Organs without Dual Innervation

Although most organs are innervated by both sympathetic and parasympathetic nerves, some—including the adrenal medulla, ar-rector pili muscles, sweat glands, and most blood vessels—receive only sympathetic innervation. In these cases, regulation is achieved by increases or decreases in the tone (firing rate) of the sympathetic fibers. Constriction of cutaneous blood vessels, for example, is produced by increased sympathetic activity that stimulates alpha-adrenergic receptors, and vasodilation results from decreased sympathetic nerve stimulation.

The sympathoadrenal system is required for nonshivering thermogenesis: animals deprived of their sympathetic system and adrenals cannot tolerate cold stress. The sympathetic system itself is required for proper thermoregulatory responses to heat. In a hot room, for example, decreased sympathetic stimulation produces dilation of the blood vessels in the skin, which increases cutaneous blood flow and provides better heat radiation. During exercise, by contrast, sympathetic activity increases, causing constriction of the blood vessels in the skin of the limbs and stimulation of sweat glands in the trunk.

Autonomic dysreflexia, a serious condition producing rapid elevations in blood pressure that can lead to stroke (cerebrovascular accident), occurs in 85% of people with quadriplegia and others with spinal cord lesions above the sixth thoracic level. Lesions to the spinal cord first produce the symptoms of spinal shock, characterized by the loss of both skeletal muscle and autonomic reflexes. After a period of time, both types of reflexes return in an exaggerated state. The skeletal muscles may become spastic in the absence of higher inhibitory influences, and the visceral organs experience denervation hypersensitivity. Patients in this condition have difficulty emptying their urinary bladders and often must be catheterized.

Noxious stimuli, such as overdistension of the urinary bladder, can result in reflex activation of the sympathetic nerves below the spinal cord lesion. This produces goose bumps, cold skin, and vasoconstriction in the regions served by the spinal cord below the level of the lesion. The rise in blood pressure resulting from this vasoconstriction activates pressure receptors that transmit impulses along sensory nerve fibers to the medulla oblongata. In response to this sensory input, the medulla directs a reflex slowing of the heart and vasodilation. Since descending impulses are blocked by the spinal lesion, however, the skin above the lesion is warm and moist (due to vasodilation and sweat gland secretion), but it is cold below the level of spinal cord damage.

The sweat glands in the trunk secrete a watery fluid in response to cholinergic sympathetic stimulation. Evaporation of this dilute sweat helps to cool the body. The sweat glands also secrete a chemical called bradykinin in response to sympathetic stimulation. Bradykinin stimulates dilation of the surface blood vessels near the sweat glands, helping to radiate some heat despite the fact that other cutaneous blood vessels are constricted. At the conclusion of exercise, sympathetic stimulation is reduced, causing cutaneous blood vessels to dilate. This increases blood flow to the skin, which helps to eliminate metabolic heat. Notice that all of these thermoregulatory responses are achieved without the direct involvement of the parasympathetic system.

236

Chapter Nine

Table 9.8 Effects Resulting from Sensory Input from Afferent Fibers in the Vagus, Which Transmit This Input to Centers in the Medulla Oblongata

Organs Type of Receptors Reflex Effects

Lungs Stretch receptors Type J receptors

Aorta Chemoreceptors

Baroreceptors Heart Atrial stretch receptors

Stretch receptors in ventricles Gastrointestinal tract Stretch receptors

Further inhalation inhibited; increase in cardiac rate and vasodilation stimulated Stimulated by pulmonary congestion—produces feelings of breathlessness and causes a reflex fall in cardiac rate and blood pressure Stimulated by rise in CO2 and fall in O2—produces increased rate of breathing, rise in heart rate, and vasoconstriction Stimulated by increased blood pressure—produces a reflex decrease in heart rate Antidiuretic hormone secretion inhibited, thus increasing the volume of urine excreted Produces a reflex decrease in heart rate and vasodilation Feelings of satiety, discomfort, and pain

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • HALLIE
    Why sweat gland receive only sympathetic innervation?
    7 years ago
  • baldassarre genovesi
    What are the organs listed without dual innervation?
    6 years ago
  • CHRISTI
    Which organ of the body is dual innervated?
    5 years ago
  • Jon
    How is blood pressure regulated with the absense of dual innervation?
    4 years ago
  • maria grazia
    How do organs without dual innervation work?
    4 years ago
  • Hal
    How do organ without dual innervation work?
    3 years ago
  • Aurora
    WHAT HAPPEND WITHOUT DUAL INNERVATION?
    2 years ago
  • Carl
    Which organ receive only sympathetic innervations?
    2 years ago
  • GUNDOBAD
    Which human organ does not have dual innervation?
    10 months ago
  • luisa
    Which blood vessels are innervated by the sympathetic system?
    6 months ago
  • jenni
    How do organs that do not have dual innervations maintain autonomic tone?
    5 months ago
  • john
    How do organs without dual innervation maintain autonomic tone?
    5 months ago
  • clare bruce
    Are most organs innervated by sympathetic nerves?
    4 months ago
  • Hayley
    Which organ of body don't receive dual innervation?
    3 months ago
  • camryn morrison
    Which organ(s) receive(s) only sympathetic innervation?
    2 months ago
  • viviano
    What are innervated by sympathetic nervous system only?
    2 months ago
  • lleyton
    Which organs lack parasympathetic innervation?
    1 month ago
  • Hamid
    Which organs dont have dual innervation?
    1 month ago

Post a comment