Spinal Reflexes


The nervous system often links sensors and effectors via a reflex arc. A reflex arc consists of an afferent pathway, a portion of the central nervous system, and an efferent pathway.

In a reflex, a stimulus to a sensor leads, via a reflex arc, to a response in an effector. Moreover, the term reflex implies that the relationship between the afferent and efferent is reasonably direct and reproducible.

The Stretch Reflex

The figure to the right shows the stretch reflex, which helps control length in skeletal muscles. It is the fastest reflex in body. Only here does the afferent neuron make direct synaptic contact with the efferent neuron. The sensor is the muscle spindle. (Go back and review this from the "Peripheral Sensors page.) The efferent neuron here goes back to the very same muscle that contains the muscle spindle.

The figure shows just one afferent neuron and one efferent neuron. But larger muscles have dozens of muscle spindles and hundreds of motor units.




QUESTION: Does the reflex above have positive or negative feedback?
ANSWER

QUESTION: Why do you say that?
ANSWER

QUESTION: You relax your biceps muscle and have a friend extend your elbow joint while you try to keep the muscle relaxed. Would this tend to excite the muscle spindles and the reflex?
ANSWER




Reciprocal Innervation

The figure to the right shows what happens to the efferent neurons that innervate an antagonist muscle. Notice an inhibitory interneuron lies in the pathway.

This, of course, prevents the antagonist from preventing the action of the stretch reflex. While not shown for the reflexes below, this principle of reciprocal innervation is part of motor reflexes in general. Exitation of the muscle in question tends to inhibit the contraction of antagonists.

Of course, this does not always apply in more complex situations. At times, we consciously contract both agonists and antagonists at a joint in order to stabilize the joint.


Golgi Tendon Organ Reflex

The figure to the right shows the Golgi tendon organ reflex, which helps control tension in a muscle. An inhibitory interneuron connects the afferent neuron to the efferent neuron going back to the same muscle.


QUESTION: Does this reflex have negative or positive feedback?
ANSWER

QUESTION: Again, why do you say that?
ANSWER

QUESTION: You relax your biceps muscle and have a friend extend your elbow joint while you try to keep the muscle relaxed. Assume for the moment that the muscle spindle reflex does not cause the muscle to contract. Would the extension tend to excite the Golgi tendon organs and the reflex?
ANSWER



Pain Withdrawal Reflex

The figure to the right shows the pain withdrawal reflex. As shown, stimulation of pain sensors in a limb tends to exite efferent neurons going to the various flexor muscles in the limb.

As described above, the principle of reciprocal innervation implies that the extensor muscles in the same limb are inhibited. But notice that the extensor muscles in the opposite limb are excited. This is called the crossed extension reflex.


QUESTION: Does this reflex have negative or positive feedback?
ANSWER

QUESTION: Again, why do you say that?
ANSWER




The figure to the right shows a person being bitten by a crab. Notice the extension of the opposite limb.


QUESTION: Is the crossed extension negative or positive feedback?
ANSWER

QUESTION: Again, why do you say that?
ANSWER