Autonomic nervous system in paralympic athletes following SCI

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Lay summary by Dante Chiesa

Edited by Crystal Han and Jocelyn Chan

This is a lay summary of the paper by ICORDians Matthias Walter and Dr. Andrei Krassioukov. Read the original article here.

The nervous system can be subdivided into the central nervous system (CNS), which includes the brain and the spinal cord, and the peripheral nervous system (PNS), which includes all of the nerves that branch out from the brain and the spinal cord to the rest of the body. The peripheral nervous system can be further divided into sensory neurons, which relay sensation information to the CNS (such as temperature), and motor neurons, which relay motor commands away from the CNS (to move your hand). However, it is important to further break down the motor component of the PNS into the somatic nervous system and the autonomic nervous system. The somatic nervous system is responsible for the voluntary control of the body via skeletal muscles. The autonomic nervous system is responsible for involuntary physiological movement and processes such as heart rate, blood pressure, respiration, and digestion. These involuntary processes are controlled in two distinct ways, first, through what we call the sympathetic component of the ANS, which stimulates the fight or flight response (ex. increases heart rate); parasympathetic, which stimulates the rest and digest response (ex. reduce heart rate and breathing).

When individuals experience a spinal cord injury (SCI), it typically results in loss or impairment of sensorimotor function that presents as paralysis and reduced sensation in certain areas. However, individuals with SCI are susceptible to developing secondary complications related to the ANS. 

How was the study done?

This study was done by investigating the various symptoms which present as a result of SCI. The study mainly focuses on physical presentations of symptoms correlated with SCI which could possibly impact the athletic capabilities of paralympic athletes.

What is the purpose of the study?

A dysfunctional ANS could impede the quality of life, and given the critical bodily functions it regulates, it is also very important for athletic performance. As the majority of people who have experienced an SCI plan to continue to exercise or engage in recreational para-sport, it is important to investigate the extent of the challenges they will face after an SCI. 

However, it is important to emphasize that the extent of the SCI (such as its location on the spinal cord) can lead to very different effects on the ANS and consequently athletic performance. This lay summary provides a general overview but for the various effects and specifics, please refer to the original article for further reading.

What were the results of the study?

One organ under significant control of the ANS is the heart. Through various nerve fibers, the ANS can influence heart rate, stroke volume (the amount of blood pumped in one contraction), and cardiac output (total amount of blood pumped in one minute). For example, when someone engages in strenuous exercise, we expect all of these factors to increase through sympathetic control by the ANS. Other aspects of the cardiovascular system are also impacted, including blood pressure. The study found that individuals who have experienced an SCI cannot manipulate these factors (heart rate, stroke volume, and cardiac output) as well as uninjured individuals are able to. Overall, these can contribute to the increased prevalence of cardiovascular disease in athletes with SCI compared to uninjured athletes.

The study also looked at the respiratory system, which is the system responsible for providing our body with oxygen during athletic performance. Individuals living with a SCI experience respiratory dysfunction that arises from dysfunction of the respiratory muscles. This can present as shortness of breath, especially when engaging in strenuous exercise. The extent of respiratory dysfunction varies significantly depending on the extent of the SCI, however, regardless of the extent of the respiratory dysfunction, a decreased ability to increase breathing rate and oxygen supply to our muscles during strenuous exercise will inhibit athletic performance.

Finally, the study also looked at thermoregulation. Thermoregulation is the mechanism by which we control our internal body temperature. As one engages in athletic activity, internal temperature attempts to rise significantly, hence, it is important for the body to thermoregulate effectively and maintain a suitable core temperature. This regulation is under the control of the thermoregulatory center in the brain. When an individual engages in strenuous exercise and internal body temperature increases, the thermoregulatory center will send a message to 2 important systems: (1) sweat glands, which release sweat and cool the skin as it evaporates, and (2) blood vessels, which widen in diameter and increases blood flow to areas near the skin where it is cooler.

Individuals with an SCI face impairments in their ability to thermoregulate and can lose control over their sweat glands and blood vessels below the level of the SCI. This means, that the higher the level of injury on the spine, the more of the body is unable to effectively have any vascular or sweating control by the ANS, and the more severe the degree of impairment in thermoregulation. Hence, people living with SCI may have a more difficult time during exercise, as they can not thermoregulate as efficiently compared to uninjured athletes.

What does this mean for the SCI community?

Traditionally, researchers within the SCI community have focused on somatic nervous system impairments that result from SCI. However, as the field progresses and the profound impact on athletic performance from a loss of autonomic control becomes apparent, it is understandable why clinicians and scientists are increasing research into documenting autonomic function after SCI.

For example, one way this novel research is being used is within major events such as the Paralympics where categorizing individuals based on their athletic inhibition is important. Having a greater understanding of how the loss of autonomic control affects athletic performance will aid in this categorization. Moreover, it is also vital for anyone who has experienced an SCI and is interested in taking part in physical activity post-injury. Being able to understand the body’s limits and where these limits are coming from is important, and even opens up the ability for the individual to explore the various treatments available. By product of this, increasing awareness of the autonomic problems that arise from SCI will drive solutions in the future.