Description
Part 1. Please help to create a concept map based on the given chapters. I’ve attached below the PDF of 4 chapters; you can pick 1 (or 2) chapters to create a Concept Map. (Please see the Concept Map criteria for more details)Part 2. Essays for Ted Talks. Please see the attached file for the Essay Requirements. Here are links to the assigned Ted Talks:1. (2) Lessons from the World Avoided | Sean Davis | TEDxBoulder – YouTube2. The politics of food: who influences what we eat? | Phillip Baker | TEDxCanberra (youtube.com)3. Mark Bittman: What’s wrong with what we eat | TED Talk
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For this assignment, you will create a concept map based on the chapters that have been covered
thus far in the course. You’re welcome to include material from all chapters we’ve covered so far,
from one entire chapter, or from a section of a chapter. This concept map will be shared with 2
of your peers so that we can appreciate each others’ approach, and help one another achieve
“Concept Map Awesome-ness!”
Here’s an Example:
Be sure to include the following criteria:
1. Start with a main topic that will serve as the focus of your concept map. I
recommend that you select a topic that is defined. In my example, my main focus is
“Sense Organs.” The reason why I recommend that you have a somewhat narrow
focus is because concept maps can get very complex quickly, especially if the focus
of the map is broad.
2. Include at least 3-4 “major subtopics” that branches out from your main topic. These
subtopics need to contribute and relate to the main topic of your map.
3. Elaborate on each of those subtopics. Add detail by adding secondary (or even
tertiary) processes that further relate to these subtopics. Perhaps you may also want
to include explanations on these processes or even images!
4. Continue to build your concept map with at least one more tier or layer of factors.
Your final map should include the main topic, at least 3-4 major subtopics, and at
least two more “layers” or “tiers” of factors in each of those subtopics.
BIOSC 48
Enzymes
Class of proteins that serve as biological catalysts
Chapter 4
Functions:
• Increase rate of a reaction
• Do not change the nature of the reaction
Enzymes and Energy
• Reaction can occur without enzyme, just slower
• Not changed by the reaction (can be reused)
• Lowers activation energy
BIOSC 48
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Activation Energy
Mechanisms of Enzyme Activity
Energy required for the reactants to engage in a reaction
1. A substrate approaches
enzyme
2. Substrates held in the
active site of the enzyme
(weak interactions)
3. The active site speeds up
the reaction (lowers EA)
4. Substrates converted into
products
5. Products released
6. Active site available for
binding of substrate
molecules
• Enzyme lowers the activation energy barrier
• Catalysts help reaction occur at lower temperatures
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BIOSC 48
Naming Enzymes
Naming Enzymes
• Suffix “-ase”
• First part indicates substrate and/or enzyme
function
Isoenzymes
• Enzymes that have the same name due to same
functions but located in different organs
• Phosphatase: removes phosphate group
• Synthetase and synthases: catalyze dehydration
synthesis
• Hydrolase: promotes hydrolysis
• Dehydrogenase: removes hydrogen atoms
• Kinase: adds phosphate group
• Isomerase: rearranges atoms
• Useful for detecting and diagnosing certain diseases
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Isoenzymes
Enzyme Activity
Diseased organs liberate different isoenzyme forms
into the blood
Measured by the rate at which substrate is converted to
product
•
Identification indicates the disease location
Influenced by:
• Temperature
• pH
• Concentration of cofactors and coenzymes
• Concentration of enzyme and substrate
• Stimulatory or inhibitory effects of products on enzyme
function
• Isoenzyme Example: Creatine phosphokinase (CK)
• CK-1 (BB): Damaged brain and lungs
• CK-2(MB): Damaged cardiac muscle in a myocardial
infarction (heart attach)
• CK-3(MM): Damaged skeletal muscle
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BIOSC 48
Enzyme Activity
Enzyme Activity
Effects of Temperature
• Increase temperature
à increase rate of
reaction
• Further increases in
temperature
denatures enzyme
Effects of pH
• Enzymes exhibit peak
activity within a narrow
pH range (pH optimum)
• Away from pH optimum
results in conformational
change
• Optimum pH reflects the
pH of the fluid the
enzyme is found in
• Ex: stomach vs. saliva vs.
