Chapter 16: Lower Motor Neurons
11. Muscles in which region of the body would be expected to have the highest ratio of slow motor units? a. Face b. Buttocks c. Finger d. Forearm e. Shoulder
Answer: b
4. As a person tries to lift a heavy box, which type of motor unit is recruited first, and which is recruited last, according to the size principle?
Answer: According to the size principle, the smallest motor units would be recruited first and the largest motor units that generate the most force would be recruited last. Thus, low-threshold slow motor units are recruited, then FR motor units, and finally FF motor units.
10. Describe central pattern generators in the spinal cord.
Answer: Central pattern generators are local circuits in the spinal cord that time and coordinate complex movements of locomotion.
1. Why are lower motor neurons referred to as the "final common pathway" for movements? Are the motor neurons that innervate head muscles upper or lower motor neurons?
Answer: Lower motor neurons are referred to as the "final common pathway" because commands for both voluntary and reflexive movement use this pathway to initiate movement in skeletal muscles. The motor neurons that innervate head muscles are lower motor neurons.
2. Do neurons in the cerebellum and basal ganglia synapse on motor neurons?
Answer: No, neurons from the cerebellum and basal ganglia synapse on upper motor neurons, not motor neurons, which are lower motor neurons.
7. What the Golgi tendon organ reflex, and how does it function?
Answer: The Golgi tendon organ, located at the junction of a muscle and tendon, is arranged in series with extrafusal muscles. Thus, as a muscle contacts, the Golgi tendon organ increases its output to the Ib afferents. These afferent neurons connect with inhibitory interneurons to inhibit the muscle that is contracted. This negative feedback is the body's way of protecting the muscle from extensive tension and helping to maintain a steady level of tone during muscle fatigue.
5. What is unique about the monosynaptic reflex arc in reciprocal innervation?
Answer: The monosynaptic reflex arc is unique because most sensory neurons from the periphery do not synapse directly on lower motor neurons. Most sensory neurons exert their effects through local circuit neurons instead.
3. What comprises a motor unit?
Answer: The motor unit is an α motor neuron and all the muscle fibers it innervates.
9. List the three sources of direct synaptic input to motor neurons. Which one is the major input?
Answer: The sources of direct synaptic input to motor neurons include the motor cortex, the mesencephalic locomotor region, and local circuit neurons. The motor cortex is the major source of input.
8. What happens to activity levels in muscle spindle afferents versus Golgi tendon organ afferents when a muscle contracts? When a muscle is passively stretched?
Answer: When a muscle contracts, afferent activity increases in a Golgi tendon organ and decreases in an intrafusal muscle fiber. When a muscle is stretched, afferent activity decreases in a Golgi tendon organ and increases in an intrafusal muscle fiber. These responses lead to the muscle spindle maintaining muscle length while the Golgi tendon organ maintains muscle force.
6. Explain how the antagonist muscle is inhibited in reciprocal innervation.
Answer: When muscle spindles are stretched, activity in the group Ia afferents in increases. These Ia afferents directly excite the α motor neurons of the stretched muscle, but inhibit the α motor neurons of the antagonist muscle via reciprocal-Ia-inhibitory interneurons.
12. As motor unit size increases, which property of the α motor neuron decreases? a. Excitability b. Cell body size c. Dendritic complexity d. Axonal diameter e. Number of axonal branches
Answer: a
13. Refer to the figure. An animal model is developed in which gamma motor neurons are dysfunctional. If you record the activity of the Ia fiber (spindle afferent), which pattern would you expect to see when the innervated muscle contracts? a. A b. B c. C d. D e. E
Answer: a
13. Which observation would a researcher expect to make after chronically stimulating a cat's leg muscle over a month-long period? a. Phenotypic change of FF motor units to S motor units b. Phenotypic change of S motor units to FF motor units c. Elimination of FR motor units d. No change in muscle phenotype e. No change in α motor neuron function
Answer: a
14. Golgi tendon organs are most sensitive to a. muscle tension. b. muscle stretch. c. absolute joint position. d. muscle contraction frequency. e. muscle fatigue.
