Download introduction to peripheral nervous system 26. 02. 2014

26. 02. 2014
Kaan Yücel
M.D., Ph.D.
http://yeditepeanatomy1.org
Figure from http://instruct.uwo.ca/anatomy/530/wholens.jpg.
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Introduction to peripheral nervous system
The nervous system comprises the central nervous system (CNS) and the peripheral nervous system
(PNS). The CNS is surrounded and protected by the skull (neurocranium) and vertebral column and consists of
the brain and the spinal cord. The PNS exists primarily outside these bony structures.
One neuron communicates with other neurons or glands or muscle cells across a junction between cells
called a synapse. Typically, communication is transmitted across a synapse by means of specific
neurotransmitters, such as acetylcholine, epinephrine, and norepinephrine, but in some cases in the CNS by
means of electric current passing from cell to cell.
The central nervous system consists of the brain and spinal cord, and the peripheral nervous system
consists of the sensory and motor nerves that are distributed throughout the body and that convey information
to and from the brain (via 12 pairs of cranial nerves) and the spinal cord (via 31 pairs of spinal nerves). The
peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.
The somatic nervous system is the part of the PNS that innervates the skin, joints, and skeletal muscles.
The autonomic nervous system (ANS) is the part of the PNS that innervates internal organs, blood vessels, and
glands.
The PNS encompasses the nervous system external to the brain and spinal cord. In the PNS,
axons (fibers) are collected into bundles supported by connective tissue to form a nerve.
The nervous system contains both the somatic system and the autonomic system, each with portions within the
CNS and PNS. The somatic system mediates information between the CNS and the skin, skeletal muscles
(voluntary movements), bones, and joints. The autonomic system, in contrast, mediates information between
the CNS and organs (involuntary movements).
In both the somatic system and autonomic system, neurons and their nerves are classified according to
function. Individual neurons that carry impulses away from the CNS are called efferent, or motor neurons. The
axons of these multipolar neurons are also referred to as efferent fibers and they synapse on muscles or glands.
Neurons that carry impulses to the CNS are called afferent, or sensory, neurons.
The term “peripheral nerve” such as sciatic nerve, ulnar nerve etc. should not be confused by the spinal
nerve. Peripheral nerve is the last product of these somatic networks; somatic plexuses.
The anterior rami form plexuses (network). All major somatic plexuses (cervical, brachial, lumbar, and sacral)
are formed by anterior rami (ramus=branch, rami=branches).
The spinal cord is a long tubular structure that is divided into a peripheral white matter (composed of
myelinated axons) and a central gray matter (cell bodies and their connecting fibers). When viewed in cross
section, the gray matter has pairs of horn-like projections into the surrounding white matter. These horns are
called ventral horns, dorsal horns, and lateral horns, but in three dimensions they represent
columns that run the length of the spinal cord.
Ventral horn of the spinal cord: Motor neurons
Lateral horn of the spinal cord: Thoracal and lumbar regions: sympathetetic system neurons
S2-S4 (S=Sacral) segments: parasympathetic system neurons
Dorsal horn of the spinal cord: Sensory neurons
A spinal nerve contains motor and sensory fibers, as well as sympathetetic and parasympathetic fibers
depending on the level of the segment. A spinal cord segment is the portion of the spinal cord that gives rise to
a pair of spinal nerves. Thus, the spinal cord gives rise to 8 pairs of cervical nerves (C1–C8), 12 pairs of
thoracic nerves (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of
coccygeal nerves (Co1). The spinal cord segments are numbered in the same manner as these nerves.
The first motor neuron is located in the precentral gyrus in the primary motor cortex. This cortex strip
is located in the frontal lobe. The somatosensory cortex, or SI, in the postcentral gyrus of each hemisphere
receives sensory information from the contralateral side of the body about touch, pain, temperature, vibration,
proprioception (body position), and kinesthesis (body movement).
2
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Introduction to peripheral nervous system
1. NERVOUS SYSTEM
The nervous system comprises the central nervous system (CNS) and the peripheral nervous system
(PNS). The CNS is surrounded and protected by the skull (neurocranium) and vertebral column and consists of
the brain and the spinal cord. The PNS exists primarily outside these bony structures.
The entire nervous system is composed of neurons, which are characterized by their ability to conduct
information in the form of impulses (action potentials), and their supporting cells plus some connective tissue.
A neuron has a cell body (perikaryon) with its nucleus and organelles that support
the functions of the cell and its processes. Dendrites are the numerous short processes that carry an action
potential toward the neuron’s cell body, and an axon is the long process that carries the action potential away
from the cell body. Many axons are ensheathed with a substance called myelin, which acts as an insulator.
Myelinated axons transmit impulses much faster than nonmyelinated axons.
