2.10: Microanatomy of the Nervous System

Lab Summary: In this lab, you will learn about nervous system histology and review the physiology of nervous system structures.

Your objectives for this lab are:

• In a neuron/ nervous tissue smear slide, identify and describe the function(s) of o Neurons, and its
  • Cell body/soma o Nucleus
  • Axon
  • Dendrite
  • Axon hillock
• In a peripheral nerve longitudinal section slide, identify and describe the function(s) of
  • Myelin sheath
  • Schwann cell
  • Neurilemma
  • Node of Ranvier
• In a peripheral nerve cross section slide, identify and describe the function(s) of o Epineurium
  • Fascicle
  • Perineurium o Myelin sheath o Neuron
  • Satellite cells
• In a spinal cord slide, identify and describe the function(s) of o Central canal
  • Dorsal horn
  • Dorsal root ganglion o Neuron
  • Satellite cell
  • Ventral horn
  • White matter
• Describe the function of the glial cells, including astrocytes, ependymal cells, Schwann cells, oligodendrocytes, and satellite cells

Background

Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.

The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else (Figure \(\PageIndex{2}\)). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities.

Activity 12.1: Neurons and Glial Cells in Histology Slides

Neurons

Neurons are the cells considered to be the basis of nervous tissue. They are responsible for transmitting the electrical signals, called action potentials, that communicate information about sensations, that produce movements in response to those stimuli, and inducing thought processes within the brain. An important part of the function of neurons is in their structure. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.

Parts of a Neuron

The neuron’s cell body, or soma, contains the nucleus and most of the major organelles. Neurons are usually described as having one, and only one, axon—a fiber that emerges from the cell body and projects to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are dendrites, which receive information from other neurons at specialized areas of contact

called synapses. Dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity—meaning that information flows in this one
direction. Figure \(\PageIndex{1}\) shows the relationship of these parts to one another.

Where the axon emerges from the cell body, there is a special region referred to as the axon hillock, a tapering of the cell body toward the axon fiber. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm.

Many axons are wrapped by an insulating substance called myelin, which is actually made from glial cells. Myelin acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an axon, called Nodes of Ranvier, and they are important to the way that electrical signals travel down the axon. At the end of the axon is the axon terminal, where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a synaptic end bulb. These bulbs are what make the connection with the target cell at the synapse.

Figure \(\PageIndex{1}\): Parts of a Neuron Major parts of the neuron are labeled on a multipolar neuron from the CNS.

Glial Cells

Just like in a movie or television show, neurons are the “stars”, who are supported by other essential players. In the nervous system, these essential helper cells that allow neurons to perform their functions and remain functioning normally are called glial cells, or neuroglia. Today, continuing research into nervous tissue has shown that there are many deeper roles that these cells play. There are six main types of glial cells: four in the CNS and two in the PNS (Figure \(\PageIndex{2}\), Figure \(\PageIndex{3}\))

Glial Cells of the CNS

Astrocytes: They are so named because they appear to be star-shaped under the microscope (astro- = “star”). Most abundant, versatile, and highly branched glial cells. These cells provide many essential support functions for neurons including,

• Cling to neurons, synaptic endings, and capillaries
• Physically support and brace neurons (“scaffolding” for neurons within nervous tissue)
• Control the chemical environment as part of the Blood Brain Barrier (BBB)
• Guide and support new neurons during nervous system development
• Serve as antigen-presenting cells to assist the nervous system in detection of and protection from disease causing agents
• Participate in information processing in the brain
• Play a role in protecting neurons from death by releasing neuronal growth factors

Oligodendrocytes: The name means “cell of a few branches” (oligo- = “few”; dendro- = “branches”; -cyte = “cell”). One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons. Myelin is a lipid-rich sheath that surrounds the axon, and by doing so, it creates
a sheath that facilitates the rapid transmission of electrical signals along the axon. Myelin can enhance the speed of action potential propagation up to 1000X normal speed.

