The Central Nervous System

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Transcript The Central Nervous System

The Nervous System
Chapter Concepts
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Homeostasis is maintained in the human
body by various parts of the nervous
system
Neural transmission occurs along axons,
due to an action potential that causes
depolarization of the neuron
Electrochemical communication occurs
between cells at the synapse
The central nervous system is the body’s
control centre. It consists of the brain and
spinal cord
Chapter Concepts (Cont.)
5.
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The brain includes centres that control
involuntary responses and voluntary responses
The cerebrum is the largest part of the brain. It
contains four pairs of lobes, each of which is
associated with particular functions
The peripheral nervous system is composed of
the somatic (voluntary) and autonomic
(involuntary) system
The autonomic nervous system is divided into the
sympathetic and parasympathetic nervous
systems
Structures and Processes of
the Nervous System
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The nervous system regulates the
human body
It coordinates with the endocrine
system to maintain homeostasis
Divisions of Vertebrate
Nervous Systems
Nervous System
CNS
PNS
Cells in the Nervous
System
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Cells within the nervous system are
either neurons or glial cells
Neurons:
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Glial Cells:
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Nerve Fibres
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Neurons and glial cells are packed
together to form nerve fibres that
extend throughout the nervous system
Neurons come in three types –
sensory, interneurons, and motor
neurons
Neural Circuits
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Messages from sensory neurons
sometimes will not travel to the brain
before action is taken
This is because we have reflex arcs
that are used for quick responses to
stimuli
The Reflex Arc
http://www.merck.com
The Purpose of Reflex
Arcs
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The purpose of a reflex arc is to prevent
serious injury
For example, if you touch a hot object, you
will often move your finger before feeling
pain
This is because the reflex arc sends the pain
message to the spinal cord interneurons,
which redirect the message instantly to the
motor neurons
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Without this reflex arc, we would have
to receive the pain signal, send it to
the brain, have it interpreted, and
then formulate the correct response
Within this time, a relatively minor
burn would become a very serious one
The Neuron
Components of the
Neuron
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Dendrites: Receive information from
adjoining cells or receptors and pass the
information along the neuron
Cell Body: Contains organelles and
processes the input from dendrites
Axon: Extension of the cytoplasm through
which nerve impulses move
Myelin Sheath: Insulative covering
surrounding the axon
Components of the
Neuron
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Schwann Cells: Structures that
produce the myelin sheath. These are
a type of glial cell
Nodes of Ranvier: Junctions between
myelin sections
Axon Terminal: Passes nerve impulse
on to the next neuron in line
Factors Affecting Nerve
Impulse Speed
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The diameter of the axon – in general,
the smaller it is, the faster the impulse
Presence of myelin sheath –
unmyelinated neurons transmit much
slower than myelinated ones
Multiple Sclerosis (MS)
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Caused by destruction of the myelin sheath
Myelinated neurons are destroyed as the
sheath turns into scar tissue
Produces a “short circuit” within the neuron
Symptoms include double-vision, speech
difficulty, jerky limb movements, and partial
paralysis of voluntary muscles
The Neurilemma
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This is a special membrane found in
the cells of the PNS
It surrounds the axon and promotes
regeneration of damaged tissue
White & Grey Matter
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White matter consists of myelinated
neurons
It is these neurons that contain the
neurilemma as well
Grey matter is unmyelinated
Therefore, damage to these neurons is
permanent
A Cross-Section of the
Spinal Cord
http://home.swipnet.se
Electrochemical Impulses
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The nerve impulses produced by neurons differ
from conventional electricity in several ways:
It moves much slower than conventional current
Cells would provide a high resistance to
conventional current
The strength of electrical currents diminish as
they move along a circuit
Conventional current requires an external source
of energy
Production of the Impulse
1. Sodium-potassium exchange pumps
use ATP to move Na+ out of the
cytoplasm of the cell and K+ into the
cytoplasm. For every 2 K+ that move
into the cell, 3 Na+ move out. This
creates high concentration gradients
across the cell membrane.
