Book - Embryology of the Pig 8
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Patten BM. Embryology of the Pig. (1951) The Blakiston Company, Toronto.
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Patten BM. Developmental defects at the foramen ovale. (1938) Am J Pathol. 14(2):135-162. PMID 19970381
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Chapter 8. The Development of the Nervous System
I. The Functional Significance of the Various Parts of the Nervous System
Without some knowledge of the functional significance of the various parts of the nervous system to serve as a basis for correlation and interpretation, its study, from either the developmental or anatomical point of view, is barren and discouraging. Therefore, even though it involves reviewing some familiar facts and also introducing some material which is more neuroanatomical than embryological, it seems advisable to summarize here certain conceptions which are essential to an understanding of the nervous system.
Neurons
The nervous system is made up of cells which are highly specialized in two of the fundamental properties of protoplasm, irritability and conductivity. These cells develop long cytoplasmic processes which extend from one part of the body to another, acting in the manner of telephone lines keeping the various parts of the organism in touch with each other and making possible prompt and coordinated response to alterations in internal or external conditions. In the nervous system of animals as complex as the vertebrates most lines of communication involve chains of such cells arranged so the ends of the processes of one cell come into close relation with the processes or cell body of another. When a change in environmental conditions (a stimulus) starts a wave of electro-chemical change (a nerve impulse) in the protoplasm of one cell in the chain, the wave traverses the processes of the cell in which it was initiated and passes on to the next cell in the chain, and so on. Each link in the chain, that is, each nerve cell with its processes, is called a neuron.
Synapses
The point at which the nerve impulse passes from the processes of one cell to the processes of another is known as a synapse. Synapses between neurons appear to be in the nature of ‘‘contacts†sufficiently intimate to permit the passage of a nerve impulse, but not ordinarily involving structural continuity of the cell processes. This contact type of relation at a synapse, which apparently is “made†or “broken†under varying physiological conditions, underlies such phenomena as alternative responses to a given stimulus. It implies the possibility of selective routing of the impulse over one of several neuron chains by the occurrence of physiological contact at certain synapses and physiological disjunction at others.
The arrangement of neuron chains or arcs, as they arc frequently called, is exceedingly complex in the vertebrate nervous system. Consideration of its details would carry us far afield, but it is quite possible to take up enough of the basic scheme of neuron arrangement so that the various parts of the nervous system assume some meaning in terms of function rather than remaining as mere names, without significance and readily confused.
Functional Classes of Neurons
All the myriad neurons which go to make up the central and peripheral nervous system are alike in that they are cells with attenuated processes specialized in conductivity. Among themselves they differ greatly as to location, relations, length, number and distribution of processes, and type and direction of impulses transmitted. Functionally neurons can be divided into three main groups. Some neurons carry impulses from sensory nerve endings and sense organs (receptors) toward the cord and brain. 'Fhesc' are said to be afferent neurons. Others conduct motor impulses away from the cord and brain to muscles or glands (effectors) which respond by appropriate activity. Such neurons are said to be efferent. In the cord, and especially in the brain, are countless neurons having many relatively short processes, which can transfer an incoming sensory impulse to any one of a number of efferent neurons with which their Various processes connect. These are association neurons. These three functional categories of neurons, afferent, association, and efferent, together with the receptors attuned to pick up various changes in internal or external conditions, and the effectors capable of carrying out the appropriate responses, constitute what we may call the action system of the organism.
Nerves
The various anatomical parts of the nervous system can be translated into terms of sensory, motor, and association neurons, and their characteristic activities. Thus, what in the dissecting room we call “nerves,†arc bundles of delicate neuron processes projected to various peripherally located structures. The nuclei and the bulk of the cytoplasm (cell bodies, cytons) of the neurons are either massed at some point on the nerve to form a ganglion, or buried in the central nervous system where they are spoken of as nuclear masses. The nerve itself consists only of the long slender neuron processes (nerve fibers) and the sheaths which protect them.
The Spinal Cord and Reflexes
The spinal cord is at once a center for automatic local responses to local stimulation, and a conduction pathway. In its capacity as a local center it receives sensory impulses coming to it by way of afferent neurons and sends out motor impulses over efferent neurons which activate effectors in the region stimulated. Such an automatic local response is known as a simple intrasegmental reflex and represents the most primitive type of mechanism in the vertebrate action system (Fig. 80, arc 1).
By transmission of a sensory impulse longitudinally along the cord through the agency of association neurons, the outgoing impulse may be imparted to efferent neurons in several adjacent metameres. Such a mechanism is termed an inter segmental reflex. It is a distinct step in advance over the strictly local reflex in that it affords a concerted response on the part of a group of effectors (Fig. 80, arc 2). The part of the cord primarily involved in segmental and intcrsegmental responses is the centrally located "gray matter." This gray matter is composed chiefly of association neurons and of the cell bodies of motor neurons which send their processes into the spinal nerves.
Cerebrospinal Conduction Paths
The peripheral "white matter" of the cord is composed of nerve fibers which run longitudinally, constituting pathways of intercommunication between the spinal cord and nerves, and the brain. The color from which this part of the cord takes its name and which sets it off so sharply from the richly cellular gray matter, is due to the sheaths enclosing the fibers. These sheaths are rich in a fatty substance (myelin) which imparts the characteristic whitish and glistening appearance.
Phylogenetically these conduction paths in the peripheral part of the cord increase in conspicuousness concomitantly with the increasing extent to which the brain assumes a coordinating control over the basic reflexes which constitute the primary function of the cord. The sensory (afferent) conduction paths are for the most part grouped in the dorsal portion, whereas the motor (efferent) paths are found in the lateral and to a less extent in the ventral portion of the cord. (Fig. 86, E.)
Reflexes Involving Organs of Special Sense
Much of the control exercised by the brain on the activities of the body as a whole depends on afferent impulses entering from sense organs of a much more highly specialized type than the endings in the general skin surfaces. Such organs as those of equilibrium, hearing, and sight play an exceedingly important part in determining the appropriate reactions of the various segmental effectors and thereby regulating the reactions of the animal as a whole.
