I)
The neuron: the basic unit of communication
A)
Basic definitions
1)
Communication
throughout the nervous system takes place via neurons, the highly specialized
cells that receive and transmit information from one part of the body to
another.
2)
The human nervous
system also includes other types of specialized cells, called glial cells,
which provide structural support, nutrition, removal of cell wastes, and
speedier communications between neurons. Glia is
Greek for “glue,” and it was once believed that they were the “glue” that held
the neurons of the brain together.
3)
There are three
basic types of neurons.
(a)
Sensory neurons
convey information to the brain from specialized receptor cells in the sense
organs, the skin, and the internal organs.
(b)
Motor neurons
communicate information to the muscles and glands of the body.
(c)
Interneurons
communicate information from one neuron to the next; they are the most common
type of neuron found in the body.
B)
Characteristics
of the neuron-most neurons have three basic components.
1)
The cell body
contains the nucleus, which provides energy for the neuron to carry out its
functions.
2)
Dendrites are
short, multiple fibers that extend from the neuron’s cell body and that receive
information from other neurons or sensory receptor cells.
3)
The axon is the
long, fluid-filled tube that carries a neuron’s messages to other neurons. Most
axons are very small, whereas others, such as the axon of the motor neuron that
controls the big toe, may be several feet long.
(a)
Many axons are
surrounded by a myelin sheath, a white, fatty covering that increase the
neuron’s communication speed; manufactured by special glial cells.
C)
The all-or-none
law is the principle that either a neuron is sufficiently stimulated and an
action potential occurs or a neuron is not sufficiently stimulated and an
action potential does not occur.
D)
Communication
between neurons: bridging the gap
1)
The point of
communication between two neurons is called the synapse.
(a)
The
message-sending neuron is the pre-synaptic neuron.
(b)
The
message-receiving neuron is the post-synaptic neuron.
(c)
Synaptic gap: the
tiny, fluid-filled space between the axon terminal of one neuron and the
dendrite of an adjoining neuron.
2)
Transmission of
information between neurons occurs in one of two ways.
(a)
Electrical: ion
channels bridge the narrow gap between neurons; communication is virtually
instantaneous.
(b)
Chemical: a
chemical substance diffuses across the synaptic gap from the presynaptic neuron
to the postsynaptic neuron (this is the most common form of communication).
(a)
Action potential
arrives at the axon terminals, branches at the end of the axon that contain
tiny pouches or sacs called synaptic vesicles, which contain chemicals called
neurotransmitters.
(b)
The synaptic
vesicles release neurotransmitters, special chemical messengers manufactured by
a neuron that communicate information to other neurons and to the muscles, into
the synaptic gap.
(c)
Synaptic
transmission defines the process in which neurotransmitters are released by one
neuron, cross the synaptic gap, and affect surrounding neurons by attaching to
receptor sites on their dendrites..
(d)
After synaptic
transmission, the following may occur.
(i)
Reuptake: the
process by which neurotransmitter molecules detach from a postsynaptic neuron
and are reabsorbed by a presynaptic neuron so they can be recycled and used
again.
(ii)
Enzymatic
destruction or breakdown.
(e)
Each
neurotransmitter has a chemically distinct, different shape. For a
neurotransmitter to affect a neuron, it must exactly match the shape of the
receptor site.
E)
Neurotransmitters
and their effects: the activation of most neurons is determined by the messages
delivered by neurotransmitters.
1)
The first
neurotransmitter discovered was acetylcholine, which produces muscle
contractions and is involved in memory, learning, and general intellectual
functioning. Levels of acetylcholine are severely reduced in people with
Alzheimer’s disease.
2)
The
neurotransmitter dopamine is involved in movement, attention, learning, and
pleasurable or rewarding sensations.
(a)
Degeneration of
neurons that produce dopamine in one brain area causes Parkinson’s disease.
Symptoms of Parkinson’s disease can be alleviated by a drug called L-dopa,
which converts to dopamine in the brain.
(b)
Excess dopamine
production in another part of the brain is involved in the hallucinations and
perceptual distortions that characterize schizophrenia.
3)
The
neurotransmitter serotonin is involved in sleep, moods, and emotional states,
including depression. Anti-depressant drugs such as Prozac increase the
availability of serotonin in certain brain regions.
4)
The
neurotransmitter norepinephrine seems to be involved in the activation of
neurons throughout the brain; it assists in the body’s response to danger or
threat, as well as in learning and memory retrieval.
5)
GABA is the
abbreviation for gamma-aminobutyric acid, a neurotransmitter that usually
communicates an inhibitory message to other neurons, helping to balance and
offset excitatory messages. Anti-anxiety medications such as Valium and Xanax
wor
6)
Endorphins are
neurotransmitters that regulate pain perception. They are also released in
response to stress or trauma and are associated with positive mood and aerobic
exercise. These are the brain’s natural opiates.
(a)
Endogenous-produced
internally in the body.
(b)
Acupuncture-an
ancient Chinese medical technique that involves inserting needles at various
locations in the body.
II)
The nervous
system and the endocrine system: communication throughout the body. The nervous
system is the complex, organized communication network of nerves; it is divided
into the central nervous system and the peripheral nervous system. Nerves are
large bundles of neuron axons, located in the peripheral system.
