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
Identify the structural components of a neuromuscular junction

Lab Content

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.

Spinal Cord

Many important features of the spinal cord are visible in this cross section. The white matter is composed of nerve fibers carrying ascending and descending information and makes up the outer regions of the cord. The gray matter, which contains cell bodies, is located in the center of the cord and is easily identified by its color and butterfly shape. The central canal lies in the center of the cord and contains cerebrospinal fluid. The dorsal root contains afferent sensory fibers that transmit signals from the periphery, through the dorsal root ganglion, to the dorsal horn. The ventral root contains efferent motor axons. The ventral and dorsal roots come together to form a spinal nerve.

Dorsal Root Ganglion

The dorsal root ganglion contains the cell bodies of sensory neurons that bring information from the periphery to the spinal cord. These neurons are pseudounipolar and contain an axon-like process that bifurcates with one branch extending toward the periphery and the other branch heading toward the grey matter of the spinal cord. Fibers heading toward the periphery leave the ganglion through the spinal nerve, where they run together with motor fibers. Fibers leading to the spinal cord travel through the dorsal root.

Motor Neuron

A motor neuron innervates one or many muscle fibers to control muscle contraction. The motor neuron in the ventral horn is easily identifiable by its large size, polygonal shape and extension from the cell body. Compare the size of the nucleolus in the motor neuron with the nuclei in the surrounding support cells.

Motor Neuron: Nissl Bodies

Nissl stain is used to label rough endoplasmic reticulum in neurons. The dark blue structures are referred to as Nissl bodies but are the equivalent of the rough endoplasmic reticulum. Note how Nissl bodies are confined to the soma and dendrites; they do not extend into the axon.

Peripheral Nerve Bundle

This slide allows you to visualize the different layers of connective tissue that comprise a nerve bundle. At the smallest level are the individual myelin sheaths covering the axons. Each of these nerve fibers is then ensheathed in a layer of endoneurium. Several fibers are surrounded and bound together by perineurium, which appears much thicker. Finally, an outer sheath of epineurium surrounds the entire trunk. You will see that muscle has a similar organization during the next laboratory.

Myelinated Axon EM

This electron micrograph shows a cross section of a myelinated nerve in the peripheral nervous system. The Schwann cell wraps around the axon to envelop it in a myelin sheath that functions as an electrical insulator.

Unmyelinated Axons EM

Schwann cells and oligodendrocytes can also associate with axons but not wrap them in myelin sheaths. This EM image shows a Schwann cell associated with several small axons but without myelinating the axons.

Nodes of Ranvier

Each Schwann cell myelinates about 100 �m of an axon. Gaps between neighboring Schwann cells are called nodes of Ranvier. Action potentials jump from node to node while traversing an axon.

Neurofibrils EM

Axons contain two prominent cytoskeletal elements: microtubules and neurofilaments. Microtubules provide tracks along which material is transported to and from the cell body. Neurofilaments are a class of intermediate filaments and provide structural support.

Synapse EM

This EM image reveals a synapse between an axon and dendrite. Note the presence of numerous synaptic vesicles and mitochondria in the axon. What triggers fusion of the synaptic vesicles with the plasma membrane? Two myelinated axons are visible on the left.