GENERAL
The cephalic and cervical flexures become more pronounced as a sharp dorsal bend called the pontine flexure appears in the middle of the rhombencephalon.
The brain and spinal cord are surrounded by a loose network of cells called the primitive meninx from which develop the meninges (protective membranes) and some bones of the skull.
BRAIN
Telencephalon
A cerebral vesicle forms on each side as an outpouching of the lateral wall leaving a midportion called the telencephalon medium.
Cerebral Vesicle
Each vesicle can be subdivided into a thick basal striatal part and a thin dorsal suprastriatal part. The rhinencephalon or smell brain that is composed of several regions of the telencephalon is becoming apparent. The primordial tuberculum olfactorium is the clear, marginal area on the ventral surface of the cerebral vesicle. In front of this area the olfactory fila from the olfactory epithelium are about to enter the telencephalon. The primordial hippocampal formation is represented by a) a clear, marginal layer in the dorsal part of the cerebral vesicle called the primordial gyrus dentatus and b) the more dense area immediately lateral called the primordial cornu Ammonis.
Telencephalon Medium
The floor of the telencephalon medium is composed of the thin lamina terminalis and the preoptic area, both of which are located just rostral to the chiasmal area of the diencephalon. The roof of the telencephalon medium is continuous with the roof of the diencephalon and with it constitutes the roof of the third ventricle. Near the junction with the diencephalon the roof bulges to produce the paraphyseal arch.
Prosocoele
That part of the prosocoele that extends into the cerebral vesicle on each side will become the lateral ventricle. The part remaining in the midline becomes the third ventricle. The interventricular foramen is developing in the junction region.
Diencephalon
The sulcus dorsalis appears in the lateral wall and separates the dorsal thalamus from the epithalamus. The roof of the epithalamus evaginates to produce the pineal bud.
The sulcus medius and sulcus ventralis become better defined. The sulcus ventralis continues rostrally as the optic groove, which passes into the optic stalk.
Two ventral bulges develop in the hypothalamus: a) a rostral, midline bulge, the neurohypophyseal bud, adjacent to the hypophyseal pouch and b) a caudal bulge on each side called the mammillary area. The third ventricle above the mammillary area forms the mammillary recess. The marginal area increases in thickness in the optic chiasmal area.
Mesencephalon
The sulcus limitans is very prominent and divides the lateral wall into a dorsal alar plate and a ventral basal plate.
Three adult subdivisions—The alar plate on each side together with the roof plate form the tectum. The basal plate on each side together with the floor plate form the tegmentum. The marginal layer ventral to the tegmentum increases in width producing the basis pedunculi area (crus cerebri).
The caudal, constricted portion that is continuous with the rhombencephalon is called the isthmus.
The mesocoele connects the third ventricle with the rhombocoele.
Rhombencephalon
The rhombocoele dilates into a diamond shaped chamber forming the fourth ventricle. It is continuous caudally with the narrow neural canal of the spinal cord. The thin roof of the fourth ventricle also enlarges and becomes diamond shaped.
The basal plates are separated in the midline by a deep median sulcus. The alar plates become separated as the roof enlarges and move to a position dorsolateral to the basal plates. The sulcus limitans marks the boundary between the alar and basal plates.
Rhombomeres are sometimes apparent on the inner surface of the ventral wall but disappear on the outer surface.
The pontine flexure divides the rhombencephalon into a rostral portion, the metencephalon, and a caudal portion, the myelencephalon.
Metencephalon—The marginal layer of the basal plate begins to thicken to produce the pons area. The lateral edge of the alar plate is called the rhombic lip where the cerebellum will later develop.
Myelencephalon—The lateral wall of the myelencephalon is divided into large alar and basal plates. In the rostral part of the myelencephalon the alar plates are separated from each other by the roof of the fourth ventricle. Here they are in a position dorsolateral to the basal plates and contribute to the formation of the floor of the fourth ventricle. In the caudal part of the myelencephalon the alar plates approach each other and are separated only by a narrow strip of roof plate. The marginal layer of the basal plate begins to thicken to form the pyramidal tract area.
