2. Nervous tissue
• 2,4% of adults body weight, of which 83% is the brain
• Large consumption of energy
• Consumption of 20% O2 from the total amount
• Content of specific complex lipids
• Rapid turnover of proteins
• Types of cells:
CNS - neurons, astrocytes, oligodendrocytes, ependymal
cells, microglia
PNS – neurons, Schwann cells, satelite cells
2
3. Types of the cells
neuron
oligodendrocyte
astrocyte
microglia
capilary
3
4. Endotel lining of vessels in brain
Blood-brain barier
Free transport of substrates from blood is restricted
The all hydrophilic substrates must have transportion systems
Only water and small lipophilic molecules are freely permeable
Kapilary beds in peripheral tissues Kapilary beds in CNS
Glc Continuous
basement
membrane
astrocytes
interstitial fluid
– free diffusion through intercellular space – numerous tight junctions limit free diffusion
– pinocytosis (transcytosis) – lack of pinocytosis
– glucose transporters – the basement membrane is highly consistent
4
– transporters GLUT have low efficiency
5. Endotelial cells in brain
• Protect the brain
• Many drug-metabolizing enzyme systems
• Active P-glycoprotein pumps
• Specific transport systems for most of compounds
• Only small lipophilic molekules readily cross blood-brain
barrier by passive difusion
• Basement membrane surrounds cappilaries
5
6. Glucose transporters in brain
• Endothelial membranes – GLUT 1
• Glial cells: GLUT1
• Neurons membranes – GLUT 3
• Rate of glucose transport is limited by KM for GLUT 1
Transport of glucose through capillary walls is much less efficient
(blood-brain barrier).
When the glucose level drops below 3 mmol/l transport is not sufficient,
hypoglycemic symptoms develop
6
7. Transport of other molecules through blood –
brain barrier
• Monocarboxylic acids (lactate, acetate, pyruvate, ketone
bodies) – specific transporters
• Esential AA (phenylalanine, leucine, tyrosine, isoleucine,
valine, tryptophan, methionine, histidine ) – rapidly enter
CFS via a single amino acid transporters
• Non essential AA – transport is markedly restricted, they
are synthetized in the brain
• Vitamins – specific transporters
• Some proteins (e.g.*insulin, transferrin, IGF) cross the
blood-brain barrier by receptor mediated endocytosis
7
8. Lipids in the brain
• Fatty acids and other lipids (cholesterol, phospholipids,
glycolipids ..) cannot penetrate blood-brain barrier
• All these lipids must be synthesized in the brain
• Only essential fatty acids (linolenic and linoleic) have
specific transporters
• Very long fatty acids are synthesized in brain (necessary
for myeline formation)
• DHA is most abundant (22:6 (n-3))
• -oxidation, peroxisomal fatty acids oxidation occur in
brain
8
9. Insulin receptors
• insulin receptors are observed in certain parts of the
mammalian brain (mostly on neurons)
• brain is an insulinsensitive tissue and, in association with other
nutrient and adiposity signals, such as fatty acids, amino acids,
and leptin, probably plays an important role in the regulation of
energy balance and glucose homeostasis.
9
10. Energy production in brain
• metabolisms of glucose ( 100 g of glucose /d)
• utilisation of keton bodies (during starvation nearly 50% )
• Citric acid cycle
• Aerobic metabolism – consumption of 20% of O2
10
11. Dendrites
Neurons with receptors of neurotransmitters.
Perikaryon – the metabolic centre of
neuron with intensive proteosynthesis, is
highly susceptible to low supply of oxygen.
Axon
– the primary active transport of Na+ and K+ ions across
axolemma and voltage operated ion channels enables
inception and spreading of action potentials.
– axonal transport (both anterograde and
retrograde) provides shifts of proteins, mitochondria,
and synaptic vesicles between perikaryon and
synaptic terminals.
- myelin sheat enables the rapid saltatory conduction of
nerve impulses.
Axon terminals - synapses
– neurotransmitters are released from synaptic vesicles
11
into the synaptic cleft by exocytosis.
12. Axonal transport
In the axon, there is a fast axonal transport along microtubules. It works
on the principle of a molecular motor, via the motile proteins.
Kinesin drifts proteins, synaptic vesicles, and *mitochondria in
anterograde transport, dynein in retrograde transport.
anterograde transport
retrograde transport
*Mitochondria are mobile organelles and cells regulate mitochondrial movement in order to
meet the changing energy needs of each cellular region. Ca2+ signaling, which halts both 12
anterograde and retrograde mitochondrial motion, serves as one regulatory input.
13. Myelin sheaths are formed by wrapping
Myelin of protruding parts of glial cells round
the axons;
oligodendrocytes (CNS)
Schwann cells (PNS).
Numerous plasma membranes are
tightly packed so that the original
intracellular and extracellular spaces
cannot be differentiated easily.
