Download Kaan Yücel M.D., Ph.D. http://fhs122.org

13. 4. 2015
Kaan Yücel
M.D., Ph.D.
http://fhs122.org
[email protected]
Dr. Kaan Yücel
fhs122.org
Introduction to neuroanatomy & Spinal cord
I do not know the question, but the brain is the answer .
As they say “You are what you eat!”, but it should be ”You are what your nervous system
permits”. From the moment your day starts and to the moment the next day starts, we are under the control
of the magic box: brain.
The humans and animals interact with the world. They also need all the systems in the body
should be working in harmony. Any stimulus is received by the nervous system. It can originate from the outer
world or the inner world inside your body. Some of these stimuli do not reach your consciousness level.
Think of a man trying to get your attention at a street! He is shouting at you! He might be
swearing at you! Not good! Or he may be actually trying to give your wallet he has just found at the
restaturant you just left! That is good!
As you see the stimulus (what the man is saying, his apperance, what is in his hand; a knife or
your wallet?) is received and your organism plans an action. You respond to that stimulus. As you can see
here, there are two ends; get the stimulus and respond. But there is also other complex processes. Your
memory, moral values, emotions, prejuduice, planning, reasoning….etc…
Welcome to the World of the Neurons & BraiN!
There are two types cells in the nervous system. Neurons & Neuroglia
Neuron: The functional unit of the nervous system
The best estimate is that the human brain contains about 1011 neurons (100 billion neurons). Although nerve
cells can be classified into different types, they share many common features. The structural base of the
functional unit of the nervous system is simple and similar in all neurons. There is the cell body. There is the
nucleus embedded within the cell body. The cell body is bounded externally by a plasma membrane. The cell
bodies of the small granular cells of the cerebellar cortex measure about 5 µm in diameter, whereas those of
the large anterior horn cells may measure as much as 135 µm in diameter.
The fibers exiting from the neuron and coming to the neuron are called neurites. When we say neurite
we understand two kinds of projections from the cell body of a neuron: A long, always single axon and a
bunch of dendirites. The dendirites are fibers bringing information to the neuron from the neurons around.
The axon carries the information to other neurons or structures. The bundles of axons (tract or peripheral
nerve) looks white thanks the myelin sheath it is covered by.
Synapse is the connection of a neuron with the neurites or the cell body of another neuron. The neuron
conveying the signal is called the presynaptic neuron. The neuron receiving the signal is called the
postsynaptic neuron. The space between these two structures is called the synaptic cleft. Another feature of
this neuronal connection is the presynaptic terminal. Most of the time, the signal is transmitted from one
neuron to another via neurotransmitters. Rarely, the connection is electrical. Popular neurotransmitters are
acetylcholine (movement, memory), adrenaline (sympathetetic system), serotonin (mood), dopamine
(addiction).
Types of neurons
Unipolar neurons are round. The cell body has one single neurite. This neurite gives branches. It gives off an
axon and a series of dendrites. There are no dendirites exiting from the cell body. These neurons are found in
some ganglia of the autonomic nervous system.
Bipolar neurons possess an elongated cell body. From each end a single neurite starts. Examples of this type
of neuron are found in the retinal bipolar cells and the cells of the sensory cochlear and vestibular ganglia,
and olfactory bulb.
Pseudo-unipolar neurons is a special form of bipolar neurons. One neuron has one short neurite with two
branches. One goes to the periphery. The other branch goes centrally. They are sensory neurons. The
peripheral branch brings the touch, pressure, pain and temperature sensations to the neuronal body. The
central branch conveys this information to the spinal cord. Examples of this form of neuron are found in the
dorsal root ganglion.
Multipolar neurons have a number of neurites arising from the cell body. With the exception of the long
process, the axon, the remainder of the neurites are dendrites. Most neurons of the brain and spinal cord are
of this type.
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Dr. Kaan Yücel
fhs122.org
Introduction to neuroanatomy & Spinal cord
The boss sits upstairs. So is the boss of the body; the brain
Nerve fiber is the name given to an axon (or a dendrite) of a nerve cell. Bundles of nerve fibers found in the
central nervous system are often referred to as nerve tracts; bundles of nerve fibers found in the peripheral
nervous system are called peripheral nerves. Two types of nerve fibers are present in the central and
peripheral parts of the nervous system: myelinated and nonmyelinated.
