Download CNS: Brain and Spinal Cord

Chapter 12
The Brain
I. Overview of Brain regions- See text
II. CSF, the cranial meninges, the ventricles, and the blood-brain barrier: how the
brain is fed and protected (out of synch with book presentation)
A. Meninges
1. Dura mater- the dura mater around the brain has two layers- an outer
(endosteal) layer that is tied to the skull, and an inner (meningeal) layer. In
some areas, the inner layer folds in between major sulci of the cerebrum.
These dural folds help to anchor the brain in place. The falx cerebri is a
dural fold that separates the cerebral hemispheres, and it attaches to the
crista galli of the ethmoid bone.
Where the dural folds occur, the space between the endosteal and
meningeal layers of dura mater form dural sinuses. These sinuses collect
CSF from the subarachnoid space and return the fluid to the blood via the
interior jugular veins (see figures from vessels chapter)
2. Arachnoid- as in the spinal arachnoid (later), the subarachnoid space is
filled with CSF. At the dural sinuses, arachnoid villi poke up through the
meningeal dura mater and deliver CSF to the sinuses.
3. Pia mater- the only covering that follows the folds of the cerebrum.
B. The ventricles: four interior chambers, where CSF is "made." They are
continuous with the central canal of the spinal cord.
1. lateral ventricles (Ventricles 1 & 2)- One is located in each of the
cerebral hemispheres. Connected to the 3rd ventricle through a small
canal.
2. Third ventricle- Located in the diencephalon. Connected to the 4th
ventricle through a canal.
3. Fourth ventricle- Located in parts of the pons, cerebellum and (mostly)
medulla. Narrows and becomes the central canal of the spinal cord. The
fourth ventricle has 3 holes that open to the subarachnoid space: two
lateral apertures and a median aperture.
C. Formation and circulation of CSF
1. Functions- please read
2. Where and how it's made- There are two groups of highly folded
capillaries lined by ependymal cells. One group is located in the 3rd
ventricle, the other is in the 4th ventricle. These complexes are called
choroid plexuses. The capillaries of the plexuses are highly permeable
(allow lots of solutes to pass across them), which is unusual for capillaries
in the brain. However, the ependymal cells surrounding the capillaries are
bound by tight junctions. The ependymal cells carefully remove solutes
from the blood and allow water from the blood to enter the ventricles.
They pick and choose the types of solutes that enter the ventricles. The
fluid and solutes that enter the ventricles will become CSF.
Because of the selective action of the ependymal cells, the solute
composition of the CSF is quite different from that of the blood. I won't
ask you about the specifics, but for your interest and understanding, some
examples include: CSF contains less protein, Ca++, and K+ than blood.
CSF contains more Na+, Cl-, and H+ (which means it has a lower pH)
than blood. No red blood cells enter CSF.
3. How it gets around- CSF leaves the ventricles through the lateral
apertures and median aperture in the 4th ventricle. It enters the
subarachnoid space and circulates. At arachnoid villi, it enters dural
sinuses and drains into the jugular veins.
D. The blood brain barrier- capillaries that enter the neural part of the brain (as
opposed to the choroid plexuses of the ventricles) contain an endothelium in
which simple squamous cells are connected by tight juctions. They have limited
protein channels. Between these two characteristics, they allow very restricted
movement of particles from the blood to the brain. In addition, astrocytes wrap
around the capillaries and somehow help to control the types of particles that
enter the brain.
This is unlike capillaries in most tissues/organs, in which the cells of the
endothelium are not bound by tight junctions and contain many types of channels,
so they are permeable to many more particles.
There are a few areas of the brain where capillaries are fairly permeable. Those
areas include the choroid plexuses (capillaries are permeable but the ependymal
lining is not), and endocrine areas where hormones are released into the blood: the
pineal gland and the hypothalamus.
