Transcript THE BRAIN

Anatomically , the brain is composed of four
subdivisions : cerebrum , diencephalon ,
brainstem , and cerebellum . The cerebrum and
diencephalons together constitute the
forebrain . The brainstem consists four
interconnected cavities , the cerebral ventricles
, which are filled with circulating cerebrospinal
fluid
• Brainstem
Brainstem is composed midbrain , pons , and medulla
oblongata .
Functions of the midbrain :
Regulation of the cerebrospinal fluid circulation .
2. The gray matter forms the nuclei of third and fourth
cranial nerves
3. Joins the cerebral hemispheres above to the pons
below.
1.
Functions of the pons :
1. It acts as a bridge between the lobes of the
cerebellum .
2. Nerve fibers pass up and down between the
midbrain above and the medulla oblongata
below .
3. The gray matter forms the nuclei of the 5th,6th
and 7th cranial nerves .
Functions of medulla oblongata :
1.
2.
3.
Connects the brain with the spinal cord .
Contains fibers passing from spinal cord , forebrain , and
cerebellum .
It contains also collection of gray matter known as vital centers
. The most important of these are :
A.
B.
C.
D.
The respiratory center which controls the rate and depth of respiration.
The vasomotor center which controls the caliber of the blood vessels .
The cardiac center which influences the rate of the heart .
Special centers such as the swallowing , vomiting centers , centers for
the movement of the stomach and the secretion of saliva and gastric
juice .
4. The gray matter forms the nuclei of 9th , 10th , 11th , 12th cranial
nerves
•
cerebellum
Function of the cerebellum :
1. Coordinates movements , including those for
posture and balance .
2. Participates in some forms of learning .
3. It helps to maintain balance and equilibrium .
• Cerebrum
The cerebrum consists of the right and left cerebral
hemispheres .
 Functions of the cerebral hemispheres :
Contain the cerebral cortex , which participates in
perception , the generation of skilled movements ,
reasoning , learning , and memory .
2. Contain subcortical nuclei , which participate in
coordination of skeletal – muscle activity .
3. Contain interconnecting fiber pathways .
1.
• Diencephalon ( between brain )
Diencephalon contains two major parts : the thalamus
and hypothalamus .
 Functions of thalamus
Is a synaptic relay station for sensory pathways
on their way to the cerebral cortex .
2. Participates in control of skeletal – muscle
coordination .
3. Plays a key role in awareness.
1.
• Functions of hypothalamus :
1. Regulates the anterior pituitary gland .
2. Regulates water balance .
3. Participates in regulation of autonomic nervous
4.
5.
6.
7.
8.
9.
system .
Regulates eating and drinking behavior .
Regulates reproductive system .
Reinforces certain behaviors .
Generates and regulates circadian rhythms .
Regulates body temperature .
Participates in generation of emotional behavior .
THE PHYSIOLOGY OF THE BRAIN
The brain is the control center of the whole human
body . Physiologically it may be divided into the
higher centers which are the seat of consciousness ,
mind , memory and will ; and the lower centers
which control many important unconscious acts .
The higher centers are situated in the cerebral
hemispheres while the lower ones are found in the
cerebellum and brainstem as well as in the basal
ganglia of the cerebrum .
The following mechanisms enable the brain to exercise
this power of control :
1. It receives sensory or afferent impulses from all parts
of the body , through the sensory pathway .
2. It is able to send out motor or efferent impulses
from all parts of the body , through the motor
pathway , which has two tracts : pyramidal
(corticospinal ) tract , which may be two kinds :
straight or crossed , and extrapyramidal tract .
3. There is a complicated system of connections
between all parts of the brain with each other .
Projection fibers transmits the impulse from the
brainstem to the cerebral cortex . Association fibers
which are situated in the cerebral cortex transmits
the impulse from one area to another within the
same hemisphere . Commissural fibers transmit the
impulse from one hemisphere to another one (from
right to the left for example )
FUNCTIONAL REGIONS
All areas of the cortex are interconnected and the
activity of each depends on the state of the entire cortex
. However , the different regions differ in function and
structure . These areas are :
1. The motor cortex area
The motor cortex area is located in the frontal lobe
from the cells of this area , voluntary motor impulses
arises and are transmitted to the various parts of the
body via spinal cord and peripheral nerves . The various
groups of muscles in the body are represented in this
area . The right cerebral hemisphere controls the left side
of the body and vice versa .
Location of pathway terminations for different parts of the body in the
somatosensory cortex , although there is actually much overlap between
the cortical regions . The left half of the body is represented on the right
hemisphere of the brain , and the right half of the body is represented on
the left hemisphere
2- The sensory (cutaneous sensitivity ) area
General sensation (pain , thermal ) is represented in the
parietal lobe .
