CONTENTS:
I. The Nervous System
II. Cells in the Nervous System
III. Neurotransmitters
DISCUSSION:
I. The Nervous System
The nervous
system is the body’s
electrochemical communication circuitry. The field that studies the nervous
system is called neuroscience,
and the people who study it are called neuroscientist.
Characteristics of Nervous System
a. Complexity - due to the orchestration of the billion of cells in the brain and nervous
system, the individual can do complex or different kinds of activities.
b. Integration - the ability of the
brain to pull information together
c. Adaptability - although the composition of
the brain and the nervous system have hereditary foundation, both have the
ability to constantly adapt to the changes in the body and the
environment.
d. Plasticity
- denotes the brain’s special capacity modification and change.
e. Electrochemical
transmission - the
brain being the information processing system, powered by electrical
impulses and chemical messages allows the individual to perceive and respond
stimuli.
Organization
of the Nervous System
The
nervous system is organized into two main parts:
1. The central
nervous system (CNS), encased in bone, consists of the brain and spinal cord.
The CNS is the nervous system’s central executive.
2. The peripheral
nervous system extends throughout the body and relays information to and from
the brain.
I.A. The Central Nervous System
The
CNS performs different functions through different networks of neurons.
Clusters of neurons are called nuclei and pathways that connect the networks
are bundles of axons called fiber tracts.
I.A.1. The Brain
1. The Hindbrain -
is found just above the spinal cord and is composed of the following structures:
a. The medulla
controls vital life functions (e.g., blood pressure, heart rate, and
breathing).
b. The reticular
formation is a web of neurons is involved in arousal and attention.
c. The cerebellum
coordinates fine motor movements, stores a memory code for well-rehearsed
behaviors, and participates in cognitive tasks such as reading.
2. The
Midbrain - relays information from the eyes, ears, and skin and controls certain types
of automatic behaviors. The midbrain and its connections to the forebrain
permit the smooth initiation of movement. The midbrain is connected to the brainstem which is the
posterior part of the brain, adjoining and
structurally continuous with the spinal cord. Reticular formation is a
region in the brainstem that
is involved in multiple tasks such as regulating the sleep-wake cycle and filtering incoming stimuli to discriminate
irrelevant background stimuli.
3. The Forebrain - the largest part of the brain regulates many complex aspects of behavior and mental
phenomena. Interior structures include the following:
a. thalamus- processes inputs from sense organs (except for smell) and then relays sensory information to appropriate “higher” forebrain areas. It is the primary sensory relay
into the rest of the brain. The brain’s “clock” that sets biological rhythms
for the body.
b. hypothalamus- is a portion of the brain that contains a number of small nuclei with
a variety of functions. Its functions are: (1) link the nervous
system to the endocrine
system via the pituitary
gland (hypophysis) and; (2) control body
temperature, hunger, important aspects of parenting
and attachment behaviors, thirst, fatigue, sleep, and circadian
cycles.
c. The limbic
system includes the amygdala and the hippocampus. The
amygdala is involved in memory and emotion. It links different kinds of sensory
information together in memory. The amygdala also plays a role in fear and
other emotions, linking emotions to sensations. The hippocampus is critical to
the ability to form new memories.
d. The cerebral cortex,
is a thin sheet of neurons comprising the forebrain’s outer surface. It folds in itself, giving the
brain a wrinkled appearance. The cerebral cortex is divided
down the middle, creating two halves called the left and right cerebral
hemispheres. The corpus callosum connects the two halves. The folds of cortex
produce gyri (ridges) and sulci,or fissures (valleys or wrinkles), on the
brain’s outer surface. Several deep sulci make convenient markers for dividing
the cortex of each hemisphere into four
anatomical areas: the frontal, parietal, occipital and temporal lobes.
d. 1. The Frontal Lobe of the brain is located deep to
the Frontal Bone of the skull. It plays an integral role in the following
functions/actions: Memory formation, Emotions, decision making/reasoning and
personality.
d. 2. The Parietal Lobe of the brain is located deep to
the Parietal Bone of the skull. It plays a major role in the following
functions/actions: Senses and integrates sensation(s), Spatial awareness and
perception (Proprioception - Awareness of body/ body parts in space and in
relation to each other).
d. 3. Occipital Lobe involves two major parts: (1) Primary Visual Cortex – the primary area of the brain responsible
for sight -recognition of size, color, light, motion, dimensions, etc; and (2) Visual Association Area – interprets information acquired through the primary
visual cortex.
d. 4. The Temporal Lobes are located on the sides of the
brain, deep to the temporal bones of the skull. They play an integral role in
the following functions: Hearing, Organization/Comprehension
of language, Information Retrieval
(Memory and Memory Formation).