III. The cerebrum- Highly folded to increase surface area and allow more neuron
connections; separated into two hemispheres (right and left); gray matter is found in the
cortex (outer surface) and nuclei (Interior, embedded in white matter)
A. Superficial landmarks
1. Gyri and sulci (singular = -us)- peaks and valleys of the folds,
respectively
2. Prominent sulci and lobes of the cerebrum
a. Longitudinal fissure separates the hemispheres. The corpus
callosum is a band of white matter (axons) that runs between the
hemispheres and allows them to communicate.
b. Central sulcus- separates the frontal lobe from the parietal lobe
c. Lateral sulcus- separates the frontal lobe from the temporal lobe
d. Parieto-occipital sulcus- separates the parietal from the occipital
lobe
e. The insula is another lobe, deep to the lateral sulcus
*Each lobe contains "centers" that process specific types of information; ex. the visual
cortex in the occipital lobe receives and processes visual information
B. The cerebral cortex- information processed here reaches our conscious
awareness. Again, specific areas of the cortex deal with specific types of
information. The two hemispheres have slightly different functions. For instance,
the left side generally deals with logic, problem solving, and so forth. The right
side generally deals with language, visual mapping and so forth (hence the
categorizing of "right-brained" vs. "left-brained" people).
1. Types of functional areas
a. Motor areas- control moter functions, found in the frontal lobes
b. Sensory areas- receive sensory information, found in the
parietal, temporal and occipital lobes
c. Association and Integration areas- integrate information between
different areas of the cortex. For instance, these areas generate
complex thoughts, cognition, planning, sympathy, language
articulation and interpretation, and personality.
2. The motor and sensory areas of the cortex generally control the opposite
sides of the body (ex, sensory info from the left hand is interpreted by the
right hemisphere).
C. White matter- bundles of myelinated axons (where are the cell bodies of these
axons?)
- can allow communication between hemispheres (ex. the corpus callosum),
between different parts of the same hemisphere, and between the cerebrum and
other parts of the brain/CNS.
D. Cerebral Nuclei- "basal nuclei"- masses of gray matter deep in the cerebrum,
embedded in white matter. Subconscious sensory and motor interpretation.
Modify motor commands issued by the cortex; help to produce smooth
movements and balance, particularly with muscle tone, postural support, and
rhythmic motion. The amygdala (which has other functions we will see later) is an
example of a cerebral nucleus.
IV. The Diencephalon- remember the 3rd ventricle is located here. Includes the
thalamus, hypothalamus, epithalamus.
A. The thalamus- Filters virtually all sensory information and determines where
and if to send it to the cerebral cortex and other parts of the brain. Most sensory
information does not make it to our conscious awareness, and the thalamus
controls what makes the cut.
B. The hypothalamus- major homeostatic control center
Monitors the blood for ion concentration, temperature, hormone levels,
etc.
Directs the release of most hormones, and releases a couple of hormones
itself.
Controls the Autonomic Nervous System; in doing so, it sends commands
to the pons and medulla which then communicate with the ANS. Any
function of the ANS is therefore a function of the hypothalamus.
Part of the limbic system, so is involved with emotional responses
Food and water intake regulation (hunger and thirst)
Sleep-wake cycles ("biological clock" is here)
And more!
C. Epithalamus- houses the pineal gland. The pineal releases melatonin, which
helps to regulate sleep-wake cycles (for your interest, melatonin release is largely
in response to day length, so during the short days of winter you really ARE more
tired because more melatonin is released).
V. The Brain Stem- basic physiological/survival functions.
A. Mesencephalon/midbrain- contains nuclei that, among other things,
influence alertness (“headquarters” of the Reticular Activating System)
produce the startle reflex (turning your head towards a loud, sudden
sound)
coordinate head/eye movement to follow moving objects
produce dopamine (Substantia Nigra)
B. Pons- most of the pons contains conduction tracts (white matter, axons
travelling to & from other areas of the CNS). Does contain a couple of nuclei,
including:
Respiratory centers (Pneumotaxic & Apneustic) that help control rate and
depth of breath.
one (shared by the medulla) that receives equilibrium info from the inner
ear via the vestibulocochlear nerve, and relays the info to the cerebellum
(Vestibular nuclei).