Fig. 80. Schematic diagram showing the nature of the activities carried out in various parts of the central nervous system. (Collaboration of Dr. L. J. Karnosh.)
Positional changes affect special sensory end organs in the semicircular canals. Through sensory neurons the impulse passes to a synaptic center (Deiters’ nucleus) at the boundary between myclencephalon and metencephalon (Fig. 80, arc 3). Thence conduction fibers carry the impulse along an efl'erent path from which it may enter the appropriate local motor neurons at any segmental level. 'J'his arc would be involved in the automatic balancing reaction to a sudden upset in equilibrium such as slipping unexpectedly on a patch of ice.
Auditory and visual stimuli are conducted to centers in the midbrain. The auditory stimuli received in the cochlea are transmitted through a series of sensory neurons to synaptic centers in the inferior colliculi on both sides of the mesencephalon and thence by motor paths to the appropriate local motor fibers at any or all segmental levels (Fig. 80, arc 5a). This arc is the one responsible for the involuntary reaction to a sudden noise.
Responses to sudden visual stimulation involve an arc much similar to that described for reactions initiated by auditory stimuli. The visual arc (Fig. 80, arc 5b) starts with sensory fibers arising in the retina of the eye. In the superior colliculi of the mesencephalon these afferent fibers have synapses with association and motor fibers. The motor fibers communicate with the effector mechanism of the body over paths paralleling those involved in the responses to auditory stimuli. Through this arc would be brought about the automatic recoil from a sudden blinding flash of light.
Coordinating Centers
So far the action mechanisms described are purely reflex in character and relatively simple. In the complex and more deliberate actions involving large groups of muscles in several segments, a coordinating effect is exercised from a center in the cerebellum. This center receives sensory impulses (position sense) from all segmental levels in the body. These impulses pass by way of a series of neurons in the cerebellum (metencephalon), to a synaptic center (red nucleus) in the mid-brain. Thence (Fig. 80, arc 4) the impulse is conducted by motor fibers through the common motor pathways to the general body musculature. Such an arc provides for a smooth and precise, but unconscious, coordination of muscular action, such as that involved in slowly bringing together the tips of the forefingers of the two hands. This is the so-called synergic type of muscular control.
Certain fairly complex responses are, nevertheless, largely automatic or unconscious, such as the rhythmic swaying of the trunk and swinging of the arms in walking. These are regulated by association neurons passing through the thalamic region of the diencephalon and the corpus striatum of the telencephalon. Sensory fibers enter the thalamus from all afl'erent pathways. Thence the impulses (Fig. 80, arc 6) are transmitted to synaptic centers in the corpus striatum. From the corpus striatum they pass over motor fibers to the red nucleus whence they enter the common motor paths along with the motor fibers from the cerebellar centers.
Voluntary and Inhibitory Control
Superimposed on the primitive types of response is a mechanism affording a wide choice of reactions in response to the stimuli entering from the various afferent pathways. The centers for this, the highest and most plastic system, are in the cortical areas of the telencephalon. Fibers from practically all receptors enter this system by way of the thalamus (Fig. 80, arc 7). From synapses in the thalamus the afferent fibers arc dispersed according to their special functions into localized areas in the cerebral cortex. By myriads of association neurons these various centers are in free intercommunication. These centers are responsible for memory and for all choices of action conditioned by previous experience. In short, they are the centers of intelligent response, in distinction to reflex reactions to existing conditions. Through the intercommunicating associatidn neurons of this system motor impulses may be transmitted by way of the common motor paths of the spinal cord to any parts of the action system.
In becoming acquainted with the various regions of the central nervous system, therefore, we should think of the spinal cord and the myelencephalon as carrying out the dual role of reflex centers and conduction pathways to and from the higher brain centers. The cerebellum acts as a coordinating center for complex muscular actions such as those concerned with the maintenance of normal posture. The floor of the mesencephalon is made up primeirily of great fiber tracts passing to and from the higher brain centers. Specialized regions in the dorsal walls of the mesencephalon are concerned with visual and auditory reflexes. Of the two pairs of conspicuous elevations (corpora quadrigemina) the more cephalic pair (superior colliculi) are visual reflex centers and the more caudal pair (inferior colliculi) are auditory reflex centers. The thalamic region of the diencephalon serves as the gateway for fibers having cerebral connections. In the deeper part of each cerebral hemisphere is the corpus striatum which is concerned with muscle tonus and automatic associated movements. The more superficial portions of the cerebral hemispheres become specialized as the cerebral cortex. Certain cortical areas are the highest terminal centers for the reception of incoming impulses which keep us in touch with our environment, such as those resulting from auditory, visual, and tactile stimulation. Other cortical areas contain the cell bodies of neurons which are the first units in efferent chains. The connection of afferent and efferent areas by association neurons affords the connecting link placing the effector mechanisms of the body under voluntary control. Thus the cerebral hemispheres act as association centers superimposed on the lower reflex mechanisms and affording the possibility of intelligent choice of response based on experience.
II. Review of the Early Stages in the Establishment of Nervous System
The initial steps in the formation of the nervous system take place very early in development. Directly or indirectly, many points of importance in connection with its establishment and early differentiation have already been dealt with. We have seen the origin of the neural groove by the infolding of a thickened plate of ectoderm in the mid-dorsal line of the embryo; the closure of the neural groove to form the neural tube, and the coincident separation of the tube from the parent ectoderm (Figs. 24, 25, 28, 29, and 35).
In the closure of the neural groove, certain cells lying near its margins remain independent, being included neither in the walls of the neural canal nor in the superficial ectoderm as it closes above the newly established neural tube. These ribands of cells come to lie on either side in the angles between the superficial ectoderm and the neural tube and constitute the neural crests (Fig. 35). They are the primordia of the sensory root ganglia of the spinal and cerebral nerves and indirectly of the sympathetic ganglia.
