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Head and Neck Anatomy: Part III – Cranial Nerves

Course Number: 598


Figure 1. Motor Neuron

Figure 1. Motor Neuron

Neurons are the working cells in the nervous system. They are metabolically very active cells and the vast majority of them are located in the central nervous system (CNS). The brain uses energy at a prodigious rate. Anyone who has taken a CPR course knows that in just minutes the brain runs out of oxygen for proper metabolism and the cells start to die off. To understand the structure of nerves we must look at these unique cells that are so important to our survival.

While there are differences in morphology between neurons whose functions are different, they all have some commonalities. They all have a central area known as the body which contains a nucleus and numerous organelles that produce energy and make proteins.

Projecting off the body are processes that extend away from the body. There are two types of these appendages: axons of which each neuron has exactly one and dendrites which are found in variable numbers from none to several depending on the function of the neuron.

Neuronal membranes carry an electrical charge based on the ionic movement across the membrane. By alternating between depolarizing and repolarizing the membrane an electric wave is created along the neuron. Directionally this wave travels from the distal end of the dendrites, through the cell body, and then passes down the axon to its terminus. If the signal is strong enough, it will cause a release of proteins known as neurotransmitters from the end of the axon into the space between the neuronal ends which is called a synapse. The electrical signal down the axon is known as the action potential and if the voltage of the action potential reaches the point at which it causes release of neurotransmitters it is said to have reached the threshold potential.

Figure 2. Action potential

Figure 2 - Action potential

You can see why the neuron has such high metabolic requirements as it has to pump ions across the membrane against the concentration gradient. It also has to produce proteins for release and transport them down the axon in addition to removing the neurotransmitter from the synapse quickly so that it does not linger in the synapse and cause unwanted effects.