Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Regulation of Posture and Movement
Posture Control System - Spinal Integration

Responses of both animals and humans following cervical Spinal Cord transection reflect the integration of Reflexes at the spinal cord level. Individual spinal reflexes are discussed in detail in Chapter 6.

Class="center">Table 12-1. Levels of Integration of Various Nervous system Functions

¹ 0 - absent; + - present; ++ - enhanced.

² Cerebral cortex removed.

³ Brainstem transected at the level of the upper Pons.

⁴ Conditioned reflexes are more difficult to elicit in decorticate animals than in normal ones.

⁵ Except for stretch reflexes.

Spinal Shock

In all vertebrates, spinal cord transection is immediately followed by a period of spinal shock, during which all spinal reflexes are profoundly depressed. During this period, the resting Membrane Potential of spinal motor Neurons is 2-6 mV higher than normal. Subsequently, reflex responses recover and even become exaggerated. The duration of spinal shock is proportional to the degree of encephalization of motor function in different species. In frogs and rats, this shock lasts for a few minutes; in dogs and cats, for 1-2 hours; in monkeys, for several days; and in humans, for at least two weeks.

The exact causes of spinal shock are not fully understood. The cessation of regulatory tonic excitatory influences descending via spinal pathways undoubtedly plays a role; however, the subsequent recovery and eventual exaggeration of reflexes (hyperreflexia) also require an explanation. The recovery of reflexes can be attributed to The Development of denervation supersensitivity to Neurotransmitters released by spinal excitatory terminals. Another plausible mechanism is The formation of collaterals from existing neurons, establishing new excitatory terminals on interneurons and motor neurons.

The first reflex to appear in humans after the resolution of spinal shock is typically a slight contraction in response to stimulation of the lower leg flexors and adductor Muscles. In some patients, the knee-jerk reflex recovers first. Approximately two weeks elapse between spinal cord transection and the onset of recovery of reflex activity, provided there are no complications. If such complications arise, spinal shock lasts significantly longer. It remains unclear why infection, malnutrition, or other post-transection complications suppress spinal reflex activity.

Complications Following Spinal Cord Transection

The care of patients with paraplegia or quadriplegia involves a complex range of problems. Like all immobilized patients, they develop a negative nitrogen balance and Catabolism of substantial amounts of body protein. Body weight compresses Skin Blood Vessels over bony prominences, and unless such patients are turned frequently, skin necrosis and pressure ulcers (bedsores) develop at these pressure points. Notably, pressure ulcers heal poorly and are prone to infection due to protein depletion. The necrotic tissue masses contain bone protein matrix, which, combined with immobilization, causes the release of large quantities of Ca²⁺. This results in hypercalcemia, hypercalciuria, and the formation of urinary calcium stones. These stones, along with paralysis of the Urinary Bladder musculature, lead to urinary retention, which promotes Urinary Tract infections—the most frequent complication of spinal cord injuries. Consequently, the prognosis for spinal cord transection was historically bleak, with mortality exceeding 80% due to septicemia, uremia, or exhaustion. Since World War II, a series of preventive measures—such as The Use of Antibiotics, proper Nutrition, maintenance of fluid and electrolyte balance, Prevention of pressure ulcers and urinary tract complications, and improvements in general nursing care—has reduced mortality in specialized medical facilities to 6%, allowing such patients to lead meaningful lives. The administration of high doses of glucocorticoids, such as 10 g of methylprednisolone per day for adults, minimizes the consequences of spinal cord injury. This Treatment is most effective when administered immediately after injury, followed by tapering due to the significant adverse side effects of long-term high-dose therapy (see Chapter 20). The rapid effect of immediate administration is presumably due to the reduction of inflammatory processes in the injured Tissues.

Table 12-2. Major Postural Reflexes

At the same time, research continues into Methods to promote the regeneration of spinal cord axons across the site of transection. The administration of neurotrophins in animal experiments is promising (see Chapter 4). Implantation of Embryonic Stem Cells into the injury site has a similar effect. However, these approaches are still in the experimental stage, although they hold great promise.

