Applications of Electrochemistry and Bioelectricity
5. Nerve Impulses and Medical Applications
Learning outcomes
- I can explain how nerve impulses are generated and transmitted.
- I can describe the role of ion movement in nerve signaling.
- I can identify medical technologies that use bioelectric principles.
- I can explain how electrochemistry contributes to healthcare.
- I can evaluate the impact of bioelectrical technologies on modern medicine.
What Is a Nerve Impulse?
The nervous system allows information to travel rapidly throughout the body.
A nerve impulse is an electrical signal transmitted along a neuron.
More precisely, neurons transmit signals using rapid changes in the electrical potential across their cell membranes called:
action potentials.
These signals depend on the controlled movement of charged particles called:
ions.
The main ions involved include:
- sodium, Na⁺
- potassium, K⁺
- calcium, Ca²⁺
- chloride, Cl⁻
Nerve signaling is therefore an excellent example of:
bioelectrochemistry.
The Structure of a Neuron
A neuron is a specialized cell that receives and transmits information.
The major parts are:
Dendrites — receive signals from other cells.
Cell body — contains the nucleus and much of the cell's machinery.
Axon — carries electrical signals away from the cell body.
Axon terminals — communicate with other neurons, muscles, or glands.
The Resting Neuron
A neuron that is not currently producing an action potential still has an electrical potential difference across its membrane.
This is called the:
resting membrane potential.
For many neurons, it is approximately:
−70 mV.
This means that the inside of the neuron is electrically negative relative to the:
outside.
Why Is There a Resting Potential?
The resting potential results from an unequal distribution of ions across the:
cell membrane.
Typically:
Na⁺ concentration is higher outside the neuron.
K⁺ concentration is higher inside the neuron.
The membrane is selectively permeable, meaning some ions can cross more easily than:
others.
Together, ion concentration differences and membrane permeability create the resting membrane potential.
The Sodium-Potassium Pump
The sodium-potassium pump helps maintain the concentration gradients needed for nerve signaling.
It uses energy from:
ATP.
During each cycle it moves:
3 Na⁺ out
and:
2 K⁺ in.
This is a form of:
active transport.
Ion Channels
The neuron membrane contains specialized proteins called:
ion channels.
These allow specific ions to move across the membrane.
Some channels respond to changes in membrane voltage and are called:
voltage-gated ion channels.
Two especially important types are:
voltage-gated Na⁺ channels
and:
voltage-gated K⁺ channels.
Opening and closing these channels produces the rapid voltage changes of an:
action potential.
Generating a Nerve Impulse
A stimulus can cause the membrane potential to become less negative.
If the membrane reaches a critical value called the:
threshold,
an action potential begins.
For many neurons, threshold is around:
−55 mV, although this varies.
Once threshold is reached, voltage-gated Na⁺ channels rapidly:
open.
Depolarization
When voltage-gated Na⁺ channels open:
Na⁺ rushes into the neuron.
The inside becomes progressively less negative.
It can eventually become temporarily:
positive.
This stage is called:
depolarization.
The membrane potential may reach approximately:
+30 mV.
Why Does Sodium Move In?
Na⁺ is influenced by its:
electrochemical gradient.
There is a concentration gradient because Na⁺ concentration is higher outside the neuron.
There is also an electrical attraction between positively charged Na⁺ and the relatively negative cell interior.
When Na⁺ channels open, these effects favor movement:
into the neuron.
Repolarization
Near the peak of the action potential, Na⁺ channels become inactivated while voltage-gated K⁺ channels open.
K⁺ moves:
out of the neuron.
Positive charge therefore leaves the cell.
The membrane potential becomes negative again.
This process is called:
repolarization.
Hyperpolarization
K⁺ channels may remain open for slightly longer than needed.
Extra K⁺ leaves the cell.
The membrane potential temporarily becomes more negative than its normal resting value.
This is:
hyperpolarization.
The neuron then returns toward its resting electrical conditions.
The Complete Action Potential
The basic sequence is:
Resting potential
↓
Stimulus
↓
Threshold
↓
Na⁺ channels open
↓
Na⁺ enters
↓
Depolarization
↓
Na⁺ channels inactivate
↓
K⁺ channels open
↓
K⁺ leaves
↓
Repolarization
↓
Hyperpolarization
↓
Return toward resting potential
This sequence happens extremely:
quickly.
