The Spinal Cord
The spinal cord is the link between the peripheral nervous system and the brain. It also helps to coordinate impulses within the CNS. The spinal cord is contained in and protected by the vertebrae, which fit together to form a continuous tube extending from the occipital bone to the coccyx (Fig. 5-11). In the embryo, the spinal cord occupies the entire spinal canal, extending down into the tail portion of the vertebral column. The column of bone grows much more rapidly than the nerve tissue of the cord, however, and eventually, the end of the spinal cord no longer reaches the lower part of the spinal canal. This disparity in growth continues to increase, so that in adults, the spinal cord ends in the region just below the area to which the last rib attaches (between the first and second lumbar vertebrae).
Structure of the Spinal Cord
The spinal cord has a small, irregularly shaped internal section of gray matter (unmyelinated tissue) surrounded by a larger area of white matter (myelinated axons) (Fig. 5-12). The internal gray matter is arranged so that a column of gray matter extends up and down dorsally, one on each side; another column is found in the ventral region on each side.
These two pairs of columns, called the dorsal horns and ventral horns, give the gray matter an Hshaped appearance in cross-section. The bridge of gray matter that connects the right and left horns is the gray commissure. In the center of the gray commissure is a small channel, the central canal, that contains cerebrospinal fluid, the liquid that circulates around the brain and spinal cord. A narrow groove, the posterior median sulcus, divides the right and left portions of the posterior white matter. A deeper groove, the anterior median fissure, separates the right and left portions of the anterior white matter.
Figure 5-11 Spinal cord and spinal nerves. Nerve plexuses (networks) are shown. (A) Lateral view. (B) Posterior view.
Ascending and Descending Tracts The spinal cord is the pathway for sensory and motor impulses traveling to and from the brain. These impulses are carried in the thousands of myelinated axons in the white matter of the spinal cord, which are subdivided into tracts (groups of fibers). Sensory (afferent) impulses entering the spinal cord are transmitted toward the brain in ascending tracts of the white matter. Motor (efferent) impulses traveling from the brain are carried in descending tracts toward the peripheral nervous system.
The Reflex Arc
As the nervous system functions, it receives, interprets, and acts on both external and internal stimuli. The spinal cord is also a relay center for coordinating neural pathways. A complete pathway through the nervous system from stimulus to response is termed a reflex arc (Fig. 5-13). This is the basic functional pathway of the nervous system. The basic parts of a reflex arc are the following:
1. Receptor-the end of a dendrite or some specialized receptor cell, as in a special sense organ, that detects a stimulus.
2. Sensory neuron, or afferent neuron-a cell that transmits impulses toward the CNS. Sensory impulses enter the dorsal horn of the gray matter in the spinal cord.
3. Central nervous system-where impulses are coordinated and a response is organized. One or more interneurons may carry impulses to and from the brain, may function within the brain, or may distribute impulses to different regions of the spinal cord. Almost every response involves connecting neurons in the CNS.
Figure 5-12 The spinal cord. (A) Cross-section of the spinal cord showing the organization of the gray and white matter. The roots of the spinal nerves are also shown. (B) Microscopic view of the spinal cord in cross-section (x5).
4. Motor neuron, or efferent neuron-a cell that carries impulses away from the CNS. Motor impulses leave the cord through the ventral horn of the spinal cord gray matter.
5. Effector-a muscle or a gland outside the CNS that carries out a response. At its simplest, a reflex arc can involve just two neurons, one sensory and one motor, with a synapse in the CNS. Few reflex arcs require only this minimal number of neurons. (The knee-jerk reflex described below is one of the few examples in humans.) Most reflex arcs involve many more, even hundreds, of connecting neurons within the CNS. The many intricate patterns that make the nervous system so responsive and adaptable also make it difficult to study, and investigation of the nervous system is one of the most active areas of research today.
Figure 5-13 Typical reflex arc. Numbers show the sequence of impulses through the spinal cord (solid arrows). Contraction of the biceps brachii results in flexion of the arm at the elbow.
Reflex Activities Although reflex pathways may be quite complex, a simple reflex is a rapid, uncomplicated, and automatic response involving very few neurons. Reflexes are specific; a given stimulus always produces the same response. When you fling out an arm or leg to catch your balance, withdraw from a painful stimulus, or blink to avoid an object approaching your eyes, you are experiencing reflex behavior. A simple reflex arc that passes through the spinal cord alone and does not involve the brain is termed a spinal reflex. The stretch reflex, in which a muscle is stretched and responds by contracting, is one example of a spinal reflex. If you tap the tendon below the kneecap (the patellar tendon), the muscle of the anterior thigh (quadriceps femoris) contracts, eliciting the knee-jerk reflex (Fig. 5-14). Such stretch reflexes may be evoked by appropriate tapping of most large muscles (such as the triceps brachii in the arm and the gastrocnemius in the calf of the leg). Because reflexes are simple and predictable, they are used in physical examinations to test the condition of the nervous system.
Figure 5-14 The patellar (knee-jerk) reflex. Numbers indicate the sequence of a reflex arc.