Autonomic dysreflexia?

Question

I know that it is an exagerated response of sympathetic nervous system to a noxious stimulus. But I don't know WHY it is exagerated. Thank you for your help.

Answer 1

Here is a website that explains it:
http://calder.med.miami.edu/pointis/automatic.html

It is basically a spinal reflex that is not counteracted by input from the brain because of the spinal cord lesion.

Autonomic dysreflexia means an over-activity of the Autonomic Nervous System. It can occur when an irritating stimulus is introduced to the body below the level of spinal cord injury, such as an overfull bladder. The stimulus sends nerve impulses to the spinal cord, where they travel upward until they are blocked by the lesion at the level of injury. Since the impulses cannot reach the brain, a reflex is activated that increases activity of the sympathetic portion of autonomic nervous system. This results in spasms and a narrowing of the blood vessels, which causes a rise in the blood pressure. Nerve receptors in the heart and blood vessels detect this rise in blood pressure and send a message to the brain. The brain sends a message to the heart, causing the heartbeat to slow down and the blood vessels above the level of injury to dilate. However, the brain cannot send messages below the level of injury, due to the spinal cord lesion, and therefore the blood pressure cannot be regulated.

Does that help?

Answer 2

eMedicine says:

Pathophysiology: This phenomenon occurs after the phase of spinal shock in which reflexes return. Individuals with injury above the major splanchnic outflow may develop AD. Below the injury, intact peripheral sensory nerves transmit impulses that ascend in the spinothalamic and posterior columns to stimulate sympathetic neurons located in the intermediolateral gray matter of the spinal cord. The inhibitory outflow above the SCI from cerebral vasomotor centers is increased, but it is unable to pass below the block of the SCI. This large sympathetic outflow causes release of various neurotransmitters (norepinephrine, dopamine-b-hydroxylase, dopamine), causing piloerection, skin pallor, and severe vasoconstriction in arterial vasculature. The result is sudden elevation in blood pressure and vasodilation above the level of injury. Patients commonly have a headache caused by vasodilation of pain sensitive intracranial vessels.

Vasomotor brainstem reflexes attempt to lower blood pressure by increasing parasympathetic stimulation to the heart through the vagus nerve to cause compensatory bradycardia. This reflex action cannot compensate for severe vasoconstriction, explained by the Poiseuille formula, where pressure in a tube is affected to the fourth power by change in radius (vasoconstriction) and only linearly by change in flow rate (bradycardia). Parasympathetic nerves prevail above the level of injury, which may be characterized by profuse sweating and vasodilation with skin flushing.

Cameron and colleagues have found that site-directed genetic manipulation of fiber sprouting in the spinal dorsal horns in a cord compression rat model could alter the extent of hyperreflexia after bowel distention, indicating that endogenous spinal cord circuitry/neural sprouting plays a role in the pathophysiology of AD