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Nervous System Part 2
VIII. Central Nervous System
A. Brain
1. Anatomical Organization
The brain contains gray and white matter
a. Gray matter
Structure:
The gray matter is made up of :
1. the cell bodies of motor neurons and interneurons
2. dendrites
3. unmyelinated axons
4. axon terminals (called telodendria in textbook)
5. neuroglia.
Gray matter in the brain is found in the nuclei of the brain (clusters of neuron cell bodies with
specific functions.
b. White Matter
Structure/Location:
The white matter consists of myelinated axons.
The white matter forms nerve tracts within the CNS. Tracts are bundles of CNS
axons that share a common origin, destination, and function.
White matter in the brain is found internal to the gray matter in the corpus callosum
and the internal capsule.
2. Structure and Function of Major Regions
There are 6 regions of the brain and many of those have multiple structures:
a. Cerebrum
Structure/Location:
1. Cerebral Hemispheres
These are the 2 halves or hemispheres of the forebrain and make up the largest part of
the brain.
2. Longitudinal Fissure
A fissure is a deep groove. The fissure called the longitudinal fissure separates the
cerebral hemispheres
3. Cerebral Cortex
The outer part made of gray matter of each cerebrum is called the cerebral cortex. The
cortex of each cerebral hemisphere receives sensory information from, and sends motor
instructions to the opposite side of the body.
The surface folds into elevated rdiges called gyri (JI-reye) which allows a greater
amount of cortex to fit in the cranium.
The gyri are separated by shallow depressions called sulci (SUL seye) or deeper
grooves called fissures.
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4. Corpus Callosum
The middle part is made up of white matter and includes the corpus callosum, which
connects the two hemispheres.
5. Nuclei
In the very middle is gray matter in collections of neuron cell bodies in the interior part
of the CNS called nuclei. Each nucleus has a specific function.
6. Lobes (5)
There are 5 lobes of the cerebrum
a. Frontal lobe
The frontal lobe is involved in
 voluntary motor function
 concentration
 verbal communication
 decision making
 planning
 personality
b. Parietal Lobe
The parietal lobe is involved in
 somatosensory processing
 evaluating shape and texture of objects being touched
c. Temporal Lobe
The temporal lobe is involved in
 hearing
 interpreting speech & language,smell
d. Occipital Lobe
 Processes incoming visual information
 stores visual memories
e. Insula
This lobe is located under the temporal lobe
It is involved in
 memory
 Interpretation of taste
7. Functional areas of the Cerebral cortex
Primary Somatosensory cortex - in post central gyrus (directly posterior to central
sulcus) of each parietal lobe receives input from somatic sensory receptors for
proprioception, touch, pain, temperature. Primary function to localize exact sites
where sensations originate Sensory homunculus – shows proportional distribution of
sensory input to the somatosensory cortex from different parts of the body based on
degree of sensory perception
Primary motor cortex - located in the precentral gyrus of the frontal lobe controls
voluntary contractions of specific muscles or groups of muscles. Size of area and
number of neurons representing each part of the body is proportional to precision and
complexity of movement of that part
Primary visual cortex - medial surface of occipital lobe receives input from the
thalamus (lateral geniculate nuclei) concerning shape, color, and movement
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Primary auditory cortex - in superior part of the temporal lobe, interprets
characteristics of sound, hearing
Primary gustatory cortex - at base of post central gyrus in the parietal lobe receives
impulses for taste
Primary olfactory cortex - medial aspect of temporal lobe - receives impulses for
smell
b. Diencephalon
(dia = through; encephalon = brain)
The diencepahalon is the region of the brain that consists of the thalamus, hypothalamus,
and pineal gland.
1. Pineal Gland (PIN-ee-ahl)(part of epithalamus)
Structure/Location: This is a cone-shaped gland located dorsal to the thalamus
Functions:
 secretes melatonin which helps to promote sleep and set biological clock
 may be involved in mood and timing the onset of puberty.
2. Thalamus
Structure/Location:
This is a large, oval structure located on either side of the third ventricle. It
consists of two masses of gray matter organized into nuclei.
Function:
It is the main relay center for all sensory impulses ascending to the cerebral cortex,
except olfactory sensations.
Thus, it is called the "gateway" to the cerebral cortex.
It filters information such as the sounds around you when you are trying to study.
