The ability to move is essential for the survival of animals. Although many forms of locomotion have evolved—swimming, flying, crawling, and walking—all use rhythmic and alternating movements of the body or appendages. This rhythmicity makes locomotion appear to be repetitive and stereotyped. Indeed, locomotion is controlled automatically at relatively low levels of the central nervous system without intervention by higher centers. Nevertheless, locomotion often takes place in environments that are either unfamiliar or present unpredictable conditions. Locomotor movements must therefore be continually modified, usually in a subtle fashion, to adapt otherwise stereotyped movement patterns to the immediate surroundings.
The study of the neural control of locomotion must address two fundamental questions. First, how do assemblies of nerve cells generate the rhythmic motor patterns associated with locomotor movements? Second, how does sensory information adjust locomotion to both anticipated and unexpected events in the environment? In this chapter we address both of these questions by examining the neural mechanisms controlling walking.
Although most information on neural control of walking has come from studying the cat's stepping movements, important insights have also come from studies of other animals as well as rhythmic behaviors other than locomotion. Therefore, we shall also consider the more general question of how rhythmic motor activity can be generated and sustained by networks of neurons.
Several critical insights into the neural mechanisms controlling quadrupedal stepping were obtained nearly a century ago when it was found that removing the cerebral hemispheres in dogs did not abolish walking—decerebrate animals are still able to walk spontaneously. One animal was observed to rear itself up in order to rest its forepaws on a gate at feeding time. It was soon discovered that stepping of the hind legs could be induced in cats and dogs after complete transection of the spinal cord. The stepping movements in these spinal preparations (Box 36–1) are similar to normal stepping. Nonrhythmic electrical stimulation of the cut cord elicits stepping at a rate related to the intensity of the stimulating current. Another important early observation was that passive movement of a limb by the experimenter could initiate stepping movements in spinal cats and dogs, suggesting that proprioceptive reflexes are crucial in regulating the movements.
Box 36–1 Preparations Used to Study the Neural Control of Stepping
The literature on the neural control of quadrupedal stepping can be confusing because different experimental preparations are used in different studies. In addition to intact animals, spinal and decerebrate cats are commonly used in studies of the neural mechanisms of locomotor rhythmicity. Moreover, each of these preparations may be used in two experimental strategies, deafferentation and immobilization, depending on what is being investigated. Finally, neonatal rat and mouse preparations have proven useful for analyzing the cellular properties of neurons generating the locomotor rhythm. Spinal Preparations
In spinal preparations the spinal cord is transected at the lower thoracic level (Figure 36–1A), thus isolating the spinal segments that control the hind limb musculature from the rest of the central nervous system. This allows investigations of the role of spinal circuits in generating rhythmic locomotor patterns.
In acute spinal preparations adrenergic drugs such as l-DOPA (l-dihydroxyphenylalanine) and nialamide are administered immediately after the transection. These drugs elevate the level of norepinephrine in the spinal cord and lead to the spontaneous generation of locomotor activity approximately 30 minutes after administration. Clonidine, another adrenergic drug, enables locomotor activity to be generated in acute spinal preparations but only if the skin of the perineal region is also stimulated.
In chronic spinal preparations animals are studied for weeks or months after transection. Without drug treatment locomotor activity can return within a few weeks of cord transection. Locomotor function returns spontaneously in kittens, but in adult cats daily training is usually required. Decerebrate Preparations
In decerebrate preparations the brain stem is completely transected at the level of the midbrain, disconnecting rostral brain centers, especially the cerebral cortex, from the spinal centers where the locomotor pattern is generated. Because brain stem centers remain connected to the spinal cord, these preparations allow investigation of the role of the cerebellum and brain stem structures in controlling locomotion.
Two decerebrate preparations are commonly used. In one the locomotor rhythm is generated spontaneously, whereas in the other it is evoked by electrical stimulation of the mesencephalic locomotor region. This difference depends on the level of decerebration. Spontaneous walking occurs in premammillary preparations, in which the brain stem is transected from the rostral margin of the superior colliculi to a point immediately rostral to the mammillary bodies. When the transection is made caudal to the mammillary bodies postmammillary or mesencephalic preparation, spontaneous stepping does not occur; rather, electrical stimulation of the mesencephalic locomotor region is required to evoke walking (Figure 36–1B).
