The document discusses the structure and function of the central nervous system, including the brain and spinal cord. It describes the layers of gray matter and white matter in the brain and spinal cord, as well as the three layers of meninges - dura mater, arachnoid mater, and pia mater - that surround and protect the brain and spinal cord. Neuroglial cells that support neurons in the central nervous system are also classified and their functions explained.

1. UNIT I
SANTHI KRUPA DASARI
ASSISTANT PROFESSOR
KRISHNA UNIVERSITY COLLEGE OF PHARMACEUTICAL SCIENCES AND RESEARCH

3.  Central nervous system (CNS) includes brain
and spinal cord. It is formed by neurons and
supporting cells called neuroglia.
 Structures of brain and spinal cord are
arranged in two layers, namely gray matter
and white matter.
 Gray matter is formed by nerve cell bodies
and the proximal parts of nerve fibers,
arising from nerve cell body. White matter is
formed by remaining parts of nerve fibers.

4.  In brain, white matter is placed in the inner part
and gray matter is placed in the outer part. In
spinal cord, white matter is in the outer part and
gray matter is in the inner part.
 Brain is situated in the skull. It is continued as
spinal cord in the vertebral canal through the
foramen magnum of the skull bone.
 Brain and spinal cord are surrounded by three
layers of meninges called the outer dura mater,
middle arachnoid mater and inner pia mater.
 The space between arachnoid mater and pia
mater is known as subarachnoid space. This
space is filled with a fluid called cerebrospinal
fluid. Brain and spinal cord are actually
suspended in the cerebrospinal fluid

10.  Neuroglia or glia (glia = glue) is the
supporting cell of the nervous system.
Neuroglial cells are non-excitable and do not
transmit nerve impulse (action potential).

11.  So, these cells are also called non-neural cells or
glial cells. When compared to the number of
neurons, the number of glial cells is 10 to 15 times
greater.
 CLASSIFICATION OF NEUROGLIAL CELLS
 Neuroglial cells are distributed in central nervous
system (CNS) as well as peripheral nervous system
(PNS).
 Accordingly the neuroglial cells are classified into
two types:
 A. Central neuroglial cells B. Peripheral neuroglial
cells. „

12.  1. Astrocytes
2. Microglia
 3. Oligodendrocytes. „
ASTROCYTES
Astrocytes are star-shaped neuroglial cells present in all
the parts of the brain. Two types of astrocytes are found
in human brain:
i. Fibrous astrocytes ii. Protoplasmic astrocytes.
Fibrous astrocytes occupy mainly the white matter.
Protoplasmic astrocytes are present mainly in gray matter.

15.  Excitability
 Conductivity „
 Refractory period „„
 Summation „
 adaptation „
 Infatigability „
 All-or-none law

16.  Excitability is defined as the physiochemical change that occurs
in a tissue when stimulus is applied.
 Stimulus is defined as an external agent, which produces
excitability in the tissues. Different types of stimulus, qualities
of stimulus and strength-duration curve.
 When a nerve fiber is stimulated, based on the strength of
stimulus, two types of response develop:
 1. Action potential or nerve impulse
 Action potential develops in a nerve fiber when it is stimulated
by a stimulus with adequate strength. Adequate strength of
stimulus, necessary for producing taction potential in a nerve
fiber is known as threshold or minimal stimulus. Action potential
is propagated.
 2. Electrotonic potential or local potential
 When the stimulus with subliminal strength is applied, only
electrotonic potential develops and the action potential does not
develop. Electrotonic potential is nonpropagated

17.  Resting membrane potential in
the nerve fiber is –70 mV.
 The firing level is at –55 mV.
Depolarization ends at +35 mV.
Usually, the action potential
starts in the initial segment of
nerve fiber

18.  Conductivity is the ability of nerve fibers to transmit
the impulse from the area of stimulation to the other
areas.
 Action potential is transmitted through the nerve fiber
as nerve impulse.
 Normally in the body, the action potential is transmitted
through the nerve fiber in only one direction. However,
in experimental conditions when, the nerve is
stimulated, the action potential travels through the
nerve fiber in either direction. „
 MECHANISM OF CONDUCTION OF ACTION POTENTIAL
Depolarization occurs first at the site of stimulation in
the nerve fiber. It causes depolarization of the
neighboring nodes.

