THE NEURON

I- INTRODUCTION

Integrity of an organism: coordinated activity of cells

intercellular communication :

- endocrine system (hormones released into the bloodstream)

- nervous system (electrical impulses transmitted from cell to cell)

Since prehistory: great interest in the brain.

Observations of brain anatomy (trepanations, dissections, surgeries...)

Nervous cell observations: pb:

- neuron size ( 20 μm Ø) microscope : late 17th century

- pb of cuts 19th : formalin + microtome

HISTOLOGY

2) Golgi coloration:Silver chrome

Pb : Uniform appearance of cells: staining techniques:

1) Nissl coloration:Distinction of neurons / glial cells

Observation of the cytoarchitecture of the brain

Pb: reveals only the cell bodies

Contribution of the Golgi coloration:

Neuron : at least 2 distinct parts:

- central part = cell body (soma or perikaryon)

- prolongations = neurites (dendrites and axon)

3 great functions: receiving, driving, transmitting

Cajal’s contribution :

Neuron theroy

(Heinrich Waldeyer)

The first to show the existence of neural circuits.

Golgi & Cajal : Nobel prize in 1906

The neuron :- excitable cell: transmits electrical signals (specific

proteins: ion channels)

- secretory cell: secretory products = neurotransmitters (synapses)

Constant and abundant supply of O2 and glucose

Longevity +++

Various forms: fundamental role of their morphology for the integration and processing of information

II- INTRACELLULAR ORGANIZATION

1) SOMA :Variable form

(spherical), 20 μm Ø

Contains cytosol (rich in K +) and organelles

2) NEURONAL MEMBRANE:

Barrier role, contains many proteins:

Pore formation: selection of inputs and outputs

Maintaining the gradient = difference in concentration on both sides of the membrane

3) CYTOSKELETON:Gives the characteristic form of the neuron, role in rigidityMade up of: microtubules, microfilaments, neurofilaments

High flexibility: elements constantly regulated, form a 3D network that structures the intracellular space

a) Microtubules :

20 nm Ø, along the neurites, compounds of tubulin (globular protein)

E.g. : Tau protein (axonal MAP) : involved in dementia(Alzheimer's)

Microtubule Associated Proteins (MAPs): role in assembling microtubule with each other or other parts of the neuron

Polymerization / depolymerization form of neurons

b) Microfilaments :

Small size (5 nm Ø), especially in neurites

Assembly of 2 actin filaments

Role in the shape of the cell

c) Neurofilaments :

Intermediate size (10 nm Ø), also called intermediatefilaments

4) AXON :

Axonic divisions= collateral

Protein composition soma membrane different functions

Neuron-specific, role in transmission of information in the NS.

Size: < 1mm > 1 m

Termination: arborization: numerous and thin branches with bulging ends synaptic contacts with target cells = terminal buttons.

Origin : conical expansion of soma = axon hillock (initial segment)

Ø : 1 to 25 μm (1 mm in calmar), constant all along the axon ( dendrites), smooth

Importance of Ø : conduction velocity

Axonic termination = terminal button:Site where the axon comes in contact with other neurons and transmits information to them.

This point of contact = synapse:

3 elements:- presynaptic (terminal button)

- synaptic cleft (intercellularspace)

- postsynaptic (dendrite or soma)

- no microtubules

- many mitochondrias (large energy needs)

- numerous synaptic vesicles(50 nm Ø)

Cytoplasm of the terminal button:

Cellular transport:Absence of ribosomes in the axon.

Protein synthesized in soma terminal button

Transport along microtubules thanks to specific proteins (ATP consumption+++)

- Anterograde:

Protein = kinesinATP dependant3 types: • fast (synthesis of neurotransmitters)• slow (renewal of axonal proteins),• mitochondrial.

- Retrograde:

Protein = dynein. Roles : signals to soma metabolic needs; waste disposal.

5) DENDRITES :

"Antennas" of the neuron

Dendritic membrane (= postsynaptic membrane) has many specialized proteins = receptors (= sites of action of neurotransmitters)

Cytoplasm close to that of the axon: elements of the cytoskeleton and mitochondria

III- CLASSIFICATION OF NEURONS

1) According to the number of neurites:

Unipolar Bipolar Multipolar

2) According to the dendrites:

Pyramidal cells

Spiny or non-spiny neurons

Star-shaped cells

3) According to the nervous connections:

Primary sensory neurons: dendrites in sensory areas

Motor neurons: synapses with muscles

Interneurons : in contact with other neurons

4) According to the neurotransmitters:

E.g. : GABAergic neurons (NT = GABA)

Cholinergic neurons (NT = acetylcholine)

IV- GLIAL CELLS

Twice as many as neurons

Occupy the space between neurons: compact tissue

5 types of cells:

In PNS : - Schwann cells

In CNS :

- interstitial cells: astrocytes, oligodendrocytes, microglia

- ependimal cells

1) Astrocytes :

The most numerous, occupy the space between the neurons and the blood vessels: represent the essential of the environment of the neurons.

