Nervous
Nervous System:The Central Nervous System
The Brain
Essential Information and Problems
Student Performance Objectives - for the lecture
1. Name the 3 initial swellings that compose the embryonic brain and the 5 final swellings
that form the mature brain.
2. Explain the value of convolution seen in the cerebral cortex.
3. Explain the difference between commissural, association and projection tracts.
4. Explain what the term "limbic system" means.
5. List the 4 major activities of the cerebrum.
6. Explain the difference between translation, interpretation and integration occurring
in the cerebral cortex.
7. Explain the significance of the angular gyrus, Wernicke's area and Broca's area.
8. Describe the significance of the precentral and postcentral gyri of the cerebral
cortex.
9. Explain why the sensory and motor homunucli look out of proportion for human bodies.
10. List and describe 4 functions for the thalamus.
11. List and describe 4 functions for the hypothalamus.
12. List and describe 2 functions for the reticular system of the midbrain.
13. List 3 sources of neural input to the cerebellum and describe the function of
the cerebellum.
14. Explain decussation.
15. Describe 4 functions for the brainstem.
Lesson Outline
A. In General: A biological discussion of the human brain could disappoint beginning
anatomy and physiology students hoping that a study of the brain will uncover what
the mind is and will reveal why they feel, remember and think the things that they
do. Even courses in psychology may not provide such answers. Questions about the mind
and consciousness with relation to the physical brain are the subject matter of important
areas of philosophy and theology and, within these disciplines, there are many points
of view. In the anatomy and physiology laboratory, students anticipate their dissection
of sheep or human brains expecting some intense revelation, or at least some sparks
to illuminate the mind's mysteries. What they get is the odor of formaldehyde and
lots of neuroanatomical terms to memorize. However, neuroscientists have learned a
great deal about the anatomical organization of the brain and the pattern of its electrochemical
circuits. Neuroanatomists have sectioned and stained the brain in great detail, neurophysiologists
have imaged living, thinking brains with CT, MRI and PET scans correlating their knowledge
with that of neurologists and neurosurgeons, and neuropharmacologists have learned
a great deal about our neurotransmitters through the use of behavior and mood altering
drugs. And so we can gain some insight into the relationship between the brain and
the mind through study of the brain's anatomy and physiology, although we may not
provide the ultimate answers.
B. In the Beginning
1. The nervous system's beginning - We have no brain or nervous system in the
first two weeks of our gestation. We do develop three primary germ layers early on
and the outermost one,ectoderm, begins forming the nervous system during our third
week of existence. For your sense of perspective on when this occurs, your mother
would be one week late in the menstrual phase of her ovulatory-menstrual cycle and
would be pretty sure she is pregnant. The early embryo's beginning nervous system
then goes through a series of rapid changes from a neural streak, where the ectoderm
changes to become a neuroectoderm, to a thickened neural plate, an infolded neural
groove with sides called neural folds, and then a neural tube, which is our primitive
spinal cord and brain, with budded off neural crest (which will form various ganglia).
http://www.brainviews.com/abFiles/AniEmdev.htm
2. The brain's beginning
a. The first three swellings - By the 4th week of gestation the anterior
region of the neural tube develops 3 swellings called, generally, the forebrain, midbrain,
and hindbrain. [In more technical terminology, the swellings are referred to, respectively,
as prosencephalon, mesencephalon, and rhombencephalon].
b. The final 5 swellings - Within a week, the forebrain divides into two
parts- telencephalon and diencephalon, as does the hindbrain - becoming metencephlon and myelencephalon.
The midbrain, or mesencephalon, does not subdivide.
3. The parts of the mature brain - The fate of the 5 embryonic brain swellings
(1) The telencephalon will form the cerebral hemispheres.
(2) The diencephalon forms the:
(a) thalamus
(b) hypothalamus
(c) pineal gland.
(3) The midbrain contains many structures: from the back of the midbrain
to the front, some important structures we will discuss are the:
(a) Corpora quadrigemina
(b) Cerebral aqueduct
(c) Reticular formation
(d) Medial lemniscus
(e) Red nucleus
(f) Substantia nigra
(g) Cerebral peduncles
(4) The metencephalon forms the:
(a) pons
(b) cerebellum
(5) The myelencephalon forms the medulla oblongata.
