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Function: Voluntary and Involuntary Responses

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.

Voluntary responses

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.

Voluntary pathways

The sensory impulses that trigger voluntary responses are dealt with in many parts of the brain.

Nerve–muscle junction

At this junction, a nerve fibre transmits signals to produce a response from muscle fibres.

Autonomic responses

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.

Autonomic pathways

Information collected by internal receptors travels along sensory nerves to the spinal cord and the brain stem for processing. Sympathetic and parasympathetic response signals have separate pathways.

Two types of responses

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.

Divisions of the autonomic system

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

Reflexes

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.

Spinal reflex pathway

Spinal reflexes involve the simplest nerve pathways: the sensory and motor neurons are directly linked together in the spinal cord.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

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