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NERVOUS SYSTEM NOTES
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I. Neurons: The Communication Specialists
A. The neuron, or nerve cell, is the basic unit of communication in vertebrate nervous systems.
1. Three classes of neurons work together:
a. Sensory neurons are receptors for specific sensory stimuli.
b. Interneurons in the brain and spinal cord integrate input and output signals.
c. Motor neurons send information from integrator to muscle or gland cells (effectors).
2. Neuroglial cells physically support, protect, and help maintain proper ion concentrations around the
neurons.
a. Schwann cells insulate lengthy axons.
B. Functional Zones of a Neuron
1. The cell body contains the nucleus and metabolic machinery for protein synthesis.
2. Dendrites are numerous, usually short extensions that receive stimuli (input zones).
3. An axon is usually a single, rather long extension (conducting zone) that transmits impulses to other cells
(output zones); electrical signals are initiated at the axon hillock (a trigger zone), the place where the axon
and cell body are joined.
C. A Neuron at Rest, Then Moved to Action
1. A neuron at rest maintains a voltage difference across the plasma membrane, called the resting
membrane potential.
2. A strong signal causes an action potential–an abrupt, brief reversal of the voltage difference across the
plasma membrane.
D. Restoring and Maintaining Readiness
1. Neurons maintain voltage differences because:
a. The lipid bilayer restricts passage of sodium, potassium and other ions.
b. Channel proteins control ion flow; some are always open, others are gated.
2. When a neuron is in between action potentials:
a. There are more potassium ions inside the membrane than outside and more sodium ions outside than
inside.
b. Pump proteins use ATP to actively pump potassium ions in and sodium ions out of the neuron to
keep the concentration of sodium ions higher outside.
II. Action Potentials
A. Approaching Threshold
1. When a stimulus reaches a certain minimum–a threshold, gated channels open and sodium rushes in.
a. The cytoplasmic side of the membrane becomes more positive.
b. In an accelerating way, more and more gates open (example of positive feedback).
2. A brief refractory period follows–sodium gates shut, potassium gates open–during which the membrane
is insensitive to stimulation.
III. High-Speed Signals Along Sheathed Axons
A. Many axons are covered by a myelin sheath derived in part from Schwann cells.
1. Each section of the sheath is separated from adjacent ones by a region (node of Ranvier) where the axon
membrane (plentiful in gated sodium channels) is exposed.
B. The myelin sheath has other functions:
1. It saves energy because the axon's ion pumps need only operate at the exposed nodes.
2. Myelination helps repair damaged neuron extensions in the peripheral nervous system.
IV. Chemical Synapses
A. A chemical synapse is a junction between a neuron and an adjacent cell, separated by a synaptic cleft into
which a transmitter substance is released.
1. The neuron that releases the transmitter into the cleft is called the presynaptic cell.
2. The transmitter binds to receptors on the membrane of the postsynaptic cell to either excite or inhibit the
membrane.
B. A Smorgasbord of Signals
1. Neurotransmitters can have excitatory or inhibitory effects:
a. Acetylcholine is the transmitter in neuromuscular junctions, glands, the brain, and the spinal cord.
b. Serotonin acts on brain cells to govern sleeping, sensory perception, temperature regulation, and
emotional states.
c. Norepinephrine affects brain regions concerned with emotions, dreaming, and awaking.
d. Dopamine and GABA are other neurotransmitters that are present in different parts of the brain.
e. Two debilitating diseases, Parkinson's and Alzheimer's, are testimony to the effects that the loss of
neurotransmitters can effect.
2. Neuromodulators can magnify or reduce the effects of a neurotransmitter.
a. These substances enhance or reduce membrane responses in target neurons.
b. Endorphins are peptides that act as potent painkillers within the brain and may have roles in
memory, learning, temperature regulation, sexual behavior, and emotional depression.
C. Removing Neurotransmitters from the Synaptic Cleft
1. Neurotransmitters must be removed from the synaptic cleft to discontinue stimulation.
2. There are three methods of removal:
a. Some amount of transmitter simply diffuses out of the cleft.
b. Enzymes, such as acetylcholinesterase, break down the transmitters.
c. Membrane transport proteins actively pump neurotransmitter molecules back into the presynaptic
cells.
V. The Nervous System: An Overview
A. The nervous system is organized into two major divisions:
1. The central nervous system (CNS) is composed of the brain and spinal cord.
2. The peripheral nervous system (PNS) includes all the nerves that carry signals to and from the brain and
spinal cord.