SPINAL CORD
The spinal cord enlarges and extends into the tail bud where it terminates adjacent to the ectoderm dorsally. The most caudal segment represents the primitive filum terminale, which will increase in length when the spinal cord fails to keep pace with the longitudinal growth of the vertebral column.
The ependymal, mantle and marginal layers become very distinct. The neuroblasts are mainly in the mantle layer. The basal portion of the mantle layer is thick and has a marginal layer of uniform thickness at its periphery. The marginal layer is thickest dorsally where the dorsal roots from spinal ganglia enter the cord. The floor plate thickens but the roof plate remains thin.
PERIPHERAL NERVOUS SYSTEM
CRANIAL NERVES
All twelve cranial nerves can be identified.
The olfactory nerve (I) is represented as fila among a cellular condensation between the olfactory epithelium and the ventral surface of the cerebral vesicle.
The optic nerve (II) is in the form of the optic stalk that joins the optic cup to the diencephalon. The stalk lumen begins to narrow.
The oculomotor (III), trochlear (IV) and abducens (VI) nerves appear as small fascicles that terminate in premuscle condensations dorsal to the optic cup.
Each of the four cranial neural crests transforms into distinct cranial nerves with motor and sensory components. The sensory ganglion (or ganglia) on each nerve becomes distinct and loses contact with its epibranchial placode. The placode disappears as the ganglion forms.
The three major divisions or branches of the trigeminal nerve (V) become apparent. The ophthalmic nerve (V1) passes dorsal to the optic stalk, the maxillary nerve (V2) courses into the maxillary process and the mandibular nerve (V3) proceeds into the mandibular process.
The facial part of the facioacoustic neural crest gives rise to the facial nerve (VII). The greater petrosal and chorda tympani branches can be traced into the first branchial arch where they communicate with the maxillary and mandibular nerves, respectively. The acoustic part of the facioacoustic neural crest contributes to the formation of the vestibulocochlear nerve (VIII). The vestibular and cochlear (spiral) ganglia develop in the region where the crest cells make contact with the otocyst.
The glossopharyngeal nerve (IX) passes through the third branchial arch and terminates in the floor of the pharynx just caudal to the foramen cecum linguae.
The vagus nerve (X) gives rise to its superior laryngeal and recurrent (inferior) laryngeal branches. It can be traced as far caudally as the stomach. The cranial part of the accessory nerve (XI) becomes a part of the vagus nerve and its recurrent laryngeal branch.
The spinal accessory nerve (XI) becomes a very distinct fascicle, which terminates in the sternocleidomastoid-trapezius premuscle mass dorsal to the cervical sinus.
The hypoglossal nerve (XII) appears as a large, discrete fascicle passing lateral to the vagus nerve and terminating in the premuscle masses of the tongue. It forms communications with the upper three cervical spinal nerves.
SPINAL NERVES
Sensory neuroblasts differentiate from neural crest cells and aggregate in each segment to form spinal ganglia. Each neuroblast initially develops two processes, one called the axon, which grows centrally and penetrates the dorsal surface of the spinal cord, and another called the dendrite, which grows into the periphery. The two processes later will join together near the cell body of the neuroblast and become one process (unipolar). The axons from each ganglion collect together to form the dorsal (sensory) root of the spinal nerve. The dendrite reaches the periphery by passing through the spinal nerve and its branches. Sensory receptors later will develop on many of the dendritic terminals.
Motor neuroblasts differentiate from cells in the basal plate portion of the mantle layer. They will form multiple processes (multipolar), one axon but many dendrites. The axon grows through the marginal layer to emerge from the spinal cord. Axons in each segment collect together to form the ventral (motor) root of the spinal nerve. They course to the periphery through the spinal nerve and its branches along with the dendrites of the sensory neuroblasts. They terminate on or near skeletal premuscle cells.
