The spinal cord performs three general functions: 1. Via spinal nerve connections, it processes afferent information from muscles, tendons, joints, ligaments, blood vessels, skin, and viscera, and it discharges efferent commands that control muscles and regulate glands. 2. The spinal cord is a reflex center, producing subconscious responses of muscles and glands to particular stimuli. 3. The spinal cord conducts information to and from the brain through a system of axonal tracts, by which the brain receives status information about the neck, trunk and limbs while dispensing commands that control posture, movement, and the visceral aspects of behavior. The pia mater, the deepest, most vascular meninx, is bound to glial cells concentrated at the spinal cord surface (glial limiting membrane) (Uehara & Ueshima, 1988). The pia mater is thickened bilaterally along the lateral margin of the spinal cord, forming denticulate ligaments. Each ligament has periodic lateral extensions that attach to dura mater, thereby suspending the spinal cord such that it is surrounded by cerebrospinal fluid (Fig. 16-3). The basis for dividing the spinal cord into segments is the attachments of dorsal (or ventral) roots. Each dorsal or ventral spinal root (radix dorsalis; radix ventralis) is composed of thousands of axons with varying amounts of Schwann cell myelin and enveloped by meninges. The axons of each root are bound together laterally where dorsal and ventral roots join to form the spinal nerve, but as roots approach the spinal cord, their axons regroup into separate bundles called rootlets (fila radicularia). The rootlets attach serially along the spinal cord (Fig. 16-4). Caudal, formerly coccygeal, roots may have only 1 or 2 rootlets, whereas segments innervating limbs may have 12 or more rootlets per dorsal or ventral root (Fletcher & Kitchell, 1966a). With respect to position within the vertebral column, four spinal cord regions can be distinguished (Fig. 16-4): (1) an initial cervical region, where at least the first cervical segment lies within its corresponding vertebra; (2) a caudal cervical through cranial thoracic region, where segments are positioned cranial to their respective vertebrae; (3) a thoracolumbar junction, where segments again lie within their corresponding vertebrae; and (4) a caudal lumbar, sacral, caudal region, where segments lie progressively cranial to their respective vertebrae (Fletcher & Kitchell, 1966a). Positions of spinal cord segments relative to vertebrae may vary by half a vertebral length cranial or caudal to the typical relationship for medium-sized and large dogs, depicted in Figure 16-4. In these dogs, the conus medullaris ends approximately at the level of the L6-7 intervertebral disc. Small dogs (weighing less than 7 kg) have relatively longer spinal cords, particularly evident in the lumbosacral and caudal regions, where segments may be one vertebra caudal to the relationship illustrated in Figure 16-5. Spinal cord projection neurons send axons into white matter to form, generally, cranial projecting pathways to the brain. The projection neurons are activated by primary afferent neurons that become excited in response to stimulation of viscera, muscles, joints, or skin. Primary afferent neurons influence the excitability of projection neurons directly or through interneurons or other projection neurons (Willis, 1985, 1986; Yaksh, 1986). The excitability of spinal cord projection neurons is also modified, directly or through interneurons, by caudally projecting axons from brain projection neurons (Noble & Riddell, 1989). A motor neuron and all of the muscle fibers it innervates constitute a motor unit. All motor units of a particular skeletal muscle have their neuronal cell bodies grouped together, forming a motor neuron pool within a motor nucleus within the ventral horn. Motor units vary in their properties. Small motor units (relatively small alpha motor neurons innervating relatively few muscle fibers) are typically activated earliest during muscle contraction; they produce small degrees of tension and contract relatively slowly, and their muscle fibers are highly resistant to fatigue. At the other extreme, the largest motor units are recruited only during maximal muscle contraction; their muscle fibers contract rapidly and are easily fatigued (Binder & Mendell, 1990; Henneman et al., 1965). The other approach involves dividing spinal cord gray matter into ten laminae (Rexed, 1952, 1954). The laminar classification scheme accounts for all spinal cord neurons; however, it is usually difficult to distinguish individual lamina in routine preparations. In general, both laminar and nuclear schemes are used, depending on which applies best to a specific situation. The lateral cervical nucleus (nucleus cervicalis lateralis) is found in the first two cervical segments of the spinal cord (Brodal & Rexed, 1953; Ha & Liu, 1966). Lateral to the dorsal horn, the profile of the nucleus forms a peninsula or island surrounded by white matter. The nucleus consists of projection neurons (third-order neurons of the spinocervicothalamic pathway). The nucleus relays cutaneous noxious stimuli and touch to conscious centers (Lu & Yang, 1989). In this text nociception is the response of an animal to a noxious stimulus, one that causes injury or has the potential to cause injury. This response indicates a patient’s discomfort or pain caused by the noxious stimulus. Pain is not a sensory modality. It is the conscious response to a noxious stimulus. Marginal nucleus (dorsomarginal nucleus) refers to flattened neurons located at the dorsal surface of the dorsal horn along the entire length of the spinal cord. Although the nucleus is not morphologically prominent, it is important as a site of nociceptive projection neurons (Craig et al., 1988; Yaksh, 1986). The substantia gelatinosa, a concentration of small neurons, forms a homogeneous crown at the apex of the dorsal horn, deep to the marginal nucleus. It extends the entire length of the spinal cord and blends cranially with the nucleus of the spinal tract of the trigeminal nerve. Most substantia gelatinosa cells are interneurons that project to the remainder of the dorsal horn, but a few larger cells are spinothalamic tract projection neurons. The substantia gelatinosa receives cutaneous input, from axons activated by noxious, tactile, or thermal stimulation (Light & Kavookjian, 1988; Rethelyi et al., 1989). The sacral parasympathetic nucleus is found in the sacral spinal cord segments, chiefly S2 and S3 (Oliver et al., 1969; Purinton & Oliver, 1979). It consists of parasympathetic preganglionic neurons that form a mediolateral band in the lateral intermediate substance. The dorsal portion of the nucleus is concerned with bowel control, the lateral portion with urinary bladder contraction (Leedy et al., 1988). The cell bodies of alpha and gamma efferent neurons that innervate a particular skeletal muscle are grouped together, forming a motor neuron pool. Related skeletal muscles have their motor neuron pools grouped into longitudinal columns that extend over one or more spinal cord segments (Horcholle-Bossavit et al., 1988). These columns are designated motor nuclei when viewed in spinal cord sections. From one to seven motor nuclei are evident in the different segments along the length of the spinal cord (Romanes, 1951).
Spinal Cord and Meninges
The Spinal Cord
Morphologic Features of the Spinal Cord
Spinal Cord Segments
Segmental Relationships to Vertebrae
Gray Matter of the Spinal Cord*
Gray Matter Organization
Gray Matter Nuclei
Dorsal Horn Nuclei
Intermediate Substance Nuclei
Ventral Horn Nuclei
White Matter of the Spinal Cord
< div class='tao-gold-member'>
You may also need
Spinal Cord and Meninges
Only gold members can continue reading. Log In or Register a > to continue