Answers to Quiz 128 · Spinal Reflexes
How many synapses lie between the afferent fibre and the motor neuron of the stretched muscle in the stretch reflex?
Answer: One
Ia fibres end directly on the α motor neurons of the stretched muscle, so the reflex is monosynaptic. Only the branch that relaxes the antagonist passes through an interneuron, making that path disynaptic. Revise stretch reflex
When the knee jerk is elicited, the hamstrings relax because:
Answer: Ia collaterals inhibit their motor neurons through an inhibitory interneuron
One branch of each Ia fibre ends on an inhibitory interneuron (the Golgi bottle neuron) that inhibits the antagonist's motor neurons. This is reciprocal inhibition, produced by reciprocal innervation. Revise stretch reflex
Exaggerated tendon jerks are an index of:
Answer: Increased γ motor neuron discharge
γ motor neurons set spindle sensitivity, so more γ discharge makes the jerks brisk and less makes them depressed. Losing the afferents or the α motor neurons breaks the arc and depresses the jerk. Revise stretch reflex
The tonic stretch reflex differs from the phasic stretch reflex in that it:
Answer: Is set off by sustained stretch and uses both Ia and II afferents
The tonic reflex answers a sustained stretch through primary and secondary endings (Ia and II), and it maintains tone and posture. Group II fibres also end directly on α motor neurons, so it too is monosynaptic. Revise stretch reflex
All of these are true of the inverse stretch reflex EXCEPT:
Answer: It is monosynaptic
Ib fibres end on inhibitory interneurons, which then inhibit the homonymous α motor neurons, so the reflex is disynaptic. The other three statements are true. Revise inverse stretch reflex and muscle tone
The Golgi tendon organ mainly signals:
Answer: Muscle tension
The tendon organ gives force feedback, whether the force comes from strong stretch or active contraction. Length feedback comes from the muscle spindle. Revise inverse stretch reflex and muscle tone
After a stroke, passive flexion of a patient's spastic elbow meets strong resistance that suddenly gives way. The sudden give is due to:
Answer: Autogenic inhibition of the triceps through its tendon organs
This is the clasp-knife phenomenon, or lengthening reaction. Further stretch of the triceps sets off the inverse stretch reflex, which relaxes the muscle. Revise inverse stretch reflex and muscle tone
Ankle clonus in upper motor neuron paralysis is best explained by:
Answer: Overactive spindles firing, then falling silent, under a held stretch
The sudden stretch fires the spindle and the muscle contracts; the contraction silences the spindle and the muscle relaxes; the held stretch then fires the spindle again. The spindles are hyperactive in upper motor neuron paralysis. Revise inverse stretch reflex and muscle tone
The flexor reflex afferents of the withdrawal reflex are:
Answer: Type III and IV somatic afferents
The withdrawal reflex starts at nociceptors, and its afferents are type III and IV fibres. Ia and II fibres serve the stretch reflex, and Ib fibres the inverse stretch reflex. Revise withdrawal reflex
Stepping on a thorn with the right foot makes the right leg flex. With a stronger stimulus, the left leg:
Answer: Extends, by the crossed extensor (Phillipson's) reflex
Commissural interneurons excite the opposite extensors and inhibit the opposite flexors, so the other leg extends and holds the body balanced. This is the crossed extensor, or Phillipson's, reflex. Revise withdrawal reflex
Afterdischarge in the withdrawal reflex is due to:
Answer: Interneuron chains of different lengths and reverberating circuits
Impulses through short chains reach the motor neuron early and through long chains later, while reverberating loops keep re-exciting it. So the motor neurons fire repeatedly after the stimulus stops. Revise withdrawal reflex
Which feature belongs to the withdrawal reflex but NOT to the stretch reflex?
Answer: Local sign
In the withdrawal reflex the pattern of response depends on the site of the stimulus: medial stimulation abducts the limb, lateral adducts it. The other three are features of the stretch reflex. Revise withdrawal reflex
Many Ia fibres from the spindles of one muscle ending on a single α motor neuron is an example of:
Answer: Convergence
Convergence is many neurons ending on one target. Divergence is the reverse, as when one flexor reflex afferent reaches many motor neurons. Revise properties of spinal reflexes
Two afferents stimulated together give a smaller response than the sum of their separate responses. This is:
Answer: Occlusion, from overlapping discharge zones
Neurons in the shared discharge zone are fired by either afferent alone, so firing both adds nothing for them. Overlapping subliminal fringes do the opposite and give facilitation. Revise properties of spinal reflexes
Renshaw cell inhibition of a spinal motor neuron is an example of:
Answer: Feedback inhibition
A collateral of the motor neuron excites the Renshaw cell, which inhibits that same motor neuron: the output feeds back on itself. Reciprocal inhibition, by contrast, acts on the antagonist's motor neurons. Revise properties of spinal reflexes
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