Neuroglia, also known as glial cells, provide support and insulation to neurons in the central and peripheral nervous systems. There are two main types of neuroglia: microglia and macroglia. Microglia are small phagocytic cells found throughout the central nervous system, while macroglia include astrocytes, oligodendrocytes, Schwann cells, and other larger glial cells. Astrocytes help form the blood-brain barrier and regulate neurotransmitters. Oligodendrocytes and Schwann cells are responsible for myelination in the central and peripheral nervous systems respectively. Nerve fibers have properties like excitability, conductivity, following the all-or-none principle, and
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Neuroglia and Nerve Fiber Properties
1. Neuroglia, Properties
of nerve fibre
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com
2. Neuroglia
Connective tissue cells of the nervous system
Provides support to the functional neurons
10-15 times more in number than functional neurons
Microglia
macroglia
4. Macroglia
Large connective tissue cells of the nervous system
Astrocytes
Oligodendrocytes
Schwann cells
Satellite cells
Ependymal cells
Muller’s cells of retina… etc
5. Astrocytes
Blood-brain barrier formation
Absorb excess amounts of NT released and prevent the spread of NT
to other areas
Take up excess potassium- stabilize RMP
Produce substances that has a trophic influence on the neighboring
neurons
13. Properties of nerve fibre
Excitability
Conductivity
All-or-None law
Summation
Refractory period
Infatiguability
Adaptation
Accommodation
14. Excitability
Ability of tissue to respond to a threshold stimulus and able to
propagate the response
Excitability of a nerve fibre is measured using the strength-duration
curve
Chronaxie is the indicator of the excitability of nerve fibres
15. Strength-duration curve
A stimulus has two quantitative characteristics
Strength or intensity
Duration
Stronger stimulus less duration required for it to excite an AP
Minimum strength of the stimulus required to excite an AP- Rheobase
The minimum duration for which a rheobase stimulus is applied for
excitation- Utilization time
16. Strength-duration curve
Minimum strength of the stimulus required to excite an AP- Rheobase
The minimum duration for which a rheobase stimulus is applied for
excitation- Utilization time
The minimum duration for which a stimulus double the rheobase has to be
applied for it is called chronaxie
Chronaxie gives us an idea about the sensitivity of excitable tissues
17.
18. Factors affecting excitability
Temperature: Increase in temperature increases excitability
Ions: Potassium , calcium, magnesium ions
Drugs: Anaesthetics abolish excitability and block the conduction by
blocking ion channels
Toxins: Tetanus and rabies increase excitability
Hypoxia: Moderate – increases, severe – decreases
pH: Alkaline- increases, acidic- decreases
19. Conductivity
Ability to conduct an impulse (AP)
Nerve fibers conduct impulses in both the directions ar variable rates 0.5-
120 m/sec)
At a chemical synapse and at NMJ conduction is unidirectional
Temperature increases the conductivity
Myelination increases speed of conduction
Conduction is directly proportional to diameter of nerve fiber
20. Conductivity
Myelinated fibers are axons covered with myelin,
a thick layer composed primarily of lipids, at regular intervals along their length
Because the water-soluble ions responsible for carrying current across the
membrane cannot permeate this myelin coating,
it acts as an insulator, just like plastic around an electrical wire, to prevent
leakage of current across the myelinated portion of the membrane.
21. Conductivity
Between the myelinated regions, at the nodes of Ranvier,
the axonal membrane is bare and exposed to the ECF.
Current can flow across the membrane only at these bare spaces to
produce action potentials.
Voltage-gated Na+ and K+ channels are concentrated at the nodes,
whereas the myelin-covered regions are almost devoid of these
special passageways
22. Conductivity
By contrast, an unmyelinated fiber has a high density of these voltage-gated channels
along its entire length.
Action potentials can be generated only at portions of the membrane furnished with an
abundance of these channels.
The distance between the nodes is short enough that local current can flow between an
active node and an adjacent inactive node before dying off.
