32 Electrochemical gradients and equilibrium potential
What drives current?
We can think of various devices—such as batteries, generators, and wall outlets —that are necessary to maintain a current. All such devices create a potential difference and are loosely referred to as voltage sources. When a voltage source is connected to a conductor, it applies a potential difference (
) that creates an electric field. The electric field in turn exerts force on charges, causing current.
Ohm’s Law
The current that flows through most substances is directly proportional to the voltage applied to it.
This important relationship is known as Ohm’s law. It can be viewed as a cause-and-effect relationship, with voltage the cause and current the effect. This is an empirical law like that for friction—an experimentally observed phenomenon. Such a linear relationship doesn’t always occur.
Resistance and simple circuits
If voltage drives current, what impedes it? The electric property that impedes current (crudely similar to friction and air resistance) is called resistance. Collisions of moving charges with atoms and molecules in a substance transfer energy to the substance and limit current. Resistance is defined as inversely proportional to current, or
Thus, for example, current is cut in half if resistance doubles. Combining the relationships of current to voltage and current to resistance gives
This relationship is also called Ohm’s law. Ohm’s law in this form really defines resistance for certain materials.
For example
Resistances range over many orders of magnitude. A dry person may have a hand-to-foot resistance of 
, whereas the resistance of the human heart is about 
.
Resistance is related to the shape of an object and the material of which it is composed.
Electric currents in the vastly complex system of billions of nerves in our body allow us to sense the world, control parts of our body, and think. These are representative of the three major functions of nerves:
- To carry messages from our sensory organs and others to the central nervous system, consisting of the brain and spinal cord
- To carry messages from the central nervous system to muscles and other organs
- To transmit and process signals within the central nervous system
Review the function of nervous tissue
- To transmit and process signals within the central nervous system
The sheer number of nerve cells and the incredibly greater number of connections between them makes this system the wonder that it is. Nerve conduction is a general term for electrical signals carried by nerve cells. It is one aspect of bioelectricity, or electrical effects in and created by biological systems. We focus on sodium ions (Na+) and potassium ions (K+). Both are cations (positively charged ions).
Flow=force/resistance
- Changes in force produce proportional changes in flow.
- Changes in resistance produce inversely proportional changes in flow.
Resting membrane potential
A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV, note that this number varies by neuron type and by species). This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium. Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell. The difference in the number of positively charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential. When the membrane is at rest, K+ ions accumulate inside the cell due to a net movement with the concentration gradient. The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell (in the extracellular fluid) relative to inside the cell (in the cytoplasm). The negative charge within the cell is created by the cell membrane being more permeable to potassium ion movement than sodium ion movement. In neurons, potassium ions are maintained at high concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell. The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. The actions of the sodium-potassium pump help to maintain the resting potential, once established. Recall that sodium potassium pumps bring two K+ ions into the cell while removing three Na+ ions per ATP consumed. As more cations are expelled from the cell than taken in, the inside of the cell remains negatively charged relative to the extracellular fluid.
Dive deeper
Watch Osmosis by Elsevier (2016, December 1) Resting membrane potential – definition, examples [Youtube, 7:49mins]
Case study
Thunder, a 4-year-old Thoroughbred gelding, presents with involuntary, repetitive head movements, including shaking, flicking, and jerking. Additional symptoms included nose rubbing, snorting, and striking at the nose. Thunder’s owner reported that these episodes were more frequent during windy conditions, bright sunlight, and after intense exercise. The behaviour had been progressively worsening over the past few months.
During the physical examination, Thunder showed no obvious signs of external injury or infection, and his vital signs were normal. Electrophysiological studies of the trigeminal nerve indicated hyperexcitability, confirming the involvement of the trigeminal nerve. Based on the clinical signs and electrophysiological findings, Thunder was diagnosed with Trigeminal-Mediated Headshaking Syndrome (TMHS). TMHS is a painful condition characterised by involuntary, repetitive head movements, often without an obvious cause. Triggers for headshaking can include wind, light, or increased exercise intensity. In addition to head movements, horses with TMHS may exhibit nose rubbing, snorting, and striking at the nose. The underlying cause of TMHS is linked to trigeminal nerve dysfunction, specifically hyperexcitability of the trigeminal nerve.
Management strategies for Thunder include environmental modifications to reduce exposure to known triggers such as wind and bright light, medications to manage pain and reduce nerve hyperexcitability, and temporary relief through nerve blocks.
Other neurological signs of nerve conduction disorders can also include impaired vision, muscle twitching, circling, incoordination, paralysis, weakness, abnormal gait, difficulty chewing or swallowing, seizures, head pressing, head tilt, and behavioural changes.
