Physiology 3120 Lecture Notes - Lecture 2: Anti-Gravity, Vestibular Nuclei, Quadriceps Femoris Muscle

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Physiology 3120
Dr. Hore
Spinal Reflexes
- Knee jerk reflex
- Occur in all skeletal muscle (Except those of eye muscle)
- Reflexes are larger, more powerful in postural, anti-gravity muscles (quadriceps and calf
muscles)
- So you can easily get reflexes by hitting the tendons of these muscles
- Hitting the tendon applies a rapid stretch to the quadriceps
Physiology of the stretch reflex
- A quick stretch of extensor muscle (quadriceps or calf) will result in receptor activation
(opening of ion channels), Na+ comes into cell, depolarization happens, receptor potential if
large enough will reach threshold and send a burst of AP in the 1a afferent fibers going up into
spinal cord and coming back down in an axon to make a monosynaptic connection on alpha
motoneuron of quadriceps muscle
- This monosynaptic connection is always excitatory causing neurotransmitter to go across
synapse and produce depolarization and if they reach threshold, we get AP in efferent axon
- 1a afferent fiber coming into the spinal cord has a branch which also releases an excitatory
neurotransmitter onto the 1a inhibitory neuron which produces IPSPs on the alpha
motoneruons on the antagonist flexor muscles (hamstrings)
- this produces a hyperpolarization in flexor alpha neurons and decreases probability of AP
- if there is a silence in AP firing, this means that there is relaxation of antagonist muscle
- spinal cord reflexes have reciprocal inhibition if the agonist is excited, the antagonist is
inhibited
o in stretch reflex, the hamstrings are inhibited this is not correct, the hamstrings relax
the only inhibition that occurs is in spinal cord
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Function of Stretch Reflex
- First line of defense in overcoming unexpected muscle stretches,
o eg., in leg muscles when standing on a bus that is starting and stopping
o In this situation tonic excitation to alpha motoneurons
- Second line of defense vestibular postural reflexes
o Signal goes from vestibular system into vestibular nuclei and straight down spinal cord
to get immediate responses
- Latency to EMG onset (electrically activity of contracting muscle)
o Stretch reflex ~ 30 ms
o Vestibular postural ~ 60 ms
o Voluntary reaction time ~ 150 ms
ElectroMyoGram (EMG)
- Recording of extracellular current flow associated with action potentials in skeletal muscles
- Simultaneous APs in hundreds of muscle fibres in each motor unit
- You have AP coming down a motor unit, (this efferent alpha motoneuron to branch out 100
times)
- All the extrafusal muscle fibers associated with this motor unit will then….
- The depolarization will travel simultaneously and the AP will travel along both ways
- There is a huge amount of extracellular electrical current for just one
- If we just fired an AP in one of these, we would have a Compound action potential from all the
extrafusal muscle fibers innervated by this
- So its not hard to record this electrical activity (get a bunch of electrodes and record
extracellularly)
o Bc there is a huge amount of extracellular carb bc of simultaneous AP in extrafusal fibers
belonging to a single motor unit
- So lets say we stuck some electrodes on the quad or calf and we applied a tendon tap to the
tendon of quadriceps, you would see that after 30ms there is a compound action potential of the
muscle that was stretched
- On the other hand, if instead of applying a tendon tap to the quad, we asked the person to shut
their eyes and contract quad every time they feel a hit to the skin, you take the tendon hammer
and hammer on the side where there is only bone and no muscle
o There is a latency of 150ms before CAP is formed
30 ms
Tendon Tap
150 ms
To oscill
30 ms
Tendon Tap
150 ms
To oscill
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Document Summary

Occur in all skeletal muscle (except those of eye muscle) Reflexes are larger, more powerful in postural, anti-gravity muscles (quadriceps and calf muscles) So you can easily get reflexes by hitting the tendons of these muscles. Hitting the tendon applies a rapid stretch to the quadriceps. 1a afferent fiber coming into the spinal cord has a branch which also releases an excitatory. First line of defense in overcoming unexpected muscle stretches: eg. , in leg muscles when standing on a bus that is starting and stopping. In this situation tonic excitation to alpha motoneurons. Second line of defense vestibular postural reflexes to get immediate responses: signal goes from vestibular system into vestibular nuclei and straight down spinal cord. Latency to emg onset (electrically activity of contracting muscle: stretch reflex ~ 30 ms, vestibular postural ~ 60 ms, voluntary reaction time ~ 150 ms. Recording of extracellular current flow associated with action potentials in skeletal muscles.

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