NERVOUS SYSTEM ANATOMY
Anatomically the nervous system is divided into the central nervous system (CNS) and the
peripheral nervous system (PNS). The CNS is housed in the dorsal body cavity and consists of the
brain and spinal cord.
The brain is a complex organ with many different parts. Embryologically, it is derived from a
structure called the neural tube as a series of five enlargement.
SOME GENERAL TERMINOLOGY
I. Gyrus: ridge of tissue on surface of cerebrum
and cerebellum
II. Sulcus: groove between gyri
III. Fissure: deep groove; usually separates
large areas of the brain from one another
IV. Gray Matter: CNS tissue where cell bodies
of neurons are found
V. White Matter: CNS tissue where neuron
fibers (axons and dendrites) are found; three
types of white matter in brain
A. Commissural Fibers: interconnect left
and right corresponding areas of two
sides of the CNS
B. Association Fibers: interconnect different areas within a given side of brain
C. Projection Fibers: tie various levels of
the CNS together
SHARK
I. Telencephalon: anterior-most enlargement of
neural tube.
A. Olfactory Bulbs: rounded masses lying
just behind olfactory sacs.
B. Olfactory Tracts: stalk-like caudal extensions connecting olfactory bulbs
with anterior brain.
C. Olfactory Lobes: rounded masses of
anterior brain which receives olfactory
tracts.
D. Cerebral Hemispheres: anterior brain
eminences just posterior to olfactory
lobes; collectively form the cerebrum.
II. Diencephalon
A. Optic Chiasm: X-shaped structure
formed by crossing of optic nerves as
these enter diencephalon.
B. Hypothalamus: forms floor of diencephalons; consists of
1. Infundibulum: located just behind
chiasm; consists of anterior inferior
lobes and posterior vascular sacs.
2. Hypophysis
C. Epithalamus: forms roof of diencephalon.
III. Mesencephalon
A. Optic Lobes: rounded bodies behind
diencephalon; visual processing centers.
IV. Metencephalon
A. Cerebellum: large oval structure partly
overlapping mes- and myelencephalon;
dorsal surface is demarcated by shallow
grooves into four quadrants.
V. Myelencephalon
A. Medulla Oblongata: greater part of
myelencephalon; continuous with spinal
cord.
MAMMALIAN
(Sheep / Human)
I. Telencephalon
anterior-most enlargement of neural tube.
A. Cerebrum: largest and most superior
part of brain; highly developed in mammals and birds; cerebral cortex is outer
part and consists of gray matter; divided
into left and right cerebral hemispheres
separated by the longitudinal fissure;
each hemisphere divided into five lobes:
1. Frontal Lobe*: posterior border is
central sulcus, inferior border is
lateral sulcus; anterior areas are
primary site of “intellect and
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thought”; posterior areas concerned
with motor activities, especially
primarymotor area of pre-central
gyrus on ventral surface are
olfactory bulbs.
2. Parietal Lobe*: anterior border is
central sulcus; posterior border is
parietooccipital sulcus; concerned
with sensory activity, especially
primary somatosensory area of
post-central gyrus.
3. Occipital Lobe*: anterior border is
parietooccipital sulcus; inferior
border is transverse fissure;
contains the primary visual area.
4. Temporal Lobe*: separated from
frontal and parietal lobes by lateral
sulcus; contains primary auditory
area.
5. Insula*: found deep to temporal
lobe through lateral sulcus.
B. Corpus Callosum: example of white
matter within cerebrum; can be seen by
gently separating left and right cerebral
hemispheres and looking into longitudinal fissures; can be seen as superior
border of a lateral ventricle on a sagittal
section.
C. Basal Nuclei
various nuclei found deep in cerebrum
receive input from cerebral cortex, subcortical structures and each other project to premotor and prefrontal areas to
start, stop, and monitor movements.
II. Diencephalon: central core of brain
surrounded by cerebral hemispheres.