One neuron communicates with other neurons or glands or muscle cells across a junction between cells
called a synapse. Typically, communication is transmitted across a synapse by means of specific
neurotransmitters, such as acetylcholine, epinephrine, and norepinephrine, but in some cases in the CNS by
means of electric current passing from cell to cell.
The central nervous system consists of the brain and spinal cord, and the peripheral nervous system
consists of the sensory and motor nerves that are distributed throughout the body and that convey
information to and from the brain (via 12 pairs of cranial nerves) and the spinal cord (via 31 pairs of spinal
nerves). The peripheral nervous system is divided into the somatic nervous system and the autonomic
nervous system.
The somatic nervous system is the part of the PNS that innervates the skin, joints, and skeletal muscles.
The autonomic nervous system (ANS) is the part of the PNS that innervates internal organs, blood vessels,
and glands.
2. PERIPHERAL NERVOUS SYSTEM
The PNS encompasses the nervous system external to the brain and spinal cord. In the PNS, axons
(fibers) are collected into bundles supported by connective tissue to form a nerve.
The nervous system contains both the somatic system and the autonomic system, each with portions within
the CNS and PNS. The somatic system mediates information between the CNS and the skin, skeletal muscles
(voluntary movements), bones, and joints. The autonomic system, in contrast, mediates information between
the CNS and organs (involuntary movements).
In both the somatic system and autonomic system, neurons and their nerves are classified according to
function. Individual neurons that carry impulses away from the CNS are called efferent, or motor neurons. The
axons of these multipolar neurons are also referred to as efferent fibers and they synapse on muscles or
glands. Neurons that carry impulses to the CNS are called afferent, or sensory, neurons.
3
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Introduction to peripheral nervous system
In the somatic system, these neurons carry impulses that originate from receptors for external stimuli (pain,
touch, and temperature), referred to as exteroceptors. In addition, receptors located in tendons, joint
capsules, and muscles convey position sense that is known as proprioception. Afferent neurons that run with
the autonomic system carry impulses from interoceptors located within visceral organs that convey stretch as
well as pressure, chemoreception, and pain.
3. SPINAL CORD
The spinal cord is a vital communication link between the brain and the peripheral nervous
system. Within the spinal cord, sensory nerves carry messages from the body to the brain for interpretation,
and motor nerves relay messages from the brain to the effectors. The spinal cord is also the
primary reflex centre, coordinating rapidly incoming and outgoing neural information.
The spinal cord is a long tubular structure that is divided into a peripheral white matter (composed of
myelinated axons) and a central gray matter (cell bodies and their connecting fibers). When viewed in cross
section, the gray matter has pairs of horn-like projections into the surrounding white matter. These horns are
called ventral horns, dorsal horns, and lateral horns, but in three dimensions they represent
columns that run the length of the spinal cord.
The ventral horns contain the cell bodies of motor neurons and their axons. A collection of neuronal cell
bodies in the CNS is a nucleus. Axons of the ventral horn nuclei leave the spinal cord in bundles called ventral
roots. These motor fibers innervate skeletal muscles.
The lateral (intermediolateral) horns contain the cell bodies for the sympathetic nervous system at spinal
cord levels T1–L2 and for the parasympathetic nervous system at spinal cord levels S2–S4. The axons from
these neurons also leave the spinal cord through the ventral root and will synapse in various peripheral
ganglia. A collection of neuronal cell bodies in the PNS is a ganglion.
The dorsal horns receive the sensory fibers originating in the peripheral nervous system. Sensory fibers
reach the dorsal horn by means of a bundle called the dorsal root. The central axons of the sensory neuron
enter the dorsal horn of the gray matter. Some of these fibers will run in tracts (a bundle of fibers in the CNS)
of the white matter to reach other parts of the CNS. Other axons will synapse with intercalated neurons
(interneurons), which in turn synapse with motor neurons in the ventral horn to form a reflex arc.
Although the dorsal root is essentially sensory and the ventral root is motor, the two roots come together
within the bony intervertebral foramen to form a mixed spinal nerve (i.e., it contains both sensory and motor
fibers). The spinal cord is defined as part of the CNS, but the ventral and dorsal roots are considered parts of
the PNS. Outside the intervertebral foramen, the mixed nerve divides into a ventral ramus (from the Latin for
“branch”) and a dorsal ramus.
4
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Introduction to peripheral nervous system
The larger ventral ramus supplies the ventrolateral body wall and the limbs; the smaller dorsal ramus
supplies the back. Since the ventral and dorsal rami are branches of the mixed nerve, they both carry sensory
and motor fibers.
