Download Central Nervous System I. Brain - Function A. Hindbrain 1. Medulla

Central Nervous System
I. Brain - Function
A. Hindbrain
1. Medulla Oblongata
a. Medulla Oblongata Centers or Nuclei
b. Pyramid
2. Pons (bridge)
3. Cerebellum - Purkinji cells
4. Reticular Formation
B. Midbrain
1. Cerebral Peduncles
2. Corpora Quadrigemina. (four twin bodies)
C. Forebrain
1. Diencephalon
a. Thalamus
b. Hypothalamus
c. Epithalamus
2. Limbic system
3. Basal Cerebral Ganglia (Nuclei)
4. Cerebrum
a. Cerebral White Matter (medulla)
Types of nerve tracts
1. Commissural tracts - Corpus Callosum
2. Association tracts
3. Projection tracts
b. Cerebral Grey Matter (cortex)
1. Gyri(us) - convolutions
2. Sulci(us) and Fissures - depressions
3. Lobes of the Cerebrum
a. frontal
b. parietal
c. temporal
d. occipital
e. insula.
The sulci and fissures separate lobes from each other.
c. Functional Areas of the Cerebral Cortex
1. Motor Areas - located mainly in the frontal lobe
a. Primary Somatic Motor Cortex - pyramdial neurons
b. Premotor Area
c. Broca’s Area
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2. Sensory Areas - located in several lobes of the cerebrum
a. Primary Somatic Sensory Cortex
b. Special Sensory Areas
1. Visual. cortex
2. Auditory cortex
3. Olfactory cortex
4. Gustatory (taste) cortex
3. Association Areas
a. Frontal Association Area
1. Somatic Motor
2. Premotor Language Areas, Frontal Eye Field Area
b. Somatic Association Area
c. Visual Association Area
d. Auditory Association Area
e. Wernicke’s Area
f. Common Integrating Area
4. Hemispheric Specialization
D. Ventricles of the Brain
E. Protection of Brain and Spinal Cord
1. Meninges
a. Dura Mater
b. Arachnoid and Subarachnoid Space
c. Pia Mater
2. Cerebrospinal Fluid (CSF)
II. Spinal Cord
A. Functions
B. General Structure of the Spinal cord
C. Composition of the Spinal Cord
1. Gray Matter of the Spinal Cord
2. Dorsal and Ventral Roots of the Spinal Nerves
3. White Matter of the Spinal Cord
a. Ascending (Sensory) Spinal Tracts
Posterior (Dorsal) Funiculus – columns or areas of white matter containing fascicles or tracts of
neurons
1. Fasciclus gracilis
2. Fasciculus cuneatus
Anterior Funiculus
3. Spinothalamic Tracts
Lateral Funiculus
4. Spinocerebellar Tracts
b. Descending (Motor) Spinal Tracts
1. Corticospinal or Pyramidal Tracts
2. Extrapyramidal Tracts
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D. Spinal Reflex Arc
A spinal reflex is a predictable, automatic, stereotypical response to stimuli.
Components of a Spinal Reflex
1. Receptor (sense organ)
2. Sensory (afferent) Neuron
3. Integrating Center – Synapses in the spinal cord
4. Motor (efferent) Neuron
5. Effector (muscle and glands)
Types of Spinal Reflexes
1. Monosynaptic Reflex
Stretch Reflex
2. Polysynaptic Reflexes
a. Flexor or Withdrawal Reflex
b. Cross Extensor Reflex
III. Distribution of Spinal Nerves
A. Rami
B. Plexuses
1. Cervical
2. Brachial
3. Lumbar
4. Sacral
C. Intercostal or Thoracic Nerves
D. Dermatomes
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Central Nervous System
I. Brain
The functions of the brain:
1. It is the center for registering sensations, integrating them with other sensation and with stored
information.
2. It is the center for intellect, emotion, behavior towards others, taking actions and memory.
Different regions of the brain are specialized for different functions and many parts of the brain work
together to accomplish a particular function.
A. Hindbrain
4. Reticular Formation
The reticular formation extends through the brain stem from the medulla oblongata, pons to the
midbrain as a netlike arrangement of small areas of gray matter (cell bodies, dendrites) interspersed
among areas of white matter (myelinated axons). The reticular formation has both sensory and motor
functions.
a. Sensory Function – The main sensory function of the reticular formation is to alert the cerebral
cortex to incoming sensory signals. This part of the reticular formation is called the reticular
activating system (RAS). The RAS maintains consciousness and arousal from sleep. Incoming
sensory impulses from the ears, eyes and skin stimulate the RAS. For example, the sound of the
alarm clock awakens us because sensory information from the ears travels in the RAS that arouses
the cerebral cortex.
b. Motor Function – The main motor function of the reticular formation is to help regulate muscle
tone (the slight degree of contraction exhibited by muscles at rest).
