Dr.Suroor Mohamed
Physiology of Neuron & Muscle
Lecture
1
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.
Dr.Suroor Mohamed
Physiology of Neuron & Muscle
Lecture
2
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.
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 “unmyelinated”. 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.
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
unmyelinated 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.
Dr.Suroor Mohamed
Physiology of Neuron & Muscle
Lecture
3
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.
Dr.Suroor Mohamed
Physiology of Neuron & Muscle
Lecture
4
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
Subthreshold stimulus
.
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
Dr.Suroor Mohamed
Physiology of Neuron & Muscle
Lecture
5
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 mor.e than the
first one
Dr.Suroor Mohamed
Physiology of Neuron & Muscle
Lecture
6
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.
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.
Dr.Suroor Mohamed
Physiology of Neuron & Muscle
Lecture
7
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
.
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.
Dr.Suroor Mohamed
Physiology of Neuron & Muscle
Lecture
8
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
.
Thank you