3. Hypothalamus (hypo = below)
Structure/Location:
This part of the diencephalon is inferior to the thalamus and forms the walls and
floors of the third ventricle.
Function:
Controls the following:
 autonomic nervous system
 body temperature
 hunger and thirst sensations (monitors levels of nutrients and then produces
sensations of hunger)
 behavior (emotional responses)-rage, pleasure, aggression, sex drive
 sleep-wake cycles
 Formation of memoryMakes many hormones which control the release of
hormones from the rest of the endocrine system.
 Secretes anidiuretic hormone (ADH) (reduces water loss at the kidney) and
oxytocin (stimulates uterine, mammary gland, and prostate gland smooth
muscle contractions)
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c. Midbrain (also called the mesencephalon)
Location:
This is the part of the brain between the pons and the diencephalon.
Function:
1. The midbrain processes visual and auditory information and generates involuntary
somatic motor responses (corpora quadrigemina--superior and inferior colliculi)
(corpora =body; quadrigemina= quadruplets)
2. has reticular activing system-arousal of the whole brain
3. has nuclei for cranial nerves II and IV
4. has ascending and descending tracts
d. Pons
(puente = bridge)
Location:
This is the part of the brain stem that forms a “bridge” between the medulla oblongata and
the midbrain. It is anterior to the cerebellum.
Function:
1. smoothes out the basic rhythm of breathing set by the medulla oblongata (i.e. the pace
and depth of breathing)
2. contains nuclei for cranial nerves V, VI, VII
3. has ascending and descending tracts.
e. Medulla Oblongata
Location:
This is the most inferior part of the brain stem.
Function:
1. relays sensory information from the spinal cord and brain stem to the cerebral cortex.
2. regulates autonomic functions such as heart rate and strength of contraction, blood
pressure (regulates contraction and relaxation of smooth muscle in arterioles), and
respiratory rate (it signals the diaphragm and intercostal muscles to contract, causing us to
inhale)
3. controls vomiting, hiccupping, swallowing, coughing, salivation, and sneezing.
4. has nuclei for cranial nerves VIII, IX, X, XI, and XII
Note: brainstem = midbrain, pons, and medulla oblongata.
f. Cerebellum
Structure/Location:
This is the part of the brain lying posterior to the medulla oblongata and pons. The
cerebellum is shaped like a butterfly. It consists of 2 lateral lobes that are connected
medially.
Function: (balance and posture)
 Smoothes and coordinates body movements (motor activities)
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
Helps maintain equilibriumThe cerebellum receives sensory input from receptors
in muscles, tendons, joints, inner ear, and retina to collect information on posture,
equilibrium and body movements. The cerebellum coordinates subconscious
contractions of skeletal muscles required for coordination, posture and balance.
3. Ventricles of the Brain
(ventricle = little cavity)
Structure/Location:
Ventricles are cavities in the brain filled with CSF.
Function:
Capillaries in the walls of the ventricles produce cerebrospinal fluid (CSF). The ventricles
circulate the CSF.
There are four ventricles. (2 lateral ventricles, a third ventricle, and fourth ventricle). The
ventricles are interconnected with each other and with the subarachnoid space and central
canal.
a. Lateral Ventricles
There is one lateral ventricle in each cerebral hemisphere.
Each is C-shaped
b. Interventricular Foramen
(inter = between)
This is an opening between the lateral ventricles.
It allows the lateral ventricles to communicate with the third ventricle.
c. Third Ventricle
located between the cerebral hemispheres in the diencephalons, around the thalamus.
d. Cerebral aqueduct (mesencephalic aqueduct)
slender canal that connects the third ventricle with the fourth ventricle.
e. Fourth Ventricle
located between the pons and cerebellum.
Within the medulla, the fourth ventricle narrows and becomes continuous with the central
canal of the spinal cord.
There are opening in the 4th ventricle that connects it to the subarachnoid space (a fluid
filled space surrounding the brain and spinal cord)
4. Cerebrospinal Fluid (CSF) (See Fig. 8-18 on pg. 246)
Structure:
The cerebrospinal fluid is a clear, lymph-like fluid produced continually by active
transport of substances from blood plasma by specialized capillaries in the roof of the
ventricles called choroid plexuses. The total volume is about 1/2 cup!
The choroid plexus is made of ependymal cells, associated capillaries and connective
tissues.
