INTRODUCTION
Animals are characterized by irritability or the ability to detect and
respond to environmental stimuli. This involves a sensory system
(detection), a nervous system (interpretation) and a motor system
(response). The nervous system is thus a connection between sensory
inputs and motor outputs.
As evolution proceeded the nervous system becomes more
complex. The radically symmetrical animals have simple nervous system
consisting of nerve net work, conducts signals from sensory cells to
muscle cells. There is no centralization of nervous system. The bilaterally
symmetrical animals have centralized nervous system with enlarged
anterior end called brain. The rest of which is the spinal cord. Generally,
invertebrate animals tend to be small and have simple nervous system,
whereas vertebrates have highly or well developed nervous system.
Especially important is that all vertebrates have a similar basic
structure of their nervous system that is divided into: (1) central nervous
system (CNS) which comprises the brain and spinal cord. (2) Peripheral
nervous system (PNS) which comprises cranial nerves that join the brain
and nerves of the spinal cord (spinal nerves). Nervous system in fish and
amphibians is poorly developed compared to higher vertebrates and the
number of cranial nerves is ten pairs only. In reptiles and birds, the
nervous system is more developed, as the brain increases in size and the
number of cranial nerves increased by two pairs (spinal accessory and the
hypoglossal). In mammals, the nervous system is more complicated than
other vertebrates and is characterized by: (1) increased brain size relative
to body size. (2) increased subdivisions and growth of forebrain,
especially the cerebrum, which is associated with the increasingly
complex behaviour of mammals. (3) cerebral cortex is present that is
concerned with the muscular activity and higher brain functions.
NERVOUS SYSTEM
Life is maintained by coordination of the functions of various
body systems. Coordination is controlled by two main systems:
1- Endocrine system (chemical regulation): is a collection of blood
carrying chemical messengers (hormones) with slow and long standing
action.
2-Nervous
system
(neural
regulation):
includes
trillions
of
interconnected neurons with rapid and short standing action.
FUNCTIONS OF NERVOUS SYSTEM:
Nervous system acts to regulate and coordinate various body
activities necessary to life by allowing us to receive stimuli (sensory
information) from various sensory receptors and then processing them
into appropriate responses made by body organs (effectors).
NERVOUS TISSUE
Nervous tissue is composed of two types of cells: (1) neurons or nerve
cells (figure 1). (2) neuroglia (also called glia or glial cells "means glue").
Figure 1: The general structure of the neuron.
GLIAL CELLS:
The glial cells (figure 2) represent about 10 to 50 times the number
of neurons and are in direct contact with neurons and often surround
them. They act to support, nourish and protect the neurons, thus aid in
their ability to do their functions.
TYPES OF GLIAL CELLS:
1-Oligodendroglia:
They are few macroglial cells that form myelin sheath around the
axons in the CNS (like the Shwann cells in the PNS). A single cell can
extend and surround large number of neurons for myelination of their
axons.
2-Astroglia:
They are the most abundant of glial cells that have star like
appearance. Astroglia fill the spaces between neurons, have numerous
projections that hold neurons to their blood supply and help to regulate
the external chemical environment of neurons by removing excess ions
(notably potassium). Also they take up neurotransmitters that are released
by neurons during synaptic transmission and recycling them.
3-Microglia:
They are of various forms that have branched processes. They are
migratory and act as phagocytes to waste products of nerve tissues.
Microglia originate outside the brain, mostly in bone marrow, unlike
other glial cells.
Figure 2: Types of glial cells.
THE NEURON:
The neuron is the structural or anatomical unit that is responsible
for the functions normally associated with the
nervous system. It
operates by generating electrical signals that pass from one part of the
cell to another part of the same cell and by releasing chemical
messengers (neurotransmitters) to communicate with other cells.
STRUCTURE OF THE NEURON:
Neurons vary considerably in size and shape according to their
sites and functions. In general, they are formed of the cell body and cell
processes (figure 1).
1) The cell body (Soma):
The cell body is the enlarged part of the neuron. It is a metabolic
center that provides nutrition for the whole neuron. The cell bodies inside
the CNS are usually collected into groups called (nuclei or centers), but
in PNS usually collect to form (ganglia). The cell body is surrounded by
the cell membrane which continues to cover its processes. The cell body
contains nucleus and surrounding cytoplasm, besides mitochondria and
other organells typical in eukaryotic cell, but no centrosomes. The
absence of centrosomes indicates that the neurons have lost its power of
division.
In addition, the neuron contains specialized structures
including:
• Nissl bodies: these are granular materials (free and attached ribosomes)
that present in the cell body and not present in dendrites or axon. Nissl
bodies are responsible for synthesis of protein in the nerve cell.
• Neurofibrils: these are thin fibers that present in the cell body and
extend into the processes of the cell. They serve as a support of the
neuron to maintain its shape.
• Microtubules: these are distributed through out the cytoplasm of the
cell body and extend into the dendrites and axon. They serve to support
the neuron (as neurofibrils) and to transport materials and organells down
from the cell body to the axon (axon transport), for regeneration of
damaged axons. Axon transport of certain materials also occurs in the
opposite direction from the axon terminals to the cell body. By this route,
growth factors and other chemical signals picked up at the terminal can
affect the neuron. This is also the route by which certain viruses taken up
by the peripheral terminals can enter the CNS.
2) The Cell Processes:
a) The dendrites: are multiple short processes which extend from the
cell body. They extend to the surrounding area to act as receptive surface.
So, the dendrites increase the surface area of the cell body. The surface of
dendrites collect impulses and transmit them to the cell body.
b) The axon (nerve fiber): is a single long process that conducts
impulses away from the cell body to its target cell. Axons vary in length
from only a millimeter to a meter or more (axons extending from spinal
cord to foot). The portion of the axon closest to the cell body plus the part
of the cell body where the axon is joined known as the initial segment or
(axon hillock). It is important in generating the electric signal that
propagates away from the cell body along the axon. The axon may have
branches called collateral branches along its length (figure 1). Near the
ends, both the axon and its collaterals undergo further branching. Each
branch ends in a terminal, which makes junction with one of the
following (figure 3):
• Dendrites or cell body of another neuron, forming a (neuro-neural
junction) or synapse.
