AP Biology: Chapter 45
Chemical Signals in Animals
Discuss the interaction of the nervous and endocrine systems in vertebrates
Identify some hormones secreted by the posterior and anterior pituitaries, and describe their
functions in vertebrates
Overview: The Body’s Long-Distance Regulators
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An animal hormone is a chemical signal that is secreted into the extracellular fluid,
circulates in the blood or hemolymph, and communicates regulatory messages within the
body.
○ A hormone may reach all parts of the body, but only specific target cells have the
receptors that enable a response.
○ A given hormone traveling in the bloodstream elicits specific responses from its target
cells—such as a change in metabolism—while other cells are unaffected.
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Chemical signaling by hormones is the function of the endocrine system, one of two basic
systems for communication and regulation throughout the body.
○ Hormones secreted by endocrine cells regulate reproduction, development, energy
metabolism, growth, and behavior.
The other major communication and control system is the nervous system, a network of
specialized cells called neurons that transmit signals along dedicated pathways.
○ These signals in turn regulate other cells, including neurons, muscle cells, and endocrine
cells.
Because signaling by neurons can regulate the release of hormones, the nervous and
endocrine systems often overlap in function.
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Hormones serve a range of functions in the body.
○ Hormones maintain steady-state conditions; mediate responses to environmental stimuli; and
regulate growth, development, and reproduction.
○ Hormones coordinate responses to stress, dehydration, and low blood glucose levels.
○ They also control the appearance of characteristics that distinguish juveniles from adults.
AP Biology Chemical signals in animals
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This diagram summarizes different types of chemical signalling in animals. Note the similarities
and differences between the different systems:
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AP Biology Chemical signals in animals
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Local regulators are chemical signals that act over short distances, reaching their target cells by
diffusion.
○ For example, immune cells communicate with each other by local regulators called cytokines.
○ Local regulators function in many other processes, including blood pressure regulation, nervous
system function, and reproduction.
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Local regulators are divided into paracrine and autocrine signals.
○ Paracrine signals act on cells near the secreting cell.
○ Autocrine signals are secreted regulators that act on the secreting cell itself.
○ Some signals have both paracrine and autocrine activity.
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Chemical signals play a critical role in the transmission of information by neurons.
Neurons communicate with target cells in other neurons and muscles at specialized junctions known as
synapses.
At many synapses, neurons secrete chemical signals called neurotransmitters, which diffuse a very short
distance to bind receptors on the target cell.
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Neurosecretory cells, specialized neurons in the brain, secrete chemical signals that diffuse from nerve cell
endings into the bloodstream.
These signals are a class of hormones called neurohormones.
○ One example is ADH (antidiuretic hormone, or vasopressin), a hormone critical to kidney function
and water balance.
Members of the same animal species may communicate with pheromones, chemical signals released into
the external environment.
○ Pheromones serve many functions, including marking trails leading to food sources, warning of
predators, and attracting potential mates.
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What is a hormone? ____________________________________________________________
What is the source gland for a hormone? ____________________________________________
What are the target cells for a hormone? ____________________________________________
What is an endocrine gland? _____________________________________________________
What two major types of molecules function as hormones? _____________________________
View water and lipid soluble animations on hard drive or website
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Polypeptides and many amine hormones are water-soluble.
Because they are insoluble in lipids, these hormones cannot pass through cell membranes.
Steroid hormones, as well as other largely nonpolar hormones such as thyroxine, are lipidsoluble and can pass through cell membranes readily.
Whether or not a hormone is able to cross cell membranes influences the location of receptors
in target cells.
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Here are two different ways that hormones can interact with their target cells. How are these two
systems the same, and how do they differ?
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Activity: Overview of Cell Signaling ch 45 website
Here is how cell surface receptors act in signal reception and signal transduction:
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Here is how steroid hormones affect their target cells directly:
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A particular hormone can affect different target cells in different ways. For example:
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Here are the main human endocrine glands:
Activity: Human Endocrine Glands and Hormones ch 45 website
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A typical hormone exerts its effect as shown below:
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Insulin and glucagon are good examples of antagonistic hormones that work to maintain glucose
homeostasis:
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The ______________________ gland is often called the ‘master gland’ because it secretes so
many different hormones. It works together with the _______hypothalamus__________ of the
brain as the neuroendocrine control centre of the body.
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Here is a diagram to show how the posterior pituitary gland works together with the
hypothalamus to exert neuroendocrine control :
ADH is antidiuretic hormone
ADH and Oxytocin act on nonendocrine
tissue.
And, this diagram shows the specific
example of how oxytocin stimulates the
milk ‘let-down’ response:
__Oxytocin hormone secretion results in
positive feedback. That is the more
hormone that is secreted the more
response and the response also stimulates
more hormone secretion. Oxytocin is
responsible for uterine contraction in
labour and for milk secretion during
lactation. _________
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Here is a diagram to show how the anterior pituitary works together with the hypothalamus to
exert neuroendocrine control:
____Tropic hormones stimulate other endocrine glands to secrete hormones. Eg FSH is follicle
stimulating hormone which stimulates the testes to secrete testosterone and the ovaries to secrete
oestrogen. TSH is Thyroid stimulating hormone and it stimulates the thyroid to secrete thyroxin.
ACTH is Adrenocorticotropic hormone and it stimulates the Adrenal cortex on the kidney to
secrete adrenalin.
Non tropic hormones do not stimulate other endocrine glands but do act on other cells and
tissues. For example: Prolactin stimulates the mammary glands to secrete milk and GH, growth
hormone stimulates growth in all tissues.
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And this diagram shows a ‘hormone cascade’ that ultimately controls the release of thyroxin
from the thyroid gland:
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Here is how the parathyroid glands control blood calcium levels using negative feedback. Notice
that the brain and hypothalamus are not used in this mechanism:
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Stress affects both the adrenal medulla and the adrenal cortex:
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