Nerve Impulses
The passage of a nerve impulse involves two key processes: the transmission of an action potential along a neuron and synaptic transmission between neurons. These processes allow for the rapid communication of signals within the nervous system, enabling the body to respond effectively to various stimuli.
Resting Potential
Before a neuron can transmit an impulse, it must maintain a resting potential, which is the stable electrical charge difference across the neuronal membrane when the neuron is not transmitting an impulse. This resting potential is typically around -70 mV, with the inside of the neuron being more negative compared to the outside.
The resting potential is maintained by the sodium-potassium pump, which actively transports three sodium ions (Na+) out of the cell and two potassium ions (K+) into the cell, creating an electrochemical gradient. Potassium ions can also leak out of the cell through potassium channels, further contributing to the negative charge inside the neuron. This imbalance of ions is necessary for setting up the conditions required for the generation of an action potential.
Transmission of an Action Potential
An action potential is an electrical signal that travels along the axon of a neuron. It begins when a neuron receives a strong enough stimulus, causing the membrane potential to reach a critical threshold approximately -55 mV. This triggers the opening of voltage-gated sodium channels, allowing sodium ions (Na+) to flow into the neuron, causing depolarisation of the membrane. The influx of positive ions generates a wave of electrical activity that moves down the axon.
Following depolarisation, potassium channels open, allowing potassium ions (K+) to exit the cell. This repolarises the membrane, restoring the resting membrane potential. The action potential travels rapidly along the axon due to saltatory conduction, where the impulse jumps from one node of Ranvier to the next, bypassing the insulated sections of the axon covered by the myelin sheath.
Synaptic Transmission
When the action potential reaches the axon terminal of the presynaptic neuron, it initiates the release of neurotransmitters. The arrival of the action potential triggers the opening of voltage-gated calcium channels, allowing calcium ions (Ca2+) to enter the axon terminal. This influx of calcium causes synaptic vesicles containing neurotransmitters to move towards and fuse with the presynaptic membrane.
Neurotransmitters are then released into the synaptic cleft, the small gap between the presynaptic and postsynaptic neurons. They diffuse across the cleft and bind to specific receptors on the postsynaptic membrane. This binding triggers a response in the postsynaptic neuron, either exciting it and generating a new action potential or inhibiting it, depending on the type of neurotransmitter and receptor involved.
Signal Transduction
The binding of neurotransmitters to receptors on the postsynaptic neuron leads to signal transduction, where the chemical signal is converted back into an electrical signal. If the neurotransmitter binding causes depolarisation of the postsynaptic membrane, an action potential may be generated, continuing the nerve impulse transmission. Alternatively, inhibitory neurotransmitters can cause hyperpolarisation, reducing the likelihood of an action potential and thereby modulating the response.
The neurotransmitters in the synaptic cleft are quickly broken down by enzymes or taken back into the presynaptic neuron through reuptake channels, ensuring that the signal is brief and allowing the synapse to be ready for the next impulse.