neural control mechanism -2

Question 1

neurons

Answer 1

The basic unit of the nervous system is the individual
nerve cell, or neuron .

Neurons operate by generating electrical signals that
move from one part of the cell to another part of the same
cell or to neighboring cells.

Question 2

nervous system is divided into

Answer 2

(1) the central nervous system (CNS) , composed of the
brain and spinal cord; and

(2) the peripheral nervous system (PNS) ,consisting of the
nerves that connect the brain and spinal cord with the body’s
muscles, glands, sense organs, and other tissues

Question 3

neurtransmitters

Answer 3

The electrical signal causes the release of chemical
messengers—neurotransmitters —to communicate with
other cells.

Question 4

cell body

Answer 4

1. Cell body (Soma): contains the nucleus and ribosomes and
   thus has the genetic information and machinery necessary for
   protein synthesis.

Question 5

dendrites

Answer 5

2. Dendrites: series of highly branched outgrowths of the cell
   body

Branching dendrites increase a cell’s surface area—some
neurons may have as many as 400,000 dendrites

Knoblike outgrowths called dendritic spines increase the
surface area

The presence of protein synthesis machinery allows dendritic
spines to remodel their shape in response to variation in
synaptic activity (like learning and memory)

Question 6

axon

Answer 6

3. Axon(nerve fiber): a long process that extends from the cell body and carries outgoing signals
   to its target cells

Range in length from a few microns to over a meter

Region of the axon that arises from the cell body (initial segment or axon hillock ) also
termed as “trigger zone”

The axon may have branches, called collaterals . The greater the degree of branching of the
axon and axon collaterals, the greater the cell’s sphere of influence

Each branch ends in an axon terminal , which is responsible for releasing neurotransmitters
from the axon

Alternatively, some neurons release their chemical messengers from a series of bulging areas
along the axon known as varicosities .

The axons of many neurons are covered by sheaths of myelin, usually consists of 20 to 200
layers of highly modified plasma membrane

In the brain and spinal cord, these myelin-forming cells are the oligodendrocytes . Each
oligodendrocyte may branch to form myelin on as many as 40 axons

Question 7

schwann cells

Answer 7

In the PNS, cells called Schwann cells form individual myelin sheaths surrounding 1- to
1.5-mm-long segments at regular intervals along some axons

Question 8

node of ranvier

Answer 8

The spaces between adjacent sections of myelin where the axon’s plasma membrane is
exposed to extracellular fluid are called the nodes of Ranvier

Question 9

maintence of structure and function of cell axon

Answer 9

Various organelles and other materials must move as far as 1 meter between the cell
body and the axon terminals. This movement, termed axonal transport , depends
on a scaffolding of microtubule “rails” running the length of the axon and
specialized types of motor proteins known as kinesins and dyneins ( Figure 6.3 ).
At one end, these double-headed motor proteins bind to their cellular cargo, and the
other end uses energy derived from the hydrolysis of ATP to “walk” along the
microtubules.

Question 10

kinesin

Answer 10

Kinesin transport mainly occurs from the cell body toward the axon terminals (
anterograde ) and is important in moving nutrient molecules, enzymes,
mitochondria, neurotransmitter-filled vesicles, and other organelles.

Question 11

dyein

Answer 11

Dynein
movement is in the other direction ( retrograde ), carrying recycled membrane
vesicles, growth factors, and other chemical signals that can affect the neuron’s
morphology, biochemistry, and connectivity. Retrograde transport is also the route
by which some harmful agents invade the CNS, including tetanus toxin and the
herpes simplex, rabies, and polio viruses.

Question 12

neurons can be divided into

Answer 12

affert neurns
effernt neurons
interneurons

Question 13

afferent neuron

Answer 13

Afferent nuerons: convey information from the tissues and organs
of the body toward the CNS

At their peripheral ends have sensory receptors , which respond to
various physical or chemical changes in their environment by
generating electrical signals in the neuron

Structurally different, two branched axon, one is peripheral process,
begins where the dendritic branches converge from the receptor
endings. The other branch, central process, enters the CNS to form
junctions with other neurons

Cell body and the long axon are outside the CNS

Question 14

efferent neurons

Answer 14

convey information away from the CNS to
effector cells like muscle, gland, or other cell types

Conventional neuronal structure (refer to figure) Their cell bodies and
dendrites are within the CNS

Question 15

interneuron

Answer 15

Interneurons: connect neurons within the CNS

Lie entirely within the CNS

They account for over 99% of all neurons and have a wide range
of physiological properties, shapes, and functions.