• Muscle fiber to form a (neuro-muscular junction).
• Secretory gland to form a (neuro-epithelial junction).
Figure 3: Termination of the axon
MYELINE (MEDULLARY) SHEATH:
All vertebrates have two types of nerve fibers, the large axons
more than 1um in diameter being myelinated and those of smaller
diameter are generally unmyelinated. Almost all invertebrates are
equipped only with non-myelinated fibers, but some differ from those in
the vertebrates in being much larger.
Myelin is a white lipoprotein material composed of several
compressed layers (20-200) which make successive wrappings around
the axon. Myelin sheath is not a continuous layer, but is interrupted at
regular internods by nodes of Ranvier (figure 4). Through these nodes,
ions and water inside the neurons can undergo exchange with
surrounding tissues.
In PNS, myelin sheath is usually covered by a thin basement
membrane (the neurilemma). Just beneath it, the Schwann cells are to be
found at the mid point of each internode. In contrast, the nerve fibers
within the central nervous system usually lack the neurilemma.
In the CNS, myelin sheaths are formed by oligodendroglia (a type
of glial cells), while in the PNS , Schwann cells are responsible for this .
Each Schwann cell only warps an internode ( i.e many Schwann cells
will myelinate one axon ), although one oligodendroglia will myelinate
many axons in the CNS. This difference allows Schwann cells to act for
regenerating damaged axons in PNS , but oligodendroglia can not guide
regeneration and damage in CNS . It is therefore irreparable.
Figure 4: Diagram of a part of vertebrate axon (ax.) showing
myelin sheath (myel.), nodes of Ranvier (n.) and Schwann cell
(S.c.) just beneath the neurilemma (N.) .
TYPES OF NEURONS:
I) STRUCTURAL CLASSIFICATION OF NEURONS:
There are four types of neurons (figure 5) according to the number
of processes extending from the cell body (polarity):
1) Unipolar neuron:
Has only one process (exists in the nuclei of cranial nerves).
2) Pseudounipolar neuron:
Has one short process that gives two branches . One is the long
peripheral process ends at the receptor . The other branch is the short
central process enters the central nervous system (exists in the spinal
ganglia).
3) Bipolar neuron:
Has one long axon and one dendrite on the opposite sides of the
cell body (less common, exists in certain tissues as retina of eye,
olfactory epithelium of the nose & in the ear).
4) Multipolar neuron:
Has several short dendrites and one long axon (exists widely in
brain & spinal cord).
Figure 5: Types of neurons. (a) unipolar neuron, (b)
pseudounipolar neuron (c) bipolar neuron and (d) multipolar
neuron.
II) FUNCTIONAL CLASSIFICATION OF NEURONS:
1-Sensory or afferent neurons:
Conduct impulses from the sensory receptors to CNS.
2-Motor or efferent neurons:
Conduct impulses from the CNS to effector organs. There are two
types of motor neurons (Figure 6):
a) Somatic motor neurons: which innervate external (voluntary)
organs (skeletal muscles).
b)Autonomic motor neurons: which innervate the internal (involuntary)
organs (smooth and cardiac muscles and glands).
3- Interneurons (about 99% of all neurons):
They connect sensory and motor neurons and serve to integrate
functions of nervous system.
Figure 6: The sensory- motor neurons.
NOTE:
• Motor and interneurons are multipolar, whereas sensory neurons
are often unipolar.
• For afferent neurons, both cell body and the long peripheral
process of the axon are outside the CNS and only a part of the
central process enters the brain or spinal cord.
• The interneurons are located inside CNS.
• For efferent neurons, the cell bodies are within the CNS, but the
axons extend out into periphery.
• The axons of both afferent and efferent neurons, except for the
small parts in the brain or spinal cord, form the nerves (nerve
trunks) of the PNS.
THE NERVE:
The nerve (or nerve trunk) is composed of a large number of nerve
fibers. Each nerve fiber is an axon covered by a myelin sheath. The fibers
are bound together in bundles by connective tissue rich in blood vessels
known as perineurium.. Bundles of individual nerve fibers form the
nerve trunk which is enclosed in a relatively strong sheath of connective
tissue known as epineurium.
REFLEX ACTION
The activities controlled by the nervous system are called reflex
actions or simply reflexes. The reflex is a reaction started by a change in
the surrounding environment which acts as a stimulus, stimulating one of
the receptors. This leads to initiation of nerve impulse which passes
through chain of sensory neurons to CNS. From the CNS, impulses pass
outwards (reflected) as a response through the motor neurons and reach
the effector organ.
REFLEX ARC:
The reflex action is carried out through a pathway called reflex arc,
which is considered the functional or (plysiological) unit of the nervous
system. The reflex arc consists of:
• A receptor organ: it is a sensory cell where stimulus is received.
• A sensory (afferent) neuron: Its function is to conduct impulse from
the receptor neuron to CNS.
• An interneuron: The afferent neurons commonly synaptically
communicate with interneurons, which in turn send impulses to motor
neurons or to other interneurons, which finally influence the activity of
the motor neurons concerned in the reflex action.
• A motor (efferent) neuron: It serves to transmit the impulse to the
effector organ.
• An effector organ: It is the organ which responds to the transmitted
impulse (e.g. the muscle or gland).
The reflex arc in which the afferent sensory neurons directly
communicate with the efferent motor neurons is a simple reflex arch with
a single synapse (monosynaptic arc) and the reflex is monosynaptic
reflex, while the reflex arc in which one or more interneurons are
imposed between the afferent and efferent neurons are polysynaptic (No.
of synapses in the arc varies from two to many hundreds).
Figure 7: Reflex arc
TYPES OF REFLEXES:
The reflexes are functionally classified into:
(1) The somatic reflexes.
(2) The autonomic reflexes.
In the two reflexes, the sensory pathway is similar, but the motor
pathway differs. In the somatic reflex the motor (efferent) branch consists
only of one motor neuron (its cell body located in CNS), while it consists
of two neurons in the autonomic reflex. They are pre-ganglionic neuron
with the cell body located in the CNS and post-ganglionic neuron with
the cell body located in a ganglion (called autonomic ganglion) outside
CNS (figure 8).
