Abstract
Respiratory system is member of the vital systems in the body; it is true for the components of this system. This system provides an environment to the body where exchange of oxygen and carbondioxid can take place. This way respiratory system maintains the acid base balance in the body.
Regulatory mechanisms of this system are also complex. The respiratory system works in two patterns: voluntary and involuntary. So there are different mechanisms for the control of these two patterns. Both the patterns are controlled by different structures in the central nervous system helped by the peripheral nervous system and components of muscular system. Also, there are two phases of breathing: inspiration and expiration. Different types of muscles are utilized in the two phases of the breathing process.
Some receptors are also involved in the regulation of respiratory system. These are either chemical or neural.
There are various types of pathological lesions, which affect the functions of neuromuscular respiratory system by disturbing various components of this system right from the higher centers up to the alveoli down in the lungs.
Neuromuscular Respiratory System
Introduction
Respiratory system composed of one of the vital organs in the body i.e. the lungs. These lungs carry out their activities in maintaining the internal milieu of the body with the help of other connecting and supporting structures. The major function of the lungs is to provide an environment to the body for the oxygenation of blood and elimination of the carbon dioxide. This way lungs also help in maintaining the acid-base balance of the body.
The performance of the respiratory system is not on simple lines but it involves multiple players to carry out the appropriate functions at a time when these are required. The whole system controlling this ventilation-perfusion process right from the higher centers up to the alveoli down in the lungs is a complex one which includes: cortex of the brain, brainstem, spinal cord, motor neurons, respiratory muscles, and feedback receptors.
Breathing is an active process, which requires muscles to complete the cycle of breathing with the help of neuromuscular control and coordination. In fact, breathing is at times a voluntary phenomenon while at other occasions it is absolutely involuntary and continues automatically. Both voluntary and involuntary breathing episodes are under the control of central nervous system (CNS).
Methods
The topic was explored on the internet, in various books on medicine and published reports. Those documents were selected which were closely related to neuromuscular respiratory anatomy, physiology and pathology.
Results
Central Nervous System
Control of Voluntary Breathing
The centers in the parietal region of cerebral cortex are the points of origin for any impulse or signal to control the process of voluntary breathing. These signals travel from the center to the motor neurons in the spinal cord via corticospinal tract, which are specific for the control of voluntary movements and remain separate form those tracts which control involuntary breathing movements (reticulospinal pathways) throughout their passage down to the motor neurons in the spinal cord; there are occasional connections between the two tracts with feeble understanding. Further, these signals travel from motor neurons to the muscles and innervate them to complete the whole process of inspiration and expiration.
A large number of disorders cause damage to the corticospinal tract as result of which the voluntary breathing movements are affected. These lesions are: mid-pontine stroke, pontine tumor, central pontine myelinosis, head injury and Parkinsonism.
Figure 1: Respiratory center
Control of Involuntary Breathing
The control of breathing during involuntary phase is not as simple as that of voluntary one. It is well-coordinated mechanism between the multiple higher centers up and alveoli down in the lungs. Two centers in the medulla and one in the pons generate rhythm and eventually drive to breathe. The feedback mechanism, which is conveyed as chemical and mechanical form also plays an important role in modulating the pattern of breathing. In fact this generation of rhythms for breathing purposes originates spontaneously in an area of medulla called Botzinger and pre-Botzinger complex. The reticulspinal tract conveys the nerve fibers from higher centers to the neurons in the spinal cord.
There are multiple pathological situations, which lead to the disturbance of involuntary breathing movements, as: injury to the autonomic respiratory centers in the brainstem like medullary infarction, bulbar poliomyelitis and bilateral cervical tractotomy.
Spinal Cord
Spinal cord as well as the major nerves is important structures for transmitting nerve impulses from the higher centers in the cortex and brain stem to the lower motor neurons in the anterior horn cells of the spinal cord. From here, the neurons supply their respective muscles. Spinal cord provides the service of providing pathways for the upward or downward transmission of neural messages through various types of nerve tracts. For the neuromuscular respiratory system, corticospinal tract facilitates the transfer of impulses for voluntary breathing movements while reticulospinal tract carries the nerve impulses for involuntary movements of the respiratory system.
Diseases of the cord at times may affect breathing because of their direct effect on the nerves. Traumatic injury at any level affects the respiratory movements accordingly.
Peripheral Nervous System
The lower motor neurons, actually, complete the link between the nervous component and the muscular component of the respiratory neuromuscular system. These neurons split into smaller fibers, called twigs, as they reach the muscle fibers. But the terminal ends of these nerve fibers are bulbous and get applied themselves to the muscle membrane at motor endplates. This is the point where nerve impulse results in release of neurotransmitter, acetylcholine, which binds to the muscle, depolarizes it and eventually the muscle contracts as a result of action potential produced due to depolarization.
Some of the disorders, which may affect respiratory drive through motor nerves, are: Guillain Barre Syndrome, botulinum toxicity and phrenic nerve dysfunction.
Respiratory Muscles
The players at the other end are respiratory muscles, which eventually contract or relax to carry out the actual process of breathing. These muscles have been divided in to three major groups based on their role in various phases of breathing and as an additional support. Inspiratory muscles contract to accomplish the task of breathing air in by increasing the volume of the thoracic cavity. These are: diaphragm, external intercostals, and accessory muscles (sternocleidomastoid, trapezii, latissimus dorsi, pectoralis major and minor). Diaphragm is supplied by the phrenic nerve while intercostals are innervated by the intercostals nerves.
Similarly, various groups of muscles take part in the expiration phase, like: internal intercostals, abdominal muscles such as rectus abdominis, internal and external obliques, and transverse abdominis. The obliques, and transverse abdominis move inwardly and displace the diaphragm inside the thoracic cavity to assist in exhalation. On the other hand, the rectus and the obliques also bring the rib cage down and inward thus decreasing the volume of the cavity and help in exhalation.
Figure 2: Thoracic cavity in respiratory phases
In addition to all these muscles, the muscles of upper airways are also important in the process of breathing when they keep the airways widely open and patent. These are innervated by cranial nerves V, VII, IX, X, and XII.
A variety of lesions affect the respiratory muscles. These are: myopathy/neuropathy, Duchene muscular dystrophy, inflammatory myopathies and myotonias.
Feedback Control
There are two types of receptors, which affect respiratory movements. The chemorecpetors, which are found in the CNS as well as peripheral areas. The peripheral receptors are found in the carotid and aortic bodies. These receptors respond to changes in the partial pressure of O2 (Pao2) more as compared to changes in Paco2. Central receptors are very important as far as the acid base balance in the body is concerned. They are located in the: locus ceruleus, nucleus tractus solitarius, midline raphe and ventrolateral quadrant of medulla. The central receptors are more sensitive to carbon dioxide.
Neural receptors are dispersed along the whole respiratory tract: upper airway, respiratory muscles, lungs, and pulmonary vessels. They send signals to the central regulatory areas of respiration via vagus nerve when they are activated. As a result the respiratory movements are adjusted by the central nervous system accordingly.
Conclusions
Various components take part to complete the neuromuscular system. This system is vital for life and is very complex in its operations. At various levels in the central nervous system, its regulation takes place, which is voluntary at occasions but becomes involuntary at other times. Different types of pathological lesions affect this system in acute or chronic way.
