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Chapter 3

PSY100H1 Chapter Notes - Chapter 3: Resting Potential, Acetylcholine, Choline

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CHAPTER 3 – Genetic and Biological Foundations
Nervous system
-A communication network comprised of billions of specialized cells, evaluates info from the external
world then produces behaviours or makes bodily adjustments to adapt to the environment
-The basic unit of the nervous system
- Cells that specialize in communication
- Operate through electrical impulses and communicate with other neurons through chemical signals
-They differ from most other cells because they are excitable
-Have 3 functions:
oReception: take in info from neighboring neurons
oConduction: integrate those signals
oTransmission: pass signals to other neurons
-Come in a wide assortment of shapes and sizes, but typically share 4 structural regions that assist
the neuron’s communication functions
1) Dendrites: branchlike extensions of the neuron that detect info from other neurons
2) Cell body: in the neuron, where info from thousands of other neurons is collected and
3) Axon: a long narrow outgrowth of a neuron by which info is transmitted to other neurons
4) Terminal buttons: small nodules at the end of axons that release chemical signals from
the neuron to an area called synapse
Synapse: the site for chemical communication between neurons
Lipids: double layer of fatty molecules of the membrane, the boundary of a
Membrane: involved in communication between neurons by regulating the
concentration of electrically charged molecules that are the basis of the
neuron’s electrical activity
-A neuron: messages are received by the dendrites, processed in the cell body, transmitted along the
axon, and sent to other neurons via chemical substances released from the terminal buttons
Types of Neurons
1) Sensory neurons: these afferent neurons detect info from the physical world and pass
that info along to the brain, usually via the spinal cord
Afferent neurons: receptors send signals from the body to the brain to
produce a response
2) Motor neurons: these efferent neurons direct muscles to contract or relax, thereby
producing movement
Efferent neurons: receptors send signal that travel from the brain to the body
3) Interneurons: these neurons communicate only with other neurons, typically within a
specific brain region, communicate within local or short-distance circuits, that is,
interneurons integrate neural activity within a single area rather than transmitting info to
other brain structures or to the body organs
- Together, sensory and motor neurons control movement
-The nerves that provide info from muscles are referred to as somatosensory, which is the general
term for sensations experienced from within the body
- Neurons don’t communicate randomly or arbitrarily, they selectively communicate with other neurons
to form circuits or neural networks. These networks develop through maturation and experience,
forming permanent alliances among groups of neurons
Action potentials cause neuronal communication
-Action potential (neuronal firing): the neural impulse (electrical signal) that passes along the axon
and subsequently causes the release of chemicals from the terminal buttons to other neurons
The resting membrane potential is negatively charged
-Resting membrane potential: the electrical charge of a neuron when it is not active
-The differential electrical charge inside and outside of the neuron is a condition known as
The roles of sodium and potassium ions

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- Two types of ions that contribute to a neuron’s resting membrane potential are sodium ions and
potassium ions
-The flow of ion through their channels is controlled by a gating mechanism
- as a result of the selective permeability of the cell membrane, there is more potassium inside the
neuron than sodium, which contributes to polarization
- The channels are specialized for specific ions
Changes in Electrical Potential Lead to Action
- Firing means passing a signal along the axon and releasing chemicals from the terminal buttons
-the signals arrive at the dendrites by the thousands
-2 types
1) Excitatory: signals stimulate the neuron to fire, depolarization
2) Inhibitory: signals reduce the likelihood of the neuron’s firing, hyperpolarization
-Signals work by affecting polarization in the cell membrane
- Depolarization causes a change in the permeability of the cell membrane, which opens the gates of
sodium channels and slows sodium to rush into the neuron, this influx of sodium causes the inside of
the neuron to become slightly more positively charged than the outside
- This change from a negative to a positive charge inside the neuron is the basis of the action potential
- Signals that are inhibitory lead to hyperpolarization of the cell membrane, in which sodium channels
become even more resistant to the passage of sodium. This hyperpolarization means that it will be
more difficult for excitatory signals to cause neuronal firing
Action potentials spread along the Axon
- Inhibitory and excitatory signals received by the dendrites are integrated within the neuron
- When the neuron fires, the depolarization of the cell membrane moves along the axon like a wave,
which is called propagation
Absolute and relative refractory periods
-Once the gating mechanism stops the flow of sodium into the neuron, potassium stops leaving,
during which a decreased concentration of potassium in the cell body momentarily creates a state of
hyperpolarization. During this brief period, known as the absolute refractory period, it is
impossible for the neuron fire, which keeps the action potential from repeating up and down the axon
in a kind of ripple effect
- The signal can’t travel backward because the preceding axonal section is in the refectory period and
its sodium ion channels are blocked.
All-or-non principle
- the firing of the neuron is all or none-a neuron cannot partially fire
-All-or-none principle: dictates that a neuron fires with the same potency each time, but at intervals
of different frequency depending on the strength of simulation.
The Myelin sheath
-Myelin sheath: a fatty material, made up of glial cells, that insulates the axon and allows for the
rapid movement of electrical impulses along the axon
-Nodes of Ranvier: small gaps of exposed axon, between the segments of myelin sheath, where
action potential are transmitted
Multiple sclerosis
-decay of myelin sheath surrounding axons, affects mostly young adults
- since the myelin insulation helps messages move quickly along axons, demyelination slows down
neural impulses
-axons short-circuit, normal neuronal communication is interrupted, brain hardening, scaring
- motor actions become jerky, lose ability to coordinate movements
-both genetic (more common in identical twins, Caucasians) and environmental (cold climates when
oMay have its origin in a slow-acting infection contracted early in childhood, no cure
Neurotransmitters bind to receptors across the synapse
Synaptic cleft
- a small space between neurons that contains extracellular fluid
-site of chemical communication between neurons
- action potentials cause neurons to release from their terminal buttons chemical that travel across the
synaptic cleft and are received by the dendrites of other neurons
- the neuron that sends the signal is called presynaptic, and the one that receives the signal is called
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