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Classification of Peripheral Nervous System

The spine has 31 pairs of nerves, the cranium has 12 pairs of nerves, and the autonomic nervous system has 12 pairs of nerves.
The peripheral nervous system consists of the following components: The spine has 31 pairs of nerves, the cranium has 12 pairs of nerves, and the autonomic nervous system has 12 pairs of nerves. The peripheral nervous system is mainly made up of sensory nerves that send afferent signals from sensory organs to the brain, and motor nerves that send efferent signals to effector organs, e.g., skeletal muscles, smooth muscles, and glands.

Spinal nerves
Each vertebral foramen contains 31 pairs of spinal nerves that emerge from the vertebral canal and then pass into the spinal canal. Eight cervical vertebrae, 12 thoracic vertebrae, and five lumbar vertebrae plus five sacral vertebrae, and one coccygeal vertebra constitute the vertebral column.

Cervical vertebrae only have seven bones, but there are 8 nerves in the cervical area. As the vertebral canal is divided between the occipital bone and the atlas, the first pair leaves between the occipital bone and the atlas, and the eighth leaves below the last cervical vertebra. Thereafter, the nerves that lie above each vertebra are given both their name and number. Located at the 1st lumbar vertebra, the motor nerve travels along with the nerve roots and into the intervertebral canal in the subarachnoid space, forming a sheaf of nerves similar to a horse's tail. A nerve's exit comes from the vertebral canal wherever it is going, depending on its destination, either lumbar, sacral, or coccygeal.

Nerve roots
Through the intervertebral foramina, the spinal nerves emerge from the spinal cord on both sides. Hence, every nerve is made up of motor (anterior) and sensory (posterior) roots and is referred to as a mixed nerve. The sympathetic nervous system contributes to the parasympathetic nervous system in the form of preganglionic fibers in the upper back and upper lumbar regions of the spine (L1 and L2).

A frontal root of the spinal cord contains motor nerve fibers, which are axons coming from the lower motor neurons of the spinal cord's anterior column, and sympathetic nerve fibers, which are axons of cells from the lateral columns of the spinal cord. Sensory nerve fibers make up the posterior nerve root. The dorsal root ganglion (posterior root ganglion) makes up a cluster of cell bodies located just outside the spinal cord. The spinal cord receives sensory impulses through these ganglia. Each nerve is associated with a section of skin known as a dermatome. Immediately following their exit from the spine, the nerve roots are covered by dura and arachnoid tissue. In the mixed spinal nerve, the roots terminate before joining together. There is no covering of the pia mater on the nerve roots.

The spinal nerves split immediately after emerging from the intervertebral foramen, into ramus communicans, rami acuminate, and rami interiors. In the autonomic nervous system, the rami communications are among the sympathetic neurons. Several smaller branches of the posterior rami provide skin, muscles, and vessels to areas on the head, neck, and trunk near the back. It supplies the neck, trunk, and upper and lower limbs on both the anterior and lateral sides.

There are large masses of nerves in the cervical, lumbar, and sacral regions, such as plexuses, in which the fibers of nerves are regrouped and rearranged before the nerves supply skin, muscles, bones, and joints of the individual areas. Consequently, damage to one spinal nerve does not lead to loss of function of any particular area of these structures, since more than one spinal nerve supplies them. A plexus does not form in the anterior rami in the thoracic region. On either side of the vertebral column, large networks of mixed nerves are formed. Specifically, the cervical plexus, brachial plexus, lumbar plexus, sacral plexus, and coccygeal plexus are involved in the human body.

Thoracic nerves
There are no thoracic plexuses formed from thoracic nerves. They are divided into 12 pairs, the first 11 of which are abdominal nerves. The stems reach the intercostal muscles via the ribs supplying them and the skin surrounding them, which is why they are called intercostal stems. There are twelve pairs of subcostal nerves. In addition to supplying the posterior and anterior abdominal walls with muscles and skin, the 7th to the 12th thoracic nerves travel through the shoulders.

12 pairs of cranial nerves extend from the brain's inferior surface in which some are sensory, some motor, and some combine the two. Names generally indicate function or distribution. The connections between neurons are ordered from anterior to posterior in the brain according to their position in the cerebral cortex. They consist of the following:
  1. Olfactory: sensory
  2. Optic: sensory
  3. Oculomotor: motor
  4. Trochlear: motor
  5. Trigeminal: mixed
  6. Abducent: motor
  7. Facial: mixed
  8. Vestibulocochlear (auditory): sensory
  9. Glossopharyngeal: mixed
  10. Vagus: mixed
  11. Accessory: motor
  12. Hypoglossal: motor.
Autonomic nervous system
Involuntary or autonomic functions are called autonomic because they originate below the brain and in the neural system. Even though stimulation occurs involuntarily, individuals may be aware of its effects, such as increased heart rate. Among the effector organs of autonomic activity are smooth muscle, for example, changes in airway diameter or blood vessel diameter cardiac muscle, for example, changes in heart rate and force glands, for example, changes in gastrointestinal secretion.

The autonomic nervous system has different types of nerves (motors), which originate in the brain and emerge between the midbrain and the sacral region of the spinal cord. The nerve fibers of these organs are often found in the same nerve sheath used by peripheral nerves. An autonomic nervous system consists of the sympathetic (thoracolumbar outflow) and parasympathetic (craniosacral outflow) divisions. Both structurally and functionally, the two divisions differ. Involuntary functions are normally balanced by working in opposition to each other. Parasympathetic nerve signals are dominant when we are at rest, while sympathetic are dominant when we are under stress.

The central nervous system and the effector organs are connected by two efferent neurons in each division. There are two types of neurons: preganglionic and postganglionic. Preganglionic neurons possess a cell body in the brain or spinal cord. Postganglionic neurons synapse with the axon terminals of autonomic ganglions outside the central nervous system. An effector organ receives messages from the postganglionic neuron.
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