1.1 The Nervous System—An Overall View

Development and Subdivision (A–D)

The nervous system serves processing information within the body in the interest of adapting its reactions. In the most primitive forms of organization (A), this function is assumed by the sensory cells (A–C1) themselves. These cells are excited by stimuli coming from the environment; the excitation is conducted to a muscle cell (A–C2) through a cellular projection, or process. The simplest response to environmental stimuli is achieved in this way. (In humans, sensory cells that still have processes of their own are only found in the olfactory epithelium.) In more differentiated organisms (B), an additional cell is interposed between the sensory cell and the muscle cell—the nerve cell, or neuron (BC3) which takes on the transmission of messages. This cell can transmit the excitation to several muscle cells or to additional nerve cells, thus forming a neural network (C). A diffuse network of this type also runs through the human body and innervates all intestinal organs, blood vessels, and glands. It is called the autonomic (visceral, or vegetative) nervous system (ANS), and consists of two components which often have opposing functions: the sympathetic nervous system and the parasympathetic nervous system. The interaction of these two systems keeps the interior organization of the organism constant.

In vertebrates, the somatic nervous system developed in addition to the autonomic nervous system; it consists of the central nervous system (CNS; brain and spinal cord), and the peripheral nervous system (PNS; nerves of head, trunk, and limbs). It is responsible for conscious perception, for voluntary movement, and for the processing of information (integration). Note that most textbooks include the peripheral nerves of the autonomic nervous system in the PNS.

The CNS develops from the neural plate (D4) of the ectoderm which then transforms into the neural groove (D5) and further into the neural tube (D6). The neural tube finally differentiates into the spinal cord (D7) and the brain (D8).

Functional Circuits (E, F)

The nervous system, the remaining organism, and the environment are functionally linked with each other. Stimuli from the environment (exteroceptive stimuli) (E9) are conducted by sensory cells (E10) via sensory (afferent) nerves (E11) to the CNS (E12). In response, there is a command from the CNS via motor (efferent) nerves (E13) to the muscles (E14). For control and regulation of the muscular response (E15), there is internal feedback from sensory cells in the muscles via sensory nerves (E16) to the CNS. This afferent tract does not transmit environmental stimuli but stimuli from within the body (proprioceptive stimuli). We therefore distinguish between exteroceptive and proprioceptive sensitivities.

However, the organism does not only respond to the environment; it also influences it spontaneously. In this case, too, there is a corresponding functional ­circuit: the action (F17) started by the brain via efferent nerves (F13) is registered by sensory organs (F10), which return the corresponding information via afferent nerves (F11) to the CNS (F12) (reafference, or external feedback). Depending on whether or not the result meets the desired target, the CNS sends out further stimulating or inhibiting signals (F13). Nervous activity is based on a vast number of such functional circuits.

In the same way as we distinguish between exteroceptive sensitivity (skin and mucosa) and proprioceptive sensitivity (receptors in muscles and tendons, autonomic sensory supply of the intestines), we can subdivide the motor system into an environment oriented somatomotor system (striated, voluntary muscles) and a visceromotor system (smooth intestinal muscles).

Position of the Nervous System in the Body (A, B)

The central nervous system (CNS) is divided into the brain, encephalon (A1), and the spinal cord (SC), medulla spinalis (A2). The brain in the cranial cavity is surrounded by a bony capsule; the spinal cord in the vertebral canal is enclosed by the bony vertebral column. Both are covered by meninges that enclose a cavity filled with a fluid, the cerebrospinal fluid. Thus, the CNS is protected from all sides by bony walls and the cushioning effect of a fluid (fluid cushion).

The peripheral nervous system (PNS) includes the cranial nerves, which emerge through holes (foramina) in the base of the skull, and the spinal nerves, which emerge through spaces between the vertebrae (intervertebral foramina) (A3). The peripheral nerves extend to muscles and skin areas. They form nerve plexuses before entering the limbs: the brachial plexus (A4) and the lumbosacral plexus (A5) in which the fibers of the spinal nerves intermingle; as a result, the nerves of the limbs contain portions of different spinal nerves (see pp. 70 and 86). At the entry points of the afferent nerve fibers lie ganglia (A6); these are small oval bodies containing sensory neurons.

When describing brain structures, terms like “top,” “bottom,” “front,” and “back” are inaccurate, because we have to distinguish between different axes of the brain (B). Owing to the upright posture of humans, the neural tube is bent; the axis of the spinal cord runs almost vertically, while the axis of the forebrain (Forel’s axis, orange) runs horizontally; the axis of the lower brain divisions (Meinert’s axis, violet) runs obliquely. The positional terms relate to theses axes: the anterior end of the axis is called oral or rostral (os, mouth; rostrum, beak), the posterior end is called caudal (cauda, tail), the underside is called basal or ventral (venter, abdomen), and the upper side is called dorsal (dorsum, back).

The lower brain divisions, which merge into the spinal cord, are collectively called the brain stem (light gray) (B7). The anterior division is called the forebrain (gray) (B8).

The divisions of the brain stem, or encephalic trunk, have a common structural plan (consisting of basal plate and alar plate, like the spinal cord, see p. 12, C). Genuine peripheral nerves emerge from these divisions, as they do from the spinal cord. Like the spinal cord, they are supported by the chorda dorsalis during embryonic development. All these features distinguish the brain stem from the forebrain. The subdivision chosen here differs from the other classifications in which the diencephalon is viewed as part of the brain stem.

The forebrain, prosencephalon, consists of two parts, the diencephalon and the telencephalon or cerebrum. In the mature brain, the telencephalon forms the two hemispheres (cerebral hemispheres). The diencephalon lies between the two hemispheres.

A9 Cerebellum.

1.2 Development and Structure of the Brain

Development of the Brain (A–E)

The closure of the neural groove into the neural tube begins at the level of the upper cervical cord. From here, further closure runs in the oral direction up to the rostral end of the brain (oral neuropore, later the terminal lamina) and in the caudal direction up to the end of the spinal cord. Further developmental events in the CNS proceed in the same directions. Thus, the brain’s divisions do not mature simultaneously but at intervals (heterochronous maturation).

Increasing growth causes the neural tube in the head region to expand and form several vesicles (p. 171, A). The rostral vesicle is the future forebrain, prosencephalon (yellow and red); the caudal vesicles are the future brain stem, encephalic trunk (blue). Two curvatures of the neural tube appear at this time: the cephalic flexure (A1) and the cervical flexure (A2). Although the brain stem still shows a uniform structure at this early stage, the future divisions can already be identified: medulla oblongata (elongated cord) (A–D3), pons (bridge of Varolius) (A–D4), cerebellum (A–D5, dark blue), and mesencephalon (midbrain) (A–C6, green). The brain stem is developmentally ahead of the prosencephalon; during the second month of human development, the telencephalon is still a thin-walled vesicle (A), whereas neurons have already differentiated in the brain stem (emergence of cranial nerves) (A7). The optic vesicle develops from the diencephalon (AB8, red) (p. 344, A) and forms the optic cup (A9). Anterior to it lies the telencephalic vesicle (telencephalon) (A–D10, yellow); initially, its anlage...