Almost as soon as it is independently established the neural tube becomes markedly enlarged cephalically. This dilated portion is the primordium of the brain. Caudaliy the neural tube remains of relatively uniform diameter as the forerunner of the spinal cord.
In its enlargement the brain at first exhibits three regional divisions — the primary fore-brain, mid-brain, and hind-brain; or, to use their more technical synonyms, the prosencephalon, mesencephalon, and rhombencephalon (Fig. 36). This three- vesicle stage of the brain is short-lived. The prosencephalon is subdivided into two regions, telencephalon and diencephalon ; the mesencephalon remains undivided; and the rhombencephalic region becomes differentiated into metencephalon and myelencephalon. Thus in place of three vesicles, five are established. This stage in the development of the brain is well shown in embryos between 9 and 12 mm. in length (Figs. 59, 60, and 65). Starting with these familiar conditions as a basis we are ready to trace the later differentiation of some of the more important parts of the nervous system.
III. The Histogenesis of the Spinal Cord and the Formation of the Spinal Nerves
The Establishment of Ependymal, Mantle, and Marginal Layers
The ectoderm of the open neural groove is at first but a single layer of cells in thickness (Fig. 81, A). These original cells proliferate very rapidly and by the time the neural tube has become closed, its wall consists of many cell layers (Fig. 81, B). The individual cells, meanwhile, tend to lose their originally clear-cut outlines. In the older studies it was generally stated that at this stage the cells merged into a syncytium as suggested by the classical illustration of Hardesty reproduced here as figure 81. More recently Sauer has restudied such material and maintains that although the membranes become delicate and inconspicuous, if sufficiently well-preserved material is c;Sirefully studied the cells can be seen to retain their membranes intact. Toward the lumen the neural tube is bounded by an internal lim it in g ' fiiciiibi ' anc, anth peripherally ^ts extent is sharply marked by an external limiting membrane (Fig. 8l, C).
Certain of the cells lying near the lumen of the neural tube can, at this stage, be seen undergoing mitosis. They are called germinal cells (Fig. 81, C) although they are probably merely the cells in this region that happened to be dividing at the time the material was fixed. Most of the new cells formed by these cell divisions are crowded somewhat away from the internal limiting membrane into a zone in the cord which becomes densely packed with nuclei. This zone is called the mantle layer (Fig. 81, D). The cells which remain nearest to the internal limiting membrane become more or less elongated and radially arranged about the lumen of the neural tube. They constitute a zone within the mantle layer known as the ependymal layer of the cord. Outside the mantle layer is a peripheral region into which practically no nuclei enter. This is the marginal layer (Figs. 81, D, and 83).
Fig. 81. Stages in the histogenesis of the spinal cord. (After Hardesty.)
A, From open neural plate of rabbit embryo.
B, From wall of recently closed neural tube, 5 mm. pig embryo.
C, From neural tube of 7 mm. pig.
D, From neural tube of 10 mm. pig. (All drawings X 550.)
Spongioblasts and Neuroblasts
Of these three primary zones in the developing spinal cord the mantle layer is the first to show striking differentiation. Its cells continue to divide rapidly and undergo divergent specialization. Some of them become spongioblasts which are destined to form merely supporting tissue, and some of them neuroblasts, which will become the functionally active nerve cells.
These two types of cells can first be differentiated from each other by the fact that the neuroblasts develop large nuclei while the nuclei of the spongioblasts remain small.
Neuroglia
The formation of supporting tissue from the spongioblasts takes place by the development of exceedingly slender and irregular cytoplasmic processes. Some of these processes may eventually lose their association with the parent cells and appear as separate fibers. The majority of them, however, retain some connection with the cells from which they were derived. In this respect as well as in its ectodermal origin, neuroglia, as this peculiar connective^ tissue of the central nervous system is called, differs from the other connective tissues of the body. The fibers and processes formed from the spongioblasts are so delicate that they are exceedingly difficult to demonstrate in material stained by routine histological methods. But when they are subjected to metallic impregnation (e.g., the Golgi silver nitrate method) the ’glia cell processes and fibers appear as blackened strands forming an elaborate tracery of supporting elements throughout the substance of the cord (Fig. 82).
All neuroglia cells exhibit processes of one sort or another and all of them are supporting in function, but the cells differ much among themselves as to shape and arrangement of processes. For convenience in description they are commonly designated as belonging and are scattered along the developing myelinated nerve tracts of the white matter of the central nervous system.
Fio. 82. Segment of the spinal cord of a 70 mm. pig showing the differentiation of the neuroglial elements. (After Hardesty.) The upper part of the segment is drawn to show the *glia cells and fibers as demonstrated by silver impregnation. The lower part of the segment indicates their appearance after routine staining with hematoxylin and eosin.
Neuroblasts
Preparations of the spinal cord made by the usual methods do not show the neuroblasts to advantage (Fig. 83). If, however, a special technique such as intra-vitam staining by methylene blue or one of the metallic impregnation methods is used, the slender processes of the neuroblasts can readily be seen (Fig. 84). With the development of these characteristic nerve fibers we can think of the neuroblasts as having become young neurons.
The Formation of the Spinal Nerves
The outgrowth of processes from neuroblasts lying in the ventral and lateral portion of the mantle layer of the cord establishes the motor fibers which compose the ventral roots of the spinal nerves (Fig. 85). Neuroblasts in the dorsal root ganglia send to the cord afferent processes which constitute the dorsal roots of the spinal nerves, and send other processes peripherally which end in connection with various types of receptors (Fig. 85). Neuroblasts which have migrated from the cord and from the neural crest to form the sympathetic ganglia develop processes which relay efferent impulses thence to their destination (Fig. 85). Reference to figure 80 will show the relations of the neurons of the spinal nerves to the central nervous mechanism as a whole. Those interested in working out in more detail the functional significance of the various types of neurons encountered in the spinal nerves will find a brief analysis appended to the legend of figure 85. For more comprehensive information along these lines reference should be made to the discussion of "spinal nerve components" in a textbook of neurology.