C. Systematic Survey of the Human Brain
1. Cerebrum
a. In general - This is the largest part of the human brain occupying its
superior surface. It is divided into hemispheres by the longitudinal fissure.
(1) Cerebral Cortex
(a) Convolutions - The surface of the cerebrum, the cerebral cortex,
is about 3 mm thick and is folded into convolutions consisting of gyri (singular =
gyrus) and sulci (singular = sulcus) that increase the cortex's functional surface
area. The 15 billion neurons located in the cerebral cortex would require a much larger
brain volume if the cortex was smooth. The folding (convolutions) permits a larger
surface area to fit into a smaller volume. It has been estimated that one's head would
have to be the size of a 5 gallon beer keg to accommodate a smooth cerebral cortex
fitting in 15 billion neurons.
(b) Neuronal types - The cortex contains two major types of neurons: pyramidal
cellsand granule cells (stellate cells), all of which form approximately 1 trillion
synapses forming the many different types of circuits required to create a mind. Unlike
the neuromuscular junctions that utilize acetylcholine as their neurotransmitter,
the majority of cerebral synapses utilize glutamate(glutamic acid) as their excitatory
neurotransmitter.
(2) Below the cortex
(a) Beneath the cerebral cortex is the cerebral white matter which
consists of commissural tracts like the corpus callosum and the anterior and posterior
commissures, all of which interconnect the two cerebral hemispheres. The cerebral
white matter also contains association tracts that interconnect different regions
of the same cerebral hemisphere, and projection tracts that link the cerebrum with
other brain regions and with the spinal cord.
(b) Also beneath the cerebral cortex and embedded within the cerebral
white matter are islands of gray matter called basal ganglia (more properly called basal
nuclei). The neurons composing these areas form feedback loops with motor areas of
the cerebrum and the cerebellum to coordinate muscular movements.
(c) An additional cerebral nucleus is the amygdala. It appears
to work with other brain regions, most notably the hypothalamus, in the area of human
emotion. Also part of the cerebrum is the hippocampus, which is involved in memory.
The term "limbic system" is often used to indicate a brain pathway, involving many
brain regions, for the initiation, response to and control of human emotions. This
system is thought to involve the cerebrocortical region called the cingulate gyrus,
and also the amygdala, hypothalamus (with its mammillary body), hippocampus, and the
fornix.
b. Cerebral activities can be divided into 4 broad areas: the processing of
sensory information, the analysis of information, the production of motor responses,
and the storage of information as memory.
(1) Processing of sensory information and information analysis involves
a number of steps that we understand in a general way.
(a) The first step in sensory information processing is translation and
it occurs in the primary sensory areas of the cerebral cortex. Bioelectrical signals
become sensations. E.g., nerve impulses reaching the primary auditory area become
sounds; those reaching the primary visual area become sights.
(b) The second step in sensory information processing is interpretation and
it occurs in the sensory association areas of the cerebral cortex. Bioelectrical signals
that have already been translated into specific sensations move into the adjacent
cortical regions called association areasand take on meaning. E.g., a noise becomes
a word with a meaning; a sight becomes a recognizable object or person.
(c) The third step in sensory information processing is integration and
it occurs in several cerebral cortical regions like the angular gyrus, Wernicke's
area and Broca's area. Here the information from various association areas are integrated
together. There is an analysis of information to give one a more complex and thorough
understanding of the information. E.g., in Wernicke's area, spoken and written sentences
take on increased meaning as information from auditory and visual association areas
is processed; in the angular gyrus, written symbols (words) we observe are processed
so they can be spoken; Broca's area prepares the muscular vocal regions of the respiratory
system for the speech we are about to deliver. This example illustrates that understanding,
writing and speaking a language is an area of cerebral function requiring much integration
of knowledge and much information analysis.
