Learning Objectives
By the end of this section, you should be able to
- 1.3.1 Correctly identify nervous system anatomical and directional orientation.
- 1.3.2 Summarize the general steps and structures in the development of the human nervous system.
- 1.3.3 Explain the basic organization of the spinal cord.
The central nervous system (CNS) is made up of the brain and spinal cord. Unlike the PNS, the CNS is encased in bone (skull and vertebral column). In addition to the brain and spinal cord, the retina (the neural tissue of the eye) is also considered to be an extension of the CNS. This section will introduce the CNS with a focus on development. You will find a detailed discussion of the brain including anatomy and function in its own section, 1.4 The Brain: Structure and Function. This section will begin by introducing some basic terminology that will be useful to you throughout this text.
Anatomical orientation and planes of orientation
Specific terminology is applied to describing anatomical structures and orientation in the nervous system. Dorsal is an anatomical direction that refers to both the back of the spinal cord and to the top of the brain. The opposite of dorsal is ventral which means toward the bottom of the brain or the front of the spinal cord. Superior and inferior refer to above or below another structure. The bend in the human neuraxis causes a dissociation between the dorsal/ventral and superior/inferior directions. Anterior means toward the face and head, and posterior means toward the back or toward the feet. Rostral is toward the brain and or the top of the spinal cord while the opposite, caudal, is towards the back of the brain. Medial means toward the middle or center while lateral means left or right of another structure (toward the outside) (Figure 1.22). Ipsilateral means the same side and contralateral means the opposite side. For example, the right motor cortex controls the left side of the body and the left motor cortex controls the right side of the body (contralateral).
There are three planes for sectioning the brain for anatomical analysis: coronal sectioning (cutting the brain front to back); sagittal sectioning (cutting the brain left to right) and horizontal or axial sectioning (cutting the brain top to bottom) (Figure 1.23).
Developmental perspective: vertebrate nervous system development
Neurodevelopment is a complex and multi-step process that requires a tremendous amount of coordination. Turning on the right genes at the right time is crucial and any missed steps or abnormal development can have catastrophic consequences. The embryonic development of the vertebrate nervous system begins with the outer tissue of the embryo called the ectoderm. At about 3 weeks in development, the ectoderm begins to thicken and form a flat structure called the neural plate. During the 3rd and 4th week of development, the neural plate begins to invaginate into a groove that eventually closes to form a neural tube. In the initial stages of formation, the neural tube is made up of one layer of cells (epithelial), which serve as the progenitors (embryonic neural stem cells) of glia and neurons. The neural tube elongates to form the spinal cord while the anterior region enlarges and expands to form the brain. Three swellings or bulges eventually become apparent, which become the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain). As development progresses, the prosencephalon subdivides into the telencephalon and the diencephalon. The mesencephalon does not further subdivide. The rhombencephalon becomes the myelencephalon and the metencephalon. The spinal cord forms from the end of the neural tube next to the myelencephalon. By 11 weeks of deveopment, the shape of the human brain is similar to a brain at birth. In the sections that follow, we will discuss the adult structures that derive from the five embryonic regions above (Figure 1.24). More details on development can be found later in this text (see Chapter 5 Neurodevelopment).
Telencephalon and diencephalon
The telencephalon becomes the cerebral hemispheres, which makes up the largest part of the brain (~85% of the total brain mass). The cerebral hemispheres include the cerebral cortex, the outermost layer of the cerebrum. The word cortex comes from Latin for ‘bark’ and represents an extremely thin layer that if flattened is the size of a regular pillowcase. Below the cortex are deeper, subcortical structures that include the hippocampus, amygdala, and basal ganglia. The lateral ventricles arise from the telencephalon. The posterior part of the forebrain is the diencephalon. This eventually becomes the thalamus, hypothalamus, and posterior pituitary gland. Additionally, the diencephalic cavity becomes the third ventricle (see 1.2 Organization of the Nervous System for ventricles).
Mesencephalon, metencephalon and myelencephalon
The mesencephalon (midbrain) forms into the tectum and tegmentum. These eventually form the roof and floor of the cerebral aqueduct which arises from the mesencephalic cavity. Behind the mesencephalon are the hindbrain divisions: 1) metencephalon, which becomes the cerebellum and the pons, and 2) myelencephalon, which becomes the medulla. The mesencephalon and myelencephalon develop from the rhombencephalon whose cavity becomes the fourth ventricle (see 1.2 Organization of the Nervous System).
Spinal cord
The spinal cord forms from the neural tube and retains this tube-like shape once fully developed. It extends from the medulla oblongata in the brainstem down the body as a solid structure that ends in the middle of the lower back and continues as a spray of fibers called cauda equina. The cauda equina is part of the PNS rather than the CNS and it sends and receives information to and from the pelvic organs and lower limbs.
The spinal cord is enclosed by three membrane layers (the meninges) and is encased in the vertebral column made up of bony vertebrae separated by intervertebral discs. Each of these vertebrae has an opening that allows 31 spinal nerves to pass on each side. The spinal cord is divided into 5 major regions: cervical, thoracic, lumbar, sacral and coccyx. Each spinal nerve is named based on which of the 4 major regions of the spinal cord it is connected to. The area of the skin innervated by the spinal nerve is the nerve’s dermatome. These are named according to the 5 major regions above (for example T1-T12 is thoracic) (Figure 1.25). The central canal (which is continuous with the 4th ventricle, see 1.2 Organization of the Nervous System) runs through the spinal cord and is filled with cerebrospinal fluid. It provides cushioning and a transportation system to the spinal cord.
Gray and white matter in the spinal cord
When looking at a cross section of the spinal cord, the center contains gray matter while the white matter is found towards the outside. The gray matter contains glia, interneurons, and motor neuron cell bodies, and it is butterfly-shaped. In keeping with this analogy, the wing areas are called horns. Dorsal and ventral horns are found on each side of the cord (Figure 1.26).
The white matter consists of myelinated nerve tracts (axons) that travel up and down the spinal cord (Figure 1.27). Along the spinal cord, the 31 pairs of spinal nerves emerge at regular intervals laterally serving each side of the body (Figure 1.25). There are two distinct nerve bundles, roots that branch off. The dorsal roots contain sensory neurons that project from the body to the spinal cord whose cell bodies are organized in a ganglion called the dorsal root ganglion which is adjacent to the spinal cord. The ventral roots house motor neurons that leave the spinal cord and project to muscles (motor) (Figure 1.26).
Ascending and descending pathways
Information that travels to the brain is divided along compartmentalized pathways via tracts. Ascending and descending tracts make up the white matter. Sensory information travels via ascending pathways from sensory receptors to processing centers in the brain. These sensory tracts carry specific information, for example pain and temperature or pressure and crude touch or fine touch, vibration, and proprioception. The sensory tracts are made of 1st, 2nd and 3rd order neurons connected to each other. The 1st order neurons receive the sensory information at the periphery. These pass information to the 2nd order neurons in the spinal cord. Eventually 3rd order neurons pick up information from the 2nd order neurons in the thalamus and carry it to the cortex (see 1.4 The Brain: Structure and Function). Motor information moves via descending pathways that carry commands from specialized CNS centers to skeletal muscles. Upper motor neurons originate in the cerebral cortex and travel down to the spinal cord, while the lower motor neurons begin in the spinal cord and send information to the periphery. Motor tracts can be specialized in regulation of voluntary movement or balance and posture (Figure 1.27).