Nervous tissue may be either gray or white. Gray nervous tissue is unmyelinated, which means it lacks a protective myelin sheath. White nervous tissue is myelinated and has a protective myelin sheath. Nervous tissues are gray or white, depending on the presence of Schwann cells (neurolemmocytes; Netter, 2022). Notches within the white myelinated tissue—called the nodes of Ranvier (also called nerve fiber nodes)— 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 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 (Netter, 2022). Nerves have connective tissue as well (see Figure 4). 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 (Netter, 2022). Figure 4: Nerve Tissue
Figure 5: Brachial Plexus Nerves
Spinal nerves that emanate from spinal column bones C4 through T1 innervate the superior limbs. These nerves stem from the inferior portion of the brachial plexus—a bundle of nerves innervating the 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 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 ● Palmaris longus
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, and endo - implies units within a structure (Netter, 2022). Nerves are classified by functionality: ● Sensory : Sensory nerves conduct electrochemical neural signals from the peripheral nerves into the central nervous system (brain and spinal cord). ● Motor : Motor nerves conduct electrochemical neural signals from the central nervous system toward the peripheral nerves. ● 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 (Netter, 2022).
● Flexor digitorum superficialis ● Flexor digitorum profundus ● Flexor pollicis longus ● Pronator quadratus ● Thenar musculature ● First and second lumbrical muscles
The mnemonic LOAF helps one remember innervation of the hand supplied by the median nerve: L = lumbricals 1 and 2; O = opponens pollicis; A = abductor pollicis brevis; F = flexor pollicis brevis. Further dissecting the median nerve, we observe as this nerve travels between the flexor digitorum profundus and the flexor digitorum superficialis muscles. This branches into two main segments: The anterior interosseous nerve , which supplies the deeper anterior antebrachial muscles, and the palmar cutaneous nerv e, which supplies the skin of the middle and lateral palm. When the median nerve travels through the carpal tunnel arch, the nerve splits into two other branches: The recurrent branch , which innervates the four thenar muscles, and the palmar digital branch , which innervates the palmar surface and fingertips of the lateral 3.5 digits, as well as the lateral two lumbrical muscle units.
Page 118
Book Code: MFL1227
EliteLearning.com/Massage-Therapy
Powered by FlippingBook