(2) The production of motor responses involves the interactions of cerebral
motor areas with basal nuclei (located deeper in the cerebrum) and the cerebellum.
Feedback loops between these brain regions (reberverating circuits) permit repetitive
activities like walking and repeated complex movements like knitting or smoothly swinging
a baseball bat. All motor movements involve the actions of several major cell types: upper
motor neurons located in the cerebral cortex's prefrontal gyrus, that synapse with lower
motor neurons in the brainstem or spinal cord that then relay the message to contract
to the skeletal muscles. Monitoring all this cerebral electrical activity are the
cerebellum's Purkinje cells that coordinate balance (proprioceptive) feedback from
joints, muscles, tendons, and the inner ear, so that cerebrum-directed movements are
smooth and appropriate for the intended outcome.
(3) Memory
(a) In general, memory is thought to be a path through the cerebral
cortex in the form of a pathway of synapses. The pathway may travel through one cerebral
lobe utilizing only local association neurons and their axons (short association fibers),
or the pathway may travel through several cerebral lobes in one hemisphere utilizing
widely separated association neurons and much longer axons (long association fibers).
(b) Types of memory - memory is commonly subdivided into short term
and long term. Short term memory is thought to involve reberverating neural circuits with
possible facilitation of synaptic transmission to permit the memory to last a bit
longer than just for the immediate moment. Long term memory is thought to involve
actual growth of new dendrites and axons on existing neurons to form new synapses
and circuits specific for that particular memory (in addition to the facilitation
of synaptic transmission).
c. Cerebral subdivisions - The cerebrum has 5 lobes, four being obvious from
a surface view. The four major cerebral lobes are named for the cranial bones that
they underlie. Some cerebral functions appear to be unique to a given cerebral lobe
(e.g., the visual sensory area is mostly in the occipital lobe and auditory sensory
area is mostly in the temporal lobe), whereas other higher brain functions such as
"learning" may occur in several cerebral areas simultaneously. Some key cerebral areas
lie at the junction of the various cerebral lobes. For example Wernicke's area and
the angular gyrus lie approximately at the intersection of the parietal, temporal
and occipital lobes. The following discussion emphasizes cerebral lobe functions that
are unique to the lobe in question.
(1) The Frontal lobes are concerned with many areas of biological intelligence
such as our ability to sense time and to plan for the future, aspects of language
(Broca's region), memory and personality traits. The frontal lobes also contain the precentral
gyri concerned with voluntary movements of skeletal muscles. There is a representation
of the human body on the precentral gyrus called the motor homunculus. This human
image is strange looking because some body parts are larger than other parts. The
face, lips, tongue and hands, particularly the thumb, are disproportionately larger
than other parts of the body because the number of motor neurons innervating and providing
fine motor control for these areas is disproportionately larger than other bodily
areas. The olfactory bulbs leading to the olfactory tracts (cranial nerve I - olfactory)
are located just beneath the frontal lobes.
(2) The Parietal lobes, containing the postcentral gyri, are concerned
with the translation and interpretation of sensory signals (touch, pressure, temperature,
pain) from skin and the tongue (sense of taste). Just as the precentral gyrus has
a motor homunculus, the postcentral gyrus has a sensory homunculus. It is as strange
looking as the sensory homunculus because some body parts are larger than others.
The face, lips, tongue and hands, particularly the thumb, are disproportionately larger
than the rest of the body because the number of sensory neurons in these areas is
disproportionately larger than other bodily areas.
(3) The Temporal lobes are concerned mainly with the senses of hearing
and smell.
(4) The Occipital lobes are mainly concerned with the sense of sight.
(5) The Insula is observed in frontal and horizontal sections of the brain,
but not from the surface. Its lobes appear to be concerned with the sense of taste
and with the processing of visceral sensations.
d. Differences between the hemispheres: The cerebral hemispheres do not work
identically. The left cerebral hemisphere appears to be more concerned with analytical
reasoning in which information is broken down and dissected in some logical way. It
is also more concerned with written and spoken language. The right cerebral hemisphere,
in contrast with the left, appears to be more concerned with the ability to see broad
spatial relationships, to grasp things intuitively, and to appreciate and demonstrate
artistic talents.
