Neurons, the fundamental units of the nervous system, play an intricate dance of electrical and chemical signals. One of the key tools to visualise these electrical activities is the oscilloscope, which offers snapshots of neuronal functions. This page delves deep into such visual representations, the phenomenon of saltatory conduction in myelinated fibres, and how the frequency of these impulses is measured.
Oscilloscope Traces for Neurons
An oscilloscope provides a real-time graphical representation of voltage against time, allowing us to monitor the transient electrical activities in neurons.
Resting and Action Potentials on Oscilloscope
- Resting Potential: The 'baseline' state of a neuron.
- Oscilloscope Display: A steady horizontal line typically at -70 mV.
- Biological Basis: At this state, the inside of a neuron is more negative compared to the outside due to the unequal distribution of ions, particularly sodium (Na+) and potassium (K+).
- Action Potential: Brief reversal of the membrane potential.
- Oscilloscope Display: Begins with a rapid ascent, indicating a positive shift (depolarisation). Peaks near +40 mV before a swift descent back (repolarisation). Occasionally, there's a slight undershoot, where the potential dips below the resting line.
- Biological Basis: Triggered by a stimulus, voltage-gated sodium channels open, allowing Na+ to rush in. This influx causes the inside to become more positive. Soon after, potassium channels open, permitting K+ to move out, returning the potential towards its resting state.
Image courtesy of Synaptidude.
Relating Oscilloscope Traces to Cellular Events
Understanding the peaks and troughs on the oscilloscope can give a comprehensive picture of what's happening within the neuron:
- Depolarisation: Marked by the steep upward slope. It's when sodium channels open, leading to a swift influx of Na+ ions, making the interior of the neuron positively charged relative to the exterior.
- Repolarisation: Corresponds to the rapid downward slope post the peak. It's when potassium channels open, causing an outflow of K+ ions, moving the charge back towards the resting potential.
- Hyperpolarisation: Reflected as a minor dip below the resting line. It indicates a slight over-exit of K+ ions, making the interior momentarily more negative than usual before ion pumps restore the balance.
Saltatory Conduction in Myelinated Fibres
The rapid and efficient transmission of neural signals is vital for organisms to respond to their environment. Myelination of axons plays a pivotal role in enhancing this efficiency.
Myelination and Its Structure
- Myelin Sheath: A multi-layered insulator made up of lipid and protein, which spirally wraps around the axon.
- Nodes of Ranvier: Small intervals or gaps in the myelin sheath, which expose the axon's membrane.
Image courtesy of OpenStax College
Saltatory Conduction: A Leap of Efficiency
- Mechanism: Instead of travelling continuously down the axon, the nerve impulse seems to 'jump' between the Nodes of Ranvier.
- Benefits:
- Rapid Transmission: Skipping the insulated parts allows for faster impulse propagation.
- Energy Efficiency: As ion exchanges (crucial for action potentials) occur mainly at the Nodes of Ranvier, the neuron uses less energy.
Measurement Techniques for Impulse Frequency
Quantifying the frequency of impulses can offer insights into stimulus intensity, making it a vital aspect of neurobiology.
Time Interval Method
- Measure the time between two consecutive peaks or troughs on the oscilloscope trace.
- The inverse of this time interval gives the frequency (in Hz).
Counting Peaks
- Fix a specific time frame, e.g., one second, on the oscilloscope.
- Count the number of action potentials (peaks) within this interval. This gives the frequency.
Advanced Analytical Tools
- Sophisticated oscilloscopes often come with built-in software capable of real-time analysis. This not only eliminates manual errors but also ensures precision in determining frequencies.
FAQ
Inhibitory neurotransmitters reduce the likelihood of the postsynaptic neuron generating an action potential. When these neurotransmitters bind to their receptors on the postsynaptic neuron, they typically cause the opening of ion channels that either allow the influx of negatively charged ions, like chloride (Cl-), or the efflux of positively charged ions like potassium (K+). Both these events make the inside of the neuron more negative, moving it further away from the threshold required to trigger an action potential. On the contrary, excitatory neurotransmitters increase the probability of an action potential by making the inside of the neuron more positive, often by promoting the influx of Na+ ions.
Saltatory conduction is exclusive to myelinated axons. Many axons, especially in smaller invertebrates, lack myelination. Myelination is a resource-intensive process requiring glial cells (like Schwann cells in the peripheral nervous system or oligodendrocytes in the central nervous system) to wrap around the axon, producing the myelin sheath. In smaller organisms or in short-distance connections, the speed advantage conferred by myelination might not outweigh its metabolic and spatial costs. In larger animals, where speed of impulse transmission becomes more crucial, myelination becomes more prevalent, thereby facilitating saltatory conduction.
The refractory period comprises two phases: the absolute refractory period and the relative refractory period. During the absolute refractory period, a neuron is incapable of initiating another action potential regardless of the strength of the stimulus. This is because the voltage-gated sodium channels, once opened and then closed during an action potential, need time to reset before they can open again. This ensures that the action potential doesn't move backwards towards a region it has just travelled through. The relative refractory period follows, where a stronger-than-normal stimulus can initiate an action potential. This sequence ensures that action potentials propagate in one direction, from the axon hillock to the axon terminals.
Voltage-gated channels are protein channels embedded in the neuron's membrane that open or close in response to changes in voltage or electrical charge across the membrane. When the membrane potential reaches a certain threshold, these channels undergo a conformational change. For example, when a stimulus causes initial depolarisation, the voltage-gated sodium channels open, allowing Na+ ions to flow into the neuron. This further depolarises the neuron. As the membrane potential approaches +40 mV, these sodium channels start closing, and voltage-gated potassium channels open, enabling K+ ions to exit the neuron and initiate repolarisation. Hence, these channels are crucial in generating and propagating action potentials.
The resting potential of about -70 mV in neurons is maintained by the relative permeability of the neuronal membrane to different ions and the action of ion pumps. At rest, the neuron's membrane is more permeable to potassium ions (K+) than to sodium ions (Na+). K+ ions tend to leak out of the cell, driven by their concentration gradient, leading to a more negative charge inside. Concurrently, the sodium-potassium pump actively transports 3 Na+ ions out of the neuron and 2 K+ ions into the neuron for each ATP molecule it hydrolyses, thereby contributing to this negative potential. Together, these processes maintain the resting potential around -70 mV.
Practice Questions
The oscilloscope trace shows the resting potential as a steady horizontal line, typically at around -70 mV. This represents the neuron's state when it's not transmitting an impulse, with the inside being more negative than the outside due to the unequal distribution of ions. The action potential is depicted by a rapid upward spike indicating depolarisation as sodium ions rush into the neuron, making the internal charge positive. This peak is near +40 mV. It's followed by a swift downward movement, signifying repolarisation, as potassium ions exit the neuron, restoring the negative internal charge. Occasionally, there might be a slight dip below the resting line, representing hyperpolarisation, where the neuron momentarily becomes even more negative before returning to the resting potential.
Saltatory conduction is a mode of nerve impulse transmission observed in myelinated fibres. In this, the action potential doesn't travel continuously down the axon; instead, it seems to 'jump' between the Nodes of Ranvier, which are small gaps in the myelin sheath. The myelin sheath acts as an insulator, preventing ion exchanges across the insulated parts of the axon. As a result, the impulse skips these parts and travels much faster. This mode of conduction offers two primary advantages. Firstly, it ensures rapid transmission, which is essential for timely responses to stimuli. Secondly, it's energy-efficient. Since ion exchanges, crucial for generating action potentials, occur primarily at the Nodes of Ranvier and not across the entire axon, the neuron uses less energy, making the process more efficient.
