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Mechanisms of Hormone Action
Figure 16–4b Effects of Intracellular Hormone Binding.
Endocrine Reflexes
• Endocrine Reflexes
– Functional counterparts of neural reflexes
– In most cases, controlled by negative feedback
mechanisms
• Stimulus triggers production of hormone whose effects
reduce intensity of the stimulus
Endocrine Reflexes
• Endocrine reflexes can be triggered by
– Humoral stimuli
• Changes in composition of extracellular fluid
– Hormonal stimuli
• Arrival or removal of specific hormone
– Neural stimuli
• Arrival of neurotransmitters at neuroglandular junctions
Endocrine Reflexes
• Simple Endocrine Reflex
– Involves only one hormone
– Controls hormone secretion by the heart, pancreas,
parathyroid gland, and digestive tract
• Complex Endocrine Reflex
– Involves
• One or more intermediary steps
• Two or more hormones
• The hypothalamus
Endocrine Reflexes
Figure 16–5 Three Mechanisms of Hypothalamic Control over Endocrine Function.
Endocrine Reflexes
• Neuroendocrine Reflexes
– Pathways include both neural and endocrine
components
• Complex Commands
– Issued by changing
• Amount of hormone secreted
• Pattern of hormone release:
– hypothalamic and pituitary hormones released in sudden
bursts
– frequency changes response of target cells
The Pituitary Gland
• Also called hypophysis
• Lies within sella turcica
– Diaphragma sellae
• A dural sheet that locks pituitary in position
• Isolates it from cranial cavity
• Hangs inferior to hypothalamus
– Connected by infundibulum
Pituitary Gland
• Releases nine important peptide hormones
• Hormones bind to membrane receptors
– Use cAMP as second messenger
Pituitary Gland
Figure 16–6a The Anatomy and Orientation of the Pituitary Gland.
Pituitary Gland
Figure 16–6b The Anatomy and Orientation of the Pituitary Gland.
Pituitary Gland
• Median Eminence
– Swelling near attachment of infundibulum
– Where hypothalamic neurons release regulatory
factors
• Into interstitial fluids
• Through fenestrated capillaries
Pituitary Gland
• Portal Vessels
– Blood vessels link two capillary networks
– Entire complex is portal system
• Ensures that regulatory factors reach intended target
cells before entering general circulation
Pituitary Gland
Figure 16–7 The Hypophyseal Portal System and the Blood Supply to the
Pituitary Gland.
Pituitary Gland
• Two Classes of Hypothalamic Regulatory Hormones
– Releasing hormones (RH)
• Stimulate synthesis and secretion of one or more hormones at
anterior lobe
– Inhibiting hormones (IH)
• Prevent synthesis and secretion of hormones from the anterior lobe
• Rate of secretion is controlled by negative feedback
Pituitary Gland
•
Anterior lobe (also called adenohypophysis)
–
–
Hormones “turn on” endocrine glands or support other
organs
Can be subdivided into three regions:
1.
2.
3.
Pars distalis
Pars intermedia
Pars tuberalis
Pituitary Gland
Figure 16–8a Feedback Control of Endocrine Secretion
Pituitary Gland
Figure 16–8b Feedback Control of Endocrine Secretion.
Pituitary Gland
Figure 16–9 Pituitary Hormones and Their Targets.
Pituitary Gland
Pituitary Gland
• Posterior lobe (also called neurohypophysis)
– Contains unmyelinated axons of hypothalamic neurons
– Supraoptic and paraventricular nuclei manufacture
• Antidiuretic hormone (ADH)
• Oxytocin (OXT)
Pituitary Gland
Figure 16–9 Pituitary Hormones and Their Targets.
Pituitary Gland