4. General Anatomy of Nervous Tissue, the Spinal Cord and the Brain Stem

Revised August 7, 2007

The objectives of this chapter are to:

  1. Identify the general features of nervous tissue, the spinal cord and the brain stem.
  2. Identify the motor nuclei associated with the cranial nerves.

I. Nervous Tissue

A. Neurons and Glia

The parenchyma of the central nervous system (CNS) consists of two cell types: neurons and glia.

Neurons. In general, neurons have a centrally placed nucleus (#9853 a) and nucleolus (#4232). Distributed in the cytoplasm are clumps of basophilic material called Nissl bodies (#6682). They are collections of granular endoplasmic reticulum. Two varieties of processes taper from the cell body: the thicker and shorter processes are the dendrites (#9853 b), and the thinner, longer process is the axon. Ordinarily, a neuron has several dendrites and a single axon. Special methods of staining such as the Golgi method show the characteristic shapes and spatial relationships of neurons (hippocampus #6662, cerebral cortex #6388). The Golgi method shows the neuronal cell body with the dendrites (#4235) and axon (#4234) extending from it. Many dendrites have tiny protrusions called dendritic spines (#3986).

Axons. As shown on the left side of #6132, axons (the round, clear spaces) have various diameters and myelin sheaths of various thicknesses (black circles). Axons conduct nerve impulses between neurons. The speed of this conduction varies with the diameter of the axon and thickness of the myelin sheath. The greater the diameter and the thicker the myelin sheath, the faster the speed of conduction. The myelin sheath is interrupted at points called the nodes of Ranvier (#6923 arrows). Nerve impulses skip from node to node; this is called saltatory (“jumping”) conduction. Obviously, the fewer the nodes, the faster the rate of conduction. However, not all axons have a myelin sheath. These are called unmyelinated axons. They are the thinnest axons and have the slowest conduction speeds.

Synapses. Neurons contact each other, as well as muscle and gland cells, at synapses. A typical synapse consists of an axon terminal (the presynaptic component #6787 d) ending on a dendrite (the postsynaptic component #6787 e). The intercellular space between the two components is the synaptic cleft. Axon terminals end not only on dendrites but also on cell bodies and other axons. These synapses are named axodendritic, axosomatic, and axoaxonic synapses, respectively. Special stains are used to show axon terminals (#4469) ending on cell bodies and dendrites. The structure of a typical synapse on a muscle cell is seen best in an electron micrograph (#6776). The axon terminal has many mitochondria (A) and small vesicles (B) that contain neurotransmitters. The neurotransmitters are released from the vesicles into the synaptic cleft (C) and cross it to affect the postsynaptic cell membrane.

Glia.  The non-neuronal cells of nervous tissue are called glia. The four major types of CNS glia (glial cells) are astrocytes, oligodendroglial cells, microglial cells, and ependymal cells. Astrocytes (#10392) act as ionic buffers between neurons. Their processes surround the blood vessels (#8648) in the CNS, separating the vessel wall from the neurons. Oligodendroglial cells (#4238) form myelin sheaths around axons in the CNS. Microglial cells (#9872) are phagocytic cells. Ependymal cells (fig 3c) line the ventricular system.

B. Gray Matter and White Matter

The gray matter of the CNS (#6132 on the right) consists of neuronal cell bodies, dendrites, and synapses intermixed with glial cells. Because this is where the synapses are, it is where information transfer occurs in the CNS. The neuronal and glial cell processes in gray matter are called the neuropil. With conventional hematoxylin and eosin stains, this area is not stained (#5533). Other preparations demonstrate the numerous neuronal and glial cell processes that compose the neuropil (#4928).  The white matter (#6132 on the left) consists of myelinated and unmyelinated axons, which conduct nerve impulses from one area of gray matter to another. What cell bodies (#4238 arrow) can be recognized in white matter? Compare the structure of gray and white matter.

II. The Spinal Cord

The spinal cord weighs only 35 g. It has two swellings, one near each end, named the cervical (#5358) and lumbosacral (#5359) enlargements. Like the brain, the cord is wrapped in meninges. The thick dura mater (#8483) forms a sac, the dural sac (#4931), in which the cord is suspended. The pia mater of the cord forms a series of collagenous ligaments, the denticulate ligaments (#8484), which anchor the cord to the dura mater helping to hold the cord in place. The pia mater also forms a ligament that extends from the caudal tip of the cord to the blind end of the dural sac. This pial ligament is named the filum terminale (#5403 ).   Extending along the length of the cord, on either side, is a series of dorsal and ventral roots( #5400). Where are the dorsal root cell bodies found (#52090)? What are dorsal root ganglia and where are they located (#12411)?

The cord ends at the level of the disc between vertebrae L-1 and L-2. The caudal tip of the cord is the conus medullaris (#4727, transected at the arrow). What cord segments comprise the conus? What do these segments innervate (#15152)? What does the cauda equina represent (#5405)?

