The first step involved in the generation of action potential is the depolarization towards threshold. It will be realized that before an action potential starts, the local currents should depolarize an excitable membrane area to threshold.
In the second step, sodium channels are made active and are further accompanied by rapid depolarization. In this step it will be recognized that the sodium activation channels usually open at threshold and cell membranes continue to turn out to be more permeable to sodium ions. The sodium ions generally sprint into the cytoplasm by being impelled by the huge electrochemical gradient. In this site, rapid depolarization begins. The inner membrane immediately changes and now holds more ions that are positive and the potential of the trans-membrane clearly alters to more positive values of +66 mV which are near the potential equilibrium of sodium. In generation of action potential the first two steps are good examples of a positive feedback because it happens that a small depolarization enhances the generation of a bigger depolarization.
In the third step the sodium channels are deactivated and potassium channels are instead activated. Sodium channels commence to close when the potential of the trans-membrane move towards +30 mV. In most cases this step is essentially referred to as the sodium inactivation channel. Potassium channels begin to open up as the sodium gates are inactivated. The chemical and electrical gradients generally tend to good turn the movement of potassium ions away from the cell. Re-polarization starts due to the unexpected positive charges lose that moves the potential of the trans-membrane to the resting levels.
In the fourth step, normal permeability returns. The sodium channels that are regulated by voltage stay to be inactivated up to the point when the membrane clearly re-polarizes to threshold. It will be realized that during this time they recuperate to their ordinary status although they are able to open. As the membranes arrive at their usual resting potential of about -70 mV, the potassium channels that are regulated by the voltage starts to close. During this time, the potassium ions get out of the cell more rapidly resulting to hyper-tension which makes the potential of the trans-membrane to be close to the potassium equilibrium potential that is usually about -90 mV. The potential of the trans-membrane go back to the ordinary resting levels as the potassium channels that are normally regulated by the voltage close. It will be noticed that the membrane gets into a pre-stimulation condition and action power ceases. It is good to note that the relative refractory period begins as the sodium channels recuperate their ordinary resting state and proceeds until the potential of the trans-membrane becomes stable at ordinary resting levels.
An example of monosynaptic reflex is the knee-jerk and patellar reflex. To demonstrate the steps that occur in a monosynaptic reflex we can use the example of a knee-jerk. The following steps occur:
- The muscle spindles normally sense a tap on a tendon that is usually fixed to the kneecap.
- The muscle spindle then creates nerve impulses patterns that are transmitted down the sensory nerve fiber.
- Neurotransmitters are then freed to the motor neutrons by the nerve terminals producing post-synaptic potentials that are excitatory to the body cells and dendrites of the motor neurons.
- Action potentials are then produced by the motor neuron that results to acetylcholine release from its muscles’ terminals.
- The muscle then reacts to the acetylcholine through contracting and depolarizing.
Examples of polysynaptic reflexes include, pulling a leg as one step on a spiky object or when pulls his/her hand after handling a hot stove. To show the steps that occur in polysynaptic reflex we can demonstrate using a case example of when you step on a sharp thorn. The following steps occur:
- The sharp thorn actually arouses pain receptors in the foot.
- This in reality sets in motion a sensory neuron.
- The sensory neurons by and large synapse with the inter-neurons inside the spinal cord making the motor neurons to be stimulated.
- The stimulated motor neurons cause contraction of the flexor muscles.
- The constricting flexor muscle raises the foot out of the thorn.