EXCITABLE TISSUES

Dentistry 07
Learning objectives:
by the end of these lectures the student should know
Morphology of the nerve cell & functional organization of neurons
Excitation & conduction along the nerve (local & propagated action potentials)
Resting membrane potential ( causes & recording)
Action potential (ionic bases & recording) electrical changes that occur on a nerve on stimulation.
Compound action potential
Changes in excitability during electronic potential (local) & action potential
All or non law
Saltatory conduction
Energy sources & metabolism of nerve
Properties of mixed nerve
Nerve types & functions

Dentistry 07
Nerve cells:
The neurons are the basic building blocks of the nervous system, their axons may or may not myelinated.
The myelin sheath is produced by the Schwan cells. It envelops the axon except at the ends & the nodes of Ranvier
The impulse is conducted faster in myelinated than unmyelinated nerves.

Dentistry 07
Resting Membrane Potential
Definition: it is the potential difference recorded across the cell membrane at rest.
Causes:
80% caused by selective permeability of the cell membrane
The K+ diffuses out the cell & Na+ diffuses inside the cell according to concentration gradient. The K+ permeability is 50-75 folds more than Na+
20% is caused by the Na+ K+ pump
an active process that needs energy taken from ATP. This is very important to maintain the concentration gradient across the cell membrane

Dentistry 07
Resting Membrane Potential (Vr)

Dentistry 07
Sodium-Potassium Exchange Pump

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Significance:
PROTEINS have a negative charge & can not leave the cell to the outside
K+ efflux is not accompanied by an equal influx of anions & membrane is maintained in a polarized state with the outside positive relative to the inside making the RMP for a nerve to be - 70 mV

Dentistry 07
It is recorded by cathode ray oscilloscope
it is negative in polarized (resting, the membrane can be excited) state with the potential difference inside the cell membrane is negative relative to the outside.
Recording of Resting and action potentials
Voltmeter

Dentistry 07
Excitation & conduction:
Nerve cells have low threshold for excitation.
The stimulus may be electrical, chemical or mechanical.
Two types of potentials may be produced
Local (Non-propagated action potential ) named after its location synaptic, generator or electronic potential
PROPAGATED ACTION POTENTIAL (nerve impulse).
Both are due to changes in the conduction of ions across the cell membrane that are produced by alternations in the ion channels

Dentistry 07
Recording membrane potential
Action potential

Dentistry 07
All or non law:
Application of a threshold stimulus either produces a full response or not at all.
Further increase in the intensity of a stimulus produces no increment or other changes in action potential.
The action potential failed to occur if the stimulus is sub-threshold, it produces only local changes with no propagation.
Latent period in a nerve: it is a period corresponding to the time taken from the site of simulation till the recording electrode.

Dentistry 07
Stimulation of a nerve produces:
ELECTRICAL CHANGES CALLS ACTION POTENTIAL
EXCITABILITY CHANGES.
THERMAL CHANGES

Dentistry 07
The action potential (AP)
An action potential is:
A regenerating depolarization of membrane potential that propagates along an excitable membrane.
[propagates = conducted without decrement (an ‘active’ membrane event)]
[excitable = capable of generating action potentials]
Action potentials:
are all-or-none events
need to reach threshold
have constant amplitude
do not summate
are initiated by depolarization
involve changes in permeability
rely on voltage-gated ion channels

Dentistry 07
Threshold and Action Potentials
Threshold – membrane is depolarized by 15 to 20 mV
Established by the total amount of current flowing through the membrane
Weak (subthreshold) stimuli are not relayed into action potentials
Strong (threshold) stimuli are relayed into action potentials
All-or-none phenomenon – action potentials either happen completely, or not at all

Dentistry 07
The Action Potential
Passive increase in positive charge
- 75 mV

Dentistry 07
The Action Potential
Opening of voltage-gated sodium channel
K
K
K
- 55 mV
threshold

Dentistry 07
The Action Potential
Opening of voltage-gated sodium channel
K
K
K
- 40 mV

Dentistry 07
The Action Potential
Inactivation of voltage-gated sodium channel
K
K
K
+ 50 mV
voltage-gated sodium channels turn to the inactivation phase

