Cell to cell communication

Cells need to communicate with each other to enable proper physiological response.  The movements of the body, for example, are controlled by the brain and the muscles. For this to happen, cells of the brain and the muscle tissues must interact.

Communication between cells are usually through chemical signals. Chemical signals, called ligand, are released from a "sending cell" into the extracellular space. The target cells must have the correct receptor for that signal. Once the signal molecule binds to the correct receptor, the target cell changes in conformation to allow the signal molecule to enter the cell. Once the ligand is inside the target cell, chemical reactions take place affecting the activity inside the cell.

I-Types of cell signaling

There are four types of cell-cell signaling mainly differing in the distance the signal travels through. These are:

1. Autocrine signaling
2. Signaling by direct contact
3. Paracine signaling
4. Endocrine signaling

1. Autocrine signaling

In this type of signalling, the "sending cell" is also the target cell. The cell releases a ligand that will bind to receptors on own it's surface. A common example of this are cancer cells. Autocrine signaling in cancer cells is thought to play a key role metastasis.

2. Signaling by direct contact

In this type of cell signaling,  there is a tiny gap between the sending cell and the target cell. This tiny gap allows signal molecules to easily diffuse between the two cells. Small molecules such as calcium ions can easily move between cells, but the larger molecules like proteins will need to move through special channels.

3. Paracrine signaling

In this is a type of signaling, there is a relatively short distance between the sending cell and the target cell. The best example of cell signaling is synaptic signaling. This occurs between two communicating nerve cells. When the sending neuron sends a signal, an electrical impulse travels down the axon. The the impulse reaches the synapse (the junction between two nerve cells), it triggers the release of the ligands called neurotransmitters.  These neurotransmitters binds to the traget neuron receptors causing changes in the cell. This results to the opening of ion channels and change in the electrical potential across the membrane.

4. Endocrine signaling

In this type of signaling, the sending cell, which are from the endocrine glands, is a long distance away from the target cell. To be able to successfully communicate, the sending cell will make use of the circulatory system. The signals used in endocrine signaling is called hormones. 

In humans, endocrine glands such as the thyroid, hypothalamus, pituitary, gonads, and pancreas release one or more types of hormones which aids in development and other important physiological functions.

II - Signal responses

Signal pathways and ligands come in many varieties. However, they have on common objective: to trigger cellular response. Cellular responses can be at the macroscopic level or at the molecular level. A macroscopic response can be changes n the appearance of the cell, i.e. cell growth or cell death. A molecular response can be increased transcription of particular genes or activity of particular enzymes.

Cellular response at the molecular level

1. Cellular metabolism

One example of a macroscopic response is cellular metabolism. Adrenaline signaling in muscle cells is achieved through cellular metabolism. Adrenaline is also known as the hormone epinephrine. This hormone is produced by the adrenal gland. They are produced when the body needs to be prepared for for short-term emergencies.

When epinephrine binds to its receptor proteins on the muscle cells, it signals a transduction cascade involving the production of cyclic AMP (cAMP). This cascade leads to the phosphorylation of two metabolic enzymes:  glycogen synthase (GS) and glycogen phosphorylase (GP).

2. Gene expression

The enzyme glycogen synthase is involved in building up glycogen. Phosphorylation of this enzyme inhibits glycogen buildup. In the phosphorylation of glycogen phosphorylase, glycogen is broken down into glucose. The breakdown of glycogen to glucose happens when energy is needed.  Thus, when epinephrine is released, glycogen buildup stops and glucose production is promoted. This enables the muscle to have a large pool of glucose molecules ready to be used in response to a sudden surge of adrenaline. This is known as the "fight or flight" response.

In gene expression, certain signaling pathways can lead to either decrease or increase of protein production in a cell. The growth factor signaling pathway activates, for example, the protein c-Myc and the translation regulator MNK1. 

c-Myc increases transcription of genes that promotes cell growth and division. This protein can lead to production of cancer cells when it is too active. Active MNK1 increases the rate of mRNA translation. Many key genes regulating cell division and survival  have mRNAs that for hairpin structures. With the presence of active MNK1, there genes are expressed at high levels driving cell growth and division.

References:

[1]https://www.khanacademy.org/science/biology/cell-signaling/mechanisms-of-cell-signaling/a/introduction-to-cell-signaling