A. Thalamus: forms roof and walls of third
ventricle composed of many nuclei
which essentially relay ascending information to cerebrum; left and right
sides are interconnected by intermediate
mass.
B. Hypothalamus: forms floor of third
ventricle controls autonomic nervous
system and major parts of endocrine
system.
C. Pineal Gland (or Body): superior to
thalamus secretes hormone melatonin.
III. Mesencephalon+: lies between diencephalon
and pons; contains cerebral aqueduct and
nuclei of cranial nerves III and IV.
A. Cerebral Peduncle: located ventrally;
contains pyramidal motor tracts.
B. Superior Colliculus: one of two protrusions from roof of mesencephalon; contains visual reflex centers.
C. Inferior Colliculus: one of two smaller
protrusion from roof of mesencephalon;
contains auditory relay and reflex
centers.
IV. Metencephalon
A. Pons+: contains projection tracts connecting motor cortex and cerebellum;
contains nuclei of cranial nerves V, VI,
and VII and part of reticular formation.
B. Cerebellum: protrudes from posterior
aspect of brain; separated from cerebrum by transverse fissure; forms roof
of fourth ventricle; processes input from
cerebral motor cortex, various brain
stem nuclei, and sensory receptors; provides precise timing and appropriate
patterns of skeletal muscle contraction.
1. Cerebellar Hemispheres: located
laterally.
2. Vermis: prominant midline landmark in sheep brain, much smaller
in human brain.
3. Arbor Vitae: white matter within
cerebellum.
V. Myelencephalon
A. Medulla Oblongata+: most inferior
portion of brain stem; contains
nuclei for cranial nerves VIII, IX,
X, XI, and XII; has pyramids
(bilateral anterior longitudinal
ridges; contains descending motor
fibers (pyramidal fibers); these
fibers decussate in medulla); has
various autonomic control centers:
1. cardiovascular
2. respiratory, coughing,
sneezing
3. swallowing, vomiting, etc.
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VI. The Meninges: exernal coverings of the
CNS; three layers:
A. Dura Mater: superficial double layered
meninx; outer layer acts as periosteum
of cranial bones; inner layer forms
superficial brain covering; subdural
space is location of veins which drain
blood from brain.
B. Arachnoid (Mater): middle meninx; has
spider web-like appearance subarachnoid space filled with cerebrospinal
fluid (CSF); CSF acts as cushion for
brain during possible trauma; CSF
drains from space to blood through
arachnoid villi.
C. Pia Mater: deepest meninx; follows
contours of brain.
VII. Ventricular System: series of four cavities
within brain; filled with CSF; each ventricle
has a choroid plexus, a network of blood
capillaries which produces CSF.
A. Lateral Ventricles: one ventricle found
in each cerebral hemisphrere; separated
from each other by septum pellucidum;
CSF drains from lateral ventricle into
third ventricle through interventricular
foramen.
B. Third Ventricle: found in midline of
diencephalons; CSF drains from this
ventricle into cerebral aqueduct.
C. Fourth Ventricle: found between cerebellum and medulla oblongata; receives
CSF from cerebral aqueduct; drains
CSF into either
1. central canal of spinal cord or
2. subarachnoid space
enters subarachnoid space by
a. one median aperature or
b. two lateral aperatures
*Found only on the human brain models, not sheep
brains.
+
The mesencephalon, pons and medulla oblongata
constitute the brain stem.
VIII. Cranial Nerves: arise directly from brain;
mainly sensory, mainly motor or mixed;
identified by a proper name or a Roman
numeral
A. Olfactory Nerve (CN I): sensory from
nasal cavity (sense of smell).
B. Optic Nerver (CN II): sensory from
retina of eye (sense of sight).
C. Oculomotor (CN III): motor to four of
extrinsic eye muscles (superior, inferior
and medial rectus and inferior oblique).
D. Trochlear (CN IV): motor to superior
oblique muscle.
E. Trigeminal (CN V): motor to muscles of
mastication; sensory from skin of face,
oral and nasal cavities and paranasal
sinuses.