Anatomically, reflexes are either: (a) spinal reflex, which
concerned with immediate withdrawal from harmful stimuli (without
involvement of brain) or (b) cranial reflex, which is more complicated
reflex action (involves association of specific brain areas).
Figure 8: Somatic and autonomic reflex arcs.
Examples Of Reflexes:
1) The sudden withdrawal of one’s hand when it comes in contact
with hot surface.
2) The act of sneezing and coughing (coughing is mainly due to
irritation of the larynx by some materials, while sneezing is due to
irritation of the mucous membrane of the nose).
3) The secretion of sweat in an increased quantity (warmth acts as
the stimulus for increased secretion).
4) The secretion of saliva and gastric juice and the movements of the
stomach (food acts as a stimulus).
RECEPTORS:
Are specialized endings of afferent (sensory) neurons or separate cells.
Generally receptors are classified into two groups: Exteroceptors and
Interoceptors.
1) Exteroceptors(for detection of external stimuli), they include:
• Touch and pressure (cutaneous) receptors.
• Thermal receptors.
• Photoreceptors.
• auditory receptors.
• Smell and taste receptors (chemoreceptors).
2) Interoceptors (for detection of internal stimuli), they inclued:
• Proprioceptors (respond to change in position of body).
• Baroreceptors (respond to change in blood pressure).
• Chemoreceptors (respond to change in circulating, CO2, O2 and H+).
NERVE IMPULSE:
The neurons (like all other cells) have difference in the concentration of
ions on the two sides of their plasma membrane. Under resting condition,
the major ions in the extracellular fluid are sodium and chloride ions,
whereas the intercellular fluid contains high concentration of potassium
ions and negatively charged nondiffusible organic molecules, particularly
proteins and phosphates. This unequal distribution of ions and charges
around the membrane results in a net negative charge inside and a
positive charge outside the membrane (i.e. the membrane is polarized).
This charge difference at rest, develops an electrical potential
(resting membrane potential). In many nerve cells the resting membrane
potential is approximately (– 0.06v) or (– 60 mv). Minus sign indicates
that inside of the cell is more negative than the extracellular fluid.
Sodium Potassium Pump :
The concentration difference for sodium and potassium on the two
sides of the cell is due to the action of plasma membrane active transport
system (sodium – potassium pump) that maintains this unequal
concentration by actively transporting ions against their concentration
gradient using ATP. Sodium–potassium pump actually pumps
three
sodium ions out of the cell for every two potassium ions that they bring.
This unequal transport of positive ions make the inside of the cell more
negative than it would be from simple ions diffusion .
The Action Potential (temporary change in the membrane
potential):
The nerve cell is especialized to respond to stimuli (change in the
surrounding environment) and to convert this stimuli into electrical signal
or nerve impulse that is conducted along the nerve fiber to its terminals.
This occurs by the rapid changes in their resting membrane potential
and the generation of action potential. Action potential is the change of
the voltage of the membrane from a negative state to a positive value for
a brief time. A process that is called depolarization and is accompanied
by transmission of electrical impulses by the nerve cell.
The changed membrane potential is as a result of the change
(activation) of the membrane Na+, k + pump in response to stimuli. If the
nerve cell is stimulated, the Na+, k + pump switches from its resting to
activation state, causing much more sodium influx than potassium
outflux. Consequently, producing a gradual depolarization of the
membrane (generation of action potential). Action potential in the nerve
cell lasts for few milliseconds, then returns so rapidly to its resting
membrane potential. This is because sodium pump undergoes
inactivation which would restore the membrane potential to its resting
level .
The action potential propagates or moves along the axon from the
initial point to the terminal ending. The sodium ions influx at one point is
considered as a stimulus for the next point (i.e. followed by movement
of sodium ions into adjacent point). As a consequence the action potential
(nerve impulse) propagates in direction from the soma to the axon
terminal at constant speed. When the action potential reaches the end of
the axon, it invades the synaptic terminal, causing the release of a
chemical transmitter (neurotransmitter) used in the communication
between neurons in a process called synaptic transmission.
NOTE:
After passage of the action potential, there is a brief period (the
refractory period) during which the membrane can not be stimulated.
This prevents the massage from being transmitted backward along the
membrane.
The Velocity of Action Potential :
In unmyelinated nerve fiber
the conduction velocity is
proportional to the diameter of the axon. The larger diameter of the axon,
the greater speed of propagation. This is because the axons with large
diameter do not offer as much resistance to the flow of ions along the
length of axon. In myelinated axon the velocity is greater and is
determined not only by the diameter of the axon, but also by distance
between nodes of Ranvier.
The formation of myelin arround an axon prevents the penetration
of ions needed for the conduction of the action potential, however there
are nodes of Ranvier, where the membrane of the nerve axon is exposed
and contains large number of sodium and potassium channels. In the
nodes of Ranvier the membrane is depolarized and action potentials are
generated. The generation of an action potential in a node of Ranvier
causes the membrane in the adjacent node of Ranvier to depolarize and
also to generate an action potential.
In this way, the propagation of an action potential along a
myelinated nerve axon appears to jump (saltus in latin) from one node of
Ranvier to the next in the process of saltatory conduction. Thus, the
greater distance between nodes of Ranvier, the greater the volecity of
action potential propagation (figure 9).
Figure 9: In a myelinated axon, the impulse jumps from
one node of Ranvier to the next (saltatory conduction).
SYNAPSES:
The communication between nerve cells occurs at junctions called
synapses. In the nervous system, there are two types of synapses
(according to presence of chemical transmitter), (1)chemical synapses
and (2)electrical synapses.
1-Chemical
synapses
mediate
communication
between
distant cells:
In the nervous system, the predominant type of synapse is the
chemical synapse (figure 10). In chemical synapses, at least two cells
participate–the cell producing the nerve impulse, called the presynaptic
cell, and the target cell receiving the impulse, called the postsynaptic cell.