Sodium-potassium Pump Animation
2. As a result of the concentration
gradients, K+ begins to diffuse out of
the cytoplasm and Na+ diffuses in.
However, there are more available K+
ion channels in the resting membrane,
so this produces a positively charged
region outside the membrane. This is
called a polarized membrane or a
resting membrane. There is a charge
difference of about -70 mV inside the
axon (there are more negative charges
inside the axon than outside)
3. As an impulse is triggered, the nerve
cell becomes more permeable to
sodium than potassium, and the
sodium rushes into the neuron. This
causes a rapid reversal of charge
known as depolarization. Once the
charge inside the axon is positive, the
sodium gates close.
Depolarization Animation – Sodium &
Potassium Channels
4. The potassium gates open again and
K+ begins to move back out of the
nerve cell. When this occurs, the Na+
and K+ are on the opposite side of the
membrane when compared to their
position before depolarization.
However, an excess of K+ move
outside of the membrane, causing
brief hyperpolarization.
5. The sodium & potassium pumps
reactivate and transport Na+ out of the
cytoplasm and K+ into the cytoplasm
to return to the resting membrane
state. This return to the original
polarity is known as repolarization.
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Because a neuron cannot fire again
before it is repolarized, there is a time
known as the refractory period where
the nerve is unable to act
This refractory period takes 1 to 10 ms
Action potentials in myelinated
neurons only occur at the Nodes of
Ranvier
The Entire Process:
Movement of an Impulse
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The nerve impulse must move along
the axon
This is achieved through the attraction
of positive and negative charges along
the nerve membrane
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The positively charged ions moving into the
cell when an action potential is produced
are attracted to the negative ions in the
neighboring regions of the cytoplasm
These positive ions begin to migrate,
triggering the opening of sodium channels
in that next region, causing depolarization
As a wave of depolarization moves along
the membrane, it causes the potassium
gates behind it to open, creating
repolarization
The Movement of an
Impulse
Action Potential Propagation Animation
Energy and Impulses
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Because active transport is used to
create the concentration gradients
needed for a resting membrane to
form, ATP must be used
Threshold Levels
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Early studies with nerve cells using
electrical currents indicated that
neurons will not produce a signal if a
stimulus is below a certain level
This lowest level that produces a
response is known as the threshold
level
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Therefore stimuli below threshold
levels will not produce a response
As well, these experiments indicated
that the response is often an all-ornone response
In other words, either the response
(such as muscle contraction) would
either not be present (when the
threshold level had not been reached)
or at maximum intensity (at any level
above the threshold level)
Detecting Intensity of
Stimuli
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This information seems to contradict
what we know from experience –
stimuli can be experienced from low to
very high intensities
For instance, we can distinguish very
cold objects from very hot objects, but
we also can feel a range of
temperatures in between
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This occurs because our brain interprets the
intensity of a stimulus based on the
frequency of the impulses it produces
Attached to each receptor are a number of
neurons, each with a different threshold
level
A low intensity message would be produced
when only the most sensitive neurons fire,
while high intensity messages occur as most
or all of the neurons are actively sending
impulses
The Synapse
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A synapse or synaptic cleft is the
space that exists between the axon
terminal of one neuron and the
dendrites of another neuron
Neurotransmitter chemicals leave the
axon terminals through vesicles in the
presynaptic neuron and travel to
receptors in the postsynaptic neuron
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The distance across the synapse is
small (about 20 nm), but
neurotransmitters must move via
diffusion
This becomes the slowest part of the
transmission of a nerve impulse
(again, this explains the quickness of a
reflex arc when compared to the
message being sent to the brain)
The Synapse
http://kvhs.nbed.nb.ca
Synapse Animation
Transmission at the
Synapse
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Excitatory transmitters trigger a nerve
impulse in a neuron
These neurotransmitters are released from
vesicles within the axon endplate and
diffuse across the synapse
As the neurotransmitter attaches to its
receptor site, it opens sodium channels on
the postsynaptic neuron
This initiates an action potential in the
neuron
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There are also neurotransmitters that
are inhibitory – they prevent the
production of a nerve impulse in the
postsynaptic neuron
These most often open potassium
gates, allowing the neuron to become
hyperpolarized
As a result, the postsynaptic neuron
cannot produce the action potential