When supported on a motorized treadmill, both preparations walk with a coordinated stepping pattern in all four limbs and the rate of stepping is matched to the treadmill speed. The motor activity can be recorded during stepping, and sensory nerves can be stimulated with implanted electrodes to examine the reflex mechanisms that regulate stepping. Deafferented Preparations
An early view of the neural control of locomotion was that it involved a "chaining" of reflexes: Successive stretch reflexes in flexor and extensor muscles were thought to produce the basic rhythm of walking. This view was disproved by Graham Brown, who showed that rhythmic locomotor patterns were generated even after complete removal of all sensory input (deafferentation) from the moving limbs.
Deafferentation is accomplished by transection of all the dorsal roots that innervate the limbs. Because the dorsal roots carry only sensory axons, motor innervation of the muscles remains intact. Deafferented preparations were once useful for demonstrating the capabilities of the isolated spinal cord but are rarely used today, principally because the loss of all tonic sensory input drastically reduces the excitability of interneurons and motor neurons in the spinal cord. Thus, changes in the locomotor pattern after deafferentation might result from the artificial reduction in excitability of neurons rather than from the loss of specific sensory inputs. Immobilized Preparations
The role of proprioceptive input from the limbs can be more systematically investigated by preventing activity in motor neurons from actually causing any movement. This is typically accomplished by paralyzing the muscles with d -tubocurarine, a competitive inhibitor of acetylcholine that blocks synaptic transmission at the neuromuscular junction.
When locomotion is initiated in such an immobilized preparation, often referred to as fictive locomotion, the motor nerves to flexor and extensor muscles fire alternately but no actual movement takes place and the proprioceptive afferents are not phasically excited. Thus the effect of proprioceptive reflexes is removed whereas tonic sensory input is preserved.
Because immobilized preparations allow intracellular and extracellular recording from neurons in the spinal cord, they are used to examine the synaptic events associated with locomotor activity and the organization of central and reflex pathways controlling locomotion. Neonatal Rodent Preparation
The spinal cord is removed from a neonatal rat or mouse (0–5 days after birth) and placed in a saline bath, where it will generate coordinated bursts of activity in leg motor neurons when exposed to NMDA and serotonin (Figure 36–1C). This preparation allows more detailed analysis of the locations and roles of the specific neurons involved in rhythm generation, as well as pharmacological studies on the rhythm-generating network.
The ability to genetically modify neurons in the spinal cord of mice allows studies on the function of identified classes of neurons in these animals.
Finally, in 1911 Thomas Graham Brown discovered that rhythmic, alternating contractions could be evoked in deafferented hind leg muscles immediately after transection of the spinal cord. He therefore proposed the concept of the half-center, whereby flexors and extensors inhibit each other reciprocally, giving rise to alternating stepping movements. Four conclusions can be drawn from these early studies.
For almost half a century following these early studies few investigations were aimed at establishing the neural mechanisms for walking. Instead, research on motor systems focused on the organization of spinal reflex pathways and the mechanisms of synaptic integration within the spinal cord (see Chapter 35). Modern research on the neural control of locomotion dates from the 1960s and two major experimental successes. First, rhythmic patterns of motor activity were elicited in spinal animals by the application of adrenergic drugs. Second, walking on a treadmill was evoked in decerebrate cats by electrical stimulation of a small region in the brain stem.
At about the same time electromyographic recordings from numerous hind leg muscles in intact cats during unrestrained walking revealed the complexity of the locomotor pattern and brought to prominence the question of how spinal reflexes are integrated with intrinsic spinal circuits to produce the locomotor pattern. Soon thereafter, investigations of stepping in spinal cats demonstrated the similarity of locomotor patterns in spinal preparations and intact animals, thus firmly establishing the idea that the motor output for locomotion is produced primarily by a neuronal system in the spinal cord.