The number of interneurons interposed between specific afferent
and efferent neurons varies according to the complexity of the
action they control.

Example: The knee-jerk reflex elicited by tapping below the
kneecap activates thigh muscles without interneurons. In
contrast, to hear a song or smell a certain perfume that evokes
memories of someone you know, millions of interneurons may
be involved

Question 16

synapse

Answer 16

Synapse: The anatomically specialized junction between two
neurons where one neuron alters the electrical and chemical
activity of another.
Most synapses occur between an axon terminal of one neuron
and a dendrite or the cell body of a second neuron

A neuron that conducts a signal toward a synapse is called a
presynaptic neuron , whereas a neuron conducting signals
away from a synapse is a postsynaptic neuron

Question 17

glial cells

Answer 17

Neurons account for only about half of the cells in the
human CNS. The remainder are glial cells ( glia, “glue”).

Glial cells surround the soma, axon, and dendrites of
neurons and provide them with physical and metabolic
support

Glial cells retain the capacity to divide throughout life.
Consequently, many CNS tumors actually originate from
glial cells

Question 18

types of glial cells

Answer 18

oligodendrocyte
astrocyte
microglia
epndymal cells
schwann cells

Question 19

oligodendrocyte

Answer 19

which forms the myelin sheath of CNS axons

Question 20

astrocyte

Answer 20

helps regulate the composition of the
extracellular fluid in the CNS by removing potassium ions
and neurotransmitters around synapses

formation of tight junctions between the cells that make up the
walls of capillaries in CNS, which forms Blood brain barrier

Astrocytes also sustain the neurons metabolically—for example,
by providing glucose and removing ammonia.

In developing embryos, astrocytes guide neurons as they migrate
to their ultimate destination, and they stimulate neuronal growth
by secreting growth factors.

Question 21

microglia

Answer 21

The microglia: a third type of glial cell, are specialized,
macrophage-like cells (Chapter 18) that perform immune functions
in the CNS, and may also contribute to synapse remodeling and
plasticity.

Question 22

eondymal cells

Answer 22

ependymal cells line the fluid-filled cavities within the brain and
spinal cord and regulate the production and flow of cerebrospinal
fluid

Question 23

schwann cells

Answer 23

Schwann cells, the glial cells of the PNS, have most of the
properties of the CNS glia.

Question 24

neural growth and regenerations

Answer 24

Neuronal cells or glia develops from stem cells in embryo

Each neuronal daughter cell differentiates, migrates to its
final location, and sends out processes that will become its
axon and dendrites.

A specialized enlargement, the growth cone , forms the
tip of each extending axon and is involved in finding the
correct route and final target for the process

Question 25

neurotrophic factors

Answer 25

Axon growth is guided by the glial cells through
attracting, supporting, deflecting, or inhibiting influences
exerted by several types of molecules (cell adhesion
molecules) or soluble neurotrophic factors (growth
factors for neural tissue)

Question 26

neuronal current

Answer 26

The predominant solutes in the extracellular fluid are
sodium and chloride ions. The intracellular fluid contains
high concentrations of potassium ions and ionized
non-penetrating molecules, particularly phosphate
compounds and proteins with negatively charged side
chains.

Electrical phenomena resulting from the distribution of
these charged particles occur at the cell’s plasma
membrane and play a significant role in signal integration
and cell-to-cell communication, the two major functions
of the neuron.

Question 27

basic elctrical facts

Answer 27

Same charges repel each other and opposite charges attract
each other if not separated by barriers

Separated electrical charges of opposite sign have the
Potential known as electrical potential

It is determined by the difference in the amount of charge
between two points, a potential difference

The units of electrical potential are volts or milli volts.

