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Midterm

BIOL273 Study Guide - Midterm Guide: Axon Hillock, Gap Junction, Growth Cone

8 pages54 viewsSpring 2018

Department
Biology
Course Code
BIOL273
Professor
Vivian Dayeh
Study Guide
Midterm

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Axonal transport
-
proteins destined for axon must be
synthesized in rough ER in soma moves by axonal
transport
Slow
Moves by axoplasmic or cytoplasmic flow; used
for components not consumed rapidly
Fast
Uses stationary microtubu
les as tracks (vesicle
transport – motor proteins)
Forward
(anterograde)
Moves vesicles +
mitochondria
Backward
(retrograde)
Returns old components (for
recycling), nerve growth
factor + some viruses
Synapse depend on chemical signals – axons of embryonic
stem cells send out growth cones to find target cell
- Growth cones depend on: growth factors, ECM molecules,
membrane protein on growth cone + cells along the path
- Once axon reaches its target cell, electrical + chemical
activity is initiated followed by synapse formation (or it
disappears)
o Depend on neurotrophic factors secreted by neurons +
glial cells
o Variation in electrical activity can cause rearrangement
of synaptic connections
Unit 1 – Introduction to Physiology
Physiology = study of structure & function of a living organism & its component parts
- Integrative science – considers many different levels of organization, most research
today focus on cellular + molecular levels
Key concepts: structure & function, biological energy, information flow, homeostasis
Organizational levels of living organisms - chemical cellular tissue organ organ
system organism community (biosphere, ecosystem)
Homeostasis – maintenance of relatively stable internal environment
- Involves series of automatic control mechanisms, achieved thru effects of different
organ systems
- Failure of maintaining homeostasis pathological state
Set point = result of homeostatic control
Acclimatization
Shift
in set point due to environmental changes, w/o genetic
change
If due to genetic changes + long period time evolution
Circadian rhythms
Daily biological rhythms
Control + Integration – homeostatic controls
Local
control
Exerted on neighbouring cells
Reflex
control
(long
distance)
Reaction in >1 organs controlled from elsewhere
- Response loops start w/ stimulus and end w/ response
- Feedback pathway control response
Negative
feedback
S
uppresses stimulus
allow homeostatic control
- Keeps system near set-point oscillation
Positive
feedback
E
nhance stimulus, until external signal shuts it off
Sends system out of control not for homeostasis
Feedforward
control
Anticipatory control; predicts
response
prevent
Ex. Body’s response to exercise, salivation reflex
Important points
- Stability = balance btw input + output
- Negative feedback restores condition
- Homeostatic systems maintain similarity
- Set points can be reset
- Some variables are controlled more closely that others
- Most require communication btw cells
Unit 2 – Neurophysiology
Nervous system – key control of communication; network of billions of nerve cells linked together in organized matter to form rapid control of system of the body
Function: receive information, integrate information and send signal
Central NS
consist of spinal column + brain
Peripheral NS
everything else
Nuclei
cluster of cell bodies
Ganglia = cluster of cell bodies
Tract
bundles of axons grouped together
Nerves = bundles of axons
Neuron – carry information
Types of neurons – classified structurally or functionally
Soma
(cell
body)
Contains nucleus + biosynthetic machinery
- Centre of chemical process keep cell functioning + alive
- Extensive cytoskeleton extends to axon + dendrites
Dendrites
Slender process that received info, transmit towards soma
- Increase SA of neuron more communication
Axon
Extension that sends signal from soma
- Axon hillock – where axon of most peripheral neuron originates from (motoneuron only)
Axon terminals = end of axon connection btw neuron + other cells, part of synapse
- Vast majority are chemical synapses; neurotransmitters
- Electrical synapses – connected by gap junctions bidirectional + a lot faster
Receptor cell
Specialized cell that converts stimuli into electrical sig
nals
Sensory/afferent
cell
Receives info from receptor cell and transmit to CNS using long cytoplasmic extensions
- Cell bodies locate outside CNS
Types: Pseudounipolar (dendrite + axon fuse during development), bipolar
Interneuron
Transmit signals wit
hin CNS: lateral (spinal cord), vertical (brain)
Integrate signal from afferent and transmit to efferent
Types: anaxonic, multipolar (numerous dendrites, no axon extensions)
Motor/Efferent
cell
Receives info from interneuron, transmit to effectors
- Cell bodies located within CNS, cytoplasmic extensions transmit info to effectors
Type: multipolar (single long axon, 5-7 dendrites)
Tissue
collecti
on of cells held together by cell junctions (desmosome, gap
junction, tight junction), with ECM synthesized & secreted by cells within the
tissue
Epithelial
Protection + exchange regulation, any material going out/in
must cross epithelia
Exchange
Exchang
e of gases (rapid)
Ciliated
Line airways + female reproductive tract
Secretory
Synthesize + release products
Transporting
