Lecture 8: Reflexes Flashcards by Emily Wallace

what is the main role of upper motor neurons

Integrates excitatory and inhibitory signals from the cortex and translates them into a signal to modulate LMN activity

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what is the main role of lower motor neurons

Receives the signal from UMN and transmits it to effector muscle to perform a movement

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what do muscle spindles do

Detects muscle length and velocity

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what are the 2 types of sensory axons in muscle spindles

group 1a axons

group 2 axons

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what are group 1a axons

Changes in length and velocity

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what are group 2 axons

measures length

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when is there a decrease in muscle spindle activity?

when there’s stimulation of the alpha motor neurone only, not actively contracting the muscle voluntarily

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do gamma motor neurons directly cause muscle contraction

they Innervate muscle spindles and dictate their sensitivity

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do alpha motor neurons or gamma motor neurons have a lower threshold for activation

gamma motor neurons

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how can gamma motor neurons be modulated

ascending pathways

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what is muscle spindle alpha-gamma co-activation

during active muscle contraction, gamma motor neurons cause intrafusal fibres to contract at the same time as extrafusal fibres

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why is Alpha-Gamma Co-activation important

Maintains sensitivity of the muscle spindles during different muscle lengths

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does Alpha-Gamma Co-activation occur during reflexes

no.

alpha-gamma co-activation only occurs in movements initiated by the CNS

only alpha motor neurons are activated during reflex

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what are Golgi tendon organs

Group 1b afferent

Designed to measure muscle tension/force

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does muscle stretch affect afferent activity in Golgi tendon organs

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does active muscle contraction affect afferent activity in Golgi tendon organs

yes activity increases

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what is the simplest level of movement

reflexes

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what is the stretch reflex

homonymous - same muscle that is stretched is excited and contracts

Spindle sensitivity modulated by UMNs

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what is the purpose of the stretch reflex

Maintain muscle length

compensate for limb
during movement

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what is reciprocal inhibition of the stretch reflex

Automatic inhibition of the antagonist alpha motor neuron (biceps) caused by contraction of the agonist muscle (triceps)

Occurs in conjunction with stretch reflex

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what is the purpose of reciprocal inhibition

to allow agonist to contract

what is the Golgi Tendon Organ (GTO) Reflex

Excitation of the inhibitory interneuron
* Inhibit “agonist” homonymous muscle LMN pool (biceps)

Excitation of the excitatory interneuron of antagonist

what is the function of the Golgi Tendon Organ (GTO) Reflex

Release load to prevent muscle tearing and
maintains muscle tone during muscle fatigue

what is the flexor withdrawal reflex?

Ipsilateral limb (painful stimulus)
* Interneuron to excite flexors
* Inhibitory interneuron to
inhibit extensors

what is the function of the flexor withdrawal reflex

Remove limb from noxious stimulus

what is the crossed extensor reflex

Occurs with flexor withdrawal reflex Input: Noxious stimulus detected * Synapse on long distance interneuron networks * Contralateral limb (cross-extensor) * Excite Extensors (postural) * Inhibit Flexors

what is the purpose of the crossed extensor reflex?

Ground stance limb to allow ipsilateral limb to withdraw from pain

which is the only reflex that crosses spinal cord

crossed extensor reflex

what is the 5 point scale of reflexes

0 - No response 1+ - Present but depressed (hyporeflexic; may or may not be normal) 2+ - Normal (average) 3+ - Brisk (increased; possibly abnormal) 4+ - Very brisk, hyperactive, with clonus (abnormal) 5+ - Sustained clonus

what do we usually see in terms of the reflex grading scale with upper motor neuron lesions

3+ - Brisk (increased; possibly abnormal) 4+ - Very brisk, hyperactive, with clonus (abnormal) 5+ - Sustained clonus

where does the Reticulospinal tract originate and terminate

originates in reticular formation and terminates bilaterally in spinal cord

what is the Dorsal/lateral reticulospinal tract responsible for

descending inhibition (hitting the breaks)

what is the Medial reticulospinal tract responsible for

descending excitation (hitting the gas)

what happens with both reticulospinal tracts we have a complete spinal cord injury

both are damaged * Excessive firing of LMNs BUT Inhibitory interneurons unable to inhibit LMNs (driving down a hill with no breaks) * Hypertonia is due to a loss of descending control

What happens when we have a stroke on the right side of our brain

Right lateral corticospinal tract damaged Reduced cortical drive to dorsal/lateral reticulospinal tract Medial reticulospinal tract increases descending drive from the contralesional side * Results in excessive (medial) reticulospinal drive (all gas no breaks)

In UMN lesions, there is excessive neural signaling due to a loss of descending control, which contributes to

Hypersensitivity of the muscle spindle Excitable  motor neuron

what happens when we do a tendon tap of the patella

muscle stretches causing increase activation of 1a afferent shortly after, alpha motor neurons are recruited, causing shortening of muscles due to stretch reflex

what happens in clonus

muscle stretches causing increase activation of 1a afferent shortly after, alpha motor neurons are recruited, causing shortening of muscles due to stretch reflex there is repeated activity of 1 a afferents, and the cycle repeats

what is the babinski test

stroke the lateral side of foot across base of toes, extension of big toes, other toes fan out normal in infants and not in adults

what is Hoffman’s Reflex

flick middle finger tip down Positive if tip flicks back thumb flexes and adducts More likely to have a false positive than Babinski

what do we usually see in terms of the reflex grading scale with lower motor neuron lesions

0 - No response (areflexia) 1+ - Present but depressed (hyporeflexic; may or may not be normal)

when would we see hyporeflexia

Lower motor neuron lesions

What happens when a muscle is stretched? There is a decrease in firing of the Golgi Tendon Organ afferent The afferent activity in the Golgi Tendon Organ changes very little There is an increase in firing of the Golgi Tendon Organ afferent There is a decrease in firing of the muscle spindle afferent

The afferent activity in the Golgi Tendon Organ changes very little

If there is an increase in cortical drive to the dorsal/lateral reticulospinal tract, what would you expect to observe?

A reduction in muscle tone

If there were an injury to the anterior horn of the spinal cord only, what type of presentation would you expect to observe? Normal reflexes Clonus Hyporeflexia Hyperreflexia

Hyporeflexia

During active muscle contraction: None of the response options are correct Only the intrafusal fibres contract The intrafusal fibres contract at the same time as extrafusal fibres Only the extrafusal fibres contract

The intrafusal fibres contract at the same time as extrafusal fibres

What is the most likely contributor to hypertonicity in stroke?

Increased medial reticulospinal tract drive

Which of the following responses is true regarding the gamma motor neuron? It modulates the sensitivity of the muscle spindles It innervates the muscle spindles All of the response options are correct It can be modulated by descending inputs

All of the response options are correct

What is the most likely contributor to hypertonicity following spinal cord injury? Increased medial reticulospinal tract drive Decreased lateral reticulospinal tract drive Insufficient firing of alpha motor neurons A lack of inhibition from interneurons to lower motor neurons

A lack of inhibition from interneurons to lower motor neurons

The muscle spindle is responsible for detecting changes in:

Muscle length and velocity