Reactions in Chronic Spinal Animals and Humans

Following spinal shock, the recovery of spinal reflexes begins, accompanied by a steady decrease in their threshold. In patients with quadriplegia, the withdrawal reflex threshold is exceptionally low. Even mild stimulation can elicit not only prolonged withdrawal of the affected limb but also noticeable flexion-extension movements in the other three limbs. Repetitive flexion movements can persist for a long time and progress into flexor muscle contractures. Stretch reflexes and many complex reactions based upon them are hyperactive. For example, if a finger is touched to the paw of a spinalized animal, its limb extends toward the finger, which has by then been withdrawn. This "magnetic" reaction (positive supporting reaction), mediated by proprioceptors and tactile afferents, transforms the limb into a rigid, gravity-defying pillar that supports the body. The cessation of this reaction is also an active phenomenon of sorts (negative supporting reaction), as it is initiated by the stretch of extensor muscles. Thanks to positive supporting reactions, spinal animals (cats or dogs) can maintain a standing posture, albeit unsteadily, for 2-3 minutes.

In cases of incomplete spinal cord transection, stimulus-evoked flexor contractures can be accompanied by distressing bouts of pain. These attacks are alleviated by baclofen, a blood-Brain-barrier-penetrating GABA_B receptor agonist that promotes inhibition (see Chapter 4). This agent is also successfully used in patients with spasticity resulting from brainstem or internal capsule lesions (see below).

Locomotor Generator

Spinal animals are capable of more than just standing. Neural circuits within the spinal cord, when appropriately stimulated, are capable of generating locomotion. There are two pattern generators for Locomotion in the spinal cord: one in the cervical region and another in the lumbar region. However, this does not mean that spinal animals or humans can walk without stimulation of these generators; generator function is driven by impulses originating from a specialized midbrain center, the mesencephalic locomotor region, and such activation presumably occurs in patients with incomplete spinal cord transections. Interestingly, these generators can be activated in experimental animals with complete spinal cord transections by administration of the noradrenaline precursor L-DOPA or the α-adrenergic receptor agonist clonidine. In humans with spinal cord injuries, significant progress in locomotion can be achieved through locomotor training utilizing supportive devices or treadmills.

Autonomic Reflexes

Reflex contractions of the urinary bladder and rectum occur in spinal animals and humans, although the bladder rarely empties completely. Hyperactive bladder reflexes can maintain the bladder in a state of prolonged contraction, leading to hypertrophy and fibrosis of its wall. Resting blood pressure is generally normal; however, due to the absence of fine feedback regulation via baroreceptor reflexes, significant blood pressure fluctuations are quite common. Episodes of sweating and skin pallor also occur.

Sexual Reflexes

Spinal animals exhibit certain reflex reactions, yet overall these are merely fragmentary responses that, in normal animals, are integrated into goal-directed sequences to perform specific functions. Sexual reflexes serve as a prime example. Coordinated sexual activity depends on a series of reflexes integrated across multiple levels of The Nervous System, METABOLISM/18.html">The Influence of which ceases after spinal cord transection. Nevertheless, by stimulating the genitals, erection and even ejaculation can be elicited in both spinal animals and humans. In spinal female dogs, vaginal stimulation causes deviation of the tail to the side and pelvic movements resembling those preceding copulation.

Mass reflex

In chronic spinal animals, afferent impulses irradiate from one reflex center to another.

Even with minor skin stimulation, simultaneous with the animal's withdrawal reaction, there is an irradiation of impulses to autonomic centers, which promotes emptying of the urinary bladder and rectum, and causes sweating, pallor, and blood pressure fluctuations. Paraplegic patients can harness this mass reflex phenomenon to partially regulate bladder and bowel function. By stroking or pinching the skin of the thighs, they are able to train the initiation of urination and defecation.