Figure 1129

Nerve cells in intracortical circuits. This simple diagram shows the organization and connections between cells in different layers of the cortex contributing to cortical afferent fibers (arrows pointing up) and cortical efferent fibers (arrows pointing down). The small interneurons are indicated in yellow.

The brain stem is not clearly separated into regions of gray matter and white matter. The nuclei of the cranial nerves located in the brain stem, however, appear as islands surrounded by more or less distinct tracts of white matter. The nuclei contain the cell bodies of the motor neurons of the cranial nerves and are both the morphologic and functional counterparts of the anterior horns of the spinal cord. In other sites in the brain stem, as in the reticular formation, the distinction between white matter and gray matter is even less evident.

Connective Tissue of the Central Nervous System

Three sequential connective tissue membranes, the meninges, cover the brain and spinal cord:

• The dura mater is the outermost layer.

• The arachnoid layer lies beneath the dura.

• The pia mater is a delicate layer resting directly on the surface of the brain and spinal cord.

I molecular layer

II external granular layer

III external pyramidal cell layer

VI multiform (polymorphic) cell layer

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