Neurons & Synapses
2nd year biology
Lecture 1: Types and classification of neurons in the nervous system
Glial cell types: provide support, nutrition, insulation, and help with signal transmission in
the nervous system.
- Central nervous system:
o Astrocytes (most abundant): support, regulate ions. Exchange of materials
between neurons and capillaries.
o Microglial cells: defend. Immune defence against invading microorganisms
o Ependymal cells: line cavities. Create, secrete, and circulate cerebrospinal
fluid
o Oligodendrocytes: wrap and insulate, form myelin sheath
- Peripheral nervous system
o Satellite cells: surround and support neuron cell bodies
o Schwann cells: wrap and insulate, form myelin sheath
Facts:
- Neurons are some of the longest-lived cells in your body
- Neurons are irreplaceable
- Neurons have huge appetites: high metabolic rate
Multipolar neurons: multiple dendrites, one axon
Bipolar neurons: one dendrite, one axon (rare)
Unipolar neurons:
Sensory neurons (afferent neurons): transmit impulses from sensory receptors toward the
CNS.
- Mostly unipolar
Motor neurons (efferent neurons): impulse moves from the CNS to the rest of the body
- Mostly multipolar
Interneurons (association neurons): impulse moves between sensory and motor neurons
- Mostly multipolar
Nervous system is a network of specialized cells
Neurons itself are not capable of producing behaviour individual, need effectors.
Nissl staining for visualization of neuronal cell-bodies
Soma size, neuron density and axo-dendritic pattern of neurons vary throughout the
nervous system.
Basic functional classification of neurons:
- Excitatory neurons secrete neurotransmitters that cause membrane depolarization
in postsynaptic neurons
- Inhibitory neurons secrete neurotransmitters that cause membrane
hyperpolarization in postsynaptic neurons
Dale’s law: 1934
Version 1: a neuron is either excitatory or inhibitory in its influence on other neurons
Version 2: a neuron secretes a single (traditional) neurotransmitter at its synapses.
1
, Neurons & Synapses
2nd year biology
Inhibition and excitation dynamics
determine rules of computation
and communication:
How can one classify neurons?
Basic features commonly used for
neuronal classification
- Morphological features
- Biochemical markers
expression
- Active and passive electrical
characteristics
Based on morphology:
Most neurons contain three major ‘compartments’:
- Soma: contacted only by few (inhibitory) synapses; metabolic centre
of the cell
- Dendrites: most synapses are made onto dendrites. The principal
compartment for spatial and temporal integration of incoming
information
- Axon: efferent part of the cell. Axonal hillock in the initial segment is
where action potential is generated. Initial segment targeted by
specific inhibitory synapses.
Spines are located on dendrites: receives input from a single axon at the
synapse. Postsynaptic site. Small protrusions.
Axons have blobs where the boutons are: making synapses onto dendrites
on the postsynaptic neurons.
Apical dendrites go to layer 1
Basal dendrites go to layer 6
Based on biochemical markers:
Typically used for classification of inhibitory neurons only.
Excitatory neurons typically express layer specific proteins
The problem: poor correlation between morphological and biochemical markers
2
, Neurons & Synapses
2nd year biology
Based on electrical properties:
Passive intrinsic electrical characteristics of neurons:
- Resting membrane potential: electrical potential across membrane at rest
- Membrane resistance: calculated as V/I. it indicates how sensitive a neuron might be
upon excitation
- Membrane time constant: i.e. time needed to reach 63% of voltage change.
Active electrical characteristics of neurons:
- Action potential rate, timing, interval
- Rate of adaptation upon sustained current injection
- Membrane potential at which action potential is generated
- Amplitude of subthreshold responses
Excitatory neurons display 2 main firing patterns:
Regular spiking neurons (RS):
- Interspike interval relatively remains constant upon sustained somatic depolarizing
current injection
Intrinsically burst spiking neurons (IB)
- Rapid sequence of action potential upon transient current injection
Functional advantages of burst spiking:
- Bursts are more reliable than single spikes in evoking postsynaptic neuronal
responses.
- Bursts overcome synaptic transmission failure
- Bursts facilitate transmitter in the short-term release whereas
single spikes do not (i.e. short-term facilitation)
- Bursts evoke long-term potentiation and hence affect synaptic
plasticity much greater, or differently than single spikes.