small intestine
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Enzyme Activity
Enzyme Activity
Cofactors and Coenzymes
• Cofactors
Enzyme Activation
• Some enzymes are produced as “inactive” forms and
activated when needed
• Help form active site
through a conformational
change of the enzyme or
helps in enzyme-substrate
binding
• Ex: metal ions
• Activation achieved by:
• Additional enzymes for phosphorylation and
dephosphorylation
• Coenzymes
• Ex: binding to protein kinase
• Binding to small, regulatory organic molecules
• Derived from water-soluble
vitamins
• Transports H+ and other
small molecules between
enzymes
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• Ex: binding to cAMP
• Enzyme inhibition controlled through turnover
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BIOSC 48
Enzyme Activity
Enzyme Activity
Reversible Reactions
• A reaction that proceeds in two directions
Substrate Concentrations
• Increasing substrate
concentration à
increase the rate of the
reaction until enzyme
saturated
• Can be driven by a single enzyme
• Dependent on substrate/product concentration
• Law of Mass Action: Reversible reactions are driven
from the side of the equation of higher
concentration à lower concentration
• Ex: Enzyme, Carbonic Anhydrase
H2O + CO2 ↔ H2CO3
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Metabolic Pathways
Metabolic Pathways
Branched Metabolic Pathways
• Few metabolic pathways are linear
Most reactions are linked together in a chain (or
web) called a metabolic pathway
1. Initial substrate
2. Intermediate enzymatic steps
3. Final product
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• Branches produce different products
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BIOSC 48
Metabolic Pathways
Inborn Errors of Metabolism
Branched Metabolic Pathways: End Product Inhibition
• Branch points inhibited by the final products via a
negative feedback loop
• Prevents the final product from accumulating
• Mutation in a
single gene that
codes for an
enzyme in the
metabolic
pathway
• Products not
formed
• Results in disease
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Bioenergetics
Bioenergetics
The flow of energy in living systems
Second Law of Thermodynamics
The amount of entropy increases in every energy
transformation
• Entropy: degree of disorganization
• Free energy decreases as entropy increases
First Law of Thermodynamics
Energy cannot be destroyed
or created, only transformed
• Transformation is not 100%
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BIOSC 48
Endergonic Reactions
Exergonic Reactions
• Low à HIGH energy
state (uphill reaction)
• HIGH à Low energy
state (downhill
reaction)
• Energy used towards
converting ADP + P à
ATP
• Hydrolysis of
ATP à ADP + P
• Requires input of
energy
• Free energy:
• Products > reactants
• Synthesis reactions
• Produces energy
• Free energy:
• Products < reactants
Ex: Plants need energy from light to turn
CO2 and H2O into glucose
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• Decomposition
reactions
Ex: Breaking glucose down into CO2 and H2O
to produce energy
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Exergonic Reactions
Coupled Reactions
Exergonic reactions drive
endergonic reactions
• ATP production is an
endergonic reaction that is
coupled to an exergonic
reaction
• Ex: Energy from glucose
breakdown (exergonic) is
coupled to make ATP
(endergonic)
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BIOSC 48
Coupled Reactions:
Oxidation - Reduction
Coupled Reactions:
Oxidation - Reduction
• Reduction: when an atom or molecule gains an
electron (is reduced)
Reduction
Xe-
• It is reduced in charge (electrons are negative)
+
Y
• Oxidation: when an atom or molecule loses an
electron (is oxidized)
X
+
Ye-
Oxidation
• Oxygen may not be involved
START
• These reactions are always coupled.
Reducing
Agent
Reduces Y by
donating e-
Xe-
“Oxidation Is Loss”
X lost an electron and
is therefore oxidized
• Reducing agent: electron donor
• Oxidizing agent: electron receiver
END
Oxidation
X
The reducing agent is
oxidized
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à
Oxidizing Agent
Oxidizes Xe - by
removing its e -
Y
Reduction
“Reduction Is Gain”
Y added an e - and is
therefore reduced
Ye-
“OIL RIG”
The oxidizing agent is
reduced
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Coupled Reactions:
Oxidation - Reduction
Hydrogen Carrier Molecules
• Oxygen is a great electron acceptor (reason why
the process is called oxidation).
Nicotinamide adenine dinucleotide (NAD)
• Comes from the vitamin niacin (B3)
NAD+ + 2H ↔ NADH + H+
• Not the only oxidizer!
• Free electrons are not passed along: Hydrogen
atoms carrying the electrons are!