Dentistry 07
The Action Potential

Dentistry 07
The Action Potential

Dentistry 07
The Action Potential
closing of voltage-gated potassium channel
repolarization due
to potassium influx
Repolarisation due
to potassium influx
Membrane potential approaches the ENa and voltage-gated sodium channels turn to the inactivation phase
Hyperpolarising
afterpotential

Dentistry 07
The Action Potential
Inactivation of
voltage-controlled sodium channel
Resting potential (-75 mV)
Equilibrium potential of sodium (+60 mV)
threshold
Hyperpolarization due to more outflux of potassium ions

Dentistry 07
The Action Potential (excitability changes)
Polarized state (resting)
Depolarisation
(due to
sodium influx)
Resting potential (-75 mV)
ENa (+60 mV)
Hyperpolarising
afterpotential
afterdepolarization
EK (-95 mV)
Absolute
refractory period
Relative refractory period

Dentistry 07
Action Potential Propagation

Dentistry 07
Saltatory Conduction: Action Potential Propagation in a Myelinated Axon

Dentistry 07
Propagation of an Action Potential (Time = 1ms)
Ions of the extracellular fluid move toward the area of greatest negative charge
A current is created that depolarizes the adjacent membrane in a forward direction
The impulse propagates away from its point of origin

Dentistry 07
Properties of action potentials
Action potentials:
are all-or-none events
threshold voltage (usually 15 mV positive to resting potential)
are initiated by depolarization
action potentials can be induced in nerve and muscle by extrinsic (percutaneous) stimulation –
APs do not summate - information is coded by frequency not amplitude.

Dentistry 07
Properties of action potentials
have constant conduction velocity
True for given fibre. Fibres with large diameter conduct faster than small fibres. As general rule:
Impulses are conducted faster in myelinated fibre than non- myelinated fibre

Dentistry 07
Functions of action potentials
Information delivery to CNS
carriage of all sensory input to CNS. Consider block APs in sensory nerves by local anaesthetics. This usually produces analgesia without paralysis. This is because LAs are more effective against small diameter (large surface area to volume ratio) C fibers than a-motorneurones.
Information encoding
The frequency of APs encodes information (remember amplitude cannot change) - covered in lecture 3.3.

Dentistry 07
Functions of action potentials
Rapid transmission over distance (nerve cell APs)
Note: speed of transmission depends on fiber size and whether it is myelinated. Information of lesser importance carried by slowly conducting unmyelinated fibers.
In non-nervous tissue APs are the initiators of a range of cellular responses
muscle contraction
secretion (eg. Adrenalin from chromaffin cells of medulla)

Dentistry 07
Conduction velocity of AP
Compound action potentials can be recorded from nerve truncks
usually done percutaneously from nerves that are close to the surface (eg. Ulnar nerve)
The passage of an action potentials in all the axons in the nerves is seen as a small (μV) voltage signal on body surface

Dentistry 07
as recordings are made further from the site of stimulation the waveform develops into several discrete peaks
Each peak was named: alpha - the first to appear; beta - the next, and so on.
The first signal to arrive at a distant recording site has travelled the fastest!
So each peak represents a set of axons with similar conduction velocity
velocity is calculated from the distance between R1 and R3 and the time taken to traverse that distance - distance/time = velocity (ranges from 0.5 to ~100 ms-1)

Dentistry 07
Recovery of membrane excitability during the refractory period
Absolute
refractory period
100
50
0
Percent recovery

Dentistry 07
Rheobase: It the least minimal threshold current, needed to excite the nerve, below it no excitation occurs whatever the duration of application of the stimulus
Utilization time: It is the time needed by Rheobase to excite
Chronaxie: It is the time needed by a stimulus double Rheobase strength to excite. It is the measure of excitability, the shorter the Chronaxie, the greater is the excitability of tissue (it is longer in smooth muscles than in skeletal)

Dentistry 07
Strength-Duration Curve for Action Potential Initiation
Duration of stimulus (msec)
5 -
4 -
3 -
2 -
1 -
0 -
Intensity of
Stimulus
(relative)
Minimal stimulation time
Rheobase
Chronaxie (σ)
Q = I x T
σ
τm = σ/ln2 = σ/0.69 = 1.44σ
Time constant = 1.44 x chronaxie

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Characteristics of
Action Potential
Threshold
All-or-none property

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Cycle of Ion Channel

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Ionic Concentration Before and After Action Potential
Potassium ion
Sodium ion