Chemical Signals in Animals Chapter 47 Chemical Signals in

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Chemical Signals in Animals

• Chapter 47

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Chemical Signals in Animals

• Hormone - chemical signal that circulates
  through the blood or other bodily fluids and
  affects distant target cells.
• The endocrine system is responsible for the
  production and secretion of hormones.
• There are six major categories of chemical
  signals in animals
• Six chemical messenger classes do not coincide
  with six structurally distinct classes of
  molecules

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Hormone Structure and Function
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Autocrine Signals

• Autocrine signals act on the same cell that
  secretes them.
• An example is cytokines most cytokines amplify
  the response of a cell to a stimulus
• Like in bone marrow cells for defense
• IL-1 is one of the most important immune
  responses, enhances the activation of T-cells in
  response to antigen

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Paracrine signals

• Paracrine signals diffuse locally and act on
  neighboring cells
• May be due to degradation
• Growth factors that are important in coordination
  of development
• Responses to allergens
• Has been linked to tumor growth and cancer

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Endocrine Signals

• Endocrine signals are produced by cells that may
  be organized into discrete organs called glands
  or may be interspersed among the cells of other
  organs
• May be carried by blood or interstitial fluid
• Act on target cells throughout the body
• Hormones such as testosterone and estrogen

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Neural Signals

• Neural signals are the chemical messengers called
  neurotransmitters
• Cause an action potential to be created in a
  neuron and create a neural signal
• Held in vacuoles within the neuron
• Acetylcholine stimulates muscle neurons
• Norepinephrine stimulates wakefulness

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Neuroendocrine Signals

• Neuroendocrine signals are released from neurons
  but act on distant cells instead of acting at the
  adjacent synapse
• Signals from stomach stimulate pituitary and
  brain cells to tell the brain and pituitary gland
  that it is full or it is empty

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Chemical Characteristicsof Hormones

• The three types of chemical messengers
  polypeptides, amino acid derivatives, and
  steroids
• All are similar in that they are
• Organic compounds
• Synthesized and secreted from the cells
• Act on target cells remote from their point of
  origin

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Chemical Characteristicsof Hormones

• Similarities
• Present in extremely small concentrations yet
  have large effects
• Key difference in these hormone types is that
  steroids are lipid soluble but polypeptides and
  amino acid derivatives are not.
• Steroids cross cell membranes much more readily
  than do other types of hormones.

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The Human Endocrine Systeman Overview

• Organs that secrete hormones into the
  bloodstream are called endocrine glands
• The major human endocrine glands are

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Hypothalamus

• The hypothalamus, a region of the lower brain
• Contains different sets of neurosecretory cells
• Receive nerve signals from throughout the body

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Pituitary Gland

• The pituitary gland, which sits just below the
  hypothalamus and has distinct anterior and
  posterior regions

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Posterior Pituitary Hormones

• The two hormones released from the posterior
  pituitary
• Act directly on nonendocrine tissues
• Oxytocin
• Induces uterine contractions and milk ejection
• Antidiuretic hormone (ADH)
• Enhances water reabsorption in the kidneys

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Anterior Pituitary Glands

• The anterior pituitary
• Produces both tropic and nontropic hormones
• The four strictly tropic hormones are
• Follicle-stimulating hormone (FSH)
• Luteinizing hormone (LH)
• Thyroid-stimulating hormone (TSH)
• Adrenocorticotropic hormone (ACTH)

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Thyroid Gland

• The thyroid gland
• Situated in the Neck
• Consists of two lobes located on the ventral
  surface of the trachea
• Produces two iodine-containing hormones,
  triiodothyronine (T3) and thyroxine (T4)

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Thyroid Hormones

• The thyroid hormones
• Play crucial roles in stimulating metabolism and
  influencing development and maturation
• Also important in maintaining homeostatic
  functions
• Hyperthyroidism, excessive secretion of thyroid
  hormones
• Can cause Graves disease in humans
• Hypothyroidism can cause weight gain

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Thyroid and Parathyroid Hormones

• The four parathyroid glands are embedded in the
  thyroid gland
• Act in opposition to thyroid
• Calcitonin, secreted by the thyroid gland
• Stimulates Ca2 deposition in the bones and
  secretion by the kidneys, thus lowering blood
  Ca2 levels
• PTH, secreted by the parathyroid glands
• Has the opposite effects on the bones and
  kidneys, and therefore raises Ca2 levels
• Also has an indirect effect, stimulating the
  kidneys to activate vitamin D, which promotes
  intestinal uptake of Ca2 from food

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Kidneys and Adrenal Glands

• The two kidneys, which lie in the posterior part
  of the abdominal cavity
• The two adrenal glands, which sit atop the
  kidneys and have an outer cortex and a central
  medulla
• The adrenal medulla secretes epinephrine and
  norepinephrine
• In response to stress-activated impulses from the
  nervous system

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Pancreas

• Located in the anterior part of the abdominal
  cavity
• Two types of cells in the pancreas
• Secrete insulin and glucagon, antagonistic
  hormones that help maintain glucose homeostasis
  and are found in clusters in the islets of
  Langerhans

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Gonadal Sex Hormones

• Suspended below the pelvic cavity, respectively
• Produce most of the bodys sex hormones
  androgens, estrogens, and progestins

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What Do Hormones Do?

• Hormones coordinate the activities of diverse
  groups of target cells
• Change according to environment or signals
• The stimuli to which hormones respond can be
  simple or complex
• Coordinate responses to environmental change
• Direct developmental processes

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Digestive Hormones

• Digestive hormones function in simple
  stimulus-and-response circuits
• When acidic food passes from the stomach to the
  upper part of the small intestine, the food
  triggers intestinal cells to release secretin and
  cholecystokinin into the bloodstream
• Secretin induces the pancreas to secrete a
  solution that neutralizes acid
• Cholecystokinin causes the pancreas to secrete
  digestive enzymes into the small intestine and
  the gallbladder to eject bile salts into the
  intestine to emulsify fats

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Responses to Stress

• When a person is in danger, hormones regulate
  both the short-term and long-term responses
• The short-term reactionthe fight-or-flight
  responseoccurs through the activation of the
  sympathetic nervous system
• Long-term stress involves glucocorticoids
  produced in the adrenal cortex.
• Ensures the continued availability of fuel
  molecules to support important body functions

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Stress and the Adrenal Gland
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How Do Hormones Direct Developmental Processes?

• Growth hormones and sex hormones promote cell
  division, increase overall body size, and promote
  sexual differentiation as an individual matures.
• Certain hormones direct the development of
  particular cells and tissues at critical
  junctures in an individuals life
• The major hormonal effects on development are

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Primary Sex Determination

• Events early in development that dictate whether
  the sex organs become male (testes) or female
  (ovaries)
• Once they develop, they begin producing
  male-specific hormones (testosterone) or
  female-specific hormones (estradiol, a member of
  the estrogen family of hormones).

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Puberty

• At puberty, surges of sex hormones lead to the
  physical and emotional changes associated with
  adolescence
• These developmental changes create the adult
  phenotype and the ability to produce offspring.

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Full Growth and Development

• In humans and other mammals, the attainment of
  full adult stature is mediated by growth factors
  that are regulated by growth hormone produced in
  the pituitary gland.

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Reproduction

• Most long-lived animals reproduce seasonally
• In many species, environmental cues trigger the
  release of sex hormones
• Although humans do not breed seasonally, sex
  hormones regulate sperm production and the
  menstrual cycle

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Hormones and Homeostasis

• Hormones that act as messengers in homeostatic
  systems include antidiuretic hormone (ADH),
  aldosterone, and erythropoietin (EPO).
• Calcitonin and parathyroid hormone work together
  to keep Ca2 levels in the blood close to a set
  point

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How are Hormones Regulated?

• In many cases, hormone production is directly or
  indirectly controlled by the nervous system.
• Adrenocorticotropic hormone (ACTH) is a
  regulatory hormone that controls release of
  glucocorticoids from the adrenal cortex
• These hormones act as regulators and all are
  involved in negative feedback, or feedback
  inhibition

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47.4 How Do HormonesAct on Target Cells?

• Differences in lipid solubility influence where a
  target cell receives the chemical message.
• Steroids often act inside the cell, whereas most
  amino acid derivatives and all polypeptides act
  at the cell surface.
• Steroid hormone-receptor complexes bind to
  specific sites in DNA called hormone-response
  elements

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Hormones That Bind to Cell-Surface Receptors

• Epinephrine and the peptide hormones are not
  lipid soluble and cannot enter the target cell
• They bind to receptors on the cell surface
• Activate the receptor on the cell surface by
  signal transduction
• Epinephrine produces two distinct patterns of
  responses because there are two types of
  receptor, alpha and beta

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Identifying theEpinephrine Receptor

• Epinephrine and its agonists (molecules that
  bind to the same receptors as the hormone itself)
  produce two distinct patterns of responses
  because there are two types of receptor, alpha
  and beta. In turn, there are two types of alpha
  and two types of beta receptor.

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Signal Transduction and the Role of Second
Messengers

• Epinephrine activates phosphorylase, the enzyme
  that catalyzes the formation of glucose from
  glycogen (Figure 47.15).

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Epinephrine Signal Transduction

• Epinephrine triggers a signal transduction
  cascade that includes cyclic adenosine
  monophosphate (cAMP) as a second messenger to
  amplify the signal