Then
we went over the organization of the nervous system, which we began
last class. I explained that the purpose to my explanation, last class, of
the evolution of the nervous system is that it helps understand how it is
structured and why it functions the way it does. The nervous system is
organized by how it developed through evolution, with the primitive
survival functions coordinated by the innermost structures, and the
neuronal cell bodies, with their vital nuclei, being largely contained
within the protective bony structures (spinal vertebrae and skull)
The nervous cells contained within these bony structures is called the
Central Nervous System, and the fibers that carry message to and from this
central system make up the peripheral nerves which are part of the
Peripheral Nervous System.
- CNS: Consists of two parts, the brain and the spinal
cord. Both the brain and spinal cord are encased in bone for
protection and the cell bodies (soma) of most neurons are contained within
the central nervous system. When they are located outside these systems in
groups, these groups or clumps of neurons are called ganglia.
The brain: central to all
human life functions, receives
information about the world, processes the information to make sense of it, and then directs
voluntary and involuntary functions of the body that support all aspects of life.
The spinal
cord: coordinates the reflex arcs and conveys information to and
from brain .
(Reflex arc: simplest, most primitive level of
nervous system's sensing/responding to
the outside world. Consists of three neurons, 1) a sensory neuron that responds to
stimuli such as pain, heat, etc), 2) a motor neuron to tell the muscles what to do, and
3) an
interneuron, which conveys information from the sensory neuron to the motor
neuron and tells the brain what is happening.)
- PNS: This consists of two separate systems that function,
the somatic
and autonomic systems, which have different
functions. Somatic ('soma' means body) nerves carry messages from the
sensory organs to the brain and from the brain to the skeletal muscles. You can consciously control some aspects of
this system, such as the muscles of motion. The autonomic
('self
governing', automatic) system controls the internal organs and glands of the body and
controls its involuntary activities. The autonomic system has two branches: one, the sympathetic, to prepare the body to deal with emergencies, and the parasympathetic, which
calms and restores the body and returns the vegetative (life sustaining) functions to normal.
The entire nervous system is made up of neurons, reactive cells that
communicate with each other and with other bodily cells (muscles, glands,
etc) and glial cells which surround, support, and insulate neurons.
Nerves are not the same thing as neurons:
'Nerves' and 'tracts' are bundles of neuronal fibers, parts of the
neuron that extend from the cell body. They can be dendrites, which
are message-receiving fibers,
or axons , which are message-sending nerve fibers. Tracts are
bundles of nerve fibers within the CNS and nerves are external to the CNS.
Nerves are 'one-way' streets, either forming afferent nerves which send messages
to the CNS or forming efferent nerves which carry
messages from the CNS. The soma, or cell bodies, of neurons are mostly in the Central
Nervous System, which is encased within protective bone structures (the skull and spinal
column); some (such as the soma of the pain-sensing neuron depicted in the diagram of the
reflex arc) are in ganglion, masses of nerve tissue outside the CNS.
Neurons are both electrical generator/transmitters and chemical 'factories': different kinds of neurons produce different kinds of
neurotransmitters, depending on their functions. (For example, motor neurons' axon
terminals go directly to muscle fiber cells in the muscle they
control. The axon terminal are adjacent to receptor sites on the muscle cell walls and
release acetylcholine into the synaptic cleft (or gap), which then attaches to the
receptor sites and triggers the muscle cell to contract.
Axons which carry messages over long distances between the brain's
hemispheres, down the spinal column or outside of the
CNS are encased in a fatty whitish sheath of tissue called myelin which speeds up
transmission of the neural impulse. Within the brain, most axons do not have a myelin
sheath. Myelin is formed by tiny glial cells that wrap themselves around and
around the axon, leaving a white fatty or waxy skin encasing and insulting the
axon. The axons and myelin also have a thin layer of cells on the outside called the
neurolemma,
which acts like a tunnel through which (outside of the Central Nervous System) damaged
nerve fibers can follow as they repair themselves. Until fairly recently, it was not
believed that nerves could re-grow once damaged, but recently, nerve regeneration has been
demonstrated outside the spinal cord. Many current textbooks still state that the axons of the CNS
"must last a lifetime". However, recent research in rats has proved some
substances ('nerve growth factors' and certain immune system cells) can enable the healing
and re-growth of axons even in the spinal cord.
