Nervous system in our body is required for controlling normal functioning of the body. The central and peripheral nervous systems work together to ensure this. While the spine, brain and nerves comprise the former, the later is a vast network of nerves all through the body. The neurons in the peripheral nervous system and be categorized as sensory neurons and motor neurons. The former build the system of nerves called afferent nerves which carry information from the nerve endings and transfer information to the brain and spinal cord.
The motor neurons constitute the efferent nerves, which carry the response signals from the brain and spinal cord back to different targeted body parts for appropriate response.
If a mosquito were to land on one’s arm, the sensory receptors at the nerve endings of the skin will receive the input of the sensation of bite and send this information to the brain and spinal cord. The information is analyzed and a response if formulated. One such response could be to contract muscles of the hand causing a slap to be planted on that part of the skin where the mosquito is sitting. This information is passed through the motor neurons of the peripheral nervous system.
The motor neurons are further classified in to somatic and autonomous neurons. The somatic neurons transfer voluntary instructions to the skeletal muscles permitting the individual to perform tasks like squashing a mosquito, sticking out their tongue and so on. The autonomic neurons transfer automatic commands to muscles and glands all over the body. The brain controls activities like heart rate, breathing rate, sweat secretion, blood pressure, hormonal release and so on.
The autonomic system can be further divided in to sympathetic and parasympathetic nervous systems which act oppositely. If the sympathetic system speeds the rate of the heart, the parasympathetic system will tend to slow it down. If the sympathetic system were to slow down the rate of digestion, the parasympathetic system will fasten it up.
If an individual is frightened, the sympathetic nervous system will trigger release of adrenalin hormones, increase the rate of heart beat and blood pressure, close the blood vessels to the stomach and open up the skeletal muscles, open the air-ways and dilate the pupil. These actions together will prepare the individual for flight or fight reaction. The body will be agile to take up any rapid action. Once the threat is not perceived, the parasympathetic nervous system will reverse these actions and restore the body to a normal resting state.
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Biology Reference: Peripheral Nervous System