Influences of Descending Pathways on Spinal Circuits
Voluntary movement. The most distinctive function of the descending motor pathways is the control of voluntary movement. These movements are initiated in the cerebral cortex, and the motor commands are transmitted to the musculature through a variety of descending pathways, including the corticospinal tract, the rubrospinal tract, and reticulospinal tracts. How voluntary movements are initiated and coordinated by the motor cortex is the subject of the next chapter.
Reflex modulation. Another critical function of the descending motor pathways is to modulate the reflex circuits in the spinal cord. The adaptiveness of spinal reflexes can change depending on the behavioral context; sometimes the gain (strength) or even the sign (extension vs. flexion) of a reflex must be changed in order to make the resulting movement adaptive. The descending pathways are responsible for controlling these variables. For example, consider the flexor reflex under two conditions.
- Imagine a situation in which you want to pick up a dish from the stove top, but you are uncertain whether it is hot or cold. You may attempt to lightly touch the surface, and this will often lower the threshold of the flexor reflex, making you more likely to pull your hand away even if the dish is not particularly hot. (You may even withdraw your hand numerous times before even touching the dish!) Descending pathways have lowered the threshold for producing the reflex in this case, making it easier for a weaker nociceptive input to trigger the reflex; these pathways can also change the gain of the reflex, making the withdrawal response greater than usual.
- Imagine now picking up the dish in order to move it to the table. As you hold the dish, more of its heat begins to transfer to your hand, and it starts to get quite hot. Rather than dropping the dish and spilling your dinner all over the floor, you rush to the table to put it down, before withdrawing your hand and wishing you had used an oven mitt. In this case, the descending pathways inhibited the flexor response.
Gamma bias. Recall from the previous chapter that there are two types of spinal motor neurons. Alpha motor neurons innervate extrafusal muscle fibers, which provide the force for a muscle contraction. Gamma motor neurons innervate the ends of intrafusal fibers and help to maintain the tautness of muscle spindles, such that they are sensitive to changes of muscle length over a wide range. In order to work adaptively, the activity of alpha and gamma motor neurons must be coordinated. Thus, whenever motor commands are sent by descending pathways to alpha motor neurons, the appropriate compensating commands are sent to gamma motor neurons. This coordination of alpha-gamma motor commands is called alpha-gamma coactivation, and the adjustment of spindle sensitivity by gamma activation is called gamma bias. Consider the following two examples:
- When a command is given to a muscle to contract, the muscle spindles become slack, thereby making them insensitive to further changes in muscle length. To compensate for this, the gamma motor neurons that innervate these intrafusal muscle fibers are activated in concert with the alpha motor neurons, allowing the intrafusal fibers to contract with the muscle. This preserves the sensitivity of the muscle to unexpected stretches of the muscle (see Figure 1.10 of the chapter on Motor Unit and Muscle Receptors).
- When a muscle contracts, the antagonist muscle is stretched during the movement. An obvious problem arises when one considers the stretch reflex of the antagonist muscle. If contraction of a muscle causes the activation of the stretch reflex of the antagonist muscle, the antagonist muscle will contract to resist the movement of the limb. How is it possible to ever flex a joint when the stretch reflex of the extensor muscle causes it to extend the joint instead? Alpha-gamma coactivation solves this problem by relaxing the contraction of the intrafusal fibers of the antagonist muscle, allowing the muscle to be stretched without triggering the stretch reflex during a voluntary movement.
Test Your Knowledge
- Question 1
- A
- B
- C
- D
- E
The lateral corticospinal tract...
A. Undergoes a 50% decussation in the caudal medulla.
B. Arises exclusively from the primary motor cortex.
C. Is an uncrossed pathway.
D. Plays a major role in the fine control of distal musculature.
E. Terminates primarily in the posterior (dorsal) horn.
The lateral corticospinal tract...
A. Undergoes a 50% decussation in the caudal medulla. This answer is INCORRECT. About 90% of the corticospinal tract fibers cross to form the lateral corticospinal tract.
B. Arises exclusively from the primary motor cortex.
C. Is an uncrossed pathway.
D. Plays a major role in the fine control of distal musculature.
E. Terminates primarily in the posterior (dorsal) horn.
The lateral corticospinal tract...
A. Undergoes a 50% decussation in the caudal medulla.
B. Arises exclusively from the primary motor cortex. This answer is INCORRECT. The corticospinal tract arises from numerous cortical areas.
