Reflexes & Gait Flashcards
What are the 2 main sensors involved in muscle sensation?
Muscle spindle (senses length), which will activate the alpha motor neuron; Golgi tendon organ (senses force or contraction), which inhibits the alpha motor neuron
What are the components of the reflex circuit?
- Receptor
- Afferent (sensory) nerve fiber
- Interneuron
- Efferent (motor) nerve fiber
- Effector organ
What are the components of the myotatic reflex?
Primary sensory endings Ia afferent fibers innervate the sensors called muscle spindles and detect rate + amount of stretch; secondary sensory endings type II fiber detects the amount of stretch
What is the function of the myotatic reflex?
To correct for unexpected loads
What is the effect of the myotatic reflex on homonymous muscles?
Muscle contraction that opposes the original stretch
What is the effect of the myotatic reflex on synergistic muscles
Activation of synergistic muscle to aid in the muscle contraction that opposes stretch
What is the effect of the myotatic reflex on antagonist muscles
Inhibition of antagonist muscles via glycinergic interneuron Ia ii inhibition of alpha motor neuron that goes to the antagonist muscle
Define reciprocal inhibition
Type 1a fiber activation of an alpha-MN resulting in the contraction of homonymous and synergistic muscles + inhibition of antagonistic muscle via interneurons (1a inhibitory neurons OR Ia ii)
Define recurrent inhibition
RC inhibition returns to the motor neuron that opposes the action of the muscle that resulted in the excitation of the RC;
What is the function of Renshaw cells (RC)
Renshaw cells are small MNs in the ventral horn that are excited by ACh release by axon collaterals of alpha-MNs; upon activation, RC axons release inhibitory glycine
How do gamma-MNs function
Will slack when alpha-MNs are activated, making it so that no further signaling can occur; normal movement requires the co-activation of both alpha and gamma -MNs
What is the gamma loop
- Gamma-MN activation conrtacts the polar region of intrafusal muscle
- Contraction stretches equatorial region by pulling ends
- Stretch of intrafusal fiber activates 1a afferents, exciting alpha-MNs innervating homonymous muscle
Give examples of multisegmental reflexes
Moro reflex, which is stimulated by sudden movement of the head backwards and results in a reflexive wide abduction of the arms and rapid breath intake + flexion of the legs (normal in infants <6 months of age)
What elicits the M wave on an EMG recording
High stimulation intensity and moderate stimulation intensity; does not occur under low stimulation intensity
What elicits the H reflex on an EMG recording?
Results from low stimulus intensity as well as moderate stimulus intensity; does not occur when there is high stimulation intensity
What do central pattern generators do?
Produces semi-automatic movements like walking, running, chewing
Where does central pattern generation occur
Spinal cord and brain stem
How is central pattern generation initiated (from which region)?
External trigger initiates it to arise from the mesencephalic locomotor region; increase output to increase speed of a slow walk to a sprint
What part of the muscle sensor system do gamma motor neurons modulate?
Muscle spindle
Where are muscle sensors located?
Between the ligament/tendon and the muscle
Why do spindles sense stretch?
They’re trapped in parallel
What is the simplest reflex?
Myotatic reflex
What is the only monosynaptic reflex in humans?
Myotatic reflex
What does the myotatic reflex look like on a physical exam?
Knee-jerk/patellar reflex
What are the steps of the myotatic reflex, starting with stimulation of the patellar tendon and ending with the leg kick (8 steps)
- Stimulate patellar tendon
- Quadriceps muscle stretches
- 1a & II afferent fibers of femoral nerve activated via muscle spindle
- Signal travels to spinal cord (L4)
- Cross the mono-synapse
- Stimulate the alpha motor neuron to the quadriceps muscle
- Contraction of the quadriceps (simultaneous inhibition of the flexor muscle)
- Leg kick
How fast is the myotatic reflex?
50 ms
Describe the central region of the intrafusal fibers
Lacks contractile proteins (doesn’t contract) and is innervated by the 1a fibers; type II fibers innervate the ends
Where are the contractile proteins of the intrafusal fibers contained?
The ends, which are innervated by the gamma (efferent) fibers as well as the type II fibers
What is the muscle spindle
A capsule containing intrafusal fibers
When does the muscle spindle stretch passively?
When the whole muscle (extrafusal) is contracted
Do intrafusal fibers contribute to the force of contraction?
NO
How are muscle spindles oriented relative to the extrafusal fibers?
Parallel
Are there more extrafusal fibers or more muscle spindles?
More muscle spindles
Which fibers detect stretch?
Ia sensory fibers
Which part of the fiber do gamma motor neurons innervate
polar regions
What is the function of gamma motor neurons
To contract the fiber (stretching it as a result)
Normal function of a muscle needs coactivation of which motor neurons?