19.  So, the action potential jumps from one node to another. Hence, it is
called saltatory conduction (saltare = jumping).
]
„
REFRACTORY PERIOD
 Refractory period is the period at which the nerve does not give any
response to a stimulus.
 „
TYPES OF REFRACTORY PERIOD
 Refractory period is of two types:
 1. Absolute Refractory Period
 Absolute refractory period is the period during which the nerve does not
show any response at all, whatever may be the strength of stimulus.
 Absolute refractory period corresponds to the period from the time when
firing level is reached till the time when one third of repolarization is
completed.
 2. Relative Refractory Period
 It is the period, during which the nerve fiber shows response, if the
strength of stimulus is increased to maximum.

20. SUMMATION
 When one subliminal stimulus is applied, it does not produce any
response in the nerve fiber because, the subliminal stimulus is very
weak. However, if two or more subliminal stimuli are applied within
a short interval of about 0.5 millisecond, the response is produced.
It is because the subliminal stimuli are summed up together to
become strong enough to produce the response. This phenomenon
is known as summation. „
ADAPTATION
 While stimulating a nerve fiber continuously, the excitability of the
nerve fiber is greater in the beginning. Later the response
decreases slowly and finally the nerve fiber does not show any
response at all. This phenomenon is known as adaptation or
accommodation.
 Cause for Adaptation
 When a nerve fiber is stimulated continuously, depolarization
occurs continuously. Continuous depolarization inactivates the
sodium pump and increases the efflux of potassium ions.

21.  „
INFATIGABILITY
 Nerve fiber cannot be fatigued, even if it is
stimulated continuously for a long time. The
reason is that nerve fiber can conduct only
one action potential at a time. At that time,
it is completely refractory and does not
conduct another action potential. „
 ALL-OR-NONE LAW
 All-or-none law states that when a nerve is
stimulated by a stimulus it gives maximum
response or does not give response at all

22.  Receptors are sensory (afferent) nerve
endings that terminate in periphery as bare
unmyelinated endings or in the form of
specialized capsulated structures. Receptors
give response to the stimulus. When
stimulated, receptors produce a series of
impulses, which are transmitted through the
afferent nerves

24.  Synapse is the junction between two
neurons. It is not an anatomical
continuation. But, it is only a physiological
continuity between two nerve cells. „
CLASSIFICATION OF SYNAPSE
 Synapse is classified by two methods:
 A. Anatomical classification
 B. Functional classification

26.  Depending upon ending of axon, synapse is
classified into three types:
 1. Axoaxonic synapse in which axon of one
neuron terminates on axon of another
neuron
 2. Axodendritic synapse in which the axon
of one neuron terminates on dendrite of
another neuron
 3. Axosomatic synapse in which axon of
one neuron ends on soma (cell body) of
another neuron

28.  Functional classification of synapse is on the
basis of mode of impulse transmission.
 According to this, synapse is classified into
two categories:
 1. Electrical synapse 2. Chemical synapse.
 However, generally the word synapse refers
to a chemical synapse.

29.  Chemical synapse is the junction between a
nerve fiber and a muscle fiber or between
two nerve fibers, through which the signals
are transmitted by the release of chemical
transmitter.
 In the chemical synapse, there is no
continuity between the two neurons because
of the presence of a space called synaptic
cleft between the two neurons.
 Action potential reaching the presynaptic
terminal causes release of neurotransmitter
substance from the vesicles of this terminal.
Neurotransmitter reaches the postsynaptic
neuron through synaptic cleft and causes the
production of potential change

33.  Spinal cord is covered by sheaths called
meninges, which are membranous in nature.
Meninges are dura mater, pia mater and
arachnoid mater.
 These coverings continue as coverings of
brain. Meninges are responsible for
protection and nourishment of the nervous
tissues.

36. The meninges envelop the brain and spinal cord and
separate them from the walls of their bony cases
(skull and vertebral column).
Based on their location, meninges are referred to as
the cranial meninges which envelop the brain,
and spinal meninges which envelop the spinal cord.
However, the cranial and spinal meninges are
continuous with each other and consist of the same
three meningeal layers.

37.  These layers bound three clinically important
potential spaces: the epidural, subdural,
and subarachnoid spaces.
 The function of the meninges is to protect
the brain and spinal cord from mechanical
trauma, to support the blood vessels and to
form a continuous cavity through which
the cerebrospinal fluid (CSF) passes.
 Specifically, the CSF passes between the
inner two meningeal layers (arachnoid and
pia) which are together called
the leptomeninges.