Lecture 18: Chapter 31 - Spinal motor control, reflexes, and locomotion

Note: the movements generated by the spinal cord are coordinated sequences of contractions organized around sensory input when sensory input is present, but the movements can be coordinated even in the absence of sensory input. This is because there are within the spinal cord, central pattern generators (CPGs) which are spinal circuits capable of generating motor sequences with or without sensory or descending input. The CPG also organizes all other sensory evoked responses when the CPG is active.

Additional note: for this chapter, there will be considerable material introduced from the lecture for which there are no figures. I will try to add the material before the lecture, but whatever is NOT here, will be in the lecture -- so either make sure you attend or get the notes. The boxes are also very important in this chapter -- make sure to read them!

Box 31.1, Figure 31.1:

"Simple" systems: The stomatogastric ganglion (STG) of the lobster or crayfish is a wonderful model system to study the relationship between a neural circuit and its motor output. The output of the system is chewing movements of the lobster gut, and the neural circuit in complete isolation will produce the rhythmic output identical to the regular pattern - for hours. The circuit is only 30 neurons, and all connections are known, as are all cells in the circuit. There are actually two rhythms that the circuit produces (actually 3, but we only discuss 2), the gastric mill rhythm (B) and the pyloric rhythm (C). The circuit configurations that give rise to each are shown above.

This simple little circuit can be "reconfigured" and produce a new pattern after being exposed to a neuromodulator. This is known to occur in this system and many others, but is particularly easily demonstrated here.

Figure 31.2

Dynamic reconfiguration of the network by a specified input (the PS neuron). The reconfiguration is not a true rebuilding of the network, but is rather a change in the properties of the neurons so that their interactions effectively silence some and activate other neurons.

Reflexes:

Figure 31.3

The basic stretch reflex circuit. The basic circuit consists of the Ia spindle afferent which is forms a monosynaptic connection upon the "homonymous" motor neurons, that is, the motor neurons that innervate its own muscle. Thus, when the muscle is stretched, say by a doctor's hammer, the spindle is activated, and activates its motor neurons to contract the muscle.

NB: Box 31.2 - on stretch reflexes, pointing out that gamma-motor neurons control the stretch reflex so that the basic reflex is almost never seen. Rather, the spindle isused to control the firing of the motor neurons.

Figure 31.4

Phasic and tonic components of the stretch reflex. The upper curve shows the effect of the stretch reflex in the decerebrate cat, but otherwise intact. The lower curve is the tension of the muscle produced by stretch in the denervated muscle, so the tension is entirely due to the passive properties of the muscle.

NB: Table 31.1 - Selected spinal interneurons - we will look at this, as these are major players in the known circuitry for the spinal cord.

Figure 31.5

Excitation of the knee or ankle extensor muscles by cutaneous receptors. The figure shows the distribution of those regions that either inhibit or excite the (A) knee extensors, or (B) ankle extensors. Thus, the particular receptors are selective in their activation of the muscles to produce an adaptive response to cutaneous stimulation.

Definition: Fictive locomotion = locomotion in the absence of movement, i.e., with the sensory input cut and the muscles either removed or paralyzed. There is also typically no descending control, but this is not necessarily the case.

Figure 31.6

EMG (electromyogram) record from three muscles of a decerebrate cat exhibiting fictive locomotion (cf. definition above). When the group I afferents (the largest afferents) are stimulated the extensor phase of the cycle is extended, and the ipsilateral flexor is delayed until the extensor is finished. The contralateral flexor is not strongly effected, but it is delayed appropriately as well.

Figure 31.7

Using optical recording methods, and calcium measuring dyes, it can be shown that in the chick embryo spinal cord, the activity during locomotion begins in the ventral horn, but then spreads throughout the spinal cord dorsal region as well. Thus, one can observe the activity of the spinal interneurons during fictive locomotion.

One of the best studied CPGs is the CPG for locomotion in vertebrates, located in the spinal cord.

Figure 31.8

The characteristics of the human gait during locomotion. Note the periods of flexion (swing) and extension (stance) when the foot is swinging forward and supporting the body, respectively. Note the relationship between the two legs and how they are strictly alternating, with some double contact during slow walk (or flight during running). Notice the different functional parts of the step cycle discussed in the text.

Figure 31.9

Motor neuron activity during locomotion in the cat. The top recording is an intracellular recording from a flexor motor neuron of the toe and ankle synchronous with the TA muscle, another flexor muscle of the ankle. Note that each muscle has its own characteristic pattern of activity, coordinated with the other muscles to produce coordinated locomotor movements. This pattern can be seen in the functionally isolated spinal cord (i.e., with the sensory feedback cut, the spinal cord cut and the animal paralyzed). Thus, the spinal cord alone can produce the pattern.

Figure 31.10

Scheme for the structure of the vertebrate CPG based on the classic "half-center" model proposed in 1914 by Brown. It posits the existence of an oscillator the flexors and extensors for each joint. These are coupled together to give the overall pattern of locomotor activity.

Figure 31.11

A model proposed for the lamprey CPG for locomotion. It is based on the known cell types and their known connections. It also includes input from the brain known to drive the CPG, and sensory input from mechanoreceptors that exist IN the spinal cord.

Figure 31.12

Real and simualted data to show that the model can produce the pattern similar to that seen in the isolated spinal cord.