(1) Handedness - this description of the differing abilities of the cerebral
hemispheres is found to hold true for the vast majority of people who are right handed.
However, in the majority of left handed people the cerebral roles are reversed. Only
in a small percentage of left handed individuals are the cerebral roles are the same
as for right handed people.
(2) Gender - women appear to have a better ability than men to have functions
from one hemisphere take over if there is loss to the other hemisphere. This is observed
with stroke victims in that men are more likely than women to suffer permanent language
impairment (a condition known generally as aphasia).
2. Diencephalon - In general, the diencephalon is located between the cerebral
hemispheres and is composed of three major structures: thalamus, hypothalamus and
the pineal gland.
(1) Thalamus - the thalamic hemispheres make up most (80%) of the mass of
the diencephalon. The cerebrum is an outgrowth of the thalamus, embryologically, and
there is a radiation of nerve fibers from the thalamus up into the cerebrum. The thalamic
hemispheres are connected by the intermediate mass.
(a) Sensory signal relay station - All ascending, sensory signals, except
for olfaction, pass through the thalamus before entering the cerebrum. It generally
requires 3 neurons for a sensory signal to reach the cerebrum: a first order neuron
carries the signal in from the receptor to the spinal cord or brainstem. A second
order neuron carries the signal from the spinal cord or brainstem to the thalamus.
Finally, a third order neuron carries the sensory signal from the thalamus into the
specific primary sensory area of the cerebrum. The brain's "emotional system" (limbic
system) also communicates with the cerebrum through impulses passing through the mammillary
bodies to the thalamus and then to the cerebrum.
(b) Rough translation - cutaneous sensation undergo some translation
in the thalamus, but without precise localization. Without the operation of the cerebral
somatosensory cortex (postcentral gyrus), you would sense "cold" if an ice cube were
placed on your neck, but you would not know exactly where the cold sensation was coming
from. The cerebrum not only translates nerve impulses into sensations, but also provides
localization of where the sensation is coming from. The thalamus does not roughly
translate visual or auditory sensory input, only cutaneous.
(c) Concentration - by acting as a filter for information traveling
into the cerebrum, the thalamus aids in our ability to concentrate: some signals reach
the thalamus and others are blocked. The thalamus works in this regard with the reticular
formation of the brainstem.
(d) Motor signal circuits - some motor signals pass directly from the
cerebrum to the brainstem or spinal cord (the corticospinal tracts, also called pyramidal
tracts), bypassing the thalamus. But some descending motor signals pass to the pons
and then to the cerebellum for processing. Also, many motor signals pass from the
cerebrum to the basal nuclei for processing. All these processed signals, from the
cerebellum and the basal nuclei, then pass back up to the cerebrum via the thalamus
for further processing before being sent to the brainstem and spinal cord.
(2) Hypothalamus - this area of the diencephalon is sometimes called "the
brain within the brain" because of the number of control centers it contains and the
influence it has on other regions of the nervous system, on the endocrine system,
and the body as a whole. The optic nerves (cranial nerve II - optic) cross at a point
called the optic chiasm that is just below the hypothalamus, although not part of
the hypothalamus. The following body functions are regulated through the hypothalamus:
(a) Thirst and body water regulation, working through the posterior pituitary.
(b) Hunger for food and feelings of satiety .
(c) Heart rate and blood pressure, working through brainstem nuclei.
(d) Body temperature regulation which controls vasodilation and vasoconstriction
of blood vessels to the skin, sweating, shivering, and control of the metabolic rate.
(e) Peristalsis and glandular secretion in the stomach and intestines.
(f) Pupillary diameter
(g) Hormone secretion, working through the anterior pituitary.
(h) Biological rhythms, including sleep and wakefulness.
(i) Basic human emotional drives, including sex, anger, joy, fear, and
pleasure.
(j) Memory, working as a pathway (through the mammillary bodies) from
the hippocampus to the thalamus and cerebrum.
