10. Chemical synapse
• Almost all synapses used for signal
transmission in the CNS of human
being are chemical synapses.
• First neuron secretes a chemical
substance called neurotransmitter
at the synapse to act on receptor on
the next neuron to excite or inhibit
its sensitivity.
11.
12. Synaptic transmission
• 1) One way conduction (one direction of impulse
transmission through a synapse (from the presynaptic neuron that
secretes the transmitter to the postsynaptic neuron on which the
transmitter acts)
• 2) Delayed conduction
• 3) Fatigue development
For nerve fiber: two-way transmission, no
delay of impulse conduction along
individual fiber, relative infatigability
13.
14.
15.
16. Brief Historical Comments
Weber (1845)-inhibitory process was
discovered for the first time (inhibition of
heart by vagal stimulation)
Sechenov(1862)-discovered inhibitory
center in the brain ( excitation of the
thalamic region inhibits spinal reflexes in
the thalamic frog)
Goltz discovered inhibitory processes in the
spinal cord (inhibition of two spinal reflexes
that arises in response to stimulation of two
different receptive zones simultaneously in
the spinal frog)
Sherrington (1900-1920) discovered
reciprocal inhibition in the spinal cord(in
mammals)
Magoun (1945-50) discovered inhibitory
zones in the reticular formation of the
brainstem.
Eccles (1950-60) discovered postsynaptic
and presynaptic inhibition (by
microelectrode method)
17. - Inhibitory synapses are axo-somatic
- the mediators are glycine or GABA
- The exit of potassium ions and influx of chloride
ions cause more negativity inside, leading to
hyperpolarization.
- Inhibitory postsynaptic potential (IPSP) is the
electrical potential in the form of hyperpolarization
that develops during postsynaptic inhibition.
- Inhibitory neurotransmitter substance acts on
postsynaptic membrane by binding with receptor.
-This complex opens the potassium and chloride
channels (instead of sodium)
-Hyperpolarized state of the synapse inhibits synaptic
transmission
-
Inhibitory synapses are axo-axonic
-The mediator is GABA
- The exit of chloride ions causes inhibitory
depolarization (cell will less negative inside) of
presynaptic part (300-400 ms)
-This prevents (decreases the quantity) of
neurotransmitter release from presynaptic part
because reduces calcium influx to presynaptic
part.
This inhibitory neuron inhibits the presynaptic
neuron and decreases the magnitude of AP (5%)
in presynaptic neuron.
So the magnitude of EPSP in postsynaptic
neuron is decreased (50%) resulting in synaptic
inhibition.
18.
19.
20.
21.
22.
23.
24. When motor neurons send motor impulses,
some of the impulses reach the Renshaw
cell by passing through collaterals.
Now, the Renshaw cell is stimulated.
In turn, it sends inhibitory impulses to a
motor neurons so that, the discharge from
motor neurons is reduced
25.
26. Nerve centers
• Nerve center is a group of neurons acting together in the performance of
a definite reflex or in the regulation of a particular function
27. Neurons are
interconnected with one
another (with the help of
neuro-neuronal
synapses) to form
circuits.
Many neural circuits
together form a
neural system
28. DIVERGENCE
Divergence is the process by which
one presynaptic
neuron terminates on many
postsynaptic neurons.
Output from one neuron onto many.
Each postsynaptic neuron receives
input from the same presynaptic
neuron, but may react to it differently.
In a divergent neural circuit, the axon
of one neuron branches to send
information to multiple target
neurons. Divergent output allows the
same signal to reach many different
neurons.
„CONVERGENCE
Convergence is the process
by which many presynaptic
neurons terminate on a single
postsynaptic neuron
Output from many neurons
onto one. Many different
presynaptic neurons provide
input to a single postsynaptic
neuron. These inputs may be
excitatory or inhibitory, and
may be active at different
times.
„
29. The basic properties of nerve centers
• 1) Unidirectional (one-way) conduction of excitation
• 2) A central delay of excitation (delayed conduction)
• 3) Summation (temporal and a spatial)
• 4) Occlusion
• 5) Post-tetanic potentiation
• 6) Aftereffect (after action)
• 7) Transformation of the rhythm of excitation
• 8) An irradiation of excitation
• 9) A plasticity of nerve centers
• 10) The tone of nerve centers
• 11) Fatigue of nerve centers
• 12) Sensitivity to an oxygen deficiency and to the chemical
drugs
30. the impulses are transmitted only in one direction in
synapse, i.e. from presynaptic neuron to postsynaptic
neuron.
31. Synaptic delay is a short delay that occurs
during the transmission of impulses through the
synapse.
It is due to the time taken for:
i. Release of neurotransmitter
ii. Passage of neurotransmitter from axon
terminal to postsynaptic membrane
iii. Action of the neurotransmitter to open the
ionic channels in postsynaptic membrane.
Normal duration of synaptic delay is 0.3 to
0.5 millisecond.
Synaptic delay is one of the causes for
reaction time of reflex activity.
32. Spatial summation occurs when many
presynaptic terminals are stimulated
simultaneously.
Temporal summation occurs when one
presynaptic terminal is stimulated
repeatedly.
Thus, both spatial summation and temporal
summation play an important role in
facilitation of response.
33. Occlusion
Occlusion is demonstrated in a flexor reflex involving a muscle, which is
innervated by two motor nerves.
These nerves can be called A and B.