Answer: a
18. While helping a friend move, you hold out your arms to take a box from your friend. To ensure that you do not drop the box, your _______ neurons send a signal to your _______ neurons to contract your biceps and relax your triceps. a. group Ia afferent; α motor b. group II afferent; α motor c. α motor; group Ia afferent d. group Ia afferent; γ motor e. group II afferent; γ motor
Answer: a
25. The local circuits in the spinal cord that are fully capable of controlling the timing and coordination of complex locomotion are called a. central pattern generators. b. the mesencephalic locomotor region. c. the flexion-crossed extension reflex. d. reciprocal innervation. e. motor units.
Answer: a
3. A motor pool (as opposed to a motor unit) consists of a. all of the motor neurons that project to a given muscle. b. all of the motor neurons within a single segment of spinal cord. c. all of the motor neurons that project to a given limb. d. a single motor neuron and all of the muscles it innervates. e. a single motor neuron and all of its afferent interneurons.
Answer: a
5. A woman sustains a spinal cord injury in a car accident and loses the ability to move her fingers and her hand. The injury most likely damaged which region of her spinal cord? a. Lateral ventral horn b. Lateral dorsal horn c. Medial ventral horn d. Medial dorsal horn e. Lateral column
Answer: a
9. Asynchronous firing of motor neurons a. provides a means by which a population of motor neurons can maintain constant force over a finite time interval. b. refers to the pattern by which more lateral motor neurons are fired first and more medial ones later. c. refers to alternating firing of fast-twitch and slow-twitch muscle fibers. d. does not occur under normal physiological conditions. e. is observed only in the γ motor neurons.
Answer: a
1. Motor neurons with cell bodies in the cerebral cortex or the brainstem are part of which motor system? a. Lower motor neurons b. Upper motor neurons c. Both lower and upper motor neurons d. Cerebellum e. Basal ganglia
Answer: b
11. The patellar tendon (knee-jerk) reflex is a. mediated by Golgi tendon organs. b. a monosynaptic reflex arc mediated by Ia afferents. c. a polysynaptic reflex arc that integrates the input from groups Ia, Ib, and II afferents. d. mediated by collaterals of somatosensory afferents. e. a volitional response to the impending impact of an object (i.e., the physician's rubber hammer) directed toward the knee.
Answer: b
17. Which neurons coil around the middle of intrafusal fibers? a. α motor neurons b. Group I afferents c. Group II afferents d. γ motor neurons e. Interneurons
Answer: b
19. In a muscle with a high gain, a _______ stretch of a muscle would lead to _______ increase in extrafusal muscle tension. a. small; a small b. small; a large c. small; no d. large; a small e. large; no
Answer: b
21. As a person squats, the quadriceps muscles contract. At the same time, the respective Golgi tendon organs _______ their firing rate, and the intrafusal muscle fibers _______ their firing rate. a. increase; increase b. increase; decrease c. decrease; decrease d. decrease; increase e. decrease; level off
Answer: b
23. During the flexion-crossed extension reflex, stimulation would lead to _______ of ipsilateral flexor muscles and _______ contralateral flexor muscles. a. excitation; excitation of b. excitation; inhibition of c. inhibition; inhibition of d. inhibition; excitation of e. inhibition; no change in
Answer: b
26. Which statement provides evidence that locomotion is organized by central pattern generators in the spinal cord? a. Retraction of a limb from a painful stimulus causes the opposite limb to extend support. b. Transection of the spinal cord does not prevent coordinated limb movements. c. Damage to descending pathways can result in the flexion reflex in response to squeezing of a limb. d. The swing phase of locomotion remains relatively constant across different speeds. e. Quadrupeds change their limb coordination patterns as they increase their locomotion speed.
Answer: b
5. Spinal interneurons that project ipsilaterally between the lumbar and cervical enlargements are most likely involved with a. ensuring left-right alternation during rhythmic behaviors. b. ensuring coordination of the forelimbs and hindlimbs. c. ensuring the proper speed of locomotion. d. coordinating activity of limb joints and extremities. e. producing alternating flexion and extension patterns in an individual limb.
