THE NERVOUS SYSTEMDescription/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.

The Axon :( See diagram 3A and 3B) The characteristic features of the axon are:It arises from the region of the cell body called the axon hillockIt is often longer than the dendrites but of uniform diameter.A typical neuron possesses only one axon.Its plasmalemma is called the axolemma and its cytoplasm the axoplasm.It contains all neuronal organelles except Nissl bodies and Golgi complex.

May have collateral branches. May be covered by myelin sheath for insulation and

rapid conduction of impulses. Conducts impulses away from the cell body. Its terminal branches are called telodendria, which

terminate in dilated terminals (Boutons terminaux) that are involved in formation of contact points called Synapses.

The initial segment is often involved in inhibitory axo-axonal synapses.

The DendriteThe characteristic features of the dendrite are:It is often shorter than the axon and tapers from the cell body hence the diameter is not uniform.It has numerous branches.Its branches bear minute projections called dendritic spines (Gemmules), which are involved in synapses.Usually conducts impulses towards the cell body.It is involved in forming contact points (Synapses).It is devoid of Golgi apparatus and Lysosomes but contains most of the other organelles found in the cell body.

Classification of Neurons:The classification of neuron is based on various criteria, which include:1. Functions Motor (Efferent).Sensory (Afferent).Interneurons (Internuncial). 2. Morphology: (See Diagram 4)Multipolar.Bipolar.Unipolar.Pseudounipolar. (Dorsal root ganglion & Mesencephalic nucleus of the trigeminal nerve)

3. Distribution:Somatic (Supplying the body wall or skin) e.g. A typical spinal nerve.Visceral (Autonomic or supplying internal organs) e.g. the vagus nerve (a cranial nerve). 4. Axonal size/Length:Golgi type I (Possesses a long axon).Golgi type II (Possesses a short axon).

Neuronal Junctional Points (SYNAPSES): (See Diagram. 5)Synapses are contact points between neurons or between neurons and non-neuronal cells/non-neuronal structures.Classification of Synapses: This is based on the structures at the contact point as well as the mode/mechanism of transmission of impulse at the contact point. Thus synaptic types include:Morphological Synapses:

Axo-somaticAxo-axonal.Axo-dendritic.Dendro-dendritic

.

Chemical Synapses: Chemical synapses are associated with the release of neurotransmitters at the contact points.Two examples of neurons based on neurotransmittertype are:

Cholinergic neuronAdrenergic neuron

Electrical Synapses:These are associated with movement of ions through the

junction. Furthermore, at electrical synapses, transmission of impulse occurs either way

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 PNS4. 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 SYSTEMFunctions: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 S4.2. It is an energy restoring/conserving system.3. It utilizes Acetylcholine as it

neurotransmitter

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 andfrom the brain and as a center for peripheral reflexactivities.For the execution of these functions, nerve fibres in thewhite matter are organized into vertical bundles called tracts. Bundle conveying information to the brain arereferred to as ascending tracts while those conveyinginformation from the brain to the spinal cord are referredto as descending tract.

THE SPINAL NERVEEach 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.

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. Prosencephalon2. 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 oblongataThe PonsThe 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 non-cranial 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.

• 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.

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.

• 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.

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 functionsassociated with each lobe are:• The Frontal lobe: This is involved in:

Initiation of all voluntary motor activitiesMotor aspect of speech (Speaking; Talking)Language comprehensionHigher 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 memoryPerception 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.

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

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:

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 injury3. Spastic paralysis of the two lower limbs4. Exaggerated tendon reflexQuadriplegia: Paralysis of the whole trunks including all four limbs: Thisoften due to spinal transection at the upper cervical level of the spinal cord. It is characterized by:1. Paralysis of all four limbs2. Loss of somatic and visceral reflexes below level of

injury3. Loss of all sensations below level of injury4. Spastic paralysis of the two lower limbs5. Exaggerated tendon reflex

The Brain: • The brain can also be damaged by:• Trauma as in gun shot, fracture and road traffic accident• Degeneration leading to loss of neurons as in Alzheimer’s and

Parkinson's disease.• Tumors leading to pressure on parts of the brain• Cardiovascular disease due to obstruction to cerebral blood flowThe commonest of these injuries is cardiovascular disease resulting in STROKE:• Stroke is cerebrovascular disease due to inadequate blood

supply (Ischemic stroke) or bleeding into the brain (Hemorrhagic stroke).

• 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