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.