Homeostatic Regulation In Sports And Exercise

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Homeostatic regulation of the musculoskeletal system, in addition to other physiologic homeostatic regulation, is an important positive process of bodily adjustment in athletes. For example, research in evidence-based sports medicine has demonstrated that weakness of core muscles hinders movement of the limbs.4 Through an understanding of homeostasis, it is clear how the musculoskel-etal system is regulated during and after sports and exercise. Homeostasis is regulated by five physiologic layers5:

1. Organs and their local reflexes: Organs can regulate their own functions with built-in reflexes that do not need any higher level of control to function effectively. For example, local stress, such as tight muscles, can be reduced by local stimulation, such as stretching, massage, and dry needling through the local reflex mechanisms.

2. Autonomic and endocrine messengers: The auto-nomic nervous system and the endocrine system form two channels of communication from the central nervous system (CNS) to individual organs. Poor microcirculation, or local ischemia, is a major physiologic deficiency in overused muscles. Stimulation by needling balances the autonomic system to improve microcirculation and promote recovery from ischemia. This process also involves neuroendocrine messengers and their receptors in capillaries.

3. Brainstem regulation: The brainstem regulates autonomic outputs to the organs through a complex network of reflex centers. For example, during exercise the working muscles and the respiratory system initiate the signals that ascend to the brainstem and higher brain centers, which produce pronounced cardiovascular and endocrine responses to the physical demand.

4. Hypothalamic integration: The hypothalamus regulates endocrine messengers to the body, the autonomic output from the brainstem, and the movement of the musculoskeletal system to maintain homeostasis in response to physiologic challenges.

5. Inputs from higher brain centers: Brain areas above the hypothalamus use information received from the external world to form memory, emotion, and awareness. These higher processes can then alter the activities of the hypothalamus and brainstem. Before or during a sports event, an athlete may compare the present situation with memories of earlier similar situations and may assess the current experience as challenging or stimulating; on the other hand, the athlete may feel that the stress is undesirable or unmanageable. This positive or negative emotional reaction will influence the output from the hypothalamus and brainstem and, subsequently, the operation of the musculoskeletal system (Fig. 2-1).

Figure 2-1 shows the hierarchical or layered regulation of homeostasis. As depicted at the bottom of the figure, the individual organs, such as muscles, have intrinsic reflexive control mechanisms that allow them to operate by themselves when external conditions are constant. This local regulation is performed by the local reflex loop, which comprises the organs, the ganglia of the autonomic nervous system, and the spinal cord. These simple reflexes are sufficient for responding to strictly local needs. When local reflexive regulation is insufficient to meet rapid changes in demand or when the separate organs need to coordinate their function, as in sports activities, two parallel systems of communication—the autonomic nervous system and the endocrine system—are activated. These two systems are regulated by the brainstem and the hypothalamus.

Hypothalamus

Figure 2-1 Schematic of homeostatic regulation over local organs. The local organs have self-regulating capacity. This self-regulation is determined by internal reflexes and actions of autonomic ganglia located in or near the organs. Local regulation is modulated in turn by descending influences from the autonomic nervous system, the brainstem, the hypothalamus, and higher centers in the central nervous system.

Figure 2-1 Schematic of homeostatic regulation over local organs. The local organs have self-regulating capacity. This self-regulation is determined by internal reflexes and actions of autonomic ganglia located in or near the organs. Local regulation is modulated in turn by descending influences from the autonomic nervous system, the brainstem, the hypothalamus, and higher centers in the central nervous system.

The hypothalamus coordinates the actions of the autonomic system and the endocrine system, and it has motor nuclei that store specific programs of survival-related behaviors. The autonomic nervous system and the endocrine system can initiate complex coordination among different organs and systems to meet changes in, or demands from, the external environment. When additional demands emerge as a result of the conscious processing of external information in the higher brain levels, as when the athlete starts to experience stressful thoughts during competition, the centers of the higher brain contribute to the shaping of inputs from the lower systems and outputs from the brain. For example, if pain or injury occurs during competition, the brain may modify the signals from the local organs, and the athlete will compare the current state with memories of previous experience to make decisions and develop coping strategies. These higher brain centers include the limbic system and the cerebral cortex.

This hierarchical regulation allows local processes to proceed on their own, leaving the human brain's finite resources for conscious processing free to manage new tasks. If more systemic coordination is needed to meet new challenges, as during intense competition, then maximal capacity of the cardiovascular system is required. In this case, endocrine and integrated autonomic regulation starts to work.

Each vital organ or organ system is capable of regulating its own function in response to slowly changing demands. If a muscle or a group of muscles is used repeatedly during a physical event, it will start to adjust. Muscles increase their metabolism, circulation, and mass in response to the physical demand. However, if the demand on the muscles exceeds what they are able to sustain, they will activate their self-protective mechanisms to resist any further stress: they become tight and inflamed, and the local reflex is inhibited. At this point, if the stressful demand is reduced or stopped, the muscles undergo a slow self-recovery process. Physical stress (e.g., tightness in soft tissue) or physiologic stress (e.g., inflammation and deficient microcirculation) suppresses self-recovery and inhibits both the reflex and feedback mechanisms that are necessary to restore homeostasis. Dry needling activates the reflex mechanisms to reduce both physical and physiologic stresses without disturbing or damaging the injured muscles, whereas stretching or massage may cause further injury.