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Christopher Reeves (actor), who was
paralyzed when his spinal cord was crushed in a fall from a horse, does extensive physical
therapy to keep his body limber and in shape in the confidence that such research will
someday heal his spinal cord and enable him to walk again. Recently, after
more than a year of electrical stimulation to the nerve endings, he
was able to regain some (very limited) feeling and motion. |
Until recently, it was also accepted theory that the brain produced no new neurons
after the first few years of life. This was disproved only a couple of
years ago, when it was found
that there is a natural regenerative mechanism in the mature brain in which new cells are
produced in one part of the brain and then migrate to the cerebral cortex
where they can establish
connections with older neurons. The implications are that degenerative diseases of the
brain such as MS (what Michael J. Fox has) and Alzheimer's may someday be treatable, if
not yet curable. (Other advances in neuroscience research have indicated that
introduction of fetal stem cells or even cells cloned from a healthy part of the
brain may allow for other modes of treating such degenerative brain diseases.)
I emphasized that we were discussing a sort of
stereotype of a neuron and how neurons, in general, work. There are many
different sizes, shapes and varieties of neurons, and showed a number of
different types, such as sensory neurons bringing messages to the brain, motor neurons
telling the muscles of the body what to do, or interneurons, connecting
neurons such as the ones that connect the sensory and motor neuron in the
spinal column in carrying out a reflex arc. For the home work, a good
paragraph on the how a neuron works will cover both the electrical processes within the
neuron and the chemical process by which the neuron receives information fro
other neurons and then sends messages to other
neurons or to muscle cells. Make sure, when you write the
paragraph, that you
make your points clear to someone who knows nothing about the subject:
be very detailed and don't assume your reader knows what you are talking about.
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The story told in class as an illustration
of how the nervous system works involved:
I was weeding a garden area under a flowering bush by a busy
street intersection as part of a volunteer project and was suddenly stung
on the hand and then found I was being stung all over, as I had disturbed
a nest of yellow-jacket wasps. First I jerked my hand back, then jumped
away from the bush out into the street, then back off the street and ran
away from that street corner, swatting at the wasps that were clinging to
my jeans, still stinging. I finally killed them all and eventually went
back to weeding with the other volunteers (but not near that same
bush!)
I used this incident to illustrate the reflex arc, the informing of the
brain, the brain's sensing- figuring out- responding aspects to the
situation (realizing I had been stung, that there were many more wasps
stinging to escape from and that I couldn't run out into the street,
deciding where to run, and then directing my body to respond to get away
from the wasps) as well as triggering the sympathetic system so that I
would be able to do so, and, last, once the brain was aware that the
crisis was over, the parasympathetic responses. |
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Structures and function of the brain. The brain as a whole is made up of
three main parts or regions, the hindbrain, the midbrain and the forebrain.
The brain has only one brainstem and midbrain, but the forebrain is
divided into two separate but almost identical halves called hemispheres.
These two halves are linked by a dense structure, the corpus callosum,
which is formed of nerve fibers and glial cells and allows the two halves to communicate.
Regions or areas of these hemispheres are then referred to as lobes.
The functions of the three major regions of the brain:
- Hindbrain: lower brain stem (pons and medulla) plus
cerebellum
Cerebellum
coordinates muscle contractions and maintains muscle tone (by regularly sending
stimulating messages to muscle fibers at rest), as well as storing and activating complex
coordinated muscle activities such as walking, riding a bike, even knitting a sweater -
any skills or habits dependent on a complex series of coordinated muscle contractions that
are learned 'by heart' (you don't have to think about it, just do it).
Medulla controls, maintains and coordinates the
vital life-sustaining ('vegetative') functions (heartbeat, breathing, digestion, etc and
reflexes (coughing, sneezing, etc) of the body. 'Pons' means 'bridge'; its'
transverse nerve fibers connect the cerebellum with the rest of the brainstem and also
contains part of the Reticular Activating System. Reticular Activating System
(RAS). This system extend from the midbrain down
through the lower brainstem, and receives information from all the sensory systems (except
smell) and controls states of consciousness, such as degree of alertness or arousal, and
sleep. This primitive system monitors incoming information and responds to sensory
stimuli with the appropriate level of
alertness demanded by incoming stimuli (Example: new mother, exhausted,
sleeps through the fire engine roaring past her bedroom window but wakes
instantly to the first soft fussing of the baby).
- The midbrain is actually the upper terminal of the brainstem; it contains the
upper portion of the RAS and is the 'switching station' that controls and directs, and
sometimes blocks, messages to the cerebrum.