The presynaptic component of the synapse consists of the terminal
ending which contains vesicles called synaptic vesicles. These vesicles
filled with chemicals referred to as neurotransmitters. When an action
potential in the presynaptic neuron reaches the end of the axon, it causes
the release of the
neurotransmitter into the synaptic cleft. The
transmitters bind to receptors located in postsynaptic cell membranes.
This results in depolarization of its plasma membrane, causing
transmission of action potential along the postsynaptic neuron.
Figure 10: The structure of the chemical synapse
2-Electrical
synapses
mediate
communication
between
adjacent cells:
Is a gap junction (with no synaptic vesicles) in which impulses are
conducted directly from one cell to another adjacent cell. Its conduction
is faster than the chemical synapse.
Electrical synapses are also found in cardiac cells of the heart,
smooth muscle cells, and other cells that display a synchronization of
activity.
I- BASIC ORGANIZATION AND FUNCTIONS OF
THE CENTRAL NERVOUS SYSTEM
The central nervous system (CNS) is composed of the brain and
spinal cord . The CNS is surrounded by bony skull and vertebrae. Both
the spinal cord and brain consist of a white matter (bundles of axons each
with a sheath of myelin) and grey matter (masses of cell bodies and
dendrites). In the spinal cord, the white matter is at the surface and the
grey matter inside. In the lower vertebrates (like fish and amphibians),
they have their white matter on the outside of their brain as well as their
spinal crod. However, in the brain of mammals this pattern is reversed.
Both the brain and spinal cord are envloped by three membranes called
the meninges (figure 11) which support and protect the central nervous
system. They are :
1- Dura mater: The outer membrane, next to the interior surface of the
skull and bony vertebrae.
2- Arachnoid : The middle membrane.
3- Pia mater: The inner membrane, covering the entire surface of the
brain and spinal cord.
Figure 11: Diagram showing the meninges surrounding the brain and
spinal cord.
The space between the arachnoid and pia mater (subarachnoid
space) is filled with fluid called cerebrospinal fluid (CSF). The rest of the
CSF lies in the ventricles of the brain.
CEREBROSPINAL FLUID (CSF):
The CSF is a clear colourless fluid secreted from the choroid plexus
(network of blood capillaries) which is found in the lining of the
ventricles of the brain.
The CSF has a composition identical to that of the nervous system
extracellular fluid (ECF) but differs from that serving as the ECF of the
cells in the rest of the body. This compositional difference of CSF is
maintained by the blood brain barrier (BBB), which is a system of tight
junctions between endothelial cells of the nervous system blood
capillaries.
The CSF circulates from the interconnected ventricular system into the
central canal of the spinal cord (figure 12). It flows through three
foramena (apertures) at the fourth ventricle into the subarachnoid space
along the brain and spinal cord, where it is directly absorbed into the
cerebral veins to enter the blood stream.
Normally, the total volume of CSF is about 150 ml and its daily
rate of secretion equals its rate of absorption. If the flow of CSF is
obstructed, CSF accumulates, causing hydrocephalus. In sever cases, the
elevation of CSF in ventricles leads to compression of the brain blood
vessels, which may lead to inadequate blood flow to the neurons,
neuronal damage and mental retardation.
Functions:
(1) It acts as bath around brain and spinal cord to protect them from
injury with position or movement.
(2) It serves to transport nutrients into the nervous system and to remove
harmful metabolilies from nervous system into blood.
(3) It keeps constant intracranial pressure, if the volume of the brain
increases, CSF drains away and if the brain shrinks, more fluid is
retained.
Figure 12: Diagram illustrating the location of the cerebrospinal in
the ventricles and the spinal canal.
1- THE BRAIN
During development , the central nervous system is formed from a
long
tube of ectoderm (neural tube). The anterior end of the tube
becomes the brain. The lumen of which becomes dilated and produces a
large ventricular system within the brain (4 ventricles), while the lumen
of the caudal end of the tube (spinal cord) remains very small and is
recognized in the adults as the central canal. In human, the brain weighs
350- 400 g at birth. As a child grows, the number of cells remains stable,
but cells grow in size and the number of connecting cells increases. So,
brain reaches 1300-1400g in adult human and is differentiated into 3
main sections, they are:
(1) Prosencephalon (forebrain): subsequently divided into the
telencephalon (cerebrum) and diencephalon (thalamus & hypothalamus).
(2)
Mesencephalon (midbrain): develops without further
subdivisions.
(3)
Rhombencephalon
(hindbrain):
subdivides
into
the
metancephalon (pons & cerebellum) and myelencephalon (medulla
oblongate).
During continuing formation of the brain, four different regions
become apparent . These regions are:
1-Cerebrum
2- Diencephalon
3- Cerebellum
4- Brainstem
(consists of midbrain, pons and medulla oblongata) (figure 13).
Figure 13: The Brain
BRAIN VENTRICLES:
The brain contains four interconnected cavities (ventricles), which
are filled with circulating CSF. This fluid is secreted into the ventricles
by the cells of the choroid plexus. These ventricles are, the first and
second (lateral) ventricles being the largest and are found in each cerebral
hemisphere. The third ventricle is a narrow cleft that lies between the
right and left thalami. It connects the lateral ventricles by means of
interventricular foramena (figure 12). The fourth ventricle is a flattened
pyramidal cavity found between the cerebellum and medulla oblongata
and is connected to the third ventricle by the cerebral aqueduct (also
called aqueduct of midbrain). The fourth ventricle is connected also with
the central canal of the spinal cord and it has three openings (two lateral
& one medial) through which CSF flows into the subarachnoid space.
THE MAIN STRUCTURES OF THE BRAIN
I- CREBRUM :
The cerebrum is the largest portion of the brain associated with the
higher brain functions, such as thought and action. It is divided into left
and right hemispheres being largely separated by a longitudinal division .
However the two hemispheres are still connected by bundels of
myelinated nerve fibers (corpus callosum) to permit transfer of
information between them.
The outer surface of each hemisphere: consists of grey matter
that represents the cerebral cortex. The cortex in each hemisphere is
about 4 mm thick. It consists mostly of cell bodies and processes having
no myelin covering. The neurons in the cortex are arranged in several
layers (six) one above the other. They are mostly pyramidal shaped cells
with dendrites extensively arborized to reach cortical surface. Cerebral
cortex is highly convoluted and this makes the brain more efficient
because it increases the surface area of the brain and the number of
neurons in it.
