Neural signals are transmitted through both electrical and chemical means. An electrical signal travels down a neuron's dendrites to its cell body. If the signal reaches the axon hillock's threshold, the axon is activated and fires, transmitting an electrical signal down the axon. At the axon terminals, neurotransmitters are released across the synaptic cleft to the next cell. The membrane potential, maintained by ion concentration gradients and sodium-potassium pumps, underlies the neuron's resting potential. When neurotransmitters bind to receptors on the post-synaptic cell, they generate graded excitatory or inhibitory post-synaptic potentials that are integrated and can trigger an all-or-none action potential for signal transmission along the ax

1. NEURONAL
CONDUCTION
Dr. M. Ramya Maheswari
Assistant Professor and Head
Department of Psychology
Ethiraj College For Women

2. HOW DO NEURONS SEND AND RECEIVE SIGNALS ?

3. THE NEURAL COMMUNICATION – AN OVERVIEW
• Neural communication actually involves both electrical and chemical—or
electrochemical—communication.
• The signal usually starts in the neuron’s dendrites, and then travels down the
length of the cell until it reaches the terminal buttons at the very tips of the
neuron’s axon.
• A single neuron normally has many dendrites. The number of dendrites being
stimulated may vary from one moment to the next. Thus, there may be more or
less electrical activity traveling down into the soma at any given time.
• If there is too little electrical activity, nothing happens. If, however, the electrical
activity reaches a certain critical amount, or threshold, the axon hillock activates
the axon. If the axon hillock’s threshold is reached and the axon is activated, an
electrical signal travels down the axon’s length until it reaches the very end of the
neuron. This activation of the axon is referred to as the neuron firing.
• From the terminal buttons , the neuron will pass the signal on to another cell, such
as another neuron or muscle cell.

4. THE NEURON

5. THE NEURAL IMPULSE CYCLE
• The nerve impulse is the reversal in the charge of the cell membrane, which
spreads along the cell membrane forming an electrical current.
• Resting Potential – Tells us about what happens when the neuron is at rest.
• Action Potential - Occurs when a neuron sends information down an axon
i) Depolarisation
ii) Repolarisation

6. THE MEMBRANE POTENTIAL
The key to understanding how neurons work is the membrane potential.
The membrane potential is the difference in electrical charge between the inside and
the outside of a cell.
Recording resting membrane potential. when the tip of the intracellular electrode is
inserted into a neuron, a steady potential of about –70 millivolts (mV) is recorded.
This steady membrane potential is called the neuron’s resting potential, and the
neuron is said to be polarized.( electrical difference across the membrane)

7. RESTING POTENTIAL
• Recording the membrane potential: difference in electrical charge between inside
and outside of cell
• Inside of the neuron is negative with respect to the outside.
• Resting membrane potential is about –70mV.
• Membrane is polarized (carries a charge)

8. TERMS TO KNOW
• Concentration Gradient: A concentration gradient occurs when
the concentration of particles is higher in one area than another. In passive
transport, particles will diffuse down a concentration gradient, from areas of
higher concentration to areas of lower concentration, until they are evenly spaced.
• Electrical Gradient: In biological solutions, electrical gradient refers to
the electrical potential that acts on an ion to drive the movement of the ion in one
or another direction

9. IONIC BASIS FOR RESTING POTENTIAL
Why are resting neurons polarised?
Salts in neural tissue separate into positively and negatively charged particles called
ions. The resting potential results from the fact that the ratio of negative to positive charges
is greater inside the neuron than outside.
Resting potential results from
(1) the concentration of Na+ is higher outside,
(2) the concentration of Cl is higher outside,
(3) the concentration of K+ is higher inside, and
(4) various negatively charged protein ions are trapped inside

10. FOUR FACTORS THAT UNDERLIE RESTING POTENTIAL
Factors contributing to even distribution of ions.
• Random motion – ions in a solution are normally under random motion. particles tend to move down
their concentration gradient.
• Electrostatic pressure – like repels like, opposites attract. It disperses accumulation of any positive or
negative charges in any area.
Factors contributing to uneven distribution of ions.
• Selective permeability to certain ions – pass through the neural membrane at specialised pores called
ion channels. When neurons are at rest, the membrane is:
a) totally resistant to the passage of protein ions,
b) extremely resistant to the passage of Na+ ions,
c) moderately resistant to the passage of K+ ions,
d) and only slightly resistant to the passage of Cl ions

11. SODIUM POTASSIUM PUMP
• Sodium Potassium Pump- energy consuming process involved in the maintenance
of the resting potential.

