Download The Nervous System

The Nervous System
The nervous system is the master
controlling and communicating
system of the body. Every thought,
action, and emotion reflects its
activity. Its cells communicate by
electrical and chemical signals,
which are rapid and specific, and
usually cause almost immediate
responses.
Functions of the Nervous System
 Sensory input – gathering information
 To monitor changes occurring inside and
outside the body.
 Changes = stimuli
 Integration
 To process and interpret sensory input and
decide if action is needed.
 Motor output
 A response to integrated stimuli
 The response activates muscles or glands
For example, when you are driving and see a
red light ahead (sensory input), your nervous
system integrates this information (red light
means “stop”), and your foot goes for the
brake (motor output).
Structural Classification of the
Nervous System
 Central nervous system (CNS)
 Brain
 Spinal cord
 Peripheral nervous system (PNS)
 Cranial nerves
 Spinal nerves
Functional Classification of the PNS
It is divided into TWO subdivisions:
1-Sensory (afferent) division
 Nerve fibers that carry information to the
central nervous system from:
- sensory receptors in the skin, skeletal
musclesand joints (somatic sensory fibers).
- Sensory receptors in the visceral organs
(visceral sensory fibers)
Figure 7.1
2-Motor (efferent) division
 Nerve fibers that carry impulses away from the
central nervous system ( to Muscles
&Glands).These impulses effect (bring about) a
motor response.
It has two subdivisions
1-Somatic nervous system = voluntary, it controls
skeletal muscles
2-Autonomic nervous system= involuntary,
it controls smooth &cardiac muscles &glands
This also is divided into
sympathetic &
parasympathetic
Organization of the Nervous System
Figure 7.2
Histology of Nervous Tissue
• Despite the complexity of the
nervous system, there are only two
functional cell types
• Neurons - excitable nerve cells that
transmit electrical signals
• Neuroglia (glial) cells - supporting
cells
Neuroglia cells - 4 types in the Central NS
1-Astrocytes
• star shaped with many processes
• connect to neurons; help anchor them to nearby blood capillaries
• control the chemical environment of the neurons
2-Microglia
•
- oval with thorny projections
- monitor the health of neurons
- if infection occurs, they change into macrophages (eating viruses,
bacteria and damaged cells)
3-Ependymal cells
• range in shape from squamous to columnar; many are ciliated
• line the dorsal body cavity housing the brain and spinal cord
• form a barrier between the neurons and the rest of the body
4-Oligodendrocytes
- have few processes
- wrap themselves around axons
- form the myelin sheath – an insulating membrane
Neuroglia cells - 2 types in the Peripheral NS
Satellite cells
- surround neuron cell bodies in the periphery
- Protective , cushioning cells
Schwann cells (neurolemmocytes)
- are vital to regeneration of damaged nerve fibers.
- adjacent Schwann cells along an axon do not touch one
another, so there are gaps in the sheath. These gaps, called
nodes of Ranvier occur at regular intervals (about 1 mm
apart) along the myelinated axon
and form the myelin sheath around larger nerve fibers in the
periphery
- it acts as an insulators.
Neuron (Nerve cell)
-The Cells are specialized
to transmit messages
-Differ structurally but
Have common features:
 A Cell body with nucleus
and the usual organelles,
Except centrioles
 One or more processes
Figure 7.4a
Neuron Anatomy
 Extensions
outside the cell
body
 Dendrites –
conduct impulses
toward the cell
body
 Axons – conduct
impulses away
from the cell body
Figure 7.4a
Axons and Nerve Impulses
 Axons end in axonal terminals
 Axonal terminals contain vesicles
with neurotransmitters
 Axonal terminals are separated
from the next neuron
(neuroneural ) junction or the
muscle (neuromuscular) junction
by a gap called Synaptic cleft
(Synapse).
Nerve Fiber Coverings
- Most long nerve fibers are
covered with a whitish, fatty
material called Myelin with waxy
appearance. It insulates the fiber
&Increases transmission rate
- Axons outside CNS are wrapped
by Schwann Cells.
Figure 7.5
Neuron Cell Body Location
 Most are found in the central nervous system
 Gray matter – cell bodies and unmyelinated
fibers
 Nuclei – clusters of cell bodies within the
white matter of the central nervous system
 Ganglia – collections of cell bodies outside
the central nervous system
 White matter- collection of myelinated fibers
(Tracts) in the CNS.
