CHAPTER
13 SPINAL CORD
AND
SPINAL
NERVES
The
spinal cord and the brain are continuous. The spinal cord is the highway for
sensory impulses going to the brain and motor impulses leaving the brain. Also,
some integration of impulses occurs in the spinal cord, and there are some
reflexes that involve the spinal cord only, not the brain.
2
connective tissue coverings protect the cord:
1. Vertebral column--vertebrae are
stacked on top of each other with the vertebral foramina forming a canal for
the cord
2. Meninges--these
are CT coverings that encircle both the spinal cord and the brain. (Spinal meninges around cord; cranial meninges around brain.) The meninges
are in 3 layers:
a. Pia
mater--innermost layer of thin CT that adheres to the surface of the cord.
It contains many blood vessels that supply the cord. Little extensions of the pia called denticulate ligaments anchor the cord to the
inside of the dura.
b. Arachnoid--CT
with a web-like arrangement of collagen and elastic fibers
c. Dura mater--tough outer
covering of dense irregular CT
In between the pia and the arachnoid is the subarachnoid space, which contains cerebrospinal fluid and helps cushion the cord.
Between the arachnoid
and the dura is the subdural
space, which contains interstitial fluid.
The epidural space is between the dura and the bony wall of the vertebral canal. This space
contains fat and CT for further protection of the cord.
Inflammation of the meninges is meningitis.
EXTERNAL ANATOMY OF THE SPINAL CORD
The
spinal cord is in the shape of a slightly flattened cylinder and extends from
the medulla oblongata to the upper border of L2 (16-18"). Its diameter
varies, but is approximately 1 inch or less. An enlargement in the cervical
area gives off nerves to supply the arms and the lumbar enlargement gives off
nerves to supply the legs.
Below
the lumbar enlargement the spinal cord tapers into a conical shape called the conus medullaris. An extension of
the pia, the filum terminale, extends downward and attaches the cord to the
coccyx.
Through
most of the length of the spinal cord, spinal nerves arise and immediately exit
from the vertebral canal. However, spinal nerves that arise from the lower
portion of the cord travel downward within the vertebral canal for a distance
before exiting. These resemble a horse's tail and are called the cauda equina.
Gray
matter of the cord contains clusters of nerve cell bodies
(nuclei). They receive and integrate incoming and outgoing information.
White
matter contains bundles of myelinated
fibers and these bundles are called tracts. Tracts contain fibers having a
common origin or destination. Ascending tracts contain sensory axons that
conduct impulses toward the brain. Descending tracts carry motor impulses
downward. Tracts in the spinal cord are continuous with tracts in the brain.
The tracts are highways for the flow of nerve impulses.
The
tracts are often named in such a way that the name alone tells:
1. Which white column the tract travels in
2. Where the tract begins
3. Where the tract ends
4. Whether it is sensory or motor
Anterior spinothalamic
The
major sensory tracts are:
1. Spinothalamic
tracts---impulses for pain, temp, crude touch, deep pressure
2. Posterior column tracts---proprioception, fine touch, pressure, vibrations
Sensory tracts bring information to the brain and spinal cord (CNS) and here this information is integrated at various levels. Motor impulses originate in the CNS and travel down 2 types of tracts:
1. Direct pathways (pyramidal tracts)—carry
motor impulses that originated in the cerebral cortex to voluntary muscle on
the conscious level
Lateral corticospinal
Anterior corticospinal
Corticobulbar
2. Indirect pathways (extrapyramidal
tracts)—carry autonomic impulses and impulses to skeletal muscle below the
conscious level (posture, muscle tone, etc.). These impulses originate in the
brain stem, hypothalamus, etc. As well as control of smooth muscle, cardiac
muscle and glands, these impulses play a major role in equilibrium and control
of postural muscles and muscle tone.
Rubrospinal
Tectospinal
Vestibulospinal
As well as connecting the PNS to the brain, the spinal cord is an integrating
center for certain reflexes (spinal reflexes). Reflexes are fast automatic
responses to changes in the environment. They allow the body to make
adjustments that will help to maintain homeostasis.
Reflex
arc--pathway for a series of nerve impulses.
