LITERATURE REVIEW ON NEUROTRANSMITTERS AND THEIR ROLE
BY
SITI NUR AMALINA BT IBRAHIM
MATRIC NO.: D10A037
ANIMAL PHYSIOLOGY III ASSIGNMENT
COURSE COORDINATOR: DR. ERKIHUN AKLILU
FACULTY OF VETERINARY MEDICINE
UNIVERSITI MALAYSIA KELANTAN
DATE: 29 NOVEMBER 2011
i
ACKNOWLEDGEMENTS
I would like to express my thanks to Dr. Erkihun Aklilu W.G., the course coordinator
of Animal Physiology III, for giving me this experience and opportunity to make this
reference journal and literature review. I could not have functioned without his assistance.
This assignment had given me the chance to do more research and reading on the specific
topic, thus enhance my knowledge and give me the ability to inform others about this topic.
Besides, I would like to express my gratitude to my entire course mates for supporting
me, give me the motivation, and help me to understand better about this literature review. We
have been together in the rough and harsh condition. So, thank you very much to each and
every one of you.
It is humbling to experience the production of a literature review and realize the effort
and friendly cooperation of the course coordinator and my friends. It is to these people that I
now recognize and extend my sincere appreciation. Above all, I thank God for these generous
people from whom I seek advice and help. Thank you very much.
ii
LIST OF TABLES
Table Page
Table 1: The name of neurons and their location 8
Table 2: Ach receptors 18
Table3: Dopamine systems in the body 20
Table 4: NE neurotransmitter 21
Table 5: The roles of excitatory neurotransmitter 23
Table 6: The roles of inhibitory neurotransmitter 24
Table 7: Responses to adrenergic stimulation 30
Table 8: Location of muscarinic receptors and the effects of stimulation by
neurotransmitters of the autonomic nerves 31
Table 9: Location of adrenergic receptors and the effects of stimulation by
neurotransmitters of autonomic nerves 32
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LIST OF FIGURES
Figure Page
Figure 1: The neurotransmitters Ach and NE associated with 6
the autonomic nervous system of mammals
Figure 2: The autonomic nervous system or autonomic neurons pathway 7
Figure 3: The diagram of the neuron pathway 7
Figure 4: The sympathetic and parasympathetic divisions 10
Figure 5: Sympathetic pathway 10
Figure 6: Parasympathetic pathway 11
Figure 7: Simple patterns to illustrate convergence and divergence
in neural networks 11
Figure 8: The diagram of a neuron with dendrites, cell body (soma),
and single axon 15
Figure 9: The transmission of neurotransmitters 15
Figure 10: The G-protein coupled receptors 17
Figure 11: Voltage gated ion channels 17
Figure 12: Basic diagram of a neuron with synapse 26
Figure 13: Chemical transmission at the synapse 27
Figure 14: The two types of synapses, (A) electrical synapse
and (B) chemical synapse 28
iv
ABBREVIATIONS
Ach acetylcholine
AchE acetylcholinesterase
ANS autonomic nervous system
AP action potential
Ca ions calcium ions
Cl ions chloride ions
CNS central nervous system
GABA gamma aminobutyric acid
K ions potassium ions
N/A not applicable
Na ions sodium ions
NE norepinephrine
PNS peripheral nervous system
SSRIs serotonin specific reuptake inhibitors
VG channel voltage gated channel
α components alpha components
β components beta components
γ components gamma components
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS i
LIST OF TABLES ii
LIST OF FIGURES iii
ABBREVIATIONS iv
TABLE OF CONTENTS v
SUMMARY vi
1. INTRODUCTION 1
2. OBJECTIVES 3
3. METHODOLOGY 4 3.1 NEUROTRANSMITTERS
3.1.1. The types of neurotransmitters 5
3.1.2. The autonomic nervous system 7
3.1.3. The preganglionic and postganglionic neurons 8
3.1.4. The parasympathetic and sympathetic nervous system 8
3.1.5. The examples of neurotransmitters 12
3.1.6. Mechanism of Transmission and Inhibition of neurotransmitters 13
3.1.7. Ion gating in axons 16
3.2. THE NEUROTRANSMITTER’S ROLE 3.2.1. The function of each amino acid and monoamine neurotransmitter 18
3.3. THE RELATIONSHIP BETWEEN NEUROTRANSMITTERS AND SYNAPSES
3.3.1. The physiologic anatomy of the synapse 25
3.3.2. The types of synapse 27
3.3.3. The synaptic transmission and the responses 28
4. CONCLUSION AND RECOMMENDATIONS 33
5. REFERENCES 34
vi
SUMMARY
This literature review is on neurotransmitters and their role. The review focuses
mainly on types of neurotransmitters, some examples of neurotransmitters, the role or
function of each neurotransmitter, and the connection between neurotransmitters and the
synapses. Neurotransmitters are the chemicals that allow the transmission of signals across
the synapses from one neuron to another neuron. They also act to stimulate the muscle fibres.
