2. the brain and spinal cord are
covered by three connective tissue layers called the meninges:
a. Outer most is the dura mater (tough mother) - dense, irregular
connective tissue with many blood vessels and nerves. It is attached to the
inside of the cranium and forms the periosteum inside the cranial cavity.
b. The dura mater extends inward between the lobes of the brain, and forms
supportive and protective
partitions.
1. an extension of the dura mater called the falx cerebri (CER-e-bree)separates the two halves of the cerebrum.
2. a similar dural extension separates the halves of the cerebellum -falx cerebelli. (cer-e-BEL-eye)
3. another dural extension separates the cerebellum from the cerebrum and is
called the tentorium cerebelli.
c. In other areas, the dura mater splits into two layers, forming channels
called dural sinuses . Venous blood flows through these channels as it
returns from the brain to the heart.
d. The dura continues down around the spinal cord from foramen magnum, where it
is continuous with the dura mater of the brain, down to level of the second
sacral vertebra where it is closed ended, forming a sac.
e. The dura mater around the spinal cord is not attached to the vertebrae, but
is separated from it by the epidural space. There is also a cushion of
fat and connective tissue in the epidural space, as well as blood
vessels.
3. The middle meninx (MEE-ninks) is
the arachnoid (mater) - named because of the delicate spider web-like
collagen and elastic fibers found in this layer.
a. between the dura mater and the arachnoid is the subdural space -
filled with interstitial fluid. (Can also become filled with blood in cases of
head trauma - compressing soft tissue underneath.)
b. The arachnoid is a thin membrane, which lacks blood vessels. It spreads over
the brain, but does not go into the grooves and depressions of the brain and
spinal cord.
c. Many thin strands extend from the underside of the arachnoid and attach to
the underlying pia mater.
d. between arachnoid and pia mater is the subarachnoid space - filled
with CSF.
4. The innermost is the pia mater (pious mother -
meant tender mother) thin, nearly transparent layer, made of interlacing bundles
of collagen fibers and some fine elastic fibers. Also has many nerves and blood
vessels that supply the brain and spinal cord.
a. The pia
mater is attached to the brain and spinal cord, and follows their contours.
b. Triangular
thickenings of the pia mater, called denticulate ligaments, help to
suspend the spinal cord within the dura mater. These ligaments are found between
the ventral and dorsal roots of the spinal nerves on both sides of the spinal
cord. They protect against shock and sudden displacement.
5. Inflammation of the meninges is called meningitis.
Cerebrospinal Fluid
The brain and spinal cord are protected and nourished by the cerebrospinal fluid (CSF).
1. The CSF contributes to homeostasis in three main ways:
a. mechanical protection - it
"floats" the brain in the cranial cavity, and acts as a shock absorber
against bumps and blows.
b. chemical protection - the
CSF provides an optimal chemical environment for the transmission of impulses in
the brain and spinal cord. It contains glucose (for energy), proteins, lactic
acid, urea, cations (Na+, K+, Ca++, Mg++)
and anions (Cl- and HCO3-) and some white blood cells.
(Basically a filtrate of blood.)
c. circulation - it brings
nutrients from the blood and removes waste products from the nervous system,
which it returns to the blood.
2. It is a circulating fluid- flows between the arachnoid and pia mater in the
subarachnoid space. Cerebrospinal fluid is formed in the choroid plexuses of
the brain. The choroid plexuses are networks of capillaries in the walls of
the ventricles in the brain The capillaries of the choroid plexuses are covered
by ependymal cells that form the CSF by filtration and secretion. (
facilitated diffusion and active transport) Because the ependymal cells are
joined by tight junctions, the only way for substances to reach the CSF from the
blood is through the ependymal cells. Thus they form a blood-cerebrospinal
fluid barrier that allows some substances through, but not others. This
protects the brain and spinal cord from harmful substances in the blood, but
also makes it more difficult to get therapeutic drugs into the CSF, say to treat
for meningitis.
There are four ventricles: two laterals ventricles located in the
center of the hemispheres of the cerebrum, the third ventricle is located
above the hypothalamus and between the halves of the thalamus, the fourth
ventricle lies between the brain stem and the cerebellum.
CSF formed in the lateral ventricles flows into the third
ventricle through the interventricular foramina . More CSF is added by
the choroid plexus in the roof of the third ventricle, and the leaves through
the cerebral aqueduct which passes through the midbrain. The fourth
ventricle adds more CSF, and then it enters the subarachnoid space by three
openings:
a. a median aperture
b. two lateral apertures
It then circulates through central canal of the spinal cord, and in the
subarachnoid space around the brain and spinal cord.