• A molecule that loses H is oxidized
• A molecule that gains H is reduced
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BIOSC 48
Hydrogen Carrier Molecules
Hydrogen Carrier Molecules
Flavin adenine dinucleotide (FAD)
• Comes from vitamin riboflavin (B2)
FAD + 2H ↔ FADH2
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BIOSC 48
Metabolism
Metabolism: all the
chemical reactions involved
in energy transformation
Chapter 5
Can be divided into:
• Anabolism:
Cell Respiration and Metabolism
• Requires input of energy
• Synthesis
BIOSC 48
• Catabolism:
• Releases energy
• Decomposition
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Catabolism Drives Anabolism
Cellular Respiration
Metabolic reactions to
convert nutrients (glucose)
into energy (ATP)
Catabolism reactions break down glucose, fatty acids,
and amino acids
• Serves as energy sources for the anabolism of ATP
• Anaerobic condition
1. Glycolysis à Lactic
Acid
• Requires many oxidation-reduction reactions
• Catabolism of glucose requires oxygen as the final
electron acceptor
• Aerobic conditions
2. (Pyruvate
decarboxylation)
3. Citric Acid Cycle
4. Oxidative
Phosphorylation &
Electron Transport
Chain
• Aerobic Cellular Respiration
• Breakdown of glucose requires enzymatically catalyzed
steps (first step: anaerobic)
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BIOSC 48
Aerobic Respiration of Glucose
C6H12O6 + O2 à 6 CO2 + 6 H2O + ATP
Occurs in 4 steps:
1. Glycolysis: cytoplasm
2. Pyruvate decarboxylation:
mitochondrial matrix
3. Citric Acid (Krebs) Cycle:
mitochondrial matrix
4. Oxidative Phosphorylation &
Electron Transport Chain:
cristae of mitochondria inner
membrane
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2&3
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https://commons.wikimedia.org/wiki/File:Cellular_respiration.gif
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Cellular Respiration
Step 1: Glycolysis
Know the following:
1. Each step of cellular respiration
Glucose à 2 pyruvic acid
C6H12O6 à 2 C3H4O3
• Anaerobic and aerobic
Location: Cytoplasm
2. Location of each step
• Cytoplasm vs. mitochondria
• Reactants:
• Glucose (C6H12O6)
• 2 NAD
• 2 ADP + 2 Pi
3. Purpose of each step
4. Reactants and products of each step
5. How many molecules of ATP are produced in
each step (if any)
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• Products:
• 2 pyruvic acid (C3H4O3 )
• 2 NADH + H+
• 2 ATP (net gain)
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BIOSC 48
Step 1: Glycolysis
Lactic Acid Pathway
After glycolysis, 2 possibilities:
Occurs when there is no oxygen to complete the
breakdown of glucose
• Anaerobic Metabolism/ Lactic Acid Fermentation
1. O2 not present: pyruvic acid converted to lactic
acid
2. O2 present: pyruvic acid enters the aerobic
respiration pathways in the mitochondria
• Reactants
• Pyruvic acid
• NADH + H+
• Products
• Lactic acid
• NAD
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Lactic Acid Pathway
Step 2: Pyruvate Decarboxylation
• Lactic Acid
Fermentation still yields
a net gain of 2 ATP!
• Location: interior mitochondrial matrix
• Reactants:
• Skeletal muscles
• Red blood cells
• 2 Pyruvic acid
• CO 2 removed to
form 2 acetic acid
• 2 Coenzyme A
• 2 NAD
• Products:
• 2 Acetyl CoA
• 2 CO2
• 2 NADH + H+
(Also called: “Swanson Conversion”)
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BIOSC 48
Step 3: Citric Acid Cycle
Step 3: Citric Acid Cycle
Also referred to as: Krebs Cycle or Tricarboxylic Acid Cycle
• Location: Inner mitochondrial
matrix
• Acetyl CoA + oxaloacetic acid
à citric acid
• Oxaloacetic acid starts and ends
the cycle
• Products (for 2 acetyl CoA):
• 6 NADH + H+
• 2 FADH2
• 2 ATP
• 4 CO2
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Step 4: Oxidative Phosphorylation
& Electron Transport Chain
Step 4: Oxidative Phosphorylation
& Electron Transport Chain
• Location: Cristae of
inner mitochondrial
membrane
• NADH and FADH2 are
oxidized
• e- transferred along
the electron transport
chain
• H+ moves through
pumps into
intermembrane space
• Reactants:
• 10 NADH + H+
• 2 FADH2
• O2
• Products:
• Pumps are reduced
and then oxidized!
• 28 ATP
• H 2O
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https://commons.wikimedia.org/wiki/File:2508_The_Electron_Transport_Chain.jpg
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BIOSC 48
Step 4: Oxidative Phosphorylation
& Electron Transport Chain
Step 4: Oxidative Phosphorylation
& Electron Transport Chain
Function of Oxygen
• Final e- acceptor
• ATP synthase
• H+ moves down
concentration
gradient
• Generates energy to
produce ATP!
