Nervous System Lab

Learning Objectives

  • Draw a neuron and label its key histological and structural features.
  • Explain the microscopic structure of a nerve fiber, including the structure of the myelin sheath and connective tissue layers.
  • Identify the four types of glial cells, their structures, and their functions.
  • Explain the general layout of the spinal cord, cerebrum, and cerebellum, and identify key cell types in each region.
  • Distinguish some important pathological examples from normal tissue.

Keywords

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Pre-Lab Reading

Introduction

The nervous system is extraordinarily complex, and it is therefore impossible to cover it in its entirety in a single laboratory. This lab will be limited to the study of the basic features of neurons and glial cells - specific organs composed of neurons, including the retina of the eye and the organ of Corti of the inner ear, will be studied in the Sensory Systems lab, in conjunction with the Neuroanatomy course.

The Neuron

An understanding of the nervous system begins with an understanding of its basic morphological and functional unit, the neuron. Neurons are nerve cells that form the conducting system that carries information throughout the central and peripheral nervous systems. Not all neurons look or act the same - they vary in size, shape, and complexity, and the important differences between the various classes of neurons will be of great importance in your study of Neuroanatomy. For now, we will focus on the common structural features that make neurons identifiable at the level of resolution of light and electron microscopes.

Neurons can be divided into four regions:

  • Dendrites. The dendrites make up the receptive portion of the neuron, and receive most synaptic afferent inputs from upstream neurons.
  • Cell body. The cell body, also the soma, is the integrative portion of the neuron, where incoming signals from dendrites are summed together. The neuron will fire or not fire based upon the results of this summation. The soma also contains the nucleus and most of the organelles of the neuron, surrounded by the cytoplasm or perikaryon.
  • Axon. The axon extends away from the soma and is the conductile portion of the neuron. Efferent signals flow down the axon in one direction, toward the terminal branches. Axons can be up to a meter long.
  • Synaptic terminal. At the end of the axon is the synaptic terminal, which is notable for its high concentration of vesicles containing neurotransmitters. This is the effector portion of the neuron; when an action potential reaches the terminal, the content of the vesicles is released and either excite or inhibit the next neuron.

While every neuron possesses these four structural features, the relative positions of these features determine the type of neuron. There are three basic neuron types:

  • A multipolar neuron has multiple dendrites extending from the cell body and a single axon extending in the opposite direction.
  • A bipolar neuron has a single dendrite that extends from the cell body, opposite the side from which the single axon extends.
  • A pseudounipolar neuron has a single axon that splits into one branch that runs to the peripheral tissues and a second branch that leads to the spinal cord.

There are a few key points to remember when you are viewing a neuron under the microscope:

  • You should begin by distinguishing the axon from the dendrites. Usually, several short dendrites extend from the cell body. The single axon tends to be longer - while the axon may split into multiple pathways, it typically originates from a single point, the axon hillock.
  • The axon hillock is a conical elevation of the cell body from which the single axon extends.
  • It is not always possible to distinguish dendrites from axons based on their shape and size alone. Instead, you can use Nissl substance to make this easier. Nissl bodies, the equivalent of the rough endoplasmic reticulum in the neuron, are found only in the soma and dendrites of the neuron - never in the axon hillock or axon. By identifying Nissl substance, you can easily distinguish the two processes.
  • Neurofibrils extend from the soma out into the dendrites. These represent aggregates of microtubules and neurofilaments, and can be visualized by EM. Neurofibrils are important because they mediate slow and fast axonal transport, the method by which cytoskeletal elements and membrane-bound organelles move to and from the soma.

Nerve Fibers

The nerve fiber consists of a neuron's axon and its myelin sheath, if present. Nerve fibers are found in the peripheral nervous system and central nervous system. In the peripheral nervous system, Schwann cells form the sheath around axons, and each Schwann cell forms the sheath for just one neuron. In the central nervous system, oligodendrocytes form the sheath, and one oligodendrocyte can myelinate multiple neurons. Schwann cells and oligodendrocytes can also associate with axons but not form a myelin sheath around the axon.