3. The CSF is reabsorbed into the blood through the arachnoid villi,(granulations)
which are fingerlike projections from the subarachnoid space into the dural
venous sinuses, especially the superior sagittal sinus.
4. The CSF is usually absorbed at the same rate it is produced, about 20 ml/ hour, so the pressure of the CSF remains constant.
5. What would happen if the CSF was produced more rapidly, or didn’t drain?
What would a blockage do? Hydrocephalus or "water on the brain" can
be caused developmental abnormalities, infection, injury or brain tumors. in
infant with fontanels, the head bulges.
In adults, whose fontanels are closed, it causes vomiting,
headache, optic atrophy, and mental disturbances. Pseudotumor cerebri, called
benign intercranial hypertension, produces all the signs of a brain tumor, even
though no tumor is present. It can be relieved by removing some of the CSF by
spinal tap, or lumbar puncture.
B. External Anatomy of the Spinal Cord
The spinal cord begins as a continuation of the medulla
oblongata and ends at about the second lumbar vertebra in an adult. In an
infant, it extends to the third or fourth lumbar vertebra. (both grow, then
spinal cord stops, and vertebral column continues to lengthen).
Although the spinal cord is not segmented, it appears
so due to the 31 pairs of spinal nerves that arise from it. These nerves branch
to various areas of the body, and connect the body parts with the CNS.
The spinal
cord contains two enlargements:
a. cervical enlargement C4 to T1 - nerves to and from the upper limbs are
found in this region.
b lumbar enlargement - T9-T12 - nerves to and from the lower limbs are
found in this region.
c. Inferior to the lumbar enlargement, the spinal cord tapers to a conical
portion called the conus medullaris which ends at the intervertebral disc
between L1 and L2.
d. Coming off of the conus medullaris is the filum terminale (terminal
filament), an extension of the pia mater that extends to anchor the spinal cord
to the coccyx.
e. After the conus medullaris the spinal cord divides into the nerves that will
leave the vertebral column at lower levels. Its appearance gives it the name cauda
equina or horse’s tail. Spinal taps are normally performed between L3 and
L4 or between L4 and L5 (to withdrawn CSF, administer anesthetics, antibiotics,
chemotherapy, contrast media for myelography.) Why?
f. Two grooves divide the spinal cord
into right and left sides:
1. the anterior median fissure - deep, wide groove on front
2. posterior median sulcus - shallow, narrow groove on back
g. Spinal nerves allow the brain and
the body to communicate. The areas where the spinal nerves attach to the spinal
cord are called roots.
1. Posterior or dorsal (sensory) roots contain sensory fibers from
the periphery of the body. Each root has a swelling called a posterior or
dorsal (sensory) root ganglion which contains the cell bodies of
these sensory neurons .
2. Anterior or ventral (motor) roots contain motor neuron axons
and conduct impulses from the brain and spinal cord to the periphery.
C. Internal Anatomy of the Spinal Cord
The gray matter in the spinal
cord forms the letter H in the center of the cord. (Cell bodies, neuroglia,
unmyelinated axons and dendrites of internerurons and motor neurons.)
a. The
crossbar of the H is called the gray commissure (2. a connecting bundle
of nerve fibers, esp. one joining the right and left sides of the brain.)
b. In the center of the commissure is a hole called the central canal - continuous with the fourth ventricle of the brain, and running the entire length of the spinal cord.
c. Anterior to the gray commissure is the white commissure which connects the white matter on the right and left sides of the spinal cord.
d. Gray matter in the spinal cord is divided into horns:
1. anterior (ventral) gray horns - cell bodies of motor neurons to skeletal muscle.
2. posterior (dorsal) gray horns
3. lateral gray horns - present only in the thoracic, upper lumbar, and sacral segments of the cord. Cell bodies of motor neurons to cardiac, smooth muscle, or glands. (part of autnomic nervous system.)
e. The white matter (myelinated nerve fibers) is divided into columns (funiculi)
1. anterior (ventral) white columns
2. posterior (dorsal) white columns
3. lateral white columns
f. Each column is divided into
bundles called nerve tracts or fasciculi. Each tract has a
common origin or destination, and carry similar information.
Ascending (sensory) tracts
- impulses toward brain
Descending (motor) tracts -
impulses from the brain
II. SPINAL CORD PHYSIOLOGY
Spinal cord has two functions:
The white matter tracts serve as
information highways to and from the brain.