(3) Pineal gland - this gland, once thought to be the location of the human
soul, secretes 2 monoamine hormones - serotonin (which you also know as a neurotransmitter
whose reuptake is inhibited by mood elevating drugs like Prozac) during the day, and melatonin at
night. Melatonin may play a role in the timing of puberty and in human sleep cycles.
3. The Midbrain is a connecting link between the forebrain (cerebrum and diencephalon),
and the hindbrain (brainstem and cerebellum). Its parts include:
a. Corpora quadrigemina - the two superior colliculi functioning in visual
reflexes in which you respond to an object that enters your visual field by turning
your eyes and head toward it, and the two inferior colliculi, functioning in auditory
reflexes in which you turn your head toward a sound you have suddenly heard.
b. Cerebral aqueduct - this is the passageway for the flow of cerebrospinal
fluid from the third ventricle to the fourth ventricle.
c. Reticular formation - this extensive region of gray matter runs through
the midbrain and also the entire brainstem (pons and medulla). It consists of over
100 nuclei that regulate:
(1) Sleep and wakefulness through their connections to the thalamus
and cerebrum. The reticular activating system learns to rouse the cerebrum from sleep
based on external signals - an alarm for instance, while ignoring even louder sounds
that might occur earlier.
(2) Concentration (working with the thalamus) through control of which
sensory signals pass through and reach the cerebrum.
(3) Heart rate and blood pressure control- these nuclei are in the part
of the reticular formation that is within the medulla oblongata. They act as reflex
centers for blood pressure control and heart rate. They also can be controlled from
the hypothalamus.
(4) Balance and posture maintenance through connections with the motor
cortex and by interconnecting signals from peripheral receptors (eyes and balance
receptors in the inner ear) with the cerebellum.
d. Medial lemniscus - consists of a continuation of sensory spinal tract
carrying cutaneous and proprioceptive signals to the thalamus.
e. Red nucleus - a large nucleus concerned with muscle control that communicates
with the cerebellum.
f. Substantia nigra - this is a motor modulating nucleus that sends inhibitory
signals to the the thalamus and basal nuclei helping to control muscle contractions.
Disease affecting this nucleus, like Parkinson's disease, results in uncontrolled
muscular movements.
g. Cerebral peduncles - these descending (motor) white matter tracts (corticospinal
tracts) are just passing through the midbrain.
h. The midbrain also contains the nuclei for cranial nerves III (oculomotor)
and IV (trochlea), which control eyeball movements (along with cranial nerve VI).
4. Metencephalon - consists of the pons and cerebellum.
a. Pons
(1) A relay station for ascending and descending signals traveling in
white matter tracts. The cerebellum receives most of its input from tracts running
through the pons.
(2) Contains the nuclei for cranial nerves V (trigeminal - motor and sensory
to the face), VI (abducens - eyeball movements), VII (facial - motor and sensory to
the face), and VIII (auditory - hearing and equilibrium).
(3) The reticular formation runs through the center of the pons and contains
nuclei that regulate aspects of respiration, posture and sleep.
b. Cerebellum - containing about 100 million neurons (1/2 of all the neurons
in the brain), the cerebellum is a major brain center for the integration of signals
relating to the smooth, precise, and coordinated movements of skeletal muscles. The
cerebellum, divided into two, finely convoluted cerebellar hemispheres, is attached
to and communicates with the brainstem and, from there, the rest of the brain, through
3 pairs of white matter tracts.
(1) Input to the cerebellum comes from:
(a) Cerebral cortex through the pons.
(b) Inner ear including signals for balance and hearing.
(c) Eyes.
(d) Reticular formation through the red nucleus.
(e) Muscle spindles and joint receptors (balance or proprioceptors).
(2) Output from the cerebellum goes to:
(a) Cerebral cortex through the thalamus.
(b) Postural muscles of the arms and legs through tracts running
through the pons, medulla oblongata and spinal cord (reticulospinal tracts).