Neurotransmitters also can be classified as amino acids, modified amino acids (monoamines),
and polypeptides. But, most of the action and stimulation that occur in the body is due to the
amino acid and monoamine neurotransmitters. There are two types of neurotransmitters,
which are the peripheral neurotransmitters and the central neurotransmitters. Peripheral
neurotransmitter is also known as excitatory neurotransmitters, while central neurotransmitter
is also known as inhibitory neurotransmitters. Neurotransmitters have close relationship with
synapses. This is because, they act as electrical and chemical messenger that passes the
information needed by the body (for action and stimulation) through the synapses. Synapses
are the functional connection between a neuron and another neuron. There are presynaptic
and postsynaptic neurons. The presynaptic neurons will release neurotransmitters. The
process will be explained further in this literature review. Overall, the system in the body
cannot function well if there are problems with the neurotransmitters, especially the nervous
system.
1
1. INTRODUCTION
Neurotransmitter is a substance that is released from the axon terminal of a
presynaptic neuron on excitation, and which travels across the synaptic cleft to either excite
or inhibit the target cell. (Blood, et.al., 2007). Examples of neurotransmitters are
norepinephrine (NE), acetylcholine (Ach), dopamine, and others. In this literature review, the
neurotransmitters and their role will be emphasized. Neurotransmitter deals with the normal
function of the body and the normal stimulation to be passed through the synapses. The brain
uses neurotransmitters to tell your heart to beat, your lungs to breathe, and your stomach to
digest. Neurotransmitters are also necessary for thought processes, emotions, and other
essential body functions including sleep, energy, and fear. (NeuroScience Lab, 2007).
There is an effect at a synapse caused by a nerve impulse. This goes the same at the
structured being innervated. Axons have branches and the branches end at a structure known
as a presynaptic terminal at the synapse. At this termination, it has vesicles containing the
chemical substance known as neurotransmitter, which is set free when there is a stimulation
of impulse. The neurotransmitter will diffused to postsynaptic neuron and influences the
entering of the sodium (Na) ions into the membrane. (Reece, 2009).
There are two types of neurotransmitters, which are the peripheral
neurotransmitter and the central neurotransmitter. Peripheral neurotransmitter is also known
as excitatory neurotransmitter. This neurotransmitter will stimulate the brain and the body. It
means that the neurotransmitter will increase the permeability of the affected membrane for
Na ions. For instance is Ach. Ach is also the preganglionic and postganglionic terminal
neurotransmitter for the parasympathetic division of the autonomic nervous system. (Reece,
2009). This is also referred to as cholinergic system. Another example is NE. It is also known
as noradrenaline. The sympathetic division is referred to as adrenergic system.
2
The central neurotransmitter is also known as inhibitory neurotransmitter. This
neurotransmitter will calm the brain and the body. It means that the neurotransmitter will
decrease the permeability of the affected membrane for Na ions. For instance are gamma-
aminobutyric acid (GABA), and glycine. The mechanism of inhibition will be explained
further in this literature review. (Reece, 2009).
In this literature review also, the monoamines and amino acids
neurotransmitters, will also be discussed. Some of the roles of the neurotransmitters and their
examples are:
• Ach act as both excitatory and inhibitory neurotransmitter.
• Serotonin regulates the mood, behaviour, appetite, and cerebral circulation.
• Dopamine acts as neurotransmitter for neurons with cell bodies in midbrain.
• NE functions in both peripheral nervous system (PNS) and central nervous
system (CNS).
• Glycine helps control skeletal movements.
• GABA helps motor functions in cerebellum.
Neurotransmitter is associated with synapse. There are two types of synaptic
transmission, which are the adrenergic and cholinergic synaptic transmissions. For each
synaptic transmission, there are different neurotransmitters that being released. Therefore,
different responses will be produced and this normal condition needs to be maintained for a
body system to function normally. (Sobti, 2008).
3
2. OBJECTIVES
The objectives of this literature review are:
1) To review information on neurotransmitters and their roles.
2) To give better understanding of neurotransmitters and their role in the body system.
3) To elaborate the mechanisms involved in neurotransmission.
4) To discuss the relationship between neurotransmitters and synapses.
