The sympathetic nervous system prepares the body for action in stressful or dangerous situations: the 'fight or flight' response. It is most active during exercise, stress, fear and excitement.Its preganglionic neurones leave the spinal cord from the thoracic and lumbar regions. They are short and synapse in ganglia that lie close to the spinal cord, in a chain on either side of it. The postganglionic neurones are long and run from these ganglia to the effectors. At most effectors the postganglionic neurone releases noradrenaline.
Effects of sympathetic stimulation include:• heart rate and force of contraction increase (through the accelerator nerve to the sinoatrial node)
• bronchioles dilate, so more air reaches the alveoli
• pupils dilate
• arterioles supplying the gut and skin constrict, so more blood goes to the skeletal muscles
• digestion and peristalsis slow down and less saliva is produced
• sweating increases
• glycogen in the liver is converted to glucose, raising blood glucoseThe sympathetic system also stimulates the adrenal medulla to release the hormone adrenaline into the blood, which reinforces and prolongs these effects throughout the body.Together these changes deliver more oxygen and glucose to the muscles and brain, so that the body can respond quickly and powerfully to a threat. Once the threat has passed, the parasympathetic system restores resting conditions.
Effects of sympathetic stimulation include:• heart rate and force of contraction increase (through the accelerator nerve to the sinoatrial node)• bronchioles dilate, so more air reaches the alveoli
• pupils dilate
• arterioles supplying the gut and skin constrict, so more blood goes to the skeletal muscles
• digestion and peristalsis slow down and less saliva is produced
• sweating increases
• glycogen in the liver is converted to glucose, raising blood glucoseThe sympathetic system also stimulates the adrenal medulla to release the hormone adrenaline into the blood, which reinforces and prolongs these effects throughout the body.Together these changes deliver more oxygen and glucose to the muscles and brain, so that the body can respond quickly and powerfully to a threat. Once the threat has passed, the parasympathetic system restores resting conditions.