Option 2.12: How does the human body use electricity?
The cells of the human body are specialised in order to conduct and utilise electric currents for many essential functions. Electrical impulses are so significant because they allow rapid responses to occur in the body; and rapid communication between sensory and applicatory systems alike. Primarily, the central nervous system that governs the relationship between our movements, responses and agency over our body’s actions operates via electric signals induced by action potentials. The depolarisation and repolarisation of our cells in order to transmit a signal can be modelled by a simple capacitor circuit. The circulatory system also relies on the electrical signals of the heart. Many scientific advances have been made towards better understanding and harnessing the electricity within the human body - pacemakers and cochlear implants are designed to artificially stimulate the heart or auditory nerves, while ECGs are essential for biomedical diagnosis to determine quality of heart function and defibrillators used to restart if it fails. Understanding the capacity of the body to carry electrical signals has allowed us to make novel advancements in medicine and technology to aid in many issues, and often improve the wellbeing of individuals.
The role of our nervous system as an essential body system relies on the property of polarisation in cells, allowing the production of an action potential in nerve cells. The key capacity of particles to freely move in response to electrical forces is referred to as their mobility, or electrical conductivity. Similar to the way metal valence electrons are free to carry an electric current, the dissolved ions that exist within human’s bodies (including Na+ , K+ , Ca2+, Mg2+, PO4 3– and Cl–) enable electric charge to be transported through the body. Fundamentally, all human cells are polarised - in an “unstimulated state,...slightly negatively charged on the inside of the cell and slightly positively charged on the outside of the cell.” (Pentland et al., 2019) This is because the lipid bilayer surrounding all cells controls diffusion across the membrane, and causes a large concentration difference of ions between the inner and outer environments. Because of the separation of charge, and the fact that the opposing charges attract, a negative potential difference exists between the inside and the outside of the cell.
Nerve cells, or neurons, are specially designed to transmit electrical impulses. The dendrites at one end receive signals, integrate them and communicate the signals to target cells to enact a response. An action potential is produced inside a neuron in response to a stimulus through the action of voltage gated channels producing an electrical event by depolarising, hyperpolarising and repolarising the nerve cell very quickly. In a resting nerve cell, hydrophilic ions are only able to cross the plasma membrane via facilitated diffusion using ATP protein pumps. These pumps keep a greater concentration of Na+ ions outside of the cell and a greater concentration of K+ ions within the cell, maintaining the typical potential difference of ≈ -30V to- 90 V. When a nerve cell experiences a stimulus in the form of Na+ ions entering the cell, the “magnitudes of the charge difference and potential difference across the cell decrease” to more than 8mV (Pentland et al., 2019) causing it to depolarise. In turn, voltage -gated channels open and allow Na+ charges to flood the neuron, until an overall positive charge exists on the inside. This triggers voltage-gated K+ channels to open and repolarise the cell and the resting state is restored by ATP pumps. Our nervous system is designed to make this process as fast as possible (2 milliseconds) allowing our body to respond to stimuli effectively and keep us alive. Even the myelin sheath that envelops the axon of neurons is designed to insulate the electrical current and make it even faster.
The resistor-capacitor circuit is a very effective model for the electrical property of body cells and systems. A capacitor is an object that can support the separation of a positive and negative charge. Capacitance is the “ratio of the charge stored on [this] component to the electrical potential difference across the component.” (Pentland et al., 2019) When a capacitor is in parallel with a resistor, charge can accumulate or dissipate from the capacitor (as a switch is opened or closed), increasing its share of the circuit’s potential difference. The time interval taken to fully charge the circuit is given by τ = RC. The time taken to charge and discharge the capacitor perfectly models the behaviour of nerve cells in depolarising and depolarising as an action potential passes through.
Our circulatory system also relies on an electrical system that regulates the heart’s pumping action, coordinating its separate chambers. The sinus node (SA), found in the atria of the heart initiates an action potential, which travels as a self-propagating wave down through conduction pathways, stimulating first the two upper chambers of the heart (atria) then the two lower chambers (ventricles) to alternatively contract. Contraction of the ventricles can only occur if the SA node stimulates the atrio-ventricular (AV) node. This process allows a steady rhythm of 60-100 heartbeats per minute to keep the body functioning properly, and it all relies on the electrical action of the SA node, our heart’s “natural pacemaker.” Artificial pacemakers work in much the same way as the SA node, utilising an RC oscillator circuit. If a switch is opened, current can flow and charge the capacitor until it closes, where this current flows through the heart as a normal impulse until repetition.
The electrical rate and rhythm of the heart is not only essential for a healthy, functioning body, but allows deviations from the normal rhythm and other medical issues to be detected by an electrocardiogram (ECG or EKG). This machine detects and amplifies miniscule electrical differences on the skin’s surface that happen during heart muscle depolarisation, producing a graph of the heart’s electrical activity that can be used to diagnose anything from decreased blood flow to present or past heart attacks. If arrhythmia does lead to fibrillation of the heart - (chaotic depolarisation and repolarisation); defibrillators can be used to shock the heart back into normal rhythm. Again, an RC circuit is used to charge a capacitor to around 5kV, and a person's heart is connected to the circuit using two leads. The capacitor delivers a significant current to depolarise a large amount of the heart muscle and allow the SA node to reset.
The normal functioning of the human body is highly impacted by the electric current that controls some of our most essential systems. Our cells naturally have a potential difference across them because of concentration gradients between ions, and neurons are specially designed to carry an electrical charge by depolarising and repolaring cells and producing an action potential. Similarly, our hearts maintain a steady rhythm with the production of an action potential by the SA node. Understanding the electrical background of these processes by modelling cell depolarisation and repolarisation as similar to a capacitor-resistor circuit is not only important for understanding these functions, but how we can help if something goes wrong. Pacemakers and defibrillators both work on the basis of charging and discharging a capacitor, and allow people affected by heart conditions to live better lives.
BIBLIOGRAPHY:
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Membrane potential (resting membrane potential) (article). (2021). Khan Academy. https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/the-membrane-potential
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