Dr. Suroor Mohammed
The Nervous System is formed of a number of cells, which are of 2 types:
1. Nerve cells = Neurons
2. Supporting cells = Glial cells
1.
NEURONS
It is the basic structural unit of the NS.
It generates electrical impulses → transmitted from one part of the body
to another.
In most neurons: electrical impulses → release of chemical messengers
(= neurotransmitters) to communicate with each other.
Neurons are integrators: their output = the sum of the inputs they
receive from thousands of other neurons that end on them.
Neurons occur in a wide variety of shapes and sizes, but they share
common features. They all possess 4 parts:
1. Cell Body ( Soma):
It contains nucleus & organelles
→ provide energy & sustain metabolic activity of cells.
2. Dendrites:-
Usually 5-7 process (or more) highly branched (up to
400,000) → to increased surface area.
receive most input & Transmit impulses toward cell body only.
3. Axon
= Nerve Fiber:
- Usually single & long (few μm to 1m).
- Transmits impulses away from soma toward target cell.
-
Axon hillock or initial segment
(= beginning of axon + part of soma
where axon joins it) is the trigger zone where electric signals are
generated in most neurons. Signals are then propagated along axon.
- Near its end the axon undergoes branching.
4. Axon Terminal
- Each branch of the axon ends in an axon terminal.
- Responsible for the release of neurotransmitters (NT) from axon. NT
diffuse out of the axon terminal to next neuron or to a target cell
2.Supporting cells:
There are
sex categories
of supporting cells:
1.Schwann cells
, which form myelin sheaths around peripheral axons.
2. Satellite cells
or ganglionic gliocytes , which support neuron cells bodies
within the ganglia of the PNS.
3. Oligodendrocytes,
which form myelin sheaths around axons of CNS.
Unlike Schwann cells, they may branch to form myelin on up to 40 axons
4. Microglia,
which migrat through the CNS and
phagocytose foreign and degenerated material.
5. Astrocytes,
which help to regulate the
external environment of neurons in the CNS.
6. Ependymal cells
, which line the cavities of the
brain and the central canal of the spinal cord.
Axons of most (but not all) neurons are coated by a protective
layer = myelin sheath termed as “
myelinated
neurons”.
Myelin sheath is formed by the following cells:
1. In peripheral NS (PNS): by Schwann cells
2. In central NS (CNS): by oligodendrocytes.
Schwann Cells
- They are glia-like cells.
- During embryonic development, these cells attach to growing axons &
wrap around them → concentric layers of plasma membrane.
- Myelin sheath of an axon is formed of many Schwann cells that align
themselves along length of axon.
- Nucleus is located in outermost layer. Each segment is separated from
the next by a small
un myelinated
segment called
node of Ranvier.
- Plasma membrane of Schwann cells is 80% lipid → myelin sheath is
mostly lipid → appears glistening white to the naked eye.
Function of myelin sheath:
1. Myelin sheath helps to insulate axons & prevents cross-stimulation of
adjacent axons.
2. Myelin sheath allows nerve impulses to travel with great speed down the
axons, “jumping” from one node of Ranvier to the next.
***Some nerve fibers are
“un myelinated
”. Their axons are covered by a
Schwann cell, but there are no multiple wrappings of membrane which
produces myelin. These axons conduct impulses at a
much lower rate
.
Nerve Impulse or Action Potential
Is the electrical current moving from the dendrites to cell body to axon.
It results from the movement of ions (charged particles) into and out a
neuron through the plasma membrane
Resting Membrane Potential *RMP*
The resting membrane potential is the potential difference that exists
across the membrane of excitable cells such as nerve and muscle in the
period between action potentials (i.e., at rest).
Is the difference in electrical charge on the outside and inside of the
plasma membrane in a resting neuron (not conducting a nerve impulse).
The
outside
has a
positive
charge and the
inside
has a
negative
charge.
We refer to this as a polarized membrane.
