Topic 6: Human Physiology

Topic 6.1

Topic 6.2

Topic 6.3

Topic 6.4

Topic 6.5

Nerve Impulse:

Resting potential:

High concentration of sodium ions outside the axon, high concentration of potassium ions on the inside. Inside is relatively more negative.

Depolarization:

Dendrite receives neurotransmitter and sodium channels open. The flowing of sodium ions into the inside of the axon makes it more positive.

Action potential:

The change in charge also triggers the next sodium channels down the axon to open. This is creating an action potential.

Also, must note, with myelinated axons, the Schwann cells act as insulators. So the action potential skips right from one node of Ranvier to the next, making the process much faster. This is called saltatory conduction

Repolarization:

Then, the difference in charge causes the potassium ion channels to open. The potassium flows down its gradient, to the outside of the cell.

Returning to resting potential:

Now, both ions are on the wrong side of the cell membrane. A sodium-potassium pump uses active transport to pump 3 sodium ions in per each 2 potassium ions pumped out.

Synaptic transmission:

When the impulse travelling down the axon eventually gets to the axon terminal, it causes calcium ion channels to open. The concentration on the outside is very high, and they flow inward. All of the calcium now in the axon terminal causes vesicles full of neurotransmitters to travel to the cell membrane, and through exocytosis, they release the transmitters in the synapse (synaptic cleft).

By doing this, the impulse is able to be continued from the pre-synaptic neuron to the post-synaptic neuron. The neurotransmitters attach to receptors on the post-synaptic neuron's dendrites, which then causes sodium channels to open, and thus the chain continues.

Motor neurons

Motor neurons, instead of having a synaptic cleft at the axon terminal which then has a post-synaptic neuron after it, it can connect to a muscle cell

It can release the neurotransmitter acetylcholine, which binds to the acetylcholine receptors on the muscle cell's membrane (also known as the sarcolemma)

Insecticides known as neonicotinoids permanently bind to the acetylcholine receptors

So this then prevents muscle from contracting, since acetylcholine is blocked from binding

This leads to paralysis of all muscles, and death

Important diagrams (from IB Biology Oxford textbook, 2014 Edition)

Topic 6.6