a well-healed spinal animal (with spinal transection in the neck above the forelimb area of the cord) is held up from the floor and its legs are allowed to dangle, as shown in Figure 54-12, the stretch on the limbs occasionally elicits stepping reflexes that involve all four limbs. In general, stepping occurs diagonally between the forelimbs and hindlimbs. This diagonal response is another manifestation of reciprocal innervation, this
Diagonal stepping movements exhibited by a spinal animal
Spinal Cord Reflexes That Cause Muscle Spasm
In human beings, local muscle spasm is often observed. In many, if not most, instances, localized pain is the cause of the local spasm.
Muscle Spasm Resulting from a Broken Bone. One type of clinically important spasm occurs in muscles that surround a broken bone. This results from pain impulses initiated from the broken edges of the bone, which cause the muscles that surround the area to contract tonically. Pain relief obtained by injecting a local anesthetic at the broken edges of the bone relieves the spasm; a deep general anesthetic of the entire body, such as ether anesthesia, also relieves the spasm. One of these two anesthetic procedures is often necessary before the spasm can be overcome sufficiently for the two ends of the bone to be set back into their appropriate positions.
time occurring the entire distance up and down the cord between the forelimbs and hindlimbs. Such a walking pattern is called a mark time reflex.
Galloping Reflex. Another type of reflex that occasionally develops in a spinal animal is the galloping reflex, in which both forelimbs move backward in unison while both hindlimbs move forward. This often occurs when almost equal stretch or pressure stimuli are applied to the limbs on both sides of the body at the same time; unequal stimulation elicits the diagonal walking reflex. This is in keeping with the normal patterns of walking and galloping, because in walking, only one forelimb and one hindlimb at a time are stimulated, which would predispose the animal to continue walking. Conversely, when the animal strikes the ground during galloping, both forelimbs and both hindlimbs are stimulated about equally; this predisposes the animal to keep galloping and, therefore, continues this pattern of motion.
An especially important cord reflex in some animals is the scratch reflex, which is initiated by itch or tickle sensation. It involves two functions: (1) a position sense that allows the paw to find the exact point of irritation on the surface of the body, and (2) a to-and-fro scratching movement.
The position sense of the scratch reflex is a highly developed function. If a flea is crawling as far forward as the shoulder of a spinal animal, the hind paw can still find its position, even though 19 muscles in the limb must be contracted simultaneously in a precise pattern to bring the paw to the position of the crawling flea. To make the reflex even more complicated, when the flea crosses the midline, the first paw stops scratching and the opposite paw begins the to-and-fro motion and eventually finds the flea.
The to-and-fro movement, like the stepping movements of locomotion, involves reciprocal innervation circuits that cause oscillation.
Abdominal Muscle Spasm in Peritonitis. Another type of local spasm caused by cord reflexes is abdominal spasm resulting from irritation of the parietal peritoneum by peritonitis. Here again, relief of the pain caused by the peritonitis allows the spastic muscle to relax. The same type of spasm often occurs during surgical operations; for instance, during abdominal operations, pain impulses from the parietal peritoneum often cause the abdominal muscles to contract extensively, sometimes extruding the intestines through the surgical wound. For this reason, deep anesthesia is usually required for intraabdominal operations.
Muscle Cramps. Still another type of local spasm is the typical muscle cramp. Electromyographic studies indicate that the cause of at least some muscle cramps is as follows: Any local irritating factor or metabolic abnormality of a muscle, such as severe cold, lack of blood flow, or overexercise, can elicit pain or other sensory signals transmitted from the muscle to the spinal cord, which in turn cause reflex feedback muscle contraction. The contraction is believed to stimulate the same sensory receptors even more, which causes the spinal cord to increase the intensity of contraction. Thus, positive feedback develops, so that a small amount of initial irritation causes more and more contraction until a fullblown muscle cramp ensues.
Many types of segmental autonomic reflexes are integrated in the spinal cord, most of which are discussed in other chapters. Briefly, these include (1) changes in vascular tone resulting from changes in local skin heat (see Chapter 73); (2) sweating, which results from localized heat on the surface of the body (see Chapter 73); (3) intestinointestinal reflexes that control some motor functions of the gut (see Chapter 62); (4) peritoneoin-testinal reflexes that inhibit gastrointestinal motility in response to peritoneal irritation (see Chapter 66); and (5) evacuation reflexes for emptying the full bladder (see Chapter 31) or the colon (see Chapter 63). In addition, all the segmental reflexes can at times be elicited simultaneously in the form of the so-called mass reflex, described next.
Mass Reflex. In a spinal animal or human being, sometimes the spinal cord suddenly becomes excessively active, causing massive discharge in large portions of the cord.The usual stimulus that causes this is a strong pain stimulus to the skin or excessive filling of a viscus, such as overdistention of the bladder or the gut. Regardless of the type of stimulus, the resulting reflex, called the mass reflex, involves large portions or even all of the cord. The effects are (1) a major portion of the body's skeletal muscles goes into strong flexor spasm; (2) the colon and bladder are likely to evacuate; (3) the arterial pressure often rises to maximal values, sometimes to a systolic pressure well over 200 mm Hg; and (4) large areas of the body break out into profuse sweating.
Because the mass reflex can last for minutes, it presumably results from activation of great numbers of reverberating circuits that excite large areas of the cord at once. This is similar to the mechanism of epileptic seizures, which involve reverberating circuits that occur in the brain instead of in the cord.
When the spinal cord is suddenly transected in the upper neck, at first, essentially all cord functions, including the cord reflexes, immediately become depressed to the point of total silence, a reaction called spinal shock. The reason for this is that normal activity of the cord neurons depends to a great extent on continual tonic excitation by the discharge of nerve fibers entering the cord from higher centers, particularly discharge transmitted through the reticulospinal tracts, vestibulospinal tracts, and corticospinal tracts.
After a few hours to a few weeks, the spinal neurons gradually regain their excitability. This seems to be a natural characteristic of neurons everywhere in the nervous system—that is, after they lose their source of facilitatory impulses, they increase their own natural degree of excitability to make up at least partially for the loss. In most nonprimates, excitability of the cord centers returns essentially to normal within a few hours to a day or so, but in human beings, the return is often delayed for several weeks and occasionally is never complete; conversely, sometimes recovery is excessive, with resultant hyperexcitability of some or all cord functions.
Some of the spinal functions specifically affected during or after spinal shock are the following:
1. At onset of spinal shock, the arterial blood pressure falls instantly and drastically—sometimes to as low as 40 mm Hg—thus demonstrating that sympathetic nervous system activity becomes blocked almost to extinction. The pressure ordinarily returns to normal within a few days, even in human beings.
2. All skeletal muscle reflexes integrated in the spinal cord are blocked during the initial stages of shock. In lower animals, a few hours to a few days are required for these reflexes to return to normal;
in human beings, 2 weeks to several months are sometimes required. In both animals and humans, some reflexes may eventually become hyperexcitable, particularly if a few facilitatory pathways remain intact between the brain and the cord while the remainder of the spinal cord is transected. The first reflexes to return are the stretch reflexes, followed in order by the progressively more complex reflexes: flexor reflexes, postural antigravity reflexes, and remnants of stepping reflexes. 3. The sacral reflexes for control of bladder and colon evacuation are suppressed in human beings for the first few weeks after cord transection, but in most cases they eventually return. These effects are discussed in Chapters 31 and 66.
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