Due MIDNIGHT: Respond/Critique two discussion posts.

            “An action potential is generated within the initial segment and propagated along an axon of the neuron” (McKinley, 2013). Most action potentials are located along the sarcolemma of muscle cells. When action potential is propagated along the plasma membrane it is called the all or none law (McKinley, 2013).  This means that the axon is decreasing intensity and only the subthreshold value is being reached (McKinley, 2013).  Action potential happens because voltage-gated channels respond to a voltage change (McKinley, 2013). The minimum voltage change is called the threshold value and any voltage below that is called subthreshold value. There are three steps when action potential occurs. The three major events of action potential are depolarization, repolarization, and hyperpolarization.  

            In action potential there is a propagation called a nerve signal (McKinley, 2013). An unstimulated axon usually has a resting membrane potential of -70 mV, also known as membrane voltage. The graded potentials then reach the axon hillock and are added to one another (McKinley, 2013). Then the first step of action potential occurs. This is called depolarization. Depolarization causes a positive charge in the membrane potential (McKinley, 2013). It occurs in the threshold (-55 mV) and voltage-gated Na+ channels open (McKinley, 2013). The Na+ enters quickly and reverses the polarity from negative to positive (+30 mV) and causes the depolarization (McKinley, 2013).

            The next event that occurs in action potential is repolarization. Repolarization involves returning a neuron to its RMP, or resting membrane potential (McKinley, 2013). It then goes through voltage-gated K+ channels, which are located in the plasma membrane (McKinley, 2013). Repolarization usually occurs because the voltage-gated Na+ channels close and the voltage-gated K+ channels open. So K+ finally exits the cell and goes in the IF and returns the neuron to its RMP (-70 mV) (McKinley, 2013). The propagation of repolarization occurs when voltage-gated K+ channels open along the length of the axon (McKinley, 2013). Finally polarity is reversed from positive to negative (-70 mV) and causes the repolarization (McKinley, 2013).

            The final event in action potential is hyperpolarization. Hyperpolarization is when everything returns to the resting membrane potential. This usually happens when the voltage-gated K+ channels stay open longer than needed (McKinley, 2013). The time reaches the membrane potential. During this time, the membrane potential is less than the resting membrane potential, therefore this makes everything return to normal (-70 mV) and causes hyperpolarization (McKinley, 2013). After that the “voltage-gated K+ channels close and the plasma membrane returns to resting conditions by activity of Na+/K+ pumps” (McKinley, 2013).

               Multiple sclerosis is a disease that affects the brain and spinal cord (WebMD, 2012). It results in loss of muscle control, vision, balance, and sensation (WebMD, 2012). Multiple sclerosis affects the nerves of the brain and spinal cord (WebMD, 2012). These are usually damaged by one's own immune system, which a condition called autoimmune disease (WebMD, 2012). Multiple sclerosis affects the action potential events because the neurons are not able to send message to the brain to function properly (WebMD, 2012). Therefore, the action potential events cannot occur properly as they should.

            All of these events sum up what action potential is and what it does. First, it depolarizes by opening Na+ channels and reversing the polarity to negative to positive (McKinley, 2013). Then, it repolarizes by closing Na+ channels and opening K+ channels and reversing back to positive to negative (McKinley, 2013). And lastly, it hyperpolarizes by reaching the resting membrane potential and returning back to normal (McKinley, 2013). These are all the events of action potential.

 

Works Cited

Mckinley, Michael P., Valerie D. O'Loughlin, and Theresa S. Bidle. Anatomy & Physiology. New York: McGraw-Hill, 2013. 459-65. Print.

WebMD. What Is Multiple Sclerosis? N.p.: Richard Senelick, 2012. Web. 15 Nov. 2013.