Motor Reflexes Flashcards
Difference between monosynaptic and polysynaptic reflexes
Monosynaptic = afferent fiber synapses directly onto efferent fiber; usually excitatory
Polysynaptic = afferent fiber synapses onto interneuron (either excitatory or inhibitory)
Differences between reflex activity and volitional movement
Reflexes: precision motions in response to afferent stimuli mediated at all levels of CNS; rapid initiation, many elicited even during unconsciousness
Volitional movement: originates in cortical areas; longer onset latency d/t higher processing, requires conscious awareness
Difference between the 2 types of lower motor neurons found in motor neuron pools of ventral horn
Alpha-motor neurons: innervate extrafusal muscle fibers (fibers that result in contraction)
Gamma-motor neurons: innervate intrafusal muscle fibers, a component of the muscle spindle
What are muscle spindles?
4-8 specialized intrafusal muscle fibers surrounded by a capsule of CT that are highly specialized mechanosensory organs found in most skeletal muscles and provide proprioceptive info about muscle LENGTH
2 types of intrafusal fibers found in muscle spindles
Nuclear bag fibers — sensory bag-like region that contains many nuclei (can be static or dynamic)
Nuclear chain fibers — central portion, containing chain-like array of nuclei
2 types of sensory afferents found coiled around central part of muscle spindle
Group Ia afferents (annulospiral endings)
Group II afferents (flowerspray endings)
2 types of y-motor neuron efferent fibers found in motor component that stimulates contraction of intrafusal fiber in muscle spindle
Which part of the muscle spindle do they innervate?
Dynamic y-motor neuron (innervates dynamic nuclear bag fibers)
Static y-motor neuron (innervates static nuclear bag fibers)
Difference between group Ia afferents and group II afferents in terms of muscle spindle changes that they are sensitive to
Group Ia —sensitive to length of muscle and how fast the length is changing (this is the primary afferent)
Group II — sensitive ONLY to the length of stretch (this is the secondary afferent)
T/F: within the spinal cord, y-motor neurons form an excitatory synapse directly on the homonymous muscle’s alpha motor neuron in the ventral horn
True; this is an unusual monosynaptic connection. Most sensory afferents do not directly contact a lower motor neuron
Note that they also form an excitatory synapse on GABAergic inhibitory interneurons that inhibit the antagonistic muscle (reciprocal innervation)
The stretch reflex appears as a contraction of a stretched muscle, initiated by muscle spindles. Why do we need this?
Protects the muscle from tearing due to over-stretch
Sequence of events in stretch reflex
- Muscle stretch
- Muscle spindle stretch
- Increase discharge of spindle Ia afferents
- Excitation of alpha-motor neurons
- Muscle contraction increases to oppose stretch
Other names for stretch reflex
Deep tendon reflex
Myotactic reflex
The ____ of the myotatic reflex refers to the amount of muscle force generated in response to a given stretch of the intrafusal fibers
Gain
The gain of the myotatic reflex refers to the amount of muscle force generated in response to a given stretch of the intrafusal fibers.
What happens when the gain is high vs. low?
If the gain is high, a small stretch produces large increase in alpha-motor neurons firing, leading to increased tension in the extrafusal fibers
If the gain is low, a greater stretch will be required to generate the same amount of tension in the extrafusal fibers
What structure participates in an inverse myotatic reflex with autogenic inhibition, appearing as a sudden relaxation of a contracted muscle?
Golgi tendon organs (GTOs)
Describe GTOs
Innervate tendons at junction with muscle
A bare nerve ending with many branches
Tension in the tendon increases APs in the GTO. What type of afferent fiber is connected to GTOs?
Ib afferent fiber
The inverse myotatic reflex appears as a sudden relaxation of a contracted muscle, why do we need this?
Protects the muscle from damage due to excessive force
Order of events in inverse myotatic reflex with GTOs
- Muscle contracts putting massive strain on tendon
- Increased tension increases APs in Ib afferent fiber
- Ib afferent synapses on inhibitory interneuron in SC
- Inhibition of a-motor neuron
- Muscle abruptly relaxes
Is the GTO reflex monosynaptic or polysynaptic? What about the muscle spindle reflex for muscle stretch?
GTO reflex is polysynaptic (segmental) reflex
Muscle spindle reflex is monosynaptic, segmental reflex
During the GTO muscle reflex, the Ib afferent synapses on an inhibitory interneuron in the SC. What else does it synapse on?
The alpha-motor neuron to the antagonist muscle, further opposing muscle activity
What is the flexor reflex
Reflexive withdrawal from noxious stimuli
The flexor reflex stimulates pain-sensing fibers and synapses on excitatory _______, which synapse on _________ that innervate flexor muscles required to withdraw from the stimulus
Inhibitory interneurons synapse on alpha-motor neurons that innervate antagonistic ____ muscles that would impede withdrawal from the stimulus
Simultaneously, flexion of the stimulate limb is accompanied by opposite reaction from the contralateral limb. This means that _____ muscles are stimulated while _____ muscles are inhibited in the contralateral limb
Interneurons; alpha-motor neurons
Extensor
Extensor; flexor [this is the crossed-extension reflex]
An acute spinal cord injury resulting in loss of reflexes and sensation below the level of the injury, requiring a very gradual recovery partly due to loss of excitatory bias from CNS
Spinal shock
____ = increased muscle tone, hyperactive stretch reflexes, and clonus
Spasticity
Difference between spasticity and rigidity
Spasticity is velocity dependent while rigidity is not
What type of rigidity is an ominous sign of severe brain damage, particularly above the superior colliculus? How does it present?
Decorticate rigidity
Arms flexed or bent inward on chest, hands clenched into fists, legs extended, feet turned inward
How is decorticate rigidity dependent on head position?
If head is in neutral, affected leg is extended and affected arm is somewhat flexed
If you rotate the head in either direction, increases extensor tonus on that side and flexion of opposite arm
What type of rigidity is an ominous sign of severe brain damage, particularly rostral to the pons? How does it present?
Decerebrate rigidity
Extensor rigidity and spasticity: head is arched back, arms extened at sides, legs extened, arms and legs internally rotated
Decerebrate rigidity results in rigidity due to an increase in ________ excitability and spasticity due to an increase in ______ firing
Alpha-motor neuron
Gamma-motor neuron
[overall loss of descending suppression yet excitatory region remains active]