The surface of the cortex in each hemisphere is marked by grooves
called sulci (singular sulcus) including two main lateral sulci and the
central sulcus. The sulci divide the cortex into four lobes visible from
outside, frontal lobe in front of the central sulcus, parietal lobe
(immediately behind the central sulcus), temporal lobe (below the lateral
sulcus) and occipital lobe at the back of the brain. Hidden beneath these
lobes of the cortex are the olfactory bulbs, which receive inputs from the
olfactory nerves (figure 14).
The deeper parts of the cerebral hemispheres: consist of white
matter that is made up of myelinated nerve fibers (nerve tracts), which in
turn overlies cell clusters (grey matter) collectively termed subcortical
nuclei. The nerve tracts may run as:
1) Association fibers from one part to another at the same hemisphere.
2) As connecting fibers from one hemisphere to the other.
3) As efferent (descending) or afferent (ascending) fibers, which carry
impulse from or to the brain.
LOBES OF THE CEREBRAL CORTEX:
1- FRONTAL LOBE:
Frontal lobe is associated with the primary motor cortex and is
important in conducting three functions. (a) speech. (b) thoughts and
make decisions. (c) initiation and control of voluntary movement.
2- PARIETAL LOBE :
Is a sensory area (primary somatosensory cortex) associated with
specialized area for taste sensation. Somatosensory cortex is responsible
for receiving impulses from somatic receptors to give sensation about
touch, pain, pressure, temperature and position of the body in the space.
The specialized sense organs of body (eyes, ears and nose) have other
functional areas on the cortex .
3- THE TEMPORAL LOBE :
Is a sensory area that receives signals from auditory nerves and
concerned with hearing, speech and memory.
4- OCCIPITAL LOBE :
Is a sensory area that receives signals from optic nerves and
concerned with many aspects of vision and reading ability.
Figure 14: Lobes of the cerebral cortex.
THE SUBCORTICAL NUCLEI (BASAL GANGLIA):
The name “basal ganglia” is an exception to the generalization that
ganglia are neuronal cell clusters that lie outside the central nervous
system. They are group of large nuclei (grey matter) that lie deep within
the cerebral hemispheres and connect with motor cortex and other brain
areas. They are important for controlling voluntary movement through
activation by the motor cortex.
II- THE DIENCEPHALON:
It is the second component of the forebrain which is divided into
two major parts (figure 15):
1) The thalamus.
2) The hypothalamus.
(1) THE THALAMUS:
Is a collection of several large nuclei (separated by the third
ventricle) into two thalami lie close together. Functionally, the thalamic
region serves as a relay station for various sensory inputs (except
olfaction) before arriving to the primary cortical areas responsible for
sensation (sensory function).
It is also important to relay motor signals coming from basal
ganglia and cerebellum on their way to the motor areas of the cortex
(motor function).
(2) THE HYPOTHALAMUS:
Is located at the base of the brain, just above the pituitary gland
and below the thalamus. Hypothalamus is a tiny region that accounts for
less than 1% percent of the brain weight. It contains different nuclei and
pathways that form the center for controlling endocrine functions (neuroendocrine coordination) and play an important role in controlling
behaviors having to do for preservation of the individual life.
Functions:
a- Control of endocrine functions:
Its secretions control the activity of the pituitary gland which in
turn regulates the action of other endocrine glands. This occurs through
secretion of releasing factors and releasing- inhibiting factors which are
carried by its blood portal system to the anterior pituitary and tigger it’s
secretion of such hormones as, adrenocorticotropic hormone (ACTH),
follicle-stimulating
thyrotropic
hormone
hormone
(TSH),
(FSH),
and
luteinizing
lactogenic
hormone
hormone
(LH),
(LTH).
Hypothalamus is also responsible for production of the posterior pituitary
hormones (antidiuretic hormone and oxytocin) and regulation of their
release.
b- Autonomic nervous control:
Hypothalamus is considered the principal higher brain center for
control of ANS. Generally, the anterior and medial portion of
hypothalamus are associated with the parasympathetic system, while the
sympathetic system is regulated by the posterior and lateral portions.
C- Cardiovascular regulation:
The hypothalamus produces regulating effects on the heart and
blood vessels indirectly, by impulses relayed to the cardiovascular
centers of medulla.
d- Body temperature regulation:
A number of temperature regulating centers is located in
hypothalamus to keep the body temperature at 37 C˚. If there is excess
heat, impulses from temperature centers in hypothalamus (through the
autonomic system) cause dilation of the blood vessels in the skin, and if
the body is colder than it should be, hypothalamic impulses cause
constriction of the cutaneous blood vessels.
e- Regulation of food intake:
There are two separate centers for regulating food intake, a hunger
center and satiety center. Stimulation of hunger center causes the animal
to eat, while stimulation of the satiety center causes an animal to ignore
food. The hunger center is stimulated by a fall of the level of glucose in
the blood. The levels of blood amino acids and fatty acids also play a
role, although less pronounced.
f- Regulation of water balance:
If there is an increase in osmotic pressure due to water lack, two
separate mechanisms go into effect: (1) The antidiuretic hormone (ADH)
is produced, traveled along the nerve fiber that link the hypothalamus
with the posterior pituitary gland and is secreted from the latter into the
blood to cause the kidney tubules to conserve water, and a more
concentrated urine is excreted. (2) Thirst center produces feeling of thirst,
which stimulates the action of drinking.
g- Control of emotional behavior:
A number of centers associated with emotional reponses (anger,
fear, pain and pleasure) are found in the hypothalamus (both in animals
and in human) for participation in the control of emotional behaviors.
PINEAL BODY:
The roof of diencephalon gives outgrowth termed pineal body (also
known as pineal gland). It usually take the form of a small oval mass
attached to the brain by a fine stalk (figure 15). Pineal gland is sensitive
to changes in light, receiving its information about light from the eyes to
secrete melatonin hormone. Melatonin secretion, therefore, undergoes a
marked 24-h cycle, being high at night and low during the day and it
seems to affect skin pigmentation in lower vertebrates and to play a role
in regulating biological rhythms in higher vertebrates.