required for an impulse to occur
Breakdown of
Neurotransmitters
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If a neurotransmitter remains in place
on a receptor, it will prevent
repolarization of the neuron
Therefore, these neurotransmitters
must be broken down
This is often accomplished through the
action of enzymes
Acetylcholine
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A good example of a neurotransmitter
and its enzyme are acetylcholine and
cholinesterase
Acetylcholine is an excitatory
neurotransmitter
Just after acetylcholine is released, the
cholinesterase enzyme is released into
the synapse
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The cholinesterase enzymes seek out
acetylcholine molecules and break
them down
As a result, there is no more
acetylcholine present and the
postsynaptic neuron can repolarize
Of course, like most enzymes,
inhibitors can be used to block their
function
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A number of insecticides and the
nerve gas sarin are cholinesterase
inhibitors which bind with
cholinesterase and prevent it from
breaking down acetylcholine
As a result, the muscles of the insect’s
heart remain contracted and will not
relax (which prevents it from beating)
Cholinesterase inhibitors have also
been considered as treatments for
Alzheimer’s Disease
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Alzheimer’s Disease is related to a
lowered production of acetylcholine
In patients with the disease, the
cholinesterase often breaks down the
low levels of acetylcholine before it
has time to act
Cholinesterase inhibitors would then
prevent the premature breakdown of
acetylcholine by inhibiting the action
of the enzymes
Common Neurotransmitters
Neurotransmitter
Function
Effects of Abnormal
Production
Acetylcholine
Excitatory
Inadequate –
Alzheimer’s Disease
Dopamine
Control of body
movements and
sensations of
pleasure
Excessive –
schizophrenia
Inadequate –
Parkinson’s Disease
Serotonin
Temperature control, Inadequate sensory perception & depression
mood
Norepinephrine
Prepares body for
stress
Excessive – anxiety,
insomnia
Inadequate – hunger,
exhaustion
Summation
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In many cases, a number of neurons come
together at a junction
Often, when this occurs, more than one of
the neurons bringing a message into the
junction must be active to produce an action
potential in the neuron leaving the junction
Summation is the effect produced by the
accumulation of neurotransmitters from two
or more neurons
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As you can see
here, both neurons
A and B must fire
at the same time to
exceed the
threshold level to
activate D (A and B
are not able to
exceed the
threshold levels
individually)
Neuron C in this
case is producing
an inhibitory
neurotransmitter
http://www.biologymad.com
The Central Nervous
System
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The brain and spinal cord make up the CNS
The brain itself is supported by three layers of
membranes known as meninges
Between the inner and middle meninges exists a
layer of fluid known as cerebralspinal fluid (CSF)
This fluid is also found in the central canal of the
spinal cord
This fluid acts as a shock absorber and as a
transport medium for nutrients and waste to and
from the brain cells
CSF and Illnesses
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The CSF can carry bacteria and viruses
These may cause inflammations of the
meninges or areas of the spinal cord
The typical method of diagnosis for
diseases such as meningitis is to
remove CSF from the spinal cord and
check it for pathogens
The Spinal Cord
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The spinal cord consists of neurons
and is approximately the diameter of a
pencil
The grey matter of the spinal cord
contains unmyelinated neurons and
the cell bodies of motor neurons
The white matter consists of
myelinated interneurons
The Spinal Cord
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The dorsal nerve
tract brings sensory
information back
into the spinal cord,
while the ventral
nerve carries motor
information to
peripheral muscles
and organs
The Brain
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The human brain has a far more
advanced forebrain than other animal
species
The brain consists three sections – the
forebrain, the midbrain, and the
hindbrain
Brain Structures
The Hindbrain
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The hindbrain is located posterior to
the midbrain and connects to the
spinal cord
It consists of three main regions: the
cerebellum, the pons, and the medulla
oblongata
The Cerebellum
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This is the largest portion of the hindbrain
It controls limb movements, balance, and
muscle tone
The cerebellum also receives information
from proprioceptors that keep track of the
location and position of the body’s limbs
This is the part of the brain that ultimately
controls excitatory and inhibitory nerve
impulses
The Pons
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The Pons serves as a relay station that
connects the two halves of the
cerebellum, and the cerebellum to the
medulla oblongata
The Medulla Oblongata
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This is the lowest part of the hindbrain
It acts as a connection between the
CNS and the PNS
It regulates involuntary muscle action
(heart rate, breathing, swallowing,
coughing, etc.)
