A
Neuroanatomy Guide
The
Central Nervous System and Peripheral Nervous System
Argosy
University Online
Kay
T. Jewels
Abstract
The human body is
controlled by two complex systems called the Central Nervous System and Peripheral
Nervous System. In a systematic breakdown of the Central Nervous System and Peripheral
Nervous System this is paper will describe the associated features including
basic structures, their location, and intended function.
A
Neuroanatomy Guide: The Central Nervous System and Peripheral Nervous System
The
human body is controlled by the nervous system including the “central nervous
system” (CNS) and the “peripheral nervous system” (PNS) (Carlson, 2009). The
central nervous system consists of the brain and spinal cord, which make up the
(CNS). The “peripheral nervous system” (PNS) consists of “cranial nerves,
spinal nerves, and peripheral ganglia” (Carlson, 2009). These two systems work
together to bring sensory and motor information from the body to the CNS and to
bring sensory and motor information from the CNS to any location within the
body (Carlson, 2009). The CNS interacts with the PNS through the use of the “cranial
nerves, spinal nerves, and peripheral ganglia” (Carlson, 2009). Effectively the
PNS works as a relay system that communicates all the information collected from
the body to the CNS, so the CNS can send signals that regulate function
throughout the whole body. In very
simple terms, the PNS can be seen as an interdependent system that connects the
brain to the body so they work simultaneously.
The
CNS and the PNS are both well protected; in fact, both the CNS and PNS are in
encased in bone and meninges. The brain, consisting of “neurons, glia, and
other supporting cells”, is found floating in cerebrospinal fluid, “chemically
guarded by the blood–brain barrier”, well protected by meninges and the skull
(Carlson, 2009, p. 72). The spinal cord is encased within the vertebral column,
which consists of the twenty-four individual vertebrae of the “cervical, thoracic, lumbar, sacral
and coccygeal” regions that
form the spine (Carlson, 2009, p. 94-95). The spinal nerves, cranial nerves and
the peripheral ganglia that compose the PNS are protected by cerebrospinal
fluid and two layers of meninges (dura mater and pia mater) (Carlson, 2009, p.
72). Meninges are a tough connective tissue that protects them from damage.
Meninges has several layers: the outer tough flexible layer called the dura
mater, the middle soft and spongy layer called the arachnoid membrane including
a spall gap called the subarachnoid space which is filled with cerebrospinal
fluid, and the pia mater which is closely attached to the brain and spinal cord
(Carlson, 2009. p 72).
Central Nervous System Breakdown
A series of hollow unified
chambers, called ventricles, full of cerebrospinal fluid (CSF) form the brain
(Carlson, 2009, p. 74-75). Located at
the midline of the brain, the walls of the third ventricle divide the brain
into symmetrical halves (Carlson, 2009). Connected to and located on each side
of the third ventricle, are the largest chambers in the brain are called the “lateral
ventricles” (Carlson, 2009). The “massa intermedia” a connective piece of
neural tissue that crosses through the middle of the third ventricle, while the
cerebral aqueduct is a long tube that connects the third ventricle to the
fourth ventricle (Carlson, 2009, p. 74-75). Within these four chambers a
special tissue called the choroid plexus protrudes and is responsible for the
production of CSF (Carlson, 2009, p. 75). The CSF continually produced by the
choroid plexus tissue located within the ventricles systematically floods the
ventricles and subarachnoid space before passing the arachnoid granulations and
reentering the blood stream through the “superior sagittal sinus” (Carlson,
2009, p. 75). Arachnoid granulations are pouch-shaped structures that extend
into a blood vessel called the superior sagittal sinus that drains into the
veins serving the brain. If there is an interruption in the flow of CSF
ventricals may enlarge, a condition called obstructive hydrocephalus may
result, causing intracerebral pressure, blood vessels to become occluded, and
permanent or possibly fatal brain damage can occur (Carlson, 2009, p. 75).
The brain is divided
into two hemispheres called left and right hemispheres. These hemispheres are
connected by the corpus callosum,
which is “a large band of axons that connects corresponding parts of the
cerebral cortex of the left and right hemispheres” (Carlson, 2009, p 87).
Typically, but not always, the left hemisphere analyzes and performs functions
that include verbal activities, such as talking, understanding the speech of
other people, reading, and writing, while the right hemisphere synthesis
information, specializes in seeing the global picture and putting things
together to make a whole as done in activities like as drawing, read maps, and
constructing complex objects out of smaller pieces (Carlson, 2009, p. 87).
However, the forebrain, the midbrain, and the hindbrain are the three major
subdivisions of the brain.