The grey matter receives and
integrates incoming and outgoing information, especially for spinal reflexes.
A. Sensory and Motor Tracts
Names of tracts indicate:
position in
the cord
where it
begins and ends
and the
direction of impulses
(anterior spinothalamic tract - in anterior column, begins in the spinal cord, ends in the thalamus - it is acending, and therefore sensory.)
REFLEXES
The second
major function of the spinal cord is to serve as an integrating center for spinal
reflexes.
Reflexes are fast,
predictable, automatic, subconscious responses to changes inside or
outside the body. Somatic reflexes involve contraction of skeletal
muscle. We also have autonomic reflexes which involve responses of
smooth muscle, cardiac muscle, and glands. (heart rate, respiration,
digestion, urination)
Reflex Arc and Homeostasis
A pathway is the route followed by a series of
nerve impulses from their origin in the dendrites or cell body of a
neuron in one part of the body to destination in another. A reflex arc
is the simplest type of pathway.
Its functional components are:
1. receptor - this is the distal end
of a sensory neuron or an associated sensory structure
which serves as a
receptor. It responds to a specific stimulus by producing a graded
potential called a generator
( or receptor) potential. If this generator potential reaches
the threshold of the
neuron, it will trigger one or more impulses.
2. sensory neuron - impulses travel from
the receptor to the axon terminals of the
sensory neuron, which are
located in the grey matter of the spinal cord or brain stem.
3. integrating center - This may be
as simple as the synapse of the sensory nerve with
a motor nerve
in the gray matter of the spinal cord. This is called a monosynaptic reflex
arc.
Usually, the integrating center consists of one or more association neurons,
which
may relay the
impulse to other association neurons as well as to the motor neuron. A
polysnaptic
reflex arc involves more than two types of neurons and more than one
CNS synapse.
4. motor neuron - The integrating center
sends impulses out to effector organs through
the motor neuron.
5. effector -this is the part of the body
which responds to the motor nerve, either muscle
or gland. The action of the
effector is called the reflex. It is a somatic reflex if
skeletal
muscle is innervated, and is an
autonomic reflex if smooth, cardiac muscle or a gland is
innervated.
We can use reflexes to test certain nerve pathways for
damage. Tapping patellar ligament activates stretch receptors in the quadriceps
femoris which synapse in the lumbar spinal cord with motor neurons which cause
contraction of the quadriceps and extention of the leg at the knee.
Absence of this reflex indicates damage to the sensory or motor neurons in the
lumbar region of the spinal cord.
Somatic reflexes can be fairly easily tested by tapping
or stroking the body surface, but most autonomic reflexes are difficult to
examine. The exception is the pupillary light reflex. Synapses in the reflex
pathway include synapses in the midbrain and brainstem, and absence of the light
reflex can indicate damage to these areas.
Physiology of the Knee-jerk ( patellar tendon or Stretch reflex):
Stretch reflex is a monosynaptic reflex. One sensory
neuron, one motor neuron and one CNS synapse. Can be tested at the elbow, wrist,
knee and ankle joints.
1. Stretch of tendon and muscle stimulate
receptors called muscle spindles which
monitor changes in the length of the
muscle.
2. The stretched muscle spindle generates one or more
impulses along a somatic sensory
nerve which enters the spinal
cord through the posterior root of the spinal nerve.
3. In the spinal cord, the sensory neuron
synapses with and activates a motor neuron in
the anterior grey horn.
4. If the excitation is great enough, the motor neuron
fires one or more times, and these
impulses travel along the axon, out
through the anterior root of the spinal nerve. This motor
nerve then synapses with muscle
fibers of the same muscle that has the activated muscle
spindle.
5. ACh is release at NMJ, and causes one or
more action potentials in the muscle, and the
muscle contracts. The contraction of
the muscle relieves the stretching of that muscle.
6. In this reflex arc, the sensory nerves enter
the spinal cord on the same side that the
motor neurons leave it. This is
called an ipsilateral reflex arc. All monosynaptic reflexes
are ipsilateral.
7. Even though this reflex is monosynaptic,
a polysnaptic reflex arc to the antagonistic
muscle occurs at the same time. An
axon collateral from the muscle spindle sensory
neuron also synapses with an
inhibitory association neuron in the integrating center.
The association neuron synapses with
and inhibits a motor neuron tha normally excites
the antagonistic muscles.
This type of circuit which simutaneously causes
contraction of one muscle and inhibition of the antagonistic muscles
is called reciprocal innervation. This coordinates body movements
and prevents conflicts.
