The PNS has three basic functions: (1) conveying motor commands to all voluntary striated muscles in the body; (2) carrying sensory information about the external world and the body to the brain and spinal cord (except visual information: the optic nerves, which convey information from the retina to the brain, are in the PNS); and (3) carrying sensory information about the external world and the body to the brain and spinal cord (except visual information: the optic nerves, which convey information from the retina to the brain, are in the PNS). The autonomic nervous system (ANS), which controls involuntary muscle action, is discussed in detail below. For now, it's enough to know that the PNS and its branches send signals to and receive signals from the brain and spinal cord that control many aspects of our lives: when we breathe, blink, sweat, or vomit; when our heart beats or tears flow; whether our digestive system works properly; and so forth.
The PNS consists of two main parts: the cranial nerve cells located in the brain and the spinal cord; and the ganglia, large collections of nerve cell bodies found mainly in the head and neck. There are eight cranial nerves and they are called "cranial" because they arise from structures within the skull. The other part of the PNS includes the sympathetic nervous system and the parasympathetic nervous system. The major difference between these two parts of the PNS is that the sympathetic nervous system prepares the body for action by activating certain organs and glands before a fight or flight response. The parasympathetic nervous system does the opposite by relaxing the body's muscles after action or during rest periods.
The nervous system is responsible for three major functions: sensory input, information processing, and motor output. Sensory receptor neurons in the PNS respond to physical stimuli in our surroundings, such as touch or temperature, and transmit messages to the CNS about the condition of the body and the external environment. The CNS processes these sensations and transmits them to different parts of the body where they are interpreted into actions that will be performed. For example, when you touch a hot stove your hand will quickly move away from it. This is your brain interpreting the message from your sensory receptors and producing a response. The part of the brain that interprets sensory information is called the cerebral cortex.
The ENS controls all of the involuntary muscles of the body. It is made up of two subsystems: the autonomic nervous system and the somatic nervous system. The parasympathetic division of the autonomic nervous system is thought to be responsible for maintaining homeostasis by preparing the body for action or reaction. It does this by causing constriction of blood vessels and increases in heart rate and blood pressure if necessary. The sympathetic division of the autonomic nervous system is believed to be responsible for arousing the body's response to danger by causing dilation of blood vessels and decreases in heart rate and blood pressure if necessary. It does this by releasing hormones such as adrenaline which cause increased heart rate and blood flow to vital organs like the brain and lungs.