Parts:
1. Sensory receptor--distal end of a sensory
neuron which responds to a stimulus by generating a nerve impulse
2. Sensory neuron--conducts impulses to its
axon terminals, located in gray matter of spinal cord or brain stem
3. Integrating center in the CNS
4. Motor neuron--carries impulse from the
integrating center to the part that will respond
5. Effector--part
that will respond--muscle or gland--its action is called a reflex
Reflexes can be classified in several ways:
1. Reflexes classified according to
where the integrating center is located
a. Cranial reflexes---integrated
in the brain
1) Turning the head suddenly in
response to a loud noise
2) Uncontrollable blinking of the
eye when the cornea is touched or painful
3) Other cranial reflexes coordinate
functions such as swallowing
b. Spinal reflexes---integrated
in the spinal cord
1) Stretch reflex---if a skeletal
muscle is suddenly stretched, it responds by quickly increasing tone
(results in kicking out of the leg)
2) Tendon reflex---if a
skeletal muscle is stretched beyond safe limits, a reflex causes the muscle to relax
before tissue is damaged
3) Flexor reflex---when pain
receptors are stimulated, reflex muscle contraction occurs
to quickly jerk the part away from the trouble
4) Crossed extensor reflex---when
you flex one leg, you automatically activate
extensors of
the other leg to support you so you won’t fall down
5) Other spinal reflexes coordinate
the muscle contractions necessary for
posture,
walking, running, etc.
2. Reflexes classified according to the
type of effector
a. Somatic reflexes---skeletal muscle
(We can also contact these same skeletal muscles voluntarily, but reflexes are
important because they occur more quickly without requiring conscious thought.)
b. Visceral reflexes---smooth muscle,
cardiac muscle, glands
Abnormal
reflexes may indicate disorders of the nervous system.
Spinal
nerves connect the CNS to the PNS. There are 31 pairs of spinal nerves.
8 cervical--numbered C1-C8--one emerges between
the atlas and the occipital bone, then 1 from below each of the 7 cervical
vertebrae
12 thoracic--numbered T1-T12
5 lumbar lower lumbar down form
5 sacral cauda
equina
1 coccygeal
There
are 3 CT coverings associated with spinal nerves:
1. Endoneurium--wraps
individual fibers
2. Perineurium--wraps
bundles of fibers called fascicles
3. Epineurium--wraps
entire nerve--the epineurium fuses with the dura mater of the cord
A
typical spinal nerve has 2 points of attachment to the cord:
1. Posterior (sensory) root
2. Anterior (motor) root
Spinal
nerves pass through the intervertebral foramina and
divide into 4 branches (rami):
1. Dorsal ramus---deep
muscles and skin of dorsal trunk
2. Ventral ramus---arms,
legs, ventral trunk
3. Meningeal
branch---reenters spinal canal
4. Rami communicantes---autonomic nervous system
Since
spinal nerves contain both motor and sensory fibers they are called mixed
nerves.
Except
for T2-T12, ventral rami do not go directly to
structures they serve. They join with ventral rami of
other spinal nerves and form networks called plexuses.
1. Cervical plexus--C1-C4 or C5--neck area--skin and muscles of head, neck, and upper shoulders and the diaphragm.
a. Phrenic
nerve--innervates diaphragm
2. Brachial
plexus--C5-C8, T1--provides the entire nerve supply of the arm
a. Axillary
nerve--deltoideus and teres
minor
b. Musculocutaneous
nerve--flexors of arm and forearm
c. Radial nerve--muscles of posterior
arm and forearm
d. Median nerve--anterior forearm and
some palm muscles
e. Ulnar
nerve--anteromedial muscles of forearm, most of
palm
3.
Lumbar plexus—L1 - L4--anterolateral abdominal
wall, external genitals, part of leg
a. Femoral--flexors
of thigh, skin on front and medial thigh, medial leg and foot
b. Obturator--adductors
of legs
4. Sacral
plexus—L4 - L5 - S1 - S4--buttocks, perineum, rest of leg
a. Sciatic--largest nerve in the
body, leg not served by femoral nerve
P.
436 through 445, various exhibits and figures show the
plexuses--read over these pages and particularly note clinical applications
regarding nerve injuries on each one.
Be
sure to read the
section on effects of spinal cord damage at various levels on Page 446.