4
3. METHODOLOGY
This literature review will be divided into three parts, the neurotransmitters,
their roles, and the relationship between neurotransmitters and synapses. In the
neurotransmitters, the types of neurotransmitters, the examples of neurotransmitters, the
mechanism of transmission and inhibition of neurotransmitters, will be emphasized and
discussed. In the neurotransmitter’s role, the function of each amino acid and monoamine
neurotransmitter will be emphasized and discussed. Last but not least, in the relationship
between neurotransmitters and synapses, the physiologic anatomy of the synapse, the types of
synapse, the synaptic transmission, and the responses will be emphasized and discussed.
All the information and data were gained through research and reading material
such as the physiology books related to neurotransmitters and their perspectives role. There
were a lot of books and related reading material that can be gained nowadays. One of the
sources was e-books about animal and human physiology. The information about
neurotransmitters and their role can be found in such books.
Other than that, there were also a lot of physiology and neurotransmitter books
in the library. One could find them and make a research and reading material out of the
books. Besides, in this technology world, information of neurotransmitters and their role can
be finding by using the internet. For examples, there are related journal articles, research
articles, and a lot more. Thus, through all the methods mentioned, this literature review was
made and can help others in finding more information and explanations about
neurotransmitters and their role.
5
3.1. NEUROTRANSMITTERS
3.1.1. The types of neurotransmitters.
Neurotransmitters are the chemicals that allow the transmission of signals
across the synapses from one neuron to another neuron. Neurotransmitters also can be
classified as amino acids, modified amino acids (monoamines), and polypeptides. But, most
of the action and stimulation that occur in the body is due to the amino acid and monoamine
neurotransmitters. (Frandson, et.al., 2008). Neurotransmitter is a substance that is released
from the axon terminal of a presynaptic neuron on excitation, and which travels across the
synaptic cleft to either excite or inhibit the target cell. (Blood, et.al., 2007). Examples of
neurotransmitters are NE, Ach, dopamine, and others. There are two types of
neurotransmitters, which are the peripheral neurotransmitter and the central neurotransmitter.
3.1.1.1. Peripheral neurotransmitters.
Peripheral neurotransmitter is also known as excitatory neurotransmitter. This
neurotransmitter will stimulate the brain and the body. It means that the neurotransmitter will
increase the permeability of the affected membrane for Na ions. For instance is Ach. Ach is
also the preganglionic and postganglionic terminal neurotransmitter for the parasympathetic
division of the autonomic nervous system. This is also referred to as cholinergic system.
Another example is NE. It is also known as noradrenaline. The sympathetic division is
referred to as adrenergic system. (Reece, 2009).
6
3.1.1.2. Central neurotransmitters.
The central neurotransmitter is also known as inhibitory neurotransmitter. This
neurotransmitter will calm the brain and the body. It means that the neurotransmitter will
decrease the permeability of the affected membrane for Na ions. For instance are GABA, and
glycine. These central neurotransmitters involves in the mechanism of inhibition. (Reece,
2009).
Figure 1. The neurotransmitters Ach and NE associated with the autonomic nervous system
of mammals.
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3.1.2. The autonomic nervous system.
The autonomic nervous system (ANS) innervates organs whose functions
involuntarily. The effectors cells or organs include cardiac and smooth muscles and glands.
The effectors are usually the part of visceral organs and blood vessels. (Sobti, 2008).
Figure 2. The autonomic nervous system or autonomic neurons pathway.
Figure 3. The diagram of the neuron pathway.
8
3.1.3. The preganglionic and postganglionic neurons.
Preganglionic neuron is the first neuron has its cell body in gray matter of brain
or spinal cord. Mean while, postganglionic neuron synapses with second neuron within an
autonomic ganglion. Autonomic ganglion has axon which extends to synapse with target
tissue. (Sobti, 2008).
Table 1. The name of neurons and their location.
Name of neurons Location
Preganglionic autonomic fibres. Midbrain, hindbrain, upper thoracic to fourth
sacral levels of spinal cord.
Autonomic ganglia. Head, neck, abdomen.
Presynaptic neuron is myelinated, and postsynaptic neuron is unmyelinated.
This information is associated with neurotransmitters because we need to know the basic of
nervous system to understand better about neurotransmitters and the terms used in explaining
neurotransmitters. The autonomic nerves release neurotransmitters that may be stimulatory or
inhibitory. (Sobti, 2008).
3.1.4. The parasympathetic and sympathetic nervous system.
Sympathetic nervous system and parasympathetic nervous system:
1. Both have preganglionic neurons that originate in CNS.
2. Both have postganglionic neurons that originate outside the CNS in ganglia.
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3.1.4.1. Sympathetic division.