The movement of electrical charge is called a current

The hindrance to electrical charge movement is known as
resistance
The effect of voltage V and resistance R on current I is
expressed in Ohm’s law : (I=V/R)

Material with high electrical resistance (insulators) vs low
electrical resistance(Conductors)

Therefore, the lipids of membranes are insulators and ion
dissolved in water (ECF connected through channels) is
conductor

Question 28

resting membrane potential

Answer 28

“All cells under resting conditions (when no current flows
through) have a potential difference across their plasma
membranes, with the inside of the cell negatively charged
with respect to the outside ( Figure 6.8 ), termed as
resting membrane potential”.

ECF is used as voltage reference point

The magnitude of the resting membrane potential varies
from about 5 to 100 mV, depending upon the type of cell.
In neurons its (40-90mV)

Question 29

ion in membrane

Answer 29

Ions that can flow across the membrane and affect its
electrical potential, Na, K, and Cl are present in the
highest concentrations, and the membrane permeability to
each is independently determined.

Na and Cl concentrations are lower inside the cell than
outside, and that the K concentration is greater inside the
cell

This difference is established by (Na/K –ATPase) Na out
of the cell and K into it

Question 30

magnitude of resting membrane potential depends upon

Answer 30

The magnitude of the resting membrane potential depends
mainly on two factors:

(1) differences in specific ion concentrations in the
intracellular and extracellular fluids; and

(2) differences in membrane permeabilities to the different
ions, which reflect the number of open channels for the
different ions in the plasma membrane.

Question 31

equlibrium potential

Answer 31

The membrane potential at which these two fluxes become
equal in magnitude but opposite in direction is called the
equilibrium potential for that ion—in this case, K+.

Question 32

permeabiity of membrane

Answer 32

Plasma membrane Na/K-ATPase pumps maintain low
intracellular Na concentration and high intracellular K
concentration.

In almost all resting cells, the plasma membrane is much
more permeable to K than to Na, so the membrane
potential is close to the K equilibrium potential—that is,
the inside is negative relative to the outside.

The Na /K -ATPase pumps directly contribute a small
component of the potential because they are electrogenic.

Question 33

graded and action potentials

Answer 33

Some cells have another group of ion channels that can be gated
(opened or closed) under certain conditions. Such channels give a
cell the ability to produce electrical signals that can transmit
information between different regions of the membrane.

This property is known as excitability , and such membranes are
called excitable membranes .

Cells of this type include all neurons and muscle cells, as well as
some endocrine, immune, and reproductive cells.

The electrical signals occur in two forms: graded potentials and
action potentials.

Graded potentials are important in signaling over short distances.

Action potentials are long-distance signals that are particularly
important in neuronal and muscle cell membranes.

Question 34

graded potential

Answer 34

“Graded potentials are local potentials whose magnitude can vary
and that die out within 1 or 2 mm of their site of origin”.

Changes in membrane potential that are confined to a relatively
small region of the plasma membrane.

Produced when some specific change in the cell’s environment acts
on a specialized region of the membrane.

They are called graded potentials simply because the magnitude of
the potential change can vary

Graded potentials are given various names related to the location of
the potential or the function they perform—for instance, receptor
potential, synaptic potential, and pacemaker potential are all
different types of graded potentials

Question 35

action potential

Answer 35

“An AP is a rapid change in the membrane potential during
which the membrane rapidly depolarizes and repolarizes. At
the peak, the potential reverses and the membrane becomes
positive inside. APs provide long-distance transmission of
information through the nervous system”.

Generally very rapid (as brief as 1–4 milliseconds)

Propagates to long distances

Large alterations in the membrane potential; might be up to
100 mV. For example, a cell might depolarize from -70 to 30
mV, and then repolarize to its resting potential.

Question 36

more about action potentials

Answer 36

APs occur in excitable membranes because these membranes contain many voltage-gated Na
channels.

These channels open as the membrane depolarizes, causing a positive feedback opening of
more voltage-gated Na channels and moving the membrane potential toward the Na
equilibrium potential.

The AP ends as the Na channels inactivate and K channels open, restoring resting conditions.

Depolarization of excitable membranes triggers an AP only when the membrane potential
exceeds a threshold potential.

Regardless of the size of the stimulus, if the membrane reaches threshold, the AP generated is
the same size.

