ROLE OF THE SKELETAL NERVES AND SKELETAL MUSCLES IN INCREASING CARDIAC OUTPUT AND ARTERIAL PRESSURE

Although most rapidly acting nervous control of the circulation is effected through the autonomic nervous system, at least two conditions exist in which the skeletal nerves and muscles also play major roles in circulatory responses.

Abdominal Compression Reflex. When a baroreceptor or chemoreceptor reflex is elicited, nerve signals are transmitted simultaneously through skeletal nerves to skeletal muscles of the body, particularly to the abdominal muscles. Muscle contraction then compresses all the venous reservoirs of the abdomen, helping to translocate blood out of the abdominal vascular reservoirs toward the heart. As a result, increased quantities of blood are made available for the heart to pump. This overall response is called the abdominal compression reflex. The resulting effect on the circulation is the same as that caused by sympathetic vasoconstrictor impulses when they con strict the veins: an increase in both cardiac output and arterial pressure. The abdominal compression reflex is probably much more important than has been realized in the past because it is well known that people whose skeletal muscles have been paralyzed are considerably more prone to hypotensive episodes than are people with normal skeletal muscles.

Increased Cardiac Output and Arterial Pressure Caused by Skeletal Muscle Contraction During Exercise. When the skeletal muscles contract during exercise, they compress blood vessels throughout the body. Even anticipation of exercise tightens the muscles, thereby compressing the vessels in the muscles and in the abdomen. This compression translocates blood from the peripheral vessels into the heart and lungs and, therefore, increases cardiac output. This effect is essential in helping to cause the fivefold to sevenfold increase in cardiac output that sometimes occurs during heavy exercise. The rise in cardiac output in turn is an essential ingredient in increasing the arterial pressure during exercise, an increase usually from a normal mean of 100 mm Hg up to 130 to 160 mm Hg.

RESPIRATORY WAVES IN THE ARTERIAL PRESSURE

 With each cycle of respiration, the arterial pressure usually rises and falls 4 to 6 mm Hg in a wavelike manner, causing respiratory waves in the arterial pressure. The waves result from several different effects, some of which are reflex in nature, as follows:

1. Many of the “breathing signals” that arise in the respiratory center of the medulla “spill over” into the vasomotor center with each respiratory cycle.

2. Every time a person inspires, the pressure in the thoracic cavity becomes more negative than usual, causing the blood vessels in the chest to expand. This reduces the quantity of blood returning to the left side of the heart and thereby momentarily decreases the cardiac output and arterial pressure.

3. The pressure changes caused in the thoracic vessels by respiration can excite vascular and atrial stretch receptors.

Although it is difficult to analyze the exact relations of all these factors in causing the respiratory pressure waves, the net result during normal respiration is usually an increase in arterial pressure during the early part of expiration and a decrease in pressure during the remainder of the respiratory cycle. During deep respiration, the blood pressure can rise and fall as much as 20 mm Hg with each respiratory cycle.

ARTERIAL PRESSURE “VASOMOTOR” WAVES—OSCILLATION OF PRESSURE REFLEX CONTROL SYSTEMS

Often while recording arterial pressure, in addition to the small pressure waves caused by respiration, some much larger waves are also noted—as great as 10 to 40 mm Hg at times—that rise and fall more slowly than the respiratory waves. The duration of each cycle varies from 26 seconds in the anesthetized dog to 7 to 10 seconds in the unanesthetized human. These waves are called vasomotor waves or Mayer waves. Such records are demonstrated in Figure 1, showing the cyclical rise and fall in arterial pressure.

Fig1. A, Vasomotor waves caused by oscillation of the CNS  ischemic response. B, Vasomotor waves caused by baroreceptor  reflex oscillation. 

The cause of vasomotor waves is “reflex oscillation” of one or more nervous pressure control mechanisms, some of which are the following.

they are usually much less intense than shown in the figure. They are caused mainly by oscillation of the baroreceptor reflex. That is, a high pressure excites the baroreceptors, which then inhibits the sympathetic nervous system and lowers the pressure a few seconds later. The decreased pressure in turn reduces the baroreceptor stimulation and allows the vasomotor center to become active once again, elevating the pressure to a high value. The response is not instantaneous, and it is delayed until a few seconds later. This high pressure then initiates another cycle, and the oscillation continues on and on.