When an action potential occurs at one node, local current flow between this node and the
oppositely charged adjacent resting node reduces the adjacent node’s potential to
threshold so that it undergoes an action potential, and so on.
23. Conductivity
Consequently, in a myelinated fiber, the impulse “jumps” from node to node,
skipping over the myelinated sections of the axon.
This process is called saltatory conduction (saltare means “to jump)
Saltatory conduction propagates action potentials more rapidly than contiguous
conduction does,
because the action potential does not have to be regenerated at myelinated
sections but must be regenerated within every section of an unmyelinated axon
from beginning to end.
24. Conductivity
Myelinated fibers conduct impulses about 50 times faster than
unmyelinated fibers of comparable size.
You can think of myelinated fibers as the “superhighways” and
unmyelinated fibers as the “back roads” of the nervous system when it
comes to the speed with which information can be transmitted.
25.
26. All-or-none phenomenon
Nerve fiber follows the all-or-none law
If it responds, it responds to the maximum extent by giving rise to an
action potential
Or else it will not respond at all
Threshold stimulus can lead to AP
Subthreshold stimulus fails to cause AP
27. Refractory period
What ensures the one-way propagation of an action potential away from the initial site of activation?
once the action potential has been regenerated at a new neighboring site (now positive inside) and
the original active area has returned to resting (again negative inside),
the proximity of opposite charges between these two areas is conducive to local current flow in the
backward direction as well as in the forward direction into as-yet-unexcited portions of the
membrane.
If such backward current flow were able to bring the previous active area to threshold again,
another action potential would be initiated here, which would spread both forward and backward,
initiating still other action potentials, and so on
28. Refractory period
But if action potentials were to move in both directions, the situation
would be chaotic, with numerous action potentials bouncing back and
forth along the axon until the neuron eventually fatigued.
Fortunately, neurons are saved from this fate of oscillating action
potentials by the refractory period
29. Refractory period
When patch of axonal membrane is undergoing an action potential, it
cannot initiate another action potential, no matter how strong the
depolarizing triggering event is.
This period when a recently activated patch of membrane is
completely refractory (meaning “stubborn” or “unresponsive”) to further
stimulation is known as the absolute refractory period.
30. Refractory period
Once the voltage-gated Na+channels are triggered to open at
threshold, they cannot open again in response to another depolarizing
triggering event, no matter how strong,
until they pass through their “closed and not capable of opening”
conformation and then are reset to their “closed and capable of
opening” conformation when resting potential is restored.
31. Refractory period
Because of the absolute refractory period, one action potential must
be over before another can be initiated at the same site.
Following the absolute refractory period is a relative refractory period,
during which a second action potential can be produced only by a
triggering event considerably stronger than usual.
32. Refractory period
Fewer voltage-gated Na+ channels are in a position to be jolted open in response to another
depolarizing triggering event.
Second, the voltage-gated K+ channels that opened at the peak of the action potential are slow to
close.
During this time, the resultant less-than-normal Na+ entry in response to another triggering event is
opposed by K+ still leaving through its slow to-close channels during the after hyperpolarization.
Thus, a greater depolarizing triggering event than normal is needed to offset the persistent
hyperpolarizing outward movement of K+and bring the membrane to threshold during the relative
refractory period
33. Refractory period
By the time the original site has recovered from its refractory period
and is capable of being restimulated by normal current flow, the action
potential has been propagated in the forward direction only and is so
far away that it can no longer influence the original site.
Thus, the refractory period ensures the one-way propagation of the
action potential down the axon away from the initial site of activation.
34. Summation
A single stimulus when applied to a nerve fiber
It does not produce AP
When several sub-threshold stimulus are applied, at a rapid rate
The responses added up
Initiate AP
This adding up of responses - summation
36. Adaptation
When a stimulus with constant strength is applied
Excitability decreases
This is called adaptation
Due to gradual inactivation of sodium channels
Sensory receptors shows adaptation
Tactile receptors shows complete adaptation
Pain nerve endings do not show adaptation