A myelinated nerve fiber is one that is surrounded by a myelin sheath. The myelin sheath is not part of
the neuron but is formed by a supporting cell. The smaller axons of the central nervous system, the
postganglionic axons of the autonomic part of the nervous system, and some fine sensory axons associated
with the reception of pain are nonmyelinated.
Neuroglia (glia)
glia, Gr. glue
There are between 10 and 50 times more glial cells than neurons in the CNS. Glial cells have other roles than
processing information. The neuroglia are non-neural supporting cells. They are considered as specialized
connective tissue cells of the nervous system. The neurons have te feature of excitability. The neuroglia do
not have this feature. The neuroglia are known as macroglia, ependymal cells and microglia. The microglia are
the macrophages of the nervous system.
1- They serve as supporting elements. They provide firmness and structures to the brain. They also separate
and occasionally insulate groups of neurons from each other.
2- Oligodendrocyte in the CNS forms myelin. Myelin is the insulating sheath that covers most large axons.
3- Some glial cells remove debris after injury or neuronal death.
4- Some glial cells take up and remove chemical transmitters released by neurons during synaptic
transmission.
5- Some glial cells have nutritive functions for nerve cells.
Central Nervous System (CNS) & Peripheral Nervous System
The nervous system is divided into two pars anatomically. This classification depends on the
locations. The central nervous system and peripheral nervous system.
The nervous system is also divided into two parts functionally. The somatic nervous system (skin &
skeletal muscles; voluntary actions) and autonomic nervous system (involuntary actions).
Central Nervous System (CNS)
Structural (Morphological) division of the central nervous system: Brain & Spinal cord
Brain in the cranium
1. Forebrain (Prosencephalon)
Telencephalon
Diencephalon
2.Midbrain (Mesencephalon)
3. Hindbrain (Rhombencephalon)
Metencephalon (Pons & Cerebellum)
Myelencephalon (Medulla oblongata-Medulla; Bulbus; Omurilik soğanı)
Spinal cord (Medulla spinalis) in the vertebral column
Brain stem (Truncus encephali) : Midbrain (Mesencephalon) + Pons + Medulla oblongata
As you see the brain is divided into five different regions/parts. They are grouped under three
bigger parts (forebrain, midbrain and hindbrain). These three major parts have embriyological references.
The brain doubles its weight when the human being becomes 2 years old. In adult, it weighs
between 1250-1450 gr.
Nucleus Group of neurons coming together for the same function in the central nervous system
Tract Group of axons coming together as bundles for the same function Fasciculus (less common)
White matter (Substantia alba; beyaz cevher): where the axons are (looks brighter thanks to the myelin sheath)
Grey matter (Substantia grisea; gri cevher) where the neurons are (looks darker) thanks to pigments within.
The grey matter is outside in the brain. The grey matter is inside, the white matter is outside in the spinal
cord.
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Dr. Kaan Yücel
fhs122.org
Introduction to neuroanatomy & Spinal cord
A neuron can be located in the cortex of the cerebral hemispheres forming a sheath. This
sheath of grey matter is also called pallium (wall, kabuk). Under the cortex there is the white matter of the
hemispheres. It is formed by the axons of the neurons in the cortex. A neuron might be located in the
subcortical structures forming nuclei. The concept of “subcortical” mean under the hemispheres level.
Thalamus, basal ganglia are all subcortical structures. A neuron can be also in groups in the periphery. This
group of neurons outside the CNS is called ganglion.
Some neurons are motor neurons. They carry information from the CNS to the parts of CNS or to the
PNS. Some are sensory neurons. They carry information to the CNS. Some neurons are interneurons. They set
the connections between the neurons of the CNS. They outnumber the motor and sensory neurons.
Peripheral Nervous System (PNS)
The peripheral nervous system is composed of 12 pairs of cranial nerves, 31 pairs of spinal nerves, their
associated ganglia, and nerve fibers of the autonomic system.
The PNS is in physical continuity with the CNS. The cell bodies of many of the nerve fibers (axons) of the
PNS are located in the CNS.
SOMATIC NERVOUS SYSTEM
The somatic nervous system is under the individual’s conscious control. The somatic nervous system is
involved in sensory information coming from the outer world, and from the skin, joints and muscles and
motor impulses to the striated muscles. It is the autonomic system involved in regulating the involuntary
actions (under the control of the brain), i.e. smooth muscles, cardiac muscles and glands.
“Peripheral nerves” is a collective term for the cranial and spinal nerves. Each peripheral nerve
consists of parallel bundles of nerve fibers.
There are 31 pairs of spinal nerves exiting from the spinal cord.
There are 12 pairs of cranial nerves exiting from the brain and brainstem.
The cranial nerves pass through openings (foramine or fissures) in the skull. Some of these nerves are
composed entirely of afferent nerve fibers bringing sensations to the brain (CNI; olfactory,CN II optic, and CN
VIII; vestibulocochlear), others are composed entirely of efferent fibers (CN III; oculomotor, CN IV; trochlear,
CN VI; abducent, CN XI; accessory, and CN XII; hypoglossal), while the remainder possess both afferent and
efferent fibers (CN V; trigeminal,CN VII; facial, CN IX; glossopharyngeal, and CN X; vagus). All cranial nerves
except vagus (CN X) supply the head and neck. The vagus nerve also supplies organs in the thorax and
abdomen.
The peripheral nerves are classified as afferent and efferent fibers. The afferent fibers carry the sensory
information coming from the receptors to the central nervous system. The efferent fibers carry the motor
responses to the effector organs from the central nervous system.
EFFERENT FIBERS
Somatic efferent fibers
Visceral (autonomic) efferent fibers
Somatic efferent fibers (Somatic motor- Somatomotor) fibers go to the striated muscles. They end in the
motor end plate. The motor end plate is in the nerve-muscle juction. The cell bodies of the somatomotor
fibers either originate from the neurons in the ventral horn of the spinal cord or somatic neurons in the brain
stem. They are called as general somatic efferent (GSE) fibers.
Special visceral efferent nerves (fibers): Muscles of the pharynx, larynx, soft palate, facial muscles, muscles of
mastication and muscles in the middle ear
Visceral (autonomic) efferent fibers go to the heart muscle (cardiomotor), smooth muscles in the organs
(visceromotor), smooth muscles in the walls of vessels (vasomotor), smooth muscles in the roots of the hair
follicles (pilomotor), glands in the digestive system (secretomotor). They are called as general visceral
efferent (GVE) fibers.
AFFERENT FIBERS
Somatic afferent fibers
Visceral afferent fibers
Somatic afferent fibers are sensory fibers. They bring information from the skin, muscle, tendon and joints.
The sensory information they bring: touch, pressure, pain, temperature and proprioception. This sensory
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Dr. Kaan Yücel
fhs122.org
Introduction to neuroanatomy & Spinal cord
information is perceived by the receptors in the body and is carried into the spinal cord. The spinal cord sends
this information to the upper levels in the central nervous system. Somatic afferent fibers are called as
general somatic afferent (GSA) fibers.
General visceral afferent (GVA) fibers carry information perceived in the viscera. This sensory information
originates from the organs, glands and membranes. The information is carried to the spinal cord. The
information can be on pain due to stretching, spasm.
Special visceral afferent (SVA) fibers carry special sensory information unique to the organs (such as nose and
tongue). Examples are gustatory (taste) and olfactory (smell) sensations.
SENSORY INFORMATION
The superficial sensory information is divided into two parts (Source # 4). One is the protopathic
sensation. These are simple sensations. The four: 1) Touch 2) Pressure 3) Temperature 4) Pain. In protopathic
sensations, you can define the degree of the sensation roughly.
The other one is the epicritic sensation. The epicritic sensation is a discriminative sensation. You can
define small differences between heat for example. You can define the difference in the heat between 10 0 and
200. You can tell the differences between the pain stimuli. One epicritic discrimation is “two-point
discrimination". Think of a compass (pergel) touching your tongue. As the receptors are dense there, you can
feel the two points of touching separetely until 1.4 mm. Less than 1.4 mm, you feel the compass at one point
of contact. In the back of the neck this distance is 36.2 mm. The superficial sensations are carried from the
exteroreceptors located in the skin or just under the skin. The discriminative (fine) touch defines the type,
intensity, localization of the touched site. The crude touch is a light touch on the skin. An example is the shirt
touching the skin lightly. There is no discriminative feature in crude (nondiscriminative) touch.
The deep sensory information (sensations) is proprioceptive, vibration and deep muscle pain. Their
receptors are located in the muscles, tendons, joint capsule and ligaments. The proprioceptive receptors are
also located in the inner ear. They give information about our location in the space. They are very important
for maintaining our balance. Not all the proprioceptive sensations reach our concious (cortex).
Vibration sensation is the repetitive sensations of pressure.
The visceroreceptors (interoceptors) carry the visceral sensory information. They are located in the
blood vessels, walls of the organs, and in glands as free nerve endings. They carry sensory information such as
pain, pressure, chemical changes, thirst, stetching, etc.
The sensations are carried by somatic afferent fibers in the spinal and cranial nerves from the
abundant receptors in the skin. These receptors are simply the dendirites of the afferent neurons (Source # 4).
RECEPTORS
An individual receives impressions from the outside world and from within the body by special sensory nerve
endings or receptors.
Sensory receptors can be classified into six types according to the sensation they receive (Source #1):
Mechanoreceptors. respond to mechanical deformation.
Thermoreceptors respond to changes in temperature; some receptors respond to cold and others to heat.
Nociceptors respond to any stimuli that bring about damage to the tissue.
Photoreceptors The rods and cones of the eyes are sensitive to changes in light intensity and wavelength.
Chemoreceptors respond to chemical changes associated with taste and smell and oxygen and carbon dioxide
concentrations in the blood.
Osmoreceptors are sensitive to differences in the osmotic pressure.
AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system is distributed throughout the central and peripheral nervous systems.
It is divided into two parts, the sympathetic and the parasympathetic and, as emphasized earlier, consists of
both afferent and efferent fibers. This division between sympathetic and parasympathetic is made on the
basis of anatomical differences, differences in the neurotransmitters, and differences in the physiologic
effects. Both the sympathetic and parasympathetic divisions produce opposite effects in most organs and are
thus considered as physiologic antagonists. However, it must be stated that both divisions operate in
conjunction with one another, and it is the balance in the activities that maintains a stable internal
environment.
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Dr. Kaan Yücel
fhs122.org
Introduction to neuroanatomy & Spinal cord
The sympathetic system is the larger of the two parts of the autonomic system and is widely
distributed throughout the body, innervating the heart and lungs, the muscle in the walls of many blood
vessels, the hair follicles and the sweat glands, and many abdominopelvic viscera.
The function of the sympathetic system is to prepare the body for an emergency. The heart rate is
increased, arterioles of the skin and intestine are constricted, arterioles of skeletal muscle are dilated, and the
blood pressure is raised. There is a redistribution of blood; thus, it leaves the skin and gastrointestinal tract
and passes to the brain, heart, and skeletal muscle. In addition, the sympathetic nerves dilate the pupils;
inhibit smooth muscle of the bronchi, intestine, and bladder wall; and close the sphincters. The hair is made
to stand on end, and sweating occurs.
The activities of the parasympathetic part of the autonomic system are directed toward conserving
and restoring energy. The heart rate is slowed, pupils are constricted, peristalsis and glandular activity is
increased, sphincters are opened, and the bladder wall is contracted.
The connector nerve cells of the parasympathetic part of the autonomic nervous system are located
in the brainstem and the sacral segments of the spinal cord.
Those nerve cells located in the brainstem form nuclei in the following cranial nerves: the oculomotor
(parasympathetic or Edinger-Westphal nucleus), the facial (superior salivatory nucleus and lacrimatory
nucleus), the glossopharyngeal (inferior salivatory nucleus), and the vagus nerves (dorsal nucleus of the
vagus). The axons of these neurons are myelinated and emerge from the brain within the cranial nerves.
The sacral part of the parasympathic systems’s nerve cells are found in the gray matter of the second,
third, and fourth sacral segments of the spinal cord.
The enteric system has beenconsidered as the third part of the autonomic nervous system.
PERIPHERAL NERVE PLEXUSES
Peripheral nerves are composed of bundles of nerve fibers. In their course, peripheral nerves
sometimes divide into branches that join neighboring peripheral nerves. A network of nerves, called a nerve
plexus, is formed. The formation of a nerve plexus allows individual nerve fibers to pass from one peripheral
nerve to another. A plexus thus permits a redistribution of the nerve fibers within the different peripheral
nerves.
At the root of the limbs, the anterior rami of the spinal nerves form complicated plexuses. The
cervical and brachial plexuses are at the root of the upper limbs, and the lumbar and sacral plexuses are at the
root of the lower limbs. This allows the nerve fibers derived from different segments of the spinal cord to be
arranged and distributed efficiently in different nerve trunks to the various parts of the upper and lower
limbs.
Cutaneous nerves, as they approach their final destination, commonly form fine plexuses. They also
permit a rearrangement of nerve fibers before they reach their terminal sensory endings.
The autonomic nervous system also possesses numerous nerve plexuses that consist of preganglionic
and postganglionic nerve fibers and ganglia. Large collections of sympathetic and parasympathetic efferent
nerve fibers and their associated ganglia, together with visceral afferent fibers, form autonomic nerve
plexuses in the thorax, abdomen, and pelvis.
GANGLIA
Neurons are found in the brain and spinal cord and in ganglia.
The sensory ganglia of the posterior spinal nerve roots and of the trunks of the trigeminal, facial,
glossopharyngeal, and vagal cranial nerves have the same structure.
The autonomic ganglia (sympathetic and parasympathetic ganglia) are situated at a distance from the
brain and spinal cord. They are found in the sympathetic trunks, in prevertebral autonomic plexuses (e.g., in
the cardiac, celiac, and mesenteric plexuses), and as ganglia in or close to viscera.
MENINGES, VASCULATURE & CSF
The brain in the skull and the spinal cord in the vertebral column are surrounded by three protective
membranes, or meninges: the dura mater, the arachnoid mater, and the pia mater. The space between the
skull and dura mater is epidural space. The space between the dura mater and arachnoid mater is
subarachnoid space. The cerebrospinal fluid is here.
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Dr. Kaan Yücel
fhs122.org
Introduction to neuroanatomy & Spinal cord
The dura mater of the brain is conventionally described as two layers: the endosteal layer and the
meningeal layer. These are closely united except along certain lines, where they separate to form venous
sinuses.
The venous sinuses of the cranial cavity are situated between the layers of the dura mater. The main
function of the dural sinuses is to receive blood from cerebral veins and the cerebrospinal fluid from the
subarachnoid space. The blood in the dural sinuses ultimately drains into the internal jugular veins in the
neck.
The brain gets 15 % of the cardiac output. It uses 20% of the oxygen (Source # 5). The brain is supplied
by the two internal carotid and the two vertebral arteries. The four arteries lie within the subarachnoid space,
and their branches anastomose on the inferior surface of the brain to form the circle of Willis.
The ventricles are four fluid-filled cavities located within the brain. These are the two lateral
ventricles, the third ventricle, and the fourth ventricle. The ventricles are lined throughout with ependyma
and are filled with cerebrospinal fluid (CSF). As you see there is no first and second ventricle. They are the
right and left lateral ventricles. The lateral ventricles are located in the telencephalon. Lobes, basal ganglia
and lateral ventricles are parts of the telencephalon. The third ventricle is between the right and left halves of
the diencephalon. The fourth ventricle is between the pons and medulla anteriorly, and cerebellum
posteriorly. The CSF is finally drained into the dural sinuses and reaches the internal jugular vein. The CSF can
be considered as a pillow of liquid around the brain and the spinal cord. Thanks to CSF, the skull feels less of
the brain it is carrying.
SPINAL CORD
part of the CNS in the superior 2/3 of the vertebral canal
It is roughly cylindrical in shape, and is circular to oval in cross-section.
Internally, the cord has a small central canal surrounded by gray and white matter.
The spinal cord extends from foramen magnum to lower border of L1 (first lumbar vertebra).
It gives rise to 31 pairs of spinal nerves (cervical, thoracic, lumbar, sacral and coccygeal).
8 cervical spinal nerves, 12 thoracic spinal nerves, 5 lumbar spinal nerves, 5 sacral spinal nerves,1
coccygeal spinal nerve
All are mixed nerves. (Motor fibers, sensory fibers, sympathetic fibers (T1-L2)/ parasympathetic fibers
(S2-S4 spinal segments)
Spinal nerves arise as rootlets then combine to form dorsal and ventral roots.
Dorsal and ventral roots merge laterally and form the spinal nerve.
Dorsal root is related to the sensory information.
Anterior (ventral) root is related to the motor information.
@ cervical region  gives origin to the brachial plexus, and in the lower thoracic and lumbar regions, where
it gives origin to the lumbosacral plexus, the spinal cord is fusiformly enlarged; the enlargements are referred
to as the cervical and lumbar enlargements.
Inferiorly, the spinal cord tapers off into the conus medullaris, from the apex of which a prolongation of the
pia mater, the filum terminale, descends to be attached to the posterior surface of the coccyx. The cord
possesses a deep longitudinal fissure called the anterior median fissure in the midline anteriorly and a shallow
furrow called the posterior median sulcus on the posterior surface.
Because of the shortness of the spinal cord relative to the length of the vertebral column, the nerve roots of
the lumbar and sacral segments have to take an oblique course downward to reach their respective
intervertebral foramina; the resulting leash of nerve roots forms the cauda equine.
Along the entire length of the spinal cord are attached 31 pairs of spinal nerves by the anterior or motor roots
and the posterior or sensory roots. Each root is attached to the cord by a series of rootlets, which extend the
whole length of the corresponding segment of the cord. Each posterior nerve root possesses a posterior root
ganglion, the cells of which give rise to peripheral and central nerve fibers.
The spinal cord is composed of an inner core of gray matter, which is surrounded by an outer covering of
white matter; there is no indication that the cord is segmented.
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Dr. Kaan Yücel
fhs122.org
Introduction to neuroanatomy & Spinal cord
Gray Matter
On cross section, the gray matter is seen as an H-shaped pillar with anterior and posterior gray columns, or
horns, united by a thin gray commissure containing the small central canal A small lateral gray column or horn
is present in the thoracic and upper lumbar segments of the cord. The amount of gray matter present at any
given level of the spinal cord is related to the amount of muscle innervated at that level. Thus, its size is
greatest within the cervical and lumbosacral enlargements of the cord, which innervate the muscles of the
upper and lower limbs, respectively.
As in other regions of the central nervous system, the gray matter of the spinal cord consists of a mixture of
nerve cells and their processes, neuroglia, and blood vessels. The nerve cells are multipolar, and the neuroglia
forms an intricate network around the nerve cell bodies and their neurites.
Anterior column (horn)  motor neurons Posterior column (horn)  sensory neurons
Intermediate column  Interneurons, in T1-L2 becomes lateral column = sympathetetic neurons, S2-S4
parasympathetetic neurons
White Matter
The white matter, for purposes of description, may be divided into anterior, lateral, and posterior white
columns or funiculi. The anterior column on each side lies between the midline and the point of emergence of
the anterior nerve roots; the lateral column lies between the emergence of the anterior nerve roots and the
entry of the posterior nerve roots; the posterior column lies between the entry of the posterior nerve roots
and the midline.
As in other regions of the central nervous system, the white matter of the spinal cord consists of a mixture of
nerve fibers, neuroglia, and blood vessels. It surrounds the gray matter, and its white color is due to the high
proportion of myelinated nerve fibers.
Although some nerve tracts are concentrated in certain areas of the white matter, it is now generally
accepted that considerable overlap is present. For purposes of description, the spinal tracts are divided into
ascending, descending, and intersegmental tracts.
On entering the spinal cord, the sensory nerve fibers of different sizes and functions are sorted out and
segregated into nerve bundles or tracts in the white matter. Some of the nerve fibers serve to link different
segments of the spinal cord, while others ascend from the spinal cord to higher centers and thus connect the
spinal cord with the brain. It is the bundles of the ascending fibers that are referred to as the ascending tracts.
The ascending tracts conduct afferent information, which may or may not reach consciousness. The
information may be divided into two main groups:
(1) exteroceptive information, which originates from outside the body, such as pain, temperature, and touch
(2) proprioceptive information, which originates from inside the body, for example, from muscles and joints.
General information from the peripheral sensory endings is conducted through the nervous system by a series
of neurons. In its simplest form, the ascending pathway to consciousness consists of three neurons. The first
neuron, the first-order neuron, has its cell body in the posterior root ganglion of the spinal nerve. A peripheral
process connects with a sensory receptor ending, whereas a central process enters the spinal cord through
the posterior root to synapse on the second-order neuron.
The second-order neuron gives rise to an axon that decussates (crosses to the opposite side) and ascends to a
higher level of the central nervous system, where it synapses with the third-order neuron. The third-order
neuron is usually in the thalamus and gives rise to a projection fiber that passes to a sensory region of the
cerebral cortex. This three-neuron chain is the most common arrangement, but some afferent pathways use
more or fewer neurons. Many of the neurons in the ascending pathways branch and give a major input into
the reticular formation, which, in turn, activates the cerebral cortex, maintaining wakefulness. Other branches
pass to motor neurons and participate in reflex muscular activity.
Imagination is more important than knowledge. Albert Einstein
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