Myelinated preganglionic fibres exit spinal cord in ventral roots from first
thoracic to second lumbar levels. Most sympathetic nerve fibres separate from somatic motor
fibres and synapse with postganglionic neurons within paravertebral ganglia. Sympathetic
division consists of two parts, which are the divergence and the convergence. (Frandson,
et.al., 2008).
Divergence is where the preganglionic fibres branch to synapse with numbers
of postganglionic neurons. Convergence is where the postganglionic neuron receives synaptic
input from large numbers of preganglionc fibres. (Frandson, et.al., 2008).
3.1.4.2. Parasympathetic division.
Preganglionic fibres originate in midbrain, medulla, pons, and in the second to
fourth sacral levels of the spinal column. Preganglionic fibres synapse in terminal ganglia
located next to or within organs innervated. Most parasympathetic fibres do not travel within
spinal nerves. For examples, they do not innervate blood vessels, sweat glands, and arrector
pili muscles. (Blood, et.al., 2007; Reece, 2009).
10
Figure 4. The sympathetic and parasympathetic divisions.
Figure 5. Sympathetic pathway.
11
Figure 6. Parasympathetic pathway.
Figure 7. Simple patterns to illustrate convergence and divergence in neural networks.
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3.1.5. The examples of neurotransmitters.
Neurotransmitters can be classified into monoamines and amino acids
neurotransmitter. Examples of monoamines neurotransmitter are:
i. Epinephrine.
ii. NE.
iii. Serotonin.
iv. Dopamine.
Examples of amino acids neurotransmitter are:
i. Glutamic acid.
ii. Aspartic acid.
iii. Glycine.
iv. GABA.
Other neurotransmitters are Ach, glutamate, histamine, glutamine, taurine,
agmatine, endorphin, nitric oxide, and neuropeptides neurotransmitter. Some of the
neurotransmitter acts as excitatory or inhibitory neurotransmitters. (Guyton, et.al., 2006).
13
3.1.6. Mechanism of Transmission and Inhibition of Neurotransmitters.
3.1.6.1. Mechanisms of transmission of neurotransmitter.
For the mechanism involves in the transmission of the neurotransmitter, the body
receptor must receives a stimulus from the environment, such as a terrified feeling. Neurons
from the receptors cell will conduct an electrical impulse to the effectors organ or muscles.
The impulses causes’ action potential (AP) passes through the axons and terminal bouton
(presynaptic neuron).
Therefore, neurotransmitter release is rapid because many vesicles form fusion
complexes at “docking site”. AP travels down axon to bouton. This causes the voltage gated
(VG) of calcium (Ca) channels to open. Hence, Ca (large numbers of positive electrical
charges) enters bouton down the concentration gradient, to interior of postsynaptic cell.
Inward diffusion triggers rapid fusion of synaptic vesicles and release of neurotransmitters.
At the postsynaptic membrane, the negatively charged gated ions (anions gated) will be
depressed. Thus, this reduces the diffusion of chloride (Cl) ions to inside of postsynaptic
membrane. (Guyton, et.al., 2006).
Besides, Ca activates calmodulin, which activates protein kinase. The protein kinase
phosphorylates (change the shape) of synapsins. Synapsins is important for aid in the fusion
of synaptic vesicles. Neurotransmitters are released and diffused across synaptic cleft. The
molecules (ligand) bind to specific receptor proteins in postsynaptic cell membrane.
Therefore, the chemically-regulated gated ion channels will open. Then, the neurotransmitter
is inactivated to end transmission. (Guyton, et.al., 2006).
14
3.1.6.2. Mechanisms of inhibition of transmission of neurotransmitter.
Once the effectors organs or muscles had been stimulated, the AP will be
decrease in the presynaptic neurons. Therefore, neurotransmitters release will decreases and
become slower. Other than that, less vesicles form fusion complexes at “docking site”. This
causes the VG of Ca channels to close. Protein kinase will be inactivated due to lack of Ca to
activate calmodulin. Less production of synapsins occurs. Hence, less or no neurotransmitters
released and diffused across the synaptic cleft. (Guyton, et.al., 2006).
The anions gated channel will open in postsynaptic membrane. Thus, Cl ions
(negatively charged ions) will enter the membrane, carrying negative charges inward and
increase the negativity of postsynaptic membrane, which is inhibitory. Besides, potassium
(K) ions channels also will open and increase the conductance of K ions out of the
postsynaptic membrane. This causes positive ions (Ca) to diffuse out of postsynaptic
membrane and decrease the positivity of the membrane. Hence, inhibits the transmission of
the neurotransmitters. (Guyton, et.al., 2006).
