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Explain, in a sequential fashion, the steps involved in the initiation and propagation of an action potential including all neurotransmitters and ions involved in the process.
An action potential is first initiated when an action potential from an adjacent neuron. This signal arrives in the presynaptic terminal and causes the voltage-gated calcium ion channels that change the permeability of the cell membrane in that terminal. This allows increased access by calcium ions through that membrane and causes a diffusion of calcium ions into the body of the neural cell. In turn, this causes synaptic vesicles within the neuron to release acetylcholine, the primary neurotransmitter of the action potential, into the presynaptic cleft. Once there, the acetylcholine diffuses across the synaptic cleft and binds to its neurotransmitter receptors on the adjacent postsynaptic neural cell. This activates the receptors and opens the ligand-gated sodium ion channels on the cell membrane. Sodium ions diffuse into the cell through these open ion channels and cause the membrane potential to become more positive. When the action potential threshold is reached, the postsynaptic action potential is generated and propagated, travelling to the next cell.
McGraw-Hill. (2006). Human Anatomy. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__chemical_synapse__quiz_1_.html
Describe the path of a somatic sensory impulse from the receptor back to the effector. Include the components of the sectional organization of the spinal cord.
An effector is the muscle or gland that the motor neurons act upon. A receptor receives somatic stimuli that initiates a chain reaction that transmits the nerve signal up the nervous system until it reaches the brain. From the receptor, the somatic sensory impulse is propagated through successive action potentials travelling towards the central nervous system. It then passes into the nerve fibers that are attached to the spinal cord. Each spinal nerve is composed of nerve fibers based on its related muscles and skin that it develops from, in this case dependent on the source of the sensory impulse, the effector tissue. The spinal cord is organized by sections that are defined by being connected to one spinal nerve that is connected to that part of the spinal cord. The somatic sensory impulse travels up the spinal nerve into the spinal cord via the dorsal root. The nerve impulses then travel up the spinal cord and into the brain where the sensory information can be processed. If necessary, a signal can be sent back to the effector tissue via the motor neural pathway, through the spinal cord, ventral root, and into the nerves.
Dafny, N. (n.d.). Chapter 3: Anatomy of the Spinal Cord. Neuroscience Online. Retrieved from: http://neuroscience.uth.tmc.edu/s2/chapter03.html
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