NERVOUS SYSTEM

Dr. Asmaa Seddek

Lecturer of Physiology
FOM/SCU
 Nervous system
Anatomical classification Functional classification

Central Somatic Sensory

Nervous Brain
Nervous Integrative
System System
Spinal Motor
(CNS) (voluntary)
Cord
Spinal Autonomic
Peripheral Sympathetic
Nerves Nervous
Nervous
System
System Cranial Para-
(ANS) sympathetic
(PNS) Nerves
(Involuntary)
 Neuron (Nerve cells)

 Neuron or nerve cell is defined as the structural and

functional unit of nervous system.
 Neuron is similar to any other cell in the body,
having nucleus and all the organelles in cytoplasm.
However, it is different from other cells by two ways:
1. Neuron has branches or processes called axon and
dendrites.
2. Neuron does not have centrosome. So, it cannot
undergo division.
On the basis of function, nerve cells are
classified into two types:
1. Sensory or afferent neurons: carry the
sensory impulses from periphery to central
nervous system (CNS).
2. Motor or efferent neurons: carry the motor
impulses from central nervous system (CNS)to
peripheral effector organs like muscles, glands,
blood vessels
 Synapse
 Synapse is the junction between two neurons. It is not an
anatomical continuation. But, it is only a physiological
continuity between two nerve cells.
 Synapse is classified by two methods:
A. Anatomical classification:
1. Axoaxonic synapse in which
axon of presynaptic neuron ends
on axon of postsynaptic neuron.
2. Axodendritic synapse in which the
axon of presynaptic neuron ends
on dendrite of postsynaptic neuron
3. Axosomatic synapse in which axon
of presynaptic neuron ends on soma
(cell body) of postsynaptic neuron.
B. Functional classification:
Based on mode of impulse
transmission, synapse is classified
into:
1. Electrical synapse:
The physiological continuity
between the presynaptic and the
postsynaptic neurons is provided by
gap junction between the two
neurons, which allows direct
exchange of ions between the two
neurons
Example is cardiac muscle fibers,
smooth muscle fibers of intestine
2. Chemical synapse: 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.
Mechanism of impulse transmission in
chemical synapse:
a. Arrival of impulse to the presynaptic
neuron, stimulates the release of
chemical neurotransmitter in the
neurotransmitter
synaptic cleft (= space between pre-
and post- synaptic neurons).
b. The neurotransmitter travel through
the synaptic cleft to the post-synaptic
neuron, where it binds to specific
receptors stimulating generation of
(=impulse) in the post-synaptic neuron.
 Central Nervous System (CNS)
 CNS is formed by neurons and
supporting cells called neuroglia.
 Structures of brain and spinal cord
are arranged in two layers:
1) Gray matter is formed by nerve
cell bodies and the proximal parts of
nerve fibers, arising from nerve cell
body.
2) White matter is formed by
remaining parts of nerve fibers.
 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 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.
Brain and spinal cord are actually suspended
in the cerebrospinal fluid (CSF), which is filling
the subarachnoid space.
 Function of brain meninges

• Protect your CNS (central nervous system)

from trauma injury to your brain , by acting as
a shock absorber. They keep your brain from
moving around within your skull.
• Provide a support system for blood vessels
(including your middle meningeal artery) that
deliver blood to your CNS tissue.
 Function of CSF
• Buoyancy(floating) : The actual mass of the human brain is about
1400–1500 grams; however, the net weight of the brain suspended in
CSF equals 25-50 grams. The brain therefore exists in neutral buoyancy
which allows the brain to maintain its density without being impaired by
its own weight, which would cut off blood supply and kill neurons in the
lower sections .
• Protection: CSF protects the brain tissue from injury , by providing a
fluid buffer that acts as a shock absorber from some forms of mechanical
injury.
• Prevention of brain ischemia: by decreasing the amount of CSF in the
limited space inside the skull. This decreases total intracranial
pressure and facilitates blood perfusion.
• Regulation: CSF allows for the homeostatic regulation of the
distribution of substances between cells of the brain,
• Clearing waste: CSF allows for the removal of waste products from the
brain.
 Brain
Cerebral
cerebellum Brain Stem Others
Cortex

Midbrain Thalamus

Pons Hypo-
thalamus
Medulla Basal
Oblongata Ganglia
 Spinal Cord
Spinal cord lies loosely in the vertebral canal.
It extends from foramen magnum, as a
continuation of medulla oblongata
 Functions of CNS

Three major levels of the CNS have specific

functional characteristics:
(1) the spinal cord level,
(2) the lower brain or subcortical level (it includes
medulla, pons, hypothalamus, thalamus,
cerebellum, and basal ganglia),
(3) the higher brain or cortical level.
 (1) The spinal cord level:

 Spinal cord has neuronal circuits which can cause:

(1) Walking movements,
(2) Reflexes that withdraw portions of the body from
painful objects,
(3) Reflexes that stiffen the legs to support the body
against gravity, and
(4) Reflexes that control local blood vessels,
gastrointestinal movements, or urinary excretion.
(2) The lower brain or subcortical level:
 It is responsible for control of subconscious
activities, including:
1) subconscious control of arterial pressure and
respiration is achieved mainly in the medulla and
pons.
2) Control of equilibrium is a combined function of
the cerebellum and the medulla, and pons.
3) Feeding reflexes, such as salivation and licking of
the lips in response to the taste of food, are
controlled by areas in the medulla, pons, and
hypothalamus.
4) Many emotional patterns such as anger, reaction
to pain, and reaction to pleasure.
 (3) The higher brain or cortical level. :
The cerebral cortex is an extremely large memory
storehouse.
The cortex never functions alone but always in
association with lower centers of the nervous
system.
Without the cerebral cortex, the functions of the
lower brain centers are often inaccurate
The storehouse of cortical information converts
the functions of lower brain centers into
determinative and accurate operations.
• Determining intelligence
• Determining personality
• Motor function
• Planning and organization
• Touch sensation
• Processing sensory information
• Language processing
 Peripheral nervous system (PNS)
 PNS is formed by neurons and their processes
present in all regions of the body.
 It consists of cranial nerves (arising from brain)
and spinal nerves (arising from the spinal cord). It
is again divided into two subdivisions:
1. Somatic nervous system: concerned with
somatic functions (somatic sensation& motor
functions).
2. Autonomic nervous system: concerned with
regulation of visceral functions (involuntary), it
consists of Sympathetic and Parasympathetic
nervous systems .
Reflex Activity
 Reflex activity is the response of the peripheral
nervous system stimulation that occurs without
our consciousness.
 For example,
1. when hand is placed on a hot object, it is
withdrawn immediately.
2. When a bright light is thrown into the eyes,
eyelids are closed and pupil is constricted to
prevent the damage of retina by entrance of
excessive light into the eyes.
 REFLEX ARC
• Reflex arc is the anatomical nervous pathway
for a reflex action.