B. General Paths of Information Flow
1. Through synaptic integration, signals at any neuron can be reinforced, dampened, sent on, or suppressed.
2. The direction in which a given signal will travel depends on the organization of neurons in different body
regions.
3. The brain deals with its many neurons in regional blocks.
a. Divergent circuits fan out from one block into another.
b. In convergent circuits, signals from many neurons are sent on to only a few other cells.
c. In reverberating circuits, neurons repeat signals among themselves.
4. Signals between brain or spinal cord and body regions travel by nerves.
a. Axons of sensory neurons, motor neurons, or both, are bundled together in a nerve.
b. Within the brain and spinal cord, bundles of interneuron axons are called nerve tracts.
5. The sensory and motor neurons of many nerves take part in reflexes, which are stereotyped movements
made in response to sensory stimuli.
VI. The Peripheral Nervous System
A. Somatic and Autonomic Subdivisions
1. The somatic nerves relay commands to and from skeletal muscles (voluntary control).
2. The autonomic nerves send signals to and from smooth muscles, cardiac muscle, and glands (involuntary
control).
B. Sympathetic and Parasympathetic Nerves
1. Parasympathetic nerves tend to slow down body activity when the body is not under stress.
2. Sympathetic nerves increase overall body activity during times of stress, excitement, or danger; they also
call on the hormone epinephrine to increase the "fight-flight" response.
VII. The Central Nervous System
A. The Spinal Cord
1. The spinal cord lies within a closed channel formed by the bones of the vertebral column.
2. The spinal cord (and also the brain) is covered with tough membranes–the meninges.
3. Signals move up and down the spinal cord in nerve tracts, which are bundles of sheathed axons.
4. The spinal cord is a pathway for signal travel between the peripheral nervous system and the brain; it
also is the center for controlling some reflex actions.
a. Spinal reflexes result from neural connections made within the spinal cord and do not require input
from the brain, even though the event is recorded there.
b. Autonomic reflexes, such as bladder emptying, are also the responsibility of the spinal cord.
B. The Brain
1. The body’s master control panel, the brain, is a continuation of the anterior end of the spinal cord; it is
protected by meninges (such as the dura mater), bones, and cerebrospinal fluid.
2. Hindbrain
a. The medulla oblongata has influence over respiration, heart rate, swallowing, coughing, and
sleep/wake responses.
b. The cerebellum acts as reflex center for maintaining posture and coordinating limbs.
c. The pons ("bridge") possesses nerve tracts that pass between brain centers.
3. Midbrain
a. The midbrain has a roof of gray matter, the tectum, where visual and sensory input converges before
being sent to higher brain centers–a sort of "switching center."
b. The midbrain, pons, and medulla together represent the brainstem, within which is a network–the
reticular formation, that coordinates muscles and higher brain activities.
4. Forebrain
a. The forebrain is the most developed portion of the brain in humans and "higher" animals.
b. The cerebrum integrates sensory input and selected motor responses.
c. The thalamus (below cerebrum) relays and coordinates sensory signals.
d. The hypothalamus monitors internal organs and influences responses to thirst, hunger, and sex.
VIII. Other Aspects of CNS Structure
A. The Cerebral Hemispheres
1. The human cerebrum is divided into left and right cerebral hemispheres, which communicate with each
other by means of the corpus callosum.
a. Each hemisphere can function separately; the left hemisphere responds to signals from the right side
of the body, and vice versa.
b. The regions responsible for verbal skills reside in the left side; nonverbal skills such as music,
mathematics and abstract abilities reside in the right.
2. The thin surface (cerebral cortex) is gray matter, divided into lobes by folds and fissures.
3. Its white matter consists of major nerve tracts that permit communication between hemispheres and the
body.
B. Brain Cavities and Canals
1. The brain and spinal cord are surrounded by the cerebrospinal fluid, which fills four cavities (ventricles)
within the brain.
2. A mechanism called the blood-brain barrier controls which substances will pass to the fluid and
subsequently to the neurons.
a. Tight junctions between endothelial cells of the capillaries force materials to pass through the cells,
not around them.
b. Lipid-soluble substances, such as alcohol, nicotine, and drugs, diffuse quickly through the lipid
bilayer of the plasma membrane.