Ventral
rami of T2-T12 directly form the intercostal
(thoracic) nerves (no plexuses), which supply intercostal
muscles, skin over the rib and abdominal area and some abdominal muscles.
Each
spinal nerve serves a specific constant area of the skin. The area of the skin
that provides sensory input to one pair of spinal nerves is a dermatome. This
knowledge can be used to determine which area of the spinal cord is
malfunctioning--if skin area is stimulated and the stimulus is not perceived
that helps locate the trouble. See chart on p. 458 (do not have
to memorize).
Myotome---all
muscles innervated by motor neurons in a single spinal segment.
CHAPTER 14 BRAIN AND CRANIAL NERVES
The
adult brain is made up of approximately100 billion neurons. It weighs about 3
pounds and can be divided into 4 principal parts:
I. Brain stem--continuous with spinal
cord--medulla, pons, midbrain
II. Cerebellum---posterior to brain
stem
III. Diencephalon--thalamus,
hypothalamus, epithalamus, subthalamus
IV. Cerebrum--occupies most of the
cranium (hollow space inside the skull)
PROTECTIVE COVERINGS
The
brain is protected by the cranial bones and the cranial meninges,
which are continuous with the spinal meninges:
1. Dura mater--outer layer--adheres
to the inside of the cranial bones and forms their periosteum
2. Arachnoid
3. Pia
mater--rich in capillaries--attached to brain and dips down into all sulci
and
fissures
The right and left sides of the cerebrum (cerebral hemispheres) are separated by a downward extension of the dura called the falx cerebri. The falx cerebelli separates the two halves of the cerebellum, and the falx tentorium extends between the cerebrum and the cerebellum.
Blood
reaches the brain by the internal carotid arteries and the vertebral arteries,
and returns by the internal jugular veins. The brain composes 2% of body weight
and consumes 20% of the oxygen at rest. Oxygen use can rise with high levels of
activity. Even a brief interruption of blood flow can cause unconsciousness.
Four minutes without oxygen can permanently injure neurons, as lysosomes begin
to break open due to low oxygen.
The
brain also requires a constant supply of glucose, since carbohydrate storage in
the CNS is extremely limited.
Low blood glucose causes confusion, dizziness, etc.
Different
materials pass from the blood into the brain at different rates due to the
blood-brain barrier (BBB). Brain capillaries have endothelial cells that are
connected by tight junctions and astrocytes that
press against the vessels also regulate passage.
Water
crosses easily
Some water-soluble substances such as
glucose are moved by special active transport mechanisms or facilitated
diffusion (which are not needed in other capillaries).
Lipid-soluble substances cross easily—examples
are oxygen, carbon dioxide, other lipid-soluble substances (alcohol, caffeine,
anesthetics)
Creatinine, urea,
ions enter slowly
Proteins and most antibiotics cannot pass
Trauma,
some toxins, and inflammation can lessen the effectiveness of the blood-brain
barrier. At times, we would like to have the ability to cause certain
substances to penetrate this barrier. Read "Breaching the blood-brain
barrier" P. 477.
The
brain and spinal cord are nourished and protected by cerebrospinal fluid (CSF).
It circulates through the subarachnoid space around
the brain and spinal cord and through cavities (ventricles) within the
brain. (Fig. 14.3 P. 478)
CSF
totals 80-150 ml (3-5 oz) and is a clear, colorless liquid that contains
glucose, proteins, urea, lactic acid, ions, and some white blood cells.
Functions:
1.
Mechanical protection--shock absorber--brain "floats" in CSF
2.
Chemical protection--provides an optimum chemical environment for accurate
neuron signaling
3.
Circulation—brings in some oxygen and nutrients, carries away some waste as it
is constantly secreted and reabsorbed.
Located
within the brain are 4 fluid-filled cavities called ventricles:
2 lateral ventricles---one within each
cerebral hemisphere
3rd ventricle—a slit at the midline
between the right and left halves of the thalamus, which means it is also between the lateral ventricles
4th ventricle--lies between brain stem
and cerebellum
Networks
of capillaries called choroid plexuses in the walls
of the ventricles are covered by ependymal cells,
which form the CSF from the blood plasma in these capillaries by filtration and
secretion. The ependymal cells can prevent certain
harmful substances that might be present in blood from entering the CSF—this is
the blood-cerebrospinal fluid barrier.
