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CBNS 106 Review
Lecture 2
Brain Cells
• Neurons (100 billion)
– Process information
– Sense environmental changes
– Communicate changes to other neurons
– Command body response
• Glia (10 times more)
– Insulate, support and nourish neurons
Staining
• Nissl Stain
– Facilitates study of cytoarchitecture
• Golgi Stain
– Shows soma and neurites (dendrites and axon)
Different classes of neurons
Afferent – INFO  CNS
ASTROCYTES!!!
CBNS 106
Lecture 3
- Astrocytes buffer
excess potassium, cycle
it from ecs to bbb
CBNS 106 Review
Lecture 4 and 5
Action Potential
Ideal
Actual
“Resting” Cell
During AP
Action Potential Conduction
The entry of positive charge during
the action potential causes the
membrane just ahead to depolarize
to threshold
Orthodromic conduction – action
potentials conduct only in one
direction
Antidromic conduction – backward
propagation
Typical conduction velocity ~ 10
m/sec
Action potentials last ~ 2 msec
Factors Influencing Conduction
Velocity
Spread of action potential along membrane
• Dependent upon axon structure
Path of the positive charge
• Inside of the axon (faster)
• Across the axonal membrane (slower)
* If the axon is narrow and there are many open membrane pores then most of
the current will flow out across the membrane.
* If the axon is wide and there are few open membrane pores, then most of the
current will flow down inside the axon
* The farther the current goes down the axon, the farther ahead the action
potential will depolarize the membrane and thus the faster the action potential.
Factors Influencing Conduction
Velocity
Axonal excitability
• Axonal diameter (bigger =
faster)
• Number of voltage-gated
channels
Myelin: Facilitates current flow
• Myelinating cells
– Schwann cells in the PNS
– Oligodendroglia in CNS
Saltatory Conduction –
The propagation of an
action potential down a
myelinated axon
CBNS 106 Review
Lecture 6
Synaptic Transmission
• The process of information transfer at a synapse
• Direction of Flow
– One direction, Neuron-to-Target Cell
• 1st Neuron called Presynaptic Neuron
• Target Cell called Postsynaptic Neuron
• Two Types of synapses
– Chemical
– Electrical
Synaptic Transmission
• Electrical Synapses:
- They allow the direct transfer of ionic current from one cell to another
- Gap junction
- Cells are said to be “electrically coupled”
• Flow of ions from cytoplasm to cytoplasm
- Very fast transmission
• Postsynaptic potentials (PSPs)
- Synaptic integration: Several PSPs
occurring simultaneously to excite a
neuron (i.e.causes AP)
Synaptic Transmission
Types of Synapses
- Axodendritic (a)
- Axosomatic (b)
- Axoaxonic (c)
- Asymmetrical membrane differentiations (a)
- Symmetrical membrane differentiations (b)
Synaptic Transmission
• Chemical Synapses:
– Neurotransmitter, the chemical is used to
transfer information from one cell to another
- Most synaptic transmission in the mature
human nervous system is chemical.
Synaptic Transmission
Principles of Chemical Synaptic
Transmission
• Basic Steps
– Neurotransmitter synthesis
– Load neurotransmitter into synaptic
vesicles
– Neurotransmitter Release
• Vesicles fuse to presynaptic
terminal
• Neurotransmitter spills into
synaptic cleft
– Binds to postsynaptic receptors
– Biochemical/Electrical response
elicited in postsynaptic cell
– Removal of neurotransmitter from
synaptic cleft
Synaptic Transmission
Neurotransmitter Release
– Exocytosis: Process by which vesicles
release their contents; 60 microsec
Mechanisms
• Process of exocytosis stimulated by
release of intracellular calcium, [Ca2+]i
• Proteins alter conformation - activated
• Vesicle membrane incorporated into
presynaptic membrane
• Neurotransmitter released
• Vesicle membrane recovered by
endocytosis
Neurotransmitter Recovery and
Degradation
– Diffusion: Away from the synapse
– Reuptake: Neurotransmitter re-enters presynaptic
axon terminal
– Enzymatic destruction inside terminal cytosol or
synaptic cleft
– Desensitization: e.g., AChE cleaves Ach to inactive
state
Neurotransmitters
• Amino acids: Small organic molecules
• e.g., Glutamate (Glu), Glycine (Gly),
gammaaminobutyric acid (GABA)
• Amines: Small organic molecules
• e.g., Dopamine (DA), Acetylcholine (Ach),
Norepinephrine (NE), Serotonin (5-HT)
• Peptides: Short amino acid chains (i.e. proteins)
stored in and released from secretory granules
• e.g., Dynorphin, Enkephalins
PSP Generation
• EPSP = excitatory postsynaptic potential
– Transient postsynaptic membrane depolarization by
presynaptic release of neurotransmitter
• IPSP = inhibitory postsynaptic potential
– Transient hyperpolarization of postsynaptic membrane
potential caused by presynaptic release of neurotransmitter
Synaptic Integration
• Process by which multiple synaptic potentials
combine within one postsynaptic neuron
-Allows for neurons to perform sophisticated computations
-Integration: PSPs added together
-Spatial: PSPs generated simultaneously in different spaces
-Temporal: PSPs generated at same synapse in rapid succession
Synaptic Integration
-
Synaptic vesicles: Elementary units of synaptic
transmission
-
Quantum: An indivisible unit
-
Miniature postsynaptic potential (“mini”)
-
Quantal analysis: Used to determine number
of vesicles that release during
neurotransmission
-
Neuromuscular junction: About 200 synaptic
vesicles, EPSP of 40mV or more
- CNS synapse: Single vesicle, EPSP of few tenths
of a millivolt
Synaptic Integration
• IPSPs and Shunting Inhibition
– Excitatory vs. inhibitory synapses:
Bind different neurotransmitters, allow
different ions to pass through channels
– Membrane potential less negative
than -65mV = hyperpolarizing IPSP
• Shunting Inhibition: Inhibiting
current flow from soma to axon
hillock
(a) Stimulation of the excitatory input causes
inward postsynaptic
current that spreads to the soma, where it can be
recorded as an EPSP.
(b) When the inhibitory and excitatory inputs are
stimulated together, the depolarizing current
leaks out before it reaches the soma.
Chemical Synaptic Transmission
Principles
Autoreceptors
– Presynaptic receptors sensitive to
neurotransmitter released by presynaptic
terminal
– Act as safety valve to reduce release when
levels are high in synaptic cleft
(autoregulation)