• Oxidizes
Cytochrome a3 (IV)
• Oxygen binds with
H+ to form H2O:
O 2 + 4 e - + 4 H + à 2 H 2O
• Oxygen is reduced and
binds to 2 H+ forming
water
https://commons.wikimedia.org/wiki/File:2508_The_Electron_Transport_Chain.jpg
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https://commons.wikimedia.org/wiki/File:2508_The_Electron_Transport_Chain.jpg
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Step 4: Oxidative Phosphorylation
& Electron Transport Chain
How much ATP is produced?
One glucose will yield 32 ATP molecules!
Cyanide Poisoning
• Binds to the enzyme
cytochrome c
oxidase
• Inhibits the transfer
of electrons from
complex IV to oxygen
• Aerobic respiration
ceases
Aerobic Respiration Process:
ATP Produced
• Cells do not have
adequate ATP
• High doses results in
death
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Glycolysis
2
Oxidative Decarboxylation of Pyruvate
0
Citric Acid Cycle
2
Electron Transport Chain
Each NADH yields 2.5 ATP x 10 (Theoretical 3 ATP)
Each FADH2 yields 1.5 ATP x 2 (Theoretical 2 ATP)
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3 (4)
Total = 32 ATP
Theoretical = 38 ATP
https://commons.wikimedia.org/wiki/File:2508_The_Electron_Transport_Chain.jpg
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BIOSC 48
Table for Cellular Respiration
Step
Location
Reactants
Products
Common Metabolic Process Terms
Info/Summary
1. Glycolysis
2. Pyruvate
Decarboxylation
3. Citric Acid
Cycle / Krebs
Cycle
4. Electron
Transport Chain
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Glucose Metabolism
Glucose Metabolism
• Glycogenesis
Glycogenesis
• Glucose is stored as
glycogen in the liver,
skeletal muscles, and
cardiac muscles
• Glycogen is formed
from glucose via
glycogenesis
• Glucose à
Glycogen
• Glycogenolysis
• Glycogen à
Glucose
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BIOSC 48
Glucose Metabolism
Glucose Metabolism
Glycogenolysis
• When the cell
needs glucose,
it breaks down
glycogen
• In the liver:
Gluconeogenesis
• Conversion of noncarbohydrate molecules through
pyruvic acid to glucose
• Ex: Lactic acid, amino acid, glycerol
• Glucose 1phosphate à
Glucose 6phosphate
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Interconversion of Lactic Acid
The Cori Cycle
• Skeletal muscles produce lactic acid
• Anaerobic conditions
• In the liver:
1. Lactic acid dehydrogenase (enzyme) converts lactic
acid à pyruvic acid + NADH
2. Pyruvic acid à glucose 6-phosphate
• Can be used to make glycogen or glucose
• Ex of gluconeogenesis!
3. If glucose is made, returns to muscles
• Completes Cori Cycle
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BIOSC 48
Lipid & Protein Metabolism
Lipid Metabolism
• Common intermediates in
the interconversion of
glucose, lipids, amino acids:
• Glucose à Fat
• Glycolysis &
Pyruvate
decarboxylation to
make acetyl CoA
• Pyruvic acid
• Citric cycle acids
• Acetyl CoA à
• Glucose à Glycogen & fat
• Citric acid cycle
• Cholesterol
• Ketone bodies
• Fatty acids
• Occurs when ATP levels rise
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Lipid Metabolism
Lipid Metabolism
Lipogenesis
• The formation of fat (triglyceride)
• Occurs in adipose tissue (and liver)
Lipolysis
• Breakdown of triglycerides à glycerol + fatty acids
• Enzyme: lipase
• Glycerol à Glucose (gluconeogenesis in the liver)
• Fatty acids à acetyl CoA
• In adipose tissue: white fat
• β-oxidation
• 3 fatty acids + glycerol à triglyceride
• Fatty acid: derived from acetyl CoA
• Glycerol: derived from glycolysis intermediates
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BIOSC 48
Lipid Metabolism
Lipid Metabolism
Lipolysis cont’d
• Every 2 carbons on the fatty acid
chain à 1 acetyl CoA
Brown Adipose Tissue (Brown Fat)
• Stored in different cells
• Functions:
• Continues onto CAC and ETC
• Each acetyl CoA produces:
• Newborns: thermogenesis
• Adults: contributes to calories and thermogenesis
• 10 ATP: from CAC and ETC
• 3 NADH (x 2.5 ATP)
• 1 FADH 2 (x 1.5 ATP)
• 1 ATP
• Contains: Thermogenin (Uncoupling protein, UCP1)
• Uncouples oxidative phosphorylation à generates heat
• Each β-oxidation produces:
• 4 ATP:
• 1 NADH (x 2.5 ATP)
• 1 FADH 2 (x 1.5 ATP)
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Lipid Metabolism
Amino Acid Metabolism
Ketone Bodies
• Water-soluble molecules that circulate in the blood
Types of Amino Acids
• Essential: 8 AAs
obtained from diet
• Nonessential: 12 AAs
made by the body
• Acetone, acetoacetic acid, β-hydroxybutyric acid
• Produced by liver when ATP levels are sufficient
• Ketogenesis: formation of ketone bodies
• Fatty acids à acetyl CoA à ketone bodies
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BIOSC 48
Amino Acid Metabolism
Transamination
