Spinal Cord: Ascending Pathways

Types of Spinal Neurons

The smallest neurons (soma diameters of 5 to 20 μm) are interneurons , and their cell bodies are contained within the cord. While the processes of some interneurons are confined within a single segment, others send their axons into the white matter surrounding the grey matter and ascend or descend two or more segments, interconnecting different spinal cord segments. These latter processes are termed propriospinal fibres and form the fasciculi proprii. Many of these smallest neurons participate in spinal reflexes. Others are intermediate cell stations interposed between fibre tracts descending from the brain and motor neurons projecting to cells controlling locomotion. Others are so placed as to influence sensory transmission from lower to higher levels of the central nervous system (CNS).

Medium-sized neurons (soma diameters of 20 to 50 μm) are found in most parts of the grey matter. Most are relay (projection) cells receiving inputs from dorsal root afferents and projecting their axons to the brain. The projections are in the form of tracts , a tract being defined as a functionally homogeneous group of fibres. As will be seen, the term ‘tract’ is often used loosely because many projections originally thought to be ‘pure’ contain more than one functional class of fibre.

The largest neurons of all are the α motor neurons (soma diameters of 50 to 100 μm) that innervate skeletal muscles. Scattered among them are smaller γ motor neurons supplying muscle spindles. In the medial part of the ventral horn are Renshaw cells , which exert a tonic inhibition upon α motor neurons.

Spinal reflex arcs originating in muscle spindles and tendon organs have been described in Chapter 10 and are part of the withdrawal reflex as described in Chapter 14 .

On the basis of cytoarchitectonic characteristics (e.g., neuronal size, staining characteristics, receptors, and connectivity), the spinal cord grey matter is divided into 10 layers, the laminae of Rexed , that serve a descriptive but not necessarily functional purpose. Their configuration differs at various levels of the spinal cord; at some spinal cord levels, specific cell columns are recognised within the laminae, whereas in others, they are less clear ( Fig. 15.3 ).

Spinal Ganglia

The spinal or dorsal root ganglia are located on the dorsal root in the intervertebral foramina, where the ventral and dorsal roots come together to form the spinal nerves. Thoracic ganglia contain about 50,000 unipolar neurons and spinocerebellar pathways serving the limbs contain about 100,000. These neurons are described as unipolar (or more correctly pseudounipolar). Their axons are morphologically indistinguishable from their dendrites because their somas are attached by a short stem axon . The individual ganglion cells are invested with modified Schwann cells called satellite cells ( Fig. 15.4 ).

Central Terminations of Dorsal Root Afferents ( Fig. 15.5 )

In the dorsal root entry zone close to the surface of the cord, the afferent fibres become segregated into medial and lateral divisions. The medial division comprises medium and large fibres that divide within the dorsal funiculus into ascending and descending branches. The branches swing into the dorsal grey horn and may synapse in the nucleus dorsalis (also known as the dorsal nucleus of Clarke). The largest ascending fibres run all the way to the dorsal column nuclei (gracilis/cuneatus) in the medulla oblongata, forming the bulk of the gracile and cuneate fasciculi.

The lateral division comprises small (Aδ and C) fibres, which upon entry divide into short ascending and descending branches within the Lissauer tract and synapse upon neurons of the substantia gelatinosa; some fibres synapse upon dendrites of cells belonging to the nucleus proprius. The nucleus proprius gives rise to the spinothalamic tract.