The peripheral nervous system can be divided into a somatic and an autonomic division, which are part of the
somatic nervous system and the
autonomic nervous system, respectively. The somatic nervous system is under voluntary control, and transmits signals from the brain to end organs such as
muscles. The
sensory nervous system is part of the somatic nervous system and transmits signals from senses such as
taste and touch (including fine touch and gross touch) to the spinal cord and brain. The autonomic nervous system is a "self-regulating" system which influences the function of organs outside voluntary control, such as the
heart rate, or the functions of the
digestive system.
Somatic nervous system The
somatic nervous system includes the
sensory nervous system (ex. the
somatosensory system) and consists of sensory nerves and somatic nerves, and many nerves which hold both functions. In the
head and
neck,
cranial nerves carry somatosensory data. There are twelve cranial nerves, ten of which originate from the
brainstem, and mainly control the functions of the anatomic structures of the head with some exceptions. One unique cranial nerve is the
vagus nerve, which receives sensory information from organs in the
thorax and
abdomen. The other unique cranial nerve is the
accessory nerve which is responsible for innervating the
sternocleidomastoid and
trapezius muscles, neither of which are located exclusively in the head. For the rest of the body,
spinal nerves are responsible for somatosensory information. These arise from the
spinal cord. Usually these arise as a web ("plexus") of interconnected nerves roots that arrange to form single nerves. These nerves control the functions of the rest of the body. In humans, there are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. These nerve roots are named according to the spinal vertebrata which they are adjacent to. In the cervical region, the spinal nerve roots come out
above the corresponding vertebrae (i.e., nerve root between the skull and 1st cervical vertebrae is called spinal nerve C1). From the thoracic region to the coccygeal region, the spinal nerve roots come out
below the corresponding vertebrae. This method creates a problem when naming the spinal nerve root between C7 and T1 (so it is called spinal nerve root C8). In the lumbar and sacral region, the spinal nerve roots travel within the dural sac and they travel below the level of L2 as the cauda equina.
Cervical spinal nerves (C1–C4) The first 4 cervical spinal nerves, C1 through C4, split and recombine to produce a variety of nerves that serve the neck and back of head. Spinal nerve C1 is called the
suboccipital nerve, which provides motor innervation to muscles at the base of the
skull. C2 and C3 form many of the nerves of the neck, providing both sensory and motor control. These include the
greater occipital nerve, which provides sensation to the back of the head, the
lesser occipital nerve, which provides sensation to the area behind the
ears, the
greater auricular nerve and the
lesser auricular nerve. The
phrenic nerve is a nerve essential for our survival which arises from nerve roots C3, C4 and C5. It supplies the
thoracic diaphragm, enabling
breathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible.
Brachial plexus (C5–T1) The last four cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1, combine to form the
brachial plexus, or
plexus brachialis, a tangled array of nerves, splitting, combining and recombining, to form the nerves that subserve the upper-limb and upper back. Although the brachial plexus may appear tangled, it is highly organized and predictable, with little variation between people. See
brachial plexus injuries.
Lumbosacral plexus (L1–Co1) The
anterior divisions of the
lumbar nerves,
sacral nerves, and
coccygeal nerve form the
lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the
twelfth thoracic. For descriptive purposes this plexus is usually divided into three parts: •
lumbar plexus •
sacral plexus •
pudendal plexus Autonomic nervous system The
autonomic nervous system (ANS) controls involuntary responses to regulate physiological functions. The brain and spinal cord of the
central nervous system are connected with organs that have smooth muscle or cardiac muscle, such as the heart, bladder, and other cardiac, exocrine, and endocrine related organs, by ganglionic neurons. Located only around the digestive tract, this system allows for local control without input from the sympathetic or the parasympathetic branches, though it can still receive and respond to signals from the rest of the body. The enteric system is responsible for various functions related to gastrointestinal system. == Disease ==