Nerve impulses Flashcards
(13 cards)
What is the structure of a myelinated motor neuron?
- Cell body: contains most organelles (e.g. nucleus, RER, ribosomes).
- Dendrites: receive electrical impulses from neighbouring neurons.
- Nucleus: contains the cell’s DNA, coding for axon proteins.
- Axon: transfers electrical impulses from cell body to synapse.
- Myelin sheath: forms an electrically-insulting cover around the axon.
- Synapses: transmit electrical impulses to neighbouring neurons via neurotransmitters.
How is a resting potential established?
3 Na+ ions are actively transported out of the axon in exchange for 2 K+ ions, via a Na+/K+ pump.
The Na+ builds up outside of the axon as the voltage-gated Na+ channels are closed, so Na+ cannot diffuse back. Voltage-gated K+ channels are closed.
Describe depolarisation in how an action potential is established.
A stimulus excites the cell-surface membrane which makes it more permeable to Na+ ions. This causes the Na+ channels to open so Na+ ions diffuse into the neuron causing the K+ channels to close. The inside of the neuron is less negative than the outside.
Describe repolarisation in how an action potential is established.
The Na+ channels close and the K+ channels open causing the K+ ions to diffuse out of the neuron. The inside of the neuron becomes more negative and the voltage decreases back to the normal level.
Describe hyperpolarisation in how an action potential is established.
The K+ channels are still open for a short time meaning too many K+ ions diffuse out of the neuron. The voltage becomes more negative than the resting potential.
Describe how the resting potential is re-established.
The K+ channels close, and the Na+/K+ pump restores resting potential.
What are the voltages at depolarisation, repolarisation, and rest.
Depolarisation = -50mV
Repolarisation = -20mV
Resting = -70mV
Describe the passage of an action potential along an unmyelinated axon.
As one region depolarises, it acts as a stimulus for the depolarisation of the next region meaning an action is a travelling wave of depolarisation. In the mean time, the previous region repolarises.
Describe the passage of an action potential along a myelinated axon.
The myelin sheaths act as electrical insulators which prevent action potentials from occurring. At regular intervals, there are breaks in this insulation known as the Nodes of Ranvier. Actional potentials only occur at the Nodes of Ranvier as the potential jumps from one node to the next as localised circuits arise between adjacent Nodes of Ranvier. This makes the actional potential pass along the neuron faster so less energy from ATP hydrolysis is needed for the Na+/K+ pump.
State the all-or-nothing principle in action potentials.
An action potential only happens if the stimulus reaches the threshold value, and they are always the same size.
What are the key factors affecting the speed of impulses?
Myelination, axon diameter, and temperature.
How does the axon diameter affect the speed of impulses?
The greater the axon diameter, the faster the conduction due to less leakage out of the axon (mostly applies to invertebrates who do not have myelination).
How does the temperature affect the speed of impulses?
Higher temperature results in faster conduction as ions have more kinetic energy meaning facilitated diffusion is faster, there is also higher enzyme activity.