Stimulus
1. Stimulus
thing or event that evokes a specific functional reaction in an organ or
tissue.
2. Receptor
Receptor is the end organ, which receives the stimulus. When receptor is
stimulated, impulses are generated in afferent nerve.
3. Afferent Nerve
Afferent or sensory nerve transmits sensory impulses from the receptor
to center.
4. Center
Center receives the sensory impulses via afferent nerve fibers and in turn,
it generates appropriate motor impulses. Center is located in the brain or
spinal cord.
5. Efferent Nerve
Efferent or motor nerve transmits motor impulses from the center to the
effector organ.
6. Effector Organ
Effector organ is the structure such as muscle or gland where the activity
occurs in response to stimulus.
Autonomic nervous system
 Autonomic nervous system
(ANS) is primarily concerned with regulation of
involuntary functions of the body. So, it is also
called involuntary nervous system.
DIVISIONS OF ANS:
From anatomical and physiological point of view,
ANS is divided into :
1. Sympathetic division
2. Parasympathetic division.
3. Enteric (visceral)
 FUNCTIONS OF ANS
Autonomic nervous system is concerned with the
regulation of involuntary functions. Thus, ANS plays an
important role in maintaining constant internal
environment (homeostasis).
Almost all the visceral organs are supplied by both
sympathetic and parasympathetic divisions of ANS and
the two divisions produce antagonistic effects on each
organ.
When the fibers of one division supplying to an organ is
sectioned or affected by lesion, the effects of fibers from
other division on the organ become more prominent.
 AUTONOMIC GANGLIA
Ganglia = a structure containing a number of nerve
cell bodies, typically linked by synapses.
Autonomic Ganglia are classified into three
groups:(according to location)
Paravertebral or sympathetic chain ganglia:
Ganglia on either side of the spinal cord are
connected with each other by longitudinal fibers, to
form the sympathetic chains.
Ganglia of the sympathetic chain on each side are
divided into four groups:
8 Cervical ganglia b. 12 Thoracic ganglia
c. 5 Lumbar ganglia d. 5 Sacral ganglia
B. Prevertebral or collateral ganglia:
Prevertebral ganglia are situated in thorax,
abdomen and pelvis, in relation to aorta and its
branches. Prevertebral ganglia are:
1. Celiac ganglion 2. Superior mesenteric ganglion
3. Inferior mesenteric ganglion.
C. Terminal or peripheral ganglia:
Terminal ganglia are mainly in the parasympathetic
division, and situated within or close to structures
innervated by them. Heart, bronchi, pancreas and
urinary bladder are innervated by the terminal
ganglia.
 Sympathetic Nervous System (SNS)
Origin: Sympathetic division has thoracolumbar
outflow because, the preganglionic neurons are
situated in lateral horns of 12 thoracic and first
two lumbar segments of spinal cord.
Supply: Sympathetic division supplies smooth
muscle fibers of all the visceral organs such as
blood vessels, heart, lungs, glands,
gastrointestinal organs, etc.
 Anatomical •
origin of
Sympathetic
nervous system
Thoracolumbar •
(T1–L2)
PARASYMPATHETIC NERVOUS SYSTEM
Origin: craniosacral outflow because, the fibers of this
division arise from brain and sacral segments of spinal
cord.
A- Cranial outflow: It innervates the blood vessels of
head and neck and many thoraco-abdominal visceral
organs. It includes:
1. Tectal or midbrain outflow (its pre-ganglionic fibers
run in 3rd cranial nerve “oculomotor”)
2. Bulbar or medulla oblongata outflow (its pre-
ganglionic fibers run in 7th “ facial”, 9th
“glossopharyngeal”, and 10th “vagus” cranial nerves).
B. Sacral outflow:
It innervates smooth muscles forming the walls of
viscera and the glands such as large intestine,
liver, spleen, kidneys, bladder, genitalia, etc.
Its preganglionic fibers arise from 2nd, 3rd, and 4th
sacral segments of spinal cord (S2, S3, S4),and run
as pelvic nerves .
Anatomical origin •
of parasympathetic
nervous system
(Craniosaccral)
Cranial nerves
(III, VII, IX, and X)
S2–S4
 Regulation Of Autonomic Nervous
System Activity
The nervous activity of the ANS is regulated by
several regions in CNS in particular by the
hypothalamus and the brainstem.
Many important variables in the body are
monitored and regulated in the hypothalamus
and the brainstem including heart rate, blood
pressure, gastrointestinal movements and
glandular secretion, body temperature, hunger,
thirst, plasma volume, and plasma osmolality.
 Two Important Features Of Both
Systems
1. Many tissues are innervated by both systems
(dual innervation)
Exception: Vascular smooth muscle and sweat
glands are regulated by the sympathetic system
only
2. Both systems are tonically active. In other
words, they provide some degree of
nervous input to a given tissue at all times.
 Each System Is Dominant Under
Certain Conditions
The sympathetic system predominates during
emergency fight-or-flight reactions and during exercise.
• The overall effect of the sympathetic system under these conditions
is to prepare the body for strenuous physical activity. More
specifically, sympathetic nervous activity will increase the flow of
blood that is well-oxygenated and rich in nutrients to the tissues
that need it, in particular, the working skeletal muscles.
The parasympathetic system predominates during
quiet, resting conditions.
• The overall effect of the parasympathetic system under these
conditions is to conserve and store energy and to regulate basic
body functions such as digestion and urination (rest and digest).
 3. Enteric nervous system
• Organized in to ganglia interconnected by 2
plexuses :
• Outer Myentreic (Auerbach’s ) plexus found
along the entire length of the GIT.
Controls GIT movement
• Inner sub mucosal (meissner’s) plexus found
only in the small and large intestine.
Controls GIT secretions and blood flow
Enteric nervous system
Thank you