Spinal reflexes (wk 10)

Question 1

What is nomenclature and the 2 classification system?

Answer 1

-There are 2 classification systems to define nerve fibres based on their size and conduction velocity:
1. Erlanger-Gasser (alphabetical)
2. Lloyd (numerical)

Question 2

Describe reflex:

Answer 2

-Automatic response to a change in the environment
-Reflexes are coordinated, involuntary (i.e. automatic) motor responses initiated by a stimulus applied to peripheral receptors
-But -> Some reflexes are complex and involve supraspinal components. Spinal reflexes are highly modifiable by input from the brain. Thus, the definition of reflex is tricky as it can be difficult to fully distinguish automatic and voluntary – they are often connected.

Question 3

What is the stretch reflex?

Answer 3

a sensorimotor loop. Evoked by tendon tap (e.g. biceps). Abolished by cutting dorsal roots. Therefore, the stretch reflex is not a mechanical response and relies upon intact sensory afferent feedback.

Question 4

What are muscle spindles?

Answer 4

-> Receptors that detect changes in muscle length during contraction. Stretch evokes an increase in spindle firing rate: this sensory signal is sent to the spinal dorsal horn by group 1a ( and II) afferent neurons. In turn, alpha-motor neurons are activated in the spinal ventral horn, leading to subsequent contraction of the agonist/ homonymous and synergist muscles. Antagonist muscles are inhibited (reciprocal inhibition).

Question 5

What does the muscle spindle consist of (4):

Answer 5

• Bundle of thin muscle fibres (intrafusal) contained within a capsule
• Situated in parallel with main ‘extrafusal’ muscle fibres but generates no force
• Wrapped around by a pair of sensory axons (group 1a and II)
• y-Motor neurons cause active contraction of spindle

Question 6

What does the muscle spindle detect:

Answer 6

• Length/ stretch of muscle (position)
• Rate of change (velocity, group 1a only)

Question 7

What are intrafusal fibres?

Answer 7

Intrafusal fibres are innervated by y-motor neurons. There are 2 main types: Nuclear bag fibres (innervated by group 1a afferents, annulospiral endings) and Nuclear chain fibres (innervated by group 1a and II afferents, flower spray endings). Nuclear bag fibres are larger due to the nuclei being bunched together. Both types of fibre respond to stretch (only 1a fibres respond to velocity).

Question 8

Describe the stretch reflex as a monosynaptic loop:

Answer 8

• It is possible to infer how many neurons are involved in a reflex by its latency – faster the response, the fewer the neurons (and synapses)
• Earliest onset response is <1ms after the stimulus – this speed is consistent with only 2 neurons (i.e. monosynaptic)
• The antagonist muscles are inhibited at slightly longer latencies (>1ms) – this speed is consistent with 3 neurons (i.e. disynaptic)

Question 9

What is reciprocal inhibition?

Answer 9

-> Antagonist muscles which may interfere with the desired movement are suppressed by contraction of the agonist muscle. Mediated by group 1a inhibitory interneuron. However, sometime we may want to activate agonist and antagonists at the same time (co-contraction) – immediately before catching a ball. Thus, reciprocal inhibition can be modulated t descending inputs from the brain (inhibition of the inhibition).

Question 10

What are supraspinal components of the stretch reflex?

Answer 10

EMG response to stretch includes 2 separate components: M1 and M2. M1 latency is consistent with monosynaptic reflex. M2 latency is consistent with a transcortical loop.

Question 11

What is Klippel-Feil syndrome?

Answer 11

-> Rare condition in which premotor neurons from the cortex bifurcate to innervate both sides of the body. Moving one hand (ipsilateral) leads to mirror movements in the other hand (contralateral). Only the long-latency M2 component is observed on the contralateral side.

Question 12

What is hyperreflexia/ spasticity?

Answer 12

-> Descending input from the brain normally acts to regulate reflex gain in the spinal cord. Spinal cord injury, stroke, and other conditions can abolish this input. The results in an increase in the reflex gain (output) known as hyperreflexia.

Question 13

What is the intrathecal baclofen?

Answer 13

-> Baclofen is a GABA-B agonist and elicits presynaptic inhibition of afferent fibres. Decreases Ca 2+ influx to presynaptic fibres. Causes hyperpolarisation (more negatively charged = less excitable) reducing neurotransmitter release.
+ Involves the following -> Catheter is placed in cerebrospinal fluid. Catheter is connected to a pump filled with baclofen that is inserted into abdominal wall. Pump delivers small amounts of baclofen into CSF causing spinal dorsal horn inhibition. Results in decreased muscle tone and spasms. Common therapy for spinal cord injury.

Question 14

What is the purpose of the stretch reflex - feedback loop?

Answer 14

-> The stretch reflex operates in a negative feedback loop. Negative feedback prevents major disturbances to the regulation of a controlled variable – in this case muscle length.

Question 15

Draw Houk’s mode of control strategy

Question 16

What is the purpose of the stretch reflex:
-Servo-assistance

Answer 16

-> The stretch reflex may maintain accuracy of movement in the face of small deviations – this is known as servo-assistance. Voluntary commands from motor cortex drive movement and the stretch reflex helps correct small deviations from the desired trajectory caused by unpredictable perturbations. An unpredictable increase in load is a disturbance detected by muscle spindles that compensates for the deviation from a desired muscle length.

Question 17

What is the Hoffman reflex?

Answer 17

-> Elicited by activation of type 1a sensory nerves. Evokes a muscle contraction via the monosynaptic spinal reflex that can be recorded using EMG. Sensory group 1a afferent fibres are activated as lower stimulation thresholds versus motor fibres because of their greater diameter axons. At higher stimulation intensities, motor axons are activated (M-wave) which abolishes the H-reflex due to antidromic signals.

Question 18

What is the Hoffman reflex?
-Associative learning

Answer 18

-> H-reflex amplitude can be altered by a conditioning stimulus. This is an example of motor learning. One group given a food reward when reflex amplitude exceeds a certain threshold (positive reinforcement). Another group rewarded when reflex is below a threshold. Simple reflex behaviour can be modified by reinforcement (operant conditioning) – likely due to plasticity in corticospinal tract.

Question 19

What is the flexion withdrawal reflex?

Answer 19

-> Painful sensory stimulus activates ipsilateral flexor muscles whilst extensors and inhibited. Group II fibres (thinly myelinated). Detected by free nerve endings (nociceptors), not spindles. Contralateral extensors are simultaneously activated. This provides extension in the contralateral limb to compensated for unloading in the ipsilateral limb. Reflex persists after spinal cord transection – not modified by supraspinal inputs

Question 20

What is reflex adaptability?
-Reversal of golgi tendon organ reflex

Answer 20

-> At rest, golgi tendon organ (GTO) input inhibits ongoing muscle activity via negative feedback. During locomotion, GTO input increases extensor muscle activity to produce positive feedback. This helps to produce sufficient force during the stance phase of locomotion. Believed to be caused by descending central motor commands.

Question 21

What is reflex adaptability?
-Gait phase-dependent golgi feedback

Answer 21

-> GTO reflex contributed to transition between stance and swing. Positive feedback during stance. Negative feedback during swing.