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Cell types

In the nervous system

Sarah Olesen & Peter Vincent

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Total number of neurons	86 billion
Total number of NON-neurons	85 billion
Neurons, cerebral cortex	16 billion (82% brain mass)
Neurons, cerebellum	69 billion (10% brain mass)

What are brains made of?

Brain size and neuron count

Cerebral cortex mass and neuron count for various mammals.

Do you think insects have brains..?

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What are the different cell types in the brain?

Neurons Glia

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Anatomy of a neuron

soma / cell body

signal integration

axon

signal conduction

dendrites

signal reception & integration

myelin sheaths

insulation

So, here we have some neurons. Neurons generally connect to one another, relay information throughout your body, not just within your brain, and the way they are built reflects that function. Firstly, there’s the axon - this is the long projection which functions to conduct, or send signals, down the line to the next neuron. For this reason, they are insulated by myelin sheaths, which we’ll come back to in a few minutes. The main portion of the neuron, the cell body or soma, integrates signals but also houses a lot of organelles, including the nucleus. Dendrites, located on the soma, are responsible for receiving and integrating signals incoming from other neurons. Importantly, these signals always travel in one direction - they are received by the dendrites and conducted along the axon to the following cell. The reception of signals, between axons and dendrites, occurs at junctions called synapses, which we will also look at in more detail in a moment. But the gist of it is, the axon sends the signal, the dendrite receives it, and the next axon passes it on again.

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Neurons transmit signals

soma / cell body

signal integration

axon

signal conduction

synapses

signal transmission

dendrites

signal reception & integration

myelin sheaths

insulation

So, here we have some neurons. Neurons generally connect to one another, relay information throughout your body, not just within your brain, and the way they are built reflects that function. Firstly, there’s the axon - this is the long projection which functions to conduct, or send signals, down the line to the next neuron. For this reason, they are insulated by myelin sheaths, which we’ll come back to in a few minutes. The main portion of the neuron, the cell body or soma, integrates signals but also houses a lot of organelles, including the nucleus. Dendrites, located on the soma, are responsible for receiving and integrating signals incoming from other neurons. Importantly, these signals always travel in one direction - they are received by the dendrites and conducted along the axon to the following cell. The reception of signals, between axons and dendrites, occurs at junctions called synapses, which we will also look at in more detail in a moment. But the gist of it is, the axon sends the signal, the dendrite receives it, and the next axon passes it on again.

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There is an electrochemical gradient across the neuronal membrane.

energy-dependent Na⁺ / K⁺ pump (3 Na⁺ out, 2 K⁺ in per 1 ATP)
selectively permeable membrane

high Na⁺

high Cl^–

low K⁺

low Na⁺

low Cl^–

high K⁺

Δ = 65 mV

Na⁺ channel

K⁺ channel

Na⁺ / K⁺ pump

+ lots of negatively charged macromolecules

At rest, membrane is mostly permeable to K⁺

Neurons have a resting membrane potential of

–65 mV

Membrane potential: neurons are excitable

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resting Vm

–65 mV

Action potentials: neurons are excitable

peak

+30 mV

If membrane potential ≥ threshold:

Depolarisation

Voltage-gated Na+ channels open

Na+ flows into the cell

Repolarisation

Vg Na+ channels inactivate

Vg K+ channels open

K+ flows out of the cell

Hyperpolarisation

K+ keeps flowing out

Absolute refractory period

Vg Na+ channels de-inactivate

Vg K+ channels close

Relative refractory period

Return to resting potential

1

2

3

4

5

1

2

3

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Voltage & Action Potentials

action potentials in axon*:

large, ‘all-or-nothing’

synaptic potentials in dendrites / soma:

Small, can be positive or negative

axon hillock

action potential initiation

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axon terminal

voltage-gated Ca²⁺ channels

synapse

postsynaptic cell

synaptic vesicle

with neurotransmitter

postsynaptic receptor

mitochondria

electromicrograph

Synapses are only 20-40 nm wide!

How do neurons talk to each other?

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How do neurons talk to each other?

The main excitatory neurotransmitter is glutamate

The main inhibitory neurotransmitter in the brain is GABA

γ (gamma)

amino butyric

acid

The effect of a neurotransmitter depends on the receptor.

Can you think of any other chemical messengers?

The main inhibitory neurotransmitter in the spinal cord is glycine

ionotropic receptor

allows ion flow,

millisecond timescale;

excitation / inhibition

metabotropic receptor

activates 2nd messengers,

second timescale,

modulation

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Circuit motifs

excitatory: promotes firing

inhibitory: prevents firing

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Circuit motifs

Which of the following will cause the last neuron in the chain to fire less frequently?

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Which of the following will decrease firing in the first neuron in the chain?

Circuit motifs

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What are the different cell types in the brain?

Neurons Glia

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Oligodendrocytes

Myelinated cells
Aids the rapid transmission of electrical signals

Astrocytes

Help keep extracellular ion concentration constant
Help recycling of neurotransmitters after high periods of local activity

Microglia

Immune cells
Remove damaged neurons and infection

From the Greek word for “glue”

Glial cells

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Which image shows demyelinated axons?

Multiple Sclerosis is a demyelinating disease

Immune system attacks the myelin sheath or the cells that produce or maintain it.

Common symptoms include:

Fatigue
Vision problems
Difficulty with walking/balance

Myelin for fast electrical transmission!

From the Greek word for “glue”

Glial cells

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Pulling it all together: functional circuits

Take home message:

Neurons and glia interact with each other as part of a complex cellular network

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Lots of different neurons!

How many neurons does this brain have?

Ramon y Cajal, drawing of the cells of the chick cerebellum. Estructura de los centros nerviosos de las aves.

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Can you think of any other properties that give neurons unique identities?

What makes neurons different from each other?

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Morphology Connectivity Firing properties Neurotransmitters

Can you think of any other properties that give neurons unique identities?

What makes neurons different from each other?

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Different morphologies:

Pyramidal Cells

Purkinje Cells

Starburst Amacrine Cells

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Different connections: Calyx of Held

Calyx of Held: �Axo-somatic synapse

"Classic" CNS Synapse:

Axo-dendritic synapses

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Different firing properties

Neurons fire action potentials (electrical activity) at different frequencies. We call these firing rates.

Neuron ID:

1

2

3

4

5

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Different neurotransmitters: Dopaminergic neurons

Dopamine

Control Parkinson’s

Dopamine-releasing neurons play an essential role in modulating movement related signals. In Parkinson’s disease, these neurons are lost.

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SUMMARY

The brain is made of different types of neurons and glia.

Glia support the health and function of the central nervous system.

Neurons communicate with each other through chemical and electrical signals as part of a circuit.

Together they form a network for sensing, information processing and movement.

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Questions?