B. Midbrain
1. Cerebral Peduncles
The cerebral peduncles are on the anterior surface of the midbrain. It contains the motor nerve
tracts that conduct nerve impulses from the cerebrum to the pons, medulla and spinal cord and
sensory nerve tracts from the medulla to the thalamus.
C. Forebrain
2. Limbic System
The limbic system encircles the upper part of the brain stem and corpus callosum on the inner
border of the cerebrum and floor of the diencephalon. It is composed of 8 structures. The limbic
system is call the “emotional brain”, because it plays a role in our emtions – pain, pleasure,
affection anger, etc. When different areas of the limbic system are stimulated there are reactions
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indicating intense pain or extreme pleasure and rage. One part of the limbic system in conjunction
with parts of the cerebrum function in memory. People with damage to certain parts of the limbic
system are forgetful and cannot commit things to memory.
3. Basal Cerebral Ganglia (Basal Cerebral Nuclei)
The basal cerebral ganglia are several pairs of nuclei (groups of cell bodies) situated in opposite
cerebral hemispheres. They are areas of gray matter scattered among white matter (myelinated
axons), the internal capsule. The basal cerebral nuclei receive input and output from and to the
cerebral cortex, thalamus and hypothalamus. Some of these nuclei control automatic movements of
the skeletal muscles, such as swinging of the arms while walking. Other of these nuclei regulates
muscle tone for specific body movements.
4. Cerebrum
c. Functional Areas of the Cerebral Cortex
1. Motor Areas – located mainly in the frontal lobes. Its function is to initiate movement.
a. Primary Somatic Motor Area
This area is located in the precentral gyrus of the frontal lobe. The neurons’ cell bodies
are pyramidal shaped. Each region in the primary motor area controls voluntary
contractions of specific muscles. Electrical stimulation of any point in the primary motor
area results in contraction of specific skeletal muscles on the opposite side of the body.
The body parts are not represented in proportion to their size. More cortical area is
devoted to those muscles involved in skilled, complex or delicate movement compared
to muscles involved in gross movements.
b. Premotor Area – Immediately anterior to the primary motor area is a motor association
area. Neurons in this area communicate with the primary motor cortex, sensory
association areas in the parietal lobe, basal cerebral ganglia and thalamus. The premotor
area deals with learned motor activities of a complex and sequential nature. It causes
specific groups of muscles to contract in specific sequences. For example, when you
write, this area controls these skilled movements and serves as a memory for such
movements.
c. Motor Speech (Broca’s) Area – Speaking and understanding language involves sensory,
association and motor areas of the cerebral cortex. For 97% of the population the Broca’s
area and the language areas (Frontal Association Area) are located in the left frontal
lobe. The Broca’s Area is located superior and lateral to the lateral cerebral sulcus.
Language Areas – From Broca’s speech area, nerve impulses pass to the premotor area
that control muscles of the larynx, pharynx and mouth resulting in specific coordinated
muscle contractions that enable you to speak. At same time impulses are sent from the
Broca’s Area to the primary motor cortex to control breathing muscles to regulate proper
flow of air past the vocal cords. The coordinated contractions of the speech and
breathing muscles allow you to speak.
2. Sensory Areas- located in several lobes of the cerebrum, but mainly in the posterior half of
the cerebral hemispheres. Secondary sensory areas and sensory association areas adjacent to
the primary areas receive input from the primary sense areas and other regions of the brain.
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Secondary sensory and sensory association areas integrate the sensations to generate
meaningful patterns of recognition and awareness leading to a meaningful, coordinated
response.
a. Primary Somatic Sensory Area
This area is located in the postcentral gyrus of the parietal lobe. It extends from the
longitudinal fissure on the superior cerebrum to the lateral sulcus. It is separated from the
precentral gyrus of the frontal lobe by the central sulcus. The primary somatic sensory
area receives nerve impulses from somatic sense organs – touch, proprioceptors, pain and
temperature. The major function of the primary somatic sensory area is to localize exactly
points on the body where sensations originate. Electrical stimulation of any point in the
primary somatic sensory area results in sensation of a specific region on the opposite side
of the body. Each point within the area spatially represents the entire body, however the
size of the cortical area depends on the number of receptors present rather than the size of
the part. For example, a larger part of the primary somatic sensory area receives impulses
from the lips and fingertips, as these structures are first to enter the environment. These
structures have a great number of receptors located in them, compared to the hip and trunk
of the body. Again body parts are not represented in proportion to their size.
b. Special Sensory Cortex Areas
1. Visual Cortex
2. Auditory Cortex
3. Olfactory Cortex
4. Gustatory Cortex
These areas receive sensory information from the eyes, ears nose, and tongue.
3. Association Areas
These areas are located on the surfaces of the occipital, parietal, temporal lobes and on the
frontal lobes anterior to the motor areas. Association areas are connected to one another and
to the primary sensory and motor areas.
a. Somatic Motor or Frontal Association Area
This area includes the premotor, language and frontal eye field areas.
b. Somatic Sensory Association Area
This area is located in the parietal lobe posterior to the primary somatic sensory area. It
integrates and interprets sensations. This area permits you to determine exact shape and
texture of an object without looking at it. It also allows you to determine orientation of
one object to another as they are felt and sense relationship of one body part to another.
This association area allows for storage of memories of past experiences, thus enabling
you to compare current sensations with previous experiences.
c. Visual Association Area
This area is located in the occipital lobe. It receives impulses from the primary visual
area. It correlates present and past visual experiences. It recognizes and evaluates what is
seen.
d. Auditory Association Area
This area is located in the temporal lobe inferior and posterior to the primary auditory
area. It discriminates whether a sound is speech, music or noise.
e. Posterior Language (Wernicke’s) Area
This area is for comprehension of written and spoken language. This area is located in
regions of the parietal and temporal lobes. This area interprets the meaning of speech by
recognizing spoken words. It translates words into thoughts, contributes to verbal
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communication by adding tonal inflections and emotional content to spoken language.
For example, due to this area you can tell if a person is angry or joyful by the tone of the
voice.
f. Common Integrative Area
This area is located bordering on the other association areas. It integrates sensory
information from the association areas, transmits this information to other parts of the
brain to cause a coordinated meaningful response to the interpretation of the sensory
information.
4. Hemispheric Specialization (Lateralization)
Although the brain is fairly symmetrical on its right and left sides, there are subtle anatomic
differences between the two hemispheres. This slight differences account for each
hemisphere being specialized to perform certain unique functions. This functional
asymmetry is termed hemispheric lateralization.
The left hemisphere controls the right side of the body; the right hemisphere controls the left
side of the body. The left hemisphere is more important for spoken and written language,
numerical and scientific skills, ability to use and understand sign language and reasoning.
The right hemisphere is more important for musical and artistic awareness, spatial and
pattern perception, recognition of faces, facial expression, and emotional content of
language and for generating mental images to compare these images to each other.
Lateralization is less pronounced in females than in males for language, visual and spatial
skills. This is possibly related to the fact that females’ brains have greater communication
tracts between the two hemispheres.
II. Spinal Cord
Function
The functions of the spinal cord are to maintain homeostasis by nerve impulse propagation and by
information integration. In the white matter of the spinal cord the spinal tracts (bundles of neurons) carry
nerve impulses to and from the brain. The spinal cord’s gray matter is the site of integration of nerve
impulses from the sense organs and brain (incoming information) leading to a coordinated and meaningful
response (outgoing information) to stimuli.
Through the spinal reflexes, the spinal cord, along with the attached spinal nerves, receptors and effectors,
allows one to respond very quickly to environmental changes.
1. Gray Matter of the Spinal Cord
The amount of gray matter of the spinal cord is greatest in segments of the spinal cord that deals with
sensory and motor control of the limbs. These areas form enlargements of the spinal cord. The cervical
enlargement supplies nerves, through the cervical and brachial plexuses, to the shoulder girdles and
upper limbs. The lumbar enlargement supplies nerves, through the lumbar and sacral plexuses, to the
structures of the pelvis and lower limbs.
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2. White Matter of the Spinal Cord – Somatic Sensory and Motor Tracts
The name of the tract indicates the position in the white matter where each tract begins and ends and the
direction of nerve impulse propagation in the tract.
a. Ascending (Sensory) Spinal Tracts – Somatic Sensory Tract
Lateral and Anterior Spinothalamic Tracts
These tracts convey sensory information for pain, temperature, touch and deep pressure from the
spinal cord to the thalamus and then onto the primary somatic sensory area in the cerebral cortex.
This pathway has three orders of neurons.
First-order neurons – connect receptors to the spinal cord by the sensory neurons in the spinal
nerves.
Second-order neurons – have their origin in the gray matter of the spinal cord. These neurons cross
to the opposite side of the spinal cord and sent their axons into the white of the spinal cord to pass
upward to the thalamus. These are the interneurons or association neurons.
Third –order neurons – they synapse in the thalamus with the second-order neurons and project to
the primary somatic sensory area of the cerebral cortex where there is conscious perception of the
sensation.
b. Descending (Motor) Spinal Tracts – Somatic Motor Tract
Corticospinal or Pyramidal Tracts
Lateral and Anterior Corticospinal Tracts are direct motor pathways. Nerve impulses for voluntary
movements propagate from the motor cortex to somatic motor neurons that innervate skeletal muscles.