The All-or-None Principle
An action potential follows the:
all-or-none principle.
If threshold is not reached:
no action potential occurs.
If threshold is reached:
a full action potential occurs.
A stronger stimulus does not normally create a taller individual action potential.
Instead, stimulus intensity can often be represented by changes in the:
frequency of action potentials.
The Refractory Period
Immediately after an action potential begins, the neuron temporarily becomes unable, or less able, to produce another action potential.
This is called the:
refractory period.
It occurs partly because voltage-gated Na⁺ channels require time to recover from:
inactivation.
The refractory period helps ensure that action potentials normally travel along an axon in:
one direction.
Propagation of the Nerve Impulse
An action potential at one part of the axon creates electrical changes in nearby membrane.
These changes can bring the neighboring region to:
threshold.
Voltage-gated Na⁺ channels then open in that region.
A new action potential is generated.
The process repeats along the:
axon.
The Signal Is Regenerated
An action potential does not simply drift down an axon and gradually disappear.
Instead, each region of membrane triggers an action potential in the:
next region.
The signal is therefore repeatedly:
regenerated.
This allows information to travel considerable distances without the action potential becoming progressively smaller.
Myelin and Faster Transmission
Many axons are covered by an insulating layer called the:
myelin sheath.
Myelin reduces ion movement across most of the covered membrane.
Small gaps in the myelin are called:
nodes of Ranvier.
Action potentials are regenerated mainly at these:
nodes.
Saltatory Conduction
In a myelinated axon, electrical activity spreads rapidly beneath the myelin and action potentials are regenerated at successive nodes.
The signal therefore appears to jump from:
node to node.
This is called:
saltatory conduction.
It allows nerve impulses to travel much faster than they would along a comparable unmyelinated axon.
What Happens at the End of the Neuron?
Eventually the action potential reaches the:
axon terminal.
Most neurons communicate with the next cell across a tiny junction called a:
synapse.
At many synapses, the electrical signal is converted temporarily into a:
chemical signal.
Calcium and Neurotransmitter Release
When the action potential reaches the axon terminal, voltage-gated Ca²⁺ channels open.
Ca²⁺ enters the terminal.
This triggers vesicles containing:
neurotransmitters
to release their contents into the synaptic cleft.
Crossing the Synapse
The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the next cell.
These receptors can affect:
ion channels.
This changes the membrane potential of the receiving cell.
The sequence can therefore be summarized as:
electrical signal
↓
chemical signal
↓
electrical response
Nerves and Muscles
Motor neurons transmit signals to:
muscle cells.
At the neuromuscular junction, neurotransmitter release causes electrical changes in the muscle membrane.
This eventually causes Ca²⁺ to become available inside the muscle cell.
Calcium helps trigger:
muscle contraction.
Therefore every deliberate movement you make depends on carefully controlled:
bioelectrical signaling.
Bioelectricity in Medicine
The body's electrical activity can be:
measured, interpreted, and modified.
This has led to many important medical technologies.
Examples include:
- ECG
- EEG
- EMG
- pacemakers
- defibrillators
- cochlear implants
- deep brain stimulation
- nerve stimulation
- brain-computer interfaces
These technologies apply principles of:
electricity, electrochemistry, and biology.
Electrocardiography
An electrocardiogram (ECG or EKG) records electrical activity associated with the:
heart.
Electrodes are placed on the skin.
They detect changing electrical potential differences produced as electrical activity spreads through heart tissue.
The ECG Trace
A typical ECG contains several recognizable features.
These include:
P wave
QRS complex
T wave
These correspond to patterns of electrical activation and recovery in different parts of the heart.
Doctors can use ECG information as one part of evaluating:
heart rhythm and electrical activity.
Electroencephalography
An electroencephalogram (EEG) records electrical activity associated with populations of neurons in the:
brain.
Electrodes placed on the scalp detect small voltage changes.
EEGs can provide useful information in areas such as:
- seizure evaluation
- sleep studies
- monitoring brain activity in certain clinical situations
Electromyography
Electromyography (EMG) measures electrical activity associated with:
muscles.
