By Elizabeth A. Murray, Mount St. Joseph University
When it comes to understanding the effect of trauma on our nervous system, what we most need to understand is how it effects our ability to control our skeletal muscles. Since the nerves of the peripheral nervous system most directly stimulate the muscles, the answer to the question of how the nervous system responds to trauma lies in the shared relationship between the cranial and spinal nerves. Read on to find out how.
The Cranial Nerves
The peripheral nerves extend from the two components of the central nervous system or CNS: Cranial nerves come off the brain, and spinal nerves come off the spinal cord. Considering both of these types is important. This is because, in general, cranial nerves govern functions in the head and neck, while spinal nerves control skeletal muscles and receive sensations from the trunk and limbs.
There are 12 pairs of cranial nerves that come directly off the brain and exit the cranial cavity by various holes in the skull, generally called foramina. But some of these passageways are more tube-like and are called canals, while others are more slit-like and are called fissures. Regardless, a few of the cranial nerve pairs are entirely sensory in their function, such as those that relay the special senses of sight and hearing.
Some other cranial nerves are entirely motor, meaning their sole function is to control muscle—whether skeletal muscle, smooth muscle, cardiac muscle, or a combination of those. And still other cranial nerves are both sensory and motor, and cover a myriad of functions, most of which are in the head and neck. But nine of the 12 pairs of cranial nerves govern some of our voluntary skeletal muscles, and all of those muscles are wholly or partly in the head or neck.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Spinal Nerves
On the other hand, there are 31 pairs of spinal nerves that come directly off the spinal cord. They exit the vertebral column by openings formed between the vertebrae, where they are stacked on each other. Because these holes form between adjacent vertebrae, they are known as intervertebral foramina.
There are 24 vertebrae, plus a sacrum and coccyx, sometimes called the tailbone—and spinal nerves also exit from those. The net result is 31 pairs of spinal nerves, which share their naming with their respective vertebrae. There are eight pairs of cervical nerves in the neck region, 12 pairs of thoracic nerves in the area of the rib cage, five pairs of lumbar spinal nerves in the low back, five pairs of sacral nerves that exit holes in the sacrum, and one pair of coccygeal nerves that exit between the sacrum and coccyx.
All 31 of these pairs of spinal nerves are considered mixed, relaying both sensory and motor information.
Risk of Damage
With so many different structures in the nervous system’s pathways, and with both electrical and chemical signals involved, there are many opportunities for disease, trauma, or other pathologies to affect its control over muscles.
Damaged brain regions from a stroke can render voluntary movements impossible, depending on the region that area of the brain controls. Surprisingly, though, if the spinal cord is not damaged, reflexive movements of those regions may still take place.
Damage Limited to Specific Regions
A massive stroke can be devastating, but small areas of brain damage from a stroke can result in only a limited body region being affected. For example, a patient had a mini-stroke during or following a surgery, and yet, the result was only a numb spot on her thumb and another one on her face.
Damage to the spinal cord—such as from a fracture or gunshot wound—could block transmission of voluntary motor pathways that originate in the brain and prevent them from reaching the peripheral nerves. Injuries to the spinal nerves themselves—like from a deep wound on a limb—can block the signals between the spinal cord and the skeletal muscle tissue. Or damage to the muscle itself can cause it to be unresponsive to signals from the nervous system. All complex pathways like this have many levels at which pathology or injury can occur.
Common Questions about the Peripheral Nervous System
There are 12 pairs of cranial nerves that come directly off the brain and exit the cranial cavity by various holes in the skull, generally called foramina. But some of these passageways are more tube-like and are called canals, while others are more slit-like and are called fissures.
There are 31 pairs of spinal nerves that come directly off the spinal cord. They exit the vertebral column by openings formed between the vertebrae, where they are stacked on each other. Because these holes form between adjacent vertebrae, they are known as intervertebral foramina.
Damaged brain regions from a stroke can render voluntary movements impossible, depending on the region that area of the brain controls. Surprisingly, though, if the spinal cord is not damaged, reflexive movements of those regions may still take place.
