FIGURE 12.29 • Posterior view of the spinal cord with surrounding meninges. Each spinal nerve arises from the spinal cord by rootlets, which merge together to form dorsal (posterior) and ventral (anterior) nerve roots. These roots unite to form a spinal nerve that, after short course, divides into larger ventral (anterior) and smaller dorsal (posterior) primary rami. Note the dura mater (the outer layer of the meninges) surrounds the spinal cord and emerging spinal nerves. The denticulate ligament of the pia mater that anchors the spinal cord to the wall of the spinal canal is also visible.
the spinal nerve of that segment, and as such is conveyed to the muscle. The axon is myelinated except at its origin and termination. Near the muscle cell, the axon divides into numerous terminal branches that form neuromuscular junctions with the muscle cell (see page 322).
The cell bodies of sensory neurons are located in ganglia that lie on the dorsal root of the spinal nerve.
Sensory neurons in the dorsal root ganglia are pseudounipolar (Plate 27, page 390). They have a single process that divides into a peripheral segment that brings information from the periphery to the cell body and a central segment that carries information from the cell body into the gray matter of the spinal cord. Because the sensory neuron conducts impulses to the CNS, it is an afferent neuron. Impulses are generated in the terminal receptor arborization of the peripheral segment.
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.
FIGURE 12.30 • Cross-section of the human spinal cord. The photomicrograph shows a cross-section through the lower lumbar (most likely L4–L5) level of the spinal cord stained by the Bielschowsky silver method. The spinal cord is organized into an outer part, the white matter, and an inner part, the gray matter that contains nerve cell bodies and associated nerve fibers. The gray matter of the spinal cord appears roughly in the form of a butterfly. The anterior and posterior prongs are referred to as ventral horns (VH) and dorsal horns (DH), respectively. They are connected by the gray commissure (GC). The white matter contains nerve fibers that form ascending and descending tracts. The outer surface of the spinal cord is surrounded by the pia mater. Blood vessels of the pia mater, the ventral fissure (VF), and some dorsal roots of the spinal nerves are visible in the section. 5.
Because arachnoid and pia mater develop from the single layer of mesenchyme surrounding the developing brain, they are commonly referred as the pia-arachnoid. In adults, pia mater represents the visceral portion, and arachnoid represents the parietal portion of the same layer. This common origin of pia-arachnoid is evident in adult meninges in which numerous strands of connective tissue (arachnoid trabeculae) pass between pia mater and arachnoid.
The dura mater is a relatively thick sheet of dense connective tissue.
In the cranial cavity, the thick layer of connective tissue that forms the dura mater (L., tough mother) is continuous at its outer surface with the periosteum of the skull. Within the dura mater are spaces lined by endothelium (and backed by periosteum and dura mater, respectively) that serve as the principal channels for blood returning from the brain. These venous (dural) sinuses receive blood from the principal cerebral veins and carry it to the internal jugular veins.