A membrane is refractory for a brief time following an AP.

APs are propagated without any change in size from one site to another along a membrane.

In myelinated nerve fibers, APs are regenerated at the nodes of Ranvier in saltatory
conduction.

APs can be triggered by depolarizing graded potentials in sensory neurons, at synapses, or in
some cells by pacemaker potentials.

Question 37

what is synapse

Answer 37

“Synapse is an anatomically specialized junction between two
neurons, at which the electrical activity in a presynaptic neuron influences
the electrical activity of a postsynaptic neuron”.

Estimated number in CNS 1014 (100 trillion)

Convergence allows information from many sources to influence a cell’s
activity; divergence allows one cell to affect multiple pathways.

Question 38

types of synapse

Answer 38

Types: Excitatory synapse & inhibitory synapse

Excitatory synapse brings the membrane of the postsynaptic cell closer to
threshold.

Inhibitory synapse prevents the postsynaptic cell from approaching threshold
by hyperpolarizing or stabilizing the membrane potential.
Whether a postsynaptic cell fires action potentials depends on the number
of synapses that are active and whether they are excitatory or inhibitory.

Question 39

electrical synapse

Answer 39

Electrical synapses consist of gap junctions that allow current
to flow between adjacent cells.

The current flows directly across the junction through the connecting
channels from one neuron to the other.

Communication between cells via electrical synapses is extremely rapid

Rare in the adult mammalian nervous system

Possibly involved in functions like synchronization of electrical activity
of neurons clustered in local CNS networks and communication
between glial cells and neurons

Multiple isoforms of gap-junction proteins have been described, and the
conductance of some of these is modulated by factors such as
membrane voltage, intracellular pH, and Ca2+concentration

Question 40

chemical synapse

Answer 40

Chemical synapses, neurotransmitter molecules are stored in synaptic vesicles in the
presynaptic axon terminal, and when released transmit the signal from a presynaptic to a
postsynaptic neuron.

The axon of the presynaptic neuron ends the axon terminal, which holds the synaptic vesicles
that contain neurotransmitter molecules.

The postsynaptic membrane adjacent to the axon terminal has a high density of membrane
proteins that make up a specialized area called the postsynaptic density .

The size and shape of the presynaptic and postsynaptic elements can vary greatly ( Figure
6.26b ).

A 10-20 nm extracellular space, the synaptic cleft , separates the presynaptic and
postsynaptic neurons and prevents direct propagation of the current from the presynaptic
neuron to the postsynaptic cell.

Instead, signals are transmitted across the synaptic cleft by means of a chemical
messenger—a neurotransmitter—released from the presynaptic axon terminal.

Sometimes more than one neurotransmitter may be simultaneously released from an axon, in
which case the additional neurotransmitter is called a co-transmitter .

These neurotransmitters have different receptors on the postsynaptic cell.

Question 41

mechanism of synapse release

Answer 41

Depolarization of the axon terminal increases the Ca+2 concentration within the
terminal, which causes the release of neurotransmitter into the synaptic cleft.
Calcium ions activate processes that lead to the fusion of docked vesicles with the
synaptic terminal membrane

The neurotransmitters are stored in small vesicles, many vesicles are docked on the
presynaptic membrane at release regions known as active zones while others are
dispersed

The vesicles are docked in the active zones by the interaction of a group of proteins,
some of which are anchored in the vesicle membrane and others that are found in the
membrane of the terminal. These are collectively known as SNARE proteins (soluble
N-ethylmaleimide-sensitive factor attachment protein receptors).

The axone terminal have voltage gated channels

The entered Ca interact with separate family of proteins associated with the vesicle,
synaptotagmins , triggering a conformational change in the SNARE complex that
leads to membrane fusion and neurotransmitter release.

The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the
postsynaptic cell; the activated receptors usually open ion channels.

Question 42

activation of post-synaptic cell

Answer 42

Fraction of neurotransmitters are released from the presynaptic axon
terminal bind to receptors on the plasma membrane of the
postsynaptic cell.