Myelin membranes contain about 80 % lipids
cytoplasmic sides
(30% of cholesterol, 16% of cerebrosides).
The main structural proteins are
the "outer“ sides
- proteolipidic protein,
- the basic protein of myelin (encephalitogen),
- high molecular-weight protein
called Wolfram's protein.
13
14. Myeline damage
• Demyelination disease - loss or damage of myelin, both
in the CNS and in the PNS
• E.g. Multiple sclerosis
The cause has yet to be determined; autoimmune proces
demyelination of white matter of brain tissue and
multifocal inflammation damaged nerve conduction
along the demyelinated area
Presence of oligoclonal immunoglobulins in cerebrospinal
fluid
14
15. Astrocytes
Roles of astrocytes:
• connected by gap junctions - syncitium
• support for migration of neurons, matrix that keeps them in
place
• synthesis and degradation of glycogen
• uptake and metabolism of neurotransmiters (e.g. GABA,
glutamate)
• by phagocytoses take up debris left behind by cells
• control the brain extracellular ionic enviroment (Na+/K+-
pump, K+-„uptake“, Na+-HCO3 – cotransport, transport of
gases.)
• secretion of neurotransmiters (NO, VIP /vasoactive
intestinal peptide/ PGE2) 15
16. Metabolism of ammonia in brain
• Glutamate released by neurons into the synaptic cleft is taken
up by astrocytes and recycled into the form of glutamine
• NH3 crosses the H-E barrier
• If the concentration of ammonia in blood is high, it is
incorporated into Glu a Gln in astrocytes (glutamate
dehydrogenase, glutamin synthetase).
• depletion of ATP, due to depletion of 2-oxoglutarate for
CAC accumulation of Gln and Glu in astrocytes.
• Accumulation of Glu in astrocytes lieds to fall in transport of
Glu from synaptic cleft (and damage of neurons by excitotoxic
effects of Glu).
• Surplus of Gln results in damage of osmotic regulation in
astrocyte, increased intake of water oedem of brain
16
17. Metabolism of ammonia in brain
Blood- Astroglial cell Neuron
Capilary brain
barier glutamate
NH4+ NH3 NH4+ NH3+ 2-oxoglutarate
ATP
glutamate GABA
NH4+ NH3 NH3 NH4+
ATP
glutamine glutamate
17
18. Nerve impulse
•Neurons are irritable cells
•The lipidic bilayer is practically impermeable to the
unevenly distributed Na+ and K+ ions.
•The resting membrane potential is –70 mV on the inner
side of the plasma membrane.
•This potential is maintained by Na+,K+–ATPase.
Channels present in the membrane:
ligand-gated Na+/K+ channel,
– voltage-operated Na+ channel, and
– voltage-operated K+ channel.
18
19. Changes of membrane
mV Spike potential during an
35 potential action potential
Na+ channels (propagation of nerve
opened
impuls along the
repolarization mebrane)
Treshhold
potential
Resting -70
potential
Time
(ms)
•stimulus triggers the depolarization
• if the depolarization reaches the „treshold value“ voltage operating Na+ channels will open,
Na+ enters massively the cell, depolarization takes place
• this triggers the slow opening K+ channels, Na+ channels will slowly close, potassium fluxes
inside the cell, repolarization takes place
• Na+/K+ pump restores the concentration gradients disrupted by action potential
•Action potential is propagated to the axon terminals 19
20. Transmiting the nerve impuls across the synaptic
cleft
•Action potential travels down the axon to the axon terminal
•Each axon terminal is swollen, forming a synaptic knob
•The synaptic knob is filled with membrane –bounded vesicles
containing a neurotransmitter
•Arrival of an action potential at the synaptic knob opens voltage-
gated Ca2+-channels in the plasma membrane
•The influx of Ca2+ triggers the exocytosis of some of the vesicles
•The nerotransmitter is released into the synaptic cleft
•The neurotrabnsmitter bind to receptors on the postsynaptic
20
membrane
21. Synaptic transmission
Neurotransmitters act as chemical signals between nerve cells
or between nerve cells and the target cells.
voltage-gated Ca2+ channel
depolarization wave
Ca2+ receptor
synaptic vesicles
(synaptosomes)
postsynaptic
membrane
synaptic cleft
The response to the neurotransmitter depends on the receptor type:
– ionotropic receptors (ion channels) evoke a change in the
membrane potential - an electrical signal,
– metabotropic receptors are coupled to second messenger
pathway, the evoked signal is a chemical one.
21
22. Neurotransmitters
A large number (much more than 30) of neurotransmitters have been described.
