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Know about your autonomic nervous system

How the autonomic nervous system normally works

The autonomic nervous system is the part of the nervous system that supplies the internal organs, such as blood vessels, stomach, intestine, liver, kidneys, bladder and genitals.

The autonomic nervous system has two main divisions:

  • Sympathetic
  • Parasympathetic

After the autonomic nervous system receives information about the body and external environment, it responds by stimulating body processes, usually through the sympathetic division, or inhibiting them, commonly through the parasympathetic division.

The autonomic nervous system controls internal body processes:

  • Blood pressure
  • Heart and breathing rates
  • Body temperature
  • Digestion
  • Metabolism which affects body weight
  • The balance of water and electrolytes, for example, sodium and calcium
  • The production of body fluids, for example, saliva, sweat and tears
  • Urination
  • Defecation
  • Sexual response.

Many organs are controlled primarily by either the sympathetic or the parasympathetic division. Sometimes the two divisions have opposite effects on the same organ. For example, the sympathetic division increases blood pressure while the parasympathetic division decreases it. Overall, the two divisions work together to ensure the body responds appropriately to different situations.

What is normal blood pressure

Normal baseline blood pressure for most people is: 120/80 mmHg.

Blood pressure changes depending on what a person is doing and other factors, including:

  • Smoking
  • Being overweight or obese
  • Lack of physical activity
  • Too much salt in the diet
  • Too much alcohol consumption (more than 1-2 drinks per day)
  • Stress
  • Older age
  • Genetics.
Blood pressure (BP) category
Systolic BP (upper number)
Less than 120
Less than 80
Less than 80
High blood pressure (Stage 1)
High blood pressure (Stage 2)
140 or higher
90 or higher
Blood pressure crisis
Higher than 180
Higher than 120
What does research tell you?

People with a higher level (cervical and upper thoracic) spinal cord injury are more likely to experience a lower blood pressure (by around 20 mmHg) than those with lower level or incomplete spinal cord injury during the first month of rehabilitation, which may persist in the longer term. 

Effects of a spinal cord injury on the autonomic nervous system

Autonomic dysreflexia is a medical condition that causes a rapid rise in blood pressure of people with a spinal cord injury, at or above the T6 spinal level. The condition is rare below the T6 spinal level.

Autonomic dysreflexia occurs when there is a problem in your body below the level of your spinal cord injury causing irritation and overactivity in the part of the nervous system responsible for controlling blood pressure. In other words, it is a response by the body when pain and discomfort are experienced, although your spinal cord injury prevents that message from getting through to your brain.

Autonomic dysreflexia is a medical emergency as the high blood pressure can lead to a stroke, fitting or death. It therefore calls for immediate action to find and treat the cause of the Autonomic dysreflexia.

Do you know?

People with a complete spinal cord injury have a much higher incidence of autonomic dysreflexia: 91% with a complete injury versus 27% with an incomplete injury.

Irritation below the level of the spinal cord injury
Signal reaches the injured spinal cord resulting in massive constriction of the blood vessels below the level of injury
Leads to very high blood pressure
Brain detects a sudden rise in blood pressure and attempts to control the rise by sending signals via spinal cord to blood vessels, which are blocked
Signals from brain cannot get through below the level of injury, but a different nerve (called the vagus) tries to compensate
This results in a lowering of the heart rate down to 40-50 beats per minute

Causes of autonomic dysreflexia

Autonomic dysreflexia is most commonly triggered by a cause related to either bladder or bowel, but can be triggered by other stimuli.

Bladder causes

  • Overfull bladder from kinked or blocked catheter or full leg bag
  • Urinary tract infection
  • Kidney stones
  • Procedures such as catheter change or tests where the bladder is distended.

Bowel and abdominal causes

  • Constipation
  • Irritation from rectal examination or
    enema insertion
  • Inflamed haemorrhoids
  • Stomach ulcers, gallstones or appendicitis.

Other common causes include:

Skin causes

  • Excess pressure from contact with hard or sharp objects
  • Pressure injuries (sores)
  • Ingrown toenails
  • Burns
  • Insect bites.

Sexual activity

In males

  • Genital stimulation, especially with a vibrator
  • Orgasm (ejaculation)
  • Problems with the testicles (e.g., pressure on testicles).

In females

  • Menstruation or labour
  • Infections of vagina or uterus.
What does research tell you?

One of the most common causes of autonomic dysreflexia is bladder distension, which alone can account for up to 85% of episodes.

female hand writing results of a blood pressure test with blood test monitor in the background