Neuronal impulses and action potentials are critical components of the nervous system that enable communication between neurons. This complex process can be broken down into several stages and is influenced by factors such as myelination and saltatory conduction.
Neurons Transmit Electrical Impulses Called Action Potentials
Action Potentials: The Basics
- Definition: Action potentials are temporary reversals of the electrical potential across a neuron's membrane.
- Stages of an Action Potential:
- Resting Potential (around -70 mV): The inside of the neuron is negatively charged relative to the outside. Sodium-potassium pumps maintain this balance by pumping three sodium ions out for every two potassium ions in.
- Depolarisation (around +40 mV): Voltage-gated sodium channels open, allowing sodium ions to enter the neuron, increasing the positive charge inside.
- Threshold Potential (around -55 mV): If this level is reached, it triggers an all-or-nothing action potential.
- Repolarisation: Voltage-gated potassium channels open, allowing potassium ions to leave the neuron, restoring the negative charge inside.
- Hyperpolarisation: A slight overshoot where the potential drops below the resting level.
- Refractory Period: A period when a new action potential cannot be initiated, ensuring one-way travel.
Propagation of the Action Potential
- Continuous Conduction: In unmyelinated axons, the action potential moves like a wave along the axon.
- Saltatory Conduction: In myelinated axons, the action potential jumps between the Nodes of Ranvier, speeding up the transmission.
Myelination and Saltatory Conduction's Impact on Nerve Impulse Transmission
Myelination
- Definition: Myelination involves glial cells wrapping around an axon to form a myelin sheath.
- Function: Myelination increases the speed of nerve impulse transmission.
- Myelin Sheath: Acts as an insulator, preventing ion leakage and increasing the speed of transmission.
- Nodes of Ranvier: Gaps between myelin where the axon is exposed and action potentials can be generated.
Saltatory Conduction
- Definition: This is the "jumping" of the action potential between Nodes of Ranvier in myelinated axons.
- Advantages:
- Speed: Increases the speed of transmission by 50 times or more compared to unmyelinated axons.
- Energy Efficiency: Less energy is needed to restore the ion balance after an action potential.
Multiple Sclerosis (MS): An Example of the Importance of Myelination
- What is MS? A disease where the immune system attacks the myelin sheath, impairing nerve transmission.
- Symptoms: Vision loss, pain, fatigue, impaired coordination.
- Treatment: There is no cure, but treatments can help manage symptoms and slow progression.
Neurotransmission at the Synapse
- Chemical Synapses: A most common type of synapse, where neurotransmitters carry the signal.
- Release of Neurotransmitters: Action potential reaches the synaptic terminal, causing calcium channels to open.
- Binding to Receptors: Neurotransmitters bind to receptors on the postsynaptic neuron, triggering or inhibiting a new action potential.
- Electrical Synapses: Less common, allow direct electrical communication between neurons.
- Gap Junctions: Channels that allow ions to flow directly between neurons.
Significance of Neuronal Impulses and Action Potentials
Understanding the intricacies of neuronal impulses and action potentials provides insights into:
- Brain Function: How thoughts, emotions, and actions are processed.
- Neurological Disorders: Like epilepsy, where abnormal action potentials occur.
- Medical Interventions: The development of treatments for disorders or injuries.
FAQ
Depolarisation is characterised by the opening of voltage-gated sodium channels, leading to an influx of sodium ions, making the inside of the axon more positive. Repolarisation occurs when voltage-gated potassium channels open, allowing potassium ions to flow out, restoring the negative internal charge. These phases are sequential and vital for the propagation of an action potential.
The refractory period is a brief time following an action potential when the neuron is unresponsive to a new stimulus. This ensures that action potentials travel in one direction along the axon and allows time for the sodium-potassium pump to restore the ionic gradients, maintaining the neuron's ability to fire again.
Nodes of Ranvier are gaps in the myelin sheath that expose the axon's membrane. They contain a high density of voltage-gated ion channels and enable the action potential to jump from node to node (saltatory conduction). This increases the speed of impulse transmission and conserves energy.
The resting potential is maintained through the sodium-potassium pump, which actively transports three sodium ions out of the neuron and two potassium ions in. Additionally, leak channels allow some potassium to leak out, creating a more negative internal environment. The combined effect establishes a resting potential of about -70 mV.
Local anaesthetics work by blocking sodium channels in neurons, preventing the influx of sodium ions during depolarisation. This inhibits the initiation and propagation of action potentials, leading to a temporary loss of sensation in the targeted area. As a result, pain signals cannot be transmitted to the brain.
Practice Questions
Saltatory conduction is the process of "jumping" action potentials between the Nodes of Ranvier in myelinated neurons. In myelinated axons, the myelin sheath acts as an insulator, allowing the action potential to leap from one node to the next. This increases the speed of transmission by 50 times or more compared to continuous conduction in unmyelinated neurons. Continuous conduction occurs like a wave, with continuous depolarisation and repolarisation along the axon. Saltatory conduction not only enhances speed but also promotes energy efficiency, as less energy is required to restore the ion balance after an action potential.
The stages of an action potential include:
- Resting Potential (around -70 mV): The neuron is at rest, with the inside negatively charged.
- Depolarisation (around +40 mV): Sodium channels open, allowing sodium ions to enter, increasing the positive charge inside.
- Threshold Potential (around -55 mV): If reached, triggers the action potential.
- Repolarisation: Potassium channels open, restoring the negative charge inside.
- Hyperpolarisation: A slight overshoot below the resting level.
- Refractory Period: Prevents a new action potential, ensuring one-way travel. The all-or-nothing principle is relevant as the action potential is triggered if the threshold is reached, and the same action potential is generated regardless of stimulus strength. If the threshold is not reached, no action potential occurs.
