General aspects of neurotransmission and steps involved in neurotransmission

1. Neurotransmission
Presented by,
Mr. Siddharam Bagalkote
Dept. of Pharmacology
SJM College of Pharmacy, Chitradurga

2. Introduction
• Neurotransmission is the process by which
signaling molecules (neurotransmitters) are
released by the axon terminal of a neuron and
bind to and react with the receptors on the
dendrites of another neuron.
• Nerves transmit their message across
synapses and neuroeffector junctions by the
release of humoral (chemical) messengers.

3. Overview of nervous system
• Nervous system forms an interconnecting
fibers of communication network.
• In the hard-wiring of the nerves, the signals
travel in the form of a flow of electrical
current called nerve impulses.

4. Overview of nervous system
• Irritability is the universal property of life
which means the capacity of organisms to
respond to changes in the environment called
stimuli.
• The specific reaction elicited by a stimulus is
termed as response.

5. Divisions of Nervous System

6. Description of a neuron
• A neuron consists of a cell body and two kinds
of processes, the dendrites and the axon.
• The cell body has neuroplasm, a nucleus, nissl
bodies, neurofibrils and a cell membrane.
• The dendrites carry impulses towards the cell
body
• The axon carries impulses away from the cell
body.

7. • The axon originates from axon hillock of the
cell body.
• The axon is surrounded by two coverings:
myelin sheath and Schwann sheath.
• These two coverings are interrupted at
intervals by nodes of Ranvier.
• The fine branches at the end of axon are
called axon terminals.

8. Neuron

9. Properties of neuron
• Excitability –stimulated by suitable stimuli-
mechanical, thermal, chemical, electrical.
• Conductivity – impulse is conducted similar to
cable conduction and digital in character.
• All or none law – the stimulus should be in
adequate threshold strength.

10. • Refractory period – when the nerve fiber is
once excited, it will not respond to a second
stimulus for a brief period.
• Indefatigability – nerve is normally not
fatigued

11. • Nerve impulse
A nerve impulse is the sum total of physical
and chemical events associated with the
transmission of a signal along an axon
• Stimulus
Is defined as a sudden change in the
environment which is strong enough to cause
a response in the living organism

12. Types of stimulus
• Mechanical – Sharp Pressure
• Physical – Heat, Cold
• Chemicals- acids and Bases
• Electrical-Application of electricity
The weaker the stimulus, the longer it will
have to be applied to produce a response
The nerve takes lesser time to respond for a
stronger stimulus

13. Action Potential
• When a nerve is stimulated, Na+ ions
suddenly move into the cell and causing a
positive potential.
• The influx of Na + ions reaches its peak in
100msec.
• Sodium permeability causes depolarization.
• The inward diffusion of Na + ions halted near
the peak of action potential.

14. • The diffusion K+ ions restores the membrane
potential –called repolarization.

15. Plot of a typical action potential

16. Step involved in neurohumoral
transmission
• Impulse conduction
• Transmitter release
• Transmitter action on postjunctional
membrane
• Post junctional activity
• Termination of the effect of released
transmitter

17. I. Impulse conduction
• The resting membrane potential (-70 mV
negative inside) is established by high K+
permeability of axonal membrane and high
axoplasmic concentration of this ion coupled
with low Na+ permeability.
• Stimulation or arrival of an electrical impulse
causes a sudden increase in Na+ conductance
leads to depolarization (reverse polarization:
MP becoming 20 mV positive)

18. • K+ ions then move out in the direction of
their concentration gradient and
repolarization is achieved.
• The ionic distribution is normalized during
the refractory period by the activation of
Na+ K+ pump.
• The action potential (AP) thus generated
activate ionic channels at the next excitable
part of the membrane and the AP is
propagated without decrement.

19. II. Transmitter release
• The transmitter (excitatory or inhibitory) is
stored in prejunctional nerve endings within
‘synaptic vesicles’
• Nerve impulse promotes fusion of vesicular
and axonal membranes through Ca2 entry
which fluidizes membranes.
• All contents of the vesicle (transmitter,
enzymes and other proteins) are released in
the junctional cleft.

20. III. Transmitter action on postjunctional
membrane
• The released transmitter combines with
specific receptors on the postjunctional
membrane
• Depending on its nature induces an excitatory
postsynaptic potential (EPSP) or an inhibitory
postsynaptic potential (IPSP)

21. • EPSP Increases permeability to cations Na+ or
Ca2+ influx causes depolarization followed by
K+ efflux.
• IPSP Increase in permeability to anions Cl¯
ions move in tend to hyperpolarize the
membrane

22. • Stabilization of the membrane or
hyperpolarization can also result from
selective increase in permeability to K+ ions,
which move out carrying +ve charges.

23. Diagrammatic representation of steps in excitatory and inhibitory
neurohumoral transmission:

24. IV. Postjunctional activity
• EPSP generates a propagated postjunctional
AP which results in
Nerve impulse (in neurone)
Contraction (in muscle) or secretion (in gland).
• An IPSP stabilizes the postjunctional
membrane and resists depolarizing stimuli.

25. V. Termination of transmitter
action
• The various mechanisms of termination of
transmitter action are,
• Following its combination with the receptor,
the transmitter is locally degraded (e.g. ACh)
• Is partly taken back into the prejunctional
neurone by active reuptake and partly diffuses
away (e.g. NA).

26. Reference
• Thripathi K.D ‘Essentials of medical
pharmacology’ 6th edition, Jaypee Brothers
Medical Publications (P) Ltd. New Delhi, 2006
Page No. 89-92.
• Goodman & Gilman’s ‘The Pharmacological
basis of Therapeutics’ 11th edition Page No.
145-150.