The Development of the White and Gray Matter of the Cord
During the period of development when the neurons are being differentiated, the appearance of the spinal cord as seen in sections undergoes very marked changes. Some of the neuroblasts in the mantle layer of the cord, as we have seen, send out processes very early in development. Others remain undifferentiated and continue to proliferate for a time, causing continued growth in the mantle layer. As it grows in mass the mantle layer takes on a very characteristic configuration, becoming butterfiy-shaped in cross-section. With this change in shape, and with the transformation of its spongioblasts into neuroglia and its neuroblasts into characteristic nerve cells, the mantle layer becomes the so-called "gray matter" of the spinal cord (Fig. 86).
During the growth of the mantle layer the originally extensive lumen of the neural tube is reduced, by obliteration of its dorsal portion, to the small central canal characteristic of the adult cord (Fig. 86). The cells of the ependymal layer now constitute a sort of epithelioid lining of the central canal.
Fig, 85, Schematic diagram indicating the various connections made by the neurons which develop in a typical spinal nerve. (Modified from Froriep.)
The neurologist classifies the fibers in a spinal nerve according to their relations and functions. The components of a typical spinal nerve on this basis are:
I. Afferent
A. General Somatic Afferent
(1) Exteroceptive i.e., fibers conducting impulses from the external surface of the body such as touch, pain, temperature. (Represented in this figure by short broken lines.)
(2) Proprioceptive i.e., fibers carrying impulses of position sense from joints, tendons, and muscles. (Not represented in this diagram.)
Meanwhile the outer or marginal layer of the cord has been increasing extensively in mass. Its growth is due to the secondary ingrowth of longitudinally disposed neuron processes which constitute the conduction paths between the various levels of the spinal cord and the brain (Fig. 80). Each of these fibers being enveloped in a sheath rich in myelin, the region of the cord in which they lie has a characteristic whitish appearance which contrasts strongly with the gray color of the richly cellular portion of the cord derived from the mantle layer. For this reason the fibers which develop in the marginal layer of the cord are said to constitute its white matter. The main groups of these fibers are more or less marked off from each other by the dorsal and ventral horns of the gray matter. They are known as the dorsal, lateral, and ventral columns of the white matter of the cord (Fig. 86). The dorsal columns contain the main tactile and proprioceptive paths to the brain; the ventral columns are primarily motor; and the lateral columns contain irnportant ascending fiber tracts to the brain and also some of the main motor paths from brain to cord and thence to spinal nerves (Fig. 80).
B. General Viscerai. Afferent
Fibers from viscera (interoceptive) by way of sympathetic chain ganglion, white ramus communicans, and dorsal root; cell bodies in dorsal root ganglion; no synapse before reaching cord. (Illustrated in this figure by dotted line.)
II. Efferent
A. General Somatic Efferent
Motor neurons to skeletal muscle; cell bodies in ventral columns of gray matter; fibers emerge by ventral roots. (Illustrated in this figure by solid lines.)
B. General Viscerai. Efferent
Two-neuron chains from cord to glands and to smooth muscle of viscera and blood vessels. I'he first neurons (preganglionic) have their cells of origin in lateral column of gray matter of cord from first thoracic to third lumbar level. Fibers leave cord by ventral root, turn off in white ramus communicans to end in synapse with the second neurons (postganglionic) of the two-neuron chain.
Note the various destinations of the visceral efferent paths, e.g.:
(1) fibers to smooth muscle of gut wall; impulse relayed by second order motor neuron.^ from synapses in mesenteric ganglion.
(2) fibers (vaso-motor) to .smooth muscle of blood vessel wall; impulses relayed by second order motor neurons from synapses in prevertebral ganglia.
(3) fibers (pilo-motor) to muscles about hair follicles, and fibers (sudomotor) to sweat glands in skin. Impulses in both these cases relayed from synapses in prevertebral ganglia by second order motor fibers passing back over the ramus communicans and thence to periphery via branches of spinal nerve.
Most of the fibers in the spinal nerve are medullated and therefore whitish in appearance. The visceral second order motor (‘‘post-ganglionicâ€) fibers, however, lack a myelin sheath atid are grayish in appearance. Certain of such second order visceral motor fibers (e.g. pilo-motor and sudo-motor) which run back along the ramus communicans after synapse in a prevertebral ganglion account for the so-called ^‘gray bundle of the ramus communicans.
Fig. 86. Transverse sections through spinal cord of the pig at various ages. Note especially the parts of the adult cord derived from the ependymal, mantle, and marginal layers of the embryonic neural tube.
IV. The Regional Differentiation of the Brain
It will be recalled that in embryos of about 5 mm. the brain was just beginning to progress from the three- to the five- vesicle stage. In embryos of the 9 to 12 mm. range we saw the five-vesicle condition of the brain well established. These same five basic regions of the brain will continue to be recognized as the major divisions of the adult brain. During their later development they become greatly altered in appearance and certain specialized parts of them receive new names, but their fundamental relations remain the same. The details of all the structural features which appear in the various parts of the brain constitute too complex a story to be satisfactorily handled in brief compass. Accordingly we shall confine ourselves to Jbecoming acquainted with the main morphological landmarks and the locations of the principal functional centers of the brain — the bare foundation on which subsequent work may build a fuller knowledge of this interesting system.
The Myelencephalon
The myelencephalon of the embryo becomes the medulla of the adult brain (Fig. 87). Very early in development the lumen of this part of the neural tube becomes dilated, foreshadowing its ultimate fate as the large cavity in the medulla known as the fourth ventricle (Fig. 88). At the same time its roof becomes very thin (Fig. 88, A). Small blood vessels develop against this membranous roof and push it ahead of them into the lumen of the fourth ventricle. The freely branching group of vessels thus formed is known as the choroid plexus of the fourth ventricle (Figs. 65, 99, and 106).