Function:

forms a protective cushion around within the CNS.
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


keeps the brain "floating" in the cranium.
transports nutrients and chemical messengers.
absorbs and removes waste products and toxins produced by the metabolic activity
of neurons.
Location:
The CSF circulates around the CNS through the ventricles of the brain, the central canal of
the spinal cord and the subarachnoid space of the meninges.
Blood Brain Barrier:
This is a barrier that keeps materials from diffusing across capillaries. It is formed by
1. the perivascular feet of astrocytes that cover and wrap around capillaries in the brain. They act as
“gate keepers” that allow or don’t allow materials that leave the capillaries to pass to the
neurons. Only a few lipid soluble substances such as nicotine, anesthetics and alcohol can pass
through to reach the neurons.
2. the basement membrane of the endothelial cells that make up the capillaries
3. the tight juncions between adjacent endothelial cells
There are only 3 places in the brain that do not have a blood brain barrier:
1. choroid plexuses (make CSF)
2. the hypothalamus (needs to let hormones into bloodstream)
3. the pineal gland (also needs to let hormones into the bloodstream)
B. Spinal Cord
Location:
The spinal cord is a continuation of the brain. It extends from the foramen magnum of the skull
to L1 vertebrae.
It is about 18" (45 cm) long
Structure:
 The spinal cord, like the brain is made up of gray matter and white matter.
 The gray matter is centrally located (buterfly)
 31 pairs of spinal nerves originate from the spinal cord (spinal nerves are part of the
PNS)
Function:
1. The spinal cord serves as the major pathway for impulses to and from the brain it conveys sensory impulses from the body (from sensory receptors) to the brain
 it conveys motor impulses to the body (to muscles and glands).
2. The spinal cord also integrates information on its own, controlling spinal reflexes that occur
without any brain involvement (ex: withdrawal from pain)
[Note If spinal cord is severed or injured between C1-C5-lose all functionality and ability to breathe because
nerves that signal diaphragm to contract to breathe travel between C3-C5, so need artificial respirator.
If injure C5-T1 are quadriplegic-lose use of arms and legs.
If injure T1-upper lumbar are paraplegic- lose use of legs.]
1. Anatomical Organization
a. Gray matter
The gray matter in cross section looks like a butterfly.
It contains the somas, dendrites, and proximal parts of the axons of neurons.
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The crossbar of the gray matter is called the gray commissure. (commissural = a joining
together)
In the center of the gray commissure is a small space called the central canal. The central
canal runs the length of the spinal cord and contains CSF.
1. Posterior (Dorsal) Gray Horns
Is the sensory part of the gray matter. Sensory info comes INTO the spinal cord here.
Contain the axons of somatic and visceral sensory neurons and the cell bodies of
interneurons.
[Laura: the cell bodies of the sensory neurons are located in the posterior root glanglia]
2. Anterior (Ventral) Gray Horns
Is the motor part of the gray matter. Motor info LEAVES the spinal cord here.
Contain the dendrites and cell bodies of somatic motor neurons (innervate skeletal
muscle) that leave the spinal cord.
These horns are thicker than the dorsal horns.
3. Lateral Gray Horns
At the thoracic and lumbar levels there are lateral horns that contain the cell bodies of
autonomic motor neurons (innervate cardiac muscle, smooth muscle, and glands) that
lead to organs (heart, digestive system, etc).
[send their axons out of the cord by way of the ventral root along with the somatic
efferent fibers.]
2. White Matter
The white matter is made mostly of myelinated axons (some unmyelinated) organized into
sensory (ascending) and motor (descending) tracts that convey impulses between the brain
and spinal cord. Think: SUB: Sensory UP the BACK; Motor DOWN the FRONT)
The white matter is divided into 3 regions, each called a funiculus or column.
Each column contains axons of both motor and sensory nerves.
1. Posterior White Funiculus (fūnicūlus) (used to be called columns, but text says this is
now not considered correct)
Consists of sensory (ascending) tracts
Function:
Convey sensory information (e.g. pain, temperature, light touch, pressure) to brain.
2. Lateral White Funiculus (fūnicūlus)
Consists of both sensory and motor tracts
3. Anterior White Funiculus (fūnicūlus)
Consists of descending (motor) tracts.