The activated receptors themselves may be ion channels (ionotropic
receptors) or may act indirectly on separate ion channels through a
G protein and/or a second messenger (metabotropic receptors)

Opening or closing of specific ion channels in the postsynaptic
plasma membrane change the membrane potential

Due to involved sequence of events, a very brief synaptic delay
—about 0.2 msec—between the arrival of an action potential at a
presynaptic terminal and the membrane potential changes in the
postsynaptic cell occur.

Neurotransmitter binding to the receptor is a transient and
reversible, non-covalent event

Question 43

unbound neurotransmitters are removed from synaptic cleft when they

Answer 43

Unbound neurotransmitters are removed from the synaptic
cleft when they

Neurotransmitter reuptake: actively transported back into
the presynaptic axon terminal

Diffuse away from the receptor site

Neurotransmitter Degradation: enzymatically
transformed into inactive substances, some of which are
transported back into the presynaptic axon terminal for
reuse.

Question 44

types of chemical synapse

Answer 44

According to effects of the neurotransmitter on the
postsynaptic cell, chemical synapses are differentiated in
two types—excitatory and inhibitory

Depends on the type of ion channel influenced, when
neurotransmitter binds to its receptor

Question 45

excitatory chemical synapse

Answer 45

At an excitatory synapse, the electrical response in the
postsynaptic cell is called an excitatory postsynaptic
potential (EPSP).

Usually at an excitatory synapse, channels in the
postsynaptic cell that are permeable to Na+, K+, and other
small positive ions open, but Na+ flux dominates, because
it has the largest electrochemical gradient.

Question 46

inhibitory chemical synapse

Answer 46

At inhibitory synapses, it is either an inhibitory
postsynaptic potential (IPSP) or a stabilization of the
membrane potential near resting levels.

At inhibitory synapses, channels like Cl- or K+ get open

Question 47

synaptic integration

Answer 47

Action potentials are generally initiated by the temporal and spatial
summation of many EPSPs.

A depolarization of the membrane toward threshold occurs when
excitatory synaptic input predominates, and either a
hyperpolarization or stabilization occurs when inhibitory input
predominates.

Assume there are three synaptic inputs to the postsynaptic cell. The
synapses from axons A and B are excitatory, and the synapse from
axon C is inhibitory.

Temporal summation

Spatial summation

The postsynaptic cell’s membrane potential is the result of temporal
and spatial summation of the EPSPs and IPSPs at the many active
excitatory and inhibitory synapses on the cell.

Question 48

synaptic strength

Answer 48

The synapse—whether excitatory or inhibitory—shows enormous variability in the
postsynaptic potentials (strength) that follow a presynaptic input.

The effectiveness or strength of a given synapse is influenced by both presynaptic
and postsynaptic mechanisms.

Question 49

presynaptic mehcanism

Answer 49

Presynaptic mechanisms:

Presynaptic terminal does not release a constant amount of neurotransmitter every
time it is activated

Due to variation in Ca concentration

The neurotransmitter output of some presynaptic terminals is also altered by activation
of membrane receptors on the terminals themselves (as in axo–axonic synapse)

Presynaptic inhibition: Decrease release neurotransmitter

presynaptic facilitation: Increase it

Presynaptic receptors are activated by neurotransmitters or other chemical messengers
released by nearby neurons or glia or even by the axon terminal itself
(autoreceptors: regulate neurotransmer release by feedback mechanism)

Question 50

postsynaptic mechanism

Answer 50

Postsynaptic mechanisms:

Different receptor types for each neurotransmitter operate
by different signal transduction mechanisms and can have
different—sometimes even opposite

Number of receptors for neurotransmitter is not constant,
varying with up- and down-regulation

Ability of a given receptor to respond to its
neurotransmitter can change (receptor desensitization)

Effect of cotransmitter (or several cotransmitters) is
released with the neurotransmitter

Question 51

Modification of Synaptic Transmission
by Drugs and Disease

Answer 51

Therapeutics (recreational) drugs that act on the nervous
system do so by altering synaptic mechanisms and thus
synaptic strength

Interfering with or stimulating normal processes in the
neuron involved in neurotransmitter synthesis, storage,
and release, and in receptor activation.