F. Abducens (CN VI): motor to lateral
rectus muscle.
G. Facial (CN VII): motor to facial and
scalp muscles; sensory from anterior
two-thirds of tongue
H. Auditory or Vestibulocochlear (VIII):
sensory from inner ear (sense of hearing
and equilibrium).
J. Glossopharyngeal (CN IX): motor to
pharynxgeal muscles; sensory from
posterior two-thirds of tongue.
K. Vagus (CN X): motor to larynx (speech)
and esophagus (swallowing) and to gut,
bronchial tree and heart; sensory from
pharynx, larynx, gut, bronchial tree and
heart.
L. (Spinal) Accessory# (CN XI): motor to
trapezius and sternocleidomastoid.
M. Hypoglossal# (CN XII): motor to
intrinsic and extrinsic tongue muscles.
#
These are not cranial nerves in the shark, but rather
spinal nerves, known as the spinal and occipital
nerves, respectively.
MAMMALIAN SPINAL CORD
and SPINAL NERVES
The spinal Cord part of the CNS continuous
with brain and is housed within the vertebral
canal of spinal column. It has certain external
features that we will not bother to learn, and
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some internal features which are important in
understanding some of the physiology of CNS
control.
I. Spinal Cord Internal Features
A. Central Canal: space in middle of spinal
cord continuous with brain’s ventricular
system; filled with cerebrospinal fluid.
B. Gray Matter: found surrounding central
canal, generally described as having a
butterfly-like shape; divided into posterior, anterior, and lateral horns.
C. White Matter: found on outside of spinal
cord; divided into posterior, anterior,
and lateral columns.
II. Spinal Nerves: arise directly from spinal
cord; formed by merging of the dorsal and
ventral roots.
A. Dorsal (or Posterior or Sensory) Root:
exclusively composed of portions of
sensory neurons; carry information into
spinal cord; cell bodies of sensory
neurons are found in dorsal root ganglia of each dorsal root.
B. Ventral (or Anterior or Motor) Root:
exclusively composed of portions of
motor neurons; carry information away
from spinal cord.
because dorsal and ventral roots merge to
form spinal nerve, a spinal nerve carries
both sensory and motor information.
III. Nerve Plexuses: as spinal nerves exit intervertebral foramina they branch and interconnect with other branches; this network of
nerves is a nerve plexus; four plexuses send
specific nerves to particular locations in the
body.
A. Cervical Plexus: arises from spinal
nerves C1 through C4 or C5.
1. Phrenic Nerve: motor to diaphragm.
B. Brachial Plexus: arises from spinal
nerves C5 through C8 or T1.
1. Axillary Nerve
motor to deltoid and teres major
sensory from skin of shoulder
2. Musculocutaneous Nerve
motor to flexor muscles of arm
sensory from skin of forearm
3. Radial Nerve
motor to extensor muscles
sensory from some skin of upper
extremity
4. Median Nerve
motor to most flexor muscles of
the forearm and hand
sensory from some skin of hand
5. Ulnar Nerve
motor to some flexor muscles of
forearm and hand
sensory from some skin of hand
C. Lumbar Plexus
1. Femoral Nerve
motor to quadriceps femoris, and
sartorius
sensory from skin of anterior and
medial aspects of lower
extremity
2. Obturator Nerve
motor to adductor muscles of thigh
D. Sacral Plexus
1. Sciatic Nerve
motor to hamstring
sensory from skin of posterior thigh
two major branches
a. Tibial Nerve
motor to flexor muscles of leg
and extensor muscles of
foot
sensory from skin of leg
b. (Common) Peroneal Nerve
motor to biceps femoris,
peroneus longus and
extensor digitorum
sensory from skin of anterior
leg and dorsum of foot
2. Pudendal Nerve
motor to muscles of the urogenital
diaphragm
sensory from penis and scrotum in
males or clitoris, labia and
vagina in females
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