Spinal Mechanisms of Motor Control

Question 1

Motor control at the spinal cord level is

Answer 1

reflexive

Question 2

3 functions of the spinal cord reflexes

Answer 2

1. To adjust for unexpected changes (going on a spinny ride)
2. Organizing coordinated movement (flexor withdrawal/crossed extension)
3. Reciprocal inhibition (rapid protection from painful stimuli)

Question 3

long and short propriospinal pathways help

Answer 3

long - coordinate postural control

short - coordinate limb movement

Question 4

are reflexes flexible or not

Answer 4

they are flexible and not rigidly fixed. many neurologic diseases/injuries result in loses of flexibility

Question 5

do you need conscious perception for a reflex

Answer 5

no

once movement is stimulated if you adjust movement, then you need perception

Question 6

Descending control from “upper” motor neuron tracts can influence or regulate reflexes by

Answer 6

increasing or decreasing the size of the reflex response but you do not need descending motor pathways to have a reflex present

Therefore, you don’t need descending motor or ascending sensory white matter pathways for a reflex to be present. But, you probably need pathways for reflexes to operate properly.

Question 7

stimulation of a reflex may be used to obtain a

Answer 7

motor response in a patient

The motor response occurs over a very short time frame and does not last much longer after the removal of the stimulus.

stimulating a spinal reflex can be helpful for someone with a SC injury

Question 8

lesions and reflexes

Answer 8

lesions in the sensory cortex or ascending sensory white matter tracts that affect our ability to detect a sensation will not abolish a reflex but it will not have its normal flexibility

Question 9

The only way to lose or abolish a reflex is to

Answer 9

1. remove the sensation that stimulates it (peripheral nerve, dorsal root, dorsal horn, sensory cranial nerve)
2. have a LMN lesion.

(UMN lesions will not abolish reflexes. They just may make the reflex presentation abnormal and less flexible.)

Question 10

lesions in which general structures can abolish the reflex

Answer 10

peripheral nerve
dorsal root
dorsal horn
sensory cranial nerve
LMN

This is because the only way to abolish a reflex is to remove or prevent the sensation that stimulates it or prevent the innervation of the muscles activated by it.

Question 11

normal response to a noxious stimuli on the bottom of the foot

Answer 11

flexor withdrawal and crossed extension

Question 12

normal response to a fast cutaneous stimulation of the skin

Answer 12

contracts the muscle deep to the area of skin being stimulated

Question 13

normal response to heating the skin over a muscle

Answer 13

relaxes or inhibits the muscle deep to the area of skin being stimulated

Question 14

receptors and afferents involved in reflex responses

Answer 14

cutaneous afferent nerve fibers (some of which are flexor reflex afferents, or FRA’s) transmit the sensory input signal to the spinal cord which then prompts descending activation of alpha motor neurons to respond reflexively to the stimulus

Question 15

GTO organizing patterns for coordinated movement

Answer 15

The GTO regulates the force of a contraction and is regulated by descending upper motor neuron pathways. For example, if you are holding a videotape in your hand and it starts to slip, the reflex can be inhibited to allow you to develop more force to hold the tape. That said cutaneous input has been shown to be the key to prevent slip.

Question 16

GTO allowing for rapid protection from painful or damaging stimuli

Answer 16

The GTO reflex protects a muscle from exerting excessive force. This is probably not a common occurrence.

Question 17

Flexor Withdrawal Reflex and Crossed Extension

Answer 17

A painful stimulus stimulates nociceptors, the cutaneous afferent nerve fibers (flexor reflex afferents or FRA’s) transmit the signal to the spinal cord and synapse on alpha motor neurons.

The flexors of the limb that is contacting the painful stimulus are excited and the extensors of the same limb are inhibited.

The flexor withdrawal reflex serves a protective function and is activated so fast that the cortex is unaware of the pain until after the limb has been withdrawn.

Question 18

The flexor withdrawal reflex is an example of an

Answer 18

intersegmental reflex as many muscles are coordinated to activate at once to withdraw the limb. Clinically, this reflex may be used to stimulate muscle contraction in the flexors. For example, it may be used in comatose patients for arousal and to initiate a motor response. In normal life the flexor withdrawal and crossed extension may utilize similar circuitry as locomotion (walking)

Question 19

cutaneous surface stimulation - fast/ice

which type of muscle is it appropriate for

Answer 19

• will cause the muscle underneath the stimulated area to contract (excites the homonymous muscle.)
• appropriate for a hypotonic muscle; avoid using over a spastic muscle.

Question 20

cutaneous surface stimulation - heat

which type of muscle is it appropriate for

Answer 20

• cause the muscle under the skin that is heated to relax or be inhibited
• appropriate to apply to the skin over a spastic muscle

Question 21

under which 2 circumstances should reflexes be used clinically?

Answer 21

initiating movement
or
inhibiting spastic muscles

• in rehab: stimulate muscle activity and give patient proprioception of movement

Question 22

limitations of choosing to use reflexes to facilitate movement

Answer 22

muscle movement initiated by a reflex is typically time-limited to the duration of the stimulus – which is to say, if you remove the stimulus then the muscle movement will also cease shortly thereafter

reflexive movement is not functional because at some point functional movement requires conscious control to make adjustments

reflexes typically activate specific/isolated muscles, rather than functional muscle groups

Question 23

rationale for placing patients on a treadmill soon following a stroke

Answer 23

doing so will activate the CPG for locomotion, and studies have shown that stroke survivors who undergo this treatment soon after the stroke may have better clinical outcomes than those receiving traditional therapy

Question 24

central pattern generators (CPG)

Answer 24

Complex spinal cord circuits that control certain rhythmical movements i.e. locomotion.

Decerebrate and decorticate cat studies-circuitry exists for locomotion.

Humans have the circuitry but need stimulus to initiate and to control speed.

Question 25

polio

Answer 25

damages the alpha motor neurons in the ventral horn of the spinal cord and cranial nerve motor nuclei in the brainstem due to a viral infection
LMN disorder
s/s = weakness (paresis), hypotonia, hyporeflexia, muscle atrophy, fasciculations, and fibrillations
the virus is self-limiting and pt’s usually recover to a point and live with residual deficits

Question 26

post-polio

Answer 26

• seen in pt’s who have been living with residual deficits for many years - likely caused by a combination of aging and overuse of unaffected muscles
  s/s = new muscle weakness and muscle fatigue
  not life threatening, but requires significant lifestyle changes reduce fatigue and conserve remaining muscle energy

Question 27

ALS (amyotrophic lateral sclerosis/Lou Gehrig’s)

Answer 27

destroys the alpha motor neurons in the ventral horn of the spinal cord and cranial nerve motor nuclei in the brainstem
- classified as LMN disorder, with possible UMN s/s if lateral columns affected
s/s = sames as polio
- may affect lateral white matter columns of SC and pt may have UMN signs such as spasticity
- fatal within 5 years

Question 28

subacute combined degeneration

Answer 28

damages the dorsal and lateral white matter columns due to a B12 deficiency - commonly seen with alcoholism
s/s = DCML affectation due to damage of dorsal columns + UMN s/s due to damage of lateral columns

Question 29

syringomyelia and s/s

Answer 29

damages various SC structures due to cavitation of the central canal
s/s = bilateral ALS affectation due to damage beginning at the VWC + eventual motor deficits due to expanding cavitation which can interrupt descending pathways

Question 30

Jendrassik response

Answer 30

• hold operatic area pose and have them pull hands apart

- exaggerates a reflex response