8. Axon collaterals from the muscle spindle
sensory neuron also synapse with other cells
which relay impulses to the brain, so
the brain can use the information about the state
of this muscle to coordinate other
muscle movements and posture. The nerve impulses
to the brain also go to centers which
allow you to perceive that the reflex has occurred.
Physiology of the Flexor (Withdrawal) Reflex and Crossed Extensor Reflex:
The flexor or withdrawal
reflex is a polysynaptic reflex.
1. Stepping on a tack stimulate the dendrites (free
nerve endings) of a pain sensing neuron.
2. The sensory neuron fires, and impulses enter the
spinal cord.
3. In the spinal cord the sensory neuron activates
association neurons that extend to
several spinal cord segments.
4. The association neurons activate motor neurons in
several spinal cord segments, which
in turn generate impulses which leave
the spinal cord through the anterior roots and go
to the muscles.
5. ACh released at NMJ's causes the flexor
muscles in the thigh to contract, withdrawing
the leg from the source of the
pain.
6. This reflex is also ipsilateral. In this reflex
about 6 different muscles are involved, and
several motor neurons must
simulateously convey impulses to several muscles. The
nerve impulses from one sensory
neuron accend and descend in the spinal cord and
activate association neurons in
different segments of the spinal cord, this type of reflex
is called an intersegmental reflex
arc.(also exhibits reciprocal innervation)
When you step on the tack and withdraw your foot, you may also start to lose
your balance. The pain impulses from the tack also initiate a
balance-maintaining crossed extensor reflex.
1. Stimulation of pain sensitive neuron in right foot.
2. Sensory neuron sends impulses into the spinal cord
(dorsal root)
3. In the spinal cord, the sensory neuron activates
several association neurons that
synapse with motor neurons on
the left side of the spinal cord in several spinal cord segments.
(Signals cross to the
opposite side thru assoc. neurons )
4. The association neurons excite the motor neurons in
the several segments, and the motor
neurons send impulses to their axon
terminals.
5. ACh release causes the extensor muscles
in the thigh of the left leg to contract, extending
the left leg. Now the left leg is
ready to support the weight of the body as you lift your
right foot. A similar
reflex occurs in either upper limb.
6. (also exhibits reciprocal innervation)
7. The crossed-extensor reflex involves a contralateral
reflex arc.
Among
clinically important somatic reflexes are the patellar reflex, the Achilles
reflex, the Babinski sign, and the abdominal reflex. Read over, but don't
memorize.
Brain Development:
The development of the nervous system begins in the third week with a
thickening of the ectoderm, called the neural plate. - forms a groove with
raised edges which grow and meet to form a neural tube.
The brain begins to form in the embryo as the neural tube, which becomes
the central nervous system.
Two neural tube defects are associated with low levels of a B vitamin called folic acid. These are - spina bifida ( failure of laminae of spine to unite) and anencephaly (absence of the skull and cerebral hemispheres) The incidence of both disorders is greatly decreased when women who may become pregnant take folic acid supplements.
BRAIN
A. Principal Parts
The four
principal parts of the brain are:
1. The brain stem is continuous with the spinal cord, and consists of:
a. the medulla oblongata
b. pons
c. midbrain
2. The cerebellum - "little brain"
3. The diencephalon - superior to the brain stem
a. thalamus
b. hypothalamus
(c. Pineal gland)
4. Cerebrum- largest section
a. right and left halves - cerebral hemispheres
Cerebrum The cerebrum is the
largest part of the brain. Its cortex, or outer layer, is gray and
contains billions and billions of neurons.It contains the cell bodies of about
75% of all the neurons in the nervous system. During embryonic
development, the cortex develops faster than the white matter tracts, and folds
in upon itself, forming gyri (convolutions), sulci, and fissures.
The cerebrum is made of two large portions called the cerebral hemispheres which
are mirror images of each other, structurally and functionally.
Between the two hemispheres is the longitudinal fissure
In the longitudinal fissure is the falx cerebri, an extension of the dura
mater that encloses the superior and inferior sagittal sinuses.
The two hemispheres are connected by a thick band
of white matter called the corpus callosum.
The cerebral
lobes are named after the bones that cover them :the frontal, parietal,
temporal, and occipital lobes. Two major sulci or fissures divide
the lobes: the lateral fissure separates the frontal and parietals
lobe from the temporal lobe. Studies of Einstein's brain show that his
inferior parietal lobe was 15% larger than normal, and the lateral fissure
was largely absent. This area processes mathematical thought, three
dimensional visualization, spatial relationships and other mental processes.