Lateral
ventricles
Foramen of Munro (interventricular
foramina)
3rd
ventricle
Cerebral aqueduct(aqueduct
of Sylvius)
4th
ventricle
Openings in roof of 4th ventricle
Subarachnoid
space
Circulates around brain & spinal cord in subarachnoid
space and in
central canal of spinal cord
Arachnoid villi--extensions of
the arachnoid located between the cerebral
hemispheres
that reabsorb the CSF
CSF
is formed and reabsorbed at a rate of 20 ml/hour. If an obstruction,
malformation or inflammation interferes (usually by blocking passage of fluid
from ventricles to subarachnoid space) the fluid
accumulates in the ventricles and causes increased pressure--hydrocephalus.
A. MEDULLA OBLONGATA--continuation
of spinal cord--lies just superior to the foramen magnum. Contains
all ascending and descending tracts that connect brain and spinal cord.
Most tracts cross from one side to the other in the medulla. The large somatic
motor tracts cross at the pyramids (2 bulges on the ventral medulla)--this
crossing is called the decussation of the pyramids.
Impulses originating in the right cerebrum cross to the left side of the body.
Right motor cortex à Pyramids à Left side of spinal cord à Left arm
On
the dorsal medulla 2 pairs of nuclei, the right and left nucleus
gracilis and
the right and left nucleus cunneatus receive sensory
fibers and relay the information to the thalamus on the opposite side by way of
a band of white matter, the medial lemniscus.
Left arm
Nucleus gracilis
or nucleus cunneatus
Right thalamus
Right sensory cortex
Also
located in the medulla:
Cardiovascular center--regulates rate and
force of heartbeat, diameter of blood vessels
Respiratory center (medullary
rhythmicity area)--rhythm of breathing
Vomiting center
Swallowing “
Coughing
“
Sneezing
“
Hiccuping “
Nuclei of origin for cranial nerves IX, X,
XII, part of VIII & XI
(What is the meaning of the term nucleus as used here?)
Nuclei involved in equilibrium &
posture---the most important of these, the olive, is involved in precise
voluntary movements, equlibrium and posture.
Damage
to the medulla can be fatal. Without these vital centers, life functions cannot
continue.
B. PONS—consists of tracts (bundles
of axons) and nuclei (clusters of nerve cell bodies)
Tracts connect various parts of the brain to
each other—one example is tracts that connect the right and left sides of the
cerebellum
Pontine nuclei relay signals for voluntary
muscle movements from cerebrum to cerebellum
Nuclei of origin for cranial nerves V, VI,
VII, part of VIII
Nuclei that help control respiration
C. MIDBRAIN (mesencephalon)—located above the pons,
the midbrain contains tracts that connect upper parts of the brain to the pons and medulla and to the spinal cord. It also contains
some important nuclei.
In the anterior part, cerebral peduncles carry motor impulses from cerebrum to lower brain stem and spinal cord. Some sensory fibers are also included.
Superior cerebellar
peduncles connect midbrain with cerebellum
(Peduncle=bundle of nerve fibers
connecting different parts of the CNS)
Dorsal portion, the tectum,
contains 4 elevations, the corpora quadrigemina,
which serve as reflex centers for movements of the eyes, head, and neck in
response to auditory and visual stimuli
Substantia nigra---nuclei that control subconscious
muscle activity. These neurons produce the neurotransmitter dopamine. If
this function is lost, the result is Parkinson disease.
Red nuclei--centers to coordinate muscular
movements
Nuclei of origin of cranial nerves III &
IV
The
brainstem also contains groups of neurons known as the reticular formation.
This consists of a netlike arrangement of small areas of gray matter connected
by threads of white matter, and extends into the spinal cord and the lower diencephalon as well as the brainstem. Its main function is
alerting the cerebral cortex to incoming sensory information. It seems to help
decide what we should consciously notice and what we should ignore. It is also
involved in regulation of muscle tone.
Part
of the reticular formation, the reticular activating system (RAS) consists of
fibers that connect directly to the cerebral cortex and allow various stimuli
to awaken us. The neurons of the RAS are active when we are awake; in fact,
they play
Second
largest portion of the brain--posterior to medulla and pons
and inferior to occipital lobes of the cerebrum. A deep groove, the transverse fissure, with an extension of the dura, the tentorium cerebelli, separate it from the cerebrum.
