Objectives
- Identify neurons in the central nervous system (CNS) and peripheral nervous system (PNS)
- Identify different kinds of supporting, or glial cells in the CNS and PNS
- Identify the basic organization of a peripheral nerve
Overview
The nervous system is specialized for communication. It conveys information-rich sensory signals from the peripheral receptors in the body to the brain and conveys motor responses from the brain to the peripheral organs. The nervous system is composed of two basic anatomical subdivisions: the central nervous system (CNS), composed of the brain and spinal cord and the peripheral nervous system (PNS), nerve structures throughout the rest of the body.
The nervous system is also divided functionally into the somatic nervous system (SNS) and autonomic nervous system (ANS). Both of these divisions consist of parts of both the PNS and CNS. The SNS provides motor and sensory innervation to parts of the body under voluntary, conscious control with the exception of reflex arcs. The ANS provides function to non-voluntary structures such as smooth muscle, the heart and glands. It also conveys pain and autonomic reflexes from viscera to the brain. The ANS is further divided into a sympathetic division (fight or flight), parasympathetic division (rest and digest) and enteric division (found only in the gastrointestinal system).
Nerve tissue is a highly modified version of epithelial tissue. The CNS begins as a simple tubular pseudostratified epithelial structure. Many features of epithelial tissues persist in the central nervous system. For example, it retains an apical to basal polarization. Apical tight junctions join the apical borders of cells of the CNS, may have cilia, and face on a lumen (brain ventricles and central canal of spinal cord). The outer boundary of the CNS is its basement membrane.
Nerve tissue consists of 2 main cell types: neurons and supporting cells.
Neurons: Nerve cells, or neurons, are the functional unit of the nervous system. They consist of a body, containing a nucleus, and processes of different lengths. They receive signals from cells and send electrical impulses to other nerves or other cells/tissue through neuronal junctions, or synapses.
Peripheral nerves are made up of nerve fascicles which consist of individual nerve cells or fibers. Connective tissue (CT) holding together gross nerve structure is organized into 3 components- endoneurium (loose CT surrounding each fiber); perineurium (specialized CT surrounding each fascicle); epineurium (dense irregular CT surrounding a peripheral nerve and fills spaces between nerve fascicles).
Supporting/neruoglial/glia cells: nonconducting cells that aid in neuron function. Central neuroglia are specifically found in the CNS and peripheral neuroglia are found in the PNS.
- Astrocytes (CNS)- largest glial cells with varying number of processes that commonly branch in gray matter but are typically straight in white matter; provide a scaffold for neurons during brain development, physical and metabolic support; maintain tight junctions for blood-brain-barrier
- Oligodendrocytes (CNS)- small cells with few processes; produce and and maintain myelin
- Microglia (CNS)- smallest glial cells, dark elongated nuclei with few processes that function as phagocytes
- Ependymal cells (CNS)- cuboidal-to-columnar cells that line the ventricles of the brain and central canal of the spinal cord
- Schwann cells (PNS)- small cells with oval nuclei, produce and maintain myelin
- Satellite cells (PNS)- small, cuboidal cells that maintain the microenvironment surrounding the neuronal body
In the brain, gray matter forms the cortex (outer covering) and white matter forms the medulla (inner core). White matter contains only axons of nerve cells and associated glial cells. Gray matter contains cell bodies, dendrites and axon terminals, or synapses. The spinal cord contains a central “butterfly” of gray matter where cell bodies are found, surrounded by white matter containing axons.
>Nerve Tissue Slides
This slide is a cross-section of the spinal cord. In the screen capture above, the central gray matter butterfly shape has been outlined. The dorsal portion is more posterior while the ventral portion is more anterior. In the ventral horns, you should find motor neuron cell bodies (or soma) which contain a nucleus and prominent nucleolus. You should be able to identify the difference between the axon and dendrites, projecting from the cell body by looking for the axon hillock. The cytoplasm of the cell body is filled with basophilic granules known as Nissl substance which represents aggregates of rough endoplasmic reticulum. You should compare the motor nerve cell bodies to sensory nerves in the dorsal horns, these are typically smaller than the motor neuron cell bodies. You should also look through the tissue to identify different central neuroglia (astrocytes, oligodendrocytes, microglial cells). Look for the central canal of the spinal cord where you should be able to identify ependymal cells.
This is a section from part of the cerebral cortex. The two gyri found as elevations on the surface of the brain are separated by the deep sulcus. The brain, like the spinal cord has gray matter and white matter. Use this slide to compare astrocytes (easily visible in this stain) in white and gray matter. Astrocytes can be divided into 2 types: protoplasmic astrocytes and fibrous astrocytes. Protoplasmic astrocytes are more prevalent in gray matter of the brain and have numerous, short, branching cytoplasmic processes. Fibrous astrocytes are more prevalent in white matter and have fewer processes that are relatively straight and are more pronounced.
These specimens are peripheral nerves cut in a longitudinal section and cross-section. The cross-section can be used to identify the CT that holds nerves together. Epineurium (e) can be found surrounding the entire gross nerve while perineurium surrounds each fascicle (F) made up of individual nerve fibers surrounded by endoneurium. When using high magnification to examine the nerve fibers, look for the “foamy” myelin sheath surrounding the eosinophilic (pink) axons, you will look for round structures with radiating lines. You will also notice capillaries throughout the tissue.
This slide contains a longitudinal section (upper right) and cross section (lower left) of isolated nerves. The Toluidine blue stains myelin sheath a dark blue color and allows easy observation of layers of Schwann cells and cytoplasm. In the longitudinal section, look for nodes of Ranvier by looking for breaks in the blue lines as you follow lines of myelin, representing where one Schwann cells ends and another begins. You will also notice mast cells, CT cells scattered throughout the endoneurium. You can also find Schmidt-Lanterman clefts, small pockets of cytoplasm. In cross section, you can appreciated concentric wrapping of the Schwann cells.
Sensory nerve cell bodies are located in the dorsal root ganglion. In this specimen, you can identify neuronal cell bodies and surrounding satellite cells, myelinated nerve fibers, and Schwann cells.
Use this specimen from the jejunum of the small intestine to identify nerve tissue. The myenteric (Auerbach) plexus can be found between the inner muscular externus (IME) and outer muscular externus (OME) layers. The large, round structures are the ganglia (collection of nerve cell bodies) and are surrounded by black fibers. There is also nerve tissue in the submucosal layer, just deep to the mucosa (epithelium + lamina propria + muscularis mucosae). The submucosal collections of nerve cell bodies and part of the submucosal (Meissner) plexus.
>More Practice
This is a section of a rat spinal cord using a stain that emphasizes Nissl substance in violet and myelin sheath in dark blue.
Ganglia contain a collection of nerve fibers (NF) and cell bodies (CB). Use this slide to determine what type of ganglion based on morphology of the CB.
Use this slide to find specialized nerve structures in skin.
>More Practice Answers
Slide MS_LO5_87 Spinal Cord
- Ventral fissure
- A) sensory neuron; B) nucleolus; C) Nissl substance (collection of rough ER)
- A) motor neuron; B) dendrite
- White matter
- A) central canal; B) ependymal cells
Slide LH 64 Ganglion
- This is an autonomic ganglion- eccentric nuclei (black arrows); few satellite cells (red arrows) compared to dorsal root (see above in “nerve tissue slides”); lipofuscin (blue arrows)
- Nerve fiber
Slide 26 Thick Skin
- A) Meissner corpuscle; B) sense touch
- A) Pacinian corpuscle; B) sense vibration/pressure; C) nerve fiber; D) myelin; E) CT
