Cardiorespiratory reflexes from muscles during dynamic and static exercise in the dog

Cardiorespiratory reflex responses during the initial phase of dynamic and static contraction of hindlimb muscles were studied in anesthetized dogs. Muscle contractions were elicited by stimulating the femoral and gastrocnemius nerves at 3 and 100 Hz with the intensity of 2.0-2.5 times the motor threshold for a 20-s period. Rhythmic contractions caused a decrease in arterial pressure (Pa) and heart rate (HR) and increased pulmonary ventilation (VE) by increasing frequency (f) without significantly changing VT. Tetanic contractions provoked an increase in Pa and HR and a hyperpnea resulting from a rise in both f and VT. Similar responses were also obtained in anesthetized dogs with carotid sinuses denervated and cervical vagi cut. The abrupt increase in VE at the start of both types of exercise was not associated with immediate significant decreases in end-tidal CO2 values. These two patterns of cardiocirculatory and respiratory responses were closely similar to those reported in anesthetized rabbits in previous studies. Both patterns of responses were reflexes initiated by activation of muscle receptors verified by interrupting the afferents from the contracting muscles. It is concluded that, during dynamic and static work, two distinct muscular reflex mechanisms might exert their drives, related to the muscular metabolic rate, on the circulatory and respiratory function.     

Cardiovascular reflexes during sustained handgrip exercise: role of muscle fibre composition, potassium and lactate

Six healthy men performed sustained static handgrip exercise for 2 min at 40% maximal voluntary contraction followed by a 6-min recovery period. Heart rate (fc), arterial blood pressures, and forearm blood flow were measured during rest, exercise, and recovery. Potassium ([K+]) and lactate concentrations in blood from a deep forearm vein were analysed at rest and during recovery. Mean arterial pressure (MAP) and fc declined immediately after exercise and had returned to control levels about 2 min into recovery. The time course of the changes in MAP observed during recovery closely paralleled the changes in [K+] (r = 0.800, P < 0.01), whereas the lactate concentration remained elevated throughout the recovery period. The close relationship between MAP and [K+] was also confirmed by experiments in which a 3-min arterial occlusion period was applied during recovery to the exercised arm by an upper arm cuff. The arterial occlusion affected MAP while fc recovered at almost the same rate as in the control experiment. Muscle biopsies were taken from the brachioradialis muscle and analysed for fibre composition and capillary supply. The MAP at the end of static contraction and the [K+] appearing in the effluent blood immediately after contraction were positively correlated to the relative content of fast twitch (% FT) fibres (r = 0.886 for MAP vs % FT fibres, P < 0.05 and r = 0.878 for [K+] vs % FT fibres, P < 0.05). Capillary to fibre ratio showed an inverse correlation to % FT fibres (r = -0.979, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Shoulder reflexes.

Dynamic shoulder stability is dependent on muscular coordination and sensory inputs. In the shoulder, mechanoreceptors are found in the coracoacromial ligament, the rotator cuff tendons, the musculotendinous junctions of the rotator cuff and in the capsule. The number of receptors in the capsule is small compared to the number in the other shoulder structures. Proprioceptive information from numerous receptors in muscles and tendons is mediated via fast conducting nervefibers and probably contribute more to kinaestethic sensation than information from capsule and ligaments. Therefore it seems likely that the joint receptors have a more distinct role for the kinaestethic sense than muscle receptors. In cats a direct reflex from the afferents innervating the shoulder to the muscles around the shoulder has been presented. The reflex had an extremely short latency (2.7-3.1 ms). In man, a very long latency (300 ms) excitatory reflex has been found when nerves in the capsule were stimulated electrically during shoulder surgery. In addition, when the anterior-inferior capsule was excited in conscious humans with modest amplitude electrical stimuli during muscle activity, a strong inhibition was found with an average latency of 33 ms. Stimulation of the sensory nerves in the coracoacromial ligament has also been found to modify muscle activity strongly. Even though our understanding of the control of shoulder motion is incomplete, it is clear that sensory inputs can strongly modify muscle activity around the shoulder. This has implications for rehabilitation and shoulder surgery.     

Cruciate ligament reflexes.

The idea of muscular reflexes elicited from sensory nerves of the cruciate ligaments is more than 100 years old, but the existence of such reflexes has not been proven until the recent two decades. First in animal experiments, a muscular excitation could be elicited in the hamstrings when the anterior cruciate ligament (ACL) was pulled, and tension in the ligament caused activity of the gamma motor neurones of the muscles around the knee. Impulses from the sensory nerves in ACL were activated during motion of the knee, in particular overstretching and combined extension and rotation. In humans, proprioception in the knee is decreased after ACL rupture. By mechanical or electrical stimulation of the ACL, an excitation in the hamstrings muscles can be elicited. During muscular activity, stimulation of the ACL or PCL results in a clear inhibition of the ongoing activity, both during static isometric and isokinetic muscle work, and also during dynamic activity (gait). This inhibitory reflex subjectively resembled giving way. The latency of the reflex was short in animals (about 3 ms) and long in humans (60-120 ms), probably caused by differences in the experimental setup and between species. The long latency in humans makes it unlikely that it is a directly protective reflex. Instead it may be involved in the updating of motor programs. Further research may characterize the reflex in details and map its pathways. The existence of this reflex indicate that the cruciate ligaments have an afferent function, which influences knee dynamics.