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Effect of ischemia on responses of group III and IV afferents to contraction
Insights
During static muscle contraction in cats, ischemia significantly increases the response of group IV afferents compared to group III afferents. These findings suggest group IV afferents play a key role in cardiovascular responses during muscle ischemia.
Area of Science:
- Neuroscience
- Cardiovascular Physiology
- Skeletal Muscle Research
Background:
- Static muscle contraction in hindlimb muscles reflexly elevates cardiovascular function.
- This cardiovascular response is amplified when blood flow to the working muscles is occluded (ischemia).
- Group III and IV afferents are known mediators of cardiovascular responses to muscle contraction, but their role during ischemic conditions is less understood.
Purpose of the Study:
- To investigate the impulse activity of group III and IV afferents in the triceps surae muscle during static contraction under both normal and ischemic conditions.
- To determine if ischemia differentially affects the response of group III versus group IV afferents.
Main Methods:
- Recorded impulse activity from 24 group III and 30 group IV afferents with endings in the cat triceps surae.
- Static contraction of the triceps surae was performed with the abdominal aorta either patent (normal blood flow) or occluded (ischemia).
- Chi-squared analysis was used to compare the percentage of afferents responding differently under ischemic versus non-ischemic conditions.
Main Results:
- A significantly higher percentage of group IV afferents (46.7%) showed increased responses to static contraction during ischemia compared to group III afferents (12.5%).
- Two distinct response patterns to ischemic contraction were observed in the afferents.
- Pattern 1: Afferents stimulated by non-ischemic contraction, but more so by ischemic contraction (n=10).
- Pattern 2: Afferents not stimulated by non-ischemic contraction, but activated by ischemic contraction (n=7).
Conclusions:
- Group IV afferents are more responsive to static muscle contraction during ischemia than group III afferents.
- Both observed patterns of afferent response during ischemia likely contribute to the reflex cardiovascular adjustments.
- These findings highlight the significant role of specific afferent pathways in the cardiovascular response to muscle ischemia.
Abstract:
Static contraction of the hindlimb muscles of cats reflexly increases cardiovascular function, an effect that is potentiated by occlusion of the arterial supply to the working muscles. Although group III and IV afferents are known to be stimulated by and to cause the reflex cardiovascular responses to static muscular contraction, little is known about the responses of these afferents to static contraction when the arterial supply to a working muscle is occluded. We therefore recorded the impulse activity of 24 group III afferents and 30 group IV afferents with endings in the triceps surae while we statically contracted this muscle group, both when the abdominal aorta was occluded and when it was patent. A chi 2 analysis revealed that ischemia increased the responses to static contractions of a significantly higher percentage of group IV afferents than group III afferents (46.7% vs. 12.5%, respectively; P less than 0.02). In addition, two patterns of responses to ischemic contraction were observed. The first pattern was displayed by afferents (n = 10) that were stimulated by nonischemic contraction but were stimulated more by ischemic contraction. The second pattern was displayed by afferents (n = 7) that were not stimulated by nonischemic contraction but were stimulated by ischemic contraction. We conclude that afferents displaying both patterns are likely to contribute to the reflex cardiovascular responses to ischemic contraction.