Spinal Mechanisms of Motor Control Flashcards
Motor control at the spinal cord level is
reflexive
3 functions of the spinal cord reflexes
- To adjust for unexpected changes (going on a spinny ride)
- Organizing coordinated movement (flexor withdrawal/crossed extension)
- Reciprocal inhibition (rapid protection from painful stimuli)
long and short propriospinal pathways help
long - coordinate postural control
short - coordinate limb movement
are reflexes flexible or not
they are flexible and not rigidly fixed. many neurologic diseases/injuries result in loses of flexibility
do you need conscious perception for a reflex
no
once movement is stimulated if you adjust movement, then you need perception
Descending control from “upper” motor neuron tracts can influence or regulate reflexes by
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.
stimulation of a reflex may be used to obtain a
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
lesions and reflexes
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
The only way to lose or abolish a reflex is to
- remove the sensation that stimulates it (peripheral nerve, dorsal root, dorsal horn, sensory cranial nerve)
- have a LMN lesion.
(UMN lesions will not abolish reflexes. They just may make the reflex presentation abnormal and less flexible.)
lesions in which general structures can abolish the reflex
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.
normal response to a noxious stimuli on the bottom of the foot
flexor withdrawal and crossed extension
normal response to a fast cutaneous stimulation of the skin
contracts the muscle deep to the area of skin being stimulated
normal response to heating the skin over a muscle
relaxes or inhibits the muscle deep to the area of skin being stimulated
receptors and afferents involved in reflex responses
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
GTO organizing patterns for coordinated movement
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.
GTO allowing for rapid protection from painful or damaging stimuli
The GTO reflex protects a muscle from exerting excessive force. This is probably not a common occurrence.
Flexor Withdrawal Reflex and Crossed Extension
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.
The flexor withdrawal reflex is an example of an
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)
cutaneous surface stimulation - fast/ice
which type of muscle is it appropriate for
- 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.
cutaneous surface stimulation - heat
which type of muscle is it appropriate for
- cause the muscle under the skin that is heated to relax or be inhibited
- appropriate to apply to the skin over a spastic muscle
under which 2 circumstances should reflexes be used clinically?
initiating movement
or
inhibiting spastic muscles
- in rehab: stimulate muscle activity and give patient proprioception of movement
limitations of choosing to use reflexes to facilitate movement
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
rationale for placing patients on a treadmill soon following a stroke
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
central pattern generators (CPG)
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.
