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.

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:

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:

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

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.

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

Sensory neurons are pseudounipolar.

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.

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.

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.

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: