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Chapter 2

PSY 220 Chapter Notes - Chapter 2: Retina, Endoplasmic Reticulum, Sensory Neuron

Course Code
PSY 220
Hurwitz Barry

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PSY220- Psychobiology
Main Ideas:
The nervous system is composed of two kinds of cells: neurons and glia. Only the
neurons transmit impulses from one location to another.
The larger neurons have branches, known as axons and dendrites that can change
their branching pattern as a function of experience, age, and chemical influences.
Many molecules in the bloodstream that can enter other body organs cannot enter
the brain.
The action potential, an all-or-none change in the electrical potential across the
membrane of a neuron, is caused by the sudden flow of sodium ions into the
neuron and is followed by a flow of potassium ions out of the neuron.
Local neurons are small and do not have axons or action potentials. Instead, they
convey information to nearby neurons by graded potentials.
Appendix A Brief, Basic Chemistry p. 486
Matter is composed of 92 elements that combine to form an endless variety of
An atom is the smallest piece of an element. A molecule is the smallest piece of a
compound that maintains the properties of the compound.
Atoms consist of protons, neutrons, and electrons.
The atoms of some elements can gain or lose an electron, thus becoming ions.
Positively charged ions attract negatively charged ions, forming an ionic bond. In
some cases two or more atoms may share electrons, thus forming a covalent
The principal carrier of energy in the body is a chemical called ATP (adenosine
The number of protons in the nucleus of the atom is the atomic number of the
The weight of an atom relative to the weight of one proton is the atomic weight
of the element.
Elements are materials that cannot be broken down into other materials.
Compounds are materials made up by combining elements.
Enzymes are proteins that control the rate of chemical reactions.
2.1- The Cells of the Nervous System
Anatomy of Neurons and Glia
The nervous system consists of two kinds of cells: neurons and glia.

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Neuron- receives information and transmits it to other cells
The adult human brain contains approximately 100 billion neurons.
The brain, like the rest of the body, consists of individual cells.
In the late 1800s, Santiago Ramón y Cajal used newly discovered staining techniques to
establish that the nervous system is composed of separate cells, now known as neurons.
Cell membrane (or plasma membrane)- the surface of a cell, which separates the inside
of the cell from the outside environment
Most chemicals cannot cross the cell membrane, but specific protein channels in the
membrane permit a controlled flow of water, oxygen, sodium, potassium, calcium,
chloride, and other important chemicals.
Nucleus- a membrane-enclosed region containing DNA
All animal cells have a nucleus, which contains the chromosomes.
Mitochondrion (pl.: mitochondria)- the structure that performs metabolic activities,
providing the energy that the cell requires for all other activities
Mitochondria require fuel and oxygen to function.
Ribosomes- the sites at which the cell synthesizes new protein molecules
Proteins provide building materials for the cell and facilitate various chemical reactions.
Some ribosomes float freely within the cell, while others are attached to the endoplasmic
reticulum, a network of thin tubes that transport newly synthesized proteins to other
Larger neurons have dendrites, a soma (cell body), an axon, and presynaptic terminals.
The smallest neurons lack axons, and some lack well-defined dendrites.
Motor neuron- a neuron that has its soma in the spinal cord; it receives excitation from
other neurons through its dendrites and conducts impulses along its axon to a muscle
Sensory neuron- specialized at one end to be highly sensitive to a particular type of
stimulation, such as light, sound, or touch
Dendrites- branching fibers that get narrower near their ends; the dendrite’s surface is
lined with specialized synaptic receptors, at which the dendrite receives information from
other neurons
The greater the surface area of a dendrite, the more information it can receive.

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Dendritic spines- short outgrowths that increase the surface area available for synapses
The soma (cell body) contains the nucleus, ribosomes, and mitochondria. Most of the
metabolic work of the neuron occurs here.
Axon- a thin fiber of constant diameter, in most cases longer than the dendrites; acts as
the neuron’s information sender, conveying an impulse toward other neurons or an organ
or muscle
Many vertebrate axons are covered with an insulating material called a myelin sheath
with interruptions known as nodes of Ranvier. Invertebrate axons do not have myelin
An axon has many branches, each of which swells at its tip, forming a presynaptic
terminal. From here, the axon releases chemicals that cross through the junction between
one neuron and the next.
Afferent axon- brings information into a structure
Efferent axon- carries information away from a structure
*Efferent starts with e as in exit; afferent starts with a as in admit.
Every sensory neuron is an afferent to the rest of the nervous system, and every motor
neuron is an efferent from the nervous system.
If a cell’s dendrites and axon are entirely contained within a single structure, the cell is an
interneuron or intrinsic neuron of that structure. (Ex. an intrinsic neuron of the
thalamus has its axon and all its dendrites within the thalamus).
Glia (or neuroglia)- cells that surround neurons and provide support for and insulation
between them; the connective tissue of the nervous system
Glia are smaller but more numerous than neurons.
The brain has several types of glia with different functions:
1. Astrocytes- star-shaped cells that wrap around the presynaptic terminals of a
group of functionally related axons; help to synchronize the activity of the axons,
enabling them to send messages in waves. They also remove waste material
excreted when neurons die and control the amount of blood flow to each brain
2. Microglia- very small cells that remove waste material as well as viruses, fungi,
and other microorganisms
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