Microglia: These are smaller than the other glial cells. Ongoing research (although not entirely conclusive), suggests that they may originate as white blood cells, called macrophages, and become part of the CNS during early development. Similar to macrophages, microglia in normal healthy tissue ingest and digest diseased cells, damaged cells, or the pathogens that cause disease. Therefore, they are sometimes referred to as CNS-resident macrophages.

Ependymal cells: These cells synthesize and filter cerebrospinal fluid (CSF), the fluid that circulates through the CNS. Ependymal cells line each ventricle, one of four central cavities that are remnants of the hollow center of the neural tube formed during the embryonic development of the brain. The choroid plexus is a specialized structure in the ventricles where ependymal cells come in contact with blood vessels and filter and absorb components of the blood to produce cerebrospinal fluid. Because of this, ependymal cells can be considered a component of the BBB, or a place where the BBB breaks down. They also have cilia on their apical surface to help move the CSF through the ventricular space.

Figure \(\PageIndex{2}\): Glial Cells of the CNS The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the CNS in several ways. It is not possible to differentiate between the neuroglial cells in the micrograph on the right.

Glial Cells of the PNS

The PNS contains two types of glial cells: satellite cells and Schwann cells. 

Satellite cells:  These cells are found in sensory and autonomic ganglia, where they surround the cell bodies of neurons.  They provide support, performing similar functions in the periphery as astrocytes do in the CNS—except, of course, for establishing the BBB.  “[These cells] are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain (Hanani M. , 2005).

Schwann cells:  These cells create myelin for PNS neurons to insulate them from damage and enhance the speed of action potential propagation.  Schwann cells are different from oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others. The relationship of these two types of glial cells to ganglia and nerves in the PNS is seen in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Glial Cells of the PNS The PNS has satellite cells and Schwann cells.

Procedure for Activity 12.1:  Neurons and Glial Cells in Histology Slides

You will now use the information you have read and the figures above to locate, identify, and describe functions of particular features in nervous system slides. 

Procedure:

1. Obtain a slide of motor neurons or nervous tissue smear. 
2. View the slide using the 10x or 40x objective, as directed by your instructor.  Use the information above to identify the structures listed in the objectives and describe their functions.   In the space below, draw a neuron seen in your slide.  Label the soma, nucleus, axon, dendrite, axon hillock, and a glial cell.

Activity 12.2:  The Spinal Cord in Histology Slides

At this point, we will look at the histology of the spinal cord.  In a future lab, you will examine the macroscopic structures of the brain and spinal cord in greater detail.

As you have read previously, the central nervous system consists of the brain and the spinal cord.  The spinal cord, which carries sensory information to the brain and conveys motor information from the brain, is continuous with the brain stem and begins at the foramen magnum of the occipital bone.  As the spinal cord continues to develop in the newborn, anatomical features mark its surface. The anterior midline is marked by the anterior median fissure, and the posterior midline is marked by the posterior median sulcus.  Axons enter the posterior side through the dorsal root, which marks the posterolateral sulcus on either side. The dorsal root carries sensory information to the spinal cord from the rest of the body.  Before sensory neurons reach the dorsal, they synapse in the dorsal root ganglion.  The axons emerging from the anterior side of the spinal cord do so through the ventral root, which carries motor information out to the spinal nerves.

In cross-section, the gray matter of the spinal cord has the appearance of an ink-blot test (or a butterfly, say some) with the spread of the gray matter on one side replicated on the other.  As shown in Figure 12.4a, the gray matter is subdivided into regions that are referred to as horns. The dorsal horn is responsible for processing sensory information and directing it to the brain. The ventral horn sends out motor signals to the skeletal muscles. The lateral horn, which is only found in the thoracic, upper lumbar, and sacral regions, contains cell bodies of motor neurons that serve the autonomic nervous system.  Interneurons (99% of the neurons in the CNS) connect the dorsal horn to the ventral horn and/or lateral horn when reflexive actions, such as pulling away from a painful stimulus, are initiated in the spinal cord.

Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. Ascending tracts of nervous system fibers in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain. Looking at the spinal cord longitudinally, the columns extend along its length as continuous bands of white matter.

Figure \(\PageIndex{4a}\): Cross-section of Spinal Cord The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Figure \(\PageIndex{4b}\): Cross Section of Dorsal Root Ganglion, TM 400x, cross-section (Photo Credit: Julie Robinson)

Procedure for Activity 12.2:  The Spinal Cord in Histology Slides

You will now use the information you have read and the figures above to locate, identify, and describe functions of particular features in nervous system slides. 

1. Obtain a slide of a spinal cord and dorsal root ganglion (both cross sections).  View the slide using the 4x or 10x objective, as directed by your instructor. 
2. Use the information above to identify the structures listed in the objectives and describe their functions.  In the space below, draw the spinal cord as seen in your slide.  Label the central canal, dorsal horn, ventral horn, dorsal root ganglion, a neuron in the dorsal root ganglion, satellite cells in the dorsal root ganglion, and the location of the white matter.

Activity 12.3:  Nerves in Histology Slides

Bundles of axons in the PNS are referred to as nerves (Figure \(\PageIndex{5}\)-Figure \(\PageIndex{8}\)). These structures in the periphery are different than the central counterpart, called a tract.  Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment.  The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium.  Within the nerve, axons are further bundled into fascicles, which are each surrounded by their own layer of fibrous connective tissue called perineurium.  Next, individual axons are surrounded by loose connective tissue called the endoneurium (Figure \(\PageIndex{5}\)).  Deep to the endoneurium is the neurilemma, the cell membrane of the neuron.  These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.

Figure \(\PageIndex{5}\): Simplified Nerve Structure (Top picture: Access for free at https://openstax.org/books/anatomy- and-physiology/pages/1-introduction ; Bottom picture: Julie Robinson)

Figure \(\PageIndex{6}\): Nerve Structure The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers (tissue source: simian). LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Figure \(\PageIndex{7}\): Cross Section of a Nerve, TM 100x, cross-section (Photo Credit: Julie Robinson)

Figure \(\PageIndex{8}\): Close-Up of Nerve, TM 400x, longitudinal section (Photo Credit: Julie Robinson)

Procedure for Activity 12.3:  Nerves in Histology Slides

You will now use the information you have read and the figures above to locate, identify, and describe functions of particular features in nervous system slides.

1. Obtain a slide of a peripheral nerve (cross section). 
2. View the slide using the 10x and/or 40x objective, as directed by your instructor.  Use the information above to identify the structures listed in the objectives and describe their functions.  In the space below, draw a section of the peripheral nerve.  Label the epineurium, perineurium, fascicle, myelin sheath, axon of the neuron, and satellite cells. You may need to draw two pictures to label all of the structures listed.
3. Obtain a slide of a peripheral nerve (longitudinal section). 
4. View the slide using the 40x objective, as directed by your instructor.  Use the information above to identify the structures listed in the objectives and describe their functions.  In the space below, draw a section of the peripheral nerve.  Label the myelin sheath, axon, Schwann cell, neurilemma, and a Node of Ranvier

Additional Learning Resources (see electronic document in Canvas to access weblinks):

• Watch this video for the neuron/nervous system slide:  https://www.youtube.com/watch?v=v0VfavClNdc
• Watch this video for the peripheral nerve cross section slide:  https://www.youtube.com/watch?v=-eoXjVerqZQ
• Watch this video for the peripheral nerve longitudinal slide:  https://www.youtube.com/watch?v=igYFOTjKSR4
• Watch this video for the dorsal root ganglion (part of the spinal cord slide):  https://www.youtube.com/watch?v=Fy8DqzU5HtQ
• Watch this video for the spinal cord slide:  http://youtu.be/F1gDA7U2JX4?hd=1
• Practice identifying the cells and their parts here:  (use questions 2, 3, 8, and 9):  http://academic.pgcc.edu/~aimholtz/AandP/PracPrac/2050_Lab17/neuron.html