Schwann cells, in the peripheral nervous system, and oligodendrocytes, in the central nervous system, wrap around the axons of neurons to form myelin sheaths. Myelin sheaths are electrical insulators and prevent current from leaving axons. The myelin sheath is interrupted at intervals along the axon by the nodes of Ranvier. These nodes represent the points of discontinuity between individual Schwann cells or oligodendrocytes arrayed along the nerve fiber. In myelinated axons, current hops between nodes and travels much faster compared to an unmyelinated axon of similar diameter.

In peripheral nerve fibers, each nerve fiber is enclosed in a delicate connective tissue covering called the endoneurium. Bundles of nerve fibers are surrounded by a more extensive layer of connective tissue called the perineurium surrounds. Finally, the entire peripheral nerve trunk is encapsulated by another connective tissue sheath called the epineurium.

Sensory Nerves

Impulses are carried to the spinal cord by the sensory nerves. Sensory nerves have various types of receptors.

  • Exteroceptors carry sensations of pain, temperature, touch, and pressure from the skin and connective tissue. They may be encapsulated or unencapsulated.
  • Proprioceptors carry impulses of stretch and position from the muscles, tendons, and joints.
  • Visceroreceptors carry stimuli from the internal organs and circulatory system.

Sensory neurons are pseudounipolar.

Motor Nerves

A motor neuron innervates one or many muscle fibers to control muscle contraction. A motor unit is defined as the neuron and the muscle fibers it supplies. Muscles that require fine control have fewer muscle fibers innervated by each neuron; muscles that participate in less controlled movements may have many fibers innervated by one neuron. Motor neurons are typically multipolar with an axon that terminates in a neuromuscular junction on the surface of skeletal muscle fibers. The neuromuscular junction will be discussed in the Muscle Lab.

Neurons in Spinal Cord

You should be familiar with the gross structure of the spinal cord from Human Anatomy. The spinal cord is composed of gray matter and white matter. The gray matter, which is shaped like a butterfly, is internal and contains the nerve cell bodies. The white matter is external and contains tracts of nerve fibers. In the center of the cord is the central canal, which is lined by ependyma, epithelial cells that produce cerebrospinal fluid. You will become very familiar with the nuclei and tracts of the spinal cord in Neuroanatomy. For now, it is important to be able to distinguish the two main horns of the spinal cord, along with their associated processes and nuclei.

  • The ventral horn of the spinal cord contains the cell bodies of motor neurons. These neurons extend out of the spinal cord through the ventral root.
  • The dorsal horn of the spinal cord contains the cell bodies of ascending secondary sensory neurons. The primary sensory neurons have their cell bodies outside, but just adjacent to, the spinal cord in the dorsal root ganglion. The sensory neurons of the dorsal root ganglia are pseudounipolar because they send out one process that splits into two branches: one that extends to the periphery (to receive sensory information) and one that extends to the spinal cord (which transmits sensory information). The dorsal root ganglion also contains satellite cells, which provide structural and metabolic support to the sensory neurons.

Neurons in the Brain

In the brain, the positions of the gray and white matter are the reverse of what they are in the spinal cord - the gray matter containing cell bodies is external, and the white matter containing nerve fibers is internal. The gray matter of the cerebral cortex is divided into 6 layers. The characteristic cell type of the cortex is the pyramidal cell, so-called because of their triangular shape. Pyramidal cells have a thick, branching dendrite located at the apex and a long axon that extends toward the white matter.

The cerebellar cortex has three layers: an outer molecular layer with nerve cell processes, a layer of Purkinje cells, and an inner granular layer with several other types of neurons. Purkinje cells are very large neurons that possess a tree of branching dendrites that extend into the molecular layer.