 Fibers outside the CNS are called nerves.
Functional Classification of Neurons
1-Sensory (afferent) neurons
Carry impulses from the sensory receptors
 Cutaneous sense organs
 Proprioceptors – detect stretch or tension in
muscles and tendons and joints
2-Motor (efferent) neurons that carry impulses from the
central nervous system to muscles and glands ,their cell
bodies are always in CNS.
3-Interneurons (association neurons)
 Their cell bodies are always found in CNS.
 Connect sensory and motor neurons in neural
pathways.
Neuron Classification
Figure 7.6
Structural Classification of Neurons
 Multipolar neurons – many extensions
from the cell body.
Figure 7.8a
 Bipolar neurons – one axon and one
dendrite
Figure 7.8b
• Unipolar neurons – have a short single
process leaving the cell body which is very
short ,divides almost immediatly
Figure 7.8c
Functional Properties of Neurons
 Irritability – ability to respond to stimuli.
 Conductivity – ability to transmit an
impulse.
 The plasma membrane at rest is
polarized i.e.,Fewer positive ions are inside
the cell than outside the cell
Starting a Nerve Impulse
• a- resting membrane electrical condition.
The external face of the membrane is slightly
positive, its internal face is slightly negative.
The chief extracellular ion is sodium wheras
the chief intracellular ion is potassium. The
membranr is relatively impermeable to both
ions.
• b- Stimulus initiates local depolarization by
changing permeability to sodium which rush
inside the cell changing polarity of the
membrane so the inside becomes more
positive, the outside become more negative at
that site.
• c- Depolarization and generation of an action
potential. If the stimulus is strong enough ,
depolarization causes membrane polarity to be
completely reversed and an action potential is
initiated.
• d- Propagation of action potential.
Depolarization of the first membrane patch causes
permeability changes in the adjacent membrane
and the events described in(b) are repeated. Thus
the action potential propagates rapidly along the
entire length of the membrane.
• e- Repolarization.potassium ions diffuse out of
the cell restoring the negative charge on the
inside of the membrane and positive charge on the
outside surface.repolarization occurs in the same
direction as depolarization.
• f- Initial ionic condition restored by the sodiumpotassium pump . Three sodium ions are ejected
for every two potassium ions carried back into the
cell
Nerve Impulse Propagation
 The nerve impulse is is an
all-or-none response, like
firing a gun. It is either
propagated over the entire
axon ,or it does not happen
at all.
 Impulses travel faster when
fibers have a myelin
sheath(saltatory conduction).
 Until repolarization occur,a
neuron can not conduct
another impulse.
Figure 7.9c–e
• HOMEOSTATIC IMBALANCE
• 1-A number of chemical and physical factors impair
impulse propagation. Sedatives and anesthetics block nerve
impulses by altering membrane permeability to sodium. As
we have seen, no Na+ entry—no AP.
• 2-Cold and continuous pressure interrupt blood circulation
(and hence the delivery of oxygen and nutrients) to neuron
processes, impairing their ability to conduct impulses. For
example, your fingers get numb when you hold an ice cube
for more than a few seconds, and your foot “goes to sleep”
when you sit on it. When you remove the cold object or
pressure, impulses are transmitted again, leading to an
unpleasant prickly feeling.
Continuation of the Nerve Impulse
between Neurons
 Impulses are able to cross the synapse to another
nerve by:
 Neurotransmitter is released from a nerve’s axon terminal
 The dendrite of the next neuron has receptors that are
stimulated by the neurotransmitter
 The response is very brief because the neurotransmitter is
quickly removed either by reuptake by the axonal trminal or
by enzymatic breakdown. This limits the period to less than
the blink of an eye.
 An action potential is started in the dendrite of the next
neuron propagating to cell body and its axon.
 Notice : impulse transmission is an electrochemical
How Neurons Communicate at
Synapses
Figure 7.10
The Reflex Arc
 Reflexes are rapid, predictable, and
involuntary responses to stimuli
 Reflex arc follows a direct route from a
sensory neuron, to an interneuron,then
to an effector neuron.
 Reflex arc have a minimum 5 elements
Simple Reflex Arc
Figure 7.11b, c
Types of Reflexes
One classification:
- Autonomic reflexes eg.
 Salivary gland secretion
 Heart and blood pressure regulation
 Changes in size of the pupil
 Digestive system regulation
- Somatic reflexes
 Activation of skeletal muscles
Other classification:
- Spinal reflexes ,involve spinal cord as the flexor reflex
-Cranial reflexes requires the brain as light reflex.