Besides that, there is also enzyme that involves in the mechanism of inhibition.
This is known as activation of the receptor enzymes. The function of the receptors are to
increase the number of inhibitory postsynaptic receptors, hence decreases the number of
excitatory receptors. For example, there is an enzyme that inactivates Ach, which is
acetylcholinesterase (AchE). This enzyme present on postsynaptic membrane or immediately
outside the membrane. It prevents continued stimulation, thus inhibit the transmission of Ach.
(Guyton, et.al., 2006).
15
Figure 8. The diagram of a neuron with dendrites, cell body (soma), and single axon.
Figure 9. The transmission of neurotransmitters.
16
3.1.7. Ion gating in axons.
The ion gating is the changes occur in the membrane potential caused by ion
flow through the ion channels, whether positively charged or negatively charged. There are
different ion gating in the presynaptic terminal and the postsynaptic terminal. Example of ion
gating is the VG channels. VG channels open in response to change in the membrane
potential of the axons. The gated channels are part of proteins that comprise the channel.
Therefore, due to the protein, the gated channels can be opened or closed in response to
change. Examples of ions channels are the K ions, Na ions, and Ca ions. There are two types
of channels for K ions, which are the always open channel and the closed channel in the
resting (no AP) cell. Mean while, channel for Na ions, is always closed in resting cells. But,
some of the Na ions do leak into the cells. (Guyton, et.al., 2006).
For presynaptic terminal, it has a lot of VG especially the VG of Ca channels.
This is because, presynaptic terminal will released neurotransmitters into synaptic cleft and
diffused into the postsynaptic membrane. (Guyton, et.al., 2006).
For postsynaptic terminal, it has more receptor protein. This is because, the
terminal will act as inhibitory site to stop or reduce the transmission of neurotransmitter
inside its membrane. There are two types of ion channels in the postsynaptic terminal, which
are the cation channels (allow diffusion of positively charged ions such as Ca and Na to
excite the neurotransmitter transmission) and the anion channels (allow diffusion of
negatively charged ions such as K to inhibit the transmission of neurotransmitter). (Guyton,
et.al., 2006).
In ions gating, there is also proteins that act as second messenger to help with
the transmission of the neurotransmitter. Second messenger is important in prolonged the
changes of the postsynaptic membrane, as ion channels will instantly closed within
milliseconds after the neurotransmitter substance is no longer present. The common second
17
messenger is the G-proteins. G-proteins composed of three components, which are alpha (α),
beta (β), and gamma (γ) components. (Guyton, et.al., 2006).
Figure 10. The G-protein coupled receptors.
Figure 11. Voltage gated ion channels.
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3.2. THE NEUROTRANSMITTER’S ROLE
3.2.1. The function of each amino acid and monoamine neurotransmitter.
3.2.1.1. Ach.
Ach is both an excitatory and inhibitory neurotransmitter, depends on the organ
involved. Ach causes the opening of chemical gated ion channels. There are two types
of Ach receptors, which are the nicotinic Ach receptors and muscarinic Ach receptors.
There is an enzyme that inactivates Ach, which is AchE. This enzyme present on
postsynaptic membrane or immediately outside the membrane. It prevents continued
stimulation, thus inhibit the transmission of Ach. Furthermore, Ach is also important
in CNS (use Ach in cholinergic neuron), and PNS. (Guyton, et.al., 2006; Frandson,
et.al., 2008).
Table 2. Ach receptors.
Ach Receptors Location
Nicotinic Found in autonomic ganglia and skeletal
muscle fibres.
Muscarinic Found in the plasma membrane of smooth
and cardiac muscle cells, and in cells of
particular glands.
3.2.1.2. Endorphin.
A neurotransmitter that allows hibernation in animals (bears and others) by
slows the heart rate, respiration, and metabolism in general. (Boeree, 2009).
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3.2.1.3. Nitric oxide.
It is responsible for long term behaviour and memory. It is secreted in the brain
by nerve terminals. Nitric oxide is instantly being synthesised and diffuses out of presynaptic
membrane (no vesicles transportation). It functions by changes postsynaptic neuron
intracellular metabolic function (modify excitability for a period of time). (Guyton, et.al.,
2006).
3.2.1.4. Neuropeptides.
It is the basic parts of large protein molecules. It is synthesised by ribosome in
neuronal cell body. This neurotransmitter causes more prolonged actions, such as prolonged
closure of the Ca channels. (Guyton, et.al., 2006).