Transport of non
-
gaseous material
Protective
Found on body’s surface
Simple Squamous cells are ideal for gas diffusion
Simple Cuboidal – secretion and absorption can take place
Simple Columnar – secretion + absorption
Stratified squamous - protects underlying tissues from wear
and tear
Connective
Structural support + barriers, has extensive ECM (containing
proteoglycans, collagen, elastin, fibronectin)
Loose
Elastic tissue (tissue under skin)
Dense
Strength (tendon)
Adipose
Contains adipocytes
Blood
Watery matrix w/o insoluble protein
Supporting
Dense substance (bone, cartilage)k
Muscular
Force production + m
ovement (result of contraction)
Skeletal
Gross body movement
Smooth
Movement of material thruout the body
Cardiac
Movement of blood
Nervous
Carry info, minimal ECM
Neurons
Sends signal (chemical and electrical)
Glial cells
Supporting cell (cond
uctive support)
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Glial cells – supportive cell (outnumber neuron) aid in impulse conduction + maintaining microenvironment
- Don’t communicate long distances; only w/ each other and nearby neurons w/ electrical + chemical signals
Electrical signals + Movement of Ions – neurons transmit electrical impulses via energy stored as an electro-chemical gradient
Electrical principles
Osmotic principles
-
Opposite attract, like charges repel
- Coulomb’s law - electrical force
# of charges (strength) &
inversely distance between charges
- Differences in charge electrical gradient
- Cell membrane = electrical insulator
- Ion channels allow current (charge) to move thru membrane
-
Osmotic
P
w/ solute
[]
- Gradients result from differences in [] across membranes concentration gradients
- Molecules tend to move down gradient w/o any driving force
- Semi-permeable membranes; impermeable to most, hydrophobic molecules can slip past
- Majority moves thru channels or pores passively, or assisted by protein pumps
Membrane potential = electrical + chemical gradients caused by ion distribution
across membrane; source of potential energy
- Can be calculated if it can have equilibrium state
- Determines whether ion moves
Resting membrane potential - set by [] + relative permeability of each ion
- Relative [] of charged ions more electronegative on inside (-70 to -90 mV)
- negatively charged ions (phosphate + proteins) excess -ve charge inside cell
Nernst equation – describe MP if membrane is only permeable to only one ion
- Ions w/ +ve equilibrium potential IN
- Ions w/ -ve equilibrium potential OUT
Factors that can influence
1. Uneven ion distribution – Na+, Cl-, Ca2+ in EC, K+ on IC
2. Differing membrane permeability
- Resting MP is more permeable to K+ K+ = major contributor to resting MP
Setting resting MP – Na+/K+ ATPase (pump)
- Protein pump offsets leak of ions to achieve equilibrium
- Active transport of Na+ out and K+ in (against their [] gradients)
Changes in MP – change in membrane permeability electrical signals
Transmission - neurons use stored energy in gradients to transmit electrical signals
Depolarization – decrease in MP difference
Hyperpolarization – increase in MP difference
- Na+ contributes minimally to resting MP due to low permeability
o But it’s critical in generating MP changes that cause electrical signals
PNS
Schwaan cell
specialized glial cells that wrap around axons, each associate w/ one axon
- Forms myelin layers of membrane wrapping around axon
- Gap junction btw layers of myelin sheath allow flow of nutrients + info
- Myelin act as electrical insulator
- Secrete neurotrophic factors
- Keep microenvironment for neurons to transmit
Satellie cells
non
-
myelinating Schwaan cells
- Support soma
- Form supportive capsules around soma located in ganglia
CNS
where
majority
is found
Oligodendria (oligodendrocytes)
CNS version of Schwaan
- One associate w/ multiple axons; branches + forms myelin around
portions of several axons
- Wrap around axon forms myelin to insulate axon
Astroglia (astrocytes)
small star
-
shaped cells
- Highly branched glial cell
- Come in several subtypes, form functional network by communicating w/
one another thru gap junctions
- Some closely associate w/ synapse, where they release chemicals
- Make contact w/ blood vessels + neurons help form blood-brain
barrier
- May transfer nutrients substrates for ATP production
- Take up K+, water + neurotransmitter maintain neuron
microenvironment homeostasis in extracellular fluid around neurons
- Source of neural stem cells
Microglial
very small specialized immune cells
- Remove damaged cells + foreign invaders scavengers
- BUT: sometimes release damaging reactive oxygen species (ROS) that
form free radicals neurogenerative disease (ex. ALS)
Ependymal cells
one source of neural stem cells
- Epithelial cells that produce cerebral spinal fluid (CSF)
- Create selectively permeable barrier btw compartments
Line portion of HS, border w/ epithelia
Communication among glial cells
- Primary chemical signals
- Glial-derived growth + trophic factors
help maintain neurons + guide them
during repair + development
- Respond to neurotransmitter +
neuromodulators secreted by neurons
Membrane permeability
certain ions are more permeable (leaky channels)
- Ion contribution to resting MP is proportional to its permeability
