Neuron Function Introduction Myelin begins to form on
Neuron Function Introduction Myelin begins to form on axons during the 1. fourteenth week of prenatal development. Yet many of the axons in newborns are not completely myelinated. As a result, an infant’s 2. nervous system cannot function as effectively as that of an older child or adult. Infants’ 3. responses to stimuli are course and undifferentiated, and may involve the whole body. All myelinated axons begin to develop sheaths by the time a child starts to walk, and myelination continues into adolescence. Deficiencies of essential nutrients during developmental years may limit myelin formation, which may impair nervous system function later in life. Why don’t infant’s nervous systems function as well as an older child or adult? By what age should all of a child’s myelinated sheaths begin to develop? What cause could limit myelin formation?
Neuron Function Explicit Instruction Cell Membrane Potential ◦ A cell membrane is usually polarized with excess negative charges on the inside. ◦ Ion distribution is controlled by membrane channel proteins. ◦ The inside of the membrane has excess negative charges creating resting potential. Potential Changes ◦ Gradual or sudden depolarization may occur causing threshold, leading to action potential. Action Potential ◦ Sodium ions rush in and potassium ions rush out of the cell, causing action potential.
Neuron Function Explicit Instruction A nerve impulses are conducted as action potential Impulse Conduction ◦ Unmyelinated fibers conduct impulses over their entire membrane surface ◦ Myelinated fibers conduct impulses from node of Ranvier to node of Ranvier, a phenomenon called saltatory conduction. ◦ Saltatory conduction is many times faster than conduction on unmyelinated neurons All-or-None Response ◦ If a nerve fiber responds at all to a stimulus, it responds completely by conducting an impulse ◦ Greater intensity of stimulation triggers more impulses per second, not stronger impulses
Neuron Function Explicit Instruction The Synapse ◦ The junction between two communicating neurons is called a synapse, where there exists a synaptic cleft between them across which the impulse must be conveyed. ◦ Neurotransmitters are utilized to relay messages across the synaptic cleft in a process called synaptic transmission. ◦ Neurotransmitters may be excitatory or inhibitory. ◦ Over 50 kinds of neurotransmitters are produced in the cytoplasm and stored in the synaptic knobs. ◦ Action potential causes neurotransmitters to be released into the synaptic cleft. ◦ Enzymes rapidly decompose neurotransmitters after their release to prevent continuous stimulation.
Neuron Function Explicit Instruction Impulse processing depends on how neurons are organized. Neurons are organized into neuronal pools in the CNS, receiving input from afferent nerves and processing them according to the characteristics of the pool. ◦ Facilitation occurs when sub-threshold stimuli are received, making them more excitable. ◦ Convergence occurs when impulses are received from more than one nerve fiber, allowing for summate impulses. ◦ Divergence allows for the amplification of an impulse and occurs when a neuronal pool passes the impulse into several output fibers.
Neuron Function Guided Practice THINK DEEPER Excess neuroglial cells can harm health. Fastgrowing gliomas are brain tumors consisting of rapidly-dividing neuroglia (recall that neurons do not divide). Immediately after a spinal cord injury, destruction of neuroglia strips axons of myelin. Subsequent overgrowth of neuroglia forms scars, which impede recovery of function. 1. What is the function of myelination of neurons? 2. Why is damage to myelin harmful? Neurons deprived of oxygen change shape as their nuclei shrink, and they eventually disintegrate. Oxygen deficiency can result from lack of blood flow (ischemia) through nerve tissue, an abnormally low blood oxygen concentration (hypoxemia), or toxins that prevent neurons from using oxygen by blocking aerobic respiration. 3. Why is damage to neurons so dangerous?
Neuron Function Independent Practice 1. Distinguish between a dendrite and an axon. 2. Describe the components of a neuron. 3. Describe how a myelin sheath forms. 4. Explain why axons of peripheral nerves can regenerate, but axons of CNS nerves cannot. 5. Name and describe three groups of neurons based on structure and three groups based on function.
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