The
cerebellum is shaped like a butterfly with a central constricted vermis and lateral wings or cerebellar
hemispheres. Each hemisphere has 3 lobes:
1. Anterior—coordination of muscle
movements
2. Posterior—coordination of muscle
movements
3. Flocculonodular—equilibrium
& balance
The
surface (cerebellar cortex) consists of gray matter
in a series of slender parallel ridges called folia. Beneath it are tracts of
white matter (the arbor vitae). The cerebellum is attached to the brain stem by
cerebellar peduncles.
The
cerebellum is concerned with subconscious movements of skeletal muscles and
equilibrium. It helps smooth and coordinate muscle movements as well as
regulating posture and balance. It receives input from proprioceptors
(which let us know where our body parts are). While it does not directly send
motor impulses to skeletal muscles, it does alert areas that do this when
adjustment is necessary. It is involved in performing skilled, learned muscular
activities. Loss of coordination due to damage to the cerebellum is called
ataxia. The cerebellum also functions in learning and language.
A.
THALAMUS--makes up 80% of the diencephalon--oval
structure above the midbrain, right and left portions are joined by the
intermediate mass. It is the relay station for sensory impulses traveling
upward from the spinal cord and from other parts of the brain into the
cerebrum. It receives sensory impulses and relays them to appropriate regions
of the cerebrum, but the thalamus itself makes us aware of crude sensations,
poorly localized--pain, temp, pressure but no precise information. It also
plays a role in awareness and learning (cognition). It is connected to the
cerebrum by the internal capsule.
Each
side has 7 groups of nuclei. Some of these are:
Medial geniculate
nuclei--hearing
Lateral geniculate
nuclei--vision
Ventral posterior nuclei—relay somatic
sensations such as taste, touch, pressure, vibration, heat, cold, pain
Medial
nuclei—emotions, learning, memory
(Fig. 14.9 P. 488)
B. HYPOTHALAMUS—most
of the rest of diencephalon (20%)--located below the
thalamus partly within the sella turcica.
A stalk-like infundibulum extends from the lower part
and the pituitary gland is suspended from this. The hypothalamus contains
nuclei in 4 regions:
1. Mammillary
region---mammillary nuclei relay smell impulses
2. Tuberal
region---infundibulum, which connects the pituitary
gland to the hypothalamus, originates here. This region synthesizes hormones
that control the pituitary gland.
3. Supraoptic
region---fibers form a tract to the posterior pituitary gland
4. Preoptic
region---regulates many autonomic activities
(Fig. 14.10 P. 490)
The
hypothalamus controls many body activities and is one of the major regulators
of homeostasis.
1. Controls and integrates activities of the
autonomic nervous system—regulation of heart rate, movement of the GI tract and
contractions of the urinary bladder
2. Secretes hormones, controls the pituitary
gland, and serves as a link between the nervous and endocrine systems
3. Feeling of rage and aggression, pain and
pleasure (works together with the limbic system)
4. Regulates body temperature
5. Regulates food intake
6. Thirst center
7. Helps maintains
waking and sleep patterns
8. Helps regulate blood pressure
9. Regulates fluid balance
C.
EPITHALAMUS-small area superior and posterior to the
thalamus
1. Pineal gland—endocrine gland located
in the roof of the 3rd ventricle—secretes a hormone, melatonin
2. Habenular
nuclei—involved in smell
D.
SUBTHALAMUS—very small area just below the
thalamus—contains tracts and the subthalamic nuclei,
which help control body movements
The circumventriculr organs, in the walls of the third and
fourth ventricles, monitor chemical changes in the blood and contribute to
regulating blood pressure, fluid balance, thirst, etc. These organs are
considered to be part of the diencephalons.
Forms
the bulk of the brain. The surface is composed of a thin layer
of gray matter and is called the cerebral cortex (or just the cortex). Below is
the cerebral white matter. The cerebrum is the seat of intelligence, the
ability to read, write, speak, do math, etc. as well as memory, future plans,
creative work.