EMG testing can help investigate how muscles and the nerves controlling them are functioning.
It may involve:
- surface electrodes
- needle electrodes
depending on the type of examination.
Measuring Nerve Conduction
Electrical stimulation can also be used to investigate how effectively signals travel through:
peripheral nerves.
In a nerve conduction study, a nerve is stimulated and the resulting electrical response is measured.
Measurements such as conduction speed and response size can provide information about:
nerve function.
Pacemakers
The heart normally generates its own electrical rhythm.
In some situations, this rhythm can become too slow or otherwise require electrical support.
A pacemaker is an implanted medical device that can deliver carefully controlled electrical impulses to help regulate the:
heartbeat.
How a Pacemaker Uses Bioelectricity
A pacemaker can:
- monitor electrical activity
- detect when pacing is needed
- deliver electrical impulses
- stimulate heart muscle
The electrical impulse changes membrane potentials in cardiac cells and can initiate electrical activation of the:
heart tissue.
The device therefore interacts directly with the body's natural:
bioelectrical system.
Defibrillators
A defibrillator delivers a controlled electrical shock in certain dangerous abnormal heart rhythms.
The purpose is not simply to "restart" a completely stopped heart.
For shockable rhythms such as ventricular fibrillation, the shock can depolarize a large amount of cardiac tissue at once.
This may allow the heart's normal electrical control system to:
re-establish an organized rhythm.
Automated External Defibrillators
An AED is designed to analyze the heart's electrical rhythm and determine whether a shock is appropriate.
AEDs can be found in places such as:
- airports
- schools
- sports facilities
- shopping centers
- workplaces
Their design allows trained responders—and in many settings members of the public following device instructions—to provide rapid assistance during certain cardiac emergencies.
Pacemaker vs Defibrillator
These technologies both use electrical stimulation but serve different purposes.
| Pacemaker | Defibrillator |
|---|---|
| Delivers relatively small pacing impulses | Can deliver a much larger therapeutic shock |
| Helps regulate certain abnormal rhythms | Treats certain dangerous arrhythmias |
| Often works repeatedly as needed | Shock delivered when specific rhythm conditions occur |
| Interacts with cardiac electrical activity | Interacts with cardiac electrical activity |
Some implanted devices can provide both pacing and:
defibrillation functions.
Cochlear Implants
A cochlear implant is an electronic medical device that can provide auditory information to some people with severe hearing loss.
It converts sound into electrical signals that stimulate the:
auditory nerve.
How a Cochlear Implant Works
A simplified sequence is:
Sound enters microphone
↓
Processor analyzes sound
↓
Signals sent to implant
↓
Electrode array stimulates different regions of the cochlea
↓
Auditory nerve carries signals toward the brain
The device therefore uses controlled electrical stimulation to interact with:
nerve cells.
Deep Brain Stimulation
Deep brain stimulation (DBS) uses implanted electrodes to deliver electrical stimulation to selected areas of the:
brain.
It is used clinically for certain neurological conditions, including some movement disorders.
The electrical stimulation modifies activity within particular:
neural circuits.
Peripheral Nerve Stimulation
Electrical stimulation can also target nerves outside the brain and spinal cord.
Depending on the medical application, stimulation may be used to modify:
- nerve activity
- muscle activation
- some pain signals
The basic principle is that an applied electrical field can influence membrane potential and therefore affect whether neurons produce:
action potentials.
Functional Electrical Stimulation
Functional electrical stimulation (FES) uses electrical impulses to activate nerves controlling muscles.
In suitable clinical and rehabilitation contexts, it can assist actions such as:
- muscle contraction
- cycling movements
- hand movements
- foot movement
FES demonstrates how an artificial electrical signal can interact with natural:
neuromuscular pathways.
Brain-Computer Interfaces
A brain-computer interface (BCI) detects patterns of neural activity and uses them to control an external system.
A BCI may involve signals measured:
- from the scalp
- from the brain surface
- using implanted electrodes
Computer systems analyze the electrical patterns and translate them into:
commands.
Possible BCI Applications
Research and clinical applications can include helping some people interact with:
- computers
- communication systems
- robotic devices
- assistive technologies
BCIs combine:
neuroscience + electrical engineering + computing + medicine.