Importance of REFEXES
• Exaggerated, Distorted or Absent
indicate nervous system disorder.
• Reflex changes often occur before
the pathological condition become
obvious.
Central Nervous System (CNS)
 CNS develops from the embryonic
neural tube.
 By the fourth week the anterior end begins
to expand and brain formation begins, The
rest of the tube becomes the spinal cord.
 The central canal becomes enlarged in 4
regions of the brain to form the ventricles
which are:
-Four chambers within the brain.
-Filled with cerebrospinal fluid(CSF).
Regions of the Brain
 Cerebral
hemispheres
 Diencephalon
 Brain stem
 Cerebellum
Figure 7.12
• The cerebral hemispheres form the superior
part of the brain. Together they account for
about 83% of total brain mass.
• Picture how a mushroom cap covers the top of
its stalk, and you have a fairly good idea of
how the paired cerebral hemispheres cover and
obscure the diencephalon and the top of the
brain stem .
Cerebral Hemispheres (Cerebrum)
Figure 7.13a
• Nearly the entire surface of the cerebral hemispheres is
marked by elevated ridges of tissue called gyri (ji′ri;
“twisters”), separated by shallow grooves called sulci
(sul′ki; “furrows”). The singular forms of these terms
are gyrus and sulcus. Deeper grooves, called fissures,
separate large regions of the brain. The more
prominent gyri and sulci are similar in all people and
are important anatomical landmarks. The median
longitudinal fissure separates the cerebral
hemispheres .Another large fissure, the transverse
cerebral fissure, separates the cerebral hemispheres
from the cerebellum below
• Several sulci divide each hemisphere into
four lobes— frontal, parietal,
temporal,and occipital. The central
sulcus, which lies in the frontal plane,
separates the frontal lobe from the
parietal lobe. Bordering the central sulcus
are the precentral gyrus anteriorly and
the postcentral gyrus posteriorly. More
posteriorly, the occipital lobe is separated
from the parietal lobe by the parietooccipital sulcus (pah-ri″ĕ-to-ok-sip′ĭ-tal).
Layers of the Cerebrum
 Gray matter
 Outer layer
 Composed mostly of
cell bodies of the
neurons
Figure 7.13a
White matter
 Fiber
tracts
inside the
gray
matter
 Example:
corpus
callosum
connects
between
the two
hemisphe
res.
Figure 7.13a
Basal nuclei , or basal ganglia –islands of
gray matter buried deep within the white
matter of the cerebral hemispheres, They
help regulate voluntary motor activities in
relation to starting or stopping movements
sent to skeletal muscles by the primary
motor cortex. Disorders of the basal nuclei
result in either too much or too little movement
as exemplified by Huntington’s chorea and
Parkinson’s disease, respectively.
Specialized Area of the Cerebrum
 Gustatory area (taste)
 Visual area
 Auditory area
 Olfactory area
 Speech/language region
 Language comprehension region
Specialized Area of the Cerebrum
Figure 7.13c
Diencephalon
 Sits on top of the brain stem
 Enclosed by the cerebral heispheres
 Made of three parts
 Thalamus
 Hypothalamus
 Epithalamus
Diencephalon
Figure 7.15
Thalamus
 Surrounds the third ventricle
 The relay station for sensory impulses
(except olfaction)
 Transfers impulses to the correct part of
the cortex for localization and
interpretation
Hypothalamus
 Under the thalamus
 Important autonomic nervous system center
 Helps regulate body temperature
 Controls water balance
 Regulates metabolism
 An important part of the limbic system
(emotions),as thirst , appetite , sex, pain and
pleasure centers.
 The pituitary gland hangs from the anterior floor
of the hypothalamus by a slender stalk.
Epithalamus
 Forms the roof of the third ventricle
 Important parts are:
- pineal body (an endocrine gland). and
- the choroid plexus : knots of capillaries
withen each ventricle, forms the
cerebrospinal fluid (CSF).
Brain Stem
 Attaches to the spinal cord
 Parts of the brain stem are:
 Midbrain
 Pons
 Medulla oblongata
Brain Stem
Figure 7.15a
Midbrain
 Mostly composed of tracts of nerve
fibers
 Anteriorly , it has two bulging fiber tracts
– the cerebral peduncles which convey
ascending and descending impulses.
