Physiology 2130 Lecture 22: Spinal Reflexes
1 May 2018
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Lecture 021: Spinal Reflexes
Reflex
● Occur in all skeletal muscle (except eye
muscle)
● Larger/more power in postural anti-
gravity muscles
○ Quadriceps
○ Calf-muscles
○ Easy to get this reflex when you
hit the tendon
Stretch Reflex
● Quick stretch of
the muscle
generates
receptor
activation
○
Generates
AP in the
1-a
afferent
fiber
● AP from the 1a
afferent neuron
goes into the
spinal cord
● 1a afferent
neuron makes a
monosynaptic
connection on
the motor neuron
○ Always excitatory
● Causes release of neurotransmitter and depolarization of the alpha-motor neuron
(efferent)
○ Generate a muscle movement in the agonist
● 1a afferent also makes a connection to the 1a inhibitory interneuron
○ 1a inhibitory interneuron produces IPSP on the antagonist/flexors muscles
■ Hyperpolarizes and decreases the potential of AP in the flexors
■ Relaxation of the antagonist
●Reciprocal inhibition
○ If the agonist is excited, the antagonist is inhibited
■The only inhibition occurs is in the spinal cord (not at the
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neuromuscular junction!!)
Function of the Stretch Reflex
●First line of defense in overcoming unexpected muscle stretches
○ E.g. in leg muscle when standing on a bust that is starting and stopping
● In this situation, tonic excitation to alpha motoneuron
○ Descending drive to get the motor neurons close to threshold so that you can
easily generate the stretch reflex when the need arise
●Second line of defence: Vestibular postural reflexes
○ Ex. Generate rapid responses to keep you from falling when you trip
● Latency to EMG onset
○ Time to see the electrical magnetic activity signals
○ Stretch reflex: 30 ms
○ Vestibular postural reflexes: 60 ms
○ Voluntary reaction time 150 ms
ElectroMyoGram (EMG)
● Recording of extracellular current flow associated with action
potentials in skeletal muscles
○Depolarization in the motor unit occurs simultaneously
■ Get hundreds of simultaneous APs from all the (extrafusal) muscle fibres
in the single motor unit
● “Compound AP”
○ Huge amount of extracellular current
○ Easy to record the electrical activity (compound AP) from a single motor unit
● Applying a tendon tap stretch reflex)
○ Detect a compound AP about 30 ms after the muscle was stretched
●Voluntary quadriceps contraction
○ 150 ms delay
○ Signals has to travel farther
■ Up spinal cord -> DC/ML -> Somatic Sensory cortex -> motor cortex ->
down spinal cord -> motor neuron
● Much faster when it a reflex
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Occur in all skeletal muscle (except eye muscle) Larger/more power in postural anti- gravity muscles. Easy to get this reflex when you hit the tendon. Quick stretch of the muscle generates receptor activation. Ap from the 1a afferent neuron goes into the spinal cord. 1a afferent neuron makes a monosynaptic connection on the motor neuron. Causes release of neurotransmitter and depolarization of the alpha-motor neuron (efferent) Generate a muscle movement in the agonist. 1a afferent also makes a connection to the 1a inhibitory interneuron. 1a inhibitory interneuron produces ipsp on the antagonist/flexors muscles. Hyperpolarizes and decreases the potential of ap in the flexors. If the agonist is excited, the antagonist is inhibited. The only inhibition occurs is in the spinal cord (not at the neuromuscular junction!!) First line of defense in overcoming unexpected muscle stretches. E. g. in leg muscle when standing on a bust that is starting and stopping. In this situation, tonic excitation to alpha motoneuron.
