The idea of introducing an electric current into your system may seem distressing. However, electrical shock is commonly used to treat patients experiencing heart conditions such as cardiac arrest. Recently, external electrical stimulation has been substituted for gym time and is used to produce "fake" stimulation of the nerves innervating muscles. This often produces the same effects that may result from an (honest) heavy-duty muscular training program. But why would someone carry a battery and some electrodes with him or even implant these devices into his body to receive these electrical stimulations?

One major reason may be to treat the consequences of spinal cord injuries (SCI). SCI can drastically change people's lives; the psychological and socialc impact of such injuries is often devastating. Some of the general disabling conditions associated with SCI are permanent paralysis of the limbs, chronic pain, muscular atrophy, loss of voluntary control over bladder and bowel, inability to produce erection, ejaculation and infertility. The consequences of SCI depend on the specific nature of the injury and its location along the spinal cord.

Bladder dysfunction associated with spinal cord injuries

The loss of genitourinary function - the dysfunction of the genital and urinary organs - is one of the most important results of SCI. Specifically, neurogenic bladder, often associated with SCI, results from damage to the nerves controlling bladder function. A lesion above the sacral level (see Figure 1) of the spinal cord, which may or may not involve the brain, could affect how the bladder stores and empties urine. These upper motor lesions are most commonly seen after cerebrovascular accidents (stroke), spinal cord injuries, neurological diseases such as multiple sclerosis, and also infections.

Until recently, the main consequences of spinal cord injury were confinement to a wheelchair and a lifetime of medical help. Treatment options for SCI used to be limited; the available care was highly unsatisfactory and frustrating due to its limited success, unwanted side effects and high costs. However, this has changed since recent advances in neuroscience have drawn considerable attention to research into SCI and have made significantly better treatment and rehabilitation options available.

Figure 1 - Spinal cord showing the divisions and specific nerves coming out of each divisions (Click to view enlarged image) Courtesy of Gerard J. Tortora (1999) Principles of Human Anatomy (eight edition) John Willy & Sons Inc. , New York

Figure 2 - Spinal cord connection of nerves to muscle groups of the human body (Click to view enlarged image) Courtesy of Gerard J. Tortora (1999) Principles of Human Anatomy (eight edition) John Willy & Sons Inc. , New York

The coordinated action of bladder muscles, also known as detrusor muscles, and external and internal sphincters are essential to the process of urination. In order to urinate, the detrusor muscles must contract, while muscles of the external and internal sphincters are relaxed to allow the outflow of the urine (see Figure 3).

The neurons innervating the smooth muscles of the detrusor and the internal sphincter are mainly under involuntary control of the autonomic nervous system, whereas the neural activity at the skeletal muscles of the external sphincter is monitored by the voluntary control of the somatic nervous system versus the autonomic nervous system regulates involuntary actions of the smooth muscles, heart and glands.

The two divisions of the autonomic nervous system are the sympathetic and parasympathetic nervous systems. The nerves of the sympathetic nervous system originates in the thoracic and lumbar regions of the spinal cord and are often recruited during stress situations, such as fear, excitement, vigorous physical activity, etc. The sympathetic nervous system tends to reduce digestive secretion, speed up the heart rate, dilate the pupils and enhance contraction of the smooth muscles along the blood vessels, decreasing blood vessel diameter and increasing blood pressure.

A branch of the sympathetic nervous system originating from T11-T12 (see Figure 1 and 2) and mainly innervating the bladder neck increases bladder storage capacity. Damage to this branch often results in urinary incontinence - the inability to store urine and lack of voluntary control over the loss of the urine. In infants less than 2 years old, urinary incontinence is normal because the neurons to the external urethral sphincter are not fully developed to allow voluntary contraction of the sphincter.

Figure 3 - Structure of the bladder (Click to view enlarged image) Courtesy of Gerard J. Tortora (1999) Principles of Human Anatomy (eight edition) John Willy & Sons Inc. , New York

The parasympathetic nervous system originates in the brain stem and lower part of the spinal cord. In general, this system inhibits or opposes the physiological effects of the sympathetic nervous system, intending to stimulate digestive secretion, slow down the heart rate, constrict the pupils and dilate blood vessels.

A branch of the parasympathetic nerve supply originates from the sacral cord at S2-S4 (see Figure 1 and 2) and travels to the bladder, governing the contraction of the smooth muscles of the detrusor. Activation of this branch of parasympathetic nerve promotes bladder emptying. Damage to this branch often leads to urinary retention - the inability to empty the bladder.