 Dorsally are four rounded protrusions –
corpora quadrigemina which are reflex
centers involved with vision and
hearing.
Pons
 The bulging center part of the brain
stem.
 Mostly composed of fiber tracts.
 Includes nuclei involved in the control of
breathing.
Medulla Oblongata




The lowest part of the brain stem
Merges into the spinal cord.
Includes important fiber tracts.
Contains important control centers
 Heart rate control
 Blood pressure regulation
 Breathing
 Swallowing
 Vomiting
-The fourth ventricle lies posterior to the pons
and medulla and anterior to the cerebellum.
Reticular Formation
 Diffuse mass of gray matter along the
brain stem.
 Its neurons are involved in motor control of
visceral organs.
 A special group of its neurons are the
reticular activating system(RAS) which
plays a role in awake/sleep cycles and
consciousness . Damage to this area can
result in permanent unconsciousness.
Reticular Formation
Cerebellum
 Two cauliflower-like hemispheres with
convoluted surfaces projects dorsally from
under the occipital lobe of the cerebrum.
 Provides the precise timing for skeletal
muscle activity
 And controls our balance and equilibrium.
So movement is smooth and coordinated.
Cerebellum
Protection of the Central Nervous
System
 Scalp and skin
 Skull and vertebral column
 Meninges
 Cerebrospial
fluid
 Blood brain
barrier
Figure 7.16a
Meninges
• Three connective tissue membranes lie external to
the CNS – dura mater, arachnoid mater, and pia
mater.
• Functions of the meninges
• Cover and protect the CNS
• Protect blood vessels and enclose venous sinuses
• Contain cerebrospinal fluid (CSF)
• Form partitions within the skull
Meninges
Figure 12.24a
Dura Mater
• Leathery, strong meninx composed of two
fibrous connective tissue layers surrounding the
brain:
• Periosteal layer – attached to the inner surface of
the skull
• Meningeal layer – outer covering of the brain
• Folds inward in several areas to form a fold that
attaches the brain to the cranial cavity.
• The two layers separate in certain areas and form dural
sinuses
Dura Mater
• Three dural septa extend inward and limit
excessive movement of the brain
• Falx cerebri – fold that dips into the longitudinal
fissure
• Falx cerebelli – runs along the vermis of the
cerebellum
• Tentorium cerebelli – horizontal dural fold extends
into the transverse fissure
Dura Mater
Figure 12.25
Arachnoid Mater
• The middle meninx, which forms a loose brain
covering.
• It is separated from the dura mater by the subdural
space.
• Beneath the arachnoid is a wide subarachnoid space
filled with CSF and large blood vessels
• Specialized projections of the Arachnoid membrane,
Arachnoid villi protrude through the dura mater and
permit CSF to be absorbed into venous blood.
Arachnoid Mater
Figure 12.24a
Pia Mater
• Deep meninx composed of delicate connective
tissue that clings tightly to the surface of the brain
and spinal cord, following every fold .
HOMEOSTATIC IMBALANCE
• Meningitis, inflammation of the meninges, is a
serious threat to the brain because a bacterial or
viral meningitis may spread to the CNS.
Meningitis is usually diagnosed by obtaining a
sample of cerebrospinal fluid via a lumbar tap and
examining it for microbes.
• This condition of brain inflammation is called
encephalitis (en′sef-ah-li′tis).
Cerebrospinal Fluid
 Similar to blood plasma composition.
 Formed by the choroid plexuses that hang from the roof of each
ventricle .These plexuses are clusters of broad, thin-walled
capillaries lining the ventricles. These capillaries are fairly
permeable, and tissue fluid filters continuously from the
bloodstream. However, The choroid plexuses also help cleanse the
CSF by removing waste products and unnecessary solutes.
 In adults, the total CSF volume of about 150 ml (about half a cup)
is replaced every 8 hours or so; hence about 500 ml of CSF is
formed daily.
 Forms a watery cushion to protect the brain.
 Circulated in arachnoid space. Ventricles , and central canal
of the spinal cord.