They also demonstrate how understanding biological electrical signals can lead to entirely new forms of:
technology.
Electrochemical Sensors in Healthcare
Electrochemistry contributes to healthcare in ways beyond nerve stimulation.
Many medical sensors use electrochemical reactions to detect:
biological molecules.
A familiar example is a:
glucose sensor.
Some glucose sensors use enzymes and electrodes to generate an electrical signal related to glucose concentration.
Biosensors
A biosensor combines a biological recognition system with a device that converts the biological interaction into a measurable signal.
Electrochemical biosensors can be designed to measure substances such as:
- glucose
- lactate
- certain metabolites
- selected biomarkers
The chemical information is converted into an:
electrical measurement.
AkRrL-4L-HKjZfa6q2?purpose=fullsize">Electrodes Connect Biology and Electronics
Electrodes are crucial in many medical technologies because they create an interface between:
ionic conduction in the body
and:
electronic conduction in a device.
Inside the body, charge is often transported by:
ions.
Inside wires and electronic circuits, charge is primarily transported by:
electrons.
Electrode interfaces allow these two electrical systems to:
interact.
Why Electrode Materials Matter
Medical electrodes must be carefully designed.
Important properties can include:
- electrical conductivity
- chemical stability
- biocompatibility
- corrosion resistance
- low electrical impedance
- mechanical durability
Implanted electrodes must function in an environment containing:
water, salts, proteins, and cells.
This makes electrode design an important application of:
electrochemistry and materials science.
Benefits of Bioelectrical Technologies
Bioelectrical technologies have transformed many areas of healthcare.
Potential benefits include:
- rapid monitoring
- non-invasive measurements
- continuous monitoring
- restoration of some lost functions
- control of abnormal electrical activity
- targeted stimulation
- improved rehabilitation
- greater independence through assistive technologies
Some devices can interact directly with the body's:
electrical signaling systems.
Limitations and Challenges
Bioelectrical technologies also have limitations.
Depending on the technology, challenges can include:
- surgery for implanted devices
- infection risk
- battery replacement or charging
- electrode degradation
- scar tissue around implants
- signal interference
- cost
- device maintenance
- differences between individual patients
Therefore a technology's usefulness must be considered alongside its:
risks and limitations.
Signal Noise
Biological electrical signals can be extremely:
small.
Electrical activity from muscles, nearby devices, movement, and the environment can interfere with measurements.
This unwanted signal is called:
noise.
Medical instruments therefore use techniques such as amplification and signal processing to separate useful information from:
interference.
Biocompatibility
An implanted device must interact safely with:
living tissue.
A material suitable for ordinary electronics may not necessarily be suitable for implantation.
Implanted materials must be selected to reduce harmful reactions while maintaining their required:
electrical and mechanical properties.
Powering Implanted Devices
Implanted medical electronics require:
energy.
Depending on the device, power may come from:
- internal batteries
- rechargeable batteries
- external power transfer
- other specialized systems
Battery chemistry is therefore another direct connection between:
electrochemistry and medicine.
Evaluating Medical Bioelectrical Technology
When evaluating a technology, consider both benefits and limitations.
Questions might include:
What medical problem does it address?
Does it measure or stimulate electrical activity?
How reliable is it?
Does it require surgery?
How long does it operate?
What are the risks?
How much does it cost?
How much does it improve function or quality of life?
Are there alternative treatments?
Good evaluation requires considering:
evidence, benefits, risks, and practical limitations.
Comparing Bioelectrical Technologies
| Technology | Main Function | Bioelectrical Principle |
|---|---|---|
| ECG | Records heart activity | Measures voltage differences |
| EEG | Records brain activity | Measures voltage changes from neural activity |
| EMG | Measures muscle activity | Detects electrical activity associated with muscles |
| Pacemaker | Regulates heart rhythm | Electrical stimulation |
| Defibrillator | Treats certain dangerous rhythms | Controlled electrical shock |
| Cochlear implant | Provides auditory information | Electrical stimulation of auditory nerve |
| DBS | Modifies neural circuits | Electrical brain stimulation |
| FES | Activates muscles | Electrical stimulation of motor nerves |
| BCI | Interprets neural activity | Electrical signal detection and processing |
| Electrochemical biosensor | Detects chemicals | Converts chemical information into electrical signals |
Worked Example 1
A neuron is at −70 mV and receives a sufficient stimulus.