Many of them are derived from simple compounds, such as amino acids and
biogenic amines, but some peptides are also known to be important
neurotransmitters. The principal transporters:
Central nervous system
inhibitory GABA (at least 50 %)
glycine (spinal cord) Peripheral neurons
excitatory glutamate (more than 10 %)
– efferent
acetylcholine (about10 %)
excitatory acetylcholine
dopamine
noradrenaline
(about 1 %, in the striatum 15 %)
serotonin
histamine – afferent sensory neurons
aspartate glutamate
noradrenaline (less than 1 %, neuropeptides
but in the hypothalamus 5 %)
adenosine
neuromodulatory endorphins, enkephalins,
endozepines, delta-sleep inducing peptide,
and possibly endopsychosins.
22
23. Neurotransmitter receptors
Contrary to various types of hormone receptors, only two basal types
of neurotransmitter receptors occur:
Ionotropic receptors – ligand-gated ion channels (ROC), e.g.
excitatory – acetylcholine nicotinic - Na+/K+ channel,
– glutamate (CNS, some afferent sensory neurons)
- Na+/Ca2+/K+ channel,
inhibitory – GABAA receptor (brain) - Cl– channel
Metabotropic receptors activating G proteins, e.g.
Gs protein – -adrenergic, GABAB receptor, dopamine D1,
Gi protein – 2-adrenergic, dopamine D3,
acetylcholine muscarinic M2 (opens also K+ channel),
Gq protein – acetylcholine muscarinic M1, 1-adrenergic.
23
24. Acetylcholine receptors
nicotinic muscarinic
acetylcholine-operated
Na+/K+ channels ; M1-M5
Metabotropic, act throuhg G-proteins
in the peripheral nervous
system M1 –Gq protein, vegetative ganglia, CNS,
b.exocrine glands
in the dendrites of nearly
M2 –Gi, in heart, opens K+-chanels
all peripheral efferent
neurons (including M3 - Gq protein, in smooth muscle
adrenergic neurons)
M4 –Gi, v CNS
at neuromuscular M5 –Gq, v CNS
junctions ion the cytoplasmic
membrane of skeletal
muscles.
Atropin is an acetylcholine antagonist at
muscarinic receptors
24
.
25. Cholinergic synapse acetylcholine receptors
depolarization wave Na+
Ca2+
choline acetyltransferase
(by axonal transport) ACETYLCHOLINE
acetyl-CoA
ATP
membrane-bound
acetylcholinesterase
reuptake
choline
acetate
Increase in intracellular [Ca2+] activates Ca2+-calmodulin-dependent protein- kinase that
phosphorylates synapsin-1; its interaction with the membrane of synaptic vesicles
initiates their fusion with the presynaptic membrane and neurotransmitter exocytosis.
The membranes of vesicles are recycled.
At neuromuscular junctions, the arrival of a nerve impulse releases about 300 vesicles (approx. 40
000 acetylcholine molecules in each), which raises the acetylcholine concentration in the cleft more
than 10 000 times.
25
26. Myastenia gravis
• Myastenia gravis is an aquired autoimmune disease caused
by production of an antibody directed against the
acetylcholine receptor in skeletal muscle
• The antibodies bind to the receptor forming receptor-
antibody complex
• Complex is endocytosed and incorporated into lysosomes
Number of receptors is reduced
Muscle weakness
26
28. Inhibitors of acetylcholinesterase
CH3 CH3
H
CH3 N N
HN
• a) reverzible: carbamates (physostigmine,
O O
rivastigmine, neostigmine) CH3
• b) irreverzible: organophosphates phyzostigmin
(diisopropylfluorphosphate, soman, sarin) e
Binding of toxic organo phosphates occurs in two phases:
reversible ( it may be affected by reactivators)
ireversible – formation of covalent bond between organophosphate and an enzyme
28
32. Acetylcholine nicotinic receptor – Na+/K+ channel
•asymmetric pentamer of four kinds of membrane-spanning homologous subunits
•activated by binding of two molecules of acetylcholine.
2-subunits bind two
acetylcholine molecules
the closed state Na+ K+
a large inward
2 2 flow of Na+
a smaller outward
synaptic flow of K+
cleft
– –
cytoplasm
binding sites for local anaesthetics, changes in conformation, the channel
psychotropic phenothiazines. etc. undergoes frequent transitions between
open and closed states in few milliseconds
D-Tubocurarine is an antagonist of acetylcholine that prevents channel opening.
Succinylcholine is a myorelaxant that produces muscular end plate depolarization.
32
33. Acetylcholine (cholinergic) receptors
of the peripheral efferent neurons
N N N
N
N Most postganglionic
neurons of the sympathetic
path are adrenergic
Adrenergic
N M1 receptors
motor neurons parasympathetic sympathetic
(neuromuscular junction) system system
33
34. Adrenergic synapse
Neurotransmitter of most postganglionic sympathetic neurons
is noradrenaline.