3.2.2. The roles of monoamines as neurotransmitters.
Monoamines neurotransmitters are usually for controlling the mood, general
behaviour, and emotion. Examples of monoamines neurotransmitters are serotonin,
dopamine, and NE. Here is the detail information of the monoamines
neurotransmitters.
3.2.2.1. Serotonin.
Serotonin derived from L-tryptophan and it is a neurotransmitter for neurons
with cell bodies. It controls the regulation of mood, behaviour, appetite, and cerebral
circulation. Serotonin specific reuptake inhibitors (SSRIs) inhibit reuptake and
destruction of serotonin, prolonging the action of neurotransmitter. It is also used as
20
an antidepressant to reduce appetite, treatment for anxiety, and treatment for migraine
headaches. (Guyton, et.al., 2006; Frandson, et.al., 2008).
3.2.2.2. Dopamine.
Dopamine is a neurotransmitter for neurons with cell bodies in midbrain. There
are two systems of dopamine that axons project into. (Guyton, et.al., 2006; Frandson,
et.al., 2008).
Table 3. Dopamine systems in the body.
Dopamine systems Functions
Nigrostriatal Initiation of skeletal muscle movement,
degenerations of neurons causes Parkinson’s
disease.
Mesolimbic Involved in behaviour and emotion.
3.2.2.3. NE.
NE is a neurotransmitter in both PNS and CNS. (Guyton, et.al., 2006;
Frandson, et.al., 2008).
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Table 4. NE neurotransmitter.
PNS CNS
At smooth muscles, cardiac muscle, and
glands. Increase in blood pressure, and
constriction of arteries.
Involves in general behaviour.
3.2.3. The roles of amino acids as neurotransmitters.
Amino acids neurotransmitters are usually for memory storage, helps in
skeletal movements, intestinal function, and in motor function. Examples of
monoamines neurotransmitters are glutamic acid, aspartic acid, glycine, GABA, and
glutamine. Here is the detail information of the amino acids neurotransmitters.
3.2.3.1. Glutamic acid and aspartic acid.
Both are the major excitatory neurotransmitters in CNS. Glutamic acid is
important in memory storage. (NeuroScience Lab, 2007).
3.2.3.2. Glycine.
Glycine is the inhibitory neurotransmitter. It helps to control the skeletal
movements. (NeuroScience Lab, 2007).
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3.2.3.3. GABA.
GABA is most prevalent neurotransmitter in brain and act as inhibitory in
motor functions in cerebellum. (Guyton, et.al., 2006; Frandson, et.al., 2008).
3.2.3.4. Glutamine.
It is made into GABA and glutamate. Glutamine is important for intestinal
function. (Guyton, et.al., 2006; Frandson, et.al., 2008).
Additional roles of excitatory neurotransmitter are shown in table 5.
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Table 5. The roles of excitatory neurotransmitter.
Source: NeuroScience Lab (2007).
Neurotransmitters Function Levels in the body
High Low
Aspartic Acid Vital for energy and
brain function.
Seizures
Anxiousness
Tiredness
Low mood
Epinephrine
(adrenaline)
Important for
motivation, energy and
mental focus.
Sleep difficulties
Anxiousness
Attention issues
Fatigue
Lack of focus
Difficult weight
loss
NE (noradrenaline) Important for mental
focus and emotional
stability.
Anxiousness
Stress
Hyperactivity
High blood pressure
Lack of energy
Lack of focus
Lack of motivation
Dopamine Responsible for feelings
of pleasure and
satisfaction, also
muscle control and
function.
Poor intestinal function
Developmental delay
Attention issues
Addictions
Cravings
Histamine Helps control the sleep-
wake cycle as well as
energy and motivation.
Allergic responses
Sleep difficulties
Feeling tired
Glutamate Body’s primary
excitatory
neurotransmitter,
necessary for learning
and memory.
Anxiousness
Low mood
Seizures
Psychological disorders
Tiredness
Poor brain activity
24
Additional roles of inhibitory neurotransmitter are shown in table 6.
Table 6. The roles of inhibitory neurotransmitter.
Neurotransmitters Function Levels in the body
High Low
GABA
Glycine
Primary inhibitory
neurotransmitter in the
brain and is necessary to
feel calm and relaxed.
Helps calm & relax the
body.
Hyperactivity
Anxiousness
Sleep difficulties
Anxiousness
Low mood
Stress-related disorders
Severe hyperactivity
Severe anxiousness
Severe sleep
difficulties
Not applicable (N/A)
Taurine Important for proper heart
function, healthy sleep and
promoting calmness.
Hyperactivity
Anxiousness
Sleep difficulties
Severe hyperactivity
Severe anxiousness
Severe sleep
difficulties
Agmatine Blocks the potentially
harmful effects of
excessive glutamate.