Answer: b
7. _______ innervate striated muscle fibers, and _______ innervate specialized fibers called _______. a. γ motor neurons; α motor neurons; muscle spindles b. α motor neurons; γ motor neurons; muscle spindles c. α motor neurons; γ motor neurons; extrafusal muscle fibers d. γ motor neurons; α motor neurons; intrafusal muscle fibers e. α motor neurons; γ motor neurons; extrafusal muscle fibers
Answer: b
1. Upper motor neurons a. control the upper half of the torso. b. synapse on muscles in the eye, neck, and head. c. synapse on local circuit neurons and/or lower motor neurons. d. affect motor patterns only indirectly via their inputs to the basal ganglia. e. have cell bodies that are located in the ventral horn of the spinal cord.
Answer: c
10. After a few minutes of repeated stimulation, _______ motor units can generate only a fraction of their maximum force, whereas _______ motor units maintain maximum force generation after an hour of repeated stimulation. a. S; FF b. FF; FR c. FF; S d. FR; S e. FR; FF
Answer: c
10. Which is not a feature of motor unit plasticity? a. Changes in the strength and endurance of individual muscle fibers b. Changes in the firing properties of motor neurons c. Changes in the size and location of motor pools d. Changes in muscle properties in response to the type of motor neurons innervating the muscle e. Capacity for a non-exercised arm to be influenced by unilateral training of the other arm
Answer: c
12. During the patellar reflex, you measure voltage change in the flexor muscle motor neuron cell body in the ventral horn of the spinal cord. After stimulation of the stretch receptors, which of the following would you observe? a. An EPSP b. An action potential c. An IPSP d. No change in voltage e. Constant action potential firing leading to fused tetanus in the flexor muscle
Answer: c
16. Which neurons are responsible for muscle tone? a. α motor neurons b. Group I afferents c. Group II afferents d. γ motor neurons e. Interneurons
Answer: c
16. Which statement about cat locomotion is false? a. Flexors play an important role during the swing phase. b. Extensors play an important role during the stance phase. c. It is controlled by a single spinal central pattern generator (CPG) that always produces left-right alternation within a segment. d. Some gaits alternate activity between the left and right legs. e. Some gaits involve synchronous use of forelimbs and hindlimbs.
Answer: c
18. Proper functioning of the lamprey central pattern generator (CPG) is dependent on which input? a. Descending inputs from the brainstem or cortex b. Excitatory interneurons that cross the midline c. Inhibitory interneurons that cross the midline d. Inhibitory interneurons that do not cross the midline e. Motor neurons that cross the midline
Answer: c
20. Which part of the lower motor neuron circuit prevents muscles from generating excessive tension and maintains steady muscle tone during muscle fatigue? a. Muscles spindles b. α motor neurons c. Golgi tendon organs d. Reciprocal innervation e. γ motor neurons
Answer: c
27. Which symptom is not caused by lower motor neuron syndrome? a. Loss of muscle tone b. Paralysis c. Impairment of fine movement only d. Muscle fibrillations e. Loss of reflexes
Answer: c
6. Local circuit neurons that run the entire length of the spinal cord and connect the medial parts of the spinal cord gray matter are primarily involved in which type of movement? a. Movement of one limb b. Coordination of upper and lower limbs c. Postural control d. Fine control of distal extremities e. Coordinated movement between upper limbs
Answer: c
7. Which statement about the organization of muscles and motor neurons is false? a. Red muscle fibers contract slowly and produce the smallest amount of force. b. Large motor neurons are recruited only at higher levels of stimulus intensity. c. The fastest, strongest muscle fibers can produce the most sustained force output. d. As stimulation of a motor unit increases, the amount of force produced by the muscle increases. e. Individual muscles can be used in a variety of different locomotor patterns or gaits.
Answer: c
9. Compared to the general population, weightlifters would be expected to have a higher ratio of which motor unit(s) in their legs? a. S only b. FR only c. FF only d. Both S and FF e. Both FR and FF
Answer: c
14. According to the size principle, in which order are motor units recruited in response to increasing synaptic input? a. FF, FR, S b. S, FF, FR c. FR, FF, S d. S, FR, FF e. FR, S, FF
Answer: d
17. The lamprey central pattern generator (CPG) a. produces dorsal-ventral flexion waves that run the length of the body. b. requires sensory feedback in order for the spinal CPG to oscillate. c. is controlled by a series of segmental ganglia. d. generates an alternating left-right bending pattern by means of crossed inhibitory fibers. e. is easy to study because there are only four cells in each hemi-segment of the spinal cord.
