Download Briefed by: Dr. Hayder The human nervous system, by far the most

Briefed by: Dr. Hayder
The human nervous system, by far the most complex system in the body, is formed by a network of many
billion nerve cells (neurons), all assisted by many more supporting cells called glial (neuroglial) cells. Each
neuron has hundreds of interconnections with other neurons, forming a very complex system for: receiving
impulses, processing information and generating responses.
Anatomically, the nervous system is divided into:
- The central nervous system (CNS) consists of the brain & the spinal cord, which are located in the
cranial cavity & spinal canal, respectively.
- The Peripheral nervous system (PNS) consists of cranial, spinal & peripheral nerves that conduct
impulses from (efferent or motor nerves) & to (afferent or sensory nerves of) the CNS. Collections of
nerve cell bodies outside the CNS called ganglia.
Functionally, the nervous system is divided into:
- The somatic nervous system (SNS) consists of somatic (Gr. soma, body) parts of the CNS & PNS. It
controls functions that are under voluntary control. It provides sensory & motor innervation to all parts
of the body except viscera, smooth & cardiac muscle & glands.
- The autonomic nervous system (ANS) consists of autonomic parts of the CNS & PNS. The ANS provides
efferent involuntary motor innervation to the smooth muscle, the conducting system if heart & glands,
it also provides afferent sensory innervation from viscera (e.g. pain). The ANS is subdivided into
sympathetic & parasympathetic divisions.
Composition of nerve tissue
Nerve tissue consists of two principle types of cells, neurons & supporting cells (neuroglial cells)
THE NEURON
The neuron is the structural & functional units of the nervous system.
The human nervous system contains more than 10 billion neurons.
Each neuron consists of a polygonal cell body (AKA soma or
perikaryon)
It is the largest part of the neuron that contains the nucleus and
surrounding cytoplasm, from which cytoplasmic processes (axon &
dendrites) arise. It is primarily a trophic center, & also has receptive
capabilities. The perikaryon of most neurons receives a great number
of nerve endings that convey excitatory or inhibitory stimuli. Most
nerve cells have a spherical, unusually large, euchromatic (pale‐
staining, reflecting the intense synthetic activity) nucleus with a
prominent nucleolus. Binuclear nerve cells are seen in sympathetic
and sensory ganglia. The cell body contains a highly developed rough
endoplasmic reticulum with numerous polyribosomes. With H&E
stains, rER and free ribosomes appear as basophilic granular areas called Nissl bodies. The number of Nissl
bodies varies according to neuronal type and functional state, being abundant in large motor neurons.
The Golgi complex is located only in the cell body (around the periphery of the nucleus). Mitochondria are
scattered throughout the cytoplasm of the cell body. Neurofilaments (intermediate filaments) are abundant in
perikarya and cell processes. The neurons also contain microtubules. Nerve cells occasionally contain inclusions
of pigments, such as lipofuscin, which is a residue of undigested material by lysosomes.
Several processes of varying length (called dendrites) that receive impulses & transmit them towards the cell
body. The cytoplasm of the dendrites is similar to that of perikaryon but is devoid of Golgi complexes.
A single long process called axon extending from the cells body to a specialized terminal (synapse) that transmits
impulses from the cells body into the synapse. Axon varies in length from 1 mm to 1 meter. All axons originate
from a pyramidal-shaped region called axonal hillock that arises from perikaryon. The cytoplasm of both axonal
hillock & the axon is devoid of the major organelles especially Nissl bodies. The plasma membrane of axon is
called axolemma & its cytoplasm called axoplasm.
Nerve cells are specialized to receive stimuli from other cells of the system via their processes.
Neurons can be grouped into three general types:
- Sensory neurons (afferent): transmit impulses form receptors to the CNS (such as somatic afferent fibers
that transmit sensations of pain, temperature, touch & pressure from body surface). While visceral
afferent transmit pain impulses from internal organs, mucous membranes, glands & blood vessels.
- Motor neurons (efferent): transmit impulses from the CNS or ganglia to effector cells. Somatic efferent
neurons send voluntary impulses to skeletal muscles, while visceral efferent neurons transmit
involuntary impulses to smooth muscle, cardiac conducting cells & glands.
