Transcript The nervous tissue is made up of

THE NERVOUS SYSTEM
Description/Definition:
The nervous system is composed of an immensely
structured, integrated and coherent network of
neurons and associated supportive cells.
Functions:
1. Orientation to and perception of the ever-changing
internal and external environments.
2. Activation, control and coordination of all the other
body systems.
3. Conceptual thought (Intelligence).
• This is a high intellectual function in which is borne
the mechanism for; imagination, abstract
reasoning, creativity, predictions, calculation and
control of violence.
• It is an exhibition of the capacity of the nervous
system to record, process, store and to relate
information received and actions taken as a fund of
experience to be used in determining future
reactions to environmental changes/situations.
The Nervous Tissue:
The nervous tissue is made up of:
• Neurons, which are the functional units of the
nervous system and
• Neuroglia and connective tissue, which support,
maintain, protect and nourish the neurons.
The Neuron (See Diagram):
• The neuron is the morphological and functional unit
of the nervous tissue/system.
• It is highly specialized in the physiological properties
of Excitability and Conductivity.
• On stimulation, it generates an electrical change
called impulse which is propagated along its cell
surface and protoplasmic extension known as:
The Axon.
The neuron is composed of: (See Diagram)
• The cell body (perikaryon) which:
Accommodates a nucleus.
Is enclosed in a plasma membrane with spinous
processes called Gemmules.
Accommodates a cytoplasm, which contains
organelles and inclusion bodies.
• Cytoplasmic processes (Neuritis or Neuropil):
These are projections from the surface of the
perikaryon called Axon and Dendrites.
• Axons and dendrites of neurons make contacts at
junctions called synapses
• The neuron contains organelles like other Eukaryotic cell.
• However it contains certain organelles and inclusion
bodies, which are peculiar to it.
• The organelles are collectively referred to as Nissl bodies
and include:
Ribosomes,
Polyribosomes and
Granular endoplasmic reticulum.
• The inclusion bodies include:
Iron and
Zinc mineral and
Lipofusin and
Melanin pigments.
• Lipofusin is of value in determining the age of the
neuron in which it is found.
• It is never found in the Purkinje neuron of the
cerebellum.
Classification of Neurons:
• Neurons are commonly classified on the basis of the
number of processes arising from the cell body and
the type of neurotransmitter used by the neuron.
• There are four types of neuron based of processes,
These are (See Diagram):
Unipolar neuron
Pseudounipolar neuron
Bipolar neuron
Multipolar neuron
• Two
examples
of
neurons
neurotransmitter type are:
Cholinergic neuron
Adrenergic neuron
based
on
Neuroglial Cells (See Diagram)
• These are the supportive cells of the nervous tissue.
• They provide physical support, nutrients, defense
and insulation and are involved in the re-absorption
of neurotransmitters in the nervous system.
• They exist in various forms and are widely
distributed throughout the peripheral and central
parts of the nervous system. They include:
1. Fibrous Astrocytes (Astroglia) - Distributed within
the white matter of the central nervous system
(CNS)
2. Protoplasmic (Velate) Astrocytes - Distributed
within the gray matter of the CNS.
3. Oligodendrocytes:
For
myelination
(Insulation) in the central nervous system.
4. Schwann cells: For myelination in the
peripheral nervous system.
5. Satellite cell: Found around neurons in the
peripheral nervous system.
6. Resting microglia: These are phagocytic cells
in the central nervous system.
7. Ependyma: These are cells which line the
ventricular cavities of the central nervous
system.
Organization of the Nervous System:
The nervous system is subdivided into:
1. The Central Nervous System (CNS), which includes
The brain and
The spinal cord
2. The Peripheral Nervous System (PNS), which includes:
• Cranial and spinal nerves
• Cranial ganglia of Cr.N: V, VII, VIII, IX and X
• Dorsal root ganglia (Spinal ganglia).
• Autonomic nerves.
• Autonomic ganglia:
Cervical.
Thoracolumbar.
Preaortic.
Visceral.
3. Autonomic nervous system (ANS):
• This serves as the meeting point of the CNS and
the PNS
4. The Enteric nervous system (ENS):
• This is the special automatic nervous system of
the gastrointestinal, which enables the tract to
operate independent of the CNS.
• It initiates and maintains peristalses and
secretions in the gastrointestinal system.
THE AUTONOMIC NERVOUS SYSTEM
Functions:
The function of the autonomic nervous system
includes:
1. Innervation of smooth muscles and secretory cells
of the viscera.
