There are many organs that are a part of the endocrine system. Many are controlled in part by the actions of the pituitary gland and hypothalamus. Many bodily functions are regulated by the endocrine system and its hormones. The pituitary gland is sometimes considered to be the master gland because it has so many hormones that ultimately regulate other endocrine glands. The pituitary gland is itself under the control of the hypothalamus in the brain. The thyroid gland is located in the front of the neck. It is important in appetite, metabolism, and growth. There are diseases of an overactive thyroid that results in increased metabolism as well as diseases of an underactive thyroid gland, which greatly slows metabolism. Medications and surgery can help to balance these thyroid disorders. Some psychological disorders are directly linked to problems with an overactive or underactive thyroid gland. The other major glands in the endocrine system include the adrenal glands, which makes stress hormones, including those responsible for the fight or flight response, the pancreas, which regulates blood sugar levels, the pineal gland, which makes melatonin as part of the Circadian rhythm of the body, and the gonads, which make the sex hormones responsible for sexual maturation and sexual behaviors.
NEUROTRANSMITTERS There are dozens of neurotransmitters in the nervous system. As mentioned, they act across synapses in order to send a signal to another neuron, a muscle cell, or some type of endocrine gland. Neurotransmitters tend to be small molecules, often made from amino acid precursors, after which they are synthesized into their final form. They are stored in presynaptic vesicles close to the axon terminal so they can be released into the synaptic cleft. When they are released into the synapse, they cross the small space and attach to specific receptors, initiating an action potential in the postsynaptic cell. Neurotransmitters can be excitatory and will turn on the target cell s action or inhibitory, which means it will block the target cell s action. If excitation exceeds inhibition, the neuron will be activated and will fire an action potential. 56
