Lecture 8: Reflexes Flashcards
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
what is the main role of lower motor neurons
Receives the signal from UMN and transmits it to effector muscle to perform a movement
what do muscle spindles do
Detects muscle length and velocity
what are the 2 types of sensory axons in muscle spindles
group 1a axons
group 2 axons
what are group 1a axons
Changes in length and velocity
what are group 2 axons
measures length
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
do gamma motor neurons directly cause muscle contraction
no
they Innervate muscle spindles and dictate their sensitivity
do alpha motor neurons or gamma motor neurons have a lower threshold for activation
gamma motor neurons
how can gamma motor neurons be modulated
ascending pathways
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
why is Alpha-Gamma Co-activation important
Maintains sensitivity of the muscle spindles during different muscle lengths
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
what are Golgi tendon organs
Group 1b afferent
Designed to measure muscle tension/force
does muscle stretch affect afferent activity in Golgi tendon organs
no
does active muscle contraction affect afferent activity in Golgi tendon organs
yes activity increases
what is the simplest level of movement
reflexes
what is the stretch reflex
homonymous - same muscle that is stretched is excited and contracts
Spindle sensitivity modulated by UMNs
what is the purpose of the stretch reflex
Maintain muscle length
compensate for limb
during movement
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
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
