Anatomy of a nerve
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 systems: the somatic nervous system and the autonomic nervous system (read more). 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.