N/A Anxiousness
Low mood
Stress
Serotonin Plays important roles in
the resolution of mood,
sleep, and appetite.
SSRI medications
Low mood
Sleep difficulties
Uncontrolled appetite
Source: NeuroScience Lab (2007).
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3.2.4. The role of neurotransmitters in sympathetic effects.
Sympathetic effect is usually involves in fight or flight response. Therefore, NE will
be released from postganglionic fibres and epinephrine from adrenal medulla. Some of the
activities done to prepare for intense activity are: (Reece, 2009).
1. Increases the heart rate.
2. Dilates bronchioles.
3. Increases the blood glucose concentration.
3.2.5. The role of neurotransmitter in the parasympathetic effects.
In this effect Ach will be released. Therefore, this causes: (Reece, 2009).
1. Heart rate to decrease.
2. Blood vessels to dilate.
3. Increases digestive activity.
3.3. THE RELATIONSHIP BETWEEN NEUROTRANSMITTERS AND SYNAPSES
3.3.1. The physiologic anatomy of the synapse.
Synapse is a functional connection between a neuron and another neuron or
effectors cell. Transmission in the synapse is in one direction only, which is from axon of
first (presynaptic) to second (postsynaptic) neuron. The synaptic transmission is through a
chemical gated channel. Moreover, the presynaptic terminal (bouton) releases a
neurotransmitter. (Guyton, et.al., 2006; Frandson, et.al., 2008).
Neuron consists of soma (main body of neuron and as nutrition centre), axon
(lies from soma and leaves spinal cord, and carry impulses away from soma), and dendrites
26
(transmit impulses to soma). The synaptic knobs (presynaptic terminals) also known as
bouton, lie on surfaces of dendrites and soma. (Guyton, et.al., 2006; Frandson, et.al., 2008).
One way direction transmits the signals from neurons that release
neurotransmitter (presynaptic neuron) to neuron on which neurotransmitter acts (postsynaptic
neuron). It is important as highly focused transmission of signals at terminals allows nervous
system to perform great numbers of functions. (Guyton, et.al., 2006; Frandson, et.al., 2008).
Figure 12. Basic diagram of a neuron with synapse.
27
Figure 13. Chemical transmission at the synapse.
3.3.2. The types of synapse.
There are two types of synapses, which are electrical synapse and chemical
synapse.
3.3.2.1. Electrical synapse.
In electrical synapse, impulses can be regenerated without interruption in
adjacent cells. This is because, it has gap junctions, which adjacent cells electrically coupled
through a channel. Each gap junction is composed of 12 connexin proteins. For examples are
the gap junction in smooth and cardiac muscles, brain, and glial cells. (Guyton, et.al., 2006;
Frandson, et.al., 2008).
28
3.3.2.2. Chemical synapse.
In chemical synapse, the terminal bouton is separated from postsynaptic cell by
synaptic cleft. Neurotransmitters are released from synaptic vesicles. The vesicles fuse with
axon membrane and neurotransmitter released by exocytosis. The amount of
neurotransmitters released depends upon frequency of AP. (Guyton, et.al., 2006; Frandson,
et.al., 2008).
Figure 14. The two types of synapses, (A) electrical synapse and (B) chemical synapse.
3.3.3. The synaptic transmission and the responses.
Synaptic transmissions have two divisions. The divisions are adrenergic and
cholinergic synaptic transmission.
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3.3.3.1. Adrenergic synaptic transmission.
NE is the neurotransmitter released by most postganglionic sympathetic nerve
fibres. Adrenal medulla will released epinephrine. These neurotransmitters are also known as
catecholamines. (Guyton, et.al., 2006; Frandson, et.al., 2008; Reece, 2009).
3.3.3.1.1. Responses.
β receptors:
NE will bind to the receptor. G-protein dissociates into α subunit or βγ
complex. The resulting action depends upon the receptors tissue.
α 1 receptors:
Epinephrine will bind to the receptors. Ca is bind to calmodulin and activates
protein kinase.
α 2 receptors:
As a negative feedback control by decreases the release of NE.
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Resulting action
Table 7. Responses to adrenergic stimulation.
Receptors Responses
α 1 Constricts smooth muscles.
α 2 Contraction of smooth muscles.
β 1 Increase heart rate and force of contraction.
β 2 Relaxes smooth muscles.
β 3 As adipose tissue.
3.3.3.2. Cholinergic synaptic transmission.
At this transmission, postganglionic parasympathetic fibres at synapse with
effectors released Ach. (Guyton, et.al., 2006; Frandson, et.al., 2008; Reece, 2009).