The telencephalon
describes the two equal “cerebral hemispheres”, covered by the “cerebral cortex”
containing the “limbic system and the basal ganglia”, which form the majority
of cerebrum located in the forebrain (Carlson, 2009, p. 83). The cerebral
cortex is organized into the frontal lobe, parietal, temporal, and occipital
lobes, with the central sulcus dividing the frontal lobe from the other three (Carlson,
2009. p. 95). 
The “central sulcus” deals with movement and the planning of movement,
while the other three lobes deal principally with perception and “learning”
(Carlson, 2009. p. 95).The limbic system, includes brain structures involved in
“emotion, motivation, and learning”, such as the frontal “thalamic nuclei,
amygdala, hippocampus, limbic cortex, parts of the hypothalamus” called
mammillary bodies, and their interwoven fiber bundles called the fornix
(Carlson, 2009, p. 88).
The limbic cortex is located near the middle edge of
the cerebral hemispheres, while the “cingulate gyrus” part of the limbic cortex
lies just along the sidewalls of a channel “separating the cerebral hemispheres”,
just above the corpus callosum (Carlson, 2009, p. 88). The hippocampus and the parts of the limbic
cortex that surround it are associated with learning and memory, while the
amygdala and other parts of limbic cortex are specifically involved in
“feelings and expressions of emotions, emotional memories, and recognition of
the signs of emotions in other people” (Carlson, 2009, p. 88). The basal
ganglia is a group of subcortical nuclei including the caudate nucleus, the
globus pallidus, and the putamen, that play an important role in the motor
system (Carlson, 2009, p. 89). Mammillary bodies refer to a protrusion of the
bottom of the brain at the back end of the hypothalamus and contain some
hypothalamic nuclei (Carlson, 2009, p. 88).
The diencephalon,
located between the telencephalon and the mesencephalon surrounding the third
ventrical, includes the thalamus and the hypothalamus (Carlson, 2009, p. 89).
The thalamus is located within the
dorsal section of the diencephalon, near the middle of the cerebral
hemispheres, toward the mid-line and back of the basal ganglia, and above the
hypothalamus (Carlson, 2009, p. 89). The thalamus contains nuclei that send
information to different regions in the cerebral cortex and receive information
from it (Carlson, 2009, p. 89). The hypothalamus
is the group of nuclei that lies at the base of the brain under the thalamus,
which governs the endocrine system, the
regulation of the autonomic nervous system, controls the anterior and posterior
pituitary glands, and integration of species-typical behaviors (Carlson, 2009,
p. 90).
The
midbrain section, located between the forebrain and hindbrain includes the
tectum and the tegmentum. Tectum is the part of the brain concerned with “audition
and the control of visual reflexes and reactions to moving stimuli” (Carlson,
2009, p. 96).The tegmentum contains the reticular formation vital to sleep,
arousal, and movement; the periaqueductal gray matter that controls various
species-typical behaviors; and the red nucleus and the substantia nigra parts
of the motor system (Carlson, 2009, p. 96). The hindbrain is located at the
back of the brain surrounding the fourth ventricle and containing the
cerebellum, the pons, and the medulla (Carlson, 2009, p. 97). The cerebellum
contributes to integrating and coordinating movements, while the pons contains
various nuclei that are important in sleep and arousal (Carlson, 2009). The
medulla oblongata also is involved with the regulation of sleep and arousal,
but also plays a significant role in the control of movement and regulating
vital functions such as heart rate, breathing, and blood pressure (Carlson,
2009, p. 96).
The spinal cord is a long tapering
structure, about as thick as the pinky finger and has various reflexive control
circuits (Carlson, 2009, p. 95). The spinal cord extends only to about
two-thirds the length of the vertebral column and the rest of the space
contains a mass of spinal roots composing the cauda equina (Carlson, 2009, p.
95).The spinal cord contains white
matter and gray matter, like the brain but on the spinal cord, unlike in the
brain, the white matter is on the outside and gray matter is on the inside (Carlson,
2009, p. 95). White matter consists
mostly of ascending and descending bundles of myelinated axons, while
the gray matter consists of neural cell bodies and short unmyelinated axons
(Carlson, 2009, p. 95). The primary function of the spinal cord is to provide motor
fibers to the organs of the body including glands and muscles and collect
somatosensory information to share with brain (Carlson, 2009, p. 94).
Peripheral Nervous System Breakdown
All communication from
the organs, glands, muscles and extremities is transmitted to the CNS from the
PNS nerves called spinal and cranial nerves. First the nerves gather sensory
information then convey this information to the central nervous system and then
the CNS conveys messages from the central nervous system to the body’s parts (Carlson, 2009, p. 95). Any cell body that takes information to the
CNS (spine or brain) is called an afferent axon, while any cell body that takes
information away from the CNS is referred to as efferent (Carlson, 2009, p. 95).