- The forebrain consists of inner, more primitive structures and the
outer cortex, a
complexly interwoven and convoluted surface or outer layer of neurons that
make possible a wide range of
higher-order functions, from making sense of sensory input ('seeing' or 'hearing), to the
conversion of incoming sensory stimuli into patterns that have meaning, to coordinating
complex behaviors such as speech and to cognitive functions such as learning,
remembering, problem solving, creative thinking and self-awareness. The size
of this outer cortex is what determines intelligence and learning capacity,
not the overall size of the brain. some other mammals have larger brains,
but their outer cortex is smoother, less convoluted, as it has less surface
area. It is the size of the skull that limits the overall size of the brain,
but our brain deals with this constricted space by folding in on itself many
times over. (Also, humans are 'neotenous'; they are not as fully formed and
fully functional at birth as many other mammalian species, and their brains
continue to grow at a prodigious rate following birth. The human brain is
full-sized, although not yet functioning at an adult level, at about three
years of age.)
The forebrain's inner, more primitive structures are related to basic needs,
drives, and emotions as well as memory. They include the various
structures of the limbic system, which is involved in emotions and memory,
the thalamus which acts as a complex information exchange between the senses
and the outer cortex, and the hypothalamus, which monitors hormonal levels
and sends messages to the endocrine system. (Many textbook show the
pituitary gland, which is located immediately under the hypothalamus, as
part of the brain structures. It is not. It is the master gland of the
pituitary system and is included because it is directly connected to the
hypothalamus by a duct that conveys the messenger neurotransmitters from the
neurons of the hypothalamus.
The two hemispheres of the forebrain are mirror structures, linked by
the corpus callosum, a dense network of nerve fibers that cocarry message
between the two halve of the brain. Each hemisphere has
inner structures that curve around the top of the brain stem or
hindbrain, which is the 'oldest' part of the forebrain, and the outer layer,
the cortex. This outer layer, or cerebrum, has four areas labeled lobes (one
of each lobe per hemisphere) which are usually associated with specific
functions:
- Occipital lobes: primary visual cortex where information from visual sensory neurons
first processed.
- Temporal lobes: primary auditory processing, Also, deep within the
temporal lobes lies the hippocampus and amygdala which are part of
the limbic system and are critical to the formation of long-term memories.
- Parietal lobes: contains primary somatosensory cortex,
where incoming sensations from the body are processed, lies along front area
of the parietal lobe. The parietal lobes also have large areas of association.
- Frontal lobes:contains primary motor cortex
alongside the motor cortex which directs the body's muscles. The
frontal lobes carry out the rational functions of the human mind. They are
what allow us to live in rule-governed societies, to understand symbols and
carry out abstract thought processes. They are the 'seat' of personality and
of moral behavior.
Hemispheric specialization: The frontal lobes are essential memory, thinking and problem solving, social
behavior, complex emotions and personality. The division of the functions between the two
hemispheres is marked in that the left hemisphere in 95% of (English
speakers) is used for language, math, time and rhythm as well as other kinds
of sequential and logical thinking, and dominance of many muscle skills on the right
side of the body ('right handedness'). (However, 'handedness' is not completely correlated
with hemispherical dominance.) The right hemisphere is usually is very limited in language
understanding and production, but excels at creative, wholistic approaches to the
challenges of the world, such as drawing pictures, recognizing faces and other patterns,
etc. It is also more sensitive to and expressive of emotions. (I discussed a
recent study that looked at infants movements of the mouth and how they
differ, depending on whether the infant is 'babbling' (language related
sounds) or cooing/crying (emotion related sounds. The study found a
correlation between the type of sound ( content-related) and the mouth
movements that were controlled by that (opposite) hemisphere of the brain.
(Left side of mouth correlated with 'emotional' sounds, right side with
'babbling' or first attempts at language.)
The 'plasticity' of the brain:
The somatosensory and motor cortex vary in the number of sensory
and motor neurons involved and the amount of each of these brain area devoted to the various body parts. Areas of
the body that are more sensitive or are capable of more complex motor behaviors have more of
these cortical areas devoted to them: the fingers, for example, take up much more neuronal
'space' than the toes, as they are more sensitive and also can carry out more fine motor skills.
However, damage or lack of development in one area and usage of another to compensate can
result in changes in the structure and function of other areas of the brain. Again, 'Use it or lose it';
the brain is plastic and responds to changes in use by changes in structure and
function.
Studies have
shown that many aspects of brain structure and function are affected by
experience. For instance, how your brain works depends on your native language,
the one you first learned as a baby. A baby's exposure to the Japanese language, which is primarily vowel sounds, results in
a different usage of the right hemisphere in the production and processing of language
than that of native English speakers, who depend upon the left hemisphere for language. This is NOT
due genetic heritage as American children
of purely Japanese heritage show the English speaker's brain use patterns. .
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