Figure 15: Diagram showing various regions of the brain.
LIMBIC SYSTEM :
The portion of the forebrain mainly concerned with emotions is
termed limbic system. It includes portions of
the cortex, thalamus,
hypothalamus, basal nuclei and areas deep within the cerebrum,
connected together in an integrated system.
FUNCTIONS :
1- The limbic system primarily concerned with initiation and regulation
of emotions and emotional homeostasis.
2- The limbic system serves to support the somatic motor reactions
evoked by the emotional state through acceleration of the heart rate,
elevation of blood pressure, release of adrenaline and increase of blood
flow to skeletal muscles.
III- CEREBELLUM :
The cerebellum (little brain in Latin) consists of two deeply
convoluted hemispheres. Each composed of an outer layer of grey matter
(the cerebellar cortex), surrounding a mass of white matter, which
contains several deeper cell clusters (nuclei). The cerebellum is another
non cortical (subcortical) area that involved in the control and integration
of the body voluntary movements.
FUNCTIONS:
Cerebellum doesn't initiate voluntary movement, however it is an
important center for coordinating movement and for controlling posture
and balance. The cerebellum accompalish these functions by receiving
information (neural inputs) from eyes, ears, skin, muscles, joints or
viscera and from areas of brain involved in control of voluntary
movement.
IV- BRAINSTEM :
The brainstem connects spinal cord to the rest of the brain. It
consists of:
1- MIDBRAIN :
The midbrain is the upper most portion of the brainstem. It consists
mainly of two large bundles of nerve fibers (white matter) called the
cerebral beduncles uniting the pons with the thalamic region of the
cerebrum. Between the beduncles, there is a narrow duct known as
cerebral aqueduct (aqueduct of midbrain) which connect the third
ventricle with the fourth ventricle.
FUNCTIONS :
a) The beduncles serve to connect motor fibers from cerebrum to
cerebellum and to the spinal cord, that gives rise to a motor pathway
conveying impulses to the skeletal muscle (concerned with the voluntary
movement).
b) The midbrain contains also the neuron cell bodies (nuclei)
giving rise to the cranial nerves III (oculomotor) and IV (trochlear).
2- THE PONS:
Is about 2.5 cm in length. It contains a large bundles of fibers
consisting of descending fibers to the spinal cord and fibers passing from
the pons to the cerebellum.
FUNCTIONS:
a) The above mentioned pathways in the bones are a part of the system
that coordinate muscular activity.
b) The pons also contains the neuron cell bodies (nuclei) giving rise to
the cranial nerves V(trigeminal), VI (abductor), VII (facial) and VIII
(auditory).
3- MEDULLA OBLONGATA:
Is the remaining portion of the brainstem, about 3 cm in length. It
continues through the foramen magnum of the skull with the spinal cord.
FUNCTIONS:
a) The descending fibers of the pons cross over in the medulla and
continue down the spinal cord as the fiber tracts involved in the voluntary
movements.
b) The medulla also contains the neuron cell bodies (nuclei) of cranial
nerves IX (glossopharyngeal), X (vagus), XI (accessory) and XII
(hypoglossal).
In the medulla, some portions of the cranial nerve nuclei form the
so called vital centers of the medulla. Lesions in the medulla are
especially serious if they involve the vital centers.
These centers include the following :
• Cardiac Centers:
They are cardioacceleratory and cardioinhibitory centers ,
receiving impulses that rise in receptors in several body areas and
sending impulses to the heart to regulate its rate of beat.
• Respiratory Centers:
Inspiratory and expiratory centers form part of the system
responsible for regulating respiratory activity.
• Vasomotor centers:
Vasodilator and vasoconstrictor centers deal with the diameter of
muscular blood vessels and thereby control the blood pressure.
All of these vital centers are integrated in their activity, so that they
complement one another to achieve a desired end result. During exercise,
for example, heart rate increases, vasocontraction increases, blood
pressure, and breathing is stimulated to increase oxygen intake and
carbon dioxide elimination.
The medulla, also including portions of the medullary cranial nerve
nuclei, that form non–vital centers for intergration of the acts of:
•Swallowing
•Vomiting
• Sneezing
• Coughing
THE RETICULAR FORMATION:
It is a series of nerve cells (nuclei) that constitute a reticular
formation, located in the brainstem.
FUNCTIONS:
The nuclei of the reticular formation receive inputs from most of the
sensory systems of the body (optic, olfactory & auditory systems). It acts
as a filter to incoming stimuli and determines important from
unimportant before arriving to higher brain centers. For example, you
may be unaware of conversation in a crowded situation, but the system
alerts you when you hear your name. Stimulation of the "formation
fibers" that pass to the thalamus and then to cerebral cortex, causes
activation and alertness of the cortex in general and aids in maintaining
the conscious state or wakefulness.
2- THE SPINAL CORD
The spinal cord lies within the bony vertebral column. It is a
selender of soft tissue with variable length among different vertebrates.
In human, it is divided into 31 segments arranged into five regions
(figure 16):
1- Cervical region (8 segments).
2- Thoracic region (12 segments).
3- Lumber region (5 segments).
4- Sacral region (5 segments).
5- Coccygeal region (1 segment).
A cross section of the spinal cord shows central butterfly shaped
area composed of interneurons, the cell bodies and dendrites of efferent
neurons, the entering fibers of afferent neurons and glial cells. It is called
grey matter because there are more cell bodies than myelinated fibers.
The grey matter is surrounded by white matter which consists of groups
of myelinated axons of interneurons. These groups of axons called fiber
tracts or pathways, run longitudinal through the cord. Some descending
(efferent) to relay information from the brain to the spinal cord and others
ascending (afferent) to transmit information to the brain. Pathways also
transmit information between different levels of the spinal cord.
Figure 16: Spinal cord segments.
A short distance from the cord, the dorsal and ventral roots from
the same level combine to form a spinal nerve, one on each side of the
spinal cord (figure 17). The spinal nerves are 31 pairs. They are
designated by the four vertebral levels from which they exit: cervical,
thoracic, lumbar and sacral.