The chemoreceptor reflex can also oscillate to give the same type of waves. This reflex usually oscillates simultaneously with the baroreceptor reflex. It probably plays the major role in causing vasomotor waves when the arterial pressure is in the range of 40 to 80 mm Hg because in this low range, chemoreceptor control of the circulation becomes powerful, whereas baroreceptor control becomes weaker.

Oscillation of the CNS Ischemic Response. The record in Figure 1A resulted from oscillation of the CNS ischemic pressure control mechanism. In this experiment, the cerebrospinal fluid pressure increased to 160 mm Hg, which compressed the cerebral vessels and initiated a CNS ischemic pressure response up to 200 mm Hg. When the arterial pressure rose to such a high value, the brain ischemia was relieved and the sympathetic nervous system became inactive. As a result, the arterial pressure fell rapidly back to a much lower value, causing brain ischemia once again. The ischemia then initiated another rise in pressure. Again the ischemia was relieved and again the pressure fell. This response repeated itself cyclically as long as the cerebrospinal fluid pressure remained elevated.

Thus, any reflex pressure control mechanism can oscillate if the intensity of “feedback” is strong enough and if there is a delay between excitation of the pressure receptor and the subsequent pressure response. The vasomotor waves illustrate that the nervous reflexes that control arterial pressure obey the same principles as those applicable to mechanical and electrical control systems. For instance, if the feedback “gain” is too great in the guiding mechanism of an automatic pilot for an airplane and there is also delay in response time of the guiding mechanism, the plane will oscillate from side to side instead of following a straight course.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الخدمية: 2000 متطوع يشاركون في تقديم الخدمات لزائري النصف من شعبان

المجمع العلمي يقيم محفلًا قرآنيًّا في النجف لإحياء ذكرى ولادة الإمام المهدي (عجل الله فرجه)

بالتزامن مع زيارة النصف من شعبان.. شعبة الخطابة الحسينية النسوية تكرم الفائزات في مسابقة (سبيل المنتظرين)

قسم الشؤون الفكرية يفتتح محطة ثقافية بمناسبة زيارة النصف من شعبان في طريق الزائرين

المزيد

الآخبار الصحية

دراسة تكشف "سر الوسادة".. ودورها في حماية البصر

بالأشعة تحت الحمراء.. "ثورة علمية" في علاج اللاعبين

مشروب أحمر يخفض ضغط الدم "بشكل ملحوظ"

الصحة العالمية تكشف حقيقة انتشار فيروس نيباه خارج الهند

المزيد

الآخبار التكنلوجية

دراسة تدفعك لشراء ساعة ذكية "فورا".. هذا ما تقوله عن قلبك

علماء حاصلون على "نوبل": الذكاء الاصطناعي لن يستبدل الإنسان

وانغ هو: القمة العالمية للعلماء تجسد تقدير الإمارات للعلم

رئيس رابطة كبار العلماء يعلن تأسيس مقر في الإمارات

المزيد

مواضيع ذات صلة

Role of the Nervous System in Rapid Control of Arterial Pressure

The Lymphatic System Plays A Key Role in Controlling Interstitial Fluid Protein Concentration, Volume, and Pressure

Formation of Lymph

Rate of Lymph Flow

Exchange of Water, Nutrients, and Other Substances Between the Blood and Interstitial Fluid

Flow of Blood in the Capillaries—Vasomotion

Structure of the Microcirculation and Capillary System

Basic Principles of Circulatory Function

Physical Characteristics of the Circulation

Abnormal Rhythms That Result From Block of Heart Signals Within the Intracardiac Conduction Pathways Sinoatrial Block

Abnormal Sinus Rhythms Tachycardia

Flow of Current Around the Heart During the Cardiac Cycle