By Elizabeth A. Murray, Mount St. Joseph University
Let’s understand the muscles in the torso—chest, thorax, and abdomen—and their functions with the case study of a serious injury. A young man was badly injured in a motor vehicle accident, and his sixth and seventh cervical vertebrae were crushed. His spinal cord was essentially severed at the level of this fracture.
Probing into the Case Study
As a result of this terrible accident, will he be able to take a deep breath? In fact, will he be able to breathe on his own at all, or will he require a respirator to breathe for him? Additionally, how will this spinal cord damage affect his sensations?
As we approach this clinical correlation, it must be remembered that, except for those supplied by cranial nerves, the spinal cord is a conduit that relays motor information from the brain to most muscles of the body. In addition, the spinal cord also relays sensory information to the brain from all areas of skin below the head.
Those sensations actually occur in the brain; in other words, when we touch something, we don’t actually feel it in our hand, we feel it in our brain. But it’s the receptors in the hand, the nerves traveling up the upper limb to the spinal cord, and the pathways in the spinal cord up to the brain, that allow for that feeling. It must be remembered that the particular patches of skin supplied by a given spinal cord level are known as dermatomes.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Effect on Normal Breathing
Do you remember the muscle of normal, quiet breathing? The nerve that supplied the diaphragm was the phrenic nerve off the cervical plexus. It is a combination of the ventral rami of cervical nerves C3, C4, and C5. This is a vital nerve that travels from the cervical plexus down through the thorax, by clinging to the pericardial sac around the heart. Together, the right and left phrenic nerves supply the respiratory diaphragm.
So, this young man will be able to breathe on his own—which is a major quality of life difference from being on a respirator.
Effect on Deep Breathing
But how about taking a deep breath? For this we need to consider which muscles allow for deep inspiration. The external intercostals, which are supplied by thoracic nerves.
So, the answer is, sadly, no—signals from his brain will no longer be able to cross the area of serious damage in the cervical region of the spinal cord to reach the thoracic nerves. This means impulses from the brain will not stimulate any of the intercostal muscles, so this man will be unable to take or expel a deep breath.
Effect on Sensations
Additionally, will this injury affect the man’s sensations? Well, intercostal nerves not only control muscles, they also supply the dermatomes of the anterolateral thoracic and abdominal walls. For generalities, the T4 intercostal nerve supplies skin of the nipple level, the ventral ramus of T10 relays sensations from the skin of the umbilical level, and the skin of the groin is supplied by the ventral ramus of the first lumbar nerve.
The intervening nerves can sort of be penciled between those. What about levels above the intercostal nerves? The cutaneous nerves of the cervical plexus, like the great auricular and supraclavicular nerves come from C2, C3, and C4, so these areas, supplied by nerves above the C6-C7 level of injury, will retain sensation.
The brachial plexus is composed of the ventral rami of C5 to T1, so those levels create the dermatomes on the shoulders and upper limbs. Ultimately, this man will have some loss of sensation in his upper limbs, but some will remain. However, he will also lose motor control over some muscles of and within the upper limb, because the brachial plexus will be affected. And, given it was the spinal cord damaged at the C6 and C7 levels, the dorsal rami of spinal nerves below the level of injury will also be affected, not just the ventral rami.
The case study described a young man whose C6 and C7 vertebrae were crushed in an accident, injuring the related regions of the spinal cord. The questions posed asked whether he would be able to do normal, quiet breathing or would be on a respirator, whether he could take a deep breath beyond normal tidal volume, and if sensations from his skin would be affected?
So, this young man sustained a devastating injury: He will have limited sensations from his upper limbs and will feel nothing from the chest on down. He will be considered a quadriplegic, since his upper limbs will have limited and impaired movements, and his trunk and lower limb muscles will also lose their stimulation from the brain.
However, due to the diaphragm’s supply by the phrenic nerve off the cervical plexus, he will be able to breathe on his own, and hopefully can learn to use a motorized wheelchair and perhaps even to drive.
Common Questions about Understanding Torso Muscles
The nerve that supply the diaphragm is the phrenic nerve off the cervical plexus. It is a combination of the ventral rami of cervical nerves C3, C4, and C5. This is a vital nerve that travels from the cervical plexus down through the thorax, by clinging to the pericardial sac around the heart. Together, the right and left phrenic nerves supply the respiratory diaphragm.
Except for those supplied by cranial nerves, the spinal cord is a conduit that relays motor information from the brain to most muscles of the body. In addition, the spinal cord also relays sensory information to the brain from all areas of skin below the head.
Particular patches of skin supplied by a given spinal cord level are known as dermatomes.