Spinal cord injuries can affect the parasympathetic supply to the detrusor muscles or the sympathetic supply to the bladder neck as well as somatic nerve supply to the external urethral sphincter.

Application of functional electrical stimulation to bladder dysfunction

Based on experimental work in baboons, G.S. Brindley approached the problem by developing a stimulation system using electrodes, placed on the bilateral S2-S4 sacral anterior nerve roots. The Brindley Vocare Bladder System is now used in combination with posterior rhizotomy and has shown good results in management of neurogenic dysfunction of the bladder.

"The Brindley bladder stimulator delivers intermittent stimulation to the anterior sacral roots," report H.E. van der Aa H.E. and colleagues.

"The stimulus parameters can be adjusted and set specifically for individual use. Its primary purpose is to improve bladder emptying, thereby to eliminate urinary infection and to preserve kidney function. It also assists in defecation and enables male patients to have a sustained full erection."

The Brindley Bladder System consists of four main components:

• The external RF-Control Unit: This unit is the only part that is placed externally. The unit sends radio waves across the surface of the skin to the stimulator that is implanted inside the abdominal cavity.

  There are three primary control options to manipulate urination, defecation, and to produce erection when needed.

• Surgically implanted Receiver-Stimulator: This component is implanted in the abdominal area. The stimulator receives the radio frequency stimulation from the transmitter and generates electrical impulses that are conducted through the leads to the electrodes and then to the nerves controlling bladder function.
  
• Surgically implanted leads: Leads or wires are used to conduct the electrical impulses from the stimulator to the electrodes attached to the nerves. The wires are about 50 cm long.
• Surgically implanted electrodes: Electrodes are mostly made of silicone and soft conductors fabricated in a way to reduce trauma and compression of the nerve. The most common type of electrodes is cuff-like and is placed around the specific nerves that need to be stimulated. The average length for the electrodes used to stimulate bladder nerves is 1-2 cm.
  
  A major challenge in the use of external electrical current to establish controlled urination is the differential responses of the bladder muscles and the uretheral sphinter muscles to these stimulations.
  
  "In the past, efforts to restore bladder control using electrical stimulation have been unsuccessful due to the fact that nerve roots that supply the sphincter muscles and the bladder muscles contract simultaneously upon electrical stimulation, thereby counter acting each other," said Brindley in 1986 and 1991 reports.
  
  In comparison to the nerves innervating the bladder, the neurons to the external sphincter muscles have a lower threshold to produce action potentials that generate muscle contraction. This means whenever electrical current is utilized to produce contraction of the bladder muscles (detrusor muscles), the muscles of the external sphincter are also stimulated, and then contraction of these muscles prevents the outflow of the urine.
  
  To allow the outflow of urine, the external sphincter muscles must relax completely while the bladder muscles are contracting, to 'force' the volume of urine from the internal cavity. If the sphincter is not allowed to relax, not only can urine not exit the bladder, but dangerously high pressure may result inside the bladder that could lead to serious and irreversible damage to the kidneys.
  
  Fortunately, researchers have established a trick to use electric current to stimulate the bladder muscles and at the same time allow relaxation of the sphincter muscles. The muscle fibers that constitute the sphincter are mainly "fast twitch" muscle fibers, producing relatively high amounts of force, but also fatigue easily and relax very quickly. In contrast, the fibers that compose the detrusor muscles are "slow twitch" and initiate contraction quite gradually and have the capacity to maintain force for a longer period of time without becoming fatigued. The trick is to deliver short bursts of high frequency electrical pulses generated by the stimulator to the sacral nerve root that is coming out of the spinal cord at the sacral regions and is innervating most of the muscles involved in the urinary system.
  
  Intermittent stimulation bursts to the anterior sacral roots can be timed in a way that the peaks of contraction of the sphincter and bladder are out of phase; the contraction of the two don't occur at the same time.While the sphincter relaxes rapidly at the end of each stimulus burst, the pressure in the bladder is still sufficiently high to produce urination.
  
  In more simple words, after the pulses reach the urinary system, the first fibers that are stimulated to contract are the muscle fibers of the sphincter. Yet they are also the ones to relax first while the muscle fibers of the bladder are still in the state of contraction. This allows the outflow of urine in between the bursts of stimulation. The Brindley bladder stimulator takes advantage of this differential contraction and relaxation characteristics of the sphincter and bladder wall muscles. Using this method has enabled patients with loss of voluntary bladder control to utilize electric stimulation effectively to regain significant control over their bladder function.