The walls of the neural tube in the brain region show the same early histological changes which occur in the walls of the spinal cord, with the resulting establishment of ependymal, mantle, and marginal layers. The ependymal layer of the myelencephalon becomes the epithelioid lining of the fourth ventricle. Its mantle layer gives rise in part to continuous columns of gray matter as in the cord, and also forms more or less distinct cell masses (nuclei) associated with the roots of the more posterior cranial nerves (Fig. 91). The marginal layer receives an ingrowth of longitudinally disposed medullated fibers constituting the conduction pathways between the spinal cord and nerves and the more rostral parts of the brain (Fig. 80).
In dealing with the topography of the neural tube as it appears in cross-sections it is customary to designate its thickened side-walls as the lateral plates, its thin dorsal wall as the roof plate, and its thin ventral wall as the floor plate. On this basis the membranous covering of the fourth ventricle represents a roof plate greatly stretched out by the divergence of the lateral plates dorsally. (Compare the configuration of the cord, as shown in figure 85, with that of the medulla as diagramed in figure 91.) The deep ventral groove in the floor of the ventricle overlies a very thin floor plate. The great bulk of the myelencephalic wall consists of thickened lateral plates. Extending along the inner surface of each lateral plate is a longitudinal sulcus {sulcus limitans) which suggests a division of the lateral plate into a dorsal part {alar plate) and a ventral part {basal plate^ Fig. 91). The sulcus limitans is especially strongly marked during the early stages of the development of the myelencephalic region. Later it becomes masked in certain regions by the growth of underlying nuclei, but wherever it persists it is a valuable landmark in dealing with the location of nuclei and fiber tracts. In the brain, as in the cord, afferent centers develop dorsal, and efferent centers ventral to the sulcus limitans (Fig. 91).
The Mctencephalon
The dorso-lateral walls of the neural tube in the metencephalic region undergo very extensive growth and give rise to the cerebellum of the adult brain. The early smooth outline of this region (Fig. 87, C) is broken up by the development of a complex series of folds (Fig. 87, D, E). Eventually three main lobes are formed, each of which is subdivided into a great number of minor folds which impart a very characteristic appearance to the cerebellum (Fig. 87, F). In this part of the brain are developed the synaptic centers concerned with the coordination of complex muscular movements (Fig. 80, arc 4).
Relatively late in development great groups of fibers which form the paths of intercommunication between the cerebellum and other parts of the nervous system appear superficially in the walls of the mctencephalon. These form the ventral prominence known as t\it pons (Fig. 87, E, F), and the cerebellar peduncles which extend over the lateral walls of the mctencephalon. Deep to, and partly intermingled with, these superficial groups of fibers lie continuations of the same longitudinal fiber tracts which, on their way to and from the brain, traverse the iparginal layer of the medulla. Still deeper are the masses of cells which originate from the mantle layer of this part of the neural tube. These cells are clustered in definite centers (nuclei) associated with the cranial nerves of the metencephalic level (see Fig. 91).
The original lumen of the neural tube in the metencephalic region remains of considerable size. Since there is no line of demarcation between it and the lumen of the medulla, it is regarded as the anterior part of the fourth ventricle (Fig. 88).
The Mesencephalon
The dorso-lateral walls of the mesencephalon give rise to two pairs of rounded elevations known as the corpora quadrigemina (Fig. 87, E). The two more rostral prominences, called the superior colliculi^ are the synaptic centers for visual reflexes (Fig. 80, arc 5b); and the two more caudal prominences, called the inferior colliculi^ are the synaptic centers for auditory reflexes (Fig. 80, arc 5a).
The ventro-lateral parts of the mesencephalic walls constitute the main pathway over which fibers pass to and from the more anterior parts of the brain (Fig. 80, arcs 6 and 7). The fact that these fiber tracts in the mesencephalon are continuations of the longitudinal tracts encountered in the myelencephalon and in the floor of the metencephalon should be especially emphasized. These tracts in the mesencephalic floor are designated as the cerebral peduncles.
With the great thickening of its walls, the lumen of the mesencephalon becomes relatively reduced to form a narrow canal joining the lumen of the metencephalon and myelencephalon (fourth ventricle) with the lumen of the diencephalon (third ventricle). This canal is known as the cerebral aqueduct or aqueduct of Sylvius.
The Diencephalon
Although the diencephalon undergoes striking local modifications, its original name is still retained in the terminology of adult anatomy. Its roof becomes thin and vessels developing on its outer surface force it ahead of them in finger-like processes which project into the third ventricle as the anterior choroid plexus (Fig. 106).
In the median part of the diencephalic roof, caudal to the point of origin of the choroid plexus, the epiphysis appears as a small local evagination (Fig. 100). Later in development the walls of the epiphysis become thickened and its lumen is practically obliterated.
In the floor of the diencephalon is formed a median diverticulum called the infundibulum. The distal portion of Rathke’s pocket loses its original connection with the stomodaeal ectoderm and becomes closely applied to the infundibulum (cf. Figs. 65 and 138). Somewhat later these two structures become intimately fused to form an endocrine gland known as the hypophysis (Fig. 100).
Very early in development the optic vesicles arise as outgrowths from the ventro-lateral walls of the prosencephalon (Figs. 36, E, and 41, D). When the prosencephalon is divided into telencephalon and diencephalon the optic stalks open into the brain very near the new boundary (Figs. 60 and 67). In fact the median depression in the floor of the brain opposite their point of entrance is regarded as the ventral landmark which establishes the demarcation between telencephalon arid diencephalon (recessus opticus^ Figs. 65 and 88, *A)« Immediately caudal to the optic recess there is a marked thickening
Fig. 88. Diagrams to show the topography of the brain shortly after the transition from the three- to the five-vesicle stage. A, Sagittal section. The conventional lines of demarcation between adjacent brain vesicles are indicated by broken lines. B, Surface view of brain with position of cranial ganglia and nerve roots indicated. C, Schematic frontal plan of brain as it would appear if the flexures had all been straightened out before cutting.
in the floor of the diencephalon where part of the fibers of each optic nerve cross to the other side of the mid-line. This point of crossing of optic nerve fibers is known as the optic chiasma (Figs. 65 and 88, A). Beyond the chiasma the crossed and uncrossed fibers on either side run together as the optic tracts. The optic tracts pass along the lateral walls of the diencephalon where some of their fibers end. Others pass to the visual reflex centers in the superior colliculi (Fig. 80, arc 5b). Still others concerned with the interpretation and memory of visual impulses pass by way of the geniculate nuclei to the visual areas of the cerebral cortex.