• Production of
nonessential AAs
• Pyruvic acid and
several citric acid cycle
intermediates (keto
acids) à amino acids
Amino Acid Metabolism
Oxidative Deamination
• Removing amine groups
from amino acids à
keto acid + ammonia
• Ammonia converted to
urea
• Keto acid:
• Can enter CAC to
produce energy
• Converted to glucose
(gluconeogenesis)
• Converted to fat
• Addition of an amine
group (NH2)
• Transaminase +
vitamin B6 (coenzyme)
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Amino Acids as Energy
Uses of Different Energy Sources
• Liver
• Glucose and
ketone bodies
• Adipose tissue
• Fatty acids and
glycerol
• Muscle
• Lactic acid and
amino acids
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BIOSC 48
Relative importance of different energy
sources to different organs
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BIOSC 48
Nervous System
Divisions of the Nervous
System:
• Central Nervous System
(CNS)
Chapter 7
• Brain
• Spinal cord
The Nervous System
• Peripheral Nervous System
(PNS)
BIOSC 48
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• Cranial nerves
• Spinal nerves
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Nervous System
Neurons
Types of Cells:
• Neurons
Structural and functional units of the nervous system
• Most neurons can’t divide, but can repair
• Receive and transmit
information
Functions:
• Responds to chemical and physical stimuli
• Conducts electrochemical impulses
• Releases chemical regulators
• Enables perception of sensory stimuli, learning,
memory, and control of muscle and glands
• Glial cells (neuroglia)
• Support cells
• Physical support and
insulation
• Supply nutrients and gases
• Remove dead cells (clean-up)
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BIOSC 48
Neurons
Neurons
General structure:
Cell body
• Organelles
General structure:
Processes outside the
body
• Dendrites
• Nucleus
• Nissl bodies
(specialized rough
ER)
• Receive impulses
and conducts a
graded impulse
towards the cell
body
• Cell body clusters:
• Axon
• CNS: nuclei
• PNS: ganglia
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• Conducts action
potentials away
from the cell body
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Neurons
Functional Classification of Neurons
Based on direction and types of impulses
Axons
• Vary in length (few
mm-m)
• Regions of an axon
• Sensory neurons
• Conducts impulses:
sensory receptors à
the CNS (afferent)
• Axon hillock
• Motor neurons
• Where action
potentials are
generated
• Conducts impulses:
CNS à target organs
(muscle or glands;
efferent)
• Initial segment
• Axon collateral
• Axon terminal
• Association/
Interneurons
• Located within CNS
• Integrates functions
of the NS
• Contains vesicles
with
neurotransmitters
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BIOSC 48
Functional Classification of Neurons
Functional Classification of Neurons
Motor Neurons
• Somatic motor neurons
• Reflexes and voluntary control of skeletal muscles
• Autonomic motor neurons
• Involuntary targets (Ex: smooth muscle, cardiac muscle,
and glands
• Subdivisions of the autonomic NS
• Sympathetic “flight or flight”: emergency situations
• Parasympathetic “rest and digest”: normal functions
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Structural Classification of Neurons
Classification of Nerves
Based on the number of processes that extend from
the cell body
• Bundles of axons
• CNS: tract
• PNS: nerves
• Pseudounipolar
• Single short process that
branches to form two longer
processes
• Ex: sensory neurons
• PNS Classification
• Bipolar
• Sensory (afferent) nerves
• Two processes, one on either
end
• Ex: special sense organs (nose
and retina of eye)
• Carry impulses towards the CNS
• Motor (efferent) nerves
• Carry impulses away from the CNS
• Multipolar
• Mixed nerves
• Several dendrites and one axon
• Most common structure
• Ex: motor neurons and
interneurons
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• Contains both sensory and motor fibers
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BIOSC 48
Nervous System Terminology
Neuroglia (Glial Cells)
Cells that are non-conducting but support neurons
Types found in the PNS:
• Schwann cells (neurolemmocytes)
• Form myelin sheaths around peripheral axons
• Satellite cells (ganglionic gliocytes)
• Support cell bodies within the ganglia of the PNS
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Neuroglia (Glial Cells)
Neuroglia (Glial Cells)
• Types found in the CNS
• Oligodendrocytes
• Forms myelin sheaths around CNS axons
• Microglia
• Migrates around CNS tissue and phagocytize foreign and
degenerated material
• Astrocytes
• Regulates neural external environment
