Sympathetic Effects on Target Organs

Main purpose of sympathetic effects on target cells

Ensures survival and maintains homeostasis during physical or emotional stress.

Sympathetic effect on heart rate and blood pressure

Increases heart rate and blood pressure.

Sympathetic effect on cardiac muscle

Norepinephrine binds to β1 receptors and opens ion channels that increase the rate and force of cardiac contractions.

Purpose of increased cardiac activity

Increases blood flow to tissues and increases blood pressure.

Sympathetic effect on blood vessels of digestive, urinary, and integumentary systems

Causes vasoconstriction.

Vasoconstriction

Narrowing of blood vessels.

Receptor causing vasoconstriction in digestive, urinary, and integumentary vessels

Norepinephrine binds to α1 receptors.

Purpose of vasoconstriction during sympathetic activity

Diverts blood toward skeletal and cardiac muscle.

Sympathetic effect on digestive and urinary blood flow

Decreases blood flow, slowing digestion and urine formation.

Sympathetic effect on skin blood flow

Reduced blood flow to the skin may cause a person to appear paler.

Sympathetic effect on bronchioles

Causes bronchodilation, increasing airway diameter and the amount of oxygen inhaled with each breath.

Receptor involved in bronchodilation

Norepinephrine binds to β2 receptors and relaxes bronchiolar smooth muscle.

Sympathetic effect on blood vessels supplying skeletal and cardiac muscle

Causes vasodilation, increasing blood flow to these muscles.

Vasodilation

Relaxation and widening of blood vessels.

Receptor causing vasodilation of vessels supplying skeletal and cardiac muscle

Norepinephrine binds to β2 receptors.

Sympathetic effect on overall blood pressure during muscle vasodilation

Overall blood pressure is minimally affected because other blood vessels are simultaneously vasoconstricted.

Sympathetic effect on digestive and urinary sphincters

Causes the sphincter muscles to contract.

Effect of sympathetic sphincter contraction

Makes emptying the bowel and bladder more difficult.

Sympathetic effect on digestive tract smooth muscle

Norepinephrine binds to β3 receptors and causes relaxation.

Effect of β3 activation in digestive tract

Decreases movement of food and slows digestion.

Sympathetic effect on pupils

Causes pupil dilation.

Receptor involved in sympathetic pupil dilation

Norepinephrine binds to α1 receptors on the dilator pupillae muscles.

Dilator pupillae response

Contracts to dilate the pupil and allow more light into the eye.

Sympathetic effect on most exocrine glands

Constriction of their blood vessels decreases secretion from most glands, with sweat glands being an exception.

Sympathetic effect on cellular metabolic rate

Increases availability of fuels needed to produce the greater amounts of ATP required during sympathetic activity.

Sympathetic effect on adipocytes

Norepinephrine binds to β3 receptors and causes breakdown of lipids, releasing free fatty acids into the bloodstream.

Sympathetic effect on liver cells

Norepinephrine binds to β2 receptors and stimulates glucose release from glycogen and synthesis of glucose from other precursors.

Sympathetic effect on pancreatic cells

Norepinephrine binds to β2 receptors and stimulates glucagon release.

Effect of sympathetic glucagon release

Increases glucose concentration in the blood.

Sympathetic effect on sweat glands

Postganglionic sympathetic neurons release ACh, increasing sweat gland secretion.

Receptor on sympathetic sweat glands

ACh binds to muscarinic receptors.

Why sweat glands are a sympathetic exception

Their postganglionic sympathetic neurons release ACh rather than the norepinephrine released by most sympathetic postganglionic neurons.

Adrenal medulla

Consists of modified sympathetic postganglionic neurons and therefore functions as a sympathetic ganglion.

Neurotransmitter released onto adrenal medulla cells

Preganglionic sympathetic neurons release ACh.

Adrenal medulla receptor

ACh binds to nicotinic receptors on adrenal medulla cells.

Response of adrenal medulla to ACh

Releases epinephrine and norepinephrine into the bloodstream.

Adrenal medulla hormone proportions

Produces about 80% epinephrine and 20% norepinephrine.

Purpose of adrenal medulla hormones

Epinephrine and norepinephrine in the blood prolong sympathetic effects and reach cells not directly innervated by sympathetic neurons.

Sympathetic effect on mental alertness

Enhances mental alertness by increasing neuronal activity in association areas of the cerebral cortex.

Sympathetic effect on blood clotting

Increases the blood's tendency to clot, which may help if injury occurs during fight or flight.

Sympathetic effect on skeletal muscle strength

May temporarily increase tension generated during skeletal muscle contraction, contributing to an “adrenaline rush.”

Sympathetic effect on arrector pili muscles

Causes contraction of arrector pili muscles, producing goose bumps.

Sympathetic effect on male reproductive ducts

Causes ejaculation of semen through effects on smooth muscle of the male reproductive ducts.

Overall sympathetic digestive effect

Decreases digestive activity by reducing digestive blood flow, relaxing digestive tract smooth muscle, and contracting digestive sphincters.

Overall sympathetic respiratory effect

Dilates bronchioles to increase airflow and oxygen intake.

Overall sympathetic cardiovascular effect

Increases heart rate and force while redistributing blood toward skeletal and cardiac muscle.

Overall sympathetic metabolic effect

Mobilizes stored fuels by increasing free fatty acids and blood glucose.