The dorsal parts of the lateral walls of the dicnccphalon become greatly thickened by multiplication of neuroblasts in the mantle layer. These thickened regions are known as the thalami (Fig. 100). The dorsal portion of the thalamus is the gateway of fibers passing from the cord and the brain-stem^ to the cerebral hemispheres (Fig. 80, arcs 6 and 7). In it are large synaptic centers acting as relay stations. Superficial t© these nuclear masses are fiber tracts radiating through the lateral diencephalic walls.
The thickening of the lateral walls of the diencephalon greatly reduces the width of its lumen. In its central portion the two walls come in contact and fuse, forming across the third ventricle a conspicuous connection known as the massa intermedia.
The Telencephalon
The telencephalon consists of the most rostral part of the neural tube together with paired dorso-lateral outgrowths from the primary median portion. These outgrowths first appear as roughly hemispherical evaginations called the lateral telencephalic vesicles (Figs. 60 and 88, C). Although the division of the lateral walls of the neural tube into alar and basal plates is not clearly marked this far forward in the brain, the telencephalic evaginations, because of their general relations, are regarded as involving the alar plates.
At first the cavities within the two telencephalic vesicles are broadly continuous with the primary lumen of the neural tube (Fig. 67). Later in development these openings into the lateral vesicles appear relatively much smaller; nevertheless they persist, even in the adult, as the so-called joramina of Monro, Thus in spite of extensive local modifications the original neural canal remains open throughout the entire length of the central nervous system. Its most rostral parts, the cavities in the telencephalic vesicles {first and second ventricles of the adult brain), communicate with the median telencephalic lumen by way of the foramina of Monro. Since there is no line of demarcation between this small median telocoele and the diocoele, both are included in the cavity of the adult brain known as the third ventricle
- Brain-stem is a commonly used term for designating those portions of the brain other than the telencephalon, diencephalon, and cerebellum.
Fig. 89. Model made from a dissection exposing the nervous system of an 18 mm. pig embryo. (After Prentiss.)
(Fig. 88, C). From the third ventricle the cerebral aqueduct leads through the mesencephalon to the fourth ventricle^ which is the adult term for the confluent lumina of the metencephalon and myelencephalon. The fourth ventricle becomes narrowed caudally and is directly continuous with the central canal of the spinal cord (Fig. 88, C).
The lymph-like fluid which fills these cavities in the nervous system appears to be derived primarily from plexuses of small blood vessels which invade thin places in the dorsal wall of the brain. One such mass of vessels we have already encountered under the name of the posterior choroid plexus or choroid plexus of the fourth ventricle. Another appears in the rostral part of the diencephalic roof (Fig. 106). This anterior choroid plexus grows into the third ventricle. Closely associated plexuses push through each foramen of Monro into the lateral ventricles of the telencephalic lobes, constituting the lateral choroid plexuses (Fig. 100). It should perhaps be emphasized that the blood vessels of the choroid plexuses do not break through the brain roof but push it ahead of them so that although they appear to lie in the ventricles they are always separated from the lumen by a thin enveloping layer derived from the ependymal layer of the dorsal wall of the neural tube.
Once established, the lateral lobes of the telencephalon undergo exceedingly rapid growth. Their extension rostrally conceals the median portion of the telencephalon, and their even greater expansion dorsajiy and caudally eventually covers the entire diencephalon and mesencephalon (Fig. 87). At first the telencephalic lobes are smooth in contour and without striking local differentiations (Fig. 87, B-E). Relatively late in development they become much convoluted and certain regional divisions become clearly marked.
A conspicuous fissure called the sulcus rhinalis divides the ventral part of the telencephalic lobes from the dorsal (Fig. 87, F). The region ventral to the sulcus rhinalis is chiefly concerned with the olfactory sense and is, therefore, often called the rhinencephalon. It includes the olfactory bulb, the olfactory tract, and the pyriform lobe (Fig. 87, F). The rhinencephalon reaches its maximum development in lower forms. In higher mammals it becomes largely overshadowed by the tremendous growth of the more dorsal portions of the cerebral cortex.
Dorsal to the sulcus rhinalis the outer walls (pallium) of the telencephalic vesicles constitute the non-olfactory portions of the cerebral cortex. These cortical areas are phylogenetically the newest portions of the brain. In them are located the suprasegmental centers concerned with memory, voluntary action, and inhibitory control (Fig. 80, arc 7).
In forms such as the mammals, where the cerebral hemispheres are especially highly developed, the cortex is extensively folded. All the principal folds (gyri) and grooves (sulci) are named and the association centers for various special functions have, with considerable accuracy, been located in specific areas. It would carry us beyond the scope of this book, however, were we to attempt to do more than become familiar with the major regional divisions of the cerebral hemispheres known as the frontal, parietal, temporal, and occipital lobes (Fig. 87).
Fig. 90. Model made from a dissection exposing the nervous system of a 35 mm. pig embryo, (After Prentiss.)
The corpus striatum is developed in the mande layer of the ventrolateral walls of the telencephalon. Its location in the tt'lencephalic floor makes it more closely associated positionally with the rhinencephalic region than with the pallial region which gives rise to the cerebral cortex. In its development it tends to bulge from either side in toward the lumen of the lateral ventricles (Figs. 99 and 100), becoming eventually one of the conspicuous internal landmarks in the telencephalic region. It derives its name from the fact that when sectioned it shows alternate layers of fibers and cellular substance arranged in more or less regular bands. Two large nuclear masses, the caudate nucleus and the lentijorm nucleus^ make up the major part of the corpus striatum. Its connections are very complex and many of them are not as yet entirely worked out. Whatever its other activities may be, it is clearly involved in the co5rdination of certain complex muscular activities (Fig. 80, arc 6). It also appears to exert a steadying influence on voluntary muscular actions generally, because interference with it is followed by the appearance of tremors during movement.
