GENERAL PSYCHOLOGY / Chapter 3
Biological Foundations of Behavior
Neurons: The Building Blocks of the Nervous System
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Neurons are individual nerve cells.
There are an estimated 100 to 180 billion neurons in the nervous system.
Each neuron is an individually functioning unit, and yet they all coordinate (or work "in concert") each other.
Nerves are made up of long groups of interconnecting neurons.
There are many different sizes and shapes of neurons.
Most neurons have the basic structural elements of (1) dendrites, which receive messages from other
neurons, (2) a cell body, which serves as the control center for the neuron, and (3) an axon, which transmits
messages to neighboring neurons.
About 50% of the axons in the nervous system have a fatty coating insulator called the myelin sheath, which
increases the speed and accuracy of the nerve impulse being transmitted.
The Synapse and Neurotransmitters
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Neurons do not actually touch each other to send their messages along a neural pathway.
The synapse or synaptic cleft is the tiny gap found between the axon (terminal buttons) of one neuron and
the dendrites of another.
When a neural message is received at the dendrites, it is processed through the cell body, transmitted along
the axon to the terminal buttons found at the end of each axon. In order for the message to be transmitted
across the synapse and continue to the next neuron, a neurotransmitter must be released at the synapse.
A neurotransmitter is a chemical messenger released at the terminal buttons of one neuron in order to carry
a message across the synapse to the dendrites of another neuron.
Some neurotransmitters are excitatory, tending to make neighboring neurons fire, while others are inhibitory,
tending to make neighboring neurons stay in a resting state. At any given instant, if a neuron is receiving
more excitatory messages, it will fire. If it is receiving more inhibitory message, it will remain in a resting
state.
Neurotransmitters and their receptor sites are like keys and locks. That is, when neurotransmitters are
released, they will bind only with their chemically matched receptor sites on the dendrites of neighboring
neurons. Just as a door lock will only accept a specific key to unlock it, neurotransmitters can only bind with
specific receptor sites chemically matched with them.
After the neurotransmitters are released, cross the synapse and bind at their receptor sites, there are two
ways the synapse is cleared for the next transmission:
1. The neurotransmitters may be chemically destroyed by enzymes thus clearing the pathway for the
message.
2. The neurotransmitters may be sucked back into the tip of the axon by tiny pump in a process known
as reuptake.
Many drugs (both prescription and illegal) achieve their effects by either inhibiting the synapse clearing
enzymes or inhibiting the process of reuptake.
Drug Effects
Agonist Drugs – chemicals when put in the body make transmission of a neurotransmitter easier by speeding
up the release of a neurotransmitter or sensitizing the receptor site (making the receptor site more chemically
ready to receive the incoming neurotransmitter).
Antagonist Drugs – chemicals when put in the body make transmission of a neurotransmitter more difficult by
making the receptor site less chemically sensitive to receive the incoming neurotransmitter.
Suspected Neurotransmitter Actions
Norepinephrine - Found in the brain, generally excitatory. Also found in the body, either excitation or
inhibition depending on the organ. Associated heavily with heightened consciousness and the experience of
pleasure. Abnormally low levels are associated with depression.
Dopamine - Found in brain, generally inhibitory. Associated with regulation of muscle movement and
regulation of perceptual processes. Abnormally high dopamine transmission and sensitivity are associated with
schizophrenia (extreme perceptual disturbance resulting in loss of contact with reality). Abnormally low levels in
certain parts of the brain are associated with Parkinson's Disease (uncontrollable muscle shaking or "tremor").
Serotonin - Found in the brain and spinal cord, generally inhibitory. Associated with levels of consciousness,
sleep cycle, regulation of mood. Unregulated levels are associated with depression, other mood disturbances
and sleep problems.
Acetylcholine - In the brain, action is excitatory. In the body, action is either excitation or inhibition depending
on the organ. In the brain, acetylcholine is associated with thinking and memory function. Abnormally low levels
and transmission are associated with Alzheimer's Disease, a type of dementia resulting in progressive
destruction of the memory and ultimately all brain function.
