Cells in the Nervous System

Recommended Reading

Read section 8.2, Cells of the Nervous System (pp. 226-233) in Silverthorn. Focus on figure 8.5a (p. 230) to learn about the different types of glial cells.

Neurons

The figure below shows the parts of a typical neuron.  Be able to identify the terms in purple boldface in the figures below. 

• The dendrites of a neuron constitute an input region where a neuron receives synapses from other neurons.  Afferent neurons that sense touch, temperature and pain in the skin have sensory dendrites containing specialized ion channels that open in response to the particular sensory modality.  For instance the sensory dendrites of touch-sensitive neurons have mechanically-gated ion channels.
  
• The cell body is the metabolic center of the neuron.  In general neurons are large cells and some have axons that are very long, so therefore, neurons have large cell bodies.  Neurons are often actively synthesizing lots of protein so they have an extensive network of rough endoplasmic reticulum (rough ER) and a large and prominent nucleolus (the organelle inside the nucleus where ribosomal RNA is made).
  
• The axon is part of the output region of the neuron.  The axon conducts action potentials and thus contains voltage-gated ion channels in its plasma membrane. Some axons can be up to a meter long.
• Many axons are myelinated.  Myelination speeds axonal conduction.  Myelin consists of many tightly wrapped layers of plasma membranes (see the electron micrographs shown in the lecture).  Myelin forms in bundles covering short segments of the axon; the space between two bundles of myelin is called a node of Ranvier.
  
• At the axon terminal, there is a synapse with a postsynaptic neuron or other target cell.  In an electron micrograph, presynaptic terminals can be identified by the presence of synaptic vesicles.  Neurons may branch near their terminals and form synapses onto multiple postsynaptic cells.
  

The Histology Guide:  Virtual Histology Guide has an excellent slide of the spinal cord that we will look at in class.  We will focus on the large somatic motor neuron cell bodies found in the ventral horn of the spinal cord.  This tissue in this slide is stained with a basic dye that stains the rough endoplasmic reticulum and the nucleolus.

At right is high magnification view of somatic motor neuron cell bodies in the spinal cord. This tissue is stained by a method that produces a dark black stain on the myelin.  The neuronal cell bodies are the pale, tan structures, and the nucleus can be made out as a slightly lighter region within the cell body.  Inside the nucleus is the dark black nucleolus, an organelle where ribosomes are made.  

The picture below is high magnification of a longitudinal section from a peripheral nerve. The axons are running across the view from right to left.  During histological processing, the lipid is mostly dissolved, so the bundles of myelin look like empty bubbles.  When the section passes through a node of Ranvier, it will appear as a line running perpendicular to the axon.