Electrical characterization of inhibitory neurons:
RS= regular spiking
IB= intrinsic burst
Fast spiking neurons respond rapidly once reaching threshold:
- Contributes to rapid truncation of excitatory network activity
3
, Neurons & Synapses
2nd year biology
Late spiking needs strong and lasting excitatory drive:
- Modulates the ongoing network activity
Currently there is not an agreed upon classification approach that takes all the variability
across variables into account
Lecture 2: Organization of local networks and the dynamics of neuronal
communication
On the inside of the cell→ more negative, this difference is known as the resting membrane
potential: -70 mV, however, not constant → can be -60 mV but also -80 mV
Outside: positive sodium ions
Inside: positive potassium ions, mingled with the bigger negatively charged proteins,
therefore inside is considered as more negative
Negative membrane potential→ it is set to be polarized
Sodium potassium pumps→ for every 2 sodium ions it pumps inside the cell, 3 potassium
ions leave the cell
Other ion channels in the membrane:
- Voltage-gated channels: open and close in response to changes in membrane
potential
- Ligand-gated channels: open when a neurotransmitter latches onto its receptor
- Mechanically-gated channels: open in response to the physical stretching of the
membrane.
Action potential
1. Sodium channels open, inside gets more positive → threshold -55 mV
2. At -55 mV, the voltage-gated channels open→ all of the sodium ions rush in, making
the inside even more positive→ depolarized till +40 mV
3. Local current is strong enough to change the voltage around them and so on.
4. Repolarization: potassium channels open up and potassium leaves the cell, making it
more negative again and rebalance the charges
5. Hyperpolarization: too much potassium ions leave the cell
Once the ion channels are open, it cannot respond to any other stimulus: refractory period.
A weak stimulus tends to trigger less frequent action potentials, compared to a strong
stimulus.
Myelinated neurons conduct action potentials faster than non-myelinated neurons. Gaps
between myelin sheaths are called nodes of Ranvier → saltatory conduction (leaping)
Membrane resistance= measure of how easy for currents flow through the membrane. It is
measured in MOhm. Current likes to flow through the path with less resistance. Membrane
resistance is a function of open ion channels; as the ion channel density increases
membrane resistance is reduced.
Membrane capacitance= measure of how easily the electrical charge balance can be
established across the intracellular and extracellular environment. It is measured in nF and
4
2nd year biology
Lecture 1: Types and classification of neurons in the nervous system
Glial cell types: provide support, nutrition, insulation, and help with signal transmission in
the nervous system.
- Central nervous system:
o Astrocytes (most abundant): support, regulate ions. Exchange of materials
between neurons and capillaries.
o Microglial cells: defend. Immune defence against invading microorganisms
o Ependymal cells: line cavities. Create, secrete, and circulate cerebrospinal
fluid
o Oligodendrocytes: wrap and insulate, form myelin sheath
- Peripheral nervous system
o Satellite cells: surround and support neuron cell bodies
o Schwann cells: wrap and insulate, form myelin sheath
Facts:
- Neurons are some of the longest-lived cells in your body
- Neurons are irreplaceable
- Neurons have huge appetites: high metabolic rate
Multipolar neurons: multiple dendrites, one axon
Bipolar neurons: one dendrite, one axon (rare)
Unipolar neurons:
Sensory neurons (afferent neurons): transmit impulses from sensory receptors toward the
CNS.
- Mostly unipolar
Motor neurons (efferent neurons): impulse moves from the CNS to the rest of the body
- Mostly multipolar
Interneurons (association neurons): impulse moves between sensory and motor neurons
- Mostly multipolar
Nervous system is a network of specialized cells
Neurons itself are not capable of producing behaviour individual, need effectors.
Nissl staining for visualization of neuronal cell-bodies
Soma size, neuron density and axo-dendritic pattern of neurons vary throughout the
nervous system.
Basic functional classification of neurons:
- Excitatory neurons secrete neurotransmitters that cause membrane depolarization
in postsynaptic neurons
- Inhibitory neurons secrete neurotransmitters that cause membrane
hyperpolarization in postsynaptic neurons
Dale’s law: 1934
Version 1: a neuron is either excitatory or inhibitory in its influence on other neurons
Version 2: a neuron secretes a single (traditional) neurotransmitter at its synapses.
1
, Neurons & Synapses
2nd year biology
Inhibition and excitation dynamics
determine rules of computation
and communication:
How can one classify neurons?