GABA - Found in the brain and spinal cord, inhibitory in nature. Serves to facilitate smooth, controlled muscle
movements and/or control uncontrolled muscle movements. Works against excitatory neurotransmitters to allow
precise, fine-tuned muscle movements. Abnormally low levels are associated with Huntington's Chorea
(uncontrollable, violent, thrashing muscle movements). Anti-anxiety drugs act as an agonist for GABA forcing an
overall inhibition and slowing of the nervous system resulting in reduced anxiety.
The Nervous System
The entire nervous system is divided into two major systems, including the Central Nervous System (CNS) and
the Peripheral Nervous System (PNS) each with its own subdivisions as noted below.
Central Nervous System (CNS) is composed of the:
Brain - Includes hindbrain, midbrain & forebrain.
Spinal Cord - Includes ascending pathways, interneurons & descending pathways.
Peripheral Nervous System (PNS) is composed of the:
Somatic Division – Made up of nerves that connect he skeletal muscles and to sensory receptors.
Includes sensory or "afferent" nerves & motor or "efferent" nerves. Provides contact with our
environment and allows for reaction to stimuli.
Autonomic Division – System that connects the brain to the heart, glands and other vital organs.
Largely acts ‘automatically’ with little influence from conscious thought. Includes the sympathetic
division that mobilizes in the face of stress or danger and the parasympathetic division that acts to
conserve body energy and return the body to a resting state after an activation of the sympathetic
division.
The Brain
Hindbrain
The structures in the hindbrain are responsible for the ‘housekeeping’ functions of the body. That is, the
hindbrain is responsible for regulating such functions as heart rate, breathing, balance and reflexes.
Medulla: Responsible for controlling mostly unconscious processes such as respiration (heart rate &
breathing), certain reflexes such as sneezing, coughing and salivating.
Pons: Plays a significant role in the maintenance of attention and arousal, and also in the sleep cycle.
Cerebellum: Coordinates fine & complex muscle movements, and also plays a role in balance and our
sense of equilibrium.
Midbrain
The midbrain is a relatively small area in the brain that serves as a reflex center for orienting the eyes and ears.
Our ability to orient ourselves to visual and auditory stimuli in our environments is dependent on the functioning
of the midbrain.
Forebrain
The forebrain is the largest division of the brain and is involved in such functions as cognition, intelligence,
creativity, memory, motivation, and emotion.
Thalamus: Relays sensory information received from the sense organs to the appropriate parts of the
brain needed for processing. The thalamus acts much like a telephone switchboard does in routing
incoming calls to the proper destination.
Hypothalamus: An extremely important part of the limbic system that plays a vital role in motivation
(especially survival-oriented motives such as food, water, sex and fighting off or fleeing from harmful
stimuli), and emotion (especially emotional experiences such as pleasure and rage). The hypothalamus
also helps regulate body temperature, sleep cycles, and plays a role in the maintaining of heart rate and
blood pressure.
Cerebral Cortex (or the Cerebrum): The cerebral cortex is the largest structure in the forebrain and
includes the two large lobes or left and right ‘hemispheres’ on the top of the brain. The cerebral cortex is
responsible for thinking, memory processes, learning, reasoning, intelligence, creativity, sensory
processing and awareness, and in general, our conscious experience. The two hemispheres (left and
right) are connected and communicate with each other by a dense network of nerve tissue called the
corpus callosum. While the two hemispheres coordinate their cognitive processes quite well with each
other, they do appear to have subtly different functions. The following represents a broad generalization
of the research on left-right brain differences:
Left Brain Oriented Functions
Right Brain Oriented Functions
Logic & Critical Reasoning
Language
Mathematics
Writing
Conception of Time
Right-handed Touch and Control
Artistic & Aesthetic Experiences
Visual Spatial Relations
Music
Fantasy
Intuition
Left-handed Touch and Control