12. SODIUM POTTASSIUM PUMP
• Sodium ions tend to be driven in as a result of both concentration gradient and the
negative internal resting potential of – 70 mv. About 120 mv of electrostatic
pressure forces sodium ions into the cell.
• Potassium ions tend to move out of the neuron because of their higher
concentration inside the cells, although this tendency is partially offset by the
internal negative potential .
• However the sodium potassium pump pumps out sodium ions as rapidly as they
pass in and pumps in potassium ions as they pass out . For every three sodium ions
in it pushes into the cell it two ottassium ions it send out

13. GENERATION OF POST SYNAPTIC POTENTIALS .
How are neural signals created?
• When neurons fire, they release chemicals called neurotransmitters
• These chemicals diffuse across the synaptic cleft and bind with the post – synaptic
receptors in a lock and key fashion.

14. WHAT ARE THE EFFECTS?
• When neurotransmitter molecules bind to post –synaptic receptors, they typically have
two effects.
a) They may depolarise the receptive membrane (decrease the resting potential, from -
70 to -67 mv)
b) They may hyperpolarise ( increase the resting membrane potential from -70 to -72
mv).
• Post synaptic depolarizations are called excitatory post synaptic potentials
(EPSP).They increase the likelihood of neuronal firing.
• Post synaptic hyper - polarizations are called inhibitory post synaptic potential
(IPSP). They decrease the likelihood of neuronal firing.
Both EPSP and IPSP are graded potentials : ie., the amplitudes of E PSP’s and
IPSP’s are proportional to the intensity of the stimulus

15. CONDUCTION OF POST SYNAPTIC POTENTIALS
• EPSP’s and IPSPs travel passively from their sites of generation at
synapse, usually on the dendrites or cell body in much the same way
that electrical signals travel through the cable.
• Transmission of post synaptic potentials has two characteristics.
• 1) It is rapid, almost instantaneous irrespective of whether they are
brief or enduring.
• 2) They are decremental, ie., they decrease in amplitude as they travel
through the neuron.

16. INTEGRATION OF POST SYNAPTIC POTENTIALS AND GENERATION OF ACTION
POTENTIALS
• A neuron's action potentials are triggered at the axon hillock when neuron
is depolarized to the point that the membrane potential at the hillock reaches about
-65 mV. This is the threshold of excitation for many neuron.
• Action potential is a massive momentary reversal of the membrane potential from
about -70 to about +50 mV. This last for 1 millisecond.
• Unlike EPSPs and IPSPs, Action potentials are not graded. They follow the all or
none law.
• Most neurons receive hundreds of synaptic contacts. Whether or not a neuron
fires is determined by the adding together (integration) of what goes on
at many presynaptic neuron synapses

17. INTEGRATION OF POST SYNAPTIC POTENTIALS AND GENERATION OF
ACTION POTENTIALS
There are two kinds of neural integration:
• Spatial summation (EPSPs + EPSPs; IPSPs + IPSPs; EPSPs + IPSPs) - It shows
how local EPSPs that are produced simultaneously on different parts of the
receptive membrane sum to form a greater EPSP .
• Temporal Summation: (EPSPs + EPSPs; IPSPs + IPSPs) – It shows how post
synaptic potentials produced in rapid succession at the same synapse sum to form
a greater signal

18. SPATIAL SUMMATION

19. TEMPORAL SUMMATION

20. CONDUCTION OF ACTION POTENTIALS - IONIC BASIS
• Conduction of action potential takes place through the action of the voltage
activated ion channels – the ion channels that open and close in response to the
changes in the level of the membrane potential.

21. CONDUCTION OF ACTION POTENTIALS - IONIC BASIS
i) Voltage activated gates present in the axon membrane become more permeable to sodium ions.
ii) Sudden influx of NA+ ions
iii)Reversed Polarity- -70 mV to +50 mV
iv)Opening of voltage activated potassium channels. Potassium driven out of cell because of the high
internal concentration and at the peak of the action potential due to positive internal charge.
v)Sodium channels close marking the end of the rising phase of the action potential and beginning of
repolarisation by continuous outflow of potassium ions to the extent that the membrane stays in a
state of hyperpolarisation for a brief period of time.

22. REFRACTORY PERIODS
• A brief period of about 1 or 2 milliseconds after the initiation of an action
potential, during which it is impossible to elicit a second one , This is called
absolute refractory periods.
• This is followed by relative refractory periods - the period during which it is
possible to fire the neuron again, but only by applying higher than normal levels
of stimulation . After which the amount of stimulation necessary to fire a neuron
returns to the baseline.

23. SALTATORY CONDUCTION

24. THANK YOU