Explain in detail how the nervous system is transmitted along the axon of a nerve cell.
Describe the transmission across the junction between one neurone and the next in detail in your answer.
Explain in detail how the impulse is transmitted across a neurone.
Explain in detail how the impulse crosses the synapse.
Draw a diagram for all of this and label it.
Describe in detail what happens along the axon and at a synapse. Structure your answer in a series of stages. Start by describing what happens at the end of one neuron in detail. Then describe in detail what happens in the synapse itself. Then describe in detail what happens in the second neuron.
Transmission of an impulse along the axon of a nerve cell:
1. The process begins at the axon hillock, or initial segment of the axon, where the action potential (electrical signal) is generated.
2. The action potential is initiated when the resting membrane potential crosses the threshold, causing an influx of positively charged sodium ions (Na+) into the axon through voltage-gated sodium channels.
3. This influx of sodium ions causes a rapid positive change in the axonal membrane potential, leading to the depolarization phase of the action potential. This change causes adjacent voltage-gated sodium channels to open, allowing more sodium ions to flow in.
4. The depolarization process propagates along the axon, moving the action potential forward in a unidirectional manner through a process called saltatory conduction, especially in myelinated axons.
5. The action potential reaches its peak, and then enters the repolarization phase, where potassium (K+) channels open, allowing potassium ions to rush out of the axon, restoring the resting membrane potential.
6. The resting state is eventually re-established by the action of the sodium-potassium pump, which moves three sodium ions out of the cell and two potassium ions in, returning the membrane potential to its resting value.
Transmission across the junction between one neuron and the next (the synapse):
1. When an action potential arrives at the presynaptic terminal, the depolarization causes voltage-gated calcium (Ca2+) channels to open.
2. The influx of calcium ions triggers synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters (such as dopamine, serotonin, or acetylcholine) into the synaptic cleft.
3. The neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins located on the postsynaptic membrane of the second neuron.
4. When the neurotransmitters bind to their receptors, ion channels are either opened or closed, producing excitatory or inhibitory postsynaptic potentials in the second neuron.
5. If the excitatory input to the second neuron is strong enough to depolarize the membrane potential above the threshold, an action potential is triggered in that neuron's axon hillock, propagating the signal along the neuron.
In summary, the transmission along the axon involves depolarization and repolarization through the sequential opening and closing of the sodium and potassium channels. At a synapse, neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron, triggering either excitation or inhibition of the second neuron.