Alpha and gamma
What does Golgi tendon organ activation do?
Inhibit contraction
How many synapses does the GTO reflex have?
2 (disynaptic)
How is the GTO activated?
Via muscle contraction or tension (force), located in a capsule by the tendon
Which fibers are involved with the GTO reflex?
It depends on Ib afferent fibers that activate an inhibitory (glycinergic) interneuron, which will inhibit homonymous and synergistic muscles via their alpha-MNs
What is the inverse of the myotatic reflex?
The GTO reflex opposes and terminates the myotatic reflex’s function
What is the impact of RC inhibition of Ia ii inhibitory interneurons?
disinhibition of MNs to the antagonistic muscles (rapidly inhibits contraction, terminates alpha-MNs, and produces a brief contraction)
What condition may result from recurrent inhibition?
Tetanus
What is the effect of forebrain damage on gamma-MNs?
Brainsteam gamma-MNs postural activation of extensor muscles takes over, producing spasticity
How does activation of gamma-MNs alone cause muscle contraction?
Via indirect excitation of alpha-MNs
Define multi-segmental reflexes
Reflexes that involve more than one segment of the spinal cord, coordinates muscles at different spinal cord levels; postural
What coordinates the Moro reflex?
Fasciculus proprius
Bilateral Moro dysfunction is indicative of what?
brain damage, spinal cord damage
unilateral Moro reflex dysfunction is indicative of what?
Clavicle damage, birth trauma (brachial plexus damage AKA Erb Duchenne Palsy, hemiplegia)
Describe the tonic neck reflex
Rotation of the head to one side elicits extension of the ipsilateral arm and flexion of the contralateral arm
persistence of the tonic neck reflex (TNR) past 4 months is predictive of what?
Poor motor development
What is the effect of a lower motor neuron injury on a reflex?
Decreases its strength
What is the effect of an upper motor neuron injury on a reflex?
Increases its strength
What do the following grades in describing reflexes mean? 0 1+ 2+ 3+ 4+
0 - absent 1+ - hypoactive 2+ - normal 3+ - hyperactive without clonus 4+ - hyperactive with clonus
What is the oscillatory motor response to muscle stretch in Hz?
3-7 Hz
What causes clonus?
A lack of normal cortical inhibition of a deep tendon reflex, resulting in rapid, strong, oscillating muscular contractions
When does clonus occur?
Whern there is sustained tension placed on one of the muscles controlling a joint, like the wrist or ankle
What is the normal Babinski reflex?
Toe flexion
What is a positive Babinski’s sign?
Toes extend and separate, this is abnormal and indicates an upper motor neuron lesion affecting the lower extremity in question
What often accompanies a positive babinski’s sign
Spastic hypertonia
The clasp-knife reflex is an example of _____________
interneuron release of spasticity
What is the clasp-knife reflex?
Gradual muscle stretch leads to reflex resistance, followed by muscle extension at a critical length. There is brief excitation followed by a powerful, long-lasting inhibition of homonymous and synergistic muscles
Which mechanoreceptors are overactive in the clasp-knife reflex
group II and group IV mechanoreceptors; central effects pronounced because of reduced descending inhibition of segmental interneurons
How is hyporeflexia related to the myotatic reflex?
Hyporeflexia is an abnormal decrease in myotatic reflex
What may be the cause of hyporeflexia?
Lesion of sensory input from muscle spindles; a motoneuron lesion to the anterior horn of the spinal cord
What are the clinical outcomes of hyporeflexia
Peripheral neuropathy, Guillain-Barre, muscular dystrophy, myasthenia gravis
What happens in an EMG recording when there is low stimulus intensity?
AP travels orthodromically, excites an alpha-MN, which travels to the muscle, causing contraction resulting in the H reflex
What happens in an EMG recording when there is high stimulation intensity?
All fibers are activated, all antidromic signals collide with orthodromic signals and cancel out; only the M response will be observed
Do central pattern generators depend on sensory feedback from the periphery?
NO
What is the crossed extensor reflex/nociceptive withdrawal
A polysynaptic reflex that involves contralateral activation/inhibition of muscle groups opposite to those innervated on the ipsilateral side of the spinal cord: muscle groups excited on one side are inhibited on the opposite side
What is the function of the crossed extensor reflex/nociceptive withdrawal
Allow for body weight support on the opposite leg of a withdrawal reflex; supports alternating leg muscle activation on the opposite side of the body during walkng/running
Do central pattern generators require supraspinal input or peripheral feedback?
NO, neither are required
Gait arises from ___________
spinal circuits
Central pattern generator of the spinal cord generates _____________
rudimentary pattern = fundamental leg stride pattern