3.3.3.2.1. Responses.
Muscarinic receptors:
Ach will bind to this receptors and G-protein is needed as mediation. Βγ-
complex will affect the opening or closing of channel or it will activate an enzyme.
Nicotinic receptors:
Ach binds to two receptors binding sites. This causes Na channel to open
within the receptor protein. Nicotinic receptor is always for excitatory function.
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Table 8. Location of muscarinic receptors and the effects of stimulation by neurotransmitters
of the autonomic nerves.
Location Effect
Heart
Sinoatrial node
Atrioventricular node
Salivary glands
Reduce heart rate
Reduce impulse conduction velocity
Increase secretion
Gastrointestinal tract Increase motility of smooth muscle in wall
and secretion of lining epithelium
Urinary bladder
Contract smooth muscle to empty bladder
Circular muscle of iris of eye
Constrict smooth muscle to reduce pupil
Ciliary muscle controlling lens of eye
Contract muscle for lens accommodation
Endothelial cells lining blood vessels Stimulate release of nitric oxide to relax
smooth muscle
Smooth muscle of lung airways (bronchiolar) Contract smooth muscle to shrink airways
Source: Frandson, et.al. (2008).
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Table 9. Location of adrenergic receptors and the effects of stimulation by neurotransmitters
of autonomic nerves.
Receptor subtype Location Effect
α1 Vascular smooth muscle
Smooth muscle sphincters in
gastrointestinal tract
Radial muscle of iris of eye
Smooth muscle sphincter of
urinary bladder
Contracts muscle to constrict
vessel
Contracts muscle to constrict
sphincters
Contracts muscle to enlarge
pupil
Contracts muscle to reduce
opening into urethra
β1 Heart: sinoatrial node
Increase heart rate
Heart: atrioventricular node Increase impulse conduction
velocity
Heart: ventricular muscle
Increase force of contraction
β2 Arterial vessels supplying blood
to skeletal muscle
Relaxes smooth muscle to
permit dilation of vessels
Smooth muscle of lung airways
(bronchiolar)
Relaxes muscle to permit
airways to open
Smooth muscle in wall of
gastrointestinal tract
Relaxes muscle to reduce
motility
Liver Increases glycogenolysis,
gluconeogenesis in some
species
Source: Frandson, et.al. (2008).
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4. CONCLUSION AND RECOMMENDATIONS
Neurotransmitters are very important in nervous system. It helps the body
system to function properly in normal state. The behaviour of some animals also is controlled
by neurotransmitters. Therefore, neurotransmitters have great value to the body system.
Neurotransmitters also help the animals to respond properly to their surroundings, such as a
fight or flight response. Synapses are the place where neurotransmitters transmission takes
place. So, the relationship between neurotransmitters and synapses must be understood to
understand how neurotransmitters function. Proper functioning of the neurotransmitters
ensures normal functioning of the nervous system which in turn helps to maintain healthy
operation of the whole body system, especially the nervous system. When neurotransmitters
function properly, the body system can be maintained healthy and normal for a long period of
time.
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5. REFERENCES
Blood, D.C., Studdert, V.P., Gay, C.C., (2007). Saunders Comprehensive Veterinary
Dictionary, Third Edition, Saunders Elsevier, Toronto. pp.1233
Boeree, G.C., (2009). General Psychology Neurotransmitters, Pennsylvania.
(http://webspace.ship.edu/cgboer/genpsyneurotransmitters.html)
Frandson, R.D., Wilke, W.L., Fails, A.D., (2008). Anatomy and Physiology of Farm
Animals, Seventh Edition, Wiley-Blackwell, Colorado. pp. 175-183
Guyton, A.C., Hall, J.E., (2006). Textbook of Medical Physiology, Eleventh Edition, Elsevier
Saunders, Pennsylvania. pp. 559-564
Marieb, Mallatt, Wilhelm, (2005). Human Anatomy, Fourth Edition, Pearson Education,
prepared by Hendon L., University of Alabama Birmingham.
(http://www.southalabama.edu/alliedhealth/biomedical/311Anatomy/Chapter15.ppt)
NeuroScience Lab., (2007).
(http://www.modernherbalist.com/brochures/neurotransmitters101-brochure.pdf)
Reece, W.O., (2009). Functional Anatomy and Physiology of Domestic Animals, Fourth
Edition, Wiley-Blackwell, USA. pp. 84-110
Sobti, R.C., (2008). Animal Physiology, First Edition, Alpha Science International Ltd.,
Oxford, U.K. pp. 17.20-17.43
