Neuroscience with Pharmacology 2Functions and Mechanisms of Nerve Conduction

“Cogito, ergo sum” “I am a machine!”

The monosynaptic stretch reflex

Initiation Conduction Transmission End effect

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1. Types of neurones and glial cells; “dynamic polarisation” of neurones; slow conduction by axonal transport

2. Extracellular recording, fast conduction by action potentials, conduction velocity and function in myelinated and unmyelinated axons; refractory period.

3. Ionic mechanisms and pharmacology of action potentials and ion channels

1. Types of neurones and glial cells; “dynamic polarisation” of neurones; slow conduction by axonal transport

2. Extracellular recording, fast conduction by action potentials, conduction velocity and function in myelinated and unmyelinated axons; refractory period.

3. Ionic mechanisms and pharmacology of action potentials and ion channels

The nervous system is composed of neurones, gliaand blood vessels

Cerebral Cortex CorticalPyramidal cell

CerebellarPurkinje cell

Neurones are diverse in form and function.

DRG : sensoryneurone

Cerebellum:Purkinje neurone

Spinal Cord:motor neurone

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Neurones also differ in the neurotransmitter molecules they secrete

glutamate

AcetylcholineGABA

glycine

astrocyte

oligodendrocyte (CNS) or Schwann cell (PNS)

Astrocytes, oligodendrocytes and Schwann cells are importanttypes of neuroglia

Importance of the Axon

SOMA AXON SYNAPSE

COMPARTMENTAL MODEL OF THE NEURON

Dendrites

Information

(“dynamic polarization” - S. Ramon y Cajal)

Motor neurones are among the largest cells in the body, withaxons that extend many times the diameter of their somas

12,600 km

384,000 km

3,475 km

Pacificisland(5km)

0.5km

RobertHartley/Adrianna Teriakidis

The (mouse) motor neurone in perspective

500 µm

50µm

27,412 µm3

Vr=1

5,88

2 µ

m3

Vr=30

2,377 µm3

Vr=4

100µm

3 cm

30µm

4DL muscle

Proteins of the axonal cytoskeleton

20 nm10 nm 5 nm

Microtubule Neurofilament Microfilament

ActinTubulin

Axonal Transport

Nucleus

Rough ER

Golgiapparatus

Synapticvesicles

Organelles

2

Active peptidePrecursorpeptide

13 4

Axonal transport of organelles and trophic factors

Kieran et al. J. Cell Biol. 2008:169:561-567© 2008 Kieran et.al.

time

distance

“Slow” axonal transport :1-8 mm/day~ 0.2 mm/hr~ 0.05 µm/sec

Molecular signals, structural proteins and organelles are transported along the axonal cytoskeleton

Orthograde (kinesin motors)

Retrograde (dynein motors)

“Fast” axonal transport :100-400 mm/day~ 5-20 mm/hr~ 1-5 µm/sec

1. Types of neurones and glial cells; “dynamic polarisation” of neurones; slow conduction by axonal transport

2. Extracellular recording, fast conduction by action potentials, conduction velocity and function in myelinated and unmyelinated axons

3. Ionic mechanisms and pharmacology of action potentials and ion channels

1. Types of neurones and glial cells; “dynamic polarisation” of neurones; slow conduction by axonal transport

2. Extracellular recording, fast conduction by action potentials, conduction velocity and function in myelinated and unmyelinated axons; refractory period.

3. Ionic mechanisms and pharmacology of action potentials and ion channels

ca 1800: Galvani and Volta demonstrate the electrical excitability of nerve

ca. 1815

Stimulatingelectrodes

Recordingelectrodes

Clinical Electromyography

Increasing stimulus intensity“recruits” (activates) more axons

Extracellular recording reveals multiple “fibre groups”

S

ms

mV

Erlanger & Gasser (1930)

First recordings of “compound” action potentials in bullfrog sciatic nerves

Fibre Groups A,B,C

Aα

Bβ

Cγ D

δ

Increasing stimulus intensityactivates axons of different types(“modalities”)

Cross sections of nerves show myelinated andunmyelinated axons of wide-ranging diameter

50µm

The myelin sheath

2 µm

Cross sections of nerves show myelinated andunmyelinated axons of wide-ranging diameter