They feel that this was a anomaly that was present from birth, and may have
allowed more neurons to establish more connections and work together more
easily.
The central sulcus separates the frontal
and parietal lobes. The gyri on either side of the central sulcus are
important. The precentral gyrus contains the primary motor area of
the cerebral cortex. The postcentral gyrus contains the somatosensory
area.(more later)
There is no distinct boundary between the occipital and
parietal lobes.
The transverse fissure separates the cerebrum and the
cerebellum. Extension of dura materis found here called the tentorium cerebelli.
The insula lies deep to the parietal, frontal
and temporal lobes., and is separated from them by the circular sulcus.
The white matter is under the cortex and
makes up the bulk of the cerebrum. It consists of mostly myelinated axons
running in three principal directions:
1. between gyri in the same hemisphere.
2. from gyri in one hemisphere to the
corresponding gyri in the opposite hemisphere of the
cerebrum. The primary commissural
fibers are the corpus callosum (Also the anterior and
posterior commissures.)
3. Projection fibers form descending and
ascending tracts that transmit impulses from the
cerebrum and other parts of the brain
to the spinal cord or from the spinal cord to the brain.
Functions of the cerebrum:
Functional Areas of the Cerebral Cortex
Specific types of signals are processed in certain
regions of the cerebrum. The sensory areas of the cerebral cortex are concerned
with the interpretation of sensory impulses. The motor areas are the
regions that govern muscular movement. The association areas are concerned
with more complex integrative functions, such as memory, emotions, reasoning,
will, judgement, personality traits, and intelligence.
The primary motor areas of the cerebral cortex are
found in the frontal lobes, just in front of the central sulcus in the
precentral gyrus and anterior wall of the sulcus. Each region controls voluntary
contractions of specific muscles or groups of muscles on the opposite side of
the body. Areas are proportional to the number of motor units in a
particular muscle of the body.
In addition to the primary motor areas, certain
other regions of the frontal lobe control motor functions. Broca's area (ant. to
prim. Motor cortex and sup. to the lat. fissure.) coordinates muscle movements
which make speech possible.
Frontal eye field - controls voluntary movements of the
eyes and eyelids
Other areas - coordinate movements of head that direct
the eyes
Controls hands and fingers and makes writing possible.
Sensory input flows mainly to the posterior half of
the cerebral hemispheres, posterior to the central sulcus. The primary sensory
areas have the most direct connection with the peripheral sensory
receptors. Secondary sensory areas and sensory association areas often are
adjacent to the primary areas and receive input from the primary areas and from
other regions of the brain.
Primary somatosensory area - postcentral gyrus.
This area receives impulses for touch, proprioception (where body parts
are) pain, and temperature. Certain spots within this region receive
input from specific parts of the body. (picture) The area given to each part
depends not on the size of the body part, but by the number of sensory receptors
in that body part. The major function of this area is to localize the points on
the body where these sensations originate.
The posterior parts of the occipital lobe are
responsible for vision; the posterior dorsal parts of the temporal lobe contain
the centers for hearing. Smell- deep with in the cerebrum (uncus). Taste- near
the base of the central sulcus, along the lateral sulcus.
Most sensory fibers cross over in the brain stem and
spinal cord, however sensory areas concerned with sight and hearing receive
information from both sides (why is this important?)
Association Areas- regions of the cerebral
cortex that are not primary motor or sensory in function, which interconnect
with other areas of the brain. The secondary and association areas interpret the
sensory input into meaningful patterns of recognition and awareness. Person with
damage to primary visual cortex has a scotoma. Damage to visual association area
may see normally, but be unable to interpret or recognize what he sees (a
friend's face).
These areas also help provide memory, reasoning,
verbalization, judgement, and emotions.(Don't memorize all of the assoc. areas.)
Hemisphere dominance:
In over 90 % of the human population, the left
hemisphere is dominant for the spoken and written language.(Controls the right
side of the body). This side of the brain also controls numerical and scientific
skills, and reasoning . The right side of the brain controls the muscles
on the left side of the body, musical and artistic awareness, space and pattern
perception, insight, imagination, and generates mental images to compare spatial
relationships.
In left handed people, the right side of the brain would be dominant, and
in some individuals they are equally dominant.
Learning and memory
Learning is the ability to acquire new knowledge
or skills through instruction or experience.
Memory is the ability to store and recall
thoughts and involves persistent changes in the brain, a capability called
plasticity. This ability to change involves changes in individual neurons,
which may make new proteins or form new dendritic connections ( new research
indicates that dendrites can and do rapidly form new synapses and break old
ones). There may also be changes in the strengths of synaptic connections.
