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

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

3. Microglia
Small dendritic cells
Found throughout the CNS
Forms 20% of total glial cells in the CNS
Phagocytic role in CNS
Secrete growth factors

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

6. Oligodendrocytes
 Myelinogenesis in CNS
Regulation of PH in CNS
Regulation of iron metabolism in CNS

7. Schwann cells
 Myelinogenesis in PNS
Participates in the regeneration of peripheral nerve fibers

8. Satellite cells
 Encapsulate the dorsal root and cranial nerve ganglion cells
Regulate their micro-environment

9. Ependymal cells and choroidal cells
 Produce CSF
Also participate in blood-CSF barrier

10. Muller’s cells of the retina
 Provides supporting framework in the retina

11. Bergman cells
 Provides supporting framework in the cerebellum and cerebellar
cortex

12. Pituicytes
 Provides support to the functional cells in the pituitary gland

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

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.

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

35. Afatiguability
Nerve does not undergo fatigue

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

38. THANK YOU