NERVE ANATOMY
To better understand how carpal tunnel signs and symptoms manifest, a basic understanding of the anatomy and physiology of nerve tissue will aid our efforts. All nerves are composed of cells called neurons . Remember that a neuron is a single nerve cell; a nerve is a bundle of neurons wrapped together. All neurons have three components that regulate nerve signal conductivity and protect anatomical structure from damage. These three parts are depicted in Figure 3: 1. Cell body : The main portion of the cell that integrates electrical messages. Integration refers to the processing of the chemical pieces of the messages received. This portion contains the nucleus and the supporting cellular elements, which include Nissl bodies which are a site of protein synthesis within the cell body. A neuron damaged at the cell body will not regenerate. 2. Dendrite: An appendage-like portion that conveys an electrical message into the cell body. The terminal branches of an adjacent axon will communicate with neighboring dendrites. Dendrite receptors receive chemical messages; a polarity switch creates a conductive electrical signal to take these messages into the cell body. A neuron damaged at the dendrite will not regenerate. 3. Axon: The tail-like portion that conducts an electrical message away from the cell body. Polarity switches guide the chemical message, down the axonal branch and toward the terminal portion. Gateways on the synaptic terminals send out the chemical messages toward the adjacent tissues. Axons vary in length from a few millimeters in the brain to several inches in the upper and lower limbs. Figure 3: Three Parts of the Neuron The axons of myelinated neuronal tissue are covered by a lipid-based structure called the myelin sheath . This sheath is a protective wrapping that conducts electrical signals properly and prevents damage due to compression or tension forces. A neuron damaged at the axon with preservation of the cell body may slowly regenerate, approximately a millimeter per month. Nervous tissue may be either gray or white. Gray nervous tissue is unmyelinated, which means it is lacking a myelin protective sheath. White nervous tissue is myelinated, and possesses a myelin protective sheath. Nervous tissues are gray or white, depending on the presence of Schwann cells, or neurolemmocytes. Notches within the white myelinated tissue – called the Nodes of Ranvier (also called nerve fiber nodes ) – will expedite nerve signals by facilitating a “trap door” mechanism by which signals may skip entire axonal segments. The opening of one “trap door” notch allows for a signal to temporarily leave the axon. It is still retained by the endoneurium tissue, however, until it enters a distal “trap door” notch of the axon before its terminal portion. Nerves are composed of connective tissue as well: These tissues both bind neurons into bundles and surround individual neurons. Epineurium, a dense connective tissue, wraps an entire nerve. Perineurium, also a dense connective tissue, segments neuron cells into bundles (called fasciculi ) within the nerve. Endoneurium is a loose connective tissue that surrounds individual neurons.
Figure 4: Nerve Tissue
This connective tissue arrangement mirrors the same connective tissue orientation of muscle: Epimysium a dense connective tissue wrapping that surrounds an entire individual muscle. Perimysium is a dense connective tissue that segments muscle cells into bundles, called fasciculi , within the muscle. Endomysium is a loose connective tissue that surrounds individual muscle fibers. Highlighting this similarity allows one to more easily learn the common language of anatomy. The prefix “epi” implies the top of an entire structure; “peri” means surrounding a portion bundle. Further, “endo” implies units within a structure. Nerves are classified by functionality: 1. Sensory: Sensory nerves conduct electrochemical neural signals from the peripheral nerves into the central nervous system (brain and spinal cord). 2. Motor: Motor nerves conduct electrochemical neural signals from the central nervous system toward the peripheral nerves. 3. Mixed: Mixed nerves carry both sensory and motor neurons. Compartment units may send messages both toward and away from the central nervous system. Nerve conductivity occurs by two processes: The slower process occurs in gray nervous tissue and allows nerve conduction at a rate of 5 to 10 miles per hour. The faster process, called saltatory conduction, occurs only in white nervous tissue. Saltatory conduction is a highly efficient function and conducts messages quickly throughout the body. Messages may travel up to 250 miles per hour within myelinated tissue. Neuronal speeds attributed to Structure and Function of Human Anatomy, Steiner Education Group. Spinal nerves that emanate from spinal column bones C4 through T1 will innervate the superior limbs. These nerves stem from the inferior portion of the brachial plexus – a bundle of nerves innervating musculature of the superior limb and upper torso. As illustrated in Figure 5, the median nerve is a convergence of the lateral and medial cords of the brachial plexus unit. Nerve signal interference along either the lateral and/or medial cord pathways can cause intermittent disruptions of signals along the median nerve. This interference can be witnessed along the spinal column or along the tracks of nerve traveling through the axilla, as well as along the anterior superior limb. Here are a few mnemonics students may use to learn the subdivisions associated with the brachial plexus:* ● “Read that Dastardly Cadaver Book.” ● “Rugby Teams Drink Cold Beer.” ● “Reach to Drink Cold Beer.” *(R = Roots; T = Trunks; D = Divisions; C = Cords; B = Branches.) Branching from the brachial plexus, the median nerve descends distally within the superior limb bilaterally. The median nerve initially travels lateral to the brachial artery that crosses medially just proximal to the elbow (cubital fossa) region. This nerve innervates the following muscles: ● Pronator teres. ● Flexor carpi radialis.
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Book Code: MLA1224
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