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		d. Defects The myelin layer degenerates in multiple sclerosis, causing weakness, lack of coordination and speech and vision disturbance. Multiple sclerosis and Guillain-Barré syndrome, an autoimmune disease that also attacks the myelin layers result in dramatically slower conduction speedof neural impulses. C. Initiation of action potential How is an action potential created? A neuron can be stimulated in a number of different ways. These stimuli are usually graded; that is, they can be of varying strength. An action potential is only generated when the stimulus is strong enough to cause the membrane potential to reach the threshold. 1. Sensory neurons For sensory neurons, a graded potential is generated in the peripheral ends where they encounter the outside world. a. Mechanoreceptors Pressure deforms membrane, openning Na+ ion channel. b. Chemoreceptors Molecule bind to receptor, directly or indirectly controlling ion channel. c. Photoreceptors Light alters membrane protein, signal transduction controls ion channel. d. Electromagnetic receptors Electric charge opens ion channel. e. Thermoreceptors Temperature affects activity of membrane enzyme that controls ion channel. 2. Spontaneous change in the neuron's membrane potential Some neurons produce an action potential spontaneously; the membrane potential rises until it hits the threshold. The speed with which this happens determines the frequency of action potentials. Q. What could this be good for? - This type of neuron often controls rhythmic behaviors such as breathing.