The transfer of sensory
information is part of the somatic
nervous system; this system governs the information from the sensory
organs and those organs that control movements of the skeletal muscles (Carlson,
2009, p. 97). The autonomic nervous
system (ANS) is also part of the
PNS, but is concerned with regulation of smooth muscle, cardiac muscle,
and glands which control regulation of “vegetative processes” in the body
(Carlson, 2009, p. 97). Autonomic nervous system uses a pathway that contains
preganglionic axons from the brain or spinal cord to the sympathetic or
parasympathetic ganglia, and postganglionic axons from the ganglia to the target
organ (Carlson, 2009, p. 101). The
autonomic nervous system is further broken down into two anatomically
separate systems: the sympathetic
division and the parasympathetic
division (Carlson, 2009, p. 97).
With
the exception of the retina, all cell bodies of axons that convey sensory
information into the brain and spinal cord are located outside the CNS and
called afferent axons (Carlson,
2009, p. 97). Dorsal roots and ventral roots are small bundles of fibers
that emerge from each side of the spinal cord in two straight lines along its
front and back surfaces (Carlson, 2009, p. 95). At the point when the dorsal
and ventral roots join together passing through the intervertebral foramens,
they become spinal nerves (Carlson, 2009, p. 95). A spinal nerve is a peripheral nerve
attached to the spinal cord that branches out along that path it travels to the
organ it supplies (Carlson, 2009, p. 97). A cranial nerve is part of the peripheral nervous system that
connects with the brain directly (Carlson, 2009). The twelve cranial nerves
that are affixed directly to the front bottom surface of the brain provide
sensory and motor functions to the head and neck regions (Carlson, 2009, p.
97). For example: The tenth and
largest cranial nerve is called the vagus
nerve and regulates the functions of organs in the thoracic and
abdominal cavities by conveying efferent fibers of the parasympathetic division
of the autonomic nervous system (Carlson, 2009, p.97).
The term peripheral
ganglia describes a group of cells found in the Peripheral Nervous System
outside the spinal cord and brain that are not protected (Carlson, 2009).
Peripheral ganglia function is to connect the central nervous system to the
different parts of the body, and they are found near the organs in the upper
area of the body, specifically the head, abdomen, thorax, stomach, spleen,
liver, kidneys, and along the pelvis (Carlson, 2009, p. 95). Dorsal root ganglia is a cell body on
the dorsal root that takes
somatosensory information to the spinal cord (Carlson, 2009, p. 97). The term
sympathetic ganglia refers to nodules containing synapses between preganglionic
and postganglionic neurons of the sympathetic nervous system (Carlson, 2009, p.
98)
The
sympathetic divisions preganglionic cells are located in the thoracic and first
two lumbar segments of the spinal cord, while the parasympathetic division
preganglionic neurons are located in the brain stem and in sacral segments of
the spinal cord (Henson, 2013). The sympathetic division also controls the adrenal medulla. The Adrenal Medulla a
set of cells located in the center of the adrenal gland, just above of the
kidney and is similar in nature to the sympathetic ganglion. The adrenal
medulla and is controlled by sympathetic nerve fibers and secretes epinephrine
and norepinephrine (Carlson, 2009, p. 100). The secretion of these hormones controls
functions like increase blood flow to the muscles, the breakdown of stored
nutrients within skeletal muscle cells into glucose and increase energy
available to these cells (Carlson, 2009, p. 100)
Parasympathetic Divisions ganglia are located right
next to the intended organs (Carlson, 2009, p. 95). The nuclei that give rise
to preganglionic axons in the parasympathetic nervous system are located in the
nuclei of the “cranial nerves and the intermediate horn of the gray matter in
the sacral region of the spinal cord” (Carlson, 2009, p. 100). The Parasympathetic Division works to increase
the body’s supply of stored energy including primary functions like salivation,
gastric and intestinal motility, secretion of digestive juices, and increased
blood flow to the gastrointestinal system (Carlson, 2009, p. 100).
Typically
speaking, the Sympathetic Division
and the Parasympathetic Division both
interact with the organs they affect, just in opposite ways. For example :
the Parasympathetic Division of the autonomic
nervous system will constrict the pupil of the eye and slow the heart, while
the Sympathetic Division will dilate
the eye and speed the heart (Carlson, 2009, p. 97). The sympathetic
division of autonomic nervous system controls functions like arousal and
expenditure of energy, while the Parasympathetic
division of autonomic nervous system controls the functions that occur during a relaxed state
(Carlson, 2009, p. 98- 100).
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