C. Medulla- contains nuclei that, among other things,
relay information about motor commands from the cerebral cortex to the
cerebellum
Respiratory centers are influenced by the pneumotaxic/apneustic centers
of the pons, which are controlled by the hypothalamus
send motor neurons to the heart and blood vessels to adjust heart rate and
vasoconstriction (Cardiovascular centers).
*remember that the 4th ventricle is located between/in the pons, medulla & cerebellum,
and at the bottom of the medulla it merges into the central canal of the spinal cord.
VI. The cerebellum- coordinates precise movements.
A. Anatomy- like the cerebrum, contains an outer gray matter and inner white
matter, and has two hemispheres.
the gray matter is composed of Purkinje cells (specialized neurons)
the white matter has a branching pattern, and is called the "arbor vitae," or
tree of life.
B. Functions- coordinates precise movements. When cerebellum is impaired
(from, for example, damage or alcohol), movements are clumsy.
the cerebellum receives info from the motor cortex about the intent to produce a
movement
it also receives info about proprioception, equilibrium and balance (from
Vestibular nuclei in the medulla and directly from sensory pathways)
it integrates all this info, then sends a "blueprint" to the cortex (via the thalamus)
about how to produce a smooth movement.
input from the cerebellum reduces the number of motor units activated for a
particular movement, so that the fewest motor units needed are used, and the
movement is smooth and not overdone.
VII. Brain Systems that incorporate many different areas:
A. The limbic system- a functional group that includes areas of many parts of the
brain (cerebrum, diencephalon, and mesencephalon). Involved in emotion,
motivation, and memory. Some (but not all) components of the limbic system:
Amygdala (a cerebral nucleus)- involved in the fear response, connecting
memory with emotion, & communicates with ANS in fight-or-flight
response
Hippocampus (part of a cerebral gyrus)- storage and retrieval of long-term
memory. The hippocampus experiences production of new neurons.
Mamillary bodies (hypothalamic nuclei)- receive unfiltered olfactory
information. That is, the olfactory information coming to the mamillary
bodies does not travel through the thalamus first. A potential explanation
for the strong emotional responses we can form with certain smells.
B. The Reticular Formation- generally, receives sensory information and controls
our level of alertness by communicating with the cerebral cortex. It’s activity can
be influenced, among other things, by the hypothalamus which helps to regulate
sleep-wake cycles.
The Spinal Cord
I. Gross Anatomy of the Spinal Cord
A. Meninges (coverings)
1. Dura mater- outermost, dense connective of collagen
a. Collagen fibers tie the dura mater to the occipital bone
superiorly, and the coccyx inferiorly. The dura mater is not
attached to the vertebral bones.
b. Epidural space- between vertebral bones & dura mater. Contains
adipose & blood vessels.
2. Arachnoid- middle
a. Simple squamous layer adjacent to dura mater
b. Connective layer of collagen and elastin fibers in a loose "web"
(this is the origin of the name, as it resembles a spider's web). This
area is called the subarachnoid space, and it is filled with CerebroSpinal Fluid (CSF)
3. Pia mater- Collagen and elastin, bound to the spinal cord. Sends tiny
ligaments that look like threads through the arachnoid to the dura mater.
Those ligaments tie to the collagen of the dura mater.
B. Interesting landmarks
1. Posterior median sulcus and Anterior median fissure- can be seen in
cross-section
2. Enlargements (swollen-looking areas)- areas where LOTS of neurons
are “coming and going”
-Cervical- neurons of upper limbs
-Lumbar- neurons of lower limbs
>>3. The end of the line:
>>>Conus medullaris, cauda equina (roots that extend from the conus), filum terminale
C. Cross-Sectional Anatomy
1. Gray matter- consists of cell bodies and unmyelinated interneurons;
resembles a butterfly. The central canal is in the center and contains CNS.