2. Innervation of the smooth muscles of the ciliary
body and the iris of the eyeball.
3. Innervating of the smooth muscles of the orbit and
the upper eyelids.
4. Innervation of the lacrimal and the salivary glands.
5. Innervation of the sweat glands.
6. Innervation of the arrector pili muscles of the skin.
7. Innervation of the blood vessels of the entire body.
8. Regulation of body functions.
Organization of the Autonomic system.
The autonomic system is subdivided into:
1. The Sympathetic Division:
This has the following features:
It originates from the thoracolumbar segment of
the spinal cord.
It is an energy consuming system.
It is utilizes in Flight and fight situations.
It utilizes Noradrenaline as its neurotransmitter.
Hence it is referred to as the noradrenergic
system.
2. Parasympathetic Division:
This has the following features:
i. It originates from cranial nerves, III, VII, IX
and X and Sacral segments S2, S3 and S4.
ii. It is an energy restoring/conserving system.
iii. It utilizes Acetylcholine as it
neurotransmitter
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The Spinal Cord
The spinal cord is a cylindrical mass of nervous tissue
located in the vertebral canal of the vertebral
column.
It is the caudal continuation of the brainstem
measuring about 45cm in the male and about 43cm
in the female (As long as the femur).
Within the vertebral canal it extends from the
foramen magnum at the base of the skull to the
lower border of the first lumbar vertebral bone (L1).
It is enclosed and protected by three membranous
sheaths called the Meninges. From within outwards
these are the Pia mater, Arachnoid mater and Dura
mater.
• A protective and nutritive fluid, the cerebrospinal fluid
which, circulates in the subarachnoid space between the
pia and arachnoid mater, also encloses the spinal cord.
• The subarachnoid space ends at the level of the second
sacral vertebral bone (S2).
• The spinal cord is divided into 5 regions
• Cervical, Thoracic, Lumbar, Sacral and Coccygeal
• The spinal cord is further subdivided into 31 segments
• Each segment gives rise to a pair of spinal nerves which,
• Supply the body wall and other peripheral areas of the
body.
• The interior of the spinal cord is composed of
1.An outer layer of white matter which is made up
of nerve fibres,
2.An inner layer of gray matter which is made up
of neuronal cell bodies and a central canal which
may contain cerebrospinal fluid.
• The gray matter is shaped like a butterfly in cross
section and consists of three parts on either side
of the midline, viz.
• Dorsal horn
• Intermediate horn and
• Ventral horn.
• The white matter is also subdivided into three parts, viz.
1. Dorsal column (Funiculus)
2. Ventral column (Funiculus) and
3. Lateral columns (Funiculus)
The spinal cord serves as a dual pathway to and
from the brain and as a center for peripheral reflex
activities.
For the execution of these functions, nerve fibres in the
white matter are organized into vertical bundles called
tracts. Bundle conveying information to the brain are
referred to as ascending tracts while those conveying
information from the brain to the spinal cord are referred
to as descending tract.
THE SPINAL NERVE
Each spinal nerve is made up of:
• A Dorsal (Sensory) root, which contains incoming
fibres whose cell bodies are located in the Dorsal
root ganglion and
• A Ventral (Motor) root which contain outgoing
fibres whose cell bodies are located in the ventral
aspect (Ventral horn) of the gray matter of the
spinal cord.
• The two roots unit just before emerging from the
vertebral canal to form the Spinal nerve Trunk
which is often referred to as the Spinal nerve.
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Development of the brain
The brain which is ectodermal in origin is derived from
the upper part of a blind-ended neural tube beginning
from the third week of gestation.
Three primary vesicles are formed at the upper part of
the neural tube. These are called:
1. Prosencephalon
2. Mesencephalon and
3. Rhombencephalon.
Following further development, the Prosencephalon
and Rhombencephalon split into two secondary
vesicles each giving rise to 5 secondary vesicles in all.
These secondary vesicles will then proceed to form
different components of the brain as shown in the chat
below:
THE BRAINSTEM
• The brainstem is the lower part of the brain which
is continuous with the spinal cord at the foramen
magnum.
• It is composed of three parts, from below upwards
these are:
The medulla oblongata
The Pons
The Midbrain
• Like the spinal cord the brainstem also serves as a
dual pathway between the brain and the periphery
via the spinal cord.
• The brainstem also contains aggregations of
neurons called nuclei, which contain the cell
bodies of cranial nerve fibres as well as noncranial nerve fibres.
• The nuclei of the brainstem are implicated in
the control of all the vital functions of the body
such as respiration, cardiac functions,
swallowing and vomiting.