Figure 17: Spinal nerve.
FUNCTIONS OF SPINAL CORD:
1- Spinal cord is responsible for all immediate reflexes in which brain is
not involved.
2- Spinal cord is the main path way connecting the brain and peripheral
nerveous system.
3- Conducts information to and from the brain.
CROSSING OVER OF THE SPINAL TRACTS:
Impulses reaching the spinal cord (ascending) from one side of the
body eventually pass over to tracts running up to the other side of the
brain and vice versa. In some cases this crossing over occurs as soon as
the impulses enter the cord. In other cases, it doesn't take place until the
tracts enter the brain it self.
Figure 18: crossing over of the ascending (a&b) and
descending (c) spinal tracts.
II-PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system (PNS) provides a communication
between the CNS and other tissues. Anatomically it is composed of 12
pairs of cranial nerves and 31 pairs of spinal nerves.
I- Cranial Nerves (12 pairs):
These are connected to the brain and composed of:
1- Nerves attached to cereberum:
a) Olfactory nerve (I).
b) Optic nerve (II).
2- Nerves attached to midbrain:
a) Occulomotor nerve (III).
b) Trochlear nerve (IV).
3- Nerves attached to pons:
a) Trigeminal nerve (V).
b) Adbucent nerve (VI).
c) Facial nerve (VII).
d) Auditory nerve (VIII).
4- Nerves attached to medulla oblongata:
a) Glossopharyngeal nerve (IX).
b) Vagus nerve (X).
c) Accessory nerve (XI).
d) Hypoglossal nerve (XII).
Note that cranial nerves may be:
• Purely sensory: I, II, VIII.
• Purely motor: III, IV, VI, XI, XII.
• Mixed (sensory and motor): the remaining (table 1).
Table 1: The cranial nerves and information they transmit.
Name
Fibers
I. Olfactory
Sensory
Carries inputs from receptors in olfactory
epithelium.
II. Optic
Sensory
Carries inputs from receptors in eye
III. Oculo-
Comments
Motor
Innervates skeletal muscles that move
eyeball.
IV. Trochlear
Motor
Innervates skeletal muscles that move
eyeball
V. Trigeminal
Sensory
motor
(mixed)
VI. Abducens
VII. Facial
Transmits information from receptors in
skin of head
Motor
Innervates muscles of Jaw.
Motor
Innervates skeletal muscles that move
eyeball.
Sensory
(mixed)
Motor
Transmits information from taste buds in
tongue and mouth.
Innervates skeletal muscles of face.
VIII. Auditory
Sensory
Transmits information from receptors in
ear.
IX. Glossopharyngeal
Sensory
Transmits information from taste buds at
tongue.
(mixed)
Motor
Innervates skeletal muscles involved in
swallowing and salivary glands.
Sensory
Transmits information from receptors in
thorax and abdomen
Motor
Innervates skeletal muscles of thorax and
abdomen.
XI. Accessory
Motor
Innervates neck skeletal muscles.
XII. Hypo-
Motor
Innervates skeletal muscles of tongue.
X. Vagus
(mixed)
glossal
In summary: all cranial nerves are involved with the head and
neck regions, but the vagus nerve manages the internal organs.
II- Spinal Nerves: (31 pairs):
• These are attached to the sides of the spinal cord.
• Each spinal nerve arises by two roots, dorsal and ventral (figure
17).
• They are designated by the four vertebral levels from which they
arise: cervical, thoracic, lumber and sacral (figure 16).
For The Spinal Nerves:
• Eight cervical nerves control the neck, shoulders, arms and hands.
• Twelve thoracic nerves are associated with the chest and abdominal
walls.
• Five lumber nerves are associated with the hip and legs.
• Five sacral nerves are associated with the genital and lower digestive
tract.
• A single pair of coccygeal nerve brings the total 31 pairs.
As noted earlier, the spinal nerves contain nerve fibers that are the
axons of both efferent and afferent neurons (mixed nerves). Afferent
neurons carry information from sensory receptors to the CNS. The long
part of their axon is outside CNS and is part of the PNS. While, efferent
neurons carry signals out from the CNS to muscles or glands. This
efferent division of PNS is more complicated than afferent being and is
subdivided into:
(1)Somatic division (voluntary).
(2)Autonomic division (involuntary) (figure 19).
The simplest distinction between somatic and autonomic divisions is that:
• Neurons of somatic division innervate mainly skeletal muscles.
• The autonomic neurons innervate smooth and cardiac muscles, glands
and the gastrointestinal tract (GT) (internal organs).
Figure 19: Divisions of nervous system .
AUTONOMIC (INVOLUNTARY OR VISCERAL)
NERVOUS SYSTEM
The nervous system is functionally divided into: (1) somatic
nervous system which controls organs under voluntary control (mainly
muscles) and (2) the autonomic nervous system (ANS) which regulates
functions of individual
visceral systems, that are not under direct
voluntary control (table 2).
The ANS is predominantly an efferent system transmitting
impulses from the (CNS) to peripheral organ systems. Its effects include
control of heart rate and force of contraction, constriction and dilation of
blood vessels, contraction and relaxation of smooth muscles in various
organs, and secretions from exocrine and endocrine glands. Autonomic
nerves constitute all of the efferent fibers which leave the CNS, except
for those which innervate skeletal muscles.
The ANS includes also some afferent fibers
which are
concerned with transmission of visceral sensation to CNS. These
afferent fibers are usually carried to the CNS by major autonomic
nerves, including the vagus, splanchnic or pelvic nerves.
The ANS is divided into two separate divisions called the
sympathetic and parasympathetic systems, on the basis of anatomical
and functional differences. Both of these systems consist of myelinated
preganglionic
fibres
which
make
synaptic
connections
with
unmyelinated postganglionic fibres, which then innervate the effector
organ (table 2). These synapses usually occur in clusters called
autonomic ganglia. Most organs are innervated by fibers from both
divisions of the ANS, and the influence is usually opposing.
Table (2): Comparison between Somatic and Autonomic
Nervous System.
