In this IBDP Biology topic, we are to understand:
- The structure of a nervous neuron
- What nerve impulses are
- Resting and Action Potential
- Synaptic Transmission
Nervous system and the Neuron
The nervous system consists of the Central Nervous System (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electric impulses.
- Neurons carry messages in the form of rapid electrical impulses
- Nerve impulses are propagated along the Axon
Propogation of the nerve impulse: An action potential in one part of the axon triggers another action potential in the next part
Local currents: Diffusion of sodium ions along the axon both inside and outside of the membrane
- Myelinated nerve cell fibres have a
myelin sheath and the small gaps are called Nodes of Ranvier
- Nucleus is loctated in the cell body
- The Nodes of Ranvier allow nerve impulses to jump from node to node
Saltatory Conduction: Nerve impulses jump from node to node which speeds up transmission with the aid of the myelin sheath
Resting and Action Potential
Resting Potential: Voltage (electrial potential) across the plasma membrane of a neuron when it is NOT conducting a nerve impulse
Resting Potential: -70 millivolts
- Using Sodium-Potassium pumps in the membrane so ions are pumped by
Active Transport
- Sodium
ions are pumped out
- Potassium
ions are pumped in
- Concentration gradients
of both ions are established
- Some Potassium ions diffuse
back OUT - outside membrane potential more positive
- The inside of the neuron develops a net-negative charge compared to the outside because the presence of chloride and other negatively charged ions
Once a current reaches a certain threshold of -50 millivolts, Action Potential takes place
Action Potential: Facilitated diffusion of ions through voltage gated ion channels
1. Depolarization: Membrane potential rises from -70 to -50 millivolts
Depolarization: Potential across membrane is reversed
Voltage gated sodium channels open and ions goes in the membrane (down concentration gradient)
- Inside of neuron now has a net positive charge compared to the outside
2. Repolarization
Repolarization: Potential across membrane is restored
Voltage gated potassium channels open and ions diffuses out of the membrane
- Inside of neuron to develop and net negative charge compared to the outside
Synaptic Transmission
Synapse: Junction between two neurons or a neuron and effector cell (muscle)
- Messages are passed across the synapse in the form of chemicals called
neurotransmitters
- Neurotransmitters move from the
pre-synaptic neuron to the post synaptic neuron
- At the dendric end end of a nerve cell each dendrite
collects the nerve impulse from the terminal end of a different nerve cell. The impulse needs to jump across a small gap (synapse) by the process of synaptic transmission
- The electrical impulse is converted to a
chemical neurotransmitter
In IBDP Biology, we need to know the proccess of the Synaptic Transmission:
- A nerve impulse reaches the end of the
- pre-synaptic neuron
- Depolarization
- of the pre-synaptic membrane causes vesicles of neurotransmitters to move to the pre-synaptic membrane and fuse with it, releasing the neurotransmitter into the synaptic cleft by
- exocytosis
- The neurotransmitter
- diffuses across the synaptic cleft
- and
- binds to receptors
- in the post-synaptic membrane
- The receptors are transmitter gated sodium channels - opens when the transmitter binds. Sodium ions
diffuse into the post-synaptic neuron. This causes depolarization of the post-synaptic membrane
- The depolarization passes down the post-synaptic neuron as an
Action Potential
- Neurotransmitter in the synaptic cleft is rapidly broken down to prevent continuous synaptic transmission
And that's the end of this sub-topic of Topic 6! To see more:
References:
- https://lh3.googleusercontent.com/proxy/mIJ1-bX2NOc452tYQ6_CMdG9YeSckD57Y0zw3Ra_H8X353qFL0ZCL43Hhv5CoUVdi1snRcw0TpZ3fAtPYn5y9Qaph6CkO4oqMWcoKoxi9lE41w
- https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.researchgate.net%2Ffigure%2FA-schematic-of-an-action-potential-When-a-stimulus-is-applied-an-action-potential-is_fig1_335650473&psig=AOvVaw1fItfuwZlzimq8cqpqq1hq&ust=1624953226530000&source=images&cd=vfe&ved=0CAoQjRxqFwoTCKinotHsufECFQAAAAAdAAAAABAD
- https://media.tenor.com/images/5940b8a10db0fd53ba1e80e81cd2d3ba/tenor.gif
Written by Venetia (Biology)