V. The Cranial Nerves
In dealing with the spinal nerves we recognized four functional types of neurons, somatic afferent, somatic elferent, visceral afferent, and visceral efferent (Fig. 85). In the cranial nerves wc find these same types of neurons and in addition other subtypes with more restricted distribution and more specialized function. The eye and the ear, for example, are very highly differentiated and sharply localized somatic sense organs. Therefore their fibers are set apart from the general somatic afferent category as special somatic afferent fibers. The musculature in the pharyngeal region differs from other visceral musculature in that it is striated. So the motor fibers to it are distinguished from other visceral efferent fibers by calling them SPECIAL visceral efferent neurons.
The spinal nerves are segmentally arranged and all of them are built on the same general plan. The cranial nerves have lost their segmental arrangement and become very highly specialized. Some of them contain both sensory and motor fibers as is the case with the spinal nerves. These are called mixed nerves. Some contain only motor fibers and others only sensory fibers. No single nerve contains ail the types of fibers which occur in the cranial nerves as a group as diagramed in figure 91.
In both spinal and cranial nerves, afferent fibers arise from cell bodies outside the neural tube (cf. Figs. 85 and 91). Thus the cranial nerves which carry afferent fibers have ganglia composed of clusters of their cell bodies situated just outside the brain wall (Fig. 92). Likewise the efferent fibers in the spinal and the cranial nerves are similar in that they arise from cell bodies inside the wall of the neural tube. In the spinal nerves these cell bodies lie in the ventral and lateral horns of the gray matter of the cord. In the cranial nerves their position is homologous for they lie in clusters (nuclei) in the basal plate of the brain wall (cf. Figs. 85 and 91).
If one considers these general facts it becomes apparent that, although the cranial nerves differ from the spinal nerves in many respects, we find the same types of neurons involved and the same characteristic difference in the position of the cell bodies which so clearly sets apart afferent and efferent fibers. Put in another way, the cranial and spinal nerves contain the same types of components differently grouped. It will be helpful to keep this in mind in considering the various cranial nerves.
The Olfactory Nerve (I)
Unlike other sensory nerves the olfactory nerve lacks a ganglion. It is peculiar, also, in that all its fibers are non-medulla ted. These fibers arise from cells in the epithelial layer lining the olfactory pits (Fig. 93). Thence they grow centripctally into the olfactory bulbs. In the olfactory bulbs the fibers of the nerves terminate in synapses with other neurons which relay the impulses along the olfactory tracts (Fig. 87) to centers in the r hinencephalon .
The Optic Nerve (II)
As is the case with the olfactory nerve, the optic nerve fibers arise from peripherally located cells and grow centripetally. Neuroblasts situated in the sensory layer of the retina in close association with its photosensitive cells send out processes which leave the optic cup through the choroid fissure. These fibers then traverse the grooved ventral surface of the optic stalk and enter the brain in the diencephalic floor. At their point of entrance the two optic nerves^ intersect. At the intersection, part of the fibers from each nerve cross over to the opposite side so that each eye has central connections with both sides of the brain. It will be recalled that the point of the nerve intersection and fiber crossing is known as the optic chiasma (Fig. 65). From the chiasma the fibers pass along the lateral walls of the diencephalon to the visual reflex centers in the superior colliculi (Fig. 80, arc 5b), and by way of relay centers (geniculate nuclei) impulses are sent also to visual correlation centers in the cerebral cortex.
- Since both the photosensitive cells themselves and the retinal ganglion cells sending out their associated nerve fibers arise in the walls of the optic cup, which is in turn an evaginalion of the embryonic fore-brain, what we commonly call the optic nerve is, strictly speaking, not a nerve but a fiber tract arising within a modified portion of the brain wall.
Fig. 91. Diagram showing the central relations of the various types of fibers in cranial nerves. (Patten: ‘‘Human Embryology,†The Blakiston Cpmpany.) It should be emphasized that the diagram is a schematic composite and that no one cranial nerve contains all the types of fibers shown.
The Oculomotor Nerve (III)
As its name implies, the oculomotor nerve contains efferent fibers to muscles moving the eye. The cluster of neuroblasts from which it arises is located in the basal plate of the mesencephalon. Its fibers have internal relations in general comparable to those indicated in figure 91 for the fibers of nerve XII leading from the somatic efferent nucleus to the intrinsic muscles of the tongue. Emerging from the floor of the mesencephalon (Figs. 92 and 93) they pass directly to the orbital region and innervate the inferior oblique, and the superior, inferior, and internal rectus muscles of the eyeball.
The Trochlear Nerve (IV)
The trochlear nerve is a motor nerve to the superior oblique muscle of the eye. Its nucleus of origin, like that of the third nerve, is located in the basal plate of the mesencephalon. It is peculiar in that its fibers do not leave directly from the ventro-lateral walls of the brain as usually happens in a motor nerve. Instead they pass to the dorsal wall of the mesencephalon (Figs. 92 and 93) and cross before emerging.
The Trigeminal Nerve (V)
The trigeminal nerve takes its name from the fact that it has three main divisions, the ophthalmic, the maxillary, and the mandibular (Figs. 89, 90, and 92). As is indicated by its large semilunar ganglion, the fifth nerve has great numbers of sensory fibers. There are, nevertheless, enough motor fibers associated with its mandibular branch so it must be regarded as a mixed nerve (Figs. 91 and 94). The names of its branches clearly indicate its distribution to the facial region. Its sensory fibers have relations of the nature indicated in figure 91 by the fiber leading from the skin of the lip to the general somatic afferent column.