Question 52

diseases can also affect synaptic mechanism

Answer 52

Diseases can also affect synaptic mechanisms

For example:

Tetanus (caused by the bacillus Clostridium tetani, which
produces a toxin ( tetanus toxin ). Symptoms are increase
in muscle contraction and a rigid, or spastic paralysis

Tetanus toxin is a protease that destroys SNARE proteins,
inhibiting neurotransmitter release

Specifically affects inhibitory neurons in the CNS that
normally are important in suppressing the neurons that
lead to skeletal muscle activation

Question 53

neurotransmistters/neuromodulators

Answer 53

In general, neurotransmitters cause EPSPs and IPSPs, and
neuromodulators cause, via second messengers, more
complex metabolic effects in the postsynaptic cell.

The actions of neurotransmitters are usually faster than
those of neuromodulators.

A substance can act as a neurotransmitter at one type of
receptor and as a neuromodulator at another.a

Question 54

aceytylcholine

Answer 54

Major neurotransmitter in the PNS at the neuromuscular
junction (Chapter 9) and in the brain

Neurons that release ACh are called cholinergic neurons

Cholinergic neurons bodies are confined to few brain
areas but axons are distributed widely

Question 55

aceytylcholine detail

Answer 55

Synthesis: synthesized from choline and acetyl coenzyme A in the
cytoplasm

Storage: stored in synaptic vesicles in presynaptic terminals

Release: Released and activates receptors on the postsynaptic
membrane

Fate:

1) Degradation by enzyme acetylcholinesterase to choline (reuptake
and reutilization) and acetate

2) Simple diffusion away from the synapse to blood which is then
degraded by an enzyme

[Note: nerve gas Sarin block acetylcholinesterase results in
uncontrolled muscle contraction and then receptor desensitization
and paralysis]

Question 56

nicotinic recptor for aceytl choline

Answer 56

Nicotinic receptor:

Responds to alkaloid nicotine(1-2% in tobacco)

Nicotinic acetylcholine receptor is ionotropic receptor (Na/k
channel)

Situated in neuromuscular junctions (blocking lead to paralysis)
and brain (role in cognitive functions and behavior.

For example: One cholinergic system that employs nicotinic
receptors plays a major role in attention, learning, and memory
by reinforcing the ability to detect and respond to meaningful
stimuli

Presence of nicotinic receptors on presynaptic terminals in
reward pathways of the brain explains why tobacco products are
among the most highly addictive substances known.

Question 57

muscaranic

Answer 57

2) Muscaranic receptor

Responds to muscarinic (mushroom poison)

Receptors couple with G proteins, which then alter the
activity of a number of different enzymes and ion channels

Situated in brain and major division of PNS innervates
peripheral glands and organs, like salivary glands and the
heart.

Atropine is an antagonist of muscarinic receptors which is
used dilation of the pupils for an eye exam.

Question 58

alzhiemer disease

Answer 58

Alzheimer disease: Neurons associated with the ACh system
degenerate associated with a decreased amount of ACh in certain
areas of the brain and even the loss of the postsynaptic neurons that
would have responded to it

Prevalence is 10% to 15% of people over age 65, and 50% of people
over age 85

Characterized by declining language and perceptual abilities,
confusion, and memory loss

Genetic cause:

Mutation in genes on chromosome 1, 14, and 21 are associated with
abnormally increased concentrations of beta-amyloid protein
(responsible for cell death)

Mutation in genes on chromosome 19 that codes for a protein involved
in carrying cholesterol in the bloodstream

Question 59

biological amines

Answer 59

The biogenic amines are small, charged molecules that are
synthesized from amino acids and contain an amino group
(R—NH 2 )

It includes
Dopamine
Norepinephrine
Serotonin
Histamine

Epinephrine, another biogenic amine, is not a common
neurotransmitter in the CNS but is the major hormone secreted
by the adrenal medulla.