Ventricles and Location of the
Cerebrospinal Fluid
Figure 7.17a
HOMEOSTATIC IMBALANCE
Ordinarily, CSF is produced and drained at a constant rate. However, if
something (such as a tumor) obstructs its circulation or drainage, it
accumulates and exerts pressure on the brain. This condition is called
hydrocephalus (“water on the brain”). Hydrocephalus in a newborn
baby causes its head to enlarge; this is possible because the skull
bones have not yet fused. In adults, however, hydrocephalus is likely
to result in brain damage because the skull is rigid and hard, and
accumulating fluid compresses blood vessels serving the brain and
crushes the soft nervous tissue. Hydrocephalus is treated by inserting
a shunt into the ventricles to drain the excess fluid into a vein in the
neck or into the abdomen.
Blood Brain Barrier
 Excludes many potentially harmful
substances
 Useless against some substances
 Fats and fat soluble molecules
 Respiratory gases
 Alcohol
 Nicotine
 Anesthesia
Blood-Brain Barrier
Is a protective mechanism that helps maintain a
stable environment for the brain. Includes the least
permeable capillaries of the body.
• It is selective, rather than absolute:
- Nutrients such as glucose, essential amino acids, and
some electrolytes move passively by facilitated
diffusion through the endothelial cell membranes.
- Bloodborne metabolic wastes, proteins, certain
toxins, and most drugs are denied entry to brain
tissue.
- Small nonessential amino acids and potassium
ions not only are prevented from entering the
brain, but also are actively pumped from the
brain across the capillary endothelium.
- The barrier is ineffective against fats, fatty
acids, oxygen, carbon dioxide, and other fatsoluble molecules that diffuse easily through
all plasma membranes. This explains why
bloodborne alcohol, nicotine, and anesthetics
can affect the brain.
Traumatic Brain Injuries
 Concussion
 Slight brain injury,may be dizzy or loose
consciousness briefly.
 No permanent brain damage
 Contusion
 Nervous tissue destruction occurs
 Nervous tissue does not regenerate
 Cerebral edema
 Swelling from the inflammatory response
 May compress vital brain tissue
Cerebrovascular Accident (CVA)
 Commonly called a stroke
 The result of blocking of blood vessel
supplying a region by a clot or ruptured
blood vessel to the brain.
 Brain tissue supplied with oxygen from
that blood source dies
 Loss of some functions or death may
result
• Not all strokes are “completed.” Temporary
episodes of reversible cerebral ischemia,
called transient ischemic attacks (TIAs), are
common. TIAs last from 5 to 50 minutes and
are characterized by temporary numbness,
paralysis, or impaired speech. While these
deficits are not permanent, TIAs do constitute
“red flags” that warn of impending, more
serious CVAs.
Spinal Cord
 Extends from the
medulla oblongata to
the region of T12
 Below T12 is the cauda
equina (a collection of
spinal nerves)
 Enlargements occur in
the cervical and lumbar
regions
Figure 7.18
Spinal Cord Anatomy
 External white mater – conduction tracts
Figure 7.19
Spinal Cord Anatomy
 Internal gray matter - mostly cell bodies
 Dorsal (posterior) horns
 Anterior (ventral) horns
Figure 7.19
Spinal Cord Anatomy
 Central canal filled with cerebrospinal
fluid
Figure 7.19
• Spinal Cord Trauma
Any localized damage to the spinal cord or its roots leads to some
functional loss, either paralysis (loss of motor function) or sensory
loss. Severe damage to ventral root or ventral horn cells results in a
flaccid paralysis (flak′sid) of the skeletal muscles served. Nerve
impulses do not reach these muscles, which consequently cannot
move either voluntarily or involuntarily. Without stimulation, the
muscles atrophy. When only the upper motor neurons of the primary
motor cortex are damaged, spastic paralysis occurs. In this case, the
spinal motor neurons remain intact and the muscles continue to be
stimulated irregularly by spinal reflex activity. Thus, the muscles
remain healthy longer, but their movements are no longer subject to
voluntary control. In many such cases, the muscles become
permanently shortened.
• Transection (cross sectioning) of the spinal cord at
any level results in total motor and sensory loss in
body regions inferior to the site of damage. If the
transection occurs between T1 and L1, both lower
limbs are affected, resulting in paraplegia (par″ahple′je-ah; para = beside, plegia = a blow). If the
injury occurs in the cervical region, all four limbs
are affected and the result is quadriplegia.
Hemiplegia, paralysis of one side of the body,
usually reflects brain injury rather than spinal cord
injury.
