1) Life as a single celled organism
a) minimal abilities -- the single cell organism can find food and ingest it, can move away from
irritating environmental factors, maybe even learn and habituate to stimuli.
b) However, there are some problems -- as a single celled organism, when improvement or
focus is given to any one ability, there is an associated decrement in others. With many functions,
too much emphasis on one function causes others to suffer.
2) The colony
a) a solution -- one day you (a single cell organism) are crawling around and run into another
(an amoeba). You make a deal. You like to crawl around, it likes to ingest. So, the two of you
team up, form cells or societies and make use of each other's skills. You compensate for its
shortcomings and it compensates for yours. Together, you are far more efficient, productive, and
thus, more likely to survive and reproduce.
b) specialization -- soon, specialization begins occurring (some movement, some sensitivity to
environmental stimuli, others to irritation from environment, other secretion) - this means a
reduction in flexibility of individual cells. Each cell becomes dependent on other cells for certain
functions - while there is an increase in the ability to deal with the environment when together,
there is a decrease in the ability to deal with the environment when cut off from the other cells.
All of this leads to advancements in cell organization and development. Now, multi-celled
organisms begin to evolve and adapt to their environments. Now we can take a closer look
at the individual cells (neurons) and their components. Let's examine the Neuron and its
I. The Neuron
The majority of neurons are located in the brain - approx. 100 billion in the brain, although this is
Each neuron receives information, on average, from tens of thousands of other neurons, making
it the most complex communications system in creation.
A. Types of Neurons - although most communicate within the central nervous system (CNS -
brain & spinal cord), some do get signals from outside the central nervous system. There are
three major types of neurons upon which information travels. In addition, the information travels
from the Sensory Neurons to the Interneurons, and then finally to the Motor Neurons.
1. Sensory Neurons
bring information from sensory receptors to the central nervous system. Brings information from
the eyes, ears, etc., as well as from within the body like the stomach.
neurons in the brain and spinal cord that serve as an intermediary between sensory and motor
neurons. They carry info around the brain for processing.
3. Motor Neurons
carry the information from the CNS to the appropriate muscles to carry out behaviors.
For example, if you hold your hand over a hot flame, the information about "heat" travels from
your hand on the sensory neurons, to the internuerons where it is brought to the appropriate brain
region to process the information (now you know it is "hot") and make a decision about a corresponding action (too hot, let's move the hand). The information then travels on the Motor
Neurons from the brain to the hand so that your muscles move the hand from the hot flame. See
how easy that is?
B. Structure of the Neuron (image of the neuron)
1. Soma - the cell body which contains the nucleus, cytoplasm, etc. Everything needed for
a. dendrites - specialized branch-like structures used to receive information from other neurons.
The more dendrites a cell has the more neurons it can communicate with.
2. Axon - thin, tail-like fiber that extends from the soma to the terminal buttons. This can range
from as small as a red blood cell to 3 ft long.
a. axon hillock - area where the axon connects to the soma.
b. myelin - a fatty substance that covers the axon that serves 2 purposes:
the myelin forms a a sheath (covering) called the myelin sheath that helps the signal travel faster
along the neuron (see Nodes of Ranvier below), and it also protects the axon from damage and
signals from other neurons.
The myelin sheath is not indestructible, but can deteriorate - For example, multiple sclerosis -
signals are impeded and don't get to and from the brain properly.
c. Nodes of Ranvier - myelin sheath is not an even cover, but there are areas that are covered
and others that aren't. The areas w/o myelin are the nodes of Ranvier. The way this helps speed
up transmission is that the electrical current/signal jumps from Node of Ranvier to Node of
Ranvier instead of traveling down the entire axon.
d. axon terminal - area at the end of the neuron where it meets another neuron.
BUT ONE NEURONALONE IS MEANINGLESS - THEY MUST TALK! They
communicate using an electrical signal called the Neural Impulse (sometimes it is combined
with chemical signals...you'll see).
II. The Neural Impulse
A. Neural impulse - takes the same path all the time - it is a process of conducting information
from a stimulus by the dendrite of one neuron and carrying it through the axon and on to the next
neuron. Let's take a look at what's involved in the neural impulse:
1) ions - we have positively (+) and negatively (-) charged particles called ions. For the neural
impulse, however, we are only concerned with Sodium (Na+) and Potassium (K+).
2) selectively permeable membrane - the outer membrane of the neuron is not impermeable,
but instead selectively allows some ions to pass back and forth. The way it selects is easy - it has
pores that are only so big. So, only very small ions can fit through.Any large ions simply can't
pass through the small pores.
3) charge of the neuron - inside the neuron, the ions are mostly negatively charged. Outside the
neuron, the ions are mostly positively charged. In this state (with mostly negative charge inside
and positive charge on the outside) the neuron is said to be Polarized.
4) resting potential - while the neuron is Polarized, it is in a stable, negatively charged, inactive
state The charge is approx. -70 millivolts, and it means that the neuron is ready to fire (receive
and send information).
5) stimulus - eventually, some stimulation occurs (ex. hand to close to a flame), and the
information is brought into the body by a sensory receptor and brought to the dendrites of a
neuron. 6) action potential - once the stimulation (the heat) reaches a certain threshold (come to later)
the neural membrane opens at one area and allows the positively charged ions to rush in and the
negative ions to rush out. The charge inside the neuron then rises to approx. +40 mv. This only
occurs for a brief moment, but it is enough to create a domino effect.
7) repolarization - the neuron tries to quickly restore it's charge by pumping out the positively
charged ions and bringing back the negative ones. Can occur fast enough to allow up to 1,000
action potentials per second.
8) absolute refractory period - after the action potential occurs, there is a brief period during
which the neuron is unable to have another action potential. Then the charge inside the neuron
drops to about -90 mv (refractory period) before restoring itself to normal.
9) speed of an action potential - can travel from 10120 meters/sec, or 2-270 miles/hour.
10) all-or-none law - a neural i