5. Myelencephalon forms only one structure - the medulla oblongata. This brain
region is continuous with the spinal cord and begins just at the foramen magnum, ending
at the pons. Ascending and descending white matter tracts run through the medulla
and it is here that most of them decussate so that sensory messages from the left
side of the body are received by the right cerebral cortex and vice versa. Similarly
motor responses originating from the right cerebral motor cortex, lead to movements
on the left side of the body, and vice versa. Decussation does not apply to sensory
and motor signals from the head, only to areas below the head. The reticular formation
runs through the medulla and contains nuclei that reflexively regulate the heart rate,
blood pressure, respiratory rate and depth and sweating. The medulla contains nuclei
for cranial nerves IX (glossopharyngeal), X (vagus, XI (accessory), and XII (hypoglossal).
A number of complex reflexes are processed through the medulla utilizing these cranial
nerves, including swallowing, coughing, sneezing and vomiting.
D. Summary of the Cranial Nerves
Cranial Nerve
General Function
Cranial Exit
I Olfactory
Smell
Cribriform Plate of the Ethmooid
II Optic
Sight
Optic Foramen
III Oculomotor
Eye Movement
Superior Orbital Fissure
IV Trochlear
Eye Movement
Superior Orbital Fissure
V Trigeminal
Face: sensory, motor
Superior Orbital Fissure
VI Abducens
Eye Movement
Superior Orbital Fissure
VII Facial
Facial Expressions
Stylomastoid Foramen
VIII Vestibulocochlear
Hearing and Balance
Internal Acoustic Meatus
IX Glossopharyngeal
Toung and Throat - motor and sensory
Jugular Foramen
X Vagus
Parasympathetic
Jugular Foramen
XI Accessory
Head, neck and shoulder movement and swallowing
Jugular Foramen
XII Hypoglossal
Speech, chewing and swallowing
Hypoglossal Canal
Biomedical Terminology: Define each term:
amygdala
angular gyrus
association fibers
Broca's area
cerebellum
cerebral peduncles
commissural fibers
corpora quadrigemina
corpus callosum
decussation
diencephalon
ectoderm
forebrain
granule cells
gyrus
hindbrain
hippocampus
homunculus
hypothalamus
insula
melatonin
mesencephalon
metencephalon
midbrain
myelencephalon
optic chiasm
pons
projection fibers
Purkinje cells
pyramidal cells
reticular formation
serotonin
telencephalon
thalamus
Wernicke's area
Nervous System Problems
1. Choose one of the problems described below.
2. Prepare your solution as a word document.
3. Send it to your professor as an email attachment. You will receive an email
response.
Problem #1: An individual approached by police during a robbery resists arrest and
is subdued only by the combined efforts of 3 very strong officers. The officer's impressions,
and later chemical analyses, reveal the individual to be heavily under the influence
of cocaine. Utilize the Internet to research the effects of cocaine on the human nervous
system and other systems of the body.
Your report should include
1. A description of cocaine and a list of other chemicals in its class.
2. An explanation of the effects of cocaine on the nervous system.
3. An explanation of why the individual was difficult to subdue.
4. The effects of cocaine use, both short and long term, on the organs of
the human body.
Problem #2: A recent book: "Women are from Venus, Men are from Mars" has popularized
the idea that there are significant differences between the brains of men and women.
Utilize the Internet to answer the following questions:
1. Are there anatomical differences between the brains of men and women?
If yes, what are they?
2. What are some of the differences cited between male and female behavioral
patterns? Are any of these differences related to anatomical differences?
3. Is there evidence that men and women are born "programmed" with different
physiological responses to stimuli, or are the different responses of men and women
to stimuli based on learning throughout life?
Problem #3: A child having problems concentrating in school is given Prozac by a doctor.
A college student suffering from depression is given Prozac to combat the depression.
In fact, 30 million Americans have been given Prozac or drugs like it. Utilize the
Internet to evaluate the pros and cons of Prozac therapy.
Your report should include
1. A description of the postulated basic mechanism of action of Prozac. What
is neurogenesis and does Prozac stimulate this process?
2. Undesirable side of effects of Prozac therapy. Is altered brain microanatomy
an indication of brain damage?
3. Alternatives to Prozac therapy.
Practice Quiz