Unit 4: Week 3 Flashcards
What is an action potential?
The electrical activity that causes a brief reversal in membrane potential (depolarisation) in a neuron. Depolarisation is the switch in the polarity in membrane potential.
They have the same amplitude no matter how strong the initial stimuli.
What happens to cause a neuron to depolarise?
The neuron is initially at rest and has a resting potential. The summation of graded potentials received by dendrites means that the threshold potential is met. Finally, there is an action potential starting at the axon hillock leading to synaptic activity.
What is resting potential?
When a neuron is polarised the neuron is more negative inside than outside due to the abundance of potassium ions. The potential difference across the membrane is -70mV.
What maintains the resting potential?
Potassium and sodium ions leak/passive channels allow ions to pass through the plasma membrane.
The cell is more permeable to potassium. Potassium and sodium ATPase pump is used to restore the membrane potential at rest.
What happens at the potassium/sodium ATPase pump?
Three sodium ions bind to the inside of the pump. ATP is hydrolysed into ADP and the shape of the pump changes to alternate confirmation. Sodium ions are released outside, and two potassium ions bind to that surface. Phosphate is released and the pump resumes original shape with the membrane potential restored.
What happens during an action potential?
When the membrane potential reaches -55mV through the summation of graded potentials, the sodium voltage-gated channels open, making the inside more positive, causing depolarisation. This is the reversal of membrane potential. At +30mV, the sodium ion channels close and the voltage-gated potassium ion channels open, and potassium leaves the cell causing repolarisation. This results in an undershoot, causing hyperpolarisation due to potassium making a large difference in embrace potential, leak channels and the sodium/potassium pump work to restore a resting potential.
What is a graded potential?
Graded potentials are the potentials that are
received by the dendrites that vary in size.They can be added together to reach the threshold potential.
They originate from the dendrites which synapse with other neurons to open up chemically gated channels. The type of neurotransmitter and receptor determines whether it is excitatory or inhibitory.
What happens during an excitatory graded potential?
This is known as an ESPS, which is the excitatory postsynaptic potential, meaning that on the postsynaptic neuron sodium channels open up (ligand-gated channels). This means that the neuron is closer to the threshold potential which triggers an action potential.
What happens during an inhibitory graded potential?
This is known as an ISPS, which is the inhibitor postsynaptic potential, meaning that on the postsynaptic neuron potassium channels open up (ligand-gated channels). This means that the neuron is further from threshold potential which doesn’t trigger an action potential. Calcium (CL-) channels may be opened, further dropping membrane potential moving away from threshold potential.
What are the two types of summation?
- Spacial summation- where a cell body via the dendrites is stimulated by several presynaptic neurons. The EPSPs spread from the synapses to the axon hillock. If threshold potential is reached the postsynaptic neuron will trigger an action potential.
- Temporal summation- the cell body via the dendrites will be stimulated by one presynaptic neuron sending high frequency signals. These EPSPs will travel from one synapse to the axon hillock and if threshold potential is reached the neuron fires an action potential.
What happens in continuous conduction?
This occurs in non-myelinated axons. Sodium voltage-gated channels open in the trigger zone causing depolarisation and this reverse in membrane potential will cause depolarisation in the next segment. Depolarisation-reversal in membrane potential. The previous section is now in the absolute refractory period, so the action potential only propagates in one direction. Since each segment needs to undergo this reaction, conduction is slow.
What happens during saltatory conduction
This occurs in myelinated neurons. The action potential causes depolarisation in one segment which is able to jump from one node of ranvier to the next node of ranvier. At the node there is a high concentration of voltage-gated channels, so the action potential is able to move further. This is faster than continuous conduction. No action potential is lost due to insulation. Increase conduction velocity.
What is a myelin sheath?
Over time, schwann cells/oligodendrocytes- types of glial cells- wrap around the axon to form the myelin sheath. The sheath itself is modified plasma membrane, a part of the schwann or oligodendrocyte cell.
What is the absolute refractory period?
This refers to the period within which it is impossible to elicit another action potential no matter how strong it is. This is because sodium channels are inactivated and remain that way until hyperpolarisation. This period stretches from when the threshold is met until hyperpolarisation.
What is the relative refractory period?
This refers to the period when a larger than normal stimulus is needed for an action potential to occur. This is due to the difference in resting potential and hyperpolarisation. This difference value in membrane potential plus the threshold of excitation needs to be met before another action potential is triggered.
