Hormones & the Endocrine System• A hormone is a chemical signal that is dumped into the

bodies circulatory system– Blood or lymph– Effects a great many receptors

• Target cells are specific to hormone type

– Excreted by the endocrine system through endocrine glands• Some hormones such as epinephrine are secreted by neurosecretory

cells

Control Pathways

• Efferent Pathway– Stimulus causes the release of the efferent signal

(hormone)– The efferent signal acts on the target eliciting a response

• Controlled by a negative feedback loop– Response reduces the stimulus causing the release of the signal to

stop

• Positive feedback loops are rare and not generally used to maintain homeostasis– Suckling and milk production are an example

Water Soluble Hormones• Polypeptides and Amines

– Reception depends on binding to a cell surface protein• Based on conformation• Causes signal transduction

Fat Soluble Hormones

• Steroids– Pass through the plasma membrane

• Most receptors are located in the nucleus– Most are non-polar and are responsible for the transduction response

» Involves the release of transcription factors in most cases

Paracrine Signaling• Creates a local response

– Cytokines • Immune response

– Growth factors• Cell division

– Prostaglandins• Inflammatory response