A
resting neuron is at about -70mV
Nernst Equation
The Nernst equation is used to calculate the
equilibrium potential for an ion at a given concentration difference across a
membrane, assuming that the membrane is permeable to that ion. By
definition, the equilibrium potential is calculated for one
ion
at
a
time
At rest, The
K+
conductance or permeability is
high
and K+ channels are
almost fully
open,
allowing K+ ions to diffuse
out
of the cell down the
existing concentration gradient. This diffusion creates a K+ diffusion
potential, which drives the membrane potential toward the K+ equilibrium
potential. At rest,
the Na+
conductance is
low,
and, thus, the resting
membrane potential is
far
from the Na+ equilibrium potential.
Because of the high ratio of potassium ions inside to outside, Therefore, if
potassium ions were the only factor causing the resting potential, the
resting potential inside the fiber would be equal to –94 mV.
The difference is due to :
1.There is 30 times more K+ inside the cell than outside and about 15 times
more Na+ outside than inside.
2.There are also large negatively charged proteins trapped inside the cell.
(This is why it is negative inside.)
3. The action of the Na+/K+ pumps , that pump out 3 Na ions for every 2 K
ions that they transport into the cell.
Why so much K+ inside
Special protein channels called sodium-potassium pumps moving 3 Na+ out
and bringing 2 K+ back in, when the cell is at rest.
**In a resting cell there are no open channels for Na+ to easily move back into
the cell. However, there are some K+ channels open at all time.
**Na+ causes the outside to be positive forcing more K+ into the cell. (Lots of
potassium ions inside the resting cell.
There is continuous pumping of three sodium ions to the outside for each
two potassium ions pumped to the inside of the membrane. The fact that
more sodium ions are being pumped to the outside than potassium to the
inside causes continual loss of positive charges from inside the membrane;
this creates an additional degree of negativity Therefore, the net membrane
potential of k+ with all these factors operative at the same time is about –
90 mV .
Alterations in the membrane potential are achieved by varying the membrane
permeability to specific ions in response to stimulations.
The physiology of neurons and muscle cells are their ability to
produce
and
conduct these changes in membrane potential, such an ability is termed
excitability or irritability.
❖
If appropriate stimulation cause positive charges to flow into the cell. This
change is called
depolarization
(hypo polarization).
❖
A return to the RMP is known as
repolarization
.
If stimulation cause the inside of the cell to become
more negative
than the
RMP this change is
called hyper polarization
which can be caused either by
positive charges leaving the cell or by negative charges enter the cell.
Any potential not the RMP called membrane potential.
Any stimulus can cause action potential
called threshold stimulus
.
Electrotonic potential
is a local potential and cannot be propagated and
produced by sub threshold stimulus.
Action potential or ( nerve impulse)
The shape of action potential is the same in all the nerves but it's magnitude change
from one nerve to another but it remain
uniform
shape.
When the axon membrane has been
depolarized
to a threshold level, the
Na+gates
open
and the membrane becomes permeable to Na+, this permits Na+ to enter the
axon by diffusion which further depolarized the membrane(make the inside less
negative or more positive).
Since the gates for the Na+channels of the axon membrane are voltage regulated,
this additional depolarization opens more Na+channels and makes the membrane
even more permeable to Na+and more Na+ can enter the cell and induce a
depolarization that opens even more voltage– regulated Na+gates
A
positive
feedback loop is thus created, the explosive increase in Na+permeability
results in a rapid
reversal
of the membrane potential in that region from(– 70mv) to
(+30mv). At that point in time, the channels of Na+ close (become inactivated).
At this time, voltage–
gated K+ channels open
and
K+ diffuse rapidly out
of the
cell, and make the inside of the cell less positive or
more negative
. This process is
called
repolarization
and represents the completion of a negative feed back loop.
Once an action potential has been completed, the
Na+– K+ pump
will extrude the
extra Na+ that has entered the axon and recover the K+ that has diffused out of the
axon.
Phases of action potential
The first portion ,
local response
is due to slowly opening of voltage
gated Na+channels.
At the
firing
level (–55mv), full complete opening of voltage gated Na+
channels, and Na+will rush very rapidly to cell and membrane potential
will reach ( +35mv). So the
depolarization
is due to opening of the
voltage gated Na+channels
.
At ( +35mv) the Na+entarce will stop because:
1. The opening of voltage gated Na+ channels are time limited for short
constant period and this limited time cause depolarization will reach only
to (+35mv) and then stop.