Fig. 91 — {Continued)
As was the case with the spinal nerves (Fig. 85), the efferent nuclei or clusters of efferent nerve cells are located ventro-laterally, in the basal plate of the neural tube wall. The afferent fibers of the cranial nerves have their cell bodies located outside the neural tube in ganglia (cf. Fig. 92). The afferent nuclei or columns in the alar plate of the walls of the neural tube are clusters of cell bodies belonging to neurons which relay the incoming impulses to other parts of the brain. (Cf. Fig. 80, arc 5a.) For further explanation see text.
- Not even sufficient knowledge of Greek to recognize in the word trochlear the root meaning pulley is of much immediate help in understanding its significance as used in anatomy. One has to find that the nerve takes its name from an old appellation of the superior oblique musdle to which it runs. This muscle was formerly known as the trochlear muscle because its tendon passes through a pulley-like fibrous loop attached to the eye-socket.
The Abducens Nerve (VI)
The abducens nerve takes its name from the fact that it controls the external rectus muscle, contraction of which makes the eyeball rotate outwards. Its nucleus lies in the basal plate of the myelencephalon from which its fibers emerge ventrally just caudal to the pons, and pass toward the orbit (Figs. 92 and 94).
The Facial Nerve (VII)
The facial nerve is primarily motor but the presence of the geniculate ganglion on its root shows that it carries also some sensory fibers. A large part of its sensory fibers pass by way of the chorda tympani branch (Figs. 91 and 92) to join the mandibular branch of the fifth nerve. These fibers are concerned with the sense of taste. Its motor fibers arise from a nucleus situated in the basal plate of the myelencephalon and innervate the muscles of facial expression.
The Auditory Nerve (VIII)
At first the ganglionic mass from which the fibers of the eighth nerve arise is closely associated with the geniculate ganglion of the seventh nerve (Fig. 92). Gradually these ganglia become entirely distinct. Still later the ganglion of the eighth nerve divides into two parts, a vestibular ganglion and a spiral ganglion. With the division of the ganglion, the nerve fibers arising from its cells becorhe grouped into two main bundles, one associated with each ganglion. Meanwhile the otic vesicle has differentiated into twp distinct parts, the cochlea, which is the organ of hearing, and the group of semicircular canals which, together with the utriculus and sacculus, constitute an organ of equilibration. The spiral ganglion and the cochlear branch of the eighth nerve become associated with the auditory part of the mechanism. The vestibular ganglion and its branch of the eighth nerve become associated with the semicircular canals. The auditory and equilibratory fibers have the general relations indicated in figure 91 by the sensory fibers connecting, respectively, with the cochlea and with the ampullae of the semicircular canals.
Fig. 93. Drawing (X 14) of parasagittal section of head of 15 mm, pig embryo. The section is to the right of the mid-line, in a plane especially favorable for showing the relations of the nasal pits and of the olfactory (I) and oculomotor (III) nerves.
The Glossopharyngeal Nerve (IX)
The glossopharyngeal is a mixed nerve but by far the greater number of its fibers are sensory. The ganglion cells from which these sensory fibers arise are grouped in two clusters, one near the root of the nerve (superior ganglion), and one farther peripherally on its course (petrosal ganglion) (Figs. 92 and 94). The cell bodies in the superior ganglion give rise to fibers which innervate a small cutaneous area of the external ear. These neurons are, therefore, general somatic afferent. The petrosal ganglion contains cell bodies which give rise to visceral afferent fibers. Some of these, being concerned with general sensibility in the region about the root of the tongue, are general visceral afferent (Fig. 91). Other fibers innervate taste buds in the back parts of the tongue and so are classified as special visceral afferent. The efferent fibers arise from nuclei in the basal plate of the myelencephalon. Some of these fibers innervate the stylopharyngeus muscles. These are special visceral efferent neurons. Others are secretory fibers to the parotid gland by way of the otic ganglion. These are general visceral efferent fibers.
The Vagus Nerve (X)
The vagus is a mixed nerve carrying five different types of fibers. General somatic afferent fibers arise from cells in its jugular ganglion and extend peripherally to the skin in the region of the external ear. General visceral afferent fibers arise from cells in the nodose ganglion and extend peripherally to the pharynx, larynx, trachea, esophagus, and the thoracic and abdominal viscera. Special visceral afferent fibers having cells of origin in the nodose ganglion carry gustatory impulses from scattered taste buds in the region of the epiglottis. From nuclei of origin in the myelencephalon special visceral efferent fibers extend to striated muscles of the pharynx and larynx. General visceral efferent fibers run to various terminal ganglia whence the impulses are relayed to the visceral musculature over second-order motor neurons (Fig. 91).
Fig. 94. Drawing (X 14) of parasagittal section of head of 15 mm. pig embryo. The plane of section is slightly farther to the right than that shown in the preceding figure. It is particularly favorable for showing the position of origin, and the ganglia, of the trigeminal (V), glossopharyngeal (IX), and vagus (X) nerves.
The Accessory Nerve (XI)
The commissural ganglion (Fig. 92) which appears so closely associated with the accessory nerve is really a continuation of the jugular ganglion of the vagus. Froriep’s ganglion usually disappears in the adult so the accessory nerve is left without ganglia. Its fibers, practically all efferent, originate not only from the posterior part of the myelencephalon but also from the first five or six segments of the spinal cord (Fig. 92). A large number of the fibers of the accessory nerve run with the general visceral efferent fibers of the vagus nerve to sympathetic ganglia from which the motor impulses are relayed to the smooth muscle of the viscera. Other fibers (special visceral efferent) from the accessory join similar vagus fibers to striated muscles in the pharynx and larynx. Most of the fibers arising from the cervical part of the cord turn off in the external ramus to end in the trapezius and sterno-cleido-mastoid muscles.
The Hypoglossal Nerve (XII)
The hypoglossal nerve is composed practically entirely of somatic motor fibers. They arise from an elongated nucleus in the posterior part of the myelencephalon (Fig. 91) and emerge in several separate roots which join to form a single main trunk (Fig. 92). Peripherally they are distributed to the muscles of the tongue.
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Cite this page: Hill, M.A. (2024, April 16) Embryology Book - Embryology of the Pig 8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Embryology_of_the_Pig_8
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