A Development of the brain

a Embryo with a greatest length (GL, see p. 4) of 10 mm, at the beginning of the 2nd month of development. Even at this stage we can see the differentiation of the neural tube into segments that will generate various brain regions (see C):

• Medulla oblongata (gray)

• Pons (gray)

• Cerebellum (light blue)

• Midbrain (mesencephalon, dark blue)

• Interbrain (diencephalon, yellow)

• Forebrain (telencephalon, red)

b Embryo with a GL of 27 mm, near the end of the 2nd month of development (end of the embryonic period). The olfactory bulb is developing from the telencephalon, part of the pituitary anlage (neurohypophysis) from the diencephalon.

c Fetus with a GL of 53 mm, in approximately the 3rd month of development. By this time the telencephalon has begun to overgrow the other brain areas. The insula is still on the surface but will subsequently be covered by the cerebral hemispheres (compare with d).

d Fetus with a crown–rump length (CRL, see p. 4) of 27 cm (270 mm), in approximately the 7th month of development. The brain has begun to develop conspicuous gyri and sulci.

Note: The telencephalon grows over all the other brain structures as development proceeds.

B Brain vesicles and their derivatives

The cranial end of the neural tube expands to form three primary brain vesicles: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon). The forebrain (telencephalon) and interbrain (diencephalon) develop from the prosencephalon. The mesencephalon gives rise to the superior and inferior colliculi and related structures. The rhombencephalon differentiates into the pons, cerebellum, and medulla oblongata. The pons and cerebellum are also known collectively as the metencephalon. Some important structures of the adult brain are listed at far right to illustrate the derivatives of the brain vesicles. They can be traced back in the diagram to their developmental precursors.

During development, the neural groove folds away from the overlying dorsal ectoderm and closes to form the neural tube. Cells migrate from the lateral portions of the neural groove to form the neural crest on each side. The central nervous system (brain and spinal cord) develops from the neural tube, while the peripheral nervous system develops from derivatives of the neural crest (see p. 76).

D Differentiation of the neural tube in the spinal cord region during development

Cross section, cranial view.

a Early neural tube, b intermediate stage, c adult spinal cord.

The neurons that form in the basal plate are efferent (motor) neurons, while those that form in the alar plate are afferent (sensory) neurons.

The area between them—the future thoracic, lumbar, and sacral cord—is another zone that gives rise to preganglionic autonomic neurons. The roof plate and floor plate do not form neurons. A knowledge of how these neuron populations are distributed is helpful in understanding the structure of the hindbrain (rhombencephalon, see E).

E Embryonic migratory movements of neuron populations and their effect on the location of the cranial nerve nuclei

Cross section, cranial view. (Visual aid: If we compare the spinal cord to a book, it would be closed in a and open in b and c.)

a In the initial stage, the motor neurons are ventral (anterior), and the sensory neurons are dorsal (posterior). The arrows indicate the directions of migration.

b In the early embryonic stage, the neurons of the alar plate migrate laterally and ventrally (anteriorly).

c In the adult brain (medulla oblongata and pons, derivatives of the rhombencephalon), we can distinguish four nuclear columns (after His and Herrick) that contain functionally distinct cranial nerve nuclei (from medial to lateral):

1. Somatomotor column (lilac)

2. Visceromotor column (orange and green stripes)

3. Viscerosensory column (light blue)

4. Somatosensory column (dark blue)

B Main migratory pathways and derivatives of the neural crest (after Christ and Wachtler)

Neural crest cells originated in different regions have different migration paths and fates. Those from cranial levels contribute to cartilage and bone in the head and neck, and to cranial parasympathetic ganglia (see C). Those from thoracolumbar levels do not become skeletal cells, but generate peripheral neurons, endocrine cells, melanocytes, and Schwann cells. This diagram shows the migration of the neural crest cells in the trunk of the early embryo (4 weeks’ development, see p. 7). They follow three main migratory pathways:

① Dorsolateral pathway (melanoblasts, which differentiate into melanocytes)

② Ventrolateral pathway (ganglioblasts, which differentiate into sensory nerve cells in the dorsal (posterior) root [spinal] ganglia)

③ Ventral pathway (cells differentiate into neurons and associated cells of the paravertebral sympathetic ganglia, into chromaffin cells of the adrenal medulla, and into autonomic plexuses in the gastrointestinal tract)

Thus, the neural crest can develop into a variety of seemingly unrelated nonneuronal cells as well as peripheral ganglion cells. These unusual characteristics of the neural crest—its pluripotential capacity and wide-ranging migration—have consequences when its differentiation or migration is defective. Disruption of neural crest development may deprive organs of their autonomic innervation (Hirschsprung disease). Tumors derived from neural crest cells tend to be highly malignant and difficult to treat. (See D.)