Glial Cell in the Central Nervous System

Neuroglia are the main non-nervous cells of the central nervous system. The are present in the extracellular space of nervous tissue, or neuropil. You will observe four types of CNS neuroglia in this lab:

  • Astrocytes are derived from the ectoderm. They are supporting cells that possess many processes and are interposed between neurons, except at the site of synapses. These cells regulate the metabolic environment of the extracellular space and are important for scarring during traumatic injury to the brain. Astrocytes occur as two histological types. Protoplasmic astrocytes have broad, symmetrical processes and are usually confined to the gray matter, whereas fibrous astrocytes have asymmetrical processes and are typically confined to white matter.
  • Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. These are derived from epithelial cells and produce cerebrospinal fluid.
  • Oligodendrocytes are derived from the ectoderm and are the myelinating cells of the central nervous system.
  • Microglia are of mesodermal origin and are situated among neurons and around capillaries. These cells are phagocytic and are the CNS counterpart of connective tissue macrophages (also of mesodermal origin).

Pre-Lab Quiz

  1. Beginning at the axon membrane, name the layers of connective tissue that build up to make a peripheral nerve.
  2. Answer:
  3. Identify the location and major function of each of these cells:
    • Astrocyte
    • Schwann Cell
    • Oligodendrocyte
    • Ependymal Cell
    • Microglia
    • Satellite Cell
    Answer:

Slides

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  1. Spinal Cord
  2. Dorsal Root Ganglion
  3. Motor Neuron
  4. Motor Neuron: Nissl Bodies
  5. Peripheral Nerve Bundle
  6. Myelinated Axon EM
  7. Unmyelinated Axons EM
  8. Nodes of Ranvier
  9. Neurofibrils EM
  10. Synapse EM
  11. Pyramidal Cell
  12. Purkinje Cells
  13. Microglia and Astrocytes
  14. Ependymal Cells

Virtual Microscope Slides

  1. Spinal Cord
  2. Before increasing the magnification, observe the general organization of the spinal cord. Identify the dorsal and ventral sides. Distinguishing the white matter from the gray matter. Identify the dorsal root ganglion.
  3. Node of Ranvier
  4. These nerves were fixed in osmium to preserve the lipids of the myelin sheath. At the highest magnification, try to find nodes of Ranvier. These are the points of discontinuity along the nerve fibers. What type of channel is found in the plasma membrane at nodes of Ranvier?
  5. Cerebellar Cortex
  6. This slide shows the three layers of the cerebellar cortex. Identify and name each layer. At high magnification, identify the single layer of Purkinje cells, the granule cells, and the molecular layer. What does the innermost region of this tissue contain?
  7. Peripheral Nerve Bundle
  8. Identify nerve bundles in cross section and longitudinal section.

Pathology

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  1. Alzheimer's Disease
  2. Parkinson's Disease

Quiz

  1. What is the name of the structures in the cytoplasm and of what are they composed?
  2. Answer: Nissl body, rough endoplasmic reticulum.
  3. Name the region of the spinal cord in which these cells are located.
  4. Answer: Ventral Horn (grey matter).
  5. What takes place at this site?
  6. Answer: Depolarization and saltatory conduction.
  7. Identify the myelinated and unmyelinated nerves.
  8. Answer: A is the unmyelinated nerve and B is myelinated nerve. Note the white region, which is myelin, around the axons in B.
  9. Identify the structure.
  10. Answer: Peripheral nerve bundle.
  11. Is this a myelinated or unmyelinated nerve?
  12. Answer: Unmyelinated nerves.
  13. Identify A, B, and C.
  14. Answer: A = neuron cell body, B = glial cell body, C = axons
  15. Identify A, B, C, and D.
  16. Answer: A= molecular layer, B = Purkinje layer, C = granular layer, D = white matter
  17. Multiple sclerosis is an autoimmune disorder of the central nervous system in which the body creates antibodies against myelin. To what type of cell is the antibody in MS produced? Since peripheral nerves also contain myelin sheaths, why are these nerves not affected?
  18. Answer: The antibody must be to the membrane of the oligodendrocyte, but not to Schwann cells. Guillan-Barre disease involves antibodies against Schwann cells but not oligodendrocytes - the peripheral nervous system is affected instead.