These lecture notes introduce the concepts of action potentials, different types of ion channels, the structure of neurons, synapses and a quick look at how drugs affect our brains. This document contains significant information that you will not only need to pass your MCQ exams but comes in handy ...
Action Potentials:
When a neuron is stimulated, it sends an impulse down the axon to the
following neuron. Each individual neuron only has one specified
action/impulse that it can only send at a particular speed and intensity. The
brain then translates this corresponding speed and intensity of signal to
match an action.
These nerve impulses are called action potentials. But how does this
happen?
The structure of a neuron:
The neuron has a thick membrane that separates positively charged ions
from negatively charged ions. It keeps these ions separated until we require
the energy that their attraction creates. A visual event or stimulus will then
result in these charges being brought together to create the energy to
perform an action potential. A resting neuron remains negatively charged
inside, with positive ions outside of the membrane.
A resting membrane potential = 70mv
The positively charged ions outside the resting neuron are sodium ions.
There are some positively charged potassium ions inside the membrane
mingled with larger, negatively charged proteins. When there is a negative
membrane potential inside the neuron, this means the neuron is polarised.
There are also sodium-potassium pumps on the surface of the neuron, along
the axon. (For every 2 potassium ions that are pumped inside the
membrane, it pumps out 3 sodium ions. This creates an electrochemical
gradient. In order to even this gradient out, more ions need to pass through
the membrane.
, Ion Channels
Ion channels are proteins that allow more ions to pass through the
membrane when the gates are open. There are 3 different types:
Voltage-gated channels: Open and close in response to changes in
membrane potentials
Ligand-gated channels: Open when a specific neuro-transmitter latches
onto a receptor- such as serotonin.
Mechanically gated channels: Open in response to a physical stretching
of the membrane.
When gates are open, ions can quickly diffuse through the membrane,
balancing out the concentration.
When only a few gates open, limited sodium ions enter the cell causing a
graded potential, which is a much smaller reaction in comparison to an
action potential. For this signal to go further, it would require a much bigger
charge. Otherwise known as an action potential.
The process of an action potential:
To create an action potential, the neuron must depolarise with a substantial
change in potential which will cause the voltage gated channels to open. But
how does this process occur?
1. A mental stimulus causes the sodium channels to open, causing an
increased charge inside the membrane.
2. The membrane must reach the threshold of –55mv to cause the
voltage gates to open.
3. The voltage gates open, causing a depolarisation.
4. At 40mv, the depolarisation causes an action potential.
After an action potential has occurred, the cell will begin to repolarise. In
fact, it will briefly hyperpolarise to –75mv causing the gates to close and the
sodium/potassium pump will return equilibrium to the cell.
This repolarisation stage can also be called the refractory period. During this
time, the neuron is unable to respond to any stimuli. This simply prevents 2
things from travelling down the axon at once.
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