I.A.2.The Spinal Cord
It receives and sends signals to and from the brain. There are 31
pairs of spinal nerves which run through the spinal cord. These nerves are
called “mixed” nerves because each nerve contains a sensory and a motor axon. The
spinal cord can also be a minor coordinating centre for some simple reflexes
like the withdrawal reflex.
Meanwhile, reflexes are simple, involuntary behaviors controlled by spinal cord neurons, without
requiring instructions from the brain. These are controlled by a feedback system. Information about the
consequences of an action goes back to the source of the action for further
adjustment, if necessary.
I.B. The Peripheral Nervous System
It has two subsystems:
I. B. 1. The
Somatic Nervous System - carries signals between the senses and CNS and between the CNS
and skeletal muscles. Sensory
neurons bring information to the brain, and motor neurons send information from
the brain to the muscles.
I. B. 2. The
Autonomic Nervous System - carries messages between the CNS and the heart, lungs, and
other organs and glands. The ANS has two divisions that may act on the same body areas, with their relative “balance”
regulating the state of the targeted organs:
I.B.2.a. The sympathetic
system directs the body to spend energy (e.g., increased heart rate, faster
breathing, sweating, sometimes called the fight-or-flight” response) to react
to stress.
I.B.2.b. The parasympathetic
system directs the body’s functions to conserve
energy (e.g., slower heart rate, increased digestive activity). Parasympathetic
activity helps “calm” a person after increased sympathetic arousal.
II. Cells in the Nervous System
There are two main cell types in the nervous system. Neurons (also known as nerve cells) are specialized to respond rapidly to signals
and send signals of their own while glial
cells provide energy, help restore damage, and respond to signals from neurons. These cells have some features in common. They both have an outer membrane that selectively allows only some
substances to pass in and out. The only notable differences between neurons and glial cells are neurons' possession of axons and dendrites, and capacity to generate action potentials.
Moreover, neurons have: cell body (also known as soma) which contains the nucleus; mitochondria turn oxygen and glucose into energy; axon which is a cell fiber that carries signals away from
the cell body and a dendrite which is a cell fiber that receives signals from
other neurons and carries information toward the neuron’s cell body. Most
neurons have one axon but have many dendrites. Some axons are wrapped in a myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann cells which serve as supportive, nutritive, and service facilities
for neurons. The gap between Schwann cells is known as the node of Ranvier, and serves as points along the neuron for generating a signal.
 |
| http://brainu.org/files/tn_about_neurons.jpg |
As mentioned, neurons have special features that permit effective
signal communication and they have the capacity to generate action potentials. Action potentials are electrochemical pulses that shoot
down the neuron’s axon. They are “all-or-none” which means that a neuron either fires an action
potential at full strength or does not fire at all. After an action potential, there is a brief recovery
time called a refractory period, during which a neuron cannot fire another
action potential. The speed of an action potential depends on the
thickness of the axon and on the presence of myelin sheath, a white, fatty substance
that speeds up action potentials. At the axon endings, the action potential causes bag-like vesicles
to release stored chemicals called neurotransmitters into a space between the
two neurons. This space is called a synapse, a connection that is a
narrow gap separating the axon of one neuron from the dendrites of another. It
is the means by which two neurons communicate. Released neurotransmitters “float” across the synapse to
“bind” with receptors and proteins on a dendrite of a receiving neuron. The
interaction between neurotransmitters and receptors is very specific, like a
lock and key. This interaction creates a signal called a postsynaptic
potential (PSP) that might make action potentials in the receiving, or
postsynaptic, neuron either more or less likely. A number of PSPs sum together
at the junction of the cell body and the axon. Whether or not an action
potential “fires” depends on the kind of signals that are most numerous.
Neurons have three kinds (as taken from wikipedia.com):
1. Sensory neurons are neurons responsible
for converting various external stimuli that come from the environment into
corresponding internal stimuli. They are activated by sensory input, and send
projections to other elements of the nervous system,
ultimately conveying sensory information to the brain or spinal cord. Unlike
neurons of the central nervous system, whose inputs come from other neurons,
sensory neurons are activated by physical modalities such as visible light,
sound, heat, physical contact, etc., or by chemical signals for example in the
case of smell or taste.