The definitive number of spinal ganglia (31 pairs) becomes evident. The dorsal and ventral primary rami of the spinal nerves can be identified. The dorsal primary rami will innervate the deep muscles of the back and the overlying subcutaneous tissue and dermis. The ventral primary rami will innervate the muscles, subcutaneous tissue and dermis of the limbs and ventrolateral body wall.
The ventral primary rami join together in all except the thoracic segments to form plexuses where nerve processes intermingle. They behave as indicated in Table 6–1.
The phrenic nerve (C-3, C-4, C-5) becomes a discrete nerve fascicle, which passes lateral to the heart and terminates in the septum transversum where the diaphragm develops.
AUTONOMIC NERVOUS SYSTEM
FUNDAMENTALS
The nerves of the autonomic nervous system course with the peripheral nervous system and consist of all the motor neurons that innervate visceral structures (viz., glands and smooth and cardiac muscles).
A unique feature of this system is that the messages from the CNS must travel through not one but two motor neurons before they reach the peripheral effector organ. The cell body of the first neuron is located inside the CNS and is called the preganglionic neuron. Its axon leaves the CNS and synapses with the second motor neuron called the postganglionic neuron. Postganglionic neurons are located outside the CNS and collect together to form motor (autonomic) ganglia. The synapse of the preganglionic neuron on the postganglionic neuron occurs in these ganglia.
The autonomic nervous system is divided into two parts depending upon the location of the cell body of the preganglionic neuron within the CNS. If the neuron cell body resides in the thoracic or upper lumbar segments of the spinal cord, it belongs to the sympathetic system; if the neuron cell body resides in either the brain or sacral segments of the spinal cord, it belongs to the parasympathetic system.
SYMPATHETIC NERVOUS SYSTEM
These preganglionic neurons differentiate in the mantle layer of the lateralmost part of the basal plate in the thoracic and upper lumbar segments of the spinal cord (T-1 to L-2). Their axonal processes grow out of the spinal cord through the ventral roots, then enter the spinal nerve. They leave the spinal nerves through the communicating rami and make contact with the postganglionic neurons located in the vicinity of the dorsal aorta.
Neural crest cells give rise to the postganglionic neurons, many of which migrate ventrally to collect into a longitudinal column on each side of the dorsal aorta to form the sympathetic trunk. Some of the neuroblasts migrate beyond the trunk and collect in front of the dorsal aorta forming preaortic ganglia. The axonal process of the postganglionic neurons terminates in viscera.
The suprarenal gland is closely related to the sympathetic nervous system and develops in the upper thoracic segments. It is subdivided into a cortex and a medulla, each having a different origin. The cortex forms from mesothelial cells at the base of the dorsal mesogastrium. The medulla develops from cells that are closely related to sympathetic neuroblasts. They are referred to as prechromaffin cells because they will stain later with chrome salts. These cells differentiate from neural crest cells that collect dorsomedial to the cortical cells and are closely related to similar cells surrounding the celiac branch of the dorsal aorta.
The functions of the sympathetic system are associated with fright and flight reactions.
PARASYMPATHETIC NERVOUS SYSTEM
These preganglionic neurons differentiate in the mantle layer in the lateral part of the basal plate in either the brain or sacral segments of the spinal cord. The axonal process of those neurons in the brain leave by way of cranial nerves III, VII, IX or X. Those in the sacral segments of the spinal cord leave by way of the S-2, S-3 or S-4 ventral roots and travel to the viscera by way of the pelvic splanchnic nerves. The axonal processes synapse on postganglionic neurons close to or within the viscera.
Postganglionic neurons probably arise from neural crest cells in the brain and sacral regions and migrate close to or within the viscera they will innervate. Their axonal processes are short.
The functions of the parasympathetic system are generally antagonistic to those of the sympathetic system.
Source: Atlas of Human Embryos.