- Interneurons: form communicating & integrating network between the sensory & motor neurons, it is
estimated that 99.9% of all neurons belong to this integrating network.
Neurons are classified on the basis of the number of processes extending from the cell body into:
- Multipolar neurons: have one axon & two or more dendrites. The direction of impulses from dendrites
to cell body to axon or from cell body to axon. Motor neurons & interneurons constitute most of the
multipolar neurons in the nervous system.
- Bipolar neurons have one axon & one dendrite. Bipolar neurons are associated with receptors for the
special senses (taste, smell, hearing, sight & equilibrium).
- Pseudounipolar (unipolar) neurons have one process, the axon that divides close to the cells body into
two long branches. The majority of pseudounipolar neurons are sensory neurons located close to CNS.
Cell bodies of sensory neurons are situated in the Dorsal Root Ganglia DRG & cranial nerve ganglia.
Although neurons do not replicate, the cellular organelles & other cellular components of neurons turn over
regularly. The constant need to replace enzymes, neurotransmitter substances, membrane components & other
complex molecules is consistent with high level of protein synthetic activity of the neurons. Newly synthesized
protein molecules are transported to distant locations within a neuron in a process called axonal transport.
Neural stem cells are also able to migrate to sites of injury and differentiate into new nerve cells. Research
studies on the animal model demonstrate that newly generated cells mature into functional neurons in the
adult mammalian brain. These findings may lead to therapeutic strategies that use neural cells to replace
nerve cells lost or damaged by neurodegenerative disorders such as Alzheimer and Parkinson diseases.
Synapses are the sites where 2 neurons contact each other. At synapses information are transmitted (in a
unidirectional way) in the form of action potentials between two neurons (or between one neuron & other
effector cell such as muscle or glandular cells). Synapse is formed by axonal terminal (presynaptic terminal)
that delivers the signal, a region on the surface of another cell at which a new signal is generated
(postsynaptic terminal) & a thin intercellular space between them (synaptic cleft). An axon terminal may
synapse with a cell body (axosomatic synapse), a dendrite (axodendritic synapse), or another axon
(axoaxonic synapse). In humans, synapses work by chemical molecules (neurotransmitters) released from
presynaptic neuron terminal & affect the postsynaptic cell. This type is called chemical synapse. On the other
hand, action potentials may be transmitted directly from the presynaptic cell to postsynaptic cell through gap
junctions (electrical synapse) which is extremely rare in human.
The neuroglia
Neuroglia are the supporting cells of the nervous system. They are nonconducting cells that are located close
to the neurons.
Function of the various neuroglial cells types include:
-
Physical support & protection of neurons
Insulation for nerve cell body & processes to facilitates rapid impulse transmission
Repair of neuronal injury
Regulation of the internal fluid environment of CNS
Clearance of neurotransmitters from synaptic clefts
Metabolic exchange vascular & neuron sides
In the PNS supporting cells are called peripheral neuroglia, while in the CNS, they are called central neuroglia.
Peripheral neuroglia
Schwann Cells
They are the major glia cells of the PNS, being responsible for PNS axon myelination. One Schwann cell
myelinates a segments of one nerve axon. Unmyelinated nerve fibers are enclosed by non-myelin forming
Schwann cells, & here, several axons are embedded in the peripheral cytoplasm of one Schwann cell,
individually or in groups.
The territory of an individual Schwann cells defines
an internode. The interval between two internodes is
a node of Ranvier (The node of Ranvier represents
the junction between two adjacent Schwann cells).
Myelination occurs when the axon initially lies in a
groove on the surface of the Schwann cell. The axon
is surrounded by a Schwann cell. Note the two
domains of the Schwann cell, the adaxonal plasmamembrane domain and abaxonal plasma-membrane
domain. The mesaxon plasma membrane links these
domains. The mesaxon membrane initiates
myelination by surrounding the embedded axon. A
sheet like extension of the mesaxon membrane then
wraps around the axon, forming multiple membrane
layers. During the wrapping process, the cytoplasm is
extruded from between the two apposing plasma
membranes of the Schwann cell, which then become
compacted to form myelin.
