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Lecture 5

PSYCH 1XX3 Lecture Notes - Lecture 5: Potassium Channel, Myelin, Neuroglia


Department
Psychology
Course Code
PSYCH 1XX3
Professor
Joe Kim
Lecture
5

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Lecture 5: Neuroscience I
Introduction to Neuroscience
Descartes
o Tried to understand the mental processes that allow a person to learn, feel and
act and relate it all to the brain
o His approach was to separate the mental processes of the mind from the
physical processes of the brain
o In his dualist framework, the mind was seen as a separate entity existing outside
of our biology, yet in control of our actions and thoughts
o The physical brain was thought to serve as a connection between the mind and
body
o In modern times, the challenge for neuroscience is to understand how the
biological brain produces the mental processes of the mind
The Neuron
Neurons are cells that are specialized for communication
Each of your 100 billion neurons are organized into signalling pathways to communicate
via synaptic transmission
What makes neurons good at communicating is their unique structure
o A typical neuron contains two distinct zones
A receptive zone designed to receive signals from other neurons
Made up of dendrites branching out from the cell body
A transmission zone designed to pass on signals to other cells
Made up of the axon and terminal boutons
The receptive zone
o Begins with the cell body
o The cell body contains most of the vital organelles, which keep the cell
functioning
o Branching from the cell body are a number of projections called dendrites
o These dendrites reach out to other neurons and receive signals to be relayed
through the dendritic branch to the cell body, where some signals will go on to
be conveyed down the axon
The axon
o Once a neuron receives a signal in the receptive zone, it is passed down a long
fiber called the axon, which can vary in length
o Some neurons have very short axons, while others have axons that can be 1m in
length as they extend from your spine to the bottom of your feet
o At the end of the axon, approaching the transmission zone of the neuron, is
another cluster of branches
o These branches at the end of the neuron are called end-feet or terminal boutons
or terminal ends

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o The terminal ends reach out and make connections with receptive zones of
nearby neurons to transmit a signal further
A neural network
o Each neuron can receive inputs from thousands of other neurons through their
dendrites and terminal boutons to form a complex network of information
transfer
o The glial cells are the hardworking, co-stars of the nervous system
They provide structural support, nourishment and insulation needed by
the high profile neurons
o The glial cells and neurons that work together, resting in a bath of ion chemicals
and blood vasculature make up the entirety of your brain
The Action Potential
A neuron’s cell membrane separates the intracellular fluid, which fills the neuron and
the extracellular fluid, which surrounds it
Each contains different concentrations of important ions, including sodium, potassium
and chloride
The cell membrane is selectively permeable, preferentially allowing different ions to
pass through it with various levels of ease
The cell membrane also contains a number of protein channels, which acts as
passageways for ions to pass through
Important channels to consider include the potassium channel and the sodium channel
The selective movement of ions across the cell membrane into and out of the neuron is
critical for neural communication
The resting potential
o The inside of a typical neuron starts of at -70mv relative to the outside of the cell
o This baseline imbalance is called the resting potential of the neuron
o The resting potential of a neuron is controlled by two forces, diffusion and
electrostatic force
o Diffusion is the force that distributes molecules evenly throughout a medium
o The diffusion force interacts with the electrostatic force between charged ions
When two similarly charged ions meet, they repel each other and when
two oppositely charged ions meet, they attract
o The net result of the diffusion and electrostatic forces leads to an overall resting
potential of -70mv inside the cell compared to the outside
o At the start of the resting potential, the negatively charged large protein
molecules within the neuron are so large that they cannot pass through the cell
membrane and so they remain trapped inside
o On the other hand, potassium, sodium and chloride ions are mobile
o Two different types of potassium channels
The leaky potassium channel is like a tap that’s always open
It allows positively charged potassium to pass through the cell
membrane out of the neuron

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However, most of the potassium remains inside the cell at rest
Overall the leaky potassium channel is a major contributor to
maintaining the resting potential of the neuron
Voltage gated channel
Important for the action potential
o The negatively charged chloride ions are also mobile and the electrostatic force
of the negatively charged protein molecules keep them primarily on the outside
of the cell
o Voltage gated sodium channels are closed in the resting state of the neuron and
so the positively charged sodium ions flow in only very low concentrations into
the cell
o Despite this subtle inward flow, most of the sodium ions remain resting on the
outside of the cell and the flow of sodium is far less important to the resting
state of the neuron than potassium
The threshold
o The forces governing the distribution of ions are not rigidly stuck in place and in
reality, the resting voltage of the neuron is constantly fluctuating somewhere
around -70mv
o Under the influence of nearby neurons and random ion flow, a large enough
change in the resting charge will occur to reach an important threshold level
o When the threshold of -50mv is reached, the action potential is triggered
The action potential
o The fundamental unit of communication for neurons
o When the -50mv threshold is reached, a cascade of events is triggered
It starts with the sodium channels along the cell membrane beginning to
open
Up to this point, most of the sodium ions are on the outside of the cell
With the sodium channels now open, the force of diffusion causes the
positively charged sodium ions to being rushing into the neuron, causing
the charge on the inside of the cell to rapidly become more positive
relative to the outside
As the positively charged sodium rushes into the cell, the electrostatic
force begins to push some of the positively charged potassium ions out of
the cell through the leaky potassium channels
Overall, the net effect is to still increase the positive charge building up
inside the cell to the point (0mv) where the voltage gated potassium
channels open, which allows more positively charged potassium ions to
rush out of the cell
After reaching a peak charge of about +40mv on the inside of the cell, the
sodium channels close
This means that sodium stops entering the cell, but potassium continues
to rush outward through the still-open voltage gated potassium channel
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