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.
Development Of An Action Potential
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).
Cells Of The Nervous System
The functional and structural units of the Nervous System are its Neurons. They arise from EctoDermally derived Cells - NeuroBlasts.
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 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.
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.
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).
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