F Structure of a peripheral nerve

A peripheral nerve consists entirely of axons (also called neurites) and sheath tissue (Schwann cells, fibroblasts, blood vessels). The axons transmit information either from the periphery to the CNS (afferents) or in the opposite direction from the CNS to the periphery (efferents). Axons may be myelinated or unmyelinated. The latter have a much slower conduction velocity and are usually fibers of the autonomic nervous system (see p. 97). Among the investing layers of the nerve, the perineurium ensheaths the nerve fascicles and provides an important tissue barrier (see p. 95).

A Topography of the nervous system

a Posterior view, b right lateral view.

The central nervous system (CNS), consisting of the brain (encephalon) and spinal cord, is shown in pink. The peripheral nervous system (PNS), consisting of nerves and ganglia, is shown in yellow. The nerves arising from the spinal cord leave their bony canal through the intervertebral foramina and are distributed to their target organs. The spinal nerves are formed in the foramina by the union of their dorsal (posterior) roots and ventral (anterior) roots (see p. 83). The small spinal ganglion in the intervertebral foramen appears as a slight swelling of the dorsal root (visible only in the posterior view; its function is described on p. 83).

In the limbs, the ventral rami of the spinal nerves come together to form plexuses. These plexuses then give rise to the peripheral nerves that supply the limbs.

D Schematic representation of information flow in the nervous system

The information encoded in nerve fibers is transmitted either to the CNS (brain and spinal cord) or from the CNS to the periphery (PNS, including the peripheral parts of the autonomic nervous system, see p. 96). Fibers that carry information to the CNS are called afferent fibers or afferents for short; fibers that carry signals away from the CNS are called efferent fibers or efferents.

C Basic forms of the neuron and its functionally adapted variants

The horizontal line marks the region of the axon hillock, which represents the initial segment of the axon. (The structure of a peripheral nerve, consisting only of axons and sheath tissue, is shown on p. 77.)

a Multipolar neuron (multiple dendrites) with a long axon (= long transmission path). Examples are projection neurons such as α-motoneurons in the spinal cord.

b Multipolar neuron with a short axon (= short transmission path). Examples are interneurons like those in the gray matter of the brain and spinal cord.

c Pyramidal cell: Dendrites are present only at the apex and base of the tridentate cell body, and the axon is long. Examples are efferent neurons of the cerebral motor cortex.

d Purkinje cell: An elaborately branched dendritic tree arises from a circumscribed site on the cell body. The Purkinje cell receives many synaptic contacts from afferents to the cerebellum and is also the efferent cell of the cerebellar cortex.

e Bipolar neuron: The dendrite branches in the periphery. Examples are bipolar cells of the retina.

f Pseudounipolar neuron: The dendrite and axon are not separated by a cell body. An example is the primary afferent (= first sensory) neuron in the spinal ganglion (see p. 91).

A Structure of a spinal cord segment with its spinal nerve

Superior view. The spinal cord is made up of 31 consecutive segments arranged one above the other (see B). An anterior root and a posterior root emerge from the sides of each segment. The anterior root consists of efferent (motor) fibers, while the posterior root consists of afferent (sensory) fibers. Both roots from one segment unite in the intervertebral foramen to form the spinal nerve. The afferent (sensory) fibers and efferent (motor) fibers intermingle at this junction, so that the branches into which the spinal nerve divides (see below) contain motor and sensory elements (except for the meningeal branch, which is purely sensory). This division into branches (rami) occurs shortly after the anterior and posterior roots unite to form the spinal nerve. Consequently, the spinal nerve itself is only about 1 cm long.

The principal branches of the spinal nerve have the following functions:

• The anterior ramus innervates the anterior and lateral body wall and the limbs.

• The posterior ramus innervates the skin of the back and the intrinsic back muscles.

• The meningeal branch reenters the spinal canal, providing sensory innervation to the spinal membranes and other structures.

• The white ramus communicans carries white (= myelinated) fibers to the ganglion of the sympathetic trunk.

• The gray ramus communicans carries gray (= unmyelinated) fibers from the sympathetic ganglion back to the spinal nerve (the functional significance of this is described on p. 97). The anterior and posterior rami of the spinal nerves subdivide into further branches.