Signal transduction is the process by which signals from outside the cell are transmitted inside the cell. There are four classes of signal transducing receptors that initiate this process: enzyme-linked receptors, 7-transmembrane receptors coupled to G proteins, nuclear receptors, and ligand-gated ion channels. Signal transduction pathways can involve either direct enzymatic activity of the receptor or activation of intracellular messenger molecules. Chemical signals between cells are transmitted by diffusion, current flow, or contact and include hormones, pheromones, and allomones. Odor signals are detected through olfactory receptors in the nose or other sensory organs.

1. Signal Transduction
Signal transduction - is the transmission of molecular signals from a cell's exterior to its
interior. Signals received by cells must be transmitted effectively into the cell to ensure
an appropriate response.
Signal Transduction Pathways - Utilizes receptors that initiate biochemical changes
can do so either directly via intrinsic enzymatic activities within the receptor or by
activating intracellular messenger molecules.
Four General Classes of Signal Transducing Receptors
 Enzyme-linked receptors-Receptors that penetrate the plasma membrane and have
intrinsic enzymatic activity or are enzyme associated
 7-TM receptors-Receptors that are coupled, inside the cell, to G proteins .
 Nuclear receptors-Receptors that are found intracellular and upon ligand binding
directly alter gene transcription .
 Ligand-gated ion channel- are a group of trans membrane ion channel proteins
which open to allow ions such as Na+, K+, Ca2+, or Cl− to pass through the
membrane in response to the binding of a chemical messenger.
Two Types of Signal Transduction
 Electrical Signal- refers to the change in electric current produced by the sum of an
electrical potential difference across a specialized tissue, organ or cell system like
the nervous system.
 Chemical Signals- refers to the movement of molecules from sender to receiver.

2. Chemical Signals
• the movement of molecules from sender to receiver
• Methods of Propagation
o Diffusion
o Current flow
o Contact with receiver
• Olfactory Reception
o Features:
 Directionality
• Generally propagate away, often irregular
 Transmission speed
• Depends on diffusion rates, wind speeds
 Temporal pattern
• Difficult to turn on and off
 Spectrum
• Multi-dimensional
• Contact Reception
o Food detection
o Social signals (vomeronasal organs)
Types of Chemical Signals
• Hormone
o Chemical signals used within individuals
o Produced by endocrine glands
• Pheromone
o Chemical signals used between conspecifics
o Produced by exocrine glands
o Pheromone examples:
 Diffusion rate is inversely related to molecule size
 Small compounds are volatile
• 5-20 carbon compounds
• Carbon (MW-12) + hydrogen is less dense than
oxygen (MW=16) + hydrogen (H20)
 Large compounds can persist
• Proteins and lipids
 No size restriction for waterborne or deposited chemicals
• Allomone
o Chemical signals used between species
Production of odors
• Endocrine glands
o Can influence waste products in urine or feces
• Exocrine secretory glands
o On skin or internal with ducts to surface

3. • Body orifices
o Food digestion (including saliva)
o Reproduction
Odor glands in mammal skin
• Sebacous - flask-shaped, sloughing cells create sebum - carries
pheromones
• Sudoriferous - coiled tubes containing liquid pheromones, faster secretion
Diffusion
• Movement of molecules from areas of high concentration to low
• Rate depends on:
o Steepness of concentration gradient
o Molecule size
o Medium type
• Described by Fick’s first law
Active space is dynamic
• There is a maximum size of active space which is set by the detection
threshold and amount of odorant released
• Independent of diffusion rate
Media affects transmissioin
• Q= number of molecules released
• K= detection threshold
• D= diffusion rate
• Diffusion is slow in water
• Need to be close, sessile, or utilize current
Diffusion from a trail
• Modeled as a series of single emissions from a moving source
• Width depends on Q/K. rmax occurs at 0.37 of length
Diffusion in laminar flow
• Laminar flow: smooth, parallel motion of media
• In theory rmax is independent of flow
Moth active space in wind
• In practice, rmax may decrease if molecule drops sufficiently fast
Diffusion in turbulent flow
• Much more common to have turbulence
• Makes it difficult to follow odor trail
Transmission of deposited odors
• Scent marks are often designed to maximize fadeout time
o Embedded in sebum matrix
o Large molecular weight
o Deposited on porous material to impede loss
• Volatile in presence of water

4. o Licking releases pheromone to receiver – lizards
o But, degrade quickly in humidity
Strategies for chemoreception
• The ideal chemosensory organ
o Responds to range of different chemicals
o Sensitive to low concentrations
• Labeled-line coding
o Individual receptors respond to single chemicals
o Organ has many different cells types
o Higher sensitivity, lower generality
• Across-neuron coding
o Receptors respond broadly but with different profiles
o Stimuli encoded by response across receptor population
o Lower sensitivity, greater generality
Olfactory receptor cell
• Found in olfactory epithelium
• Receptor cells are short-lived (<60 d0
• Axons travel to olfactory bulb where there is an odor-topic map
Olfactory receptor genes
• Largest gene family in vertebrates
o 1296 different genes in mice
o Less than 400 genes in humans
o Why?
• Olfactory receptor proteins have 7 trans- membrane domains (like
opsins)
o Each receptor type binds a specific odor molecule
Gradient detection and orientation
• Simultaneous sampling
o Requires paired olfactory receptors at sides of body.
o Need wide head or nose on appendage (antenna)
• Sequential sampling
o Animals follow concentration gradient, requires tracking back and
forth across trail.
Moth scent tracking
• Animals follow concentration gradient, requires tracking back and forth
across trail. Some have paired olfactory receptors at sides of body.

5. Reference:
http://www.tocris.com/pharmacologicalBrowser.php?ItemId=187888#.VUtNg47t2ko
http://www.ncbi.nlm.nih.gov/books/NBK11150/

6. Reference:
http://www.tocris.com/pharmacologicalBrowser.php?ItemId=187888#.VUtNg47t2ko
http://www.ncbi.nlm.nih.gov/books/NBK11150/