The document provides an overview of the nervous system, including its functions and major components. It discusses the two principal cell types - neurons and neuroglia. Neurons are specialized to transmit electrical signals through a process called an action potential. All neurons have dendrites, a cell body, an axon, and presynaptic terminals. The document also describes the basic structure and function of a nerve, including the myelin sheath that surrounds many axons. Nerve fibers are classified in different ways, including based on their structure, function, neurotransmitter secretion, origin and distribution.

1. Introduction To Nervous System
Dr. Kanwal Abbasi

2. Nervous System Function
• The nervous system has three functions.
• 1- monitors the internal and external
environments
• 2- integrates sensory information
• 3- coordinates voluntary and involuntary
responses.

3. Division of Nervous system

5. Neuron and Neuroglia
Two principal cell types
Neurons
Excitable nerve cells that transmit electrical signals
– Supporting cells
Smaller cells surrounding and wrapping neurons
“Neuroglia”

6. Neurons
• Cells of the nervous system, called nerve cells
or neurons, are specialized to carry
"messages" through an electrochemical
process. The human brain has about 100
billion neurons that carry out the nerve
impulses through a process called action
potential.

7. Neurons are similar to other cells in the body because:
1. Neurons are surrounded by a cell membrane.
2. Neurons have a nucleus that contains genes.
3. Neurons contain cytoplasm, mitochondria and other "organelles".
4. Neurons carry out basic cellular processes such as protein
synthesis and energy production.
Neurons differ from other cells in the body because:
1. Neurons have specialized extensions called dendrites and axons.
Dendrites bring information to the cell body and axons take
information away from the cell body.
2. Neurons communicate with each other through an
electrochemical process.
3. Neurons contain some specialized structures (for example,
synapses) and chemicals (for example, neurotransmitters).

9. The Cells of the Nervous System
• All neurons have the following major
components:
– Dendrites.
– Soma/ cell body.
– Axon.
– Presynaptic terminals.

10. The Cells of the Nervous System
• Dendrites- branching fibers with a surface
lined with synaptic receptors responsible for
bringing in information from other neurons.
• Some dendrites also contain dendritic spines
that further branch out and increase the
surface area of the dendrite.

11. Fig. 2-7, p. 33

12. The Cells of the Nervous System
• Soma - contains the nucleus, mitochondria,
ribosomes, and other structures found in
other cells.
– Also responsible for the metabolic work of the
neuron.

13. The Cells of the Nervous System
• Axon - thin fiber of a neuron responsible for
transmitting nerve impulses away to other
neurons, glands, or muscles.
• Some neurons are covered with an insulating
material called the myelin sheath with
interruptions in the sheath known as nodes of
Ranvier.

14. The Cells of the Nervous System
• Presynaptic terminals refer to the end points
of an axon responsible for releasing chemicals
to communicate with other neurons.

15. Axons
• Take information away
from the cell body
• Smooth Surface
• Generally only 1 axon per
cell
• No ribosomes
• Can have myelin
• Branch further from the cell
body
Dendrites
• Bring information to the
cell body
• Rough Surface (dendritic
spines)
• Usually many dendrites
per cell
• Have ribosomes
• No myelin insulation
• Branch near the cell body
There are several differences between
axons and dendrites

16. Organization of Nerve

17. Nerve
• Each nerve is a cable-like structure that contains
many axons that are sometimes referred to as
"fibers. " Within a nerve, each axon is surrounded
by a layer of connective tissue called the
endoneurium. The axons are bundled together
into groups called fascicles. Each fascicle is
wrapped in a layer of connective tissue called the
perineurium. Finally, the entire nerve is wrapped
in a layer of connective tissue called the
epineurium

18. Myelin Sheath

19. Myelin is an insulating layer,
or sheath, that forms around
nerves, including those in the
brain and spinal cord. It is made
up of protein and Lipids e.g
Cholesterol, lecithin and
cerebroside (spingomyelin).
Functions of Myelin Sheath
1)Faster Conduction
The purpose of the myelin sheath is to allow electrical impulses
to transmit quickly and efficiently along the nerve cells. If myelin
is damaged, the impulses slow down.
2)Insulating Capacity
Myelin sheath has a high insulating capacity means its restricts
the nerve impulses within the signal never fiber and prevents the
stimulation of neighboring nerve fibers.

