting Potential and is largely if not wholly, distributed by ElectroChemical Processes only.

The wide discrepancy between calculated and actual Potential for Na+ discourages any attempt to explain its distribution by diffusion or voltage.

The variance has to suggest, the anomalous distribution of Na+ is the result of an active process, since both voltage and diffusion must tend to move Na+ into the Cell.

Low IntraCellular Sodium is maintained by a Na+ positive Pump, utilizing metabolic energy derived from ATP, to remove Sodium actively from the Cell against gradients.

The small difference in K+ Potential may be explained by assuming K+ pumping into the Cell.

It would seem that the same pump that removes Na+ from the Cell also brings K+ into the Cell; that is, there is a coupled pump operating.

In summary, the establishment of a Resting Potential of the observed magnitude is the result of active transport of Na+ and K+, with the Membrane more permeable to K+ than to Na+.


Development Of An Action Potential

p.77

The second question to be answered is: how the all or nothing Action Potential is generated, given the origin of the Resting Potential described above. In short, how does the Membrane of an excitable Cell become DePolarized?

During an Action Potential the Membrane Potential nearly reverses itself. This suggests that there has been an increase in Membrane permeability to Na+ and that it moves into the Cell to create a positive internal charge.

An effective stimulus produces a 500 fold increase in Membrane permeability to Na+, perhaps by shutting down the Na+/K+ pump.

About a 40 fold increase in permeability to K+ also occurs but does not contribute much to the Potential change.

The relationship of Ion flow to electrical changes is presented in removal of the stimulus, the pump resumes, and the original state is restored (RePolarization).


Transmission Of An Action Potential

It is a statement of fact that, once formed, an Action Potential is relentlessly conducted along a Membrane until it reaches some terminus. In Nerve Cell Axons that do NOT have insulating Myelin Sheaths, the process occurs as follows.

An area of DePolarization lies next to a still Polarized area. Utilizing the Ion-ladden ExtraCellular Fluid (ECF) and IntraCellular Fluid (ICF), a local current flow develops from positive to negative areas.

It is as if, two tiny batteries had been connected by Ionic Flow; indeed, this phenomenon is sometimes called the Battery Effect.

The current flow is of sufficient strength to DePolarize the next segment, which develops an Action Potential that causes DePolarization, current flow, DePolarization of the next segment, and so on.

Axons are normally stimulated at one end, so the impulses is conducted in one direction. RePolarization occurs as the impulse propagates along the Axon's Membrane.

p.78

In Axons WITH Myelin Sheaths, the current flow can occur only where there are breaks in the sheaths - that is, at Nodes of Ranvier (View Image).

This results in the Action Potential being developed at the Nodes that lie 2 to 3 mm apart.

The impulse will then cover a given distance in jumps (from Node to Node, bypassing InterNodes) rather than steps, speeding its rate of passage along the Axon. This type of conduction is called Saltatory Conduction.


Basic Neuronal Properties

p.79

Neuronal Membranes have a Membrane Potential value to which the Membrane must be DePolarized to initiate an Action Potential. A stimulus, to be effective, must posses a certain strength.

The term Threshold applies to both quantities. Thresholds of Neuronal Membranes vary according to the chemical and physical environment of the Neuron and thus are not always constant.

p.81

Once initiated, an Action Potential is conducted in Decrementless fashion along a normal Axon. That is, there is no decrease in strength of the impulse.

Since the propagation of the impulse is caused by biological processes, it should not lose strength, unless something interferes with those processes.

If a stimulus can cause DePolarization of the Neuron's Membrane, the response is all or none. It is like pulling the trigger of a gun. The gun will fire if the pull is strong enough; if not, nothing happens.

Axons possess a Refractory Period, the time during which it is in the DePolarized State. The length of the Refractory Period is about 1 msec.

The Axon is ready to conduct another impulse very quickly. Many Nerve fibers show Accommodation, evidenced by a rise in the threshold of the fiber.