3- The visual area is situated in the occipital lobe .
4- The auditory area is situated in the temporal lobe
5- The sense 0f smell and taste is located in the medial
surface of the hemisphere .
Special centers also exist for speech , reading , writing ,
etc.
THE CEREBROSPINAL FLUID (CSF)
The cerebrospinal fluid fills the subarachnoid space , the
ventricles of the brain and the canal of the spinal cord . This
is a clear , colourless fluid . It has the following functions :
It acts as a ”water cushion “ protecting the brain
and spinal cord from jars and shocks due to
body movement .
2. It conveys nutritive material to brain and spinal
cord .
3. It removes waste products from these structures .
4. Its examination is used as diagnostic tool for
some diseases as meningitis .
PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system consists of :
1. Cranial nerves.
2. Spinal nerves.
1.
CRANIAL NERVES
There are twelve pairs of cranial nerves which arise
from the brain and brain stem . Some of them are
sensory , bringing impulses to the brain , others are
motor , carrying impulses from the brain to the
periphery , while a few are mixed and contain both
motor and sensory fibers . The cranial nerves and a
summary of the information they transmit were
listed in Table 1/3 .
THE CRANIAL NERVES
Name
Fibers
comments
I. Olfactory
Afferent
Carries input from
receptors in olfactory
{smell} neuroepithelium
.. Not true nerve .
II. Optic
Afferent
Carries input from
receptors in eye. .not true
nerve .
III. Oculomotor
Efferent
Innervates skeletal muscles
that move eyeball up , down ,
and medially and raise upper
eyelid ; innervates smooth
muscles that constrict pupil
and alter lens shape for near
and far vision .
Afferent
Transmits information
from receptors in
muscles .
V. Trigeminal
E
SPINAL NERVES
There are thirty one pairs of spinal nerves coming off
The spinal cord at intervals and that they classified
according to the regions from which they arise .
Further . that each spinal nerve consists of anterior
and posterior roots which unite to form the main truck
as it leaves the vertebral column . All sensory nerve
posterior fibers {afferent}reach the spinal cord by the
root and all motor nerve fibers {efferent}leave by the
anterior root . It follows that the spinal nerves are
mixed nerves containing both sensory and motor
fibers{afferent and efferent}
Efferent neurons carry signal from the CNC out to
muscles and glands and subdivided into a somatic
nervous system and an autonomic nervous system .
The simplest distinction between the somatic and
autonomic system is that the neurons of the somatic
division innervate skeletal muscle whereas the
autonomic neurons innervate smooth and cardiac
muscle glands and neurons in the gastrointestinal
tract.
AUTONOMIC NERVOUS SYSTEM
Autonomic means<self controlling>.the autonomic
nervous system innervates cardiac and smooth muscle
–glands and gastrointestinal tract neuron.
Each autonomic pathway consist of a preganglionic
neuron with its cell body in the CNS and a
postganglionic neuron with its cell body in an
autonomic ganglion outside the CNS.
1-Sympathetic component
2-parasympathetic component
The autonomic nervous system is divided in to:
Autonomic nervous system exercises its
functions independently of the central nervous
system they are both closely associated and are
able to influence each other.
The autonomic nervous system is concerned
mainly with the control of all the involuntary
unstriped muscles in the body.
The autonomic nervous system sends two sets of
directly opposite character to the same organ .
Thus there are sympathetic fibers which
increase the rate of the heart beat and
parasympathetic fibers which decrease the rate.
Table 4 is a reference list of the effects of
autonomic
Nervous system activity . Not that the heart and many
glands and smooth muscles are innervated by both
sympathetic and parasympathetic fibers –that is-they
receive dural innervations.
Whatever effect one division has on the effector cells the
other division usually has the opposite effect .
Moreover the two division are usually activated
reciprocally -that is- as the activity of one division is
increased the activity of the other is decreased . Dual
innervations by nerve fibers that cause opposite
responses provides a very fine degree of control over
the effector organ.
A useful generalization is that the sympathetic system
Increases its response under conditions of physical
Or psychological stress . Indeed a full –blown
sympathetic response is called the fight or flight
response describing the situation of an animal forced
to challenge an attacker or run from it. All resources
are mobilized : heart rate and blood pressure
increases blood flow to the skeletal muscles -heartand brain increases the liver releases glucose and the
pupils dilate . Simultaneously activity of the
gastrointestinal tract and blood flow to the skin are
decreased by inhibitory sympathetic effects.
Autonomic responses usually occur without conscious
control or awareness as though they w
ere indeed autonomous{in fact the autonomic nervous
system has been called the{involuntary}nervous
system}.
THE SENSORY SYSTEM
Awareness of our internal and external world is brought
about by the sensory system.