Feature
Somatic Nervous
Autonomic Nervous
System
System
1- Control
Voluntary functions
Involuntary functions.
2- Afferent
Carry cutaneous sensation
Carry visceral sensations.
(sensory)
fibers
3- Center
In the brain and anterior In the brain and lateral horn
horn in the spinal cord.
in the spinal cord.
4-Efferent
-One neuron.
-Double neurons.
(motor) fibers
-No ganglia (i.e. no synapse -Presence of ganglia (i.e.
synapse outside CNS).
outside CNS).
-Thick
myelinated
nerve -Preganglionic
is
thin
myelinated nerve fibers and
fibers.
postganglionic
is
non
myelinated nerve fibers.
-Fast conducting.
-Leads
only
to
exitation (contration).
-Slowly conducting.
muscle -Either excitatory or
inhibitory to effector organs.
5- Effector
Mainly skeletal muscles.
organs
Smooth
muscles,
cardiac
muscles and glands.
6- Chemical
Acetylcholine.
transmitter
At preganglionic nerve
endings: acetylcholine and
at
postganlionic
endings:
nerve
acetylcholine
norepinephrine.
Parasympathetic Nervous System:
The preganglionic outflow of the parasympathetic nervous system
arises from the cell bodies of the motor nuclei of the cranial nerves III,
VII, IX and X in the brain stem and from the second, third and fourth
sacral segments of the spinal cord. It is therefore also known as the
cranio-sacral outflow (figure 20).
Preganglionic fibres run long distant to the effector organ and
synapse in ganglia close to or within that organ, giving rise to
postganglionic fibres which then innervate the effector organ.
The chemical transmitter at both pre and postganglionic synapses in the
parasympathetic system is acetylcholine.
Functions:
The parasympathetic system is dominant when the individual is
relaxed and non threatened (normal situations). It is concerned with
preservations energy in the body, as it causes a reduction in heart rate and
blood pressure, and facilitates digestion and absorption of nutrients, and
consequently the excretion of waste products.
or
Figure 20: Parasympathetic nervous system.
Sympathetic Nervous System:
The cell bodies of the sympathetic preganglionic fibres are in the
lateral horns of the spinal cord in the thoracic and in the frist and second
lumbar segments (T1-L2), so called thoraco-lumbar outflow (finger 21).
The preganglionic fibres travel a short distance in the mixed spinal nerve,
and then branch off to enter the sympathetic ganglia, which lie close to
the spinal cord and from two chains of ganglia extending from the
cervical to the sacral region. They are called the sympathetic chains
(trunks). The short preganglionic fibres which enter the chain make a
synapse with a postsynaptic fiber either at the same level, or at a higher
or lower level, and then the postganglionic fibers extend long distance to
innervate the effector organ.
Figure 21: Sympathetic nervous system.
In the sympathetic system, some preganglianic fibres do not
synapse in the sympathetic chains but terminate in separate ganglia,
called collatral ganglia (the celiac, superior mesenteric & inferior
mesenteric ganglia) located in the abdominal cavity close to the
innervated organ (figure 22). The neurotransmitter at the perganglionic
synapse is acetylcholine, while norepinephrine is the neurotransmitter in
the postsynaptic endings.
Functions:
In contrast to the parasympathetic system, the sympathetic system
enables the body to be prepared for fear, flight or fight (emergency
situations). Sympathetic stimulation causes an increase in heart rate,
blood pressure and cardiac output, a diversion of blood flow from the
skin and splanchnic vessels to those supplying skeletal muscle, increased
pupil size, broncho-dilation, contraction of GIT sphincters. Sympathetic
activity also increases energy expenditure of the body by increasing
mobilization of glucose from glycogen and inhibition of insulin secretion
from pancreas.
Figure 22: Course of sympathetic fibers.
Adrenal Medulla:
The adrenal medulla is a modified sympathetic ganglion, whose
cell bodies do not send out post ganglionic axons, but instead, upon
activation by preganglionic axons release their transmitters into the blood
stream (figure 23). This “ganglion”, called the adrenal medulla, therefore
functions as an endocrine gland whose secretion is controlled by
sympathetic preganglionic nerve fibers. It releases a mixture of about
80% adrenaline (epinephrine) and 20 % noradrenaline (norepinephrine)
into the blood (plus small amounts of other substances including
dopamine.
These
catecholamines,
called
homones
rather
than
neurotransmitters.
Figure 23: Transmitters used in the various components of the
autonomic nervous system. They are: Ach, acetylcholine; NE,
norepinephrine; E, epinephrine.
The Physiologic Actions Of Sympathetic And
Parasympathetic Systems.
The action of the sympathetic and parasympathetic inputs to an
organ generally have opposite effects, however, actions may be parallel
(table 3). These actions operate to control visceral functions. For
instance, when the blood pressure becomes too high, the parasympathetic
inputs are activated to decrease the heart rate, by releasing acetylcholine.
When acetylcholine binds to receptors located on pacemaker cells (which
are cardiac cells that regulate heat rate), this slows their pacemaker
activity, decreases the heart rate, and ultimately decreases the blood
pressure. When the blood pressure is too low, the sympathetic inputs to
the heart release norepinephrine. The binding of norepinephrine to
receptors on cardiac cells resulting in an increase in heart rate.
Table 3: Actions of sympathetic and parasympathetic
divisions.
(I)Antagonistic action:
Sympathetic
Parasympathetic
1- Pupil
Dilation
Constriction
2- Air passages.
Dilatation
Constriction
3- Heart
4- GIT: Wall
rate of contraction
Relaxation &
secretory activity.
rate of contraction
Contraction &
secretory activity.
5- Rectum
Retention of faeces
Defecation
6-Urinary bladder
Retention of urine
Micturition
7- Blood vessels
Vasoconstriction
Vasodilatation
Little and viscid
Large and watery
salivary secretion
(II) Parallel action:
Salivary secretion
salivary secretion
In summary: (1) whole sympathetic actions tend to go off together,
while parasympathetic actions do not go off together. (2)
stimulation of sympathetic actions is useful, while parasympathetic
stimulation is fatal