The fiber becomes HyperPolarized when a Threshold Voltage is applied to it, the more slowly the stimulus DePolarizes the fiber, the greater the strength of the stimulus required to initiate an Action Potential.

This phenomenon may enable the fiber to ignore stimuli that persist once the CNS has been notified of the presence of the stimulus.

Under proper circumstances, two or more stimuli that are only SubThreshold can add together to initiate an Action Potential. This is called Summation.

Temporal Summation occurs when two stimuli are applied in close succession to a single fiber. There is a Local Partial DePolarization from one stimulus that is furthered by the second stimulus.

Spatial summation occurs when two SubThreshold stimuli are applied simultaneously but at different points on a Neuron. They combine to cause DePolarization and development of an Action Potential.


Cells Of The Nervous System

The functional and structural units of the Nervous System are its Neurons. They arise from EctoDermally derived Cells - NeuroBlasts.

SpongioBlasts Cells, also derived from EcoDerm, give rise to Glial Cells that have supportive and nutritive functions. One variety of Glial Cell is of MesoDermal origin (Microglia).

Neurons
Have a variety of shapes and sizes in different parts of the Nervous System. They may be generally divided by function into: Motor, Sensory, and InterNunical Neurons.

For understanding the basic structure of Neurons in general, a Multipolar Motor and a Unipolar Sensory Neuron will be described.

A Cell Body is surrounded by a typical Plasma Membrane. The Membrane encloses the NeuroPlasma or Perikaryon, the CytoPlasm of the Cell.

The NeuroPlasm contains the usual Cellular Organelles (Golgi Bodies, Mitochondria, ER, etc.) but appears, several years after birth, to lose or develop a nonfunctioning Cell center.

This implies that, after a period of time, the Cells lose their capacity to divide Mitotically and replace lost Cells. The NeuroPlasm has its ER in the form of irregular masses of Ribosome-studded Vesicles called Nissl Bodies.

Hollow MicroTubules called NeuroTubules run through the CytoPlasm and into the Processes of the Neuron, probably helping maintain the form of the process.

The Nucleus of the Cell is surrounded by a Membrane that encloses the Karyoplasm of the Nucleus. Chromatin and large Mucleoli reside in the Nuclear Fluid.

    The Cell Body gives rise to one or more elongated processes. In a MultiPolar type of Neuron, there are clearly two Morphologically distinct types of Processes:

    1. Multiple, highly branched, short, irregular diameter, Afferent (towards Cell Body) Conducting Processes are known as Dendrites.

    2. The Axon is a long sparsely branched, regular diameter, Efferent (away from Cell Body) conducting process that commonly bears one or more sheaths - a Lamellated interrupted fatty covering called the Myelin Sheath.

The breaks in the sheath are Nodes Of Ranvier, and the segments between Nodes are designated as InterNodes. Each InterNode appears to be the product of a Glial Cell (Oligodendrocyte) in the Central Nervous System.


Glial Cells

Glial Cells outnumber Neurons by about five to one in the Nervous System. They have processes but do NOT form or conduct Nerve impulses. They possess the capacity to divide throughout life. The following are included as types of Glial Cells and their assigned functions:

  1. Astrocytes are of two types, depending on number and degree of branching of their processes:
    • Fibrous Astrocytes
        - have fewer and less branched Processes
    • ProtoPlasmic (Mossy) Astrocytes
        - have more, highly branched Processes

    Both types of Cells are believed to be the major force creating Cohesion of the Central Nervous Tissue. In other words, they hold things together and maintain the structural relationship of the Cells and their Vascular supply.

    p.83

  2. Oligodendrocytes are Myelin forming Cells of the CNS. An InterNode of CNS Myelin is the product of a single Oligodendrocyte.

  3. Ependymal Cells act as an Epithelial lining for the cavities within the CNS (Ventricles of the Brain and Central Canal of the Spinal Cord). Very small amounts of CerebroSpinal Fluid are formed by secretion by these Cells.