Astrocytes type 1 :

Contact between neurons and blood vessels

Swellings = feet

functional barrier between neurons and vessels

External surface of the CNS: barrier = blood-brain barrier

Form an envelope around the synapses, have NT receivers.

Role : regulation of composition of the extracellular medium

Astrocytes type 2 :

Contact only with neurons: in somato-dendritic regions, around Ranvier nodes and axonal terminations.

2) Oligodendrocytes & Schwann cells :Role : form the myelin sheaths allows the acceleration of the influx.

Oligodendrocytes : only in CNS, 1 oligodendrocyte for

several neurons.

Schwann cells : only in PNS, 1 cell for 1 axon.

3) Other non-neuronal cells:

Microglial cells: role of phagocytes

Ependymal cells: lining the cerebral ventricles and the ependymal canal.

RESTING POTENTIAL

End of 19th : Galvani showed that the nervous information was electric

Contraction of a muscle by applying an electric current on its nerve.

Excitability: fundamental property at the base of the functioning of nerves and muscles.

Membrane Potential= Vm(Voltage of membrane)

Neuron inside: negative compared to the outside

Measurement of the resting potential:

This constant difference= resting potential

I- ROLE OF CELLULAR COMPONENTS

1) Cytosol and extracellular environment:

Water: main component, polar molecule solvent of other polar molecules

The more importants : K+, Na+, Ca2+, Cl-

Ions = atoms and molecules that have an electrical charge.

charge + = cation

charge - = anion

2) Membrane phospholipids:Form a barrier that prevents the passage of molecules (ions and water) from one medium to another.

Organized in double layer = phospholipidic bilayer

Contain a hydrophilic head (facing intra- and extra-cellular aqueous media) and a hydrophobic tail (inside the membrane)

3) Membrane proteins:

Ion selective pathways (selective permeability)

Incorporated into membrane transmembrane

Ionic channels:

Important properties: - ionic selectivity (potassium channels, sodium, calcium ...)

Assembly of 4 to 6 transmembrane proteinmolecules.

"Passive" transport

Ligand or voltage sensitive

- opening / closing (depending on the local environment of the membrane)

Ionic pumps:

Transmembrane

E.g. : Na + / K + pump

Crucial role in neuronal signaling via Na + or Ca2 + iontransport.

Enzymatic (hydrolysis of ATP)

II- ION MOVEMENTS

1) Diffusion force:

Tendency to distribute the ions evenly in the solution.

Depends on the temperature

Diffusion : concentration gradient + ion channels

The difference in concentration = concentration gradient

Movements of ions from medium of high concentration to medium of lower concentration = diffusion

2) Electric force:

Ions = electrically charged particles

opposite charges attract, similar charges repel each other

Ohm law : I = gV

Voltage dependent (= ddp) which reflects the charge difference between the anode and the cathode (V) and the conductance (g (S)) which represents the ability of the electrical charge to pass from one point to another (g = 1 / R).

Electric power = amplitude of displacement of charges (= I (A))

III- MEMBRANE POTENTIAL

Holds as long as the neuron does not generate action potential.

1) Overview :

Membrane at rest = electric battery, current generator

(negative pole inside, positive pole outside)

2) Balance potentials:

Waterproof membrane Selectivepermeability

Diffusion force

Electric force

Diffusion force

Equilibriumpotential

Vm = 0 Difference of potential

Intracellular Extracellular

Equilibrium: when electrical and diffusion forces are equal and opposite.

To generate a P : - ion concentration gradient

- ionic permeability

Nernst Equation :

3) Ion distribution:

[K+]i > [K+]e

[Na+]e > [Na+]i

[Ca2+]e > [Ca2+]iMaintained by ion pumps

Eg : Na\K pump; Ca2+ pump

Concentration gradients

Na\K pump:

Enzyme (ATP), exchange 3 Na+ against 2 K+

Against the concentration gradient

Ca2+ pump:

Enzyme (ATP)

Ca2+ also chelated by intracellular proteins and organelles (mitochondria + RE)

Maintain ionic concentration gradients

Role of ion pumps:

4) Membrane permeability:

Membrane highly permeable to K+ (EK+ = -80 mV)

Membrane resting potential: close to the equilibrium potential of the K+

Take into account the relative permeability of the membrane to certain ions

Resting potential = -65 mV

Also permeable to Na+ (ENa+ = 62 mV)

hence: Resting potential = - 65 mV

Goldman – Hodgkin – Katz voltage equation:

Takes into account the relative permeability of the membrane to certain ions

Vm = RT

FLn

( Pk [K+]e + PNa [Na+]e )

( Pk [K+]i + PNa [Na+]i )

5) Importance of potassium channels:

K+ : most important ion in the resting potential

Importance of selective permeability of potassium channels

Ion Selectivity: AA arrangement that forms the pore of the channel

Very large number of potassium channels

Role in pathologies: ex. epilepsy

Protection of neurons against extracellular increase of [K+]:

- blood-brain barrier

- glial cells (astrocytes: pumps and potassium channels)

Ex. of unprotected cells: cardiac cells

If hyperkalaemia: plasma [K+] > 5,0 mmol/L depolarization of cells loss of excitability death

ACTION POTENTIAL

Traduction neurons’ excitation

Have universal properties

Recording method:

The different phases of the AP:Ascending phase: rapid depolarization( + 40 mV)

Descending phase: repolarization of the membrane ( < - 65 mV)

Hyperpolarization

Return to the resting potential.

