The anterior and posterior surfaces of the spinal cord have several longitudinally running fissures and sulci. Along the midline on the anterior surface of the spinal cord is a deep separation, the anterior median fissure (eFig. 9.47). Posteriorly the spinal cord has a shallower separation, the posterior median sulcus, which is flanked on either side by a posterolateral sulcus (eFig. 9.47). Emerging from the spinal cord are a series of rootlets, which coalesce to form anterior and posterior roots at the corresponding cord segment (eFig. 9.48). These anterior and posterior roots converge to form 31 pairs of spinal nerves, which extend the length of the spinal cord (eFig. 9.44). Along the length of the spinal cord two regions are enlarged to accommodate the numerous neurons innervating the upper and lower extremities. The cervical enlargement extends from C5 to T1 and innervates the upper extremities, whereas the lumbar enlargement extends from L2 to S3 and innervates the lower extremities (eFig. 9.44).
A cross-section of the spinal cord reveals an inner H-shaped gray matter consisting of neuronal cell bodies and an outer white matter composed of myelinated neuronal axons. The ventral or anterior horns of gray matter contain cell bodies of motor neurons, whereas the dorsal or posterior horns contain cell bodies receiving sensory information (eFig. 9.47). An enlargement of the lateral portion of the gray matter, termed the intermediolateral cell column, can be seen in the T1 to L2 region of the spinal cord (eFig. 9.49). This region enlarges to accommodate the preganglionic cell bodies of the sympathetic nervous system. To further define the diverse cytoarchitecture of the gray matter, it is divided into 10 zones known as Rexed’s laminae (eFig. 9.50). These will be referred to as they relate to later discussions of the ascending and descending tracts within the spinal cord.
The anterior funiculus of the white matter consists of motor axons, whereas the posterior funiculus consists of axons conveying sensory information (eFig. 9.47). The lateral funiculus has a mixture of axons conveying both sensory and motor information.
Sensory information entering the CNS from peripheral sensory receptors is conducted through a series of neurons that synapse with targets in the spinal cord, cerebral cortex, and other brain structures. The sensory modalities carried in these pathways include pain, temperature, tactile, and proprioceptive input. Conscious perception of sensory input is transmitted through neuronal pathways, which reach the primary somatosensory region of the cerebral cortex. In addition to conscious sensory input, there is subconscious sensory input, which is transmitted to other structures such as the cerebellum. For simplicity, in this section we will review the sensory pathways that reach conscious perception and discuss the pathways conveying subconscious sensory input in the section on cerebellum.
Two somatosensory pathways ascend within the spinal cord to reach the cortex: (1) the anterolateral pathways, which convey sensations of pain, temperature, and crude touch; and (2) the posterior column–medial lemniscal pathway, which conveys sensations of discriminative or fine touch, vibration, and conscious proprioception (eFig. 9.51). Both of these pathways transmit information through a series of three neurons. We will review the anterolateral pathway first.