A sensory system is a part of the nervous system that consists
of sensory receptors that receive stimuli from the external
or internal environment , the neural pathways that
conduct information from the receptors to the brain ,and
those parts of the brain that deal primarily with processing
the information.
Information processed by a sensory system may
or may not lead to conscious awareness of the
stimulus . Regardless of whether the
information reaches consciousness , it is called
sensory information .If the information does
reach consciousness , it can also be called a
sensation.
The understanding of the sensations meaning is
called perception.
Sensation may be classified
To these subdivision
1-General sensations
2-Special sensation
General sensations(Somatic sensation)
Sensation from the skin, body, wall
muscles, bones, tendon and joints
is termed somatic sensation and is
initiated by a variety of somatic
receptors.some respond to
mechanical stimulation of the
skin, hairs, and underlying tissues.
Whereas others respond to temperature
or chemical changes . Activation of
somatic receptor gives rise to the sensation
of touch pressure, warmth, cold, pain, and
awareness of the position of the body parts
and their movement. The receptors for visceral
sensations, which arise in certain organs of the
thoracic and abdominal cavities, are the same
as the receptors that give rise to somatic
sensation. Some organs such as liver have no
sensory receptors at all.
Each sensation is associated
with a specific receptor
type . In other words, there
are distinct receptor for
heat, cold, touch, pressure
,limb position or movement
,and pain.
Special sensation
1-Vision
The eyes are composed of an optical
portion,
which focuses the visual image on the
receptor
cells, and a neural component, which
transforms the visual image into a
pattern of
neural discharges.
The receptors of the eye are sensitive only to
visible light. The wavelengths of visible light
are between 400 and 700nm.photoreceptor
Cells in the retina are called rods and cones.
The rods are extremely sensitive and respond
to very low levels of illumination , whereas
the cones are considerably less sensitive and
respond only when the light is brighter
(daylight). Light (stimulus) activates
photoreceptors, action potentials transmitted
through the visual pathway to the visual
cortex in the occipital lobe.
It is important to note some common
defects of vision requiring the use of
spectacles. Whereas the normal eye is
practically spherical ,in some people it
tend to be slightly elongated and in
others flattened. In other words , in the
former case the distance from the lens
To the retina is increased and in the latter
It is decreased. It follows, therefore, that
The lens will not naturally focus
the image accurately on the retina in
these conditions.
In the former, elongated or myopic eye
(short sight) the image will tend to fall
in front of the retina, while in the
hypermetropic eye )Long sight) it will
behind the retina. In both instances the
objects seen will be blurred and out
of focus.
The increasing stiffness of the lens that
occurs with aging makes accommodation
for near vision increasingly difficult.
This condition, known as presbyopia ,is
a normal part of the aging process and
is the reason that people around 45 years
of age may have to begin wearing
reading glasses.
Accommodation is adjustment
Of eye for viewing various by
Changing shape of lens.
2-Hearing
The sense of hearing is based on the
Physics of sound and physiology of the
ear, the nerves to the brain, and the brain
parts involved in processing acoustic
information.
The entire range of frequencies audible
sound to human beings extend from
20 to 20000 Hz. Human can distinguish
about 400000 different sounds.
The sequence of sound transmission
is as follows:
Sound waves enter the external auditory
canal and press against the tympanic
membrane, causing it to vibrate.
The vibrating membrane causes
Movement of the three small middle
Ear bones ,the stapes vibrates against
the oval window membrane.
Movement of the oval window membrane
Set up pressure waves in the fluid-filled
scala vestibule, which cause vibrations
in the cochlear duct wall, setting up
pressure waves in the fluid there.
These pressure waves cause vibration,
in the basilar membrane, which is
located on one side of the cochlear
duct.
As this membrane vibrates, the hair cells
of the organ of Corti move in relation
to the tectorial membrane.
Movement of the hair cells stereocilia
stimulates them to release
neurotransmitter, which activates
receptors on the peripheral ends off the
afferent nerve fibers (8th cranial nerve).
The auditory centre of
the brain situated in
the temporal lobe of
the opposite side.
• Vestibular system (sensation
Of body position and movement)
The position and displacement
of the Body in space are perceived
by various Sense organs . The
vestibular apparatus plays an
important part in producing
sensations of body position
and movement.
The receptors of the vestibular
Apparatus , which perceive the
Changes in the position of the body
in space, are imbedded in the
maculas and cristae which are
located in the membranous vesicles
of the vestibule and in the
membranous semicircular canals.
Changes in the position of the head
and the speed of movement are
accompanied by changes in the
pressure of the endolymph on the
sensory calls of the maculas and
cristae .This produces excitation in
the vestibular nerves, which is
transmitted along the acoustic
nerve (8th cranial nerve)to the brain.