A)
The central
nervous system
1)
The central
nervous system consists of the brain and the spinal cord, which are suspended
in cerebrospinal fluid for protection.
2)
Spinal reflexes
are simple, automatic behaviors that are processed in the spinal cord without
any brain involvement.
3)
One of the simplest
spinal reflexes involves a three-neuron loop of rapid communication-a sensory
neuron that communicates sensation to the spinal cord, an interneuron that
relays information within the spinal cord, and a motor-neuron leading from the
spinal cord that signals muscles to react.
B)
The peripheral
nervous system : the peripheral nervous system
includes all the nerves lying outside the central nervous system; it is divided
into the somatic nervous system and the autonomic nervous system.
1)
The somatic
nervous system communicates sensory information received by sense organ
receptors to the central nervous system and carries messages from the central
nervous system along motor nerves to the muscles.
2)
The autonomic
nervous system regulates involuntary functions such as heartbeat, blood
pressure, breathing, and digestion; its two branches are the sympathetic
nervous system and parasympathetic nervous system.
(a)
The sympathetic
nervous system produces rapid physical arousal in response to perceived threats
or emergencies, of in response to emotions such as anger or anxiety.
Physiological changes collectively represent the fight-or-flight response- they
physically prepare you to fight or flee from a perceived danger.
(b)
The
parasympathetic nervous system maintains normal bodily functions and conserves
the body’s physical resources.
C)
The endocrine
system is a group of glands located throughout the body that secrete hormones
into the bloodstream. It is a relatively slow-acting communication system.
1)
Hormones are
chemical messengers that influence many physical and behavioral processes.
2)
A brain structure
called the hypothalamus is the main lin
3)
The pituitary
gland is an endocrine gland attached to the base of the brain that secretes
hormones that affect the functioning of other glands as well as hormones that
act directly on physical processes.
4)
The paired
endocrine glands that are involved in the human stress response are called the
adrenal glands.
(a)
The adrenal
cortex is the outer portion of the adrenal gland.
(b)
The adrenal
medulla is the inner portion of the adrenal gland; it secretes epinephrine (or
adrenaline) and norepinephrine.
D)
Imaging the brain
1)
The
electroencephalograph (EEG) is an instrument that uses electrodes placed on the
scalp to record the brain’s electrical activity.
2)
The computerized
axial tomography scan, called CAT or CT scan, produces pictures of the brain
structures from any angle using multiple x-rays that are reassembled by a
computer.
3)
Magnetic
resonance imaging (MRI) provides highly detailed views of the brain using
electrical signals generated by the brain in response to magnetic fields.
4)
The positron
emission tomography scan PET scan, provides color-coded images of brain
activity by measuring the amount of radioactively tagged compounds such as
glucose, oxygen, or a particular drug used in different brain regions.
5)
Functional
magnetic resonance imaging (FMRI) tracks changes in the brain’s blood flow as
measured by fluctuation in blood oxygen levels.
E)
The forebrain is
the largest and most complex brain region. The cerebral cortex is the
quarter-inch thick, wrinkled outer portion of the forebrain; it is grayish in
color-which is why it is sometimes referred to as gray matter. It contains the
most sophisticated brain centers for complex behaviors and mental processes.
Extending inward from the cerebral cortex are white myelinated axons that are
sometimes referred to as white matter.
1)
The two divisions
of the cerebral cortex are the cerebral hemispheres.
2)
The corpus
callosum is a thic
3)
The hippocampus
is a large structure embedded in the temporal lobe that plays a role in the
ability to form new memories. Neurogenesis was first detected in this area.
4)
Aphasia is the
partial or complete inability to articulate ideas or understand spoken or
written language due to brain injury or damage.
5)
Researchers once
believed that by early adulthood the brain’s physical structure was hardwired,
or fixed for life. Now we know that exposure to stimulating or unstimulating
environmental conditions actually causes some brain structure to change
physically- a phenomenon called structural plasticity.
Neurotransmitter function deficit
surplus agonist antagonist
|
Acetylcholine |
Excitatory: controls muscle
contraction; involved in memory formation in (hippocampus) |
Alzheimer’s |
|
Nicotine |
curare |
|
Dopamine |
Inhibitory: involved in
voluntary muscle movements, learning, memory, and emotional arousal. |
Parkinson’s |
Schizophrenia |
l-dopa |
Phenothiazine |
|
Serotonin |
Inhibitory: involved in
emotional states, sexual behavior, pain, perception, and sleep. |
Anxiety, mood disorders,
insomnia |
Autism |
LSD |
Prozac |
|
Endorphins |
Inhibitory: involved in pain perception and positive
emotions. |
Body experiences pain. |
Body may not give adequate
warning about pain |
Opiates |
|
|
Norepinephrine |
Excitatory: involved in
increasing heartbeat, arousal, learning, memory, and eating |
Depression |
Anxiety |
Lithium |
Caffeine, cocaine, amphetamines |
|
GABA |
Inhibitory:associated with allergies |
Anxiety |
Sleep and eating disorders |
Anti-histamines |
|