Spinal Ganglion and Posterior Root
The posterior spinal root contains a spindle-shaped bulge, the spinal ganglion (A), an accumulation of cell bodies of sensory neurons; their bifurcated processes send one branch to the periphery and the other branch to the spinal cord. They lie as cell clusters or as cell rows between the bundles of nerve fibers.
Development of the ganglia (C).The cellsoriginate from the lateral zone of the neuralplate (C1); however, they do not participatein the formation of the neural tube but re-main at both sides as the neural crest (C2). Hence, the spinal ganglia can be regarded as gray matter of the spinal cord that became translocated to the periphery. Other deriva-tives of the neural crest are the cells of the autonomic ganglia, the paraganglia, and the adrenal medulla.
From the capsule (A3) of the spinal gan-glion, which merges into the perineurium of the spinal nerve, connective tissue extends to the interior and forms a sheath around each neuron (endoganglionic connectivetissue) (B4). The innermost sheath, however,is formed by ectodermal satellite cells (BE5) and is surrounded by a basal membrane comparable to that around the Schwann cells of the peripheral nerve. The large nerve cells (B6, E) with their myelinated process conglomerated into a glomerulus represent only one-third of the ganglion. They trans-mit impulses of epicritic sensibility. The remainder consists of medium-sized and small ganglion cells with poorly myeli-nated or unmyelinated nerve fibers which are thought to conduct pain signals and sen-sations from the intestine. There are also some multipolar nerve cells.
Development of the ganglion cells (D). Thespinal ganglion cells are initially bipolarcells. During development, however, thetwo processes fuse to form a single trunk which then bifurcates in a T-shaped man-ner. The cells are therefore called pseudo-unipolar nerve cells.
The posterior root is thicker than the ante-rior root. It contains fibers of various cali-bers, two-thirds of them being poorly my-elinated or unmyelinated fibers. The thin poorly myelinated and unmyelinated fibers, which transmit impulses of the protopathicsensibility, enter through the lateralpart of the root into the spinal cord (F7). The thick myelinated fibers transmit impulses of the epicritic sensibility and enter through the median part of the root into the spinal cord (F8).
At the entrance into the spinal cord, there is a narrow zone where the myelin sheaths are very thin so that the fibers appear unmyeli-nated. This zone is regarded as the boundarybetween the central and the peripheral nervous systems (Redlich – Obersteiner zone)(G). In the electron-microscopic image (H), however, this boundary does not exactly coincide with the Redlich – Obersteiner zone. For each axon, the boundary is marked by the last node of Ranvier prior to the en-trance into the spinal cord. Up to this point, the peripheral myelin sheath is surrounded by a basal membrane (blue in H). The next internode no longer has a basal membrane. For unmyelinated fibers, the boundary is also marked by the basal membrane of the enveloping Schwann cell. Thus, the basal membranes around the spinal cord form a boundary that is only penetrated by the axons.