EXCITABLE TISSUES

Содержание

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Dentistry 07

Learning objectives:

by the end of these lectures the student should know
Morphology

Dentistry 07 Learning objectives: by the end of these lectures the student
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

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Dentistry 07

Nerve cells:
The neurons are the basic building blocks of the nervous

Dentistry 07 Nerve cells: The neurons are the basic building blocks of
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.

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Dentistry 07

Resting Membrane Potential

Definition: it is the potential difference recorded across

Dentistry 07 Resting Membrane Potential Definition: it is the potential difference recorded
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

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Dentistry 07

Resting Membrane Potential (Vr)

Dentistry 07 Resting Membrane Potential (Vr)

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Dentistry 07

Sodium-Potassium Exchange Pump

Dentistry 07 Sodium-Potassium Exchange Pump

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Dentistry 07

Significance:
PROTEINS have a negative charge & can not leave the

Dentistry 07 Significance: PROTEINS have a negative charge & can not leave
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

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Dentistry 07

It is recorded by cathode ray oscilloscope
it is negative in

Dentistry 07 It is recorded by cathode ray oscilloscope it is negative
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

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Dentistry 07

Excitation & conduction: Nerve cells have low threshold for excitation. The stimulus may

Dentistry 07 Excitation & conduction: Nerve cells have low threshold for excitation.
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

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Dentistry 07

Recording membrane potential

Action potential

Dentistry 07 Recording membrane potential Action potential

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Dentistry 07

All or non law:
Application of a threshold stimulus either produces a

Dentistry 07 All or non law: Application of a threshold stimulus either
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.

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Dentistry 07

Stimulation of a nerve produces:

ELECTRICAL CHANGES CALLS ACTION POTENTIAL
EXCITABILITY CHANGES.
THERMAL

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

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Dentistry 07

The action potential (AP)

An action potential is:
A regenerating depolarization of membrane

Dentistry 07 The action potential (AP) An action potential is: A regenerating
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

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Dentistry 07

Threshold and Action Potentials

Threshold – membrane is depolarized by 15 to

Dentistry 07 Threshold and Action Potentials Threshold – membrane is depolarized by
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

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Dentistry 07

The Action Potential

Passive increase in positive charge

- 75 mV

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

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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

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Dentistry 07

The Action Potential

Opening of voltage-gated sodium channel

K

K

K

- 40 mV

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

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Dentistry 07

The Action Potential

Inactivation of voltage-gated sodium channel

K

K

K

+ 50 mV

voltage-gated sodium

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

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Dentistry 07

The Action Potential

Dentistry 07 The Action Potential

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Dentistry 07

The Action Potential

Dentistry 07 The Action Potential

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Dentistry 07

The Action Potential

closing of voltage-gated potassium channel

repolarization due to potassium influx

Repolarisation due to

Dentistry 07 The Action Potential closing of voltage-gated potassium channel repolarization due
potassium influx

Membrane potential approaches the ENa and voltage-gated sodium channels turn to the inactivation phase

Hyperpolarising afterpotential

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Dentistry 07

The Action Potential

Inactivation of voltage-controlled sodium channel

Resting potential (-75 mV)

Equilibrium potential

Dentistry 07 The Action Potential Inactivation of voltage-controlled sodium channel Resting potential
of sodium (+60 mV)

threshold

Hyperpolarization due to more outflux of potassium ions

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Dentistry 07

The Action Potential (excitability changes)

Polarized state (resting)

Depolarisation (due to sodium influx)

Resting potential

Dentistry 07 The Action Potential (excitability changes) Polarized state (resting) Depolarisation (due
(-75 mV)

ENa (+60 mV)

Hyperpolarising afterpotential

afterdepolarization

EK (-95 mV)

Absolute refractory period

Relative refractory period

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Dentistry 07

Action Potential Propagation

Dentistry 07 Action Potential Propagation

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Dentistry 07

Saltatory Conduction: Action Potential Propagation in a Myelinated Axon

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

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Dentistry 07

Propagation of an Action Potential (Time = 1ms)

Ions of the extracellular

Dentistry 07 Propagation of an Action Potential (Time = 1ms) Ions of
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

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Dentistry 07

Properties of action potentials

Action potentials:
are all-or-none events
threshold voltage (usually 15 mV

Dentistry 07 Properties of action potentials Action potentials: are all-or-none events threshold
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.

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Dentistry 07

Properties of action potentials

have constant conduction velocity
True for given fibre. Fibres

Dentistry 07 Properties of action potentials have constant conduction velocity True for
with large diameter conduct faster than small fibres. As general rule:

Impulses are conducted faster in myelinated fibre than non- myelinated fibre

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Dentistry 07

Functions of action potentials

Information delivery to CNS
carriage of all sensory input

Dentistry 07 Functions of action potentials Information delivery to CNS carriage of
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.

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Dentistry 07

Functions of action potentials

Rapid transmission over distance (nerve cell APs)
Note: speed

Dentistry 07 Functions of action potentials Rapid transmission over distance (nerve cell
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)

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Dentistry 07

Conduction velocity of AP

Compound action potentials can be recorded from nerve

Dentistry 07 Conduction velocity of AP Compound action potentials can be recorded
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

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Dentistry 07

as recordings are made further from the site of stimulation the

Dentistry 07 as recordings are made further from the site of stimulation
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)

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Dentistry 07

Recovery of membrane excitability during the refractory period

Absolute refractory period

100
50
0

Percent

Dentistry 07 Recovery of membrane excitability during the refractory period Absolute refractory
recovery

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Dentistry 07

Rheobase: It the least minimal threshold current, needed to excite the

Dentistry 07 Rheobase: It the least minimal threshold current, needed to excite
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)

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Dentistry 07

Strength-Duration Curve for Action Potential Initiation

Duration of stimulus (msec)

5 -
4 -
3

Dentistry 07 Strength-Duration Curve for Action Potential Initiation Duration of stimulus (msec)
-
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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Dentistry 07

Characteristics of Action Potential

Threshold
All-or-none property

Dentistry 07 Characteristics of Action Potential Threshold All-or-none property

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Dentistry 07

Cycle of Ion Channel

Dentistry 07 Cycle of Ion Channel

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Dentistry 07

Ionic Concentration Before and After Action Potential

Potassium ion

Sodium ion

Dentistry 07 Ionic Concentration Before and After Action Potential Potassium ion Sodium ion
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