Abnormal anatomy of the thoracic outlet causes compression or tension of the vital structures in the thoracic outlet. As a result, patients experience the symptoms of thoracic outlet syndrome. While the anatomy of the thoracic outlet is complex, understanding the anatomy of the thoracic outlet enables understanding of TOS. We can strive to understand the structures that cause compression as well as the structures that become compressed.

Understanding of the anatomy of the thoracic outlet is challenging. We have created videos to help with this challenge, which you can find on our TOS Video page, or on TOS education.org.

We can categorize the essential anatomy of the thoracic outlet into these categories:

Understanding of the anatomy of the thoracic outlet begins with understanding the anatomy of the spine. The spine supports our body weight while allowing flexibility in multiple directions. The brachial plexus nerves arise from the spinal cord, pass through the central canal of the spine, and then exit the spine at multiple levels through neural foramina to enter each thoracic outlet.

The vertebral body is the basic building block of the spine. Vertebral bodies (also known as vertebrae) are stacked one upon the other from the skull to the pelvis. The vertebral bodies support our body weight.

Between each vertebral body lies an intervertebral disc. Each disc is firmly attached to the vertebral body above it and to the body below it. The disc has a fibrous outer layer that stabilizes each vertebral body to the next. The center of each disc contains a jelly-like substance that absorbs impacts from walking, jumping and other movement. The flexible discs allow movement in multiple directions:

The spine comprises five anatomic regions, from top to bottom:

Each region of the spine has a different number of vertebral bodies:

A bony arch arises from the back of each vertebral body. These arches line up to form a flexible bony tunnel, the spinal canal. The spinal canal runs from the base of the skull to the pelvis, and it contains and protects the spinal cord. At each spinal level, a nerve leaves the spinal cord on each side. Each nerve exits the spinal canal through a tunnel between the bony arches to reach the body or extremities. This tunnel is called a neural foramen.

On the back and sides of each arch we see a number of bony levers, which allow for attachments of the numerous muscles that stabilize and move the spine.

A fluid filled sac (the thecal sac) fills the skull and central spinal canal, and contains the brain and spinal cord. Similar to the bony vertebral levels, the spinal cord comprises cervical, thoracic, lumbar, sacral and coccygeal segments. Thus, cervical nerve roots exit the upper spinal cord, and the coccygeal nerve roots exit the distal spinal cord. It should be noted that the levels in the spinal cord do not align with the levels in the bony spine. Therefore, exiting nerve roots travel a variable distance in the thecal sac before exiting at their respective neural foramina.

Nerve fibers are vulnerable to compression or tension as they travel in narrow spaces throughout the body. Within the spinal canal, a bulging disc or herniated disc can press on the spinal cord. A narrowed neural foramen can compress an exiting nerve root. These are examples of nerve entrapment, where compression or tension on a nerve causes symptoms in the territory of that nerve. Neurogenic thoracic outlet syndrome is another, more complex, form of nerve entrapment.

Once a nerve exits its neural foramen, it enters the thoracic outlet. Read more about the anatomy of the thoracic outlet.

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Nerves connect the brain and the body. At its simplest, a nerve can be thought of as a simple wire that transmits information. At a deeper level, however, nerves are complex and varied. A basic understanding of the anatomy of a nerve will help us understand the varied symptoms of TOS.

In general, the human nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system comprises the brain, the brainstem (the lowest and most primitive part of the brain) and the spinal cord. These structures are contained within the fluid-filled thecal sac, which sits inside the skull and spinal canal. The peripheral nervous system comprises all other nerves and nervous system structures.

The peripheral nervous system is divided into two parallel nervous systems: the somatic nervous system and the autonomic nervous system . The somatic nervous system controls voluntary bodily functions, while the autonomic nervous system controls involuntary bodily functions. Voluntary functions include motor functions, such as walking, and conscious sensations, such as feeling light touch against the skin. Involuntary functions include unconscious motor functions like blood pressure regulation, and unconscious sensory regulation, such as pupils constricting in bright light.

The basic building block of a nerve is the nerve cell, the neuron. A neuron is a specialized, electrically excitable cell that receives, processes, and transmits information throughout the body. Each neuron includes a cell body (control center), dendrites (which receive signals), and an axon (which transmits signals). The axon, or nerve fiber, can be extremely long relative to the size of the body. There are many different types of neurons, resulting in a large variety of nerve fibers. The body of each neuron may be located in the brain, the spinal cord, or in a ganglion. A ganglion is a cluster of neuron cell bodies arising outside the central nervous system.

