Action Potential
Dendrites receive electrochemical pulses from other external stimuli particularly from other neural cells, and therefore, they act as intermediary electrochemical-pulse transmitters. After receiving an electrochemical stimulus, the dendrites become stimulated. The process of dendrite stimulation occurs as a result of either inhibitory or excitatory actions. An inhibitory activity causes the overall effect of the nerve cell to be regulated thus resulting in dormancy of the nerve cell. In contrast, an excitatory action results in increased activity of the nerve cell. Notably, local stimulus in a nerve cell is mainly generated at the cell membrane after an inhibitory or excitatory synapse has occurred.
Polarization in a cell occurs when there is a change in the charge distribution of a cell. In contrary, the depolarization action happens when there is a surge in the net charge of a cell. In other instances, the net charge in a cell may decline just after depolarization; an action referred to as repolarization. Hyperpolarization occurs when a high degree of depolarization occurs suddenly resulting in an action potential. In instances when the charges in a cell are well-distributed and stable, the cell is said to be in a state of resting potential (Martini et al., 2018). Therefore, the generation of an electrochemical stimuli results in the depolarization action, and as such, depolarization is the most fundamental way that neuron-cells convey pulses and signals.
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The pulses in synapses are minute electrochemical voltages generated when there is an influx of the ion concentration within the cell membranes and synapses. Depending on the rate of influx of the ions, local potentials sum up incrementally to reach the threshold potential. Once this threshold is achieved, an action potential is formed in that particular cell. Therefore, an action potential can only be realized under two conditions (Martini et al., 2018). First, a local stimulus has to occur and form a graded potential. Secondly, the sum of the graded potential has to reach the threshold potential.
Voltage-gated ion channels are a type of transmembrane proteins that form ion channels, which are further stimulated when there is an influx in the electrical potential at the cell membrane. In contrast, ligand-gated ion channels are another type of transmembrane proteins that act as a gate to ions that relay impulses. Some of these ions include K + , Na + , and Ca 2+ . The synaptic vesicles are sometimes referred to as the ‘store’ of neurotransmitter vesicles owing to the fact that they contain different types of neurotransmitters (Martini et al., 2018) . Specifically, synaptic vesicles aid in propagating electrochemical impulses in addition to assisting in the transition of materials from the cell through exocytosis.
Neuromuscular Junction
A neuromuscular junction is a transitional zone composed of a chemical synapse. Particularly, it links a muscle fiber with a motor neuron and is sometimes referred to as a myoneural junction. The ends of nerve fibers exist in networks composed of single branches. In a neuromuscular junction, each nerve end connects to a section of a muscle fiber known as an end plate. Between a skeletal muscle and a motor neuron, there is an existent gap known as a synapse that is the basis of the existence of a neuromuscular junction (Martini et al., 2018). Indeed, the neuromuscular junction allows the synaptic transmission to take place. Notably, the action of synaptic transmission is coherent with the functioning of neurotransmitters.
End plates contain a lot of neuron-receptors whose primary function is to receive electrochemical impulses. The transmission of electrochemical signals in the neuromuscular junction occurs in one model. First, an electrochemical signal is relayed from other nerve cells that receive different forms of external or internal physical stimuli. Specifically, the signal is as a result of the action potential. When an action potential is achieved, it means there is a huge flux of charge distribution in the nerve cells. Since the nervous system of animals functions in an action-reaction model, every stimulus is treated as an action and the response accorded to the action is a reaction.
At the neuromuscular junction, synaptic activity begins with the instigation of an action potential. For synaptic transmission to occur, the electrochemical signal encoded through action potential has to reach the presynaptic terminal part of a motor neuron. When it gets to this point, it is relayed to the relative muscle fiber. Consequently, synapses and synaptic vesicles come into play. Action potential triggers the synapses to have an electrochemical imbalance that is further relayed into the subsequent neurotransmitter. Accordingly, neurotransmitters and receptors bind leading to depolarization (Martini et al., 2018). In the end, receptors in the muscle fibers receive pulses that are decoded and implemented as muscle action (Martini et al., 2018). It is important to note that the initial stimulus is treated as a ‘message’ throughout this relaying process. Therefore, while the methods and models of relay change, the nature of the message is maintained.
Skeletal Muscle Contraction
Muscle reaction to a signal or impulse can be through contraction or relaxation, which is determined by the nature of action potential created at the elementary stages of stimuli perception and processing in the nerve cells. While the result of neurotransmitter and receptor binding leads to depolarization, this action leads to the creation of another action potential but on the side of the muscle fiber. Essentially, it is like one athlete passing a baton to another in athletics competition. Therefore, the binding between neurotransmitters and receptors relays the net effect associated with action potential. The induction of action potential causes an ion release through depolarization creating an imbalance in the electrochemical charges of the muscle cells.
Action potential affects membrane permeability. Under normal cell activity, the concentration of Potassium ions is higher on the intracellular fluids of the cell. Sodium ions, on the other hand, have a higher concentration on the extracellular fluids of the cell. As such, the potassium and sodium ions are regulated at the membrane of cells. Calcium ions are also regulated across the cell membrane but are involved in skeletal muscle contraction. Particularly, the induction of action potential triggers the sarcoplasmic reticulum to release Ca 2+ which further initiate the release of Acetylcholine (ACh) (Martini et al., 2018). In the cells, myofibrils contain an organic compound called the troponin. The released Ca 2+ bind with the troponin thus regulating the activity of tropomyosin by limiting cross-bridge binding sites.
By limiting the action that results from tropomyosin, cross-bridges are enabled. The result of this action is the institution of ready-state cross-bridges fundamental to the occurrence of ATP. Once there are sufficient cross-bridges, ATP binds to the cross-bridges. Through this bond, the cross-bridge breaks the actin-myosin bond. The process proceeds as another ATP is produced to bind with the cross-bridge. It is through this series of cycles between the ATP, cross-bridges, and Calcium ions that muscle contractions continue to occur (Martini et al., 2018). It is thus crucial to note that the Calcium ions act as the initiators of skeletal muscle contractions. As such, the process of muscle contraction continues as long as Ca 2+ ions are present in the sarcoplasm. Once the Ca 2+ ions are exhausted, muscle contractions cease.
Reference
Martini, F. H., Ober, W. C., Nath, J. L., Bartholomew, E. F. & Petti, K. F. (2018). Visual Anatomy & Physiology. 3rd Edition; Pearson Publishing.