IX. Focus On Our Environment: An Environmental Assault on the Nervous System
X. A Closer Look at the Cerebral Cortex
A. The cerebral cortex is a thin layer of gray matter.
1. The cell bodies of interneurons are located at or near the surface of the cerebrum.
2. The neurons of the cerebral cortex are localized in four lobes.
a. The occipital lobe in the rear has centers for vision.
b. The temporal lobe near each temple is a processing center for hearing and influences emotional
behavior.
c. The parietal lobe houses the somatosensory cortex–the main receiving area for signals from the skin
and joints.
d. The frontal lobe includes the motor cortex and the areas used in thinking and memory.
B. States of Consciousness
1. The central nervous system governs sleeping, dozing, daydreaming, and full alertness.
a. The constant electrical activity can be measured by an electroencephalogram (EKG).
b. PET scans can show the precise location of brain activity.
2. Neurons of the reticular activating system control the changing levels of consciousness by releasing
serotonin.
C. Connections with the Limbic System
1. Our emotions are governed by the limbic system, which consists of several brain regions.
2. The cerebral cortex and thalamus plus the "gatekeeper" hypothalamus are able to regulate reactions to
emotional situations.
XI. Science Comes to Life: Sperry's Split-Brain Experiments
XII. Memory
A. Memory is the storage and retrieval of information about previous experiences.
1. Association is the linkage of information in packages due to chemical and structural changes in brain
regions.
a. Short-term memory lasts from seconds to hours and is limited to a few bits of information.
b. Long-term memory is more permanent and seems to be limitless.
2. In amnesia, portions of time are lost to memory, while others may be retained.
B. Information that seems most important to the individual moves most rapidly into long-term storage.
1. Epinephrine stimulates glucose production, fueling the brain's memory work.
2. Memory information is encoded in a form resistant to degradation.
XIII. How Psychoactive Drugs Affect the Central Nervous System
A. A drug is any substance introduced into the body to provoke a specific physiological response.
1. Drug abuse is defined as use of a drug in a way that harms health or interferes with a person's ability to
function in society.
2. In general, drug abuse involves psychoactive drugs, which act on the CNS to alter mental and physical
states.
B. Drug Effects
1. As the body develops tolerance to a drug, larger and more frequent doses are needed to produce the same
effect; this reflects physical drug dependence.
2. Psychological drug dependence, or habituation, develops when a user begins to crave the feelings
associated with using a particular drug and cannot function without it.
3. Habituation and tolerance are evidence of addiction.
C. Drug Action and Interactions
1. In a synergistic interaction, two drugs used together have a much more powerful effect than they would
have if used separately.
2. Some drugs combinations are antagonistic–one drug blocks the effects of another.
3. Others are potentiating, in that one drug enhances the effects of the other.
XIV. Commonly Abused Drugs
A. Stimulants
1. Caffeine first stimulates the cerebral cortex, causing increased alertness; higher doses act at the medulla
oblongata to disrupt motor coordination and intellectual functions.
2. Nicotine mimics acetylcholine, directly stimulating receptors such as those in the brain that in turn
stimulate the adrenals to release hormones that increase the heart rate and blood pressure.
3. Amphetamines resemble dopamine and norepinephrine, causing increased alertness initially, but
increased dependence over prolonged use.
4. Cocaine stimulates the pleasure centers by blocking natural signaling molecules released at synapses;
thus the body needs a rather constant supply to achieve the "feeling."
B. Drugs That Reduce Brain Activity
1. Sedatives, hypnotics, and antianxiety drugs lower the activity of nerves and the brain.
2. Alcohol acts directly on the plasma membrane to alter cell activity leading initially to relaxation but
eventually to disorientation, diminished judgment, and uncoordinated muscular movements.
3. Barbiturates are prescribed to reduce epileptic seizures and induce relaxation; but can be used abusively
as "downers".
C. Analgesics
1. The body produces its own pain relievers, such as endorphins and enkephalins.
2. Narcotic analgesics such as codeine, morphine, and heroin are derived from opium; they have medical
uses but are habit-forming and seriously addictive.
D. Psychedelics and Hallucinogens
1. LSD affects the activity of serotonin, a brain hormone, causing increases in blood pressure as well as
hallucinations.
2. Marijuana contains the active ingredient THC, which acts as a depressant.
E. Deliriants
1. PCP leads to toxic psychosis or delirium, impairment of perceptions, and outbursts of violent behavior.
2. Inhalants of chemicals in cleaning fluids and spray paint work their magic on the brain but also lead to
toxicity in the liver, kidneys, respiratory system, and heart with longterm use.