The brainstem and hypothalamus receive information about the state of organs such as muscles, joints, and viscera, and they restore homeostatic physiology by sending back commands to the same organs by way of the autonomic nervous system and endocrine messages. Although they play different roles, all the autonomic, endocrine, and muscu-loskeletal systems are involved in the functioning of the motor system.

The skeletal motor system is a conscious and voluntary system. It has sensory nerves that give the brain information about the position and motion of the limbs. These nerves project to sensory areas of the cerebral cortex, and these cortical sensory projections allow people to be aware of the position and function of their muscles and joints.

The sensory and motor nerves are located directly in the cerebral cortex. The motor nerves run from the motor area of the cerebral cortex directly to individual muscle fiber bundles to command their movements, which enables people to be conscious of the position of their limbs and to have voluntary control of their movements. Each motor nerve fiber connects to a single muscle bundle to enable precise control of the target muscle.

The autonomic nervous system also has sensory and motor nerves. However, its sensory nerves ascend to the brainstem, not to the level of the cerebral cortex, and its motor nerves originate in the brainstem. As a result, humans have limited awareness of the state of their vital organs and, consequently, very little control over them. The autonomic nerve fibers reaching a target organ or tissue are highly branched, and the smooth muscle cells themselves are highly interconnected. This results in more widespread response on the part of the effectors.

The endocrine response to stress has two parallel pathways: the adrenocortical response, controlled by the hypothalamus and pituitary gland, and the adrenomedullary response, controlled by the sympathetic nervous system. During stress, the adrenal medulla secretes epinephrine and norepi-nephrine. This process is activated by sympathetic preganglionic fibers originating in nucleus of the solitary tract of the brainstem, influenced by messages from the paraventricular nucleus of the hypothalamus. Epinephrine, an endocrine messenger, acts on P-adrenoreceptors existing in many tissues and organs. It reinforces the activities of sympathetic nerves. Norepinephrine's role as a stress hormone is small.

The second major stress hormone is cortisol. Unlike epinephrine, cortisol plays a role in both normal physiologic activity and during periods of stress. Cortisol is needed for normal autonomic function and therefore for all forms of physiologic regulation at cellular level, as well as for metabolism. Some of the effects of cortisol on target tissues are listed in Table 2-1.

Without the tonic influence of cortisol, the action of the autonomic nervous system would be greatly diminished, and epinephrine would be less

TABLE 2-1

Effects of Cortisol on Target Tissues

TABLE 2-1

Effects of Cortisol on Target Tissues

Tissue

Effect

Neuron

Enhanced catecholamine

synthesis

Hippocampus

Enhanced memory function

Thalamus

Sensitivity to incoming stimuli

Adrenoreceptors

Enhanced P-receptor sensitivity

Adrenal medulla

Enhanced catecholamine

synthesis

Immune system

Enhanced or inhibited

Inflammation

Inhibited

Glucose

Enhanced production

Fatty acid

Enhanced liberation from

stored fat

Kidney

Water diuresis and sodium

retention

From Lovallo WR: Stress & health: biological and psychological interaction, ed 2, Thousand Oaks, CA, 2005, Sage Publications, p 58.

From Lovallo WR: Stress & health: biological and psychological interaction, ed 2, Thousand Oaks, CA, 2005, Sage Publications, p 58.

effective at its target tissues. In addition to influencing normal tissue physiology, cortisol also participates in the stress response. During stress, cortisol potentiates the activity of the sympathetic nervous system to increase the release of stored glucose and fats. It performs the regulatory function of controlling stress so that an acute stress response will not threaten homeostasis. This important regulatory function is seen in experimental animals, which die as a result of poor regulation of the stress response if their adrenal glands are removed.

Cortisol and epinephrine reach all tissues via systemic circulation, and they work together to significantly alter the background environment in which all the tissues can function normally and to coordinate activity and responses across many tissues. Sports activity is an example of how coordinated activity of the musculoskeletal system is regulated by cortisol and epinephrine. Norepinephrine plays a less significant role in homeostatic regulation.

A third stress hormone is P-endorphin. In response to corticotropin-releasing factor (CRF) from the hypothalamus, the pituitary produces both P-endorphin and adrenocorticotropin hormone (ACTH). P-endorphin is an agonist of opiate receptors in the nervous system, producing analgesia and performing other physiologic roles such as balancing cardiovascular function. Supposedly P-endorphin functions as a physiologic and psychologic analgesic.

CENTRAL NERVOUS SYSTEM INTEGRATION OF THE STRESS RESPONSE IN SPORTS AND REHABILITATION

Mentality can influence emotions and cause psychologic stress and stress responses. Positive and negative emotions do not occur in isolation; they are part of particular patterns of brain activity, leading to distinct physiologic responses and behavioral reactions. These responses and reactions have a significant effect on the process of adjusting to stress during both sports activities and rehabilitation from injury.

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