What happens first?
The membrane reaches:
threshold.
Voltage-gated Na⁺ channels then open.
Na⁺ moves:
into the neuron.
The membrane begins to:
depolarize.
Worked Example 2
Why does the membrane repolarize?
Voltage-gated K⁺ channels open.
K⁺ moves:
out of the neuron.
Positive charge leaves the cell, causing the membrane potential to become more negative again.
Worked Example 3
Why does myelin increase nerve conduction speed?
Myelin electrically insulates much of the axon.
Action potentials are regenerated primarily at:
nodes of Ranvier.
The electrical signal therefore travels rapidly between nodes through:
saltatory conduction.
Worked Example 4
Why is Ca²⁺ important at many synapses?
When an action potential reaches the axon terminal, Ca²⁺ enters through voltage-gated channels.
This triggers the release of:
neurotransmitters.
Worked Example 5
A doctor places electrodes on a patient's chest.
What technology might be used?
An:
ECG.
The electrodes detect potential differences associated with the heart's electrical activity.
Worked Example 6
How does a pacemaker interact with heart cells?
It delivers controlled electrical impulses.
These alter membrane potentials and can trigger electrical activation of:
cardiac cells.
Worked Example 7
How does a cochlear implant use nerve signaling?
Its electrodes provide controlled electrical stimulation to portions of the auditory system.
The auditory nerve then transmits information toward the:
brain.
Worked Example 8
Why is an implanted electrode an electrochemical system?
The electronic device carries charge mainly using electrons, while body fluids carry charge mainly using ions.
The electrode forms an interface where these two forms of electrical conduction:
interact.
Worked Example 9
Why are medical electrical signals amplified?
Signals such as those measured in ECGs, EEGs, and some neural recordings can be:
very small.
Amplification makes them easier to measure and analyze.
Worked Example 10
Give one benefit and one challenge of implanted bioelectrical devices.
Possible benefit:
They can restore, support, or modify important biological functions.
Possible challenge:
They may require surgery and long-term device maintenance.
A complete evaluation considers:
both.
Common Mistake: Nerve Impulses Are Electrons Flowing Down Nerves
They are not.
Nerve impulses depend primarily on controlled movements of:
ions across cell membranes.
Common Mistake: The Sodium-Potassium Pump Causes Rapid Depolarization
The pump helps maintain ion gradients over time.
Rapid depolarization occurs mainly because:
voltage-gated Na⁺ channels open.
Common Mistake: Stronger Stimulus Means a Larger Action Potential
Once threshold is reached, an action potential follows the:
all-or-none principle.
A stronger stimulus can instead influence how frequently action potentials are:
generated.
Common Mistake: A Defibrillator Simply Restarts a Stopped Heart
Defibrillation is primarily used for certain dangerous electrical rhythms such as:
ventricular fibrillation.
Its purpose is to interrupt disorganized electrical activity so organized electrical control may resume.
Common Mistake: ECG Measures Heart Contraction Directly
An ECG measures:
electrical activity.
Mechanical contraction is related to that electrical activity, but the ECG does not directly measure the force of:
contraction.
Common Mistake: Bioelectric Medical Devices Are Purely Electronic
They operate at the interface between:
electronics and living electrochemical systems.
Understanding ions, electrodes, potentials, and chemical reactions is therefore essential to many medical technologies.
Check Your Understanding
- Define a nerve impulse.
- What is an action potential?
- Name four ions involved in bioelectrical signaling.
- What is the approximate resting potential of many neurons?
- Why is the inside of a resting neuron negative relative to the outside?
- What does the sodium-potassium pump do?
- Why does the pump require ATP?
- What is an ion channel?
- What is a voltage-gated ion channel?
- Define threshold.
- What happens when threshold is reached?
- Which ion moves into the neuron during depolarization?
- Why does Na⁺ move inward?
- Define depolarization.
- Which ion moves out during repolarization?
- Define repolarization.
- What is hyperpolarization?
- Explain the all-or-none principle.
- What is the refractory period?
- How does the refractory period help nerve signals travel in one direction?
- Explain how an action potential propagates along an axon.
- What is myelin?