• Forms the blood-brain-barrier between capillaries and neurons
• Ependymal cells
• Lines ventricles and secretes cerebrospinal fluid
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BIOSC 48
Myelination
Blood-Brain Barrier
• Provides myelination
• Structural components:
• Increases rate of nerve
impulses
• Tight junctions between endothelial cells
• Restricts paracellular (between cell) transport
• Movement is transcellular and selective
• Forms white matter
• PNS: Schwann Cells
• Gaps: nodes of Ranvier
• CNS: oligodendrocytes
• One oligodendrocyte
sends extensions to
several axons
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Blood-Brain Barrier
Astrocytes & the BBB
• Molecules that can cross the BBB
• Influences interactions between neurons and
between neurons and blood
• Secretes regulatory molecules to produce:
• Nonpolar molecules (O 2 and CO2)
• Organic molecules (alcohol, barbiturates, essential
amino acids)
• Ions and polar molecules (water, glucose)
• Tight junctions, carrier proteins, ion channels, enzymes to
destroy toxic molecules
• Requires channels and carrier proteins
• Ex: Glucose transport using GLUT1 carrier proteins
• Molecules that cannot cross the BBB
• Metabolic wastes
• Most drugs
• Challenge for treatment of brain diseases
• Proteins, nonessential amino acids
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BIOSC 48
Electrical Activity in Axons
Excitability or Irritability
• Ability of neurons and muscle cells to change their
membrane potentials
• Caused by changes in permeability to ions
Electrical Activity in Axons
Resting Membrane Potential
• Resting potential of -70 mV is
maintained by:
• Anions within the cell
• Na+/K+ pumps
• Permeability of the
membrane to positively
charged, inorganic ions (i.e.,
K+)
• Ions follow their electrochemical gradient
• Concentration of ions:
• High concentration of K+
inside the cell
• High concentration of Na+
outside the cell
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Electrical Activity in Axons
Ion Channels
At rest, a neuron is polarized
• Inside is more negative
Changes in membrane potential are controlled by
changes in the flow of ions through channels
Changes in Membrane Potential:
• Depolarization
• Leakage Channels
• Always open
• Ex: K+ leakage channels
• Membrane potential becomes more positive
• Ex: positive ions enter the cell (i.e., Na+)
• Gated Channels
• Repolarization
• Open: membrane more permeable
• Closed: membrane less permeable
• Types:
• A return to resting potential
• Hyperpolarization
• Membrane potential becomes more negative
• Ex: positive ions
leave the cell (i.e., K+) or negative ions enter
the cell (i.e., Cl-)
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• Ligand Gated (Ex: Na+)
• Voltage Gated (Ex: Na+ and K +)
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BIOSC 48
Voltage-Gated Channels
Generating an Action Potential
• Closed at resting membrane
potential (-70 mV)
• Open by depolarization
Depolarization
1. Stimulus causes dendritic membrane to
depolarize
• -70 mV (rest) à positive voltage
• If membrane potential reaches threshold (-55 mV) an
action potential occurs
• Na+ Channels
• Open: -55 mV
• K+ Channels
2. Voltage-gated Na+ channels open at the axon
hillock
• Open: +30 mV
• Influx of Na+
• -55 mV à +30 mV
• Positive feedback
• Movement of ions due to
electrochemical gradient
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Generating an Action Potential
Generating an Action Potential
Repolarization
1. Voltage-gated Na+ channels become inactivated
• Decreases Na+ permeability
2. Voltage-gated K+ channels open
• +30 mV à -70 mV
• Efflux of K+
• Negative feedback
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BIOSC 48
Generating an Action Potential
Generating an Action Potential
Hyperpolarization
1. K+ efflux continues
• -70 mV à -85 mV
2. Voltage-gated K+ channels inactivated
Recovery
1. Voltage-gated channels closed
2. Resting membrane potential reestablished
• Na+/K+ pumps
• -85 mV à -70 mV
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All-or-None Law
Coding for Stimulus Intensity
• If threshold is
reached à action
potential
• Stimulus intensity
does not affect
action potential
amplitude
Amplitude = Size of waveform
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• Increasing stimulus
intensity:
• Increases frequency
of action potentials
• Frequency modulated
• Activates more axon
fibers in a nerve
• Recruitment
Frequency = Number of occurrences/sec (Hz)
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BIOSC 48
Refractory Period
Refractory Period
Absolute Refractory
Period
• Neuron incapable of
responding to stimulus
• Temporary
inactivation of voltagegated Na+ channels
• Makes a single AP
unidirectional!
