CNS Anatomy: Brain

The central nervous system consists of the brain and spinal cord. This week we will focus on the anatomy of the brain; next week we will examine the anatomy of the spinal cord, and identify the different parts of the peripheral nervous system.

The photographic images of the brain are courtesy of John Sundsten and Kathleen Mulligan, Digital Anatomist Project, Department of Biological Structure at the University of Washington.  These images are from the collection in the Neuroanatomy Interactive Syllabus, which can be found at this link.

Recommended Reading

Read pp. 274-280 in Silverthorn. Important concepts to focus on are:

• how the central nervous system develops from a tube (see figure 9.2)
• the arrangement of the meninges (see figure 9.3b and 9.3c)
• the production of cerebrospinal fluid at the choroid plexus and its path of flow through the ventricles (see figure 9.4)
• the blood-brain barrier

Section 9.5, pp. 282-288, covers brain anatomy.

Anatomy Terms

The following terms of direction are useful for our study of brain anatomy:
anterior/posterior
ventral/dorsal
medial/lateral
superior/inferior
rostral/caudal

You should also know the anatomical planes of reference:
sagittal
frontal (coronal)
horizontal (transverse)

Structures to Identify

The central nervous system develops from a tube. As a consequence, the final structure of the central nervous system contains hollow spaces within it: the ventricles within the brain, and the central canal within the spinal cord. The ventricles are filled with cerebrospinal fluid, which is produced by the choroid plexus, a network of capillaries and epithelial tissue attached to the walls of the ventricles. CSF flows through the ventricles and into the subarachnoid space surrounding the brain.

A notable feature of the human brain is the large cerebrum, which is divided into two highly folded cerebral hemispheres. The folds on the surface of the cerebrum are called gyri (singular: gyrus) and the grooves are called sulci (singular: sulcus). The different lobes of the cerebrum (frontal lobe, parietal lobe, occipital lobe, temporal lobe) are named for the overlying skull bones.  The large groove that divides the two cerebral hemispheres is called the longitudinal fissure (visible in the inferior view). The large groove between the temporal lobe and the frontal and parietal lobes is called the lateral sulcus (also called lateral fissure or Sylvian fissure). The central sulcus extends from the lateral sulcus to the longitudinal fissure, and is bordered by two fairly continuous gyri on either side of it. The central sulcus is the landmark that separates the frontal lobe from the parietal lobe.  Anterior to the central sulcus is the precentral gyrus, which is the location of the primary motor cortex.

In the midsagittal view of the brain, an obvious feature is the corpus callosum. The corpus callosum consists of many axons and it connects the two cerebral hemispheres.

The diencephalon is the region of the brain that lies between the cerebrum and the brainstem. This region contains the thalamus, an oval structure situated just inferior to the corpus callosum. The thalamus acts as a relay station, receiving inputs from almost all sensory systems and relaying that information to the cerebrum. The thalamus is also involved in motor control. Anterior and inferior to the thalamus is the hypothalamus. The hypothalamus is important for homeostasis, the maintenance of a steady and ideal internal environment. It contains centers that regulate hunger, thirst, sleep, and other drives. The hypothalamus is also important in endocrine control, regulating hormone release by the pituitary gland.

Caudal to the diencephalon is the brainstem, which contains the midbrain, pons, and medulla oblongata. The brainstem is considered the most primitive region of the brain and is concerned with the control of basic processes such as the regulation of breathing, heart rate, and blood pressure.  The cerebellum is the highly folded structure that sits below the cerebrum and posterior to the brainstem.  The cerebellum integrates sensory and motor information to provide coordination in movement.

Visual information is detected and processed in the retina at the back of the eye. This information travels into the brain via the axons of cells called the retinal ganglion cells. The retinal ganglion cell axons leave the eye via the optic nerve (cranial nerve II).  In a location just anterior to the pituitary gland and ventral to the hypothalamus, some of these axons cross over to the opposite side. The place where the axons cross (the center of the X) is called the optic chiasm. (The sectioned optic chiasm is visible in the midsagittal view of the brain.) The bundle of axons that extends from the optic chiasm to the thalamus is called the optic tract.

Sections of the brain show the division of the tissue into gray matter and white matter.  Gray matter is where neuronal cell bodies are found, while white matter regions mainly contain axons.  White matter appears lighter due to the fact that many axons are myelinated.  In the cerebral hemispheres, much of the gray matter is located in the outermost tissue layer called the cerebral cortex.  A cluster of neuronal cell bodies within the CNS is called a nucleus.  The basal ganglia are a group of cerebral nuclei involved in the control of movement.  Bundles of axons within the CNS white matter are called tracts.  The corresponding term for a bundle of axons that extends outside the central nervous system is nerve (see the cranial nerves above).  The corticospinal tract is found within the internal capsule, a white matter structure that extends from the primary motor cortex (found in the precentral gyrus) past the basal ganglia.

There are two frontal sections; one is more rostral (closer to the nose), the other is more caudal (closer to the occipital lobe).

The basal ganglia are located adjacent to the lateral ventricles, and are characteristically divided by the white matter of the internal capsule.  In a more caudal location, the round gray matter territory adjacent to the ventricles is the thalamus. The more caudal section also shows the substantia nigra, a nucleus in the midbrain that is important in motor control.  "Substantia nigra" means "black substance", and you can identify it by the black appearance which is due to a pigment produced by cells in this nucleus.