A spinal cord segment is the portion of the spinal cord that gives rise to a pair of spinal nerves. Thus,
the spinal cord gives rise to 8 pairs of cervical nerves (C1–C8), 12 pairs of thoracic nerves (T1–T12), 5 pairs of
lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of coccygeal nerves (Co1). The spinal cord
segments are numbered in the same manner as these nerves.
The first cervical nerve (C1) emerges from the vertebral canal between the skull and vertebra CI. Therefore
cervical nerves C2 to C7 also emerge from the vertebral canal above their respective vertebrae. Because there
are only seven cervical vertebrae, C8 emerges between vertebrae CVII and TI. As a consequence, all remaining
spinal nerves, beginning with T1, emerge from the vertebral canal below their respective vertebrae.
The term “peripheral nerve” such as sciatic nerve, ulnar nerve etc. should not be confused by the spinal
nerve. Peripheral nerve is the last product of these somatic networks; somatic plexuses.
The anterior rami form plexuses (network). All major somatic plexuses (cervical, brachial, lumbar, and
sacral) are formed by anterior rami (ramus=branch, rami=branches).
The peripheral nervous system contains two systems; one working voluntarily; somatic nervous system (soma, in ancient Greek,
body), and one involuntarily, as it name implies, autonomic nervous system.
4. SOMATIC NERVEOUS SYSTEM
The somatic system is largely under voluntary control, and its neurons service the head, trunk, and limbs.
Its sensory neurons carry information about the external environment inward, from the receptors in the skin,
tendons, and skeletal muscles. Its motor neurons carry information to the skeletal muscles.
Your decision to turn this page in order to continue reading exemplifies the action of the somatic motor
nerves.
The somatic system includes 12 pairs of cranial nerves and 31 pairs of spinal nerves, all of which are
myelinated. The cranial nerves are largely associated with functions in the head, neck, and face.
An exception is the vagus nerve, which connects to many internal organs, including the heart, lung, bronchi,
digestive tract, liver, and pancreas.
The somatic motor system includes skeletal muscles and the parts of the nervous system that control
them; the somatosensory system involves the senses of touch, temperature, pain, body position, and body
movement.
5. AUTONOMIC NERVEOUS SYSTEM
The autonomic nervous system also has a motor component, sending motor output to regulate and
control the smooth muscles of internal organs, cardiac muscle, and glands (autonomic motor). The autonomic
5
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Introduction to peripheral nervous system
nervous system also includes sensory input from these internal structures that is used to monitor their status
(autonomic sensory).
The major sensory modalities other than touch (vision, audition, smell, and taste) are sometimes referred
to as special sensory.
The hypothalamus controls the autonomic system, which has neurons that are bundled together with
somatic system neurons in the cranial and spinal nerves. The sympathetic and parasympathetic
divisions of the autonomic system carry information to the effectors. In general, these two divisions have
opposing functions.
The sympathetic nervous system is typically activated in stressful situations and is often referred to as the
fight-or flight response. The sympathetic neurons release a neurotransmitter called norepinephrine, which
has an excitatory effect on its target muscles. As well, the sympathetic nerves trigger the adrenal
glands to release epinephrine and norepinephrine, both of which also function as hormones that activate the
stress response. At the same time, the sympathetic nervous system inhibits some areas of the body. For
example, in order to run from danger, the skeletal muscles need a boost of energy. Therefore, blood pressure
increases and the heart beats faster, while digestion slows down and the sphincter controlling the bladder
constricts. The increase in the sympathetic tone is related to vasoconstriction (the contraction of smooth
muscles of the vessels), and the opposite the decrease in the sympathetic tone (or an increase in the
parasympathetic tone) is related to vasodilatation (the relaxing of smooth muscles of the vessels). Some of
these physiological changes in response to stress are detectable by lie detectors.
The parasympathetic nervous system is activated when the body is calm and at rest. It acts to restore and
conserve energy. Sometimes referred to as the rest-and-digest response, the parasympathetic nervous
system slows the heart rate, reduces the blood pressure, promotes the digestion of food, and stimulates the
reproductive organs by dilating blood vessels to the genitals. The parasympathetic system uses a
neurotransmitter called acetylcholine to control organ responses.
The two branches of the autonomic system are much like the gas pedal and brake pedal of a car.
The sympathetic cell bodies are located in the lateral horns of thoracic spinal cord segments 1 through 12 plus
lumbar segments 1 and 2. The axons of these cells leave the spinal cord along with the somatic motor axons
by means of the ventral horn and root at each of these levels (T1–L2) to join the mixed spinal nerve. This
outflow is referred to as the thoracolumbar outflow. The sympathetic nervous systems neurons supply blood
vessels, arrector pili muscles, and sweat glands in the skin.
