Spinal reflexes Flashcards
1
Q
what is nomenclature
A
- there are 2 classification systems to define nerve fibres based on their size and conduction velocity:
- erianger-gasser (alphabetical)
- lloyd (numerical)
2
Q
what is the definition of a reflex
A
- 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 somewhat tricky as it can be difficult to fully distinguish automatic and voluntary - they are often connected
3
Q
what is the sensorimotor loop in the stretch reflex
A
- evoked by tendon tap (e.g. biceps)
- abolished by cutting dorsal roots
- thus, the stretch reflex is not simply a mechanical response
- relies upon intact sensory afferent feedback
4
Q
what are the muscle spindles in the stretch reflex
A
- muscle spindles are 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 type 1a afferent neurons
- in turn, a-motor neurons are activated in the spinal ventral horn, leading to subsequent muscle contraction of the agonist / homonymous and synergist muscles
- antagonist muscles are inhibited (reciprocal inhibition)
5
Q
what is the muscle spindle
A
- consists of
- 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 (type 1a and 2)
- y-motor neurons cause active contraction of spindle
- detects:
- length / stretch of muscle (position)
- rate of change (velocity)
6
Q
what are intrafusal fibres
A
- intrafusal fibres are innervated by y-motor neurons
- there are two main types:
- nuclear bag fibres (innervated by type 1a afferents, annulaspiral endings)
- nuclear chain fibres (innervated by 1a and 2 afferents, flower spray endings)
- nuclear bag fibres are longer due to the nuclei being bunched together, whereas nuclear chain fibres have the nuclei in series
- both types of fibre respond to stretch
- only 1a fibres respond to velocity
7
Q
what is the monosynaptic loop in the stretch reflex
A
- 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)
8
Q
what is reciprocal inhibition
A
- antagonist muscles which may interfere with the desired movement are suppressed by contraction of the agonist muscle
- mediated by 1a inhibitory interneuron
- however:
- sometimes we may want to activate agonists and antagonists at the same time (co-contraction)
- immediately before catching a ball
- thus, reciprocal inhibition can be modulated by descending inputs from the brain (inhibition of the inhibition)
- sometimes we may want to activate agonists and antagonists at the same time (co-contraction)
9
Q
what are the supraspinal components of the stretch reflex
A
- 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
10
Q
what is the klippel-feil syndrome in the stretch reflex
A
- rare condition in which premotor neurons from 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
11
Q
what is hyperreflexia / spasticity
A
- 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
- this result is an increase in the reflex gain (output) known as hyperreflexia
12
Q
what is intrathecal baclofen
A
- baclofen is a GABA-B agonist and elicits presynaptic inhibition of afferent fibres
- decreases Ca2+ influx into presynaptic terminals
- 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
13
Q
what is the purpose of the feedback loop in the stretch reflex
A
- 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
- Houk’s model of control strategies
14
Q
what is the purpose of the stretch reflex (servo-assistance)
A
- 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 spindle that compensates for the deviation from a desired muscle length
15
Q
what is the Hoffman reflex
A
- elicited by activation of type 1a sensory nerves
- evoked a muscle contraction via the monosynaptic spinal reflex that can be recorded using EMG
- sensory type 1a afferent fibres are activated at 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