2. At (+35mv) K+ channels are opened.
So
depolarization
from (–70mv to +35mv) is due to activation of Na+
channels. At (+35mv)
opening of K+ voltage gated
channels and K+ go
outside according to concentration gradient by diffusion. The channels are
opened completely from the first time and
repolarization
will start from
(+35mv) to (–55mv), at this point there will be in activation ( closure) of K+
channels.
Na+ ions
concentration inside will
increase
and this will cause
stimulation to Na+–K+ pump to exclude Na+ and carry K+ inside, till it
reach to (–70mv) again
( RMP),
so that after potential ( after
depolarization) phase due to Na+– K+ pump.
There will be loss of energy during action potential, so at after
depolarization to put the membrane potential again equal to RMP by Na+–
K+ pump is called {
recharging of nerve},
so any stimulus at this phase the
nerve will
not
response to it.
Why at( –55mv)Na+ channels will not open again ?
When Na+ channels inactivated, they need time more than 0.1msec. to
return to their original conformation, and to open Na+ channels again at
(–55 mv) must apply stimulus more than the first one
Repolarization of the action potential
.
The upstroke is terminated, and
the membrane potential repolarizes to the resting level as a result of two
events.
❖
1.The
inactivation gates on the Na+ channels
respond to depolarization by
closing, but their response is slower than the opening of the activation gates.
❖
2. Depolarization opens K+ channels and increases K+ conductance
to
a
value even higher than occurs at rest.
The combined effect of closing of the Na+ channels and greater opening of the
K+ channels makes the K+ conductance much higher than the Na+
conductance. Thus, an outward K+ current results, and the membrane is
repolarized.
Hyperpolarizing afterpotential (undershoot).
For a brief period
following repolarization, the K+ conductance is higher than at rest and the
membrane potential is driven even closer to the K+ equilibrium potential .
Eventually, the K+ conductance returns to the resting level, and the membrane
potential depolarizes slightly, back to the resting membrane potential.
All or Non law of action potential
If we apply
sub threshold
stimulus for the nerve, we get
no action
potential because it is un able to bring RMP to firing level. But if we
apply threshold stimulus, action potential will produced, and any
increase in the stimulus, there is no change in the magnitude and shape
or duration of action potential of the same nerve.
The shape, magnitude, duration and amplitude of action potential is the
same always all the same all the time and not change regardless to the
strength of stimulus to the same nerve
If a stimulus
is strong enough
to generate an action potential (reaches
threshold), the impulse is
conducted
along the entire length of the
neuron at the
same strength
Refractory periods:
Means the nerve will
not
respond to stimulus
during action
potential and it is of two types:
➢
Absolute RP
.
→located between the start of
depolarization
until one third of repolarization. The
nerve never
respond to
any stimulus whatever it's strength, due to full, complete
activation of Na+ channels and so no extra channels are
opened, and then at (+35mv), there will be in activation of
Na+ channels and it need time to return back to it's original
condition.
Each nerve has got specific absolute RP, and this is important
to limit the number of action potential generated by the
neurons.
➢
Relative RP
.
→ This period involve from third of
repolarization
to the end of repolarization. If we apply stimulus stronger
than the original stimulus, the nerve will respond by new
action potential, because the Na+ channels will open and can
overcome the repolarization effects of the open K+ channels.
Factors effecting the conduction velocity of
nerve impulses
1)_Diameter of the axon
: which is directly proportional with the
speed of conduction.
All peripheral nerves are mixed nerves ( the nerve contain many
axons with different threshold levels and different diameter).
Maximal stimulus
:
is the stimulus when applied to nerve it will
stimulate all axons in the nerve.
Compound action potential:
Algebraic summation of all action
potentials of all the axons in the mixed nerve.
2)_ Myelin sheath
: myelinated nerve is faster than un myelinated
nerve because, myelin sheath is an insulator material, so the
depolarization and repolarization will occur between two nods of
Ranveir, the action potential in myelinated nerve will jump and
called Saltotary conduction, while in un myelinated nerve the action
potential will walk.
3). Hypoxia
( low O2 to the tissue) , it depress the conduction.
4). Local anesthesia.
5). Temperature
.