These tracts are either two-ordered neuron or three-ordered neuron pathways.
Upper motor neurons – their origin is located mostly in the precentral gyrus. The cell bodies of
these neurons are pyramidal shape. These neurons descend through the white matter (internal
capsule) of the cerebrum, the cerebral peduncles, and pons, to the medulla. In the medulla these
neurons cross over (decussate) to the opposite side of the medulla and descend through the spinal
cord. In the medulla the axon bundles form ventral bulges known as pyramids. The motor cortex of
the right side of the brain controls muscles on the left side of the body and vice versa.
Lower motor neurons – extend from the anterior horns of the gray matter of the spinal cord to the
skeletal muscles. Most upper motor neurons synapse with interneruons, which in turn synapse with
the lower motor neurons. A few upper motor neurons synapse directly with the lower motor
neurons. Since the lower motor neurons receive and integrate information from upper motor
neurons and interneurons, the lower motor neurons are called the final common pathway to the
skeletal muscles. This is an example of a convergent neuronal pool.
III. Distribution of Spinal Nerves
A. Rami
A short distance after passing through its intervertebral foramen, a spinal nerve divides into branches or
rami. The posterior or dorsal ramus serves muscles and skin of the dorsal surface of the trunk. The
anterior or ventral ramus serve muscles and structures of the upper and lower limbs and skin of the
lateral and ventral surface of the trunk.
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B. Plexuses
The anterior rami of the spinal nerves, except for thoracic nerves T2-T12 do not go directly to the body
structures they supply. Instead they form networks or plexuses on each side of the body by joining
various neurons from the anterior rami of adjacent spinal nerves. Emerging from the plexuses is large
bundles of nerves that serve specific region of the body. Each of the nerves in turn sends branches to
specific structures they innervate.
Advantage of the Plexus
Within the plexus the neurons from different ventral rami become redistributed so that each branch of
the plexus contains neurons from several different spinal nerves. In addition, neurons from each spinal
nerve travel to a body structure by several different branches. Thus each muscle in a limb receives its
nerve supply from more than one spinal nerve. The advantage is that damage to a spinal cord segment or
a spinal nerve does not completely paralyze a limb muscle.
There are four plexuses:
1. Cervical Plexus – is formed from the anterior rami of cervical nerves C1-C4 with a contribution from
C5. The cervical plexus supplies the skin and muscles of head, neck superior part of the shoulders,
chest and diaphragm. The phrenic nerves arise from the cervical plexus to supply motor neurons to
the diaphragm.
2. Brachial Plexus – is formed from the anterior rami of cervical nerves C5-C8 and thoracic nerve T1.
The brachial plexus enters the axilla and provides nerve supply to the shoulder and upper limbs.
The neurons of brachial plexus form the nerves supplying the muscles of the arm and hand.
In the brachial plexus there is intricate intermingling of neurons from each spinal cord segment.
3. Lumbar Plexus – is formed from the anterior rami thoracic nerve T12 and lumbar nerves L1-L4.
Compared to the brachial plexus, there is no intricate intermingling of neurons from each spinal
cord segment. The spinal nerves pass outward and then give rise to the peripheral nerves. The
lumbar plexus supplies the anterior and lateral abdominal wall, external genitals and anterior parts
of the lower limbs.
4. Sacral Plexus – is formed from the anterior rami lumbar nerves L4-L5 and sacral nerves S1-S4. The
sacral plexus lies anterior to the sacrum. The sacral plexus supplies the buttocks, perineum and
posterior and lateral lower limbs. The largest nerve in the body, the sciatic nerve arises from the
sacral plexus.
C. Intercostal or Thoracic Nerves
The anterior rami of thoracic nerves T2-T12 do not enter into the formation of plexuses. These nerves
directly innervate the structures in the intercostals spaces and abdominal muscles and skin. The posterior
rami of the intercostals nerves supply the back muscles and skin of the posterior area of the thorax.
D. Dermatomes
A dermatome is a specific region of the body that is monitored by a pair of spinal nerves. The posterior
ramus of each spinal nerve provides sensory and motor innervations to muscle and skin of the back. The
anterior ramus of each spinal nerve provides innervations to the ventrolateral body surface and structures
and the limbs. The simple pattern of posterior and anterior rami is observed in thoracic nerves T2-T12.
On the other hand, the spinal nerves controlling skeletal muscles of the neck, upper and lower limbs have
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their anterior rami intermingling their neurons producing a nerve plexus.
Knowing which spinal cord segment supplies each dermatome makes it possible to locate damaged
regions of the spinal cord. If sensation is not perceived on stimulating a particular region of the skin, the
spinal cord segment and/or the spinal nerves supplying that dermatome are probably damaged.
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