Case 1
Neurotransmission happens along neurons in the CNS. Neurons are the functional, signal conducting
cells. There are also glia cells which functions are: supporting cells, synaptic transmission, homeostasis of
the microenvironment and immune surveillance.
Neurons
In the nucleus there is a soma which contains Nissl bodies. Nissl bodies are highly specialized RER.
Dendrites are thin branches that come from the soma. When there is an incoming signal from axons of
other neurons, they get accepted here. There are also dendritic spines which increases surface area
Each neuron has 1 axon. The initiation of action potential occurs at the axon hillock which leads the
action potential from the soma to the end of the axon. The axons are sheathed with myelin in order to
make the signal transduction possible to travel at a larger speed.
Neurons
• Soma
o Has nucleus and a nucleolus
o Has Nissl bodies
▪ Highly specialized rough endoplasmatic reticulum (RER):
complex of RNA & protein, stained by crezyl violet
o Mitochondria → neurons are active cells therefore need much energy
o Grey matter
▪ Cortical layers
▪ Nuclei (neuron clusters in CNS) → concentrations of soma that are collected
together
▪ Ganglia (neuron clusters in PNS)
• Dendrites
o Thin, tapering, branched processes from soma
o Receptive area for incoming signals (axons from
other neurons)
o Sends out graded potentials
▪ Excitatory postsynaptic potentials (EPSPs)
▪ Inhibitory postsynaptic potentials (IPSPs)
o Dendritic spines → connection point for actions
▪ Synapses
▪ Increase surface area
▪ Highly dynamic
▪ Increase after birth
▪ Decrease with age
, • Axon
o 1 axon per neuron
o Axon hillock
▪ Initiation of action potential
o Transmission of action potential
▪ From soma to end of axon
o Collaterals towards the end
o Sheathed with myelin
▪ CNS : oligodendrocytes
▪ PNS : Schwann cells
▪ Between two myelin sheaths → node of Ranvier
• Where the axolemma is exposed to the extracellular space
• They are uninsulated and highly enriched in ion channels, allowing them
to participate in the exchange of ions required to regenerate the action
potential
• Axon terminal (bouton)
o collaterals
• Synapse
Neurotransmission
Neurotransmission happens at the synapse and synaptic cleft in the presence of an action potential.
There can either be electrical or chemical transmission. Electrical transmission is in the presence of ions
and can happen in 2 directions. The chemical transmission is in the presence of a neurotransmitter and
can happen in only 1 direction. The rate of an action potential depends on the magnitude of current.
The resting membrane potential is normally -65 mV. K+ is inside the cell and Na+ and Ca2+ are outside
the cell.
An action potential can happen by the all-or-none principle when the membrane is depolarized enough.
,1. Ligand gated Na+ ion channels open, allowing some Na+ ions to go into the cell, which depolarizes the
membrane. When the threshold of -55 mV is reached, the voltage-gated Na+ channels open and many
more Na+ ions enter the cell (rising phase) → this is only a stimulation response, there is not yet a full
action potential.
2. At this potential difference, there will be an opening in the sodium channels which leads to a large
influx of sodium ions. When the potential is at +40 mV, there is an action potential. There is positive
charge inside (overshoot). → more permeable to sodium (Na+) than to potassium (K+) and therefore the
membrane potential ,mV, is closer to ENa than EK
3. The cell is too positive and therefore the Na+ channels close and the K+ channels open.
4. Then there comes the repolarization as the cell is too positive and K+ goes out of the cell (falling
phase)
5. The membrane will become hyperpolarized when the potential difference reaches -90 mV, because
there is new permeability to K+ → the membrane potential will be close to EK
6. The voltage gated K+ channels close which leads to the restoration of the original potential difference
of the cell membrane.
Gradual restoration: The hyperpolarization stage is the relative refractory period. At this time another
action potential cannot occur immediately since the ions are still moving in and out of the cell. The
absolute refractory period is in the middle of the depolarization and repolarization where nothing that
can occur will make another action potential, because one is already happening.
When the K+ channels close, the resting membrane potential is reached again. This is maintained by
active sodium/potassium pumps that allows 3 sodium ions into the cell and 2 potassium ions out of the
cell. The voltage gated sodium channels are closed and therefore less sodium ions diffuse back in, while
the potassium ions can diffuse back out of the cell.
Nernst equation can be used to calculate the equilibrium potential.
, • Continuous conduction →Propagation along unmyelinated axon. It’s slow because there are
always voltage-gated Na+ channels opening and more and more Na+ is rushing into the cell. Na+
and K+ channels are distributed along the axon thus; each neighboring patch of membrane also
generate an action potential.
• Saltatory conduction → Propagation along myelinated axon. It’s faster because the action
potential jumps from one node to the next and the new Na+ influx renews the depolarized
membrane.
Neurotransmitters
• Need to be synthesized and stored in presynaptic neuron
• Need to be released by a neuron when there is stimulation
• When used during an experiment, it should give the same response
• There needs to be a mechanism for removal
. Neurotransmitter Precursor
Excitatory Glutamate Glutamine
Inhibitory GABA Glutamaat
Modulatory Dopamine Tyrosine
Serotonin Trypthophan
Noradrenalin Tyrosine
Acetylcholine Choline
Dopamine : thalamus, hypothalamus, prefrontal cortex
Nigrostriatal circuit: regulation of motor functions
Mesolimbic-cortical circuit: regulation of emotional behavior
Serotonin: emotion regulation, mood (depression), self-confidence, sleep, aggression
Noradrenaline: arousal, mood, euphoric behavior, stress, fear
Acetylcholine: concentration, learning, behavior
