The Peripheral Nervous System

Free Response

What are the main differences between the sympathetic and parasympathetic branches of the autonomic nervous system?


The sympathetic nervous system prepares the body for “fight or flight,” whereas the parasympathetic nervous system allows the body to “rest and digest.” Sympathetic neurons release norepinephrine onto target organs; parasympathetic neurons release acetylcholine. Sympathetic neuron cell bodies are located in sympathetic ganglia. Parasympathetic neuron cell bodies are located in the brainstem and sacral spinal cord. Activation of the sympathetic nervous system increases heart rate and blood pressure and decreases digestion and blood flow to the skin. Activation of the parasympathetic nervous system decreases heart rate and blood pressure and increases digestion and blood flow to the skin.

What are the main functions of the sensory-somatic nervous system?


The sensory-somatic nervous system transmits sensory information from the skin, muscles, and sensory organs to the CNS. It also sends motor commands from the CNS to the muscles, causing them to contract.

Describe how the sensory-somatic nervous system reacts by reflex to a person touching something hot. How does this allow for rapid responses in potentially dangerous situations?


A person’s skin comes into contact with a hot object, and the high temperature is recognized by the thermoreceptors of a sensory neuron. The signal is relayed to the spinal cord, and sent to a motor neuron. The motor neuron relays the signal to its axon, and produces acetylcholine to contract the muscle that will pull the person away from the hot object. By connecting the sensory and motor neurons in the spinal cord (instead of integrating the signal in the brain) the body can respond faster.

Scientists have suggested that the autonomic nervous system is not well-adapted to modern human life. How is the sympathetic nervous system an ineffective response to the everyday challenges faced by modern humans?


Many events in modern human life are not physical dangers; instead they are events we think of as “stress.” Finding the money to pay your student loans or being nervous before a test still activate the sympathetic nervous system, but these situations do not require the fight-or-flight response to survive.