The Human Nervous System

WebLecture Topics

Nerve Systems
Nerve Cells: Neurons
Nerve Signals

Nerve Systems

Nerve systems are peculiar to the animal kingdom, and increase in nerve system complexity parallels increasing complexity in digestive and circulatory systems. Single-celled proteins to have no sensory or nerve-type processes, although the cell as a whole may react to light or chemical stimulus. Sponges have no nerve cells. Cnidaria have only simple nerve systems or neural nets, which allow them to coordinate tentacle movement in response to changes in water pressure that could signal the approach of prey. Flatworms such as the planarian have eye spots that can detect light. Earthworms have central nerve cords that extend through the length of the worm, allowing the individual segments to communicate with each other.

In more advanced organisms, nerve systems consist of individual cells joined together in networks or clusters to form systems of communication. In humans, there are two types of nerve cells: neurons and glial cells. Neurons produce and transmit electrical signals called nerve impulses, and glial (sometimes called neuroglia) cells support neurons by maintaining homeostasis, insulation from other neurons, producing myelin, and providing nutrients and oxygen to neurons.

We talk about the human nerve systems in two ways, one anatomical (identifying the structures) and one physiological (identifying the functions). Structurally, the central nervous system or CNS contains the brain and spinal cord (when present in vertebrates), while the peripheral nervous system or PNS contains all the sensory receptors and connecting nerves and neuron cell bodies or ganglia. Functionally, we divide the nervous system into those parasympathetic functions which support autonomic functions like breathing and digestion, and those sympathetic functions that redirect energy for flight-or-fight situations, suppressing digestion and stimulating circulation systems. Each of the parasympathetic and sympathetic functions works through both CNS and PNS components.

Nerve Cells: Neurons

Neurons are cells that respond to electron stimulus. The most common type of neuron is a multipolar neuron, consisting of cell body with radiating dendrites, and an axon terminating in synaptic knobs that contain vesicles which can excrete neurotransmitters.

Cell body: this contains the bulk of the cytoplasm, the nucleus, but of course the other organelles such as the mitochondria, which perform the same function here as they do in other cells of releasing energy.
Dendrite: these are short highly, branched fibers which extend outward in all directions from the cell body to receive incoming stimuli.
Nucleus: as in other types of cells, the nucleus contains the DNA required for cell cloning.
Axon: The axon is a single fiber extending from the cell body for up to a meter, and ending in the synaptic knobs.
Schwann cell: these glial cells surround the axon like beads around a string
Myelin sheath: a white, fatty substance forming the membrane of the Schwann cell, and acting as an electrical insulator.
Node of Ranvier: Exposed areas of the axon between Schwann cells
Synaptic Knobs: Bulky ends of the axon containing vesicles that produce neurotransmitters.

Besides the common multipolar neuron with many dendrites (diagrammed above), there are bipolar neurons which have only one dendrite opposite the axon; Betz cells or large motor neurons; pyramidal cells with triangular cell bodies, and Renshaw cells, which connect alpha motor neurons. Nerve tissues made of these cells are specialized to perform specific functions most efficiently.

External Website Optional Reading: There is more information on the structure of neurons and their assembly into nervous systems and the Human Nervous System site, and somewhat more detail (along with excellent diagrams) at the website for Jaakko Malmivuo's book on Bioelectromagnetism. Click on Chapter 2, Nerve and Muscle Cells, to see the diagrams.

Nerve Signals

To understand how nerve cells work, we need to review some basic physics, electrical theory. There are two kinds of charge, positive and negative. Objects with like charges repel one another, while objects with opposite charges attract one another.

You recall that atoms in a neutral state consistent equal number of negatively-charged electrons and positively-charged protons. If the atom gains electrons (net negative charge) or loses electrons (net positive charge), it becomes an ion. [Atoms that gain or lose positive charges through nuclear reactions such as radioactivity, fission, or fusion, change their element type and give off lots of energy; atoms that Dean or lose neutrons change their mass and become a different isotope of the same element. At the moment, we are only concerned with ions and electrical charge changes.]

In most forms of matter, the positive charges in the nucleus of the atoms are not free to move, since the atoms are held in place by chemical bonds to other atoms forming molecules, or by intermolecular forces, and the positive-charge-bearing protons are stuck in the nucleus of the atom. In solutions, however, such as cytoplasm and the human cell, ions of either charge will move according to the basic rules of charge behavior: like repels like and opposites attract. If we bring a positively charged object near a neutrally charged object, the positive charge will attract any negative charges that are free to move in the second object and repel it any positive charges that are free to move, creating a local net negative charge area. This is called induction, and is the basis of how neurons work.

Now we put the electrical theories together with what we know about how cells work. The lipid bilayer of most membranes repels ions, so the only way that they can cross the bilayer is through a protein gate, either by passive transport from an area of high concentration to an area of low concentration, or by active transport from an area of low concentration to an area of high concentration.

Nerve impulse process

Normally, the outer material (extracellular fluid) is more positive than the inside of the neuron, which has a resting potential about 70mV. A sodium-potassium pump keeps the differential in place by pumping 3 sodium ions out for every 2 potassium ions it lets in.
When an impulse occurs, the so sodium gate opens to allow more sodium in. This raises the potential in the cell. If it reaches +35 mV, the potassium gates open topic potassium out.
The local disturbance creates a local excess concentration of sodium inside and potassium outside, so neighboring potassium gates open, and the cycle is repeated. The "signal" or disturbance travels across the surface of the neuron membrane until it reaches the axon, then travels down the axon.
Meanwhile, the site of the original impulse recovers, pumping potassium and sodium out until normal resting potential is restored.

Harvard medical course has an excellent animation on how action potentials create nerve signals, so I'm going to send you there.

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