The medulla oblongata also acts as a
coordinating center for the ANS
The Midbrain
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The midbrain consists of four small
spheres of grey matter
It relays visual and auditory
information between areas of the
forebrain and the hindbrain
It also plays a role in eye movement
and the control of skeletal muscles
The Forebrain
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The forebrain contains a number of
different parts
The olfactory lobes, which detect smell
are part of the forebrain
The majority of the forebrain consists
of the cerebrum, which stores and
interprets sensory information and
initiates voluntary motor activities
Supplying the Brain
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Blood is separated from the brain by a
blood-brain barrier
The blood that travels to the brain
never enters the nervous tissue itself
The capillaries in the brain are made
up of tightly-fused cells
This blocks the passage of many
toxins and infectious agents
Transport & The BloodBrain Barrier
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Substances such as glucose and oxygen are
supplied to the brain through special
transport mechanisms
However, lipid-based molecules move
across the lipid bilayer of the capillary cells
Therefore, lipid-soluble materials (caffeine,
nicotine, alcohol, heroin) have rapid effects
on brain function
Parts of the Forebrain
Lobe
Parts and Functions
Function
Frontal Lobe
Associated with conscious thought, intelligence, memory,
personality; controls voluntary muscle movement
Temporal
Lobe
Involved in auditory reception
Parietal Lobe Receive sensory information from the skin, processes
information about body position
Occipital
Lobe
Processes visual information
Mirror Neurons
Hemispheres of the Brain
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The brain consists of a right and left
hemisphere
These two hemispheres are connected
by a bundle of nerves known as the
corpus callosum
Right vs. Left Brain…
Left
Brain
uses logic, detail oriented, facts rule, words and
language, present and past, math and science, can
comprehend, knowing, acknowledges, order/pattern
perception, knows object name, reality based, forms
strategies, practical, safe.
Right
Brain
uses feeling, "big picture" oriented, imagination
rules, symbols and images, present and future,
philosophy & religion, can "get it" (i.e. meaning),
believes, appreciates, spatial perception, knows
object function, fantasy based, presents possibilities,
impetuous, risk taking.