Relative Refractory Period
• Some Na+ channels have
recovered
• Voltage-gated K+ channels
are still open
• A second AP is possible
but needs a greater
stimulus
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Cable Properties of Neurons
Conduction of Nerve Impulses
The ability of neurons to conduct
charges through their cytoplasm
• AP initiates at the axon
hillock
• Voltage-gated Na+ channels
open
• Poor due to:
• High internal resistance to the
spread of charges
• Leaking of charges through the
membrane
• Na+ efflux causes
depolarization
• Neurons cannot depend on cable
properties to move an impulse
down the length of an axon
• The AP at one region serves
as the depolarization
stimulus for the next region
• Change in potential would be
localized to 1-2 mm in length
• Some neurons are a meter or more
in length!
• Process continues along axon
length
• Suggests neurons do not rely on cable
properties to propagate an AP!
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BIOSC 48
Action Potentials & Dominoes
Action potentials travel in one direction
Action potentials only travel in
one direction!
• The temporary inactivation
of Na+ channels prevents the
AP from moving backwards!
• All-or-none: 1st domino has to be pushed with enough force
(reach threshold) to knock down the row of dominoes
(action potential)
• Dominoes fall sequentially from one end to another similar
to an AP propagating down an axon
• Fallen dominoes represent the refractory period where an
axon cannot immediately initiate a new AP
• APs travel in only one
direction: from the axon
hillock à synaptic terminals
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Conduction in a Myelinated Neuron
Conduction in an Unmyelinated Neuron
Action potentials produced
down the entire length of the
axon, at every patch of
membrane
• Slow conduction rate
• Many APs are generated, each
one an individual event
• Amplitude of each AP is the
same
Saltatory Conduction:
Action potentials that
“leap” from node to node
• Myelin provides
insulation, improving the
speed of cable properties
• Nodes of Ranvier allow
Na+ and K+ to cross the
membrane every 1-2 mm
• Na+ ion channels are
concentrated at the node
• APs “leap” from node to
node
• Conduction without decrement
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BIOSC 48
Synapse
Action Potential Conduction Speed
Increased by:
• Increased axon diameter
The functional connection
between a neuron and
second cell
• Reduces resistance
• Myelination
• Saltatory conduction
• CNS: 2nd cell will be
another neuron
• PNS: 2nd cell will be a
neuron, muscle or gland
• Ex:
• Thin, unmyelinated
neuron speed: 1.0m/sec
• Thick, myelinated neuron
speed 100m/sec
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Synapse
Synapse
• Neuron-neuron connection: 1st neuron is the
presynaptic neuron and 2nd neuron is the postsynaptic
neuron
Synaptic transmission can be electrical or chemical:
• Presynaptic neuron can signal the dendrite (axodendritic), cell
body (axosomatic), or axon (axoaxonic) of a second neuron
• Most synapses are axodendritic
• Chemical
• Most common
• Neurotransmitters diffuse across synaptic cleft
• Electrical (gap junctions)
• Cells joined by gap junctions (connexin proteins)
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BIOSC 48
Electrical Synapse
Chemical Synapse
• Location:
• Transmission across
synaptic cleft
• Involves the release of
neurotransmitters (NTs)
from the presynaptic cell’s
terminal boutons
• Smooth & cardiac
muscle
• Between some
neurons in the brain
• Between glial cells
• Cells joined by gap
junctions (connexin
proteins)
• Allows ions and
molecules to pass
between cells
• NTs are stored in synaptic
vesicles
• Released via exocytosis
into the synaptic cleft
• Cells stimulated
together
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Chemical Synapse
Actions of Neurotransmitter
Process
1. AP reaches the terminal
bouton
2. Voltage-gated
Ca2+ channels
open & Ca2+ influx
3. Ca2+2+binds to synaptotagmin
(Ca sensor)
4. Vesicles containing NT are
docked at the plasma
membrane by 3 SNARE
proteins
5. Ca2+ synaptotagmin complex
displaces part of the SNARE,
and the vesicle fuses
6. Pores form and release NT
