transmitters are released into the synapsewhere they attach to specific receptor sites on
membranes of dendrites of your postsynaptic neurons, like a key fitting into the tumbler of
a lock (the lock and key concept). Some of your synapses are excitatory, the neurotrans-
mitters cause the neuron on the other side of the synapse to generate an action potential (to
fire); other synapses are inhibitory, reducing or preventing neural impulses. The sum of all
excitatory and inhibitory inputs determines whether your next neuron will fire and at what
rate. The constant flow of these neurochemical impulses gives your behavior its amazing
complexity. It regulates your metabolism, temperature, and respiration. It also enables you
to learn, remember, and decide.Reflex Action
The simplest form of your behavior, called a reflex, involves impulse conduction over a few
(perhaps three) neurons. The path is called a reflex arc. Sensory or afferent neuronstrans-
mit impulses from your sensory receptorsto the spinal cord or brain. Interneurons, located
entirely within your brain and spinal cord, intervene between sensory and motor neurons.
Motor or efferent neuronstransmit impulses from your sensory or interneurons to muscle
cells that contract or gland cells that secrete. Muscle and gland cells are called effectors.
Examples of your reflexes include your pupillary reflex, knee jerk, sneezing, and blinking.
Neural impulses travel one way along the neuron from dendrites to axons to terminal but-
tons, and among neurons from the receptor to the effector.The Endocrine System
Your endocrine system interacts with your nervous system to regulate your behavior and
body functions. Your endocrine systemconsists of glands that secrete chemical messengers
calledhormonesinto your blood. The hormones travel to target organs where they bind to
specific receptors. Endocrine glands include the pineal gland, hypothalamus, and pituitary
gland in your brain; the thyroid and parathyroids in your neck; the adrenal glands atop
your kidneys; pancreas near your stomach; and either testes or ovaries.Endocrine Gland Function
PINEAL GLAND Produces melatonin that helps regulate circadian rhythmsand
is associated with seasonal affective disorder.
HYPOTHALAMUS Produces hormones that stimulate or inhibit secretion of
hormones by the pituitary.
PITUITARY GLAND Sometimes called “master gland” because it produces
stimulating hormones that promote secretion by other
glands including:
TSH—thyroid-stimulating hormone
ACTH—adrenocorticotropic hormone stimulates adrenal
cortex
FSH—stimulates egg or sperm production
Produces ADH (antidiuretic hormone) to help retain water in
your body and HGH (human growth hormone).
THYROID GLAND Produces thyroxine, which stimulates and maintains metabolic
activities.
Lack of thyroxine in children can result in mental retardation.
PARATHYROIDS Produce parathyroid hormone that helps maintain the calcium
ion level in blood which is necessary for normal functioning
of neurons.Biological Bases of Behavior 73