Varicosities of the postganglionic sympathetic
depolarization wave axons are analogous to the nerve terminals.
DA -hydroxylase
synaptic vesicles
(axonal transport)
Ca2+
presynaptic
adrenergic mitochondrial
receptors NORADRENALINE
monoamine oxidase
partial reuptake
adrenergic receptors in extracellular COMT
membranes of the target cells (catechol O-methyltransferase)
34
35. Synthesis and storage of catecholamines
• Synthesis from tyrosine DOPA dopamin in cytosol
• Dopamin is transported into vesicles (ATP dependent
transport)
• Vesicles are transported to nerve terminals
• -hydroxylation to noradrenalin occurs in vesicles
35
36. Adrenergic receptors
of all types are receptors cooperating with G proteins.
-Adrenergic receptors
After binding an agonist, all types of -receptors activate Gs proteins so
that adenylate cyclase is stimulated, cAMP concentration increases,
and proteinkinase A is activated. Particular types differ namely in
their location and affinity to various catecholamines:
1 are present in the membranes of cardiomyocytes,
2 in the smooth muscles and blood vessels of the bronchial stem,
3 in the adipose tissue.
2-Adrenergic receptors
The effect is quite opposite to that of -receptors, binding of
catecholamines results in the interaction with Gi protein,
decrease in adenylate cyclase activity and in cAMP concentration.
1-Adrenergic receptors
activate Gq proteins and initiate the phosphatidylinositol cascade by
stimulation of phospholipase C resulting in an increase of
intracellular Ca2+ concentration and activation of proteinkinase C.
36
37. Adrenergic receptors 1, 2, and 3
noradrenaline/adrenaline
receptor AMP cyclase
Gas
bg
ATP
H2O cAMP
phosphodiesterases
inactive
AMP proteinkinase A
phosphorylations active proteinkinase A
The typical effects of -stimulation:
1 – tachycardia, inotropic effect in the myocard,
2 – bronchodilation, vasodilation in the bronchial tree,,
3 – mobilization of fat stores, thermogenesis.
37
38. Adrenergic receptors 2 a 1
Receptors 2 Receptors 1
adenylate cyclase phospholipase C
PL C
Gi protein Gq protein
IP3 and diacylglycerol
cAMP decrease
increase in [Ca2+]
activation of PK C
The typical effects of adrenergic
2-stimulation: 1-stimulation:
glandular secretion inhibited vasoconstriction
bronchoconstriction
motility of GIT inhibited
38
39. Inhibitory GABAA receptor
ligand-gated channel (ROC) for chloride anions. The interaction with
-aminobutyric acid (GABA) opens the channel. The influx of Cl– is
the cause of hyperpolarization of the postsynaptic membrane and thus
its depolarization (formation of an action potential) disabled.
Cl–
1 2
2 1
2
–
– –
– – – –
The receptor is a heteropentamer.
Besides the binding site for GABA, it has at least eleven allosteric modulatory
sites for compounds that enhance the response to endogenous GABA – reduction
of anxiety and muscular relaxation: anaesthetics, ethanol, and many useful drugs,
e.g. benzodiazepines, meprobamate, barbiturates. Some ligands compete for the
diazepam site or act as antagonists (inverse agonists) so that they cause discomfort
and anxiety, e.g. endogenous peptides called endozepines.
39
40. Inhibitory synapse
GABA (-aminobutyric acid) is the major inhibitory neurotransmitter in CNS.
Gabaergic synapses represent about 60 % of all synapses within the brain.
Ca2+
depolarization wave
GABA / benzodiazepine
GABA
receptors
mitochondrial synthesis
of GABA from glutamate
partial reuptake uptake of GABA into glial cells
(transporters GAT 1,2,3,4) and breakdown to succinate
In the spinal cord and the brain stem, glycine has the similar function as GABA in the
brain. The inhibitory actions of glycine are potently blocked by the alkaloid strychnine, a
40
convulsant poison in man and animals.
41. Receptors for the major neurotransmitters
Ion channels Receptors cooperating with G-proteins
(ROC) Gs (cAMP increase) Gi (cAMP decrease) Gq (IP3/DG formation)
Na+/K+ – acetylcholine – acetylcholine acetylcholine
nicotinic muscarinic M2,4 muscarinic M1,3,5
– adrenergic β1, β2, β3 adrenergic α2 adrenergic α1
Na+/Ca2+/K+ glutamate mGluR glutamate mGluR
– glutamate ionophors – group II and III group I
– dopamine D1,5 dopamin D3,4 dopamine D2
– serotonin 5-HT3 serotonin 5-HT4,6 serotonin 5-HT1 serotonin 5-HT2
– histamine H2 histamine H3,4 histamine H1
– – – tachykinin NK-1
for substance P
Cl– – GABAA GABAB (metabotropic) –
– glycine
41