Spinal cord segment L-4 is not adjacent to vertebra L-4. This is the "vertebrosegmental discrepancy." At the end of the embryonic period, eight weeks post-fertilization, the cord and vertebral column are the same length. Then the rate of growth of the vertebral column exceeds that of the cord. The result is obvious: In the adult, the vertebral column is 70 cm long and the spinal cord, 45 cm. The lower portion of the dural sac contains only the dorsal and ventral roots of the cauda equina. Therefore, this is a relatively safe place to insert a needle in the subarachnoid space to withdraw a sample of cerebrospinal fluid.

Spinal segments are distinguished by their pattern of gray and white matter. The differences are seen by comparing representative sections of each level: cervical (fig 4a), thoracic (#6610), lumbar (#6136), and sacral (#6137). Note that in the cervical section there is more white matter relative to gray matter. The opposite is true in the sacral section. Why is this? Toward the upper end of the cervical cord (#6158), the gray matter and white matter are rearranged (#6211) until the structural features of the medulla emerge (#6164). The gray matter and white matter of the medulla is continuous with that of the cord. Some features are deleted and others are added.

III. The Brain Stem and its Motor Nuclei

The structure of the brain stem (fig 4b, below) is usually studied by examining a series of gross and microscopic transverse sections through the medulla, pons, and midbrain. The sections are typically stained for myelin to show white matter as dark.  Review the major external features and cranial nerves associated with the brain stem by clicking on each of the labels below.  This should serve as a reference point for the following descriptions of cross sections.

 

fig 4b Ventral View of Brain Stem with Cranial Nerves

In a myelin-stained section, the gray matter appears pale. Other stains actually show the nerve cell bodies. The organization of the gray matter and white matter is characteristic for each level of the brain stem. The motor nuclei of the cranial nerves are emphasized in the following description. It is important to know which motor nuclei are present in each level of the brain stem. Knowing this and knowing what motor nuclei are not functioning allow you to predict what area of the brain stem is affected in a patient with a neurologic problem.

A. The Medulla

The first motor nucleus to be identified is that of the spinal accessory nerve, cranial nerve XI. This cranial nerve actually originates in the upper five cervical segments, #4302 (fig 4c). The axons leave the cord by forging their way across the white matter to exit halfway between the dorsal and ventral roots. The axons then turn rostrally, go along the side of the cervical cord (#4169), and pass through the foramen magnum into the posterior cranial fossa. Here they join the axons of cranial nerves IX and X to exit the posterior cranial fossa through the jugular foramen (fig 4d, #5415) The spinal accessory nerve innervates the ipsilateral sternocleidomastoid muscle and the upper portion of the trapezius muscle.

The medulla (fig 4e) contains the motor nuclei of cranial nerves XII, X, and IX. Two prominent structures that are characteristic of the medulla are the pyramids (#6411), composed of descending axons, and the inferior olivary nuclei (#6541). All of the medulla dorsal to the pyramids is called the tegmentum ("cover") of the medulla. On either side of the midline just beneath the ventricle floor (= the dorsal surface of the medulla) are the hypoglossal nuclei, shown in a myelin-stained section (#6319), and seen with a Kluver-Berrera stain for cell bodies (pink dots #6233). They correspond to motor nuclei in the ventral horn of the spinal cord. Where do the axons of the hypoglossal motor neurons emerge on the surface of the brain stem? (#5606). How do the axons exit from the cranial cavity? (#5187). Where do they terminate? (#51549)

The dorsal motor nucleus of X (fig 4e) (#6242) is just lateral to the hypoglossal nucleus (#6240). The dorsal motor nucleus of X is a collection of preganglionic parasympathetic cell bodies. It is one of the two motor nuclei that contribute axons to the vagus nerve (#7954). Where do the axons that originate in this nucleus terminate?

The other motor nucleus that contributes axons to cranial nerve X (#4242) is the nucleus ambiguus (#6239), located dorsal to the inferior olivary nucleus (fig 4e). The nucleus ambiguus also contributes axons to the glossopharyngeal nerve (cranial nerve IX) (#5601, #7950). Where do the axons originating in this nucleus terminate? What is the functional significance of this nucleus? Cranial nerve IX also contains preganglionic parasympathetic axons that originate from the inferior salivatory nucleus, which is directly rostral to the dorsal motor nucleus of X, but is not visible.

B. The Pons

A section of the pons (#5285) shows the fourth ventricle dorsally and, most characteristic, a large fiber bundle on each side, the middle cerebellar peduncle. The pons consists of a ventral part, the basilar pons (pons proper) and a dorsal part, the pontine tegmentum (fig 4f). The basilar part has a unique appearance with nuclei, transverse bundles of fibers, and fibers descending to the medullary pyramids. The transverse fibers cross to the cerebellum as the middle cerebellar peduncle and give the pons (“bridge”) its name. Within the tegmentum are the cranial nerve nuclei.