The parasympathetic portion of the autonomic nervous system is called the craniosacral outflow because
it has its cell bodies in the brainstem and in the sacral portion of the spinal cord. Parasympathetic fibers run in
some of the cranial nerves. The sacral parasympathetic outflow arises from the intermediolateral horn of
sacral spinal cord segments 2, 3, 4.
6
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Introduction to peripheral nervous system
6. M1 & S1
The cerebral cortex, forming the outer covering of the cerebral hemispheres (cortex comes from the
Latin word for “bark”) is truly vast, particularly in humans, where it is estimated to contain 70% of all the
neurons of the CNS. And if one considers that the medulla oblongata—with a diameter little more than that of
a dime and a length of only a few inches—can mediate physiological functioning sufficient to sustain life, the
relative enormity of the cerebral cortex, estimated to have an area of about 2,300 square centimeters (cm2),
can be appreciated.
Each cerebral hemisphere is traditionally divided into four lobes: the occipital, parietal, temporal, and
frontal lobes. These areas, taking their names simply from the bones of the skull that overlie them, were
defined long before anything significant was known about the functional specialization of the cerebral cortex.
Nevertheless, it turns out that these general areas are often useful in describing areas of the cortex that are
involved in particular behaviors.
The vast majority of cerebral cortex in humans has six layers: five layers of neurons and an outermost
layer of fibers, termed the plexiform layer. This six-layered cortex, which appeared relatively late in
evolution, is called neocortex. It is also termed isocortex (from the Greek iso, meaning “same”) because all of
it is composed of six layers, although, the relative thickness of the different layers varies across the cortex.
The relative thickness and cell composition of each of the six cortical layers varies across the neocortex,
with different areas of the neocortex having characteristic patterns. The study of these patterns, called
cytoarchitectonics (literally, “cell architecture”), was begun in the early 20th century. The two most widely
accepted cytoarchitectonic maps of the cortex are those developed by Korbinian Broadmann and by von
Economo and Koskinas. Broadmann’s map is used most widely.
The Broadmann numbering system is often used simply to identify a particular area of cortex. It turns out
that there is significant correspondence between areas defined by cytoarchitectonic studies and areas
identified as having specialized function by other methods. At least some of the areas defined by Broadmann
are also areas specialized for particular psychological processes, although this is not always the case.
The frontal lobes play a major role in the planning and execution of movement. The precentral
gyrus, just anterior to the central sulcus (the border between frontal and parietal lobes) , is known as the
motor strip, motor cortex, or M1 and is involved in the execution of movement. Lesions of the motor cortex
result in a loss of voluntary movement on the contralateral side of the body, a condition known as hemiplegia.
Electrical stimulation of this cortex reveals a complete motor representation of the body on the precentral
gyrus, the so-called motor homunculus (“little man”). Such mapping of a neural structure in terms of
associated behaviors is called a functional map. There are many functional maps in the cortex and in other
brain regions.
7
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Introduction to peripheral nervous system
Just anterior to the motor cortex are the premotor area, on the lateral surface of the hemisphere, and the
supplementary motor area, on the medial surface. These areas are involved in the coordination of sequences
of movement. Frontal areas anterior to the premotor cortex, the prefrontal cortex, are involved
in the higher-order control of movement, including planning and the modification of behavior in response
to feedback about its consequences.
The somatosensory cortex, or SI, in the postcentral gyrus of each hemisphere receives sensory
information from the contralateral side of the body about touch, pain, temperature, vibration, proprioception
(body position), and kinesthesis (body movement). As with the motor cortex, there is
an orderly mapping of the body surface represented on the postcentral gyrus, a mapping termed the sensory
homunculus. Ventral to SI is the secondary somatosensory cortex, or SII, which receives input
mainly from SI. Both SI and SII project to posterior parietal areas where higher-order somatosensory and
spatial processing take place.
6. SPINOCORTICAL & CORTICOSPINAL PATHWAYS
Information from sensory receptors in the body arrives at SI via two major systems, both relaying
through the thalamus. The spinothalamic system conveys information about pain and temperature via
a multisynaptic pathway, whereas the lemniscal system conveys more precise information about touch,
proprioception, and movement via a more direct pathway.
The corticospinal motor projections form the corticospinal pathway and start from the Betz pyramidal
neurons of Broadmann Area 4 (Primary motor cortex). They end up at the motor neurons located in the
ventral horn of the spinal cord. The journey of the “motor signal” for the movement would end at the
neuromuscular junction between the related peripheral nerve and the target muscle.
REFERENCES
https://www.us.elsevierhealth.com/media/us/samplechapters/9781416031659/9781416031659.pdf
http://highered.mcgraw-hill.com/sites/dl/free/0070960526/323541/mhriib_ch11.pdf
http://highered.mcgraw-hill.com/sites/dl/free/155934623x/28694/rai4623x_ch03.pdf
8