o More easily it can cross, more important its contribution to resting MP
Goldman-Hodgkin-Katz (GHK) equation – mathematical equation based on ion [] +
membrane permeability
- Calculate MP that results from contribution of all ions that can cross membrane
o Includes membrane permeability values b/c it influences contribution to MP
- If membrane isn’t permeable to an ion, its value is 0 (drops out of equ)
- Describes how slight permeability change affects MP
- Cell membrane is 40x more permeable to K+ than Na+
Controlling permeability – Gated
- Neurons contain a variety of gated ion channels that regulate ion movement
Mechanically
-
gated
Found in sensory neurons, respond to physical forces
Chemically
-
gated
Respond to ligand
binding
Voltage
-
gated
Respond to voltage changes
Important in initiation and conduction of electrical
signals
Changing permeability
- Total number of protein channels – involves synthesizing new proteins
extremely slow
- Open or close existing protein channels – requires change in protein channel
conformation fast
o
Mechanically
-
gated, chemically
-
gated, or voltage
-
gated
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Types of signals generated by neurons
Graded Potentials
-
o
ccur in den
drites and soma
- Started by ions entering cell from ECF; triggered by opening/closing of ion channels
- “graded” amplitude is proportional to stimulus strength
- Travel only a short distance due to:
o Current leak – some +ve charges leak back w/ depolarization wave
o Cytoplasmic resistance – cytoplasm restricts flow
Neurotransmitters – initiate ions to enter the cell after binding to receptor
1. Ion channels open & ions move along electrochemical gradient
2. Wave of depolarization/hyperpolarization spreads thruout the cell
- Strength is determined by number of ions entering cell
- Signal diminish in strength as distance increases
Action Potential
-
all identical, doesn’t diminish in strength as they travel
Set-up - initiation factors
1. Location of initiation – begin at trigger zone (integrating centre of neuron)
- Sensory – adjacent to receptor
- Motoneuron – axon hillock + initial segment (very first part of axon)
2. Threshold potential (-55mV) – min depolarization necessary to trigger action potential
- Graded potentials sum together at trigger zone (why it’s called integrating centre)
- Space spatial summation
- Time temporal summation; if 2 occurred at the same time constructive interference
- If potential sum reach threshold at trigger zone initiates AP
3. Graded potentials
Depo
larization (stimulating)
Hyperpolarization (inhibiting)
-
Depolarize MP
less
-
ve
closer to
threshold
Increase chance of exciting axon
excitatory post synaptic potentials (EPSPs)
-
Hyperpolarize MP
more
-
ve
farther
away from threshold
- Decrease chance of exciting axon
inhibitory post synaptic potentials (IPSPs)
Role of Sodium in AP
Sodium activation - AP cannot fire unless voltage-gated Na+ channels open
- opens at -55 mV sets threshold potential
Resting MP
Typically
-
70mV, b
oth channels closed
R
ising phase
(Depolarization_
Depolarization stimulus
reach threshold value
-
55 mV
Starts +ve feedback loop:
- Depolarization Na+ channels open further depolarization
Time-dependent inactivation gate – external signal that stops loop
- Activation is same as Na+ channels
- After a certain time, gate closes to prevent Na+ entry (around +30
mV)
Falling phase
(repolarization)
Gating potential (+30 mV) of K
+
channels is reached
K+ OUT repolarization
NOTE: K+ channels are slower to open and close
Hyperpola
rization
Refractory period
sets direction of current flow, prevent temporal
summation, and AP from going backwards
1. Absolute – no AP can be triggered
- Na+ channels are inactive
- Membrane must repolarize for them to open
2. Relative – suprathreshold is required to bring forth AP
- K+ channels are still open more Na+ needed to reach threshold
Reset
Repolarized back to resting via Na
+
/K
+
ATPase
AP Conduction - positive feedback loop of Na channels allow AP to travel long distances
- Depolarization in one area depolarizes region next to it wave of depolarization
- Speed depends on membrane resistance (result of membrane leakiness)
o Some invertebrates use giant axons great diameter reduce resistance
o Vertebrates use myelinated axons reduce resistance
- Domino effect – wave of depolarization too slow dissipates
- Saltatory conduction = increase in conduction speed due to myelin sheaths
Synapse
Presynaptic cell
Axon terminal
- Contains many vesicles (filled w/ NT)
- Vesicles fuse w/ presynaptic membrane to release NT into synaptic cleft via
exocytosis
Synaptic cleft
Space between cells
Postsynaptic
cell
Membrane of any target cell
,
NT create 2 types of responses
Direct
Fast synaptic potential
quick response, short
-
term
Interaction w/ ion channel
Indirec
t
Slow synaptic potential
slow response, long
-
lasting
Signal-transduction mechanisms (G-proteins & 2nd messengers)
Types
Electrical
Gap junctions allow direct signalling
Rapid conduction info travelling in both
directions
Found only in CNS, essential in NS development +
transmission
Chemical
Info carried via neurotransmitter
Used by peripheral neurons + vast majority uses it
Exist btw neurons or effector neuroeffector
junction
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