Caudal pons.  Locate motor nuclei VI (abducens) and VII (facial) in a section through the caudal pons (fig 4f). The abducens nucleus is found next to the midline directly beneath the fourth ventricle in a position corresponding to the hypoglossal nucleus in the medulla. Check the intracerebral course of the abducens axons (#6698). The axons form the nerve near the midline at the pontomedullary junction (#7955, #5280). The facial nucleus (VII) (#6176) is more ventral and lateral. It corresponds in position to the nucleus ambiguus. The axons of VII take a peculiar course within the brain stem. They go medially and dorsally and loop over the abducens nucleus before turning laterally (#6699). The abducens nucleus and overlying facial nerve rootlets form a bulge in the floor of the fourth ventricle called the facial colliculus (#4487). Portions of the course of the facial nerve within the tegmentum are seen in #9759. The facial nerve emerges between the medulla, pons, and cerebellum in a region called the cerebellopontine angle (#5305). The nerve next enters the temporal bone. What is the name of the foramen it enters (#5440)? The abducens nerve passes along the clivus (#5185) into the cavernous sinus (#7885). What is innervated by the axons originating in the abducens and facial nuclei?

Recall that cranial nerve VII also contains preganglionic parasympathetic axons. They should not be confused with the axons arising in the facial nucleus. In what nucleus are the cell bodies for these autonomic fibers located? The nucleus is so tiny that it cannot easily be identified in these sections.

Mid pons.  The motor nucleus of V (fig 4g) is rostral to the abducens and facial nuclei. Its axons go through the middle cerebellar peduncle to emerge on its surface on the side the pons (#6701, #4736). What is the distribution of axons arising in this nucleus?

Nerve cells in the nucleus ambiguus and in the facial and trigeminal motor nuclei innervate skeletal muscle that is derived from the pharyngeal (branchial) arches instead of myotomes, the typical source of skeletal muscle. The pharyngeal arch-derived skeletal muscle is called branchiomeric muscle to emphasize its homology with the branchial (gill) arch muscle system of fish. Our facial and pharyngeal muscles are branchiomeric. Are there other examples? The three nuclei (ambiguus, facial, motor nucleus of the trigeminal) really represent one nuclear group that has been interrupted so that instead of seeing one, long continuous nucleus, we see three separate ones.

C. The Mesencephalon or Midbrain

The features that identify a section through the midbrain are 1) the narrow aqueduct, 2) the large ventral cerebral peduncles, containing axons descending to the basilar pons and medullary pyramids, and 3) rounded eminences dorsally (fig 4h). The rounded eminences (#8520) are the superior colliculi and inferior colliculi, collectively called the quadrigeminal plate or tectum ("roof"). The aqueduct is surround by periaqueductal gray (central gray). The substantia nigra is dorsal to the cerebral peduncles. The midbrain tegmentum is between the substantia nigra and the aqueduct (fig 4h).

Caudal midbrain at level of inferior colliculi.  The nuclei and fibers of cranial nerve IV are seen in fig 4i and fig 4j. The axons of this nerve have a unique intracranial course. They leave the trochlear nucleus to go dorsally along the central gray (#6704) and cross dorsal to the fourth ventricle as it is about to become the aqueduct (#4461). They emerge on the opposite dorsal side of the brain stem just below the inferior colliculi (#4357, #11715). The axons then pass lateral to the cerebral peduncle (#5641) and go forward through the cavernous sinus (#7885) and superior orbital fissure (#6958) into the orbital cavity. Which muscle(s) do these axons innervate? Does the right trochlear nerve innervate muscle(s) on the left side or on the right side?

Rostral midbrain at level of superior colliculi.  The axons of cranial nerve III are shown in fig 4k. They originate in the oculomotor nuclei (#6310), which are next to the midline just ventral to the central gray at the level of the superior colliculus. This is a complex nuclear group consisting of several nuclei. Each nucleus gives rise to axons that innervate a specific extraocular muscle as well as the levator palpebrae superioris. The axons leave the nucleus and travel ventrally through the tegmentum. They leave the brain stem and enter the interpeduncular fossa (#5322, #11724). As the nerve goes into the cavernous sinus, it passes near the free edge of the tentorium cerebelli (#15236). The nerve enters the orbit through the superior orbital fissure (#6958).

In addition to the somatic motor fibers, cranial nerve III contains preganglionic parasympathetic axons from the Edinger-Westphal nucleus for autonomic innervation of the eye. Depending on the author, this nucleus is either part of the oculomotor nucleus, or an adjacent separate nucleus.

By way of review, identify all levels of the brain stem by using the dorsal view of the brain stem (fig 4m) with links to each level (click on each view arrow). It is important to remember what cranial nerves are associated with each level. Specifically, what cranial nerves are associated with the medulla #7920, pons #7919, and midbrain #7918?

Click for the Syllabus Quiz for Chapter 4.

Movies

From The Digital Anatomist Interactive Brain Syllabus. John Sundsten and Kate Mulligan, Univ. Washington School of Medicine. 1998 ©

From David Morton, University of Utah.  These drawings can help you understand the complex distribution and components of a cranial nerve (IX)  or the innervation of a particular region.  You can enlarge and add different structures to the diagram.

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