C. Is an uncrossed pathway.
D. Plays a major role in the fine control of distal musculature.
E. Terminates primarily in the posterior (dorsal) horn.
The lateral corticospinal tract...
A. Undergoes a 50% decussation in the caudal medulla.
B. Arises exclusively from the primary motor cortex.
C. Is an uncrossed pathway. This answer is INCORRECT. The lateral corticospinal tract crosses at the pyramidal decussation.
D. Plays a major role in the fine control of distal musculature.
E. Terminates primarily in the posterior (dorsal) horn.
The lateral corticospinal tract...
A. Undergoes a 50% decussation in the caudal medulla.
B. Arises exclusively from the primary motor cortex.
C. Is an uncrossed pathway.
D. Plays a major role in the fine control of distal musculature. This answer is CORRECT!
E. Terminates primarily in the posterior (dorsal) horn.
The lateral corticospinal tract...
A. Undergoes a 50% decussation in the caudal medulla.
B. Arises exclusively from the primary motor cortex.
C. Is an uncrossed pathway.
D. Plays a major role in the fine control of distal musculature.
E. Terminates primarily in the posterior (dorsal) horn. This answer is INCORRECT. Most lateral corticospinal fibers terminate in the intermediate zone.
- Question 2
- A
- B
- C
- D
- E
In reciprocal excitation of the Golgi tendon reflex, stimulation of...
A. Ia afferent fibers causes inhibition of synergistic muscles.
B. Ib afferent fibers causes inhibition of antagonist muscles.
C. Ia afferent fibers causes inhibition of antagonist muscles.
D. Ib afferent fibers causes excitation of antagonist muscles.
E. Ia afferent fibers causes excitation of muscles on the contralateral side.
In reciprocal excitation of the Golgi tendon reflex, stimulation of...
A. Ia afferent fibers causes inhibition of synergistic muscles. This answer is INCORRECT. Ia afferents innervate the muscle spindle, not the Golgi tendon organ.
B. Ib afferent fibers causes inhibition of antagonist muscles.
C. Ia afferent fibers causes inhibition of antagonist muscles.
D. Ib afferent fibers causes excitation of antagonist muscles.
E. Ia afferent fibers causes excitation of muscles on the contralateral side.
In reciprocal excitation of the Golgi tendon reflex, stimulation of...
A. Ia afferent fibers causes inhibition of synergistic muscles.
B. Ib afferent fibers causes inhibition of antagonist muscles. This answer is INCORRECT. Ib afferents inhibit the homonymous muscle, not the antagonist muscle.
C. Ia afferent fibers causes inhibition of antagonist muscles.
D. Ib afferent fibers causes excitation of antagonist muscles.
E. Ia afferent fibers causes excitation of muscles on the contralateral side.
In reciprocal excitation of the Golgi tendon reflex, stimulation of...
A. Ia afferent fibers causes inhibition of synergistic muscles.
B. Ib afferent fibers causes inhibition of antagonist muscles.
C. Ia afferent fibers causes inhibition of antagonist muscles. This answer is INCORRECT. Ia afferents innervate the muscle spindle, not the Golgi tendon organ.
D. Ib afferent fibers causes excitation of antagonist muscles.
E. Ia afferent fibers causes excitation of muscles on the contralateral side.
In reciprocal excitation of the Golgi tendon reflex, stimulation of...
A. Ia afferent fibers causes inhibition of synergistic muscles.
B. Ib afferent fibers causes inhibition of antagonist muscles.
C. Ia afferent fibers causes inhibition of antagonist muscles.
D. Ib afferent fibers causes excitation of antagonist muscles. This answer is CORRECT!
E. Ia afferent fibers causes excitation of muscles on the contralateral side.
In reciprocal excitation of the Golgi tendon reflex, stimulation of...
A. Ia afferent fibers causes inhibition of synergistic muscles.
B. Ib afferent fibers causes inhibition of antagonist muscles.
C. Ia afferent fibers causes inhibition of antagonist muscles.
D. Ib afferent fibers causes excitation of antagonist muscles.
E. Ia afferent fibers causes excitation of muscles on the contralateral side. This answer is INCORRECT. Ia afferent fibers innervate the muscle spindle, and the Golgi tendon reflex affects the ipsilateral side.
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