    Ependymal Cells form a part of the Choroid Plexuses of the Brain, wherein the vast bulk of CerebroSpinal Fluid is formed by filtration and secretion from the Blood vessels composing the Plexuses.

  4. Microglia are sometimes called Brain Macrophages. They seem to migrate into nerve tissue from the Bloodstream, perhaps being derived from Blood Cells called Monocytes.

    They come to lie around both Neurons and Fibers, and remain quiesent until there is injury or inflammation in the CNS.

    Microglia then become mobile, Phagocytic, and assume a role in cleaning up the traumatized area. They are the only MesoDermally derived Cells of the Nervous System.

  5. Satellite Cells are formed in Peripheral Ganglia and serve to support the Cell Bodies of Neurons in those Ganglia.

  6. Schwann Cells are peripheral in location and are involved in Peripheral Myelin formation and in the formation of the Neurilemma.


The Synapse

The term Synapse refers to the anatomically specialized junction between two Neurons. The end branches of a Presynaptic Neuron's Axons make contact with the Dendrites, Cell Body, or Axon of a Postsynaptic Neuron. There is no anatomical continuity between the two Neurons.

    Synapses are of two types:

    • Chemical
    • Electrical

The chemical type predominates in Mammalian Nervous Systems.


The Cranial Nerves

ch.12
p.138

The Brain gives rise to 12 pairs of Cranial Nerves, which supply Sensory and Motor Fibers to structures in the head, neck and shoulder regions. One pair does supply Viscera.

These Nerves include some with Somatic components, others with Visceral components, and still others that serve the Organs of Special Sense and are sometimes called Special Visceral Afferents.

Purely Sensory Cranial Nerves have Ganglia outside or inside the CNS that contain their Nerve Cell Bodies. Motor Cranial Nerves have their Cell Bodies in the CNS. This situation is similar to that of the Spinal Nerves with Sensory Nuclei to the right and Motor Nuclei to the left.

p.139

The individual Cranial Nerves and a brief description of each Nerve follows. The Nerves are designated by Roman numeral, by name, and by composition as being purely Sensory (S), purely Motor (M), or both (Mixed).

Pons
V. Trigeminal Nerve

- A mixed nerve with a large Sensory component, the Trigeminal supplies the Cranium and Face with Sensory Nerves and and several muscles of Mastication with Motor Nerves.

    The Sensory component has three divisions:
  • The Ophthalmic,
  • Maxillary and
  • Mandibular Nerves.
The Masseter, Pterygoids, and Temporalis Muscles are the major muscles of Mastication supplied by the Motor component of the Nerve.

Pons
VI. Abducent Nerve

- A mixed nerve, this carries Proprioceptive information from and Motor Impulses to the Lateral Rectus Muscle of the Eyeball. ( Note that three (3) Cranial Nerves - III, IV, and VI - are required to serve all Extrinsic Eye Muscles).

Pons
VII. Facial Nerve

- A mixed nerve with the Motor component predominating, the Facial supplies the muscles of facial expression with Motor fibers and serves the Anterior two thirds of the Tongue with Sensory Taste Fibers.

Lateral Pons
VIII. VestibuloCochlear Nerve

- A purely Sensory Nerve, it contains fibers from the Cochlea for the Sense of Hearing and fibers from the Vestibular Area (Semicircular Canals, Utricle, Sacculi) concerned with Balance and Equilibrial reactions.

Pons
IX. GlossoPharyngeal Nerve

- A mixed Nerve, this conveys Motor Impulses to Pharyngeal (Throat) muscles involved in Swallowing and contains Sensory Fibers for Taste from the Posterior one third (1/3) of the Tongue.

Medulla
X. Vagus Nerve

- The name means Wanderer, and the Nerve certainly does wander or send branches to nearly all Body Viscera (Internal Organs). A Mixed Nerve, it conveys Sensory and Motor Fibers to Viscera, particularly Motor Fibers to Digestive Glands, and to Smooth and Cardiac Muscle. It is the most important nerve of the ParaSympathetic division of the Autonomic Nervous System.




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