20. Neurilemma
• Surrounding the myelin sheath, there is a thin membrane
called neurilemmal sheath. This is also called neurilemma
or sheath of Schwann.
• This contains Schwann cells, which have flattened and
elongated nuclei. The cytoplasm is thin and modified to
form the thin sheath of neurilemma enclosing the myelin
sheath.
• One nucleus is present in each internode of the axon. The
nucleus is situated between myelin sheath . At the node of
Ranvier (where myelin sheath is absent), the neurilemma
invaginates and runs up to axolemma in the form of a finger
like process.
• In nonmyelinated nerve fiber, the neurilemma continuously
surrounds axolemma. Neurilemma is absent in central
nervous system. Neurilemma is necessary for the formation
of myelin sheath (myelinogeneis).

22. Neurotrophins
• Promotes neuron growth and Proteins in
nature
• Nerve growth factors include:
• Nerve growth factor (NGF), brain-derived
neurotrophic factor (BDNF), glial-derived
neurotrophic factor (GDNF), neurotrophin-3,
and neurotrophin-4/5

23. 23
Structural classification of neurons
1. A multipolar
neuron has multiple
processes extending
away from the cell
body. These are
very common in the
CNS.

24. 24
2. A unipolar
neuron, the
dendrites and axon
are continuous, and
the cell body lies off
to one side. In a
unipolar neuron, the
action potential
begins at the base of
the dendrites and
the rest of the
process is considered
an axon

25. 25
3. Bipolar neurons
have two processes,
one dendrites and
one axon, with the
cell body between
them. Bipolar
neurons are rare but
occur in special sense
organs such as the
eye and the ear.

26. Neurons can also be classified by the direction
that they send information:
• Sensory (or afferent) neurons: send information
from sensory receptors (e.g., in skin, eyes, nose,
tongue, ears) TOWARD the central nervous
system.
• Motor (or efferent) neurons: send information
AWAY from the central nervous system to
muscles or glands.
• Interneurons: send information between
sensory neurons and motor neurons. Most
interneurons are located in the central nervous
system.

27. INTRODUCTION
ABOUT NERVE FIBER
Dr. kanwal Abbasi

28. INTRODUCTION ABOUT NERVE
FIBER
• A nerve fiber is a thread like
extension of a nerve cell and
consists of an axon and
myelin sheath (if present) in
the nervous system.

30. BASIS OF
CLASSIFICATION
DEPENDING
UPON
DISTRIBUTION
DEPENDING
UPON
ORIGIN
DEPENDING
UPON
SECRETION
OF
NEUROTRAN
-SMETTER
DEPENDING
UPON
FUNCTION
ERLANGER
AND
GRASSER’S
CLASSIFICATI
ON
DEPENDING
UPON
STRUCTURE

31. •NERVE FIBERS THOSE
ARE COVERED BY
MYELIN SHEATH
MYELINATED
NERVE FIBERS
•THOSE ARE NOT
COVERED BY MYELIN
SHEATH
UNMYELINATED
NERVE FIBERS
Depending upon STRUCTURE

32. MYELINSHEATH
In peripheral nervous system it is formed by
schwann’s cell. While in case of central nervous system it
is formed by oligodendroglia.
COMPOSITION
PROTEINS
LIPIDS(CHOLESTEROL,
LECITHIN &
SPHINGOMYELIN)

33. •Supply the
skeletal muscles
of the body.
SOMATIC
NERVE FIBERS
•Supply the various
internal organs of
body.
VISCERAL OR
AUTONOMIC
NERVE FIBERS
Depending upon DISTRIBUTION

34. •Arising from
brain.
CARNIAL
NERVE
FIBERS
•Supply the various
internal organs of
body.
SPINAL
NERVE
FIBERS
Depending upon ORIGIN

35. • Afferent nerve fibers
carry sensory impulses
from different part of
body to the CNS
SENSORYNERVE
FIBERS
• Efferent nerve fibers
carry motor impulses
from CNS to different
part of body
MOTOR NERVE
FIBERS
Depending upon Function

