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Chapter 3

PSY100H1 Chapter Notes - Chapter 3: Axon Hillock, Axon Terminal, Diffusion Mri


Department
Psychology
Course Code
PSY100H1
Professor
Michael Inzlicht
Chapter
3

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Psychology Module 3.2,3.3,3.4
The human body is composed of many different types of cells. Psychologists are most
interested in neurons: one of the major types of cells found in the nervous system, that
are responsible for sending and receiving messages throughout the body. The primary
purpose of euros is to fire to reeie iput fro oe group of euros ad to the
transmit that information to other neurons. Doing so allows single neurons to work
together as part of networks, involving thousand and even millions of other cells, this will
eventually lead to some form of behavior.
All neurons have a cell body also known as a soma, is the part of a neuron that contains
the uleus that house the ell’s geeti aterial. Gees i the ell ody sythesize
proteins that form the chemical and structures that allow the neurons to function. The
activating of these genes can be influenced by the input coming from other cells. This
input is received by dendrites: small, branches radiating from the cell body that receive
messages from other cells and transmit those messages toward the rest of the cell.
At any given point in time neurons will receive input from several other neurons. These
impulses from other cells will travel across the neuron the base of the cell body known as
the axon hillac. If the axon hillac receives enough stimulation from other neurons it will
initiate a chemical reaction will flow down the rest of the neurons.
This chemical reaction is the initial step into a neuron communicating with other cells
(influencing if others will fire or not). The activity will travel from the axon hillock along a
tail like structure that protrudes from the cell body.
This structure the axon: transports information in the form of electrochemical reactions
from the cell body to the end of the neurons. When the activity reaches the end of the
axon, it will arrive at axon terminals: bulb like extensions filled with vesicles (little bags
of molecules). These vesicles contain neurotransmitters: the chemicals that function as
messenger allowing neurons to communicate with each other. The impulse travelling
down the axon will stimulate the release of these neurotransmitters, thus allowing
neural communication to take place.
Sensory Neurons: Receive information from the bodily senses and bring it toward the
brain. Neurons that respond to touch or pain sensation of the skin bring the message
toward the spinal cord and to the brain. In contrast Motor neurons: carry messages
away from the brain and spinal cord and toward muscles in order to control their flexion
and extension
Sensory and Motor Neurons: Sensory neurons carry information toward the spinal cord
and the brain whereas motor neurons send message to muscles of the body. The
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2
interneuron links the sensory and motor neurons. This is the pathway of simple
withdrawal response to painful stimulus.
Although neurons are essential for our ability to sense, move and think, they cannot
function without support from other cells. This support comes from different types of
cells collectively known as Glial Cells: Are specialized cells of the nervous system that are
involved in the mounting immune responses in the brain removing waste and
synchronizing the activity of the billions of neurons that constitute the nervous system.
Myelin: A fatty sheath that insulates axons from one another, resulting in increased
speed and efficiency and neural communication.
Resting Potential: Relatively stable state during which the cell is not transmitting
message is known as resting potential. This seemingly stable state involves a great deal
of tension. This is because of two forces electrostatic gradient: the inside and outside of
the cell have different charges, and the concentration gradient: different types of ions
are more densely packed on one side of the membrane than on the other
The surge of positive ions into the cell changes the potential of the neuron. These
changes flow down to the dendrite and cross the cell body to the axon hillock, where the
cell body meet the axons. If enough positively charged ions reach the axon hillock to
push its charge past the cells firing threshold the neurons will then initiate the action
potential: A wave of electrical activity that originates at the base of the axon and rapidly
travels down its length. When action potential occurs the charge of that part of the axon
changes from approx. 70MV to 35 Mv in other words the cell changes from being
negatively charged to positively charged. This change does not occur at once along entire
axon. Rather as one part gets depolarized it forces open the ion channels ahead of it thus
causing the action potential to move forward down the length of the axon as positively
charged ions rush membrane pores. This pattern continues until the action potential
reaches the axon terminal.
At each point of the axon, the ions channels slam shut as soon as the action potential
occurs. The sodium ion that had rushed into the axon are then rapidly pumped out of the
cell. This process of removing sodium ions from the cell often causes the neurons to
become hyperpolarized: This means that the cell is more negative than its normal resting
potential. This additional negativity makes the cell less likely to fire. It normally takes 2-3
milliseconds for the membrane to adjust back to normal resting potential. This brief
period in which a neuron can not fire is known as the refractory period.
When the action potential reaches the axo terial, it triggers the release of that ell’s
neurotransmitter into the synapse: The microscopically small spaces that separate
Individual nerve cells. The cell that releases these chemical is known as the presynaptic
cell (Before the synapse) whereas the cell that receives this input is postsyaptic after
the syapse)
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The dendrites of the postsynaptic cells contain specialized receptors that are designed to
hold specific molecules, including neurotransmitters. Then the process of neural
communication will begin again.
Nerve cells fire once threshold of excitation is reached. During the action potential,
positively charged ion are then forced out of the cell as it returns to resting potential.
All or none principle: Individual nerve cells fire at the same strength every time an action
potetial ours. Neuros do’t sort of fire or oer fire-they just fire
The Lock and Key Analogy for Matching Neurotransmitters and Receptors: The
molecular structure of different neurotransmitters must have specific shapes in order to
bind with receptors on a neuron.
Synaptic Cleft: the minute space between the axon terminal (terminal buttons) and the
dendrite This process is almost as important as the action potential itself. Prolonged
stimulation of the receptors makes it more difficult for the cells to return to its resting
potential.
Reuptake: A process whereby neurotransmitters molecules that have been released into
the synapse are reabsorbed into the axon terminal of the presynaptic neuron. Reuptake
serves as a sort of natural recycling system for neurotransmitters. Also a process used by
many commonly used drugs like antidepressant drugs, known as selective serotonin
reuptake inhibitors, increase the amount of serotonin available in synapse. Result
decrease in depression
Dozens of neuro transmitters.
The most common one is Glutamate: is the most common excitatory neurotransmitter in
the brain of vertebrates. It is involved in a number of processes including our ability to
form new memories. Abnormal function of glutamate releasing neurons has also been
implicated in a number of brain disorder including the triggering of seizures in epilepsy.
Excites nervous system: memory and autonomic nervous system reactions.
GABA (Gamma-amino butyric acid): is the primary inhibitory neurotransmitter of the
nervous system, meaning that it prevents neurons from generating action potentials.
Reduces negative charge on neighboring neurons even further than their rest state.
Inhibits brain activity: lowers arousal, anxiety and excitation, facilitates sleep. Low levels
have been linked with to epilepsy.
Acetylcholine: Is one of the most widespread neurotransmitters within the body found
at the junction between nerve cells and skeletal muscles; it is very important for
voluntary movement. Released from neurons connected to the spinal cord binds to
reeptors o usles. Cogitie defets assoiated ith agig ad Alzheier’s disease.
Movement; attention
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