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Process
1. NTs (ligand) diffuses across the synaptic cleft and
binds to receptor proteins
2. Ion channels open, changing the membrane
potential
• Opening Na+ or Ca2+ channels results in a graded
depolarization: Excitatory Postsynaptic Potential (EPSP)
• Opening K+ or Cl- channels results in graded
hyperpolarization: Inhibitory Postsynaptic Potential
(IPSP)
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BIOSC 48
EPSP & IPSP
• EPSP
• Moves membrane potential
closer to threshold
• Produces a graded potential
• May require EPSPs from
several neurons to produce
an AP
• IPSP
• Moves membrane potential
further from threshold
• Inhibits an AP from forming
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EPSP
The inward flow of Na+
depolarizes the cell, creating
an EPSP
• EPSPs occur in dendrites
and cell bodies
• EPSPs can be summed to
produce a greater
depolarization (graded)
• If enough EPSPs are
summed to reach
threshold à stimulates
opening of voltage-gated
channels in the axon
hillock, leading to an AP
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EPSP
Comparison of EPSPs and Action Potentials
Summation:
• Spatial summation
• Simultaneous
stimulation by
several presynaptic
neurons
• All EPSPs and IPSPs
are added at the
axon hillock
• Temporal summation
• High frequency
stimulation by one
presynaptic neuron
• EPSPs add together
at the axon hillock
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BIOSC 48
Actions of Neurotransmitter
Chemically Regulated Channels
Binding of a NT to a receptor can open an ion channel in
one of two ways:
1. Ligand-gated channels
• Receptor protein is also the ion channel
• Binding of a NT directly to an ion channel, opens the channel
• Ex: Nicotinic ACh receptors
2. G-protein coupled channels
• NT receptor is separate from the protein that serves as the
ion channel
• Binding at the receptor opens ion channels indirectly by using
a G-protein
• Ex: Muscarinic ACh receptors, dopamine receptors,
norepinephrine receptors
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G-Protein Coupled Receptor
Neurotransmitter
Process: G-proteins have 3 subunits ( , β, and ɣ)
1. Binding of ACh results in the dissociation of the subunit
2. The or the β - ɣ diffuses through the membrane to the ion
channel
3. The channel opens (or closes) for short period of time
4. The G-protein subunits dissociate from the channel and it closes
(opens)
Acetylcholine (ACh)
• Directly or indirectly opens ion channels when it
binds to its receptor
• Can be excitatory or inhibitory depending on the
organ involved:
• Excitatory
• CNS
• Some autonomic motor neurons
• All somatic motor neurons
• Inhibitory
• Some autonomic motor neurons
• Receptor Types: Nicotinic and Muscarinic
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BIOSC 48
ACh Receptors
Nicotinic ACh Receptors
• Ligand-gated channels
• Excitatory
• Locations:
• Motor end plate of skeletal
muscle cells
• Autonomic ganglia
• Some parts of the CNS
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ACh Receptors
Muscarinic ACh
Receptors
• G-protein coupled
receptors
• Excitatory (closed) or
Inhibitory (open)
• Locations:
• CNS
• Smooth and cardiac
muscles
• Glands innervated by
autonomic motor
neurons
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Agonists vs. Antagonists
Acetylcholinesterase (AChE)
1. Agonists: drugs that can stimulate a receptor
AChE: enzyme that inactivates ACh
• ACh à acetate and choline
• ACh reuptake into presynaptic cell for reuse
• Nicotine for nicotinic ACh receptors
• Muscarine for muscarinic ACh receptors
2. Antagonists: drugs that inhibit a receptor
• Curare is an antagonist for nicotinic ACh receptors
• Atropine is an antagonist for muscarinic ACh receptors
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BIOSC 48
ACh in the PNS
ACh in the PNS
• Somatic Nervous System
Interruption of Neuromuscular Transmission
• Certain drugs can block neuromuscular
transmission
• Form neuromuscular junctions: synapses with skeletal
muscle fibers (motor end plate); contains nAChRs
• EPSPs: end-plate potentials
• Induces muscle contraction
• Ex: Curare: antagonist of acetylcholine
• Autonomic Nervous System
• Blocks nACh rece