a. Horns- the "wings" of the gray matter. This is where sensory
neurons coming in end and motor neurons going out begin. Know,
generally, that somatic (body wall & skeletal muscle) and visceral
(organs & glands) sensory and motor neurons synapse in specific
areas of the gray matter, but you won't need to identify where
somatic and visceral synapses are. (You do need to know where
sensory vs. motor synapses are)
b. Posterior horns- consist of interneurons that synapse with
terminals of sensory neurons
c. Lateral and Anterior horns- consist of cell bodies of motor
neurons
2. The roots and ganglia of the spinal nerves- sensory axons coming into
the spinal cord are separated from motor neurons leaving. They enter and
leave via roots. These roots will merge to become mixed nerves that serve
the body.
a. Dorsal root and dorsal root ganglia- the dorsal roots carry
sensory neurons. The sensory neurons are primarily unipolar, and
their cell bodies reside in the dorsal root ganglia.
b. Ventral roots- carry motor neurons. The motor neurons are
multipolar, and their cell bodies are in the lateral and anterior horns
of the gray matter.
3. White matter & neuron pathways/tracts: white matter consists of
myelinated axons running vertically to & from the brain.
a. Axons with similar form and function form tracts, or bundles of
axons. Most tracts cross sides as they ascend or descend the cord.
b. Ascending tracts: carry sensory info up to the brain. Typically
contain several synapses:
-First-order neurons: the actual sensory neurons. In the
posterior horns, they synapse with:
-Second-order neurons: interneurons originating in the
posterior horns. They send axons up through tracts to
synapse with:
-Third-order neurons: interneurons in thalamus. They
analyze the sensory info and figure out where in the brain
to send the info (if at all)
c. Descending tracts: carry motor commands down from the brain.
May or may not go through a series of synapses (depends on the
tract). For example, a tract that has no intermediaries is the
pyramidal tract: axons come directly from the motor cortex of the
cerebrum through the white matter of the spinal cord and synapse
with motor neurons in the anterior horns.
II. Ascending and Descending Pathways
A. Somatic Sensory (afferent) Pathways
1. General info
a. Sensory information- can be specialized (sight, sound, etc) or
general (touch, temp, pain, stretch, position)
b. Sensory stimuli affect receptors on sensory neurons; for
example, a neuron that sends info about touch experiences EPSPs
in response to pressure. A neuron that sends info about sound
experiences EPSPs in response to sound waves.
c. Types of receptors- these can be intero, extero, or (in some
cases) proprio- ceptors.
-Themoreceptors- stimulated by temperature change
-Mechanoreceptors- stimulated by pressure & stretch
-Chemoreceptors- stimulated by specific chemicals, for which the
neuron contains receptors
-Nociceptors- stimulated by tissue damage (chemicals released by
damaged cells, getting squashed, etc), messages perceived as pain
d. !st, 2nd, and 3rd order neurons- present in all the sensory
pathways other than the Spinocerebellar, which only contains 1st
and 2nd order.
2. The sensory homunculus- see fig.
3. Specific pathways for the general senses- cross sides at some point in
their journey to the brain. Remember that sensory neurons are usually
unipolar with their cell bodies residing in the dorsal root ganglia. Their
axons enter the dorsal horns via the dorsal root.
a. Posterior Column/ Medial Lemniscal Pathways- localized touch,
vibration, proprioception
- Information from peripheral fibers travels through the thalamus.
The information that makes it to the cortex will be routed to the
primary sensory cortex (parietal lobes).
b. The Spinothalamic/ Anterolateral- Non-localized (can't pinpoint
exact location) touch, pressure, pain, temperature
-Information from peripheral fibers travels through the thalamus.
The information that makes it to the cortex will be routed to the
primary sensory cortex.
c. Spinocerebellar pathway- proprioception
-Information ends up in the cerebellum, and you know what the
cerebellum does with that info!
-Not all fibers cross sides
d. Visceral Sensory Pathways- (Sensory information from organs
and internal cavities; most will not reach our conscious awareness)
-1st order neurons (interoceptors) travel in nerves with those of
somatic sensory 1st order neurons
-interneurons send axons up the anterolateral pathway, to a nucleus
in the medulla. Most of that information will get routed to other
parts of the brain, but not to the cortex.