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The right side of the brain is
associated with visual patterns and
spatial awareness, while the left side is
associated with verbal skills
The ability of a person to learn, and
the learning style that suits them, may
be partially dictated by which side of
the brain is dominant
However, not all people have a
dominant hemisphere of their brain
Broca’s Area & Wernicke’s
Area
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On the left side of the cerebral cortex
are found Broca’s area (Frontal lobe)
and Wernicke’s area (Temporal lobe)
Broca’s area coordinates the muscles
for speaking and translates thought
into speech
Wernicke’s area stores the information
involved in language comprehension
Speech in Birds & Humans
Other Parts of the
Forebrain
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The forebrain also contains the thalamus and the
hypothalamus
The thalamus, which is directly below the
cerebrum, coordinates and interprets sensory
information
The hypothalamus is connected to the pituitary and
regulates a number of the body’s responses such as
blood pressure, heart rate, temperature, basic
drives (thirst & hunger) and emotions
Damage to the hypothalamus can lead to a person
demonstrating unusual or violent behaviour
Mapping Brain Functions
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Early information on the function of various
parts of the brain was gathered from
patients who recevied brain injuries or
diseases
Later, Canadian Nobel Prize winner Wilder
Penfield mapped the motor areas of the
cerebral cortex by stimulating different parts
of the brain through probing
Non-Intrusive Mapping
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PET (positron-emission tomography) and
MRI (magnetic resonance imaging) are now
used to study and map the brain
The PET can track glucose consumption in a
brain during particular activities
MRIs can produce high-detail images of the
brain structure in three dimensions
The Peripheral Nervous
System
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The peripheral nervous system includes all
nerves outside of the central nervous
system
The somatic nervous system, which is
mostly under voluntary control, controls
movement and receives information about
the environment
This system contains 12 pairs of cranial
nerves and 31 pairs of spinal nerves, all of
which are myelinated
Cranial Nerves
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Some nerves exit the brain itself –
these are known as cranial nerves
One of the most important of these
cranial nerves is the Vagus nerve
This nerve regulates the heart, the
bronchi of the lungs, the liver,
pancreas and digestive tract
The Autonomic Nervous
System
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The Autonomic Nervous System controls
involuntary functions within our body
This system helps to maintain homeostasis
despite a changing internal environment
It consists of sympathetic and
parasympathetic nerves, which are
controlled by the hypothalamus and the
medulla oblongata
Sympathetic vs.
Parasympathetic Nerves
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Sympathetic nerves prepare the body for
stress, while parasympathetic nerves return
the body to its normal state
Sympathetic nerves use norepinephrine as
an excitatory neurotransmitter which
activates muscles
A number of different organs and organ
systems are involved in ANS responses:
Effects of the ANS
Organ
Sympathetic
Parasympathetic
Heart
Increases heart rate
Decreases heart rate
Digestive
Decreases peristalsis
Increases peristalsis
Liver
Increases release of
glucose
Stores glucose
Eye
Dilates pupil
Constricts pupil
Bladder
Relaxes sphincter
Contracts sphincter
Skin
Increases blood flow
Decreases blood flow
Adrenal Gland
Released epinephrine
No effect
Neuron Anatomy
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Sympathetic nerves have a short
preganglion and a long postganglion
Parasympathetic nerves have a long
preganglion and a short postganglion
Sympathetic nerves originate from the
thoracic and lumbar vertebrae
Parasympathetic nerves originate from
the cervical and caudal vertebrae
Natural and Artificial
Painkillers
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The body produces its own natural
painkillers in response to injury
Endorphins and enkephalins are
manufactured in the brain
Specialized cells called SG (substantia
gelanosa) cells produce a transmitter
chemical that signals that damage or
injury has occurred
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The endorphins and
enkephalins fit into
receptor sites on the SG
cells, reducing the amount
of transmitter that is
produced
Opitates such as heroin,
morphine and its
derivatives have a shape
that is similar to the
body’s nautral painkillers
Endorphin structure
Morphine structure
www.bio.davidson.edu
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As a result, opiates can also fit into the
receptor sites that are usually used by
endorphins
However, the use of opiates reduces the
body’s production of the natural endorphins
Therefore, after the opiate breaks down,
there is little or none of the natural
painkiller being produced
This results in a return of pain, often
perceived as being greater than the pain
associated with the original injury
Activating Your Natural
Painkillers
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A number of different stimuli (not
necessarily all extremely painful) will release
endorphins and other similar chemicals:
Acupuncture
Consumption of capsaicin (the active
ingredient in chili peppers – this is probably
why I have hot sauce on everything…)
Strenuous exercise (although the chemical
released is actually anandamide – which is
related to the THC found in marijuana)
Other Drugs…
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Depressants such as Valium and Librium will
enhance the action of inhibitory synapses
This increases the production of the
inhibitory neurotransmitter, GABA
Alcohol actually changes the neuron
membrane, and does not act as a
neurotransmitter – it increases the effect of
GABA
http://www.cerebromente.org.br