Satellite cells
Small flattened squamous neuroepithelioid cells surrounding perikarya of the ganglionic cells in the PNS
ganglia. Their deep cytoplasm interdigitates with similar folding in the perikaryon. In psuedounipolar neurons,
satellite cells provide an insulation to ganglion cell bodies & maintain metabolic exchange pathway for
ganglionic cells.
Central Neuroglia
Astrocytes
Astrocytes are the largest of the neuroglial cells. They form a network of cells within the CNS and
communicate with neurons to support and regulate many of their activities. Some astrocytes provide a
scaffold for migrating neurons during brain development. Other astrocytes stretch their processes from
neurons to blood vessels, at which the ends of the processes expand forming end feet that cover large areas
of the vessel or axolemma. Astrocytes do not form myelin.
Two types of astrocytes are identified:
- Fibrous astrocytes are more prevalent in the inner core of the brain called white matter of the brain
- Protoplasmic astrocytes are more common in the outermost layer of the brain called gray matter.
Astrocytes contribute into formation of a tight barrier that separates the internal environment of the CNS
from that of blood, preventing any harmful drug, toxin, invading virus or bacteria to permeate through
capillaries & enter the CNS. This barrier called Blood Brain Barrier BBB, it is formed by the foot processes of
the astrocytes, endothelial basement membrane & a tight junctions between the adjacent endothelial cells.
Furthermore, astrocytes line the pia matter (tender mother) that covers the whole CNS surfaces, forming the
glia limitans.
Tumors arising from fibrous astrocytes (fibrous astrocytomas) account for about 80% of adult primary brain
tumors.
Oligodendrocytes
These are the cells responsible for producing CNS myelin, The myelin sheath in the CNS is formed by
concentric layers of oligodendrocytes plasma membrane. They usually have round nucleus & a fewer
cytoplasmic processes compared with astrocytes. The multiple processes of a single oligodendrocyte may
myelinate one axon or several nearby axons, whereas, single Schwann cell myelinates a segment of a single
nerve axon in the PNS.
Microglia
These are the phagocytic cells of the CNS. They are the smallest with elongated nucleus & normally account
for about 5% of all glial cells in the adult CNS, however they can proliferate & become active in phagocytosis in
regions of injury & disease. Microglia originate from granulocyte-monocyte CFU. Recent studies suggest that
microglia play a critical role in defense against invading microorganisms and neoplastic cells. They remove
bacteria, injured cells, and the debris of cells that undergo apoptosis.
Ependymal cells
They form the epithelium-like lining of the fluid-filled cavities of the CNS. They form a single layer of cuboidalto-columnar cells with apical cilia & microvilli that involve in absorbing cerebrospinal fluid (fluid that fill CNS
cavities i.e. ventricles & central canal & surround the CNS), these cells are modified in certain regions of brain
ventricles to form a special structure called choroid plexus which its main function is to produce cerebrospinal
fluid.
Demyelinating Diseases can affect either PNS or CNS, which are characterized by damage to myelin sheath,
resulting in decreased or lost ability to transmit impulses along nerve fibers.
Guillian-Barre syndrome (GBS) is a PNS demyelinating disease, affected patient suffers from ascending muscle
paralysis & loss of cutaneous sensation
Multiple sclerosis (MS) is a disease that attacks myelin in the CNS, in which oligodendrocytes are targeted by
immune response. It is characterized by neurological deficits such as vision loss, lack of muscle coordination.
The Peripheral nerve
A peripheral nerve is a bundle of nerve fibers held together by connective tissue. The nerves of the PNS are
made up of many nerve fibers that carry sensory and motor (effector) information between the organs and
tissues of the body and the brain and spinal cord. The cell bodies of peripheral nerves may be located within
the CNS or outside the CNS in peripheral ganglia. Ganglia contain clusters of neuronal cell bodies and the
nerve fibers leading to and from them
The bulk of a peripheral nerve consists of nerve fibers and their supporting Schwann cells. The individual
nerve fibers and their associated Schwann cells are held together by connective tissue organized into three
distinctive components, each with specific morphologic and functional characteristics:
- Endoneurium: Includes loose connective tissue surrounding each individual nerve fiber.
- Perineurium: connective tissue surrounding each nerve fascicle.
- Epineurium: Includes dense connective tissue that surrounds the whole peripheral nerve.