Outside brain and cord, nerves and ganglia still run the body
The peripheral nervous system is everything neural beyond brain and spinal cord—nerves and ganglia linking center to edge in bilateral animals. It splits into somatic and autonomic realms, each with sensory and motor traffic, so voluntary muscle and automatic organs share related cabling.
In bilateral animals the nervous system has two parts. The central system is the brain and spinal cord; the peripheral system is every nerve and ganglion outside them, acting as the relay that connects the centre to limbs and organs. That outer network lacks the armour the centre enjoys. It has no skull or vertebral column around it and no blood-brain barrier screening out toxins and pathogens, which leaves it more exposed to harm.
It splits into a somatic division and an autonomic division, and each has sensory and motor sides. The somatic side is under voluntary control, sending commands to muscles and carrying senses such as taste and both fine and gross touch back to the cord and brain. The autonomic side regulates itself, steering smooth muscle, glands, heart rate and digestion without conscious effort.
In the head and neck, cranial nerves carry sensation. There are twelve, and ten of them start in the brainstem, mostly serving head structures. Two stand out: the vagus nerve collects signals from organs of the chest and abdomen, and the accessory nerve drives the sternocleidomastoid and trapezius muscles, which are not confined to the head. Oddly, the optic nerve and the smell nerve with its epithelium are counted as central rather than peripheral, because of how they develop; the optic nerve is really a tract of the diencephalon. All cranial ganglia, though, are peripheral.
Below the head, spinal nerves take over. They leave the cord as a web of interlinked roots, a plexus, that regroups into individual nerves. Humans have 31 pairs, grouped by region as 8 in the neck, 12 in the chest, 5 lumbar, 5 sacral and a single coccygeal pair. Each is named for its neighbouring vertebra. In the neck the roots exit above their vertebra, so the one between skull and first vertebra is C1, while from the thoracic region down they exit below, which leaves the root between C7 and T1 needing a special label.
Source: Peripheral nervous system