Portions of the brain associated with memory are the frontal, temporal, parietal
and occipital lobes, the limbic system, and the diencephalon.
Memory is generally classified into two kinds:
short-term and long-term memory.
Short-term memory is the temporary ability to recall a
few pieces of information. If the information has no particular significance, or
if it is not repeated and reinforced, it is forgotten. Short-term memory
is related to electrical events, and lasts as long as the stimulation continues.
It may be caused by reverberating circuits.
Long-term memory lasts from days to years.
The reinforcement due to the frequent retrieval of a piece of information is
called memory consolidation. Long term memory is related to anatomical
and biochemical changes at synapses.
When neurons are kept active they show an increase in
the number and size of synaptic end bulbs, and an increase in the
branching of the dendrites. There may be loss of neurons or their abilities if
they go unused.
The basal ganglia are paired masses of gray
matter (nuclei) in the cerebral hemispheres.
These are the caudate nucleus ,putamen and globus
pallidus.
The basal ganglia are interconnected by many nerve
fibers an d receive input from and provide output to the cerebral cortex,
thalamus and hypothalamus ( to spinal cord). They produce most of the inhibitory
neurotransmitter dopamine. They help to control automatic muscular movements
and muscle tone. These degenerate in Parkinson's disease.
The diencephalon is located between the
cerebral hemispheres and above the brain stem. It surrounds the third ventricle,
and is mostly gray matter. Here we find the dumb-bell shaped thalamus, and the
hypothalamus which is made up of many small nuclei. Here we also find the optic
tracts and optic chiasm, the infundibulum, the posterior pituitary, mammillary
bodies and the pineal gland. The mamillary bodies serve as relay stations for
the sense of smell. Infundibulum attaches pituitary gland and makes regulating
hormones for the anterior pituitary.
The pineal gland, named for its resemblance to a
pine cone) lies on the midline in the third ventricle. This structure is usually
easy to find on the brain when you are dissecting, just remember it lies
dorsally, and don't get it confused with the pituitary gland. This gland is part
of the endocrine system, and is sometimes referred to as the "third
eye" because it responds to light. It secretes the hormone melatonin,
and more of this hormone is released during darkness. It is thought to promote
sleepiness. The pineal gland also seems to contribute to the body‘s biological
clock. In some animals it controls seasonal cycles, such as breeding.
The thalamus makes up about 80 % of the
diencephalon. It is the principle relay station for sensory impulses to the
cerebral cortex. These sensations include hearing, vision, taste and the somatic
sensations of touch, pressure, vibration, heat, cold and pain. It also plays a
role in certain emotions and memory. As shown by the case of Karen Ann Quilan,
the thalamus plays an important role in awareness, as well as the acquisition of
knowledge, or congition.
The hypothalamus is one of the major regulators
of homeostasis. Nerve fibers connect the hypothalamus with the cerebral cortex,
thalamus and parts of the brain stem.
Its chief functions are:
1. Control of the Autonomic nervous system.
(Heart rate and arterial blood pressure, and control of movements and glandular
secretions of the stomach and intestines) It controls and integrates the
activities of the ANS: heart rate, contraction of smooth muscle in the G.I tract
and elsewhere, and the secretion of glands.
2. Control of the pituitary gland. The pituitary
gland is often referred to as the"master gland" because it controls so
many functions of the body : it regulates hormones which affect the gonads, the
basal metabolic rate, the adrenal secretions, and lactation. But all of
the hormones released by the anterior pituitary are regulated by releasing and
inhibiting factors produced by the hypothalamus. These factors are carried to
the anterior pituitary by their own blood vessel network. The posterior
pituitary is actually an extension of the hypothalamus, and the hormones
released by the post. pit. are manufactured in the hypothalamus. Oxytocin and
antidiuretic hormone ( water and electrolyte balance)
3. Regulation of eating and drinking. The
feeding (hunger) center sends you feelings of hunger. When you have eaten
enough, the satiety center sends impulses that inhibit the hunger center.
The thirst center responds to increased osmotic pressure in the blood but
producing the sensation of thirst. Increased water intake reduces the
osmolarity of the blood.
4. Control of body temperature - the
hypothalamus measures the temperature of the blood flowing through it, and if it
is too high or too low, it instructs the ANS to bring about changes to fix it.
(What would these changes be?)
5. Regulation of diurnal (daily) rhythms and states
of consciousness.
It other words, it establishes patterns of sleep that occur on a daily
basis.