Total duration :

About 2 ms

2 main features::- Obey the law of all or nothing

- Propagates without amplitude attenuation all along the fiber

Importance of Na+ ions:Vm of the AP peak tends towards the equilibrium potential of Na+

ions

Triggering the AP:

Opening of sodium channels entry of Na + ionsDepolarization of the membrane

If reaching a critical level PA

This critical level = threshold

If depolarization > thresholdAP

Different types of Na+ channels: sensitive to voltage, stretching or neurotransmitters

Law of "all or nothing"

If continuous stimulation of neurons:

triggering a burst of PA

AP frequency depends on the intensity of stimulation

Max frequency = about 100 Hz (delay between 2 AP = 1 ms)

Refractory periods (absolute and relative)

The theory of AP

Depolarization: Na+ ion entry Repolarization: exit of K+ ions

At rest: membrane essentially permeable to K+

Vm = - 65 mV

Activation of sodium channels ↑ sodium permeability

depolarization

Activation of K+ channels

Repolarization

Na+ channel depending on the potential:Structure :

4 domains (I to IV) composed of 6 transmembran propellers

The 4 domains are gathered together and form a pore.

Pore closed: negative VmPore open: when reaching the depolarization threshold: modification of channel structure ions passage

Notion of ionique selectivity (let only Na+ ions pass)

Stereotyped operation :

- Only open when depolarized membrane (threshold)

- Fast opening (explaining fast ascending phase)

- Remains open about 1 ms (hence the brevity of the PA)

- No new opening as long as the potential does not find a value close to the threshold (= inactivation, hence absolute refractory period)

The membrane has hundreds of channels per μm²

Patch-clamp method:Isolates a small fragment of membrane to study the functioning of a single channel (1991 Nobel Prize)

Shows that the ion channels oscillate at least between 2 states: open and closed.

Toxins :

Tetrodotoxin (TTX) blocks Na+ channels (clogs the pore)blocks the spread of impulses into the muscle or nerve

In Japan, some restaurants serve Fugu, a fish rich in tetrodoxinethat only some certified cooks can prepare (one fish contains enough toxin to kill 30 people). All the art of the cook is to serve the flesh of the fish without contaminating it with the toxin.

From a Japanese fish (Fugu), can be fatal ...

Potassium channels depending on the potential:

Descending phase of the AP :

- inactivation of Na+ channels

- later: opening of channels K+ exit of K+repolarization

Structure :

4 Sub-units forming a pore

These K+ channels are also voltage sensitive but open later than the Na+ channels.

The different phases of the AP:

- Relative refractory period: hyperpolarized membrane, need for a large depolarizing current

- Absolute refractory period: depolarizedmembrane, inactive Na+ channels

- Inférieur point

- Descending phase : inactivation of Na+

channels and activation of K+ channels exit of K+ Repolarization

- Positif potential

- Ascending phase : entry of Na+Rapide depolarization

- Threshold

AP propagation:

- Constant amplitude

- No turning back (because refractory membrane due to inactivation of sodium channels)

- In one direction: to terminal button

Variable conduction velocity:

greater speed if myelin is present

speed increases with diameter (role also diameter in excitability)

- Depends on:- the diameter of the axon - the presence or absence of myelin (large diameter

axons are more excitable)

- Mean value = 10 m/s

Influence of myelin:

Node-to-node propagation = saltatory conduction

Interruptions at the nodes of Ranvier: at this level: passage of the ions AP

Constitutes an insulator

Pathologies :

- Muscle weakness, lack of coordination, sensory disturbances (vision) + language disorders

- Impairment of myelin sheaths; autoimmune

- Impairment of groups of axons: brain, spinal cord, optic nerves

- Detection: speed of conduction (ex: optic nerve: sti + EEG)

Multiple sclerosis:

Guillain-Barré syndrome :

Autoimmune, myelin impairment in peripheral nerves

Velocities of conduction of peripheral nerves (electrical stimulation of the nerves) muscular response)

at the level of the axonal cone = zone of initiation of the nerve impulses

AP initiation:

If depolarization of the axon cone > threshold AP that spreads

No AP in dendrites or in the cell body

Local anesthesia :

- Injected into the CSF of the spinal cord (spinal anesthesia)

- Injected into tissue = infiltration, or at the level of the nerves

- Dissolves: numbs the nerve endings = topical anesthesia

Derivative: lidocaine

First local anesthetic: cocaine (coca leaf extract, 1860 by Niemann, problem: toxic and addictive effects)

Small diameter axon: the most sensitive

Anesthetics are administered in the tissue to be anesthetized, hence: "local"

Principle: blocks the spread of AP along the axons by blocking the pores of the Na+ channels.