Basic features commonly used for
neuronal classification
- Morphological features
- Biochemical markers
expression
- Active and passive electrical
characteristics
Based on morphology:
Most neurons contain three major ‘compartments’:
- Soma: contacted only by few (inhibitory) synapses; metabolic centre
of the cell
- Dendrites: most synapses are made onto dendrites. The principal
compartment for spatial and temporal integration of incoming
information
- Axon: efferent part of the cell. Axonal hillock in the initial segment is
where action potential is generated. Initial segment targeted by
specific inhibitory synapses.
Spines are located on dendrites: receives input from a single axon at the
synapse. Postsynaptic site. Small protrusions.
Axons have blobs where the boutons are: making synapses onto dendrites
on the postsynaptic neurons.
Apical dendrites go to layer 1
Basal dendrites go to layer 6
Based on biochemical markers:
Typically used for classification of inhibitory neurons only.
Excitatory neurons typically express layer specific proteins
The problem: poor correlation between morphological and biochemical markers
2
, Neurons & Synapses
2nd year biology
Based on electrical properties:
Passive intrinsic electrical characteristics of neurons:
- Resting membrane potential: electrical potential across membrane at rest
- Membrane resistance: calculated as V/I. it indicates how sensitive a neuron might be
upon excitation
- Membrane time constant: i.e. time needed to reach 63% of voltage change.
Active electrical characteristics of neurons:
- Action potential rate, timing, interval
- Rate of adaptation upon sustained current injection
- Membrane potential at which action potential is generated
- Amplitude of subthreshold responses
Excitatory neurons display 2 main firing patterns:
Regular spiking neurons (RS):
- Interspike interval relatively remains constant upon sustained somatic depolarizing
current injection
Intrinsically burst spiking neurons (IB)
- Rapid sequence of action potential upon transient current injection
Functional advantages of burst spiking:
- Bursts are more reliable than single spikes in evoking postsynaptic neuronal
responses.
- Bursts overcome synaptic transmission failure
- Bursts facilitate transmitter in the short-term release whereas
single spikes do not (i.e. short-term facilitation)
- Bursts evoke long-term potentiation and hence affect synaptic
plasticity much greater, or differently than single spikes.
Electrical characterization of inhibitory neurons:
RS= regular spiking
IB= intrinsic burst
Fast spiking neurons respond rapidly once reaching threshold:
- Contributes to rapid truncation of excitatory network activity
3
, Neurons & Synapses
2nd year biology
Late spiking needs strong and lasting excitatory drive:
- Modulates the ongoing network activity
Currently there is not an agreed upon classification approach that takes all the variability
across variables into account
Lecture 2: Organization of local networks and the dynamics of neuronal
communication
On the inside of the cell→ more negative, this difference is known as the resting membrane
potential: -70 mV, however, not constant → can be -60 mV but also -80 mV
Outside: positive sodium ions
Inside: positive potassium ions, mingled with the bigger negatively charged proteins,
therefore inside is considered as more negative
Negative membrane potential→ it is set to be polarized
Sodium potassium pumps→ for every 2 sodium ions it pumps inside the cell, 3 potassium
ions leave the cell
Other ion channels in the membrane:
- Voltage-gated channels: open and close in response to changes in membrane
potential
- Ligand-gated channels: open when a neurotransmitter latches onto its receptor
- Mechanically-gated channels: open in response to the physical stretching of the
membrane.
Action potential
1. Sodium channels open, inside gets more positive → threshold -55 mV
2. At -55 mV, the voltage-gated channels open→ all of the sodium ions rush in, making
the inside even more positive→ depolarized till +40 mV
3. Local current is strong enough to change the voltage around them and so on.
4. Repolarization: potassium channels open up and potassium leaves the cell, making it
more negative again and rebalance the charges
5. Hyperpolarization: too much potassium ions leave the cell
Once the ion channels are open, it cannot respond to any other stimulus: refractory period.
A weak stimulus tends to trigger less frequent action potentials, compared to a strong
stimulus.
Myelinated neurons conduct action potentials faster than non-myelinated neurons. Gaps
between myelin sheaths are called nodes of Ranvier → saltatory conduction (leaping)
Membrane resistance= measure of how easy for currents flow through the membrane. It is
measured in MOhm. Current likes to flow through the path with less resistance. Membrane
resistance is a function of open ion channels; as the ion channel density increases
membrane resistance is reduced.
Membrane capacitance= measure of how easily the electrical charge balance can be
established across the intracellular and extracellular environment. It is measured in nF and
4