Question 60

catecholamines

Answer 60

Dopamine , norepinephrine (NE) , and epinephrine all
contain a catechol ring (a six-carbon ring with two
adjacent hydroxyl groups) and an amine group, which is
why they are called catecholamines

Question 61

fate of catecholamine

Answer 61

Actively transports the catecholamine back into the axon
terminal

Broken down in both the extracellular fluid and the axon terminal
by enzymes such as monoamine oxidase (MAO)

[Note: MAO inhibitors increase the amount of norepinephrine
and dopamine in a synapse by slowing their metabolic
degradation. They are used in the treatment of mood disorders
such as some types of depression]

Question 62

location of cadecholine

Answer 62

Location: The cell bodies of neurons lie brainstem and
hypothalamus in brain, few in number and axons extends to
almost all parts of brain

These neurotransmitters play a vital role in states of
consciousness, mood, motivation, directed attention,
movement, blood pressure regulation, and hormone release

Adrenaline: [epinephrine and norepinephrine]

Question 63

receptors of cadecholine

Answer 63

Receptors: There are two major classes of receptors for
norepinephrine and epinephrine: alpha-adrenergic receptors
(α1, α2) and beta-adrenergic receptors.

All catecholamine receptors are metabotropic, and thus use
second messengers to transfer a signal
Beta-adrenoceptors act via stimulatory G proteins to
increase cAMP in the postsynaptic cell. There are three
subclasses of beta-receptors, (β1, β2, β3) , which function
in different ways in different tissues

They act presynaptically to inhibit norepinephrine release
(α2) or postsynaptically to either stimulate or inhibit the
activity of different types of K channels (α1).

Question 64

seretonin

Answer 64

5-hydroxytryptamine, or 5-HT) is an important biogenic amine,
produced from tryptophan

Slow onset of action, therefore works as a neuromodulator.

Serotonergic neurons innervate virtually every structure in the
brain and spinal cord and operate via at least 16 different
receptor types.

Excitatory effect on pathways that are involved in the control
of muscles, and an inhibitory effect on pathways that mediate
sensations.

They contribute to motor activity and sleep, serotonergic
pathways also function in the regulation of food intake, bone
remodeling, reproductive behavior, and emotional states such
as mood and anxiety.

Cont.

Selective serotonin reuptake inhibitors such as paroxetine
(Paxil) are thought to aid in the treatment of depression by
inactivating the 5-HT transporter and increasing the
synaptic concentration of the neurotransmitter.

The drug lysergic acid diethylamide ( LSD ) stimulates the
5-HT 2A subtype of serotonin receptor and alters its
interaction with glutamate receptors in the brain.

Produces the intense visual hallucinations that are
produced by ingestion of LSD

Question 65

amido acid neurotransmitter

Answer 65

Several amino acids themselves function a
neurotransmitters

The most prevalent neurotransmitters in the CNS, and they
affect virtually all neurons

Types:

Excitatory amino acids e.g., aspartate, glutamate.

Inhibitory amino acids e.g., GABA, glycine.

Question 66

glutamate

Answer 66

Primary neurotransmitter at 50% of excitatory synapses in
the CNS

Receptors: majority are ionotropic, although metabotropic
also exist, two important subtypes are

AMPA (α -amino-3 hydroxy-5 methyl-4 isoxazole
propionic acid)

NMDA receptors (bind N -methyl- D -aspartate).

Cooperative activity of AMPA and NMDA receptors has
been implicated in phenomena called long-term
potentiation (LTP).

Question 67

mechanism of LTP

Answer 67

Mechanism of LTP:

Step 1: presynaptic neuron fires action potentials

Step 2: glutamate is released from presynaptic terminals

Step 3: binds to both AMPA and NMDA receptors on postsynaptic
membranes

Step 4: AMPA receptors function just like the excitatory postsynaptic
receptors, the channel becomes permeable to both Na+ and K+, but the
larger entry of Na+ creates a depolarizing EPSP

Step 5: NMDA- receptor channels mediate a substantial Ca2+ flux. A
magnesium ion blocks NMDA channels when the membrane voltage is
near the negative resting potential, and to drive it out of the way the
membrane must be significantly depolarized by the current through AMPA
channels

Step 6: When the depolarization is sufficient, however,
NMDA receptors do open, allowing Ca2+to enter the
postsynaptic cell.