As
the brain develops the gray matter of the cerebrum enlarges more rapidly than
the white matter and forms rolls and folds upon itself.
Folds--gyri or
convolutions
Grooves--fissures if deep, sulci if shallow
Longitudinal
fissure--separates the cerebrum into right and left halves (hemispheres). A
large bundle of myelinated fibers connects the two
hemispheres (corpus callosum).
Each
cerebral hemisphere is further subdivided into 4 lobes which are named after
the bones that cover them:
Frontal
Parietal
Temporal
Occipital
Each
lobe has particular functions.
The
central sulcus separates the frontal and parietal lobes.
The precentral gyrus, just
anterior to the central sulcus, is the primary motor
area of the cortex. The postcentral gyrus, posterior to the central sulcus,
is a major sensory area of the cortex.
Cerebral
white matter consists of myelinated axons running in
3 directions:
1. Association fibers--transmit impulses
within the same hemisphere
2. Commissural fibers--one hemisphere to
corresponding area of the other hemisphere---corpus callosum
is an example
3. Projection fibers--ascending and
descending tracts to other parts of the brain or to the spinal cord---internal
capsule is an example
The
basal ganglia are groups of nuclei in the cerebral hemispheres. They are
concerned with automatic movements of skeletal muscles and muscle tone, as well
and starting and stopping movements. The red nuclei and the substantia
nigra of the midbrain are linked to the basal
ganglia. Basal ganglia are paired (left & right) and include:
1 Caudate nucleus
2. Putamen
3. Globus pallidus
This
involves parts of the brain that operate on both the conscious and the
subconscious level. It consists of a ring of structures including regions of
the cerebrum and the diencephalon. (Fig. 14.14 P. 495) Myelinated axons that link these parts.
The
limbic system is sometimes called the emotional brain because it functions in
pain, pleasure, rage, affection, sexual feelings, fear and sorrow. By causing
pleasant or unpleasant feelings about experiences, the limbic system apparently
guides the individual into behavior that is likely to increase chance of
survival. It is also involved in learning and memory. One part, the
hippocampus, is essential for normal memory (although we don’t know exactly
what it does). It has links to the
senses of touch, smell, vision, hearing and taste, so these senses can awaken
complex memories. The reticular formation is also part of the limbic system.
FUNCTIONAL
ORGANIZATION OF THE CORTEX
SENSORY
AREAS---receive and interpret sensory impulses. Primary somatosensory area in the parietal lobes, just posterior to
the central sulcus--receives sensory impulses for
touch, proprioception, pain and temperature. Each
point in the area receives sensations from a specific body area.
Special senses:
Primary visual area (occipital
lobes)--sight
Primary auditory area
(temporal lobes)--hearing
Primary gustatory area
(parietal lobes)--taste
Primary olfactory area
(temporal lobes)--smell
MOTOR
AREAS---control motor movements---specific groups of neurons
control specific muscles
1. Primary motor area (frontal lobes)--many
of these fibers cross at the pyramids
2. Motor speech area (Broca’s area)—translating speech or written words into thoughts involves both sensory and association areas. Producing speech is the responsibility of Broca’s area, which will be located in one frontal lobe, usually the left. Aphasia is the inability to use or comprehend words. This may occur due to damage to Broca's area.
ASSOCIATION
AREAS---responsible for memory, emotions, reasoning,
personality, intelligence, etc. Several different
association areas may combine their specialties to come up with one response.
Figure 14.15 P. 497
|
LOBE |
MOTOR MOVEMENTS |
SENSORY |
ASSOCIATION |
|
FRONTAL |
MOVEMENT OF VOLUNTARY SKELETAL MUSCLES |
SMELL (PART) |
HIGHER INTELLECTUAL PROCESSES, PERSONALITY |
|
PARIETAL |
|
GENERAL SENSATIONS SUCH AS TOUCH AND PAIN,
TASTE |
UNDERSTANDING SPEECH AND USING WORDS,
INTERPRETATION OF SENSORY INPUT |
|
TEMPORAL |
|
HEARING AND SMELLING, MOST IMPORTANT AREA
FOR SMELL |
INTERPRETATION OF SENSORY EXPERIENCES AND MEMORY
OF VISUAL SCENES AND MUSIC |
|
OCCIPITAL |