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Biological processes are the basis of our thoughts, feelings, and actions. All of our behaviors are kept in tune with our surroundings and coordinated with one another through the work of two interacting systems: the nervous system and the endocrine system.
Neurons: The Messengers
What types of cells are found in the nervous system?
The basic building block of the nervous system is the neuron, or nerve cell. Neurons have several characteristics that distinguish them from other cells. Neurons receive messages from other neurons through short fibers, called dendrites. A longer fiber, called an axon, carries outgoing messages from the cell. A group of axons bundled together forms a nerve or tract. Some axons are covered with a myelin sheath, made up of glial cells; this increases neuron efficiency and provides insulation.
A typical myelinated neuron
The Neural Impulse
What "language" do neurons speak?
When a neuron is at rest, a state called the resting potential, there is a slightly higher concentration of negatively charged ions inside its membrane than there is outside. The membrane is said to be polarized , that is, the electrical charge inside it is negative relative to its outside. When an incoming message is strong enough, this electrical imbalance abruptly changes (the membrane is depolarized), and an action potential (neural impulse) is generated. Incoming messages cause graded potentials, which, when combined, may exceed the minimum threshold of excitation and make the neuron "fire." After firing, the neuron briefly goes through the absolute refractory period, when it will not fire again, and then through the relative refractory period, when firing will occur only if the incoming message is much stronger than usual. According to the all-or-none law, every firing of a particular neuron produces an impulse of equal strength. More rapid firing of neurons is what communicates the strength of a message.
What happens as information moves from one neuron to the next?
Neurotransmitter molecules, released by synaptic vesicles, cross the tiny synaptic space (or cleft) between an axon terminal (or terminal button) of a sending neuron and a dendrite of a receiving neuron. Here they latch on to receptor sites, much as keys fit into locks, and pass on their excitatory or inhibitory messages. Psychologists need to understand how synapses function because neurotransmitters affect an enormous range of physical and emotional responses. Certain drugs produce psychological effects by increasing or decreasing the amount of neurotransmitters at synapses. Other drugs work on receptor sites, blocking the receptors or interfering with the removal or re-absorption of neurotransmitters.
Neural Plasticity and Neurogenesis
How does experience change the brain?
Can the brain and the nervous system repair themselves?
Research demonstrates that experiences in our environments can produce changes in the brain, a principle called neural plasticity. Human brains also are capable of neurogenesis, the production of new brain cells. The study of neurogenesis may help treat neurological disorders, but also raises ethical questions.