*Before you move on to motor pathways, take a minute to draw the general routes of the
sensory pathways. Where do 1st and 2nd order neurons generally synapse? (The posterior
column pathway is an exception to this, but I will not focus on that).
B. Somatic Motor (efferent) Pathways- remember, the alternative is
Autonomic/visceral (we will consider in Ch 14). See also “the projection level”
1. General info- cross sides at some point in their journey from the brain.
There are no ganglia associated with somatic motor neurons.
a. Somatic motor pathways involve at least 2 motor neurons:
Upper (from brain) and lower (originates in brain stem or ventral
horns of spinal cord). The upper neuron can excite or inhibit the
lower neuron. The lower neuron excites muscle cells at
neuromuscular junctions/motor end plates (remember those?).
Lower neurons always secrete Ach, which is always excitatory to
skeletal muscle.
b. Upper neuron activity is monitored and adjusted by the
cerebellum and cerebral nuclei. What do they do?
c. The motor homunculus- see fig. and accompanying explanation
2. Specific pathways
a. Corticospinal Pathway- the pyramidal system. Fine movements.
-Upper neurons originate at pyramidal cells in the primary
motor cortex (frontal lobes).
-Upper neurons synapse with lower neurons in the anterior
horns of the spinal cord gray matter
b. Extrapyramidal Pathways- subconscious, gross movements,
reflex movement of head and eyes, & muscle tone
-Upper neurons originate in cerebral nuclei,
mesencephalon, and throughout the brain stem
-Upper neurons receive sensory information about body
position and equilibrium. They use this information to
adjust motor commands. To do so,
-Upper neurons synapse both with the upper neurons of the
corticospinal pathway and with the lower neurons of the
corticospinal pathway (so, the lower neurons are the same
for corticospinal, medial and lateral pathways; the medial
and lateral pathways make adjustments to the basic
commands of the corticospinal pathway).
VIII. Control by cerebral nuclei & one of the roles of dopamine
>Cerebral nuclei send axons to upper neurons of the medial pathway
>They also send messages to the primary motor cortex (via a multisynaptic loop through
the thalamus, motor association area, and finally the primary motor cortex)
>They receive information from sensory pathways about body position, proprioception,
equilibrium. They are particulary involved in rhythmic movement, muscle tone and
balance (activating opposing muscle groups to compensate for weight shifts), and
starting/stopping activity.
> Neurons from the cerebral nuclei are constantly influencing the activity of the motor
cortex, and with upper motor neurons of the corticospinal and medial pathways. Some of
these neurons are excitatory, and some are inhibitory.
> Only some excitatory neurons are active at any given time. If all of the excitatory
neurons were active, muscle tone would be uncontrolled, as opposing muscle groups
would contract without compensating for each other; that is, opposing muscle groups
would have strong contractions and would "compete." This would produce
shakiness/twitching.
> Normally, many of the excitatory neurons are inhibited. This way, opposing muscle
groups allow each other to contract smoothly. For example, if you straighten your back,
your abs will relax to some extent to allow a smooth contraction of your back muscles. If
your abs stayed contracted, your back muscles would have to work really hard to get you
straightened up.
> The inhibition of excitatory neurons of the cerebral nuclei comes from neurons of the
mesencephalon (in the substantia nigra). They release dopamine onto the excitatory
neurons. Dopamine inhibits them. Without enough dopamine, muscle tone increases and
opposing muscle groups don't compensate for each other; they just all contract.
Parkinson's disease is caused by the underproduction of dopamine.
>L-dopa is a precurser molecule to dopamine (cells can use L-dopa to make dopamine).
Dopamine can't cross the blood-brain barrier, but L-dopa can, so it is used as a treatment
for Parkinson's.
(Dopamine is also involved in mood and perception...too much is associated with
schizophrenia, although there are other neural differences seen in schizophrenics)