Neurotransmitter

1. Neuro-Transmitters & Reflex
Arc
RASHID KHAN MOHMAND
ANESTHESIA LECTURER

2. Neuro-Transmitters

3. Definition
■ A chemical substance that acts as a mediator for the transmission of nerve
impulse from one neuron to another neuron through a synapse
■ To consider a substance as a neurotransmitter, it should fulfill certain criteria as
given below:
1. It must be found in a neuron
2. It must be produced by a neuron
3. It must be released by a neuron
4. After release, it must act on a target area and produce some biological effect
5. After the action, it must be inactivated

4. Classification
1. Depending Upon Chemical Nature
a. Amino Acids
– Fast synaptic transmission and are inhibitory and excitatory in action
– E.g. GABA, glycine, glutamate and aspartate
b. Amines
– Slow synaptic transmission and are also inhibitory and excitatory in action
– E.g. Noradrenaline, adrenaline, dopamine, serotonin and histamine
c. Others
– Some neurotransmitters do not fit into any of the above category such as
acetylcholine, soluble gas nitric oxide (NO)
– Choline + acetyl co-A choline acetyltransferase Acetylcholine

6. Classification
2. Depending Upon Function
a. Excitatory Neurotransmitters
– Chemical substance, responsible for the conduction of impulse from presynaptic
to postsynaptic neuron e.g. Ach, adrenaline etc.
b. Inhibitory Neurotransmitters
– Chemical substance, which inhibits the conduction of impulse from the
presynaptic to the postsynaptic neuron e.g. GABA and dopamine
– When it is released from the presynaptic axon terminal due to the arrival of
action potential, it causes inhibitory postsynaptic potential and the action
potential is not generated

7. Transport and Release
■ Neurotransmitter is produced in the cell body of the neuron and is transported
through axon
■ At the axon terminal, the neurotransmitter is stored in small packets called
vesicles
■ Under the influence of a stimulus, these vesicles open and release the
neurotransmitter into synaptic cleft
■ It binds to specific receptors on the surface of the postsynaptic cell
■ Receptors are G proteins, protein kinase or ligand-gated receptors

8. Inactivation & Reuptake
■ After the execution of the action, neurotransmitter is inactivated by four different
mechanisms:
1. It diffuses out of synaptic cleft to the area where it has no action
2. It is destroyed or disintegrated by specific enzymes
3. It is engulfed and removed by astrocytes (macrophages)
4. It is removed by means of reuptake into the axon terminal
■ Reuptake process involves a specific carrier protein for each neurotransmitter

9. Neuromodulators
■ The chemical messenger, which modifies and regulates activities that take place
during the synaptic transmission
■ These peptides do not propagate nerve impulses like neurotransmitters
■ e,g, enkephalins, dynorphins, endorphins etc.
S.No. Neurotransmitters Neuromodulators
1 Propagate nerve impulse
through synapse
Modify and regulate synaptic
transmission
2 Packed in small synaptic
vesicles
Packed in large synaptic vesicles
3 Found only in axon terminals Found in all parts of the body
4 Generally, neuron has only one
neurotransmitter
Neuron may have one or more
neuromodulators
5 May be amino acids, amine or
others Chemically
only peptides

10. Reflex Arc

11. Reflexes
■ Reflex activity is the response to a peripheral nervous stimulation that occurs
without consciousness
■ It is a protective mechanism that protects the body from damages
■ For example:
1. when hand is placed on a hot object, it is withdrawn immediately.
2. When a bright light is thrown into the eyes, eyelids are closed and pupil is
constricted to prevent the damage of retina by entrance of excessive light
into the eyes.

12. Reflex Arc
■ The anatomical nervous pathway for a reflex action
■ A simple reflex arc includes five components:
1. Receptor
2. Afferent Nerve
3. Center
4. Efferent Nerve
5. Effector Organ

13. Classification of Reflexes
1. Depending upon whether inborn or acquired
2. Depending upon situation of center
– Cerebellar, Cortical, Midbrain, Medullary, Spinal
3. Depending upon purpose
– Protective, Antigravity
4. Depending upon number of synapse
– Mono, polysynaptic
5. Depending upon whether visceral or somatic
6. Depending upon clinical basis
– Superficial, Deep, Visceral and Pathological