- What is a node of Ranvier?
- Define saltatory conduction.
- Why does myelin increase conduction speed?
- What happens when an action potential reaches an axon terminal?
- What role does Ca²⁺ play at a synapse?
- What is a neurotransmitter?
- Describe the sequence of communication across a chemical synapse.
- Explain how nerve impulses cause muscle contraction.
- What does an ECG measure?
- What does an EEG measure?
- What does an EMG measure?
- What information can a nerve conduction study provide?
- Explain how a pacemaker uses bioelectricity.
- Explain how a defibrillator uses bioelectricity.
- Compare a pacemaker with a defibrillator.
- Explain how a cochlear implant uses electrical stimulation.
- What is deep brain stimulation?
- What is functional electrical stimulation?
- What is a brain-computer interface?
- Give two possible applications of BCIs.
- What is an electrochemical biosensor?
- Explain how electrochemistry can be used in glucose sensing.
- Why are electrodes important in medical technology?
- Why is biocompatibility important for implanted electrodes?
- Give three benefits of bioelectrical medical technologies.
- Give three challenges associated with implanted bioelectrical devices.
- Explain how electrochemistry connects nerve impulses, batteries, electrodes, and medical devices.
- Evaluate the impact of one bioelectrical technology on modern healthcare by considering its benefits, limitations, and applications.
Key Terms
Nerve impulse: Electrical signal transmitted by a neuron.
Action potential: Rapid, temporary change in membrane potential used for cellular signaling.
Resting membrane potential: Electrical potential difference across the membrane of a resting excitable cell.
Threshold: Membrane potential that must be reached to trigger an action potential.
Depolarization: Change that makes the membrane potential less negative or more positive.
Repolarization: Return of the membrane potential toward its resting negative value.
Hyperpolarization: Temporary increase in membrane negativity beyond the resting level.
Refractory period: Period following initiation of an action potential when another action potential cannot easily occur.
Voltage-gated channel: Ion channel controlled by changes in membrane potential.
Myelin: Insulating material surrounding many axons.
Node of Ranvier: Gap between sections of myelin where action potentials are regenerated.
Saltatory conduction: Rapid propagation of an action potential between nodes of Ranvier.
Synapse: Junction through which a neuron communicates with another cell.
Neurotransmitter: Chemical messenger released by neurons.
ECG: Recording of electrical activity associated with the heart.
EEG: Recording of electrical activity associated with the brain.
EMG: Measurement of electrical activity associated with muscles.
Pacemaker: Device that provides electrical impulses to help regulate certain abnormal heart rhythms.
Defibrillator: Device capable of delivering an electrical shock to treat certain dangerous cardiac rhythms.
Cochlear implant: Device that electrically stimulates the auditory system to provide auditory information.
Deep brain stimulation: Electrical stimulation of selected brain regions using implanted electrodes.
Biosensor: Device that converts biological or chemical information into a measurable signal.
Key Takeaways
- Nerve impulses are electrochemical signals produced by controlled ion movement across neuron membranes.
- The resting membrane potential depends on unequal ion distributions and selective membrane permeability.
- The sodium-potassium pump helps maintain the Na⁺ and K⁺ gradients needed for signaling.
- Na⁺ entering the neuron produces depolarization, while K⁺ leaving contributes to repolarization.
- Action potentials follow the all-or-none principle.
- Action potentials are regenerated as they travel along an axon.
- Myelin and nodes of Ranvier allow rapid saltatory conduction.
- Ca²⁺ movement at many synapses helps trigger neurotransmitter release.
- Bioelectrical signals can be measured using technologies such as ECG, EEG, and EMG.
- Medical devices can also modify bioelectrical activity.
- Pacemakers, defibrillators, cochlear implants, DBS, and FES use controlled electrical stimulation for different purposes.
- Electrochemical biosensors convert chemical information into measurable electrical signals.
- Medical electrodes connect the body's ionic electrical system with electronic devices that primarily conduct using electrons.
- Electrochemistry contributes to healthcare through diagnosis, monitoring, stimulation, sensing, implants, and energy storage.
- Bioelectrical technologies have had a major impact on modern medicine, but their benefits must be considered alongside factors such as risk, reliability, cost, maintenance, and biocompatibility.