Function:
Convey motor impulses from brain; these motor impulses help maintain skeletal
muscle tone, posture. They play a major role in equilibrium by regulating muscle tone
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in response to movements of the head and they convey impulses to skeletal muscles
that result in precise movements.
2. Meninges in Spinal Cord (and Brain)
(singular = meninx) (See Fig. 8-13, pg. 239)
(meninx = membrane)
Structure/Location:
Meninges are connective tissue membranes that cover the entire CNS (brain and spinal
cord) and separate the CNS from the bones of the vertebrae and skull.
Function:
1. separate the soft tissue of the brain from the bones of the cranium
2. enclose and protect blood vessels that supply the brain
3. contain and circulate cerebrospinal fluid.
There are 3 meninges:
a. Dura Mater (MAH ter or MAY ter-textbook)
(Dura = hard; mater = mother)
This is the outermost covering made of dense irregular connective tissue.
It is made of 2 layers in the brain (periosteal and meningeal) and the 2 layers are fused
except at the dural venal sinuses (blood filled spaces that drain blood from the brain to the
internal jugular vein)
In the spinal cord it is made of only 1 layer
Function: It gives structural support.
Recall when you cut the dura mater to observe the sheep brains..
1. Epidural Space
In the spinal cord, between the dura mater and the wall of the vertebral column is a
space called the epidural space. This space is filled with areolar connective tissue,
adipose tissue and blood vessels.
This is the space where anesthesia is injected.
In the brain the epidural space contains blood vessels that nourish the meninges and
bones of the cranium.
2. Subdural Space
(subdural hematoma)
b. Arachnoid Mater
(arachnoid = spider; mater = mother)
This is the middle meninges layer and it is made of delicate web of elastic and collagen
fibers..
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Arachnoid villi capillaries function in the absorption of CSF.
1. Subarachnoid Space
The subarachnoid space is located between the arachnoid and the pia mater.
The subarachnoid space is where the CSF circulates.
(sub = below)
c. Pia Mater
(pia = delicate; mater= mother)
This is the innermost layer of meninges made of areolar connective tissue. It is a transparent
fibrous membrane that adheres to the surface of the spinal cord and brain. It contains
numerous blood vessels that supply oxygen and nutrients to the cells of the spinal cord.
IX. Reflex Arcs
A. Definition of Reflex
A reflex is a rapid, automatic motor response to a stimulus. They are unlearned, unpremeditated, and
involuntary. They help protect you and maintain homeostasis.
E.g. you may jerk your hand away from a hot stove or vomit in response to a food that irritates your
stomach.
Many reflexes are independent of the brain, but the information picked up by the interneurons in the
spinal cord is transmitted to the brain so you are aware of what stimulus caused the reflex (e.g.
boiling water.)
All reflexes have similar properties:
1. A stimulus is required
2. They are fast.
3. They are automatic and occur the same way every time
4. They are involuntary. It is not until after the reflex has been completed that you are even
aware it has happened.
B. Structure of a Reflex Arc
The neural wiring of a single reflex is called a reflex arc. A reflex arc is a neural pathway that begins
at a sensory receptor and ends at an effector (muscle or gland). Reflex arcs consist of the following
components:
1. Receptor
The stimulus acts on a receptor (e.g. sensory receptors of a sensory neuron). The receptor
responds to a stimulus and generates an impulse.
2. Sensory Neuron
The sensory neuron transmits the afferent impulses from the sensory receptor to the CNS.
3. Integration Center
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The simplest reflexes do not involve interneurons. Instead the sensory neuron synapses directly
with a motor neuron in the anteior gray horn of the spinal cord.
More complex reflexes however use one or more interneurons in the CNS to integrate and
processing incoming sensory information and transmit a command to the motor neuron.
Interneurons also transmit information concerning the reflex to the brain.
4. Motor Neuron
The motor neuron transmits a nerve impulse through the anterior root of the gray horn and then
the spinal nerve to the effector organ.
[In somatic reflex arcs there is a single motor neuron. In autonomic reflex arcs there are 2 motor
neurons.
5. Effector
The effector (organ that produces the response) then responds to the stimulus from the motor
neuron.
This is the muscle or gland cell that responds to the efferent impulses by contracting or
secreting.
The response is the reflex.
Effectors in the somatic nervous system are skeletal muscles.
Effectors in the ANS are smooth muscle, cardiac muscle, or glands.
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