Classification of the different types of peripheral nerve fibers is based on the diameter of the nerve fiber and whether the nerve fiber is myelinated. Larger diameter nerve fibers conduct signals at a higher velocity than smaller diameter nerve fibers. In addition, some nerve fibers are insulated with a myelin sheath. This is a sheath of fatty tissue produced by a support cell adjacent to the nerve fiber. The myelin sheath helps to increase nerve conduction velocity. Somatic motor fibers are usually myelinated. Certain types of somatic sensory fibers and autonomic fibers are myelinated, while others are unmyelinated. All nerve fibers in the central nervous system are myelinated.

Whether the nerve fibers arise in the brain, spinal cord, or peripheral ganglion, they eventually form a peripheral nerve with hundreds or thousands of other nerve fibers. Peripheral nerves almost always include a mix of motor and sensory fibers, somatic and autonomic fibers, large and small, myelinated and unmyelinated fibers. The nerve fibers cluster together in fascicles, and multiple fascicles cluster together within a peripheral nerve.

The somatic nervous system controls voluntary bodily functions, while the autonomic nervous system controls involuntary bodily functions. Voluntary functions include motor functions, such as walking, as well as conscious sensations, such as touching a hot surface or feeling touch against your skin. Involuntary functions include motor functions, such as regulating blood vessels and blood pressure, as well as unconscious sensory regulation, such as the pupils constricting in bright light.

Somatic motor nerves control the voluntary muscles of the body, and allow us to move our bodies at will. Bear in mind that we have both voluntary muscles and involuntary muscles. Voluntary muscles are those we control with conscious thoughts, such as flexing one’s biceps or picking up a cup of coffee. Involuntary muscles are muscles that function on a subconscious level, usually to regulate functions of the body, such as moving food through the gut, or narrowing the pupil when bright light hits it.

Somatic sensory nerves transmit information from different types of receptors throughout the body back to the brain. Examples of sensory nerves include a nerve that senses a pinprick, or one that senses a hot stove. At the end of each sensory nerve is a receptor that is specialized for a specific function. Receptors are specialized for heat, pain, pressure, position sense, and other stimuli.

Autonomic nerves function as a subconscious, primitive nervous system that controls many involuntary functions of the human body. The autonomic nerve fibers control such functions as sweating, regulation of blood pressure, dilation or contraction of the pupils, and the function of the gut. Autonomic nerve fibers include both motor and sensory nerve fibers. Autonomic motor fibers control involuntary muscles. For example, autonomic motor fibers control the muscles in the walls of our blood vessels or bowel, regulate sweating, and control the adrenal glands. We have no conscious control of these functions. Autonomic sensory fibers transmit information about heart rate, body temperature, blood oxygen, or blood pressure back to the central nervous system. We have no direct. conscious sensation of these functions

To sum things up so far, hundreds or thousands of different nerve fibers compose a typical peripheral nerve. Each of these fibers possesses different qualities, and each has a unique function. Compression or tension on the peripheral nerve may affect many different types of nerve fibers, resulting in a variety of symptoms.

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A plexus is a network or cluster of nerves. The brachial plexus is the second largest plexus in the human body. On each side, the brachial plexus passes through the thoracic outlet to reach the arm. The anatomy of the thoracic outlet determines the course of each brachial plexus. After passing through the thoracic outlet, the brachial plexus provides the nerve supply to the upper extremity, including portions of the chest wall and shoulders.

Medical students spend a lot of time memorizing the detailed anatomy of the brachial plexus. Few doctors use such detailed information when diagnosing and treating patients. While it is important to understand the basic structure of the brachial plexus, it is also important to understand its relationship to the surrounding structures of the thoracic outlet.

Each brachial plexus comprises 5 nerve roots: C5, C6, C7, C8 and T1. Once these roots leave their respective neural foramina on each side of the cervical spine, they enter the thoracic outlet. The roots from three trunks. Each of the three trunks splits into two divisions. The divisions rejoin to form cords. Finally, the cords produce five terminal nerves:

The five terminal nerves extend down the arm. There are additional smaller nerves arising from the brachial plexus, which have been excluded for simplicity. It is worth remembering that each of the five major terminal nerves contains a mix of somatic and autonomic nerves, both motor and sensory. Given the complex branching pattern of the brachial plexus, as well as the complex mixture of nerve fibers within each branch, it should be easy to understand why compression or tension on the brachial plexus would produce complex symptoms in neurogenic thoracic outlet syndrome.

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