If nerve A is stimulated alone, the arbitrary unit of tension developed is 9.
If the nerve B is stimulated then 9 units of tension is developed too.
So, the sum of tension developed when the nerves A and B are separately
stimulated = 9 + 9 = 18 units.
When both the nerves, A and B, are stimulated simultaneously, the tension
developed by the muscle is less (12) than the sum of the tension developed
when each nerve is stimulated separately (18).
This phenomenon is called occlusion (is due to the overlapping of the nerve
fibers during the distribution)
34.
35. After – action:
excitation lasts many times longer in the
nerve center then the stimulation of ifs
receptive field, because
1) after depolarization of the neuron causes
prolonged rhythmic discharge – short-term
after-action
2) circulation of nerve impulses through the
closed neuronal chain (reverberatory circuit)
provokes long-term after-action
36. PTP – increase of excitability
of the nerve center just after
rapidly repetitive stimulation
It results from excess of Ca 2+
ions in the presynaptic terminals
(because Ca –pump acts
slowly).
The more Ca 2+ concentration,
the more vesicular release is
observed ( 2 times)
37. Tone of nerve centers – is
a constant excitation of
their at rest
Factors that maintain the
tone:
1) Constant afferent
impulses from
receptors
2) Action of various
humoral substances
(oxygen, carbon
dioxide,,,)
3) Automatic activity of
neurons
38. High sensitivity to hypoxia: when the cerebral blood flow
suddenly interrupted, cortical neurons within 5 to7 sec becomes
unexcitable (the person becomes unconscious) and within 5 to 7
min they completely die.
High sensitivity to drugs and toxins: caffeine, theophylline,
theobromine (which are found in coffee and tea) increase neuronal
excitability (they reduce the threshold potential)
When areas of the NS became overexcited fatigue is
development and causes them to lose this excess excitability.
Thus the development of fatigue is a protective mechanism
against excess neuronal activity.
43. Moving systems of spinal cord
Moving control in spinal cord occurs with the help of reflex arcs (is the anatomical
nervous pathway for a reflex action). A simple reflex arc includes 5 components:
There are 3 types of neurons in reflex arc:
1) Afferent neuron - transmit information
from periphery
2) Interneuron – combine afferent and
motoneuron
3) Motoneuron – control activity of muscles
Afferent and efferent nerve fibers may be
connected directly to the center.
1. Receptor is the end organ, which receives the
stimulus. When receptor is stimulated, impulses are
generated in afferent nerve.
2. Afferent or sensory nerve transmits sensory
impulses from the receptor to center.
3. Center (is located in the brain or spinal cord)
receives the sensory impulses via afferent nerve
fibers and generates motor impulses.
4. Efferent or motor nerve transmits motor impulses
from the center to the effector organ.
5. Effector Organ is the structure such as muscle or
gland where the activity occurs in response to
stimulus.
44. There are 3 parts of the brain
stem:
- Midbrain (mesencephalon)
- Pons
- Medulla Oblongata
Three parts of the hindbrain:
- Pons
- Medulla Oblongata
- Cerebellum
The (critical collecting link between spinal
cord and high brain centers).
It is sometimes called “the reptilian brain”
because is the oldest and most basic region
of the brain.
45. 1) Nuclei of 12th, 11th, 10th and some nuclei of 8th and 5th cranial nerves are located in
the medulla oblongata (hypoglossal nerve controls the movements of tongue,
accessory nerve - movements of shoulder, vagus nerve controls almost all the vital
functions in the body (CVS, RS, GIS ). 8th cranial nerve (the cochlear division) - is
concerned with the auditory function.
Nuclei of 8th, 7th, 6th and 5th cranial nerves also are located in pons
2) Brainstem serves as a conduct for ascending and descending tracts ( connects the brain
with the spinal cord ) and as a way station for signals from higher neural centers.
3) Brainstem provides many special control functions:
- control of respiration ( Medulla oblongata, Pons);
- control of CVS - vasomotor center controls blood pressure and HR (Medulla oblongata);
- control of GI function - salivation, deglutition and vomiting centers (Medulla oblongata);
- control of stereotyped movements of the body (pons and midbrain)
- control of equilibrium (vestibular nuclei of medulla and pons)
- control of vision, hearing, eye movements and body movements (Midbrain)
4) Information from the ears enters the pons first (Pons)
5) control of muscle tone, complex muscular movements, righting reflexes, movements of
eyeballs, skilled movements (red nucleus of midbrain)
6) It has parts that are important for the level of consciousness and for sleep, regulates
awareness (reticular formation)
7) Especially important are the reticular formation and vestibular nuclei of the brain
stem.
BRAINSTEM Functions
46. 3) Brainstem provides many special control functions:
- control of respiration ( Medulla oblongata, Pons);
- control of CVS - vasomotor center controls blood pressure and HR (Medulla
oblongata);
- control of GI function - salivation, deglutition and vomiting centers
(Medulla oblongata);
- control of stereotyped movements of the body (pons and midbrain)
- control of equilibrium (vestibular nuclei of medulla and pons)
- control of vision, hearing, eye movements and body movements
(Midbrain)
4) Information from the ears enters the pons first (Pons)
5) control of muscle tone, complex muscular movements, righting reflexes,
movements of eyeballs, skilled movements (red nucleus of midbrain)
6) It has parts that are important for the level of consciousness and for sleep,
regulates awareness (reticular formation)