Step 7: Calcium ions then activate a second-messenger
cascade in the postsynaptic cell that includes persistent
activation of multiple different protein kinases, stimulation
of gene expression and protein synthesis, and ultimately a
long-lasting increase in the sensitivity of the postsynaptic
neuron to glutamate

Question 68

GABA

Answer 68

Major inhibitory neurotransmitter in the brain derived from
glutamate.

GABA neurons in the brain are small interneurons that dampen
activity within neural circuits.

Postsynaptically, GABA may bind to ionotropic or
metabotropic receptors

The ionotropic receptor increases Cl- flux into the cell,
resulting in hyperpolarization of the postsynaptic membrane. In
addition to the GABA binding site, this receptor has several
additional binding sites for other compounds, including
steroids, barbiturates, and benzodiazepines to reduce anxiety,
guard against seizures, and induce sleep.

Question 69

mechanism of GABA

Answer 69

Mechanism: Ethanol stimulates GABA synapses and simultaneously
inhibits excitatory glutamate synapses, with the overall effect being global
depression of the electrical activity of the brain.

Question 70

characteristics of alcoholics as dose increases

Answer 70

Characteristic of alcohlics as dose increases:

Reduction in overall cognitive ability, along with sensory perception
inhibition (hearing and balance) loss of motor coordination, impaired
judgment, memory loss, and unconsciousness.

Suppression of brainstem centers responsible for regulating the
cardiovascular and respiratory systems

Dopaminergic and endogenous opioid signaling pathways (discussed in the
next section) are also affected by ethanol, which results in short-term mood
elevation or euphoria.

Question 71

Glycine

Answer 71

Major neurotransmitter released from inhibitory interneurons
in the spinal cord and brainstem

Binds to ionotropic receptors on postsynaptic cells that allow
Cl- to enter leading to hyperpolarization.

Maintain a balance of excitatory and inhibitory activity in
spinal cord integrating centers that regulate skeletal muscle
contraction

Neurotoxin strychnine , an antagonist of glycine receptors
sometimes used to kill rodents. Victims experience
hyperexcitability throughout the nervous system, which leads
to convulsions, spastic contraction of skeletal muscles, and
ultimately death due to impairment of the muscles of
respiration

Question 72

neuropeptides

Answer 72

The neuropeptides are composed of two or more amino acids
linked together by peptide bonds.

About 100 neuropeptides have been identified, but their
physiological roles are not all known.

Synthesis: as other proteins

Neurons that release one or more of the peptide
neurotransmitters are collectively called peptidergic

After release, peptides can interact with either ionotropic or
metabotropic receptors.

They are eventually broken down by peptidases located in
neuronal membranes.

A group of neuropeptides (endogenous opioids) that
includes beta-endorphin , the dynorphins , and the
enkephalins —have attracted much interest because their
receptors are the sites of action of opiate drugs such as
morphine and codeine .

Play a role in regulating pain

Substance P , another of the neuropeptides, is a
transmitter released by afferent neurons that relay sensory
information into the CNS. It is known to be involved in
pain sensation

Question 73

gases

Answer 73

Certain very short-lived gases also serve as neurotransmitters [ Such as
Nitric oxide, carbon monoxide and hydrogen sulfide]

Produced by enzymes in axon terminals (in response to Ca entry) and
simply diffuse from their sites of origin in one cell into the intracellular
fluid of other neurons or effector cells, where they bind to and activate
proteins.

For example, nitric oxide released from neurons activates guanylyl cyclase
in recipient cells, which increases the concentration of the
second-messenger cyclic GMP.

Nitric oxide plays a role in a bewildering array of neutrally mediated
events—learning, development, drug tolerance, penile and clitoral erection,
and sensory and motor modulation, to name a few.

Paradoxically, it is also implicated in neural damage that results, for
example, from the stoppage of blood flow to the brain or from a head
injury.

Question 74

purines

Answer 74

Other nontraditional neurotransmitters include the purines,
ATP and adenosine , which act principally as
neuromodulators.

ATP is present in all presynaptic vesicles and is coreleased
with one or more other neurotransmitters in response to Ca
influx into the terminal.

Adenosine is derived from ATP via enzyme activity occurring
in the extracellular compartment. Both presynaptic and
postsynaptic receptors have been described for adenosine, and
the roles these substances play in the nervous system and other
tissues are active areas of research.