6. Regulation of emotional and behavioral
patterns. Together with the limbic system, the hypothalamus regulates
feelings of rage, aggression, pain, and pleasure, and the behavioral patterns of
sexual arousal.
The
limbic system is found in the cerebral hemispheres and diencephalon. Some of
the visible parts are the mammary bodies of the diencephalon and the
olfactory bulbs.
The limbic system governs emotional aspects of behavior and also in
memory. Because this system controls emotions and memory, events
which produce a strong emotional response are remembered better that
those that do not. (worm in your dinner?) Damage to the limbic system can
result in the loss of short-term memory and the ability to learn.
Stimulation of different parts of the limbic system in animals can produce
behaviors associated with rage, pain, pleasure, fear, anger, affection and
sexual feelings. The limbic system is sometimes called the "emotional
brain."
Brain Stem
The brain stem connects the spinal cord to the diencephalon, and is made
up by the medulla oblongata, pons, and midbrain.
1.The midbrain or mesencephalon, runs from the
pons to the diencephalon, and contains the cerebral aqueduct that connects
the third and fourth ventricles. Area for passage of fibers between areas of the
brain, and for coordination of motor activity.
a. The cerebral
peduncles, located on the anterior surface of the midbrain, contain tracts
which
carry some of the motor fibers (corticospinal tracts) from the cerebral cortex
to
the pons,
medulla, and spinal cord. These tracts also have sensory fibers going from the
medulla to
the thalamus.
b. The posterior portion of the midbrain is called the tectum (roof), and
has four
round elevations called the corpora quadrigemina. The superior
colliculi are
reflex
centers for movements of the eyes, head and neck in response to
visual and
other
stimuli. The inferior colliculi are reflex centers for movements of
the head
and
trunk in response to sounds.
c. The right and left red
nuclei are so named because of their rich blood supply and an
iron containing pigment in their cell bodies. Fibers from the cerebrum and
the
cerebellum
synapse here, and this area functions to help coordinate muscle movements
and provides
reflexes that maintain posture.
The pons ("bridge") is superior to the medulla.
It connects the spinal cord with the brain and links parts of the brain with one
another by way of tracts.
There are several nuclei in the pons. Several of these nuclei are the
origins of cranial nerves.
Other nuclei along with the medullary rhythmicity area, control
breathing.
The medulla oblongata
1. The medulla is continuous with the upper part
of the spinal cord. It begins at the foramen magnum, and extends upward to
the pons. In the medulla are all the ascending (sensory) and descending (motor)
tracts that connect the brain to the spinal cord.
a. Most sensory and
motor tracts cross over from one side of the body to the other
as they pass
through the medulla. On the anterior side of the medullar are two
bulges
called the pyramids. These contain the largest motor tracts which
travel from
the cerebrum
to the spinal cord. Just above where the spinal cord joins the medulla,
these tracts
cross over, right to left and left to right. This is called the decussation
of
pyramids.
(Which side of the brain controls the left side of the body?)
It also contains nuclei that regulate some vital body functions:
The cardiac center
regulates the heart rate and force of contraction of the heart.
The vasomotor center
sends impulses to the smooth muscle in the walls of the blood vessels, causing
them to constrict and raise blood pressure. A decrease in the activity of these
cells allows the smooth muscle to relax and the vessels dilate.
The respiratory center
(The medullary rhythmicity area) which controls the rate, rhythm and depth of
breathing.
Other centers
coordinate swallowing, vomiting and some modified
respiratory movements, such as coughing, sneezing,
vomiting and hiccuping
The reticular formation is a net-like formation of white and gray
matter that extends through a large portion of the brain stem, running
from the spinal cord up into the diencephalon. It has both sensory and motor
functions. It receives input from higher brain centers that control skeletal
muscle, and contributes to regulating muscle tone. It also alerts the cerebral
cortex to incoming sensory signals. This part of the reticular formation
is called the reticular activating system (RAS) and is responsible for
maintaining consciousness and awakening from sleep. Incoming signals from the
ears, eyes and skin stimulate the RAS.( RAS wakes up the cerebral cortex)
Cerebellum
The
cerebellum is the second largest part of the brain, and is located interiorly
and posteriorly.
It is separated from the cerebrum by the transverse fissure and by
an extension of the dura mater called the tentorium cerebelli. It consists of
two hemispheres and a central, constricted area called the vermis ("worm").