Cross-section of HumanPeripheral Nerve

Aα

Aδ

Myelinated Nerve FibreDiameter Spectrum

Development of the myelin sheath

Myelin sheath and node of Ranvier

Node ofRanvier

Myelinsheath

K / Na / Caspr

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Saltatory conduction

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Myelinsheath

Node ofRanvier

Axon

7111143

t=0

Unmyelinated Myelinated

<0.5 m/second 2-100 m/second

Conduction velocity =

Distance (m)Latency(s)

Myelination optimises nerve conduction

The “g-ratio” is roughly constant (0.6) inmyelinated axons

Refractory period

No action potential can occur during the absoluterefractory period. A stronger stimulus is requiredduring the relative refractory period

1. Types of neurones and glial cells; “dynamic polarisation” of neurones; slow conduction by axonal transport

2. Extracellular recording, fast conduction by action potentials, conduction velocity and function in myelinated and unmyelinated axons

3. Ionic mechanisms and pharmacology of action potentials and ion channels

1. Types of neurones and glial cells; “dynamic polarisation” of neurones; slow conduction by axonal transport

2. Extracellular recording, fast conduction by action potentials, conduction velocity and function in myelinated and unmyelinated axons; refractory period.

3. Ionic mechanisms and pharmacology of action potentials and ion channels

Initial Segment

http://tainano.com/chin/Molecular%20Biology%20Glossary.htm

The action potential propagates as a wave of depolarisation,due to switching from closed,open and inactivated states involtage-gated ion channels.

The action potential recorded intracellularly (squid giant axon)

Resting potential

Threshold

Upstroke: DepolarisationOvershoot

Downstroke: Repolarisation

Afterhyperpolarisation

Refractory Period

Stimulus artefact

Em=RT

F.Ln

(Pk.[K

+

]o+ P

Na.[Na

+

]o+ P

Cl.[Cl

!

]i)

(Pk.[K +]

i+ P

Na.[Na+]

i+ P

Cl.[Cl !]

o)

Na+

K+

Inside Outside

Cl-

Goldman- Hodgkin-Katz(GHK;or “Goldman”) Equation

Membrane potential is determined by selective permeability and ion gradients

1950-55: The “sodium hypothesis”

1. Low sodium reduces AP

[Na+]o

2. “Voltage-clamp” analysis reveals transient inward currentscarried by sodium ions and delayed outward currents carriedby potassium ions when membranes are depolarised.

Alan Hodgkin

AndrewHuxley

BernardKatz

Erwin Neher Bert Sakmann

1976: Neher & Sakmann make the first recordings from single ion channels

T=0 ms; Rest

Na+

- -- -

m

h

K+

n

- -- -

T=1 ms; Activation

Na+

+ ++ +

K+

T=2 ms; Inactivation/Rectification

Na+

- -+ +

K+

-- -

- -- -

T=4 ms; Absolute Refractory Period

Na+

K+

-- -

- -- -

T=5-10 ms; Relative Refractory Period

Na+

K+

-- -

T=20 ms; Recharged = Rest

Na+

- -- -

K+

Voltage-sensitive ionic currents are blocked by specific drugs

Channel Antagonist/blocker

Na+ currents tetrodotoxin (TTX)lignocaineµ-conotoxin

K+ currents tetraethylammonium (TEA)4-aminopyridine (4-AP)Cs+

Ca2+ currents Cd2+, Co 2+ , Mg2+

L-type dihydropyridines (DHP) N-type ω-conotoxin P-type ω-agatoxin

1. There are several types of neurones and glial cells. Neurones show“dynamic polarisation”. The function of axons is to carry information: fromthe soma-dendritic compartment to the axon terminals; and from axonterminals to cell bodies.Axonal transport along cytoskeletal proteins carries orthograde andretrograde signals, molecules and organelles slowly, at velocities of~ 0.05 µm/sec (slow axonal transport) to ~ 5 µm/sec (fast axonaltransport).

2. Extracellular recording and microscopy show that action potentialconduction in axons is determined by their diameter and myelination(oligodendrocytes in the CNS, Schwann cells in the PNS).Axon membranes conduct action potentials rapidly, at ~0.5-100 m/sec.

3. Ion channel properties explain AP shape, threshold, all-or-nothingresponse and refractory period.Voltage-clamp analysis and pharmacology show that ion channels in axonmembranes have specificity (Na, K, Ca) and voltage- and time-dependentpermeability.

Summary