Autonomic Nervous System and Spinal Cord Injury

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Anatomy of the Autonomic Nervous System (ANS)

There are two main functions of the autonomic nervous system:

1. Regulating visceral functions
2. Maintaining homeostasis within the human body

Sympathetic Nervous System (SNS)

Sympathetic Nervous System (SNS) arises from the thoracolumbar regions of the spinal cord. The intermediolateral horns of the spinal cord is the location of the preganglionic cell bodies. The sympathetic nerves run parallel to the spinal cord on both sides of the vertebral column.

The functions of the SNS include:

• "Fight or flight" response: blood pressure and heart rate increases, glucose is poured into the bloodstream, and gastrointestinal peristalsis ceases.
• Innervating the tissues in almost every organ system by fibres from the SNS and providing physiological regulation over diverse body processes including pupil diameter, gut motility (movement), and urinary output.
• Preparing the body for physical activity, a whole-body reaction affecting many organ systems throughout the body to redirect oxygen-rich blood to areas of the body needed during intense physical demand.

Parasympathetic Nervous System (PNS)

Craniosacral axis is a starting point for the preganglionic cell bodies of the parasympathetic nervous system (PNS). It is characterised by long efferent preganglionic fibres, and short post-ganglionic fibers to effector organs. It is smaller than SNS and it innervates only the head, the viscera, and the external genitalia.

The functions of the PNS include:

• Promoting the rest and digest process
• Lowering heart rate and blood pressure
• Gastrointestinal peristalsis or digestion restarting

ANS and Spinal Cord Injury (SCI)

After a spinal cord injury, all three subsystems of the autonomic nervous system are affected due to their anatomical location, loss of supraspinal influence, and sustained responses to afferent stimuli. The pathophysiological responses from the ANS that contribute to the comorbidities and mortality of SCI include the following systems:

• Cardiovascular due to parasympathetic dominance with weakening influence of the sympathetic nervous system in cervical and high thoracic levels of spinal cord injury. The following responses may occur:
  • Low resting arterial blood pressure
  • Postural hypotension
  • Autonomic dysreflexia (Acute hypertension)
  • Bradycardia or arrhythmia
• Thermoregulatory as a result of sympathetic nervous system subsiding which inhibits sweating below the level of injury in high thoracic and cervical SCI. When body attempts to dissipate heat, excess sweating may be present. It can cause the following pathologies:
  • Poikilothermia
    • Absorbing an encompassing temperature as a result of inability to regulate core body temperature
  • Quad fever (idiopathic hyperpyrexia)
  • Exercise-induced fever
  • Hyperhidrosis or hypohidrosis
• Respiratory following dominance of the parasympathetic system over weakened supraspinal sympathetic drive. High cervical and thoracic spinal cord injury causes intercostal and abdominal muscle paralysis, which leads to neurogenic restrictive lung disease which results in the following:
  • Bronchiolar constriction
  • Hyper-reactive airways
  • Increased mucus secretion
• Gastrointestinal as a result of ENS impairment. ENS is influenced by SNS and PNS. PNS influence can lead to:
  • Acute and chronic increased in gastric acid secretions
  • High rates of biliary sludge, cholelithiasis and cholecystitis
  • Increased transit time at the distal colon
  • Reflex colorectal contractions
    • Constipation or bowel incontinence
• Genitourinary due to increased uninhibited activation of the sympathetic and parasympathetic systems together with the somatic nervous system, responsible for bladder storage and emptying. The results of the genitourinary system impairment are:
  • Bladder and bowel dysfunction
    • High bladder pressures, and probable vesicoureteral reflux associated with hydroureter, hydronephrosis and urinary incontinence
  • Impaired sexual function affecting arousal, ejaculation, and orgasm
  • Problems during pregnancy, labour, and breastfeeding

The pathophysiological responses from the ANS can also be present during procedures or when pain or injury occurs. The examples include:

• Functional electrical stimulation

• Surgery
• Invasive investigational procedures such as urodynamic studies.
• Stretches
• Fractures,
• UTIs.

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