Answer: d
2. Ava notices that when she consumes two alcoholic beverages within an hour she has trouble controlling her movements. She stumbles occasionally and may knock over a glass that she intended to pick up. She doesn't have any trouble initiating movement, but all of her movements seem a little "off." Which motor system is most affected by Ava's alcohol consumption? a. Lower motor neurons b. Upper motor neurons c. Both lower and upper motor neurons d. Cerebellum e. Basal ganglia
Answer: d
2. The part of the motor system that acts as a servomechanism to minimize motor error is a. Brodmann's area 4. b. Brodmann's area 6. c. the brainstem. d. the cerebellum. e. the basal ganglia.
Answer: d
4. Refer to the figure. Where are lower motor neurons located? a. A b. B c. C d. D e. E
Answer: d
4. The motor neurons innervating the medial gastrocnemius muscle of the cat are found a. concentrated within a single segment of cervical spinal cord. b. concentrated within a single segment of lumbar spinal cord. c. spanning several segments of medial lumbar and sacral spinal cord. d. spanning several segments of lateral lumbar and sacral spinal cord. e. distributed diffusely throughout the dorsal and ventral horns of lumbar and sacral spinal cord.
Answer: d
6. A specialized motor unit, with an average size of three muscle fibers, controls a. finger movements. b. postural muscle. c. the large, fast contracting muscles used in sprinting. d. eye muscles. e. muscles of high-endurance athletes.
Answer: d
8. The condition called "fused tetanus" refers to a. lockjaw symptoms resulting from exposure to the tetanus toxin. b. a muscle firing pattern resulting from one's willing a muscle to contract at maximum velocity. c. the response of single muscle fibers to a single action potential input. d. a sustained maximal contraction due to intense motor unit firing. e. an excessive contraction that results in the fusing together of muscle fibers.
Answer: d
15. Which activity would recruit all three types of motor units? a. Walking b. Sleeping c. Kayaking d. Skipping e. Sprinting
Answer: e
15. Which statement about the withdrawal reflex is false? a. It can be initiated by pain- and temperature-sensitive sensory fibers. b. It has opposing effects within a limb. c. It has opposing effects in the left and right limbs. d. It controls muscles by means of polysynaptic pathways. e. It results in extensor muscle activation within the stimulated limb.
Answer: e
19. Which symptom would you expect to see in a patient with lower motor neuron damage? a. Hyperactive deep reflexes b. Increased muscle tone c. Babinski's sign d. No development of atrophy e. Hypoactive deep reflexes
Answer: e
20. One candidate hypothesis for the selective degeneration of lower and upper motor neurons in ALS is that a. these neurons share distinct sets of G-protein-coupled receptors. b. these neurons exhibit high resting firing rates. c. motor neurons are used far more intensively than other CNS cell types. d. these cells are exposed to infectious agents via their peripheral projections. e. their very long axons make them more vulnerable to defects in axonal transport.
Answer: e
22. The flexion-crossed extension reflex would occur in response to which activity? a. Holding a box steady as more items are added to it b. Coordinating your legs to squat c. A knee tap with a reflex hammer d. Squinting when you move from a dark room to bright sunlight e. Stepping on a sharp object
Answer: e
24. Which neurons receive converging inputs from several different sources before they relay signals to the muscles in the flexor reflex? a. α motor neurons b. Group I afferents c. Group II afferents d. γ motor neurons e. Local circuit neurons
Answer: e
3. Huntington's disease and Parkinson's disease indicate the importance of which motor system for proper motor functioning? a. Lower motor neurons b. Upper motor neurons c. Both lower and upper motor neurons d. Cerebellum e. Basal ganglia
Answer: e
8. Which motor units are resistant to fatigue? a. Slow only b. Fast fatigue-resistant only c. Fast fatigue only d. Both slow and fast fatigue e. Both slow and fast fatigue-resistant
Answer: e