3-SMILL
The organ of smell is composed of
special sensory cells located in the
mucous membrane of the upper part
of the nasal cavity. The processes of
these calls are filaments of the
Olfactory nerve(1st cranial nerve).
The olfactory cells are stimulated
By odoriferous substances.
The appropriate or adequate stimulus for
the sense of smell must be either in the
form of gas or minute particles which are
soluble in the secretions of the nasal
mucous membrane.
The resultant excitation is transmitted
along the olfactory nerve to the brain
where corresponding sensations arise.
The cerebral part of smell is located
in the cortex of the temporal lobe.
The quality of food may be
determined by it smell.
During eating olfactory sensation
supplement taste sensation.
When olfactory is disturbed, for
example in cases of rhinitis, the
ability to determine taste is
impaired and the food seems
tasteless
The human has 1000 different
Olfactory receptor types, but he
can discriminate between
10000 different odorants.
4- Taste
The organ of taste is represented by the
so called taste buds which are located in
the taste papillae of the tongue ,in the soft
palate and in the pharynx .
the taste buds consist of special cells which
sensory nerve fibers terminate .food
substances in the oral cavity come into
contact with the taste buds and give rise to
excitations in the ending of the
gustatory(sensory)
nerves(facial, glosso -pharyngeal ,trigeminal)
,the resultant excitations are transmitted to
the brain . taste sensations arise in the
cerebral cortex(temporal lobe).
There are four types of tastes sensation:
sweet ,bitter, salt , and sour. All the other
sensation are combination of these basic
sensations .
It should be remembered that the taste
buds are stimulated only by food
substances in a dissolved
state. Their solvent in the oral cavity is the
saliva.
Muscles
The primary function of the muscles is to
generate the forces and movements used by
multicellular organisms in the regulation of their
internal environments and to produce the
movements of the entire organism in the
external environment . In
humans , the ability to communicate ,
Whether by speech , writing , or artistic
expression also depends on muscle contraction
. Indeed , it is only by controlling the activity of
muscles that the human mind ultimately
expresses itself.
Three types of muscles tissue can
be identified on the basis of
structure , contractile properties
and control mechanisms;
1-skeletal muscle(voluntary , striated)
2-smooth muscle(involuntary ,
unstriated)
3-cardiac muscle(involuntary , striated)
Properties of muscle;
1- Excitability
2- The power of contraction if any stimulus applied to the
muscle or its nerves.
3- Elasticity . Muscle tissue is elastic and can be stretched by
a weight . When this is removed , the muscle ret
Removed , the muscle returns to its normal length.
4- Fatigue . After a number of contraction the supply of
glucose immediately available is used up and a certain
amount of lactic acid accumulates . The muscle then
becomes tired and is unable to contract with the same
degree of efficiency . It requires rest in order to replenish its
supply of glucose and to remove the lactic acid.
5- Muscular tone . when the muscle appears to be
at rest it is always partially contracted and therefore
ready for immediate action . This state of partial
contraction is called muscle tone .
skeletal muscle
sliding-filament Mechanism
When force generation produces shortening of a skeletal-muscle
fiber , the overlapping thick and thin filaments in each
sarcomere(repeating structural unit of myofibril , composed of
thick and thin filaments , extends between two adjacent Z lines)
move past each other , propelled by movements of the cross
bridges . During this shortening of the sarcomeres , there is no change
in the lengths of either the thick or thin filaments. This is known as
the sliding-filament mechanism of muscle contraction . There is an
important role to ATP and calcium in the
process of contraction.
the neuromuscular junction ;
Synapse – like junction between an axon terminal of an
efferent (motor ) nerve fiber and a skeletal – muscle fiber .
The nerve cells whose axons innervate skeletal – muscle
fibers are known as motor neuron (efferent ) , and their
cell body are located in either the brainstem or the
spinal cord . The axons of motor neurons are myelinated
and are the largest – diameter axons in the body .
Upon reaching a muscle , the axon of a motor neuron
divides into many branches , each branch forming a single
junction with a muscle fiber .
A single motor neuron innervates many muscle fibers , but
each fiber is controlled by a branch from only one motor
neuron .
A motor neuron plus the muscle fibers it innervates is
called a motor unit . Motor unit is the basic functional
unit of the muscle .
Work of muscle
When muscles contract they do work . The
work of muscle depends on their strength and
length . The strength of a muscle is directly
proportional to the cross – section of all the
muscle fibers forming the given muscle . In
other words , the thicker the muscle , the
greater the load which it can lift . The height
to which a load can be lifted depends on the
length of the muscle . Consequently , the
thicker and longer the muscle , the more work
it can do .