36. •Secrete
noradrenaline
ADRENERGIC
NERVE FIBERS
•Secrete
Acetylcholine
CHOLINERGIC
NERVE FIBERS
Depending upon Secretion Of Neurotransmitter

37. ERLANGER AND GRASSER’S CLASSIFICATION
• Erlanger and Grasser studied the
action potential of mixed nerve trunk
by means of cathode ray oscilloscope
and they obtained the compounded
spike. So they divided nerve fibers into
3 groups. They observed that the main
cause of difference in nerve fibers is
diameter

38. • A GROUP
• B GROUP
• C GROUP
GROUPS OF NERVE FIBERS
Depending of Diameter

39. PROPERTIES CORELATED WITH DIAMETER
AS Diameter increases
• Velocity of conduction increases.
• Magnitude of electrical response increases.
• Threshold of excitation decreases.
• Duration of response decreases.
• Refractory period decreases.

40. A GROUP
• A group is composed of largest fibers.
• The fibers of this group are myelinated.
• Both sensory and motor in function.
• It is found in somatic nerves
It is further classified into 4 sub groups.
• Aα (afferent and efferent fibers)
• Aβ (afferent and efferent fibers)
• Aγ (efferent fibers)
• Aδ (afferent fibers)

41. B GROUP
• The fibers of this group are myelinated.
• The B fibers are found solely in preganglionic
autonomic nerve.

42. C GROUP
• It is composed of smallest fibers.
• All the fibers of this group are unmyelinated.
• Mostly found in visceral and cutaneous nerve.
• They have high threshold i.e. 30 folds that of A group.
• Generally they are found in postganglionic
sympathetic nerve.

43. Type Of Nerve Fibers

44. Preganglionic and postganglionic
neurons

45. Properties of Nerve Fibers
1) Excitability:
It is the ability of generating electrochemical
impulse (action potentials) at the cell
membrane in response to any stimulus.
Stimulus*
The stimulus is defined as an external agent
that produce excitabilty in tissues.

46. Types of Stimulus
-Chemical transmitters
- Hormones.
- Drugs.
-Ions (Na+, K+, .... etc).
- Gases (O2 and CO2).
-Thermal.
e.g. cooling
or warming.
- Mechanical.
e.g. stretch, touch,
pressure and injury.
- Electromagnetic.
e.g. light rays
Chemical Physical1 2 Electrical3
- Galvanic Current:
Low intensity
Long Duration
- Faradic Current:
High intensity
Short duration

47. Resting membrane potential
A voltage exists across the plasma membrane
– Due to separation of oppositely charged ions
Potential difference in a resting membrane is
termed its “resting membrane potential”
~ -70 mV in a resting
neuron
Membrane is “polarized”

48. Action Potential Or Nerve Impulus
The synchronized opening and closing of Na+ and
K+ gates result in the movement of electrical
charges that generates a nerve impulse or action
potential.
Action potentials reach the end of each neuron
where these electrical signals are either
transmitted directly to the next cell in the
sequence via gap junctions, or are responsible for
activating the release of specialized
neurotransmitter chemicals

49. A nerve impulse is “all-or-none:” it either goes or
not, and there’s no halfway.
A neuron needs a threshold stimulus, the
minimum level of stimulus needed, to generate
action potential to go and the impulse to travel.
A neuron cannot immediately fire again; it needs
time for the sodium and potassium to return to
their places and everything to return to normal.
This time is called the refractory period.

52. Conductivity
• Conductivity:
• It is the ability to propagate action potential
from the point of generation to the rest of the
membrane.

53. The Nerve Impulse
• The myelin sheath of axons are interrupted
by short unmyelinated sections called
nodes of Ranvier.
• At each node of Ranvier, the action
potential is regenerated by a chain of
positively charged ion pushed along by the
previous segment.

54. The Nerve Impulse
• Saltatory conduction is the word used to
describe this “jumping” of the action
potential from node to node.
– Provides rapid conduction of impulses
– Conserves energy for the cell
• Multiple sclerosis is disease in which the
myelin sheath is destroyed and associated
with poor muscle coordination.