2. Motor neuron (or motoneuron)
classically applies to neurons located
in the central nervous
system (CNS) that project
their axons outside
the CNS to directly or indirectly control muscles. Motor neurons are efferent nerves also called effector neurons that carry signals from
the spinal cord to the muscles to produce (effect) movement.
3. An interneuron (also called relay neuron, association neuron, connector neuron or local circuit neuron) is a neuron that
forms a connection between other neurons. Interneurons are neither motor nor sensory. The
term is also applied to brain and spinal cord neurons whose axons connect
only with nearby neurons, to distinguish them from "projection"
neurons, whose axons (projection fibers) project to more distant regions of the brain or spinal cord.
III. Neurotransmitters
There are about 100 neurotransmitters
that have been identified (which means some are still undiscovered). A group of neurons that communicate using the same neurotransmitter
is called a neurotransmitter system. Neurotransmitters are chemical messengers that traverse the synaptic gaps between neurons. When released, these travel across the synapse and bind to receptor sites on the receiving neuron, thereby influencing whether it will generate a neural impulse. Below are some of its types:
a) Acetylcholine is used by sets of
neurons involved in controlling movement of the body, in making memories, and
in slowing the heartbeat and activating the digestive system. Alzheimer’s
disease may result from disruptions of this system.
b) Norepinephrine
affects arousal, wakefulness, learning, and mood. Disruptions of this system
have been linked to depression.
c) Serotonin affects sleep, mood,
aggression, and impulsive behaviors. Serotonin levels can be affected by what
is eaten.
(1)
Malfunctions in serotonin systems can result in mood and appetite problems seen
in some types of obesity, premenstrual tension, and depression.
(2) Antidepressant medications such as Prozac,
Zoloft, and Paxil are thought to act on serotonin systems to relieve some of
the symptoms of depression.
d) Dopamine is used by sets of neurons
involved in controlling movement, and damage to these systems contributes to
shakiness experienced by people with Parkinson’s disease. Other dopamine
systems are involved in the experiencing of reward, or pleasure, which is vital
in shaping and motivating behavior. Certain other dopamine systems are
suspected to be responsible for the perceptual, emotional, and thought
disturbances associated with schizophrenia.
e) GABA (gamma-amino butyric acid) is the
main inhibitory neurotransmitter in the brain—it slows down the brain’s neural
activity.
(1)
Some drugs amplify the inhibitory action of GABA. One example is alcohol, which
results in impairments of thinking, judgment, and motor skills. Drugs that
interfere with GABA’s inhibitory effects produce intense repetitive electrical
discharges, known as seizures
(2)
Impaired GABA systems are thought to contribute to severe anxiety, Huntington’s
disease, and epilepsy.
f. Endorphins
- natural opiates that mainly stimulates the firing of neurons. It shields
the body from pain and elevates feelings of pleasure.
References:
Carlson,
N. R. (1997). Psychology: the science of behavior. Allyn & Bacon
Gazzaniga,
M. S., Ivry, R. B., and Mangun, G. R. (1998). Cognitive neuroscience: the
biology of the mind. New York,
NY: W. W. Norton & Company,
Inc.
Goldstein,
E. B. (1999). Sensation and perception (5th ed.). Pacific Grove, CA:Brooks/Cole
Pub.
Kolb,
B. and Wishaw, I.Q. (1996). Fundamentals of human neuropsychology (4th ed.). New York, NY:
Freeman.
Rosenzweig,
M. R., Leiman, A.L., and Breedlove, S. M. (1999). Biological psychology: An
introduction to behavioral, cognitive, and clinical neuroscience (2nd ed.). Sunderland, MA:
Sinauer Associates, Inc.
http://serendip.brynmawr.edu/-
Articles and links on the brain and behaviour.
http://www.neuroguide.com/
- Links to journals, images, and
resources.
http://anatomy.umas.edu/HTMLpages/anatomyhtml/neuro_atl
as.html – Complete pictorial atlas of the brain.
http://www.cc.emory.edu/ANATOMY/AnatomyManual/nervous_system.html
– Illustrated tutorial of the nervous system
http://en.wikipedia.org/wiki/Neuroglia
http://www2.estrellamountain.edu/faculty/farabee/biobk/biobooknerv.html