The falx cerebelli separates the two hemispheres, and each hemisphere is divided
into lobes by deep fissures.
a. The cerebellar cortex is made of gray matter in as series of
ridges called folia.
b. The white matter tracts in the cerebellum are called the arbor
vitae ("tree of life")
c. Cerebellar peduncles connect the cerebellum to other parts of
the brain..
d. The cerebellum compares intended movement programmed by the cerebrum,
with what is actually happening. It receives input from proprioceptors
(senses where body parts are), receptors for equilibrium, and visual
receptors through the inferior peduncles. The middle peduncle sends information
from the cerebral cortex about the desired position of theses body parts.
The cerebellum then compares the two sources. If the intent of the
cerebrum is not being met, the cerebellum detects the variations, and from
the dentate nucleus and out through the superior peduncles sends
correcting impulses to the midbrain. The corrections are incorporated into
the motor impulses traveling through the brain stem. This produces smooth,
coordinated muscle movements. The cerebellum also coordinates posture and
balance.
Most of what we know about what the cerebellum does comes
from people who have damage to that region. Patients with cerebellar damage walk
with the feet wide apart, in part due to loss of equilibrium., they show
inaccurate movements of voluntary muscles and "intention tremors."
They also show a loss of muscle tone.
Peripheral nerves:
Consists of nerves that branch from the central nervous
system, and connect the CNS to other parts of the body. The PNS includes the 31
pairs of spinal nerves and the cranial nerves that arise from the brain. We can
have nerves that are sensory nerves and motor nerves, but most are mixed
nerves.
Composition and Coverings
Each cranial and spinal nerve is surrounded by connective tissue
coverings.
Individual neurons, myelinated or unmyelinated, are
wrapped in endoneurium.
Groups of axons are arranged in bundles called fascicles,
and each fascicle is wrapped in perineurium.
The fascicle which form the nerve are all wrapped
together in epineurium.
The dura mater of the spinal meniges fuses with the
epineurium as the nerve passes through the intervertebral foramen.
Note the many blood vessels in the coverings.
Dermatomes
Each spinal nerve contains both sensory and motor
neurons, and serves a specific, constant area of the body. Most of the skin of
the face and scalp is served by cranial nerve V, the trigeminal nerve.
The area of skin that provides sensory input to one pair of spinal nerves or to
cranial nerve V is called a dermatome. (Shingles - herpes zoster - dorsal
root ganglion) The nerve supply in neighboring dermatomes over laps somewhat, in
some cases considerably. This overlap provides a backup system in case one of
the nerves supplying a dermatome is damaged.
Skeletal muscles receive their innervation from the
motor neurons in a single spinal segment called a myotome. Myotomes
roughly underlie the corresponding dermatomes.
Dermatomes and myotomes help physcians locate areas of nerve damage.
Because of the overlap, if a doctor wishes to achieve complete anaesthesia of a
region, at least three adjacent spinal nerves must be cut or blocked by drugs.
Autonomic Nervous system:
Is divided into sympathetic and parasympathetic nervous
systems.
All fibers are efferent or motor fibers and consist of
two nerves: a preganglionic fiber, whose cell body is in the CNS and a
postganglionic fiber, whose cell body is in an autonomic ganglion outside the
CNS. In the sympathetic NS the preganglionic fibers arise from the lateral
horns. Also associated with the symp. N.S. is the medulla of the adrenal
glands, which releases epinephrine and norepinephrine.
The Autonomic vs. The Somatic Nervous System
Somatic Nervous System The Autonomic Nervous System
Both Sensory and motor neurons Motor neurons
Input from the special senses, proprioreceptors,
General visceral sensory neurons
and general somatic receptors
Operates with conscious control Usually without conscious control
Effect of motor neurons is excitation Effects can be excitatory or inhibitory
Single motor neuron Two motor neurons in series
Releases Acetylcholine at synapses
Releases acetylcholine or
norepinephrine at synapses
Sympathetic vs Parasympathetic nervous systems:
Sympathetic N.S.
Parasympathetic N. S.
Originates in the thoracic and lumbar segements
Originates in the brain stem and sacral
of the spinal cord : Thoracolumbar system
segments: Craniosacral system
Synapses near the spinal cord Synapses near visceral effector
Preganglionic neuron synapses with neurons to
Preganglionic neuron synapses with
several organs
neurons to a single organ
Effect is wide spread Effect is local
"Fight-or-Flight" - "E's":
"Feed and Breed" - "SLUD"
emergency, excitement, exercise, embarrassment
salivation, lacrimation, urination,
defecation
Postganglionic fibers release norepinephrine Postganglionic fibers release ACh
Receptors are alpha and beta
Receptors are nicotinic and
muscarinic