• Special sensory functions such as vision,
hearing, taste and balancing of the body are
also under the influence of the brainstem.
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The brainstem is also associated with the source and
termination of cranial nerve fibres.
• Ten out of the twelve pairs of cranial nerves are
attached to the brainstem. From above downwards:
1. The occulomotor nerve (Third cranial nerve) which
controls eye movements is attached to the Midbrain
2. The Trochlear nerve (Fourth cranial nerve) which also
controls eye movements is attached to the Midbrain
3. The Trigeminal nerve (Fifth cranial nerve) which
contain sensory fibres from the eyes, nose and mouth
and motor fibres to muscles of mastication is attached
to the Pons
• The Abducent nerve Sixth cranial nerve) which
also contributes to the control of eye
movements is connected to the Pons
• The Facial nerve (Seventh cranial nerve) which
control the muscle for facial expression and
contain special sensory fibres to the tongue is
connected to the Pons
• The Vestibulochochlear nerve (Eight cranial
nerve), which is responsible for hearing and
body balancing is connected partly to the pons
and the medulla oblongata
• The Glossopharyngeal nerve (Ninth cranial nerve)
which contain sensory fibres to the tongue and palate
is connected to the Medulla oblongata
• The Vagus nerve (Tenth Cranial nerve), which contains
sensory and motor fibres to the gastrointestinal,
respiratory and cardiovascular systems, is connected to
the Medulla oblongata.
• The Accessory nerve (Eleventh cranial nerve) which
contains motor fibres to the neck and voice box is
connected to the Medulla oblongata
• The Hypoglossal nerve (Twelve cranial nerve), which
contains motor fibres for the muscles of the tongue, is
connected to the Medulla oblongata.
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The Cerebellum:
The cerebellum is part of the hindbrain and is
connected to the posterior aspect of the Pons and
Medulla oblongata.
It is separated from these two parts of the brainstem
by the fourth ventricle of the brain.
The cerebellum is shaped like cauliflower with an
irregular surface and subdivide into two hemisphere.
The main function of the cerebellum is the
coordination of muscular movements and the
maintenance of body balance and equilibrium.
Cerebellar damage often result in clumsy and
disorganized movement referred to as Ataxia.
Ataxic patients walk as if they are drunk.
The Diencephalon:
• The diencephalon is the rostral continuation of
the brainstem thus serving as a bridge between
the cerebrum and the brainstem.
• The diencephalon is derived originally from the
lower part of the Procencephalon of the
embryonic neural tube.
• The upper part of the Procencephalon is
referred to as the Telencephalon and forms the
cerebral hemisphere of the adult brain.
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The diencephalon is subdivided into four
components, namely
1. The Thalamus,
2. The Hypothalamus,
3. The Epithalamus and
4. The Subthalamus.
These four component surround the third
ventricle of the brain which in turn contains
cerebrospinal fluid.
• The main function of the diencephalons is the
integration of sensory and motor activities of
the brain.
• Crude awareness of sensation may be felt at the
level of the diencephalons but a comprehensive
awareness only occurs when the sensation
reaches the cerebrum.
• The hypothalamic part of the diencephalons
also constitutes the center of control of all
autonomic and endocrine functions of the body.
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The Cerebrum: (See Diagram)
This is the most rostral and largest part of the brain.
It is roughly spherical in shape with irregular
convolutions on its surface.
The convolutions are referred to as gyri and are
separated from one another by grooves referred to as
sulci.
The cerebrum is further subdivided into two
hemispheres separated in the median plane by a very
deep groove called the longitudinal fissure.
The two hemispheres are further separated from the
cerebellum below by a transverse deep groove called
the transverse fissure.
Each hemisphere is further subdivided into lobes which
are named after the skull bone that overlie them.
The lobes of the cerebrum and the functions
associated with each lobe are:
• The Frontal lobe: This is involved in:
Initiation of all voluntary motor activities
Motor aspect of speech (Speaking; Talking)
Language comprehension
Higher mental functions
• The Parietal lobe: This is involved in:
– Recognition and interpretation of all sensory
information reaching the brain.
The occipital lobe: This is involved in
Perception and interpretation of visual
information
• The temporal lobe: This is involved in:
Acquisition and storage of memory
Perception and interpretation of the sense of
smell (Olfaction)
Perception and interpretation of the sense of
taste (Gustation)
Perception and interpretation of the sense of
hearing (Auditory function).
The Insula lobe:
 This lobe overlies the group of nuclei
referred to as the Basal Nuclei.
