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Download Readings to Accompany “Nerves” Worksheet (adapted from France
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Readings to Accompany “Nerves” Worksheet (adapted from France pp 324-328) Types of Nervous Tissue Nervous tissue is composed of two main cell types: neurons and neuroglial cells. Neurons transmit nerve messages. Neuroglial cells are in direct contact with neurons and often surround them. They serve to insulate, support and protect the neurons. The Neuron The neuron is the functional unit of the nervous system. Neurons possess the characteristics of being able to react when stimulated (irritability) and of being able to pass an impulse on to other neurons (conductivity). While variable in size and shape, most neurons have three parts. Dendrites receive information (from the environment or another cell) and transmit the message to the neuron cell body (AKA soma). The cell body contains the nucleus, mitochondria and other organelles. After reaching the cell body, the message is transmitted down the axon and can then be transmitted to another neuron or to a muscle or gland. Axons can be covered with a substance called “myelin” which greatly increases the transmission speed of impulses. An axon can branch at its end and can thus contact many other cells. The terminal end of an axon is called the terminal bouton (or terminal knob). Most terminal boutons contain neurotransmitters (chemicals involved in the transmission of nerve impulses). Types of Neurons Functionally, there are three types of neurons. Sensory neurons (also called afferent neurons) carry messages from sensory receptors in the periphery (such as those in the skin) to the central nervous system (spinal cord and brain). Motor neurons (efferent neurons) transmit messages from the central nervous system to the muscles or glands. . Interneurons are found in the central nervous system where they connect neurons. The vast majority of neurons in the human body are interneurons (approximately 99%). Nerves A nerve is a group of neuron fibers (axons and dendrites) which is bundled together. A nerve can be strictly a motor nerve (a bundle of motor neurons which stimulates only muscles or glands), a sensory nerve (a bundle of sensory neurons which is associated with only sensory organs) or a mixed nerve (a bundle of both motor and sensory neurons). Structure of a Nerve Two Neurons & Synapse between them Illustration of Neuromuscular Junction (AKA Synapse btn neuron & muscle fiber) Nerve Impulse Transmission & Muscle Fiber Contraction To understand how impulses are carried along nerves or throughout a muscle to cause contraction, we need to learn a little more about membrane excitability. Nerves transmit impulses by movement of electrically charged particles. Neurons have a membrane that separates the cytoplasm inside from the extracellular fluids outside the nerve cell, thereby creating two chemically different areas. Each area has differing amounts of potassium and sodium ions and other charged substances, with the inside of the cell being more negatively charged than the outside when the neuron is in a resting state. When dendrites of a neuron receive sufficient stimulation, the axon hillock of the neuron will transmit that impulse toward the axon. This is the first step in transmitting a stimulus called the action potential. Sodium (Na+) ions will rush into the axon through Na+ channels resulting in a change in the neuron’s charge from negative to positive with respect to the outside of the neuron. This change in polarity is called depolarization. This depolarization will occur sequentially at adjacent regions of the axon (or at nodes of Ranvier in myelinated axons). The depolarization is quickly followed by diffusion of K+ ions out of the neuron to “repolarize” the axon (re-establish a negative charge inside the axon as compared with the outside). The term “diffusion” means the movement of ions from an area of greater concentration to an area of lesser concentration. Following repolarization of the neuron by diffusion of K+, a re-establishment of ion concentrations to that of the resting state of the neuron is achieved by an active transport mechanism called the Na+ / K+ pump. The sodium/potassium pump actively pumps 3 Na+ ions out of the neuron for every 2 K+ ions it pumps back into the neuron. The sodium potassium pump helps return the ion concentrations inside and outside of the neuron back to their original resting states. When the depolarization reaches the terminal bouton of a neuron, Calcium (Ca ++) ions rush into the terminal bouton and cause the release of neurotransmitter into the synaptic cleft. Neurotransmitter will cross the synaptic cleft and bind to receptors on the post-synaptic membrane (which can be a muscle fiber, gland, or another neuron). The neurotransmitter (depending on its type) can either have an inhibitory or excitatory effect on the post-synaptic membrane. If the post-synaptic membrane is a muscle fiber and the neurotransmitter has an excitatory effect on it, Na+ ions will rush into it and down tubes which traverse the muscle fiber called t-tubules. This causes the release of Ca++ ions from another organelle in the muscle fiber called the sarcoplasmic reticulum which is located adjacent to the t-tubules. These Ca++ ions bind to troponin molecules found on thin myofilaments within the muscle fiber. This causes the protein tropomyosin to move off of binding sites on the actin. Myosin heads attach to the binding sites on actin and use energy from ADP + P to “ratchet”, sliding the thin filament and resulting in muscle fiber contraction. This is called a “power stroke”. If ATP is available, it will bind with the myosin head and cause it to detach from the actin molecule. Energy from ATP will be used to “re-cock” the myosin head in preparation for another muscle fiber contraction. Nerve Damage Nerves are fragile and can be damaged by compression, tension, or cutting. Injury to a nerve can stop signals to and from the central nervous system, causing impaired muscle function and loss of (or abnormal) sensation in the injured area. When a nerve is cut, both the nerve and its insulating myelin sheath are disrupted. Compression or tensile injuries can cause nerve fibers to break without damaging the insulatory sheath surrounding the fibers. When nerve fibers are damaged, the end of the fiber distal to the site of injury dies, although the insulation (myelin sheath) stays healthy. The end of the nerve fibers proximal to the injury (and thus closer to the brain and spinal cord) does not die. After some time damaged nerve fibers will begin to heal and may grow down their empty myelin sheath until they reach a muscle, gland or sensory receptor. If both the neurons and the myelin sheath have been disrupted and the nerve sheath is not repaired surgically, the new nerve fibers may grow into a ball at the location of injury, forming a scar or neuroma. A neuroma is a ball-like growth of nerve fibers that can be painful and cause an “electrical” sensation when touched. The treatment for a cut nerve is to sew together the myelin sheath that is around both ends of the nerve. The goal in fixing the nerve is to save the sheath (cover) so that new fibers may heal and function may be regained. Once the myelin sheath is fixed, the nerve generally begins to heal within it. Nerves usually grow one inch every month, depending on the patient’s age and other factors. This means that with an injury to a nerve in the arm above the fingertips, it may take up to a year before feeling returns to the fingertips.