55. Fig. 2-19, p. 46

57. DEGENERATION OF NERVE FIBERS
When a peripheral nerve is injured, the
degenerative changes occurs in the nerve cell
body and in the nerve fibres.
Degenerative Changes are Classified in to Three
Types.
Wallarian Degeneration
Retrograde Degeneration
Transneural Degeneration

58. Wallerian Degeneration
The degenerative changes in the distal cut end of nerve fiber ( AXON) is known as
Wallerian degeneration or orthograde degeneration
1) In the part of nerve fiber distal to injury, the degenerative changes occur
within 24 hours of injury.
2) Axis cylinder swells and neurofibrils and axis cylinder breaks up into small
pieces. After few days the broken pieces appear as debris in the space
occupied by axis cylinder.
3) The myelin sheath is slowly disintegrated into fat droplets. The changes in
myelin sheath occur from 8th to 35th day.
4) The region is invaded by macrophages that remove degenerating axons,
myelin and cellular debris. These macrophages probably secrete substances
that causes proliferation of Schwann cells and also produce nerve growth
factors. All these changes takes place for about 2 months from the day of
injury. The schwann cells of distal side increase in size and proliferate to form
series of tubes. When one of the regenerating axonal branch succeeds in
reaching tube, it enters and grows rapidly inside it

61. Retrograde Degeneration
The Retrograde degeneration changes in
the nerve cell body and part of axon
attached to nerve cell body, axon proximal
to the cut are together known as
RETROGRADE DEGENERATION .

62. Transneuronal Degeneration
The degenerative changes occur in the neuron
with which the afferent nerve fiber synapses it is
called transneuronal degeneration

63. Regeneration Of Nerve Fiber
The injured and degenerated nerve fiber can
regenerated, but regeneration is possible only
If degenerated nerve fiber meets with following
criteria
1)The between the cut ends of the nerve should
not exceed 3mm
2) The neurilemma should be present
3) The nucleus must be intact
4) The cut ends should remain in same line

64. NEUROGLIA
“Nerve glue”
Six types of small cells associated with neurons
– 4 in CNS
– 2 in PNS
Most have central cell body and branching
processes
Several functions
– e.g., Supportive scaffolding for neurons
– e.g., Electrical isolation of neurons
– e.g., Neuron health and growth

65. CNS NEUROGLIA
Astrocytes
Microglia
Ependymal cells
Oligodendrocytes

66. CNS NEUROGLIA
Astrocytes
Most abundant and versatile glial cells
Numerous processes support branching
neurons
– Anchor neurons to capillary blood supply
Guide migration of young neurons
Facilitate nutrient delivery to neurons
– (blood  glial cell  neuron)
Control chemical environment
around neurons
– Uptake of K+, neurotransmitters
Communicate with astrocytes
& neurons
– Gap junctions

67. CNS NEUROGLIA
Microglia
Small ovoid cells
Relatively long “thorny”
processes
– Processes touch nearby neurons
Migrate toward injured neurons
Transform into macrophage
– Phagocytize microorganisms, debris
– (Cells of immune system cannot enter the CNS)

68. CNS NEUROGLIA
Ependymal Cells
Line central cavities of brain and spinal cord
– Form permeable barrier between cerebrospinal fluid inside
these cavities and tissue fluid of CNS tissue
Shapes range from squamous to columnar
Many are ciliated
– Beating helps circulate cerebrospinal fluid cushioning brain
and spinal cord

69. CNS NEUROGLIA
Oligodendrocytes
Fewer processes than astrocytes
Wrap processes tightly around thicker neuron
fibers in CNS
– “Myelin sheath”
– Insulating covering

70. PNS NEUROGLIA
Satellite cells
Schwann cells

71. PNS NEUROGLIA
Satellite cells
Surround neuron cell bodies within ganglia
– (A ganglion is a collection of nerve cell bodies
outside of the CNS)
Function poorly understood

72. PNS NEUROGLIA
Schwann cells
 “Neurolemmocytes”
Surround and form myelin sheaths around larger nerve
fibers of PNS
– Functionally similar to oligodendrocytes
Vital to regeneration of peripheral nerve fibers