The responses of the nervous system to stimuli may be voluntary or involuntary. Voluntary responses are mainly under conscious control, but some voluntary movements, such as walking, require less conscious attention. There are two types of involuntary response, autonomic and reflex. Autonomic responses regulate the body’s internal environment. Reflexes mainly affect those muscles that are normally under voluntary control.
All voluntary activities involve the brain, which sends out the motor impulses that control movement. These motor signals are initiated by thought and most also involve a response to sensory stimuli. For example, people use sight and sense of position to help them coordinate the action of walking.
The autonomic nervous system controls the body’s internal environment without conscious intervention and helps to regulate vital functions, such as blood pressure. The two types of autonomic nerves, sympathetic and parasympathetic, have opposing effects but balance each other most of the time. At certain times, such as during stress or exercise, one system dominates.
Sympathetic and parasympathetic nerves each produce different responses in a particular organ. The sympathetic responses prepare the body to cope at times of stress. Parasympathetic responses help conserve or restore energy.
|Organ Affected||Sympathetic response||Parasympathetic response|
|Eyes||Pupils dilate||Pupils constrict|
|Lungs||Bronchial tubes dilate||Bronchial tubes constrict|
|Heart||Rate and strength of heartbeat increase||Rate and strength of heartbeat decrease|
|Stomach||Enzymes decrease||Enzymes increase|
|Liver||Releases glucose||Stores glucose|
A reflex is an involuntary response to a stimulus, such as withdrawing your hand from a hot surface before you become aware of the heat. Most reflexes are processed in the spine, although some, such as blinking, are processed in the brain. In a spinal reflex, the stimulus signal travels along a sensory nerve to the spinal cord, and a response signal travels back by means of a motor nerve.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.