 Basal nuclei are involved in modification of
motor functions initiated by the frontal
lobe.
 Damage to these nuclei leads to
abnormalities in motor activities such as in
Huntington’s disease and Parkinson’s
disease.
The insula lobe also plays a role in the perception
of the:
Sense of smell (Olfaction)
Sense of taste (Gustation)
These two senses are actually coordinated in the
insula.
• Each cerebral hemisphere has a ventricular
cavity called the lateral ventricle which
produces and contains cerebrospinal fluid.
• The interior of each hemisphere is made up of
an outer layer of Gray matter called the cortex
and an inner layer of White matter called the
medulla.
• The outer gray matter contains cell bodies of
neurons of various sizes and shapes while
• The inner medulla contains nerve fibres running
in different directions.
• The two hemispheres are connected across the
midline by a group of fibres referred to as the
Corpus callosum while
• Lobes of the same hemisphere are
interconnected by fibres referred to as
Association fibres.
• The entire cerebrum is connected to other parts
of the brain and spinal cord by Projection fibres.
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Blood supply of the Brain
The brain and spinal cord receive their blood
supply from 4 major arteries.
These are 2 internal carotid arteries and two
vertebral arteries.
These 4 arteries (through their branches) form a
copious network of arteries at the base of the
brain.
The network of arteries is referred to as the
CIRCLE OF WILLIS
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Formation of the Circle of Willis
The two vertebral arteries converge to form the Basilar
artery. The basilar artery runs on the ventral surface of
the Pons towards the ventral aspect of the midbrain.
The basilar artery then bifurcate to form the posterior
cerebral arteries (Left and right)
Anteriorly, each internal carotid artery gives rise to an
anterior cerebral artery.
The two anterior cerebral arteries are connected in the
midline by an anterior communicating artery.
The circle is then completed by the connection of the
internal carotid artery on each side to the posterior
cerebral artery through posterior communicating
arteries as shown in the diagram below:
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Blood Supply of the Spinal cord
The spinal cord receives its blood supply from two major
arteries, one anterior spinal artery and two posterior
spinal arteries.
The anterior spinal artery is derived by the union of the
anterior spinal artery from each vertebral artery.
It runs in a midline groove (Ventral median fissure) on the
ventral aspect of the spinal cord.
Each posterior spinal artery is a branch the vertebral
artery or the posterior inferior cerebellar artery.
Each posterior spinal artery runs in the groove
(Dorsolateral sulcus) on either sides of the midline on the
posterior aspect of the spinal cord.
Segmental arteries from the vertebral, posterior
intercostal, and lumbar arteries also supplement the
arterial supply of the spinal cord (see diagram).
Applied Anatomy of the Nervous System
• A. The Spinal Cord:
The spinal cord can be damaged by:
• Trauma as in gun shot, stabbing and road traffic
accidents.
• Degeneration leading to loss of neurons
• Tumors leading to pressure on spinal cord and
nerve.
• Cardiovascular disease due to obstruction to
spinal blood flow.
These injuries could result in:
– Monoplegia: Paralysis of one lower limp
– Hemiplegia: Paralysis of one side of the body
including the limbs
Paraplegia: Paralysis of the two lower limbs:
This is usually due to complete section of the
spinal cord between the lower thoracic and
upper lumbar regions. It is characterized by:
1. Loss of somatic and visceral reflexes below level of
injury
2. Loss of all sensations below level of injury
3. Spastic paralysis of the two lower limbs
4. Exaggerated tendon reflex
Quadriplegia:
Paralysis of the whole trunks including all four limbs: This
often due to spinal transection at the upper cervical level
of the spinal cord. It is characterized by:
1. Paralysis of all four limbs
2. Loss of somatic and visceral reflexes below level of
injury
3. Loss of all sensations below level of injury
4. Spastic paralysis of the two lower limbs
5. Exaggerated tendon reflex
The Brain:
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The brain can also be damaged by:
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Trauma as in gun shot, fracture and road traffic accident
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Degeneration leading to loss of neurons as in Alzheimer’s and
Parkinson's disease.
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Tumors leading to pressure on parts of the brain
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Cardiovascular disease due to obstruction to cerebral blood flow
The commonest of these injuries is cardiovascular disease
resulting in STROKE:
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Stroke is cerebrovascular disease due to inadequate blood
supply (Ischemic stroke) or bleeding into the brain (Hemorrhagic
stroke).
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It usually results in paralysis of one side of the body opposite
the site of injury. In severe cases it may affect the speech of the
patient