Peripheral Nervous System
 Nerve = bundle of
neuron fibers outside
the central nervous
system
 Neuron fibers are
bundled by connective
tissue
Classification of Nerves
 Mixed nerves – both sensory and motor
fibers
 Afferent (sensory) nerves – carry
impulses toward the CNS
 Efferent (motor) nerves – carry impulses
away from the CNS
Cranial Nerves
 12 pairs of nerves that mostly serve the
head and neck
 Numbered in order, front to back
 Most are mixed nerves, but three are
sensory only
Distribution of Cranial Nerves
Figure 7.21
Cranial Nerves
 I Olfactory nerve – sensory for smell
 II Optic nerve – sensory for vision
 III Oculomotor nerve – motor fibers to
eye muscles
 IV Trochlear – motor fiber to eye
muscles
Cranial Nerves
 V Trigeminal nerve – sensory for the
face; motor fibers to chewing muscles
 VI Abducens nerve –
motor fibers to eye muscles
 VII Facial nerve – sensory for taste;
motor fibers to the face
 VIII Vestibulocochlear nerve –
sensory for balance and hearing
Cranial Nerves
 IX Glossopharyngeal nerve – sensory
for taste; motor fibers to the pharynx
 X Vagus nerves – sensory and motor
fibers for pharynx, larynx, and
abdominal viscera
 XI Accessory nerve – motor fibers to
neck and upper back
 XII Hypoglossal nerve – motor fibers to
tongue
Spinal Nerves
 There is a pair of spinal nerves at the
level of each vertebrae for a total of 31
pairs
 Spinal nerves are formed by the
combination of the ventral and dorsal
roots of the spinal cord
 Spinal nerves are named for the region
from which they arise
Spinal Nerves
Figure 7.22a
Distribution of major peripheral nerves of upper and
lower limbs
Figure 7.23
Autonomic Nervous System
 The involuntary part of the peripheral
nervous system
 Consists of motor nerves only
 Divided into two divisions
 Sympathetic division
 Parasympathetic division
Differences Between Somatic and Autonomic
Nervous Systems
 Nerves
 Somatic – one motor neuron
 Autonomic – two, preganglionic and postganglionic
nerves
 Effector organs
 Somatic – skeletal muscle
 Autonomic – smooth muscle, cardiac muscle , and
glands
 Nerurotransmitters
 Somatic – always use acetylcholine
 Autominic – use acetylcholine, epinephrine, or
norepinephrine
Figure 7.24
Anatomy of the Sympathetic
Division(thoracolumber)
 Originates from T1 through L2
 Ganglia are at the sympathetic trunk (near the spinal
cord)
 Short pre-ganglionic neuron and long postganglionic
neuron transmit impulse from CNS to the effector
 Acetylcholine is the transmitter at the ganglion.
 Norepinephrine and epinephrine are neurotransmitters
to the effector and are classified as adrenergic fibers.
Slide 7.70
Sympathetic Pathways
Figure 7.26
Slide 7.71
Anatomy of the Parasympathetic
Division(craniosacral)
 Originates from the brain stem and S1
through S4
 Terminal ganglia are at the effector
organs
 Always uses acetylcholine as a
neurotransmitter so called cholinergic
fibers (ko″lin-er′jik) fibers.
Anatomy of the Autonomic Nervous System
Figure 7.25
Autonomic Functioning
 Sympathetic – “fight-or-flight”
 Response to unusual stimuli
 Takes over to increase activities
 Parasympathetic – housekeeping
activities
 Conserves energy
 Maintains daily necessary body functions
• An easy way to remember the most important roles
of the two ANS divisions is to think of:
- the parasympathetic division as the D division
[digestion, defecation, and diuresis (urination)], and
- the sympathetic division as the E division (exercise,
excitement, emergency, embarrassment).
• Remember, however, that their is a dynamic
antagonism exists between the two divisions, and
fine adjustments are made continuously by both.
Development Aspects of the Nervous System
 The nervous system is formed during the first month of
embryonic development.
 Any maternal infection can have extremely harmful
effects as rubella .
 The hypothalamus is one of the last areas of the brain
to develop.
 No more neurons are formed after birth, but growth
and maturation continues for several years.
 The brain reaches maximum weight in Young adults.
• Many elderly people complain fainting episodes due
to orthostatic hypotension (ortho = straight; stat =
standing), a form of low blood pressure that occurs
because :
1-the aging pressure receptors respond less to
changes in blood pressure following changes in
position.
2- of slowed responses of sympathetic
vasoconstrictor centers.
These problems can be managed by changing
position slowly to gives the sympathetic nervous
system time to adjust the blood pressure.