1. Cell Communication
Cells need to communicate with one another, whether they are located close to each other or far apart.
Extracellular signaling molecules regulate interactions between cells.
The basic mechanism requires a ligand (signaling molecule) to interact with its receptor so as to convert the extracellular signal to an intracellular signal. This process is called signal transduction and can occur in several forms.

2. Types of Cell Signaling
If a signal is needed to communicate with the entire organism; the signaling molecule is secreted into the bloodstream.
Endocrine signaling requires a cell to synthesize and secrete a hormone into the circulatory system.
That hormone is then recognized by a specific target cell protein (receptor) either on the plasma membrane or within the cytoplasm.

3. Types of Cell Signaling
In other situations the signal is required to act locally.
Paracrine signaling molecules (local mediators) can be released by a neighboring cell, diffuse locally through the ECM, and stimulate a close target cell.
Growth and differentiation factors are thought to act primarily as paracrine signaling molecules.

4. Types of Cell Signaling
A third form of communication is neuronal signaling.
This type of signal transduction can occur over long distances, however the delivery is by way of long cellular processes called axons.
Neuronal signals can act on target cells or on other neuronal cells.
The signal travels through the axon as an electrical impulse that upon reaching the axon terminus it is converted into a chemical signal called a neurotransmitter.

5. Types of Cell Signaling
The fourth style of signal transduction is the most short-range of all. It typically does not require the release of a secreted molecule.
Contact-dependent signaling requires the transduction to be completed when the signaling molecule anchored in the PM of the signaling cell to bind to the receptor molecule embedded in the PM of the target cell.
Contact-dependent signaling can also occur in the form of a cell interacting with the ECM.

6. Cellular Responses to Signal Molecules
Each cell is able to respond to a limited set of signals due to its specialized function as well as its limited types of receptors.
Additional, each cell responds to a signal molecule differently.
In this case, the neurotransmitter, acetylcholine can stimulate one type of muscle cell to contract (skeletal) or inhibit contraction in another (cardiac). Acetylcholine can also stimulate certain cells to secrete the contents of their secretory vesicles.
Similar receptors can activate different intracellular signaling pathways, or the ligand binds to different receptors.

7. Cell Responses to Extracellular Signals
Often, cells require multiple signals to elicit the proper response.
The possible cellular responses include:
Survival and/or normal cellular functioning.
Proliferation
Differentiation
Cell death
The different combinations of signals allows a relatively small number of signaling molecules to be used to control the complex behavior of a cell.

8. Intracellular Signaling Cascades
Signal transduction begins when the ligand or signaling molecule binds to its receptor.
This binding event elicits an intracellular signaling mechanism that passes the signal from one intracellular signaling molecule to another until the final outcome or response of the cell is achieved.
This signaling cascade allows for the amplification, modulation, and distribution of the signal inside the cell.

9. Intracellular Signaling Cascades
Features of signal cascades include:
1. They can physically transfer the signal from where its received, either on the plasma membrane or in the cytoplasm, to the cell machinery where the response occurs.
2. They can transform the signal into the molecular form that is able to stimulate that response.
3. They can amplify the signal received, making it stronger, so that fewer signaling molecules are needed to produce the response.
4. They can distribute the signal to several cellular locations in order to influence several processes simultaneously.
5. Each step in the signaling cascade is open to modification or alteration by other factors whether they are outside or inside the cell.

10. Types of Signaling Molecules
Most signaling molecules are hydrophilic and therefore unable to cross the plasma membrane. They instead bind to cell-surface receptors which generate intracellular signals inside the target cell.
In contrast, some small hydrophobic signaling molecules can and do diffuse across the PM and bind to receptors located within the cytosol or nucleus of the target cell.

11. Hydrophobic Hormones

12. Steroid Hormone Signaling Pathway
Steroid hormones, being hydrophobic molecules are able to diffuse across the PM where they can encounter their receptor located within the cytoplasm.
The ligand-hormone complex can then move into the nucleus where it acts as a gene regulatory complex by binding to a specific regulatory sequence in the DNA and activating gene transcription.
Other steroid hormone receptors are already located in the nucleus.

13. Cell-Surface Receptors
Most cell-surface receptors belong to one of three large families.
Ion-channel-linked receptors - when a ligand binds to this receptor it opens or closes an ion channel.
G-protein-linked receptors - when its ligand binds, this receptor it activates a G-protein that initiates a cascade of effects.
Enzyme-linked receptors - when activated by ligand binding, this receptor switches on an enzyme on its cytoplasmic side and generates a downstream cascade of signals.