“Introduction to cell signaling (Article).

” Khan Academy,

https://khanacademy.org/science/biology/cell-signaling/mechanisms-of-cell-signaling/a/

introduction-to-cell-signaling. Accessed 24 Sept. 2017.

細胞不僅是靜止的建築磚塊，實際上它們能夠感知環境並即時作出反應。在我們體內，細胞正通過化學信號分子不斷發送和接收數百萬條信息 。

這篇文章介紹了細胞之間的基本通信原理，首先探討了細胞間信號傳遞的方式，接著分析不同的短距離和長距離信號傳遞。

細胞間的通信通常依賴於化學信號，這些信號由發送細胞產生並釋放到細胞外空間中，然後擴散到相鄰的細胞中。然而，只有具備特定受體的

細胞才能接收到這些信號。當信號分子（配體）與受體結合時，會引發細胞內的變化，最終導致基因活動變化或細胞分裂等反應。

細胞通信的形式主要分為四類：旁分泌、自分泌、內分泌和直接接觸。旁分泌信號發生在相鄰細胞之間，例如突觸信號傳遞，神經元通過釋放

神經遞質來傳遞信息。自分泌信號作用於自身細胞，它對細胞發育和癌症擴散有重要影響。內分泌信號則是通過血液系統長距離傳遞，像激素

這樣的信號分子能影響全身不同部位的細胞。直接接觸信號則通過細胞間的微小通道或細胞表面的互補蛋白進行，這在免疫系統中尤為重要。

了解這些不同類型的信號傳遞，能幫助我們深入理解細胞如何協同工作來維持生命的正常運作。

Introduction

Think your cells are just simple building blocks, unconscious and static as bricks in a wall? If

so, think again! Cells can detect what's going on around them, and they can respond in real

time to cues from their neighbors and environment. At this very moment, your cells are

sending and receiving millions of messages in the form of chemical signaling molecules!
In this article, we'll examine the basic principles of how cells communicate with one another.

We'll first look at how cell-cell signaling works, then consider different kinds of short- and

long-range signaling that happen in our bodies.

Overview of cell signaling

Cells typically communicate using chemical signals. These chemical signals, which are

proteins or other molecules produced by a sending cell, are often secreted from the cell and

released into the extracellular space. There, they can float – like messages in a bottle – over to

neighboring cells.

Not all cells can “hear” a particular chemical message. In order to detect a signal (that is, to

be a target cell), a neighbor cell must have the right receptor for that signal. When a signaling

molecule binds to its receptor, it alters the shape or activity of the receptor, triggering a

change inside of the cell. Signaling molecules are often called ligands, a general term for

molecules that bind specifically to other molecules (such as receptors).

The message carried by a ligand is often relayed through a chain of chemical messengers

inside the cell. Ultimately, it leads to a change in the cell, such as alteration in the activity of a

gene or even the induction of a whole process, such as cell division. Thus, the

original intercellular(between-cells) signal is converted into an intracellular (within-cell)

signal that triggers a response.

relay, and cellular responses.

Forms of signaling

Cell-cell signaling involves the transmission of a signal from a sending cell to a receiving

cell. However, not all sending and receiving cells are next-door neighbors, nor do all cell

pairs exchange signals in the same way.

There are four basic categories of chemical signaling found in multicellular organisms:

paracrine signaling, autocrine signaling, endocrine signaling, and signaling by direct contact.

The main difference between the different categories of signaling is the distance that the

signal travels through the organism to reach the target cell.

Paracrine signaling

Often, cells that are near one another communicate through the release of chemical

messengers (ligands that can diffuse through the space between the cells). This type of
signaling, in which cells communicate over relatively short distances, is known as paracrine

signaling.

Paracrine signaling allows cells to locally coordinate activities with their neighbors. Although

they're used in many different tissues and contexts, paracrine signals are especially important

during development, when they allow one group of cells to tell a neighboring group of cells

what cellular identity to take on.

Synaptic signaling
One unique example of paracrine signaling is synaptic signaling, in which nerve cells

transmit signals. This process is named for the synapse, the junction between two nerve cells

where signal transmission occurs.

When the sending neuron fires, an electrical impulse moves rapidly through the cell, traveling

down a long, fiber-like extension called an axon. When the impulse reaches the synapse, it

triggers the release of ligands called neurotransmitters, which quickly cross the small gap

between the nerve cells. When the neurotransmitters arrive at the receiving cell, they bind to

receptors and cause a chemical change inside of the cell (often, opening ion channels and

changing the electrical potential across the membrane).

College, Biology (CC BY 3.0).

taken back up by the sending cell. This "resets" the system so they synapse is prepared to

respond quickly to the next signal.

Image modified from "Signaling molecules and cellular receptors: Figure 1," by OpenStax

College, Biology (CC BY 3.0).

Autocrine signaling

In autocrine signaling, a cell signals to itself, releasing a ligand that binds to receptors on its

own surface (or, depending on the type of signal, to receptors inside of the cell). This may

seem like an odd thing for a cell to do, but autocrine signaling plays an important role in

many processes.

For instance, autocrine signaling is important during development, helping cells take on and

reinforce their correct identities. From a medical standpoint, autocrine signaling is important

in cancer and is thought to play a key role in metastasis (the spread of cancer from its original

site to other parts of the body)\[^6\]. In many cases, a signal may have both autocrine and

paracrine effects, binding to the sending cell as well as other similar cells in the area.

Endocrine signaling
When cells need to transmit signals over long distances, they often use the circulatory system

as a distribution network for the messages they send. In long-distance endocrine signaling,

signals are produced by specialized cells and released into the bloodstream, which carries

them to target cells in distant parts of the body. Signals that are produced in one part of the

body and travel through the circulation to reach far-away targets are known as hormones.

In humans, endocrine glands that release hormones include the thyroid, the hypothalamus,

and the pituitary, as well as the gonads (testes and ovaries) and the pancreas. Each endocrine

gland releases one or more types of hormones, many of which are master regulators of

development and physiology.

For example, the pituitary releases growth hormone (GH), which promotes growth,

particularly of the skeleton and cartilage. Like most hormones, GH affects many different

types of cells throughout the body. However, cartilage cells provide one example of how GH

functions: it binds to receptors on the surface of these cells and encourages them to divide\

[^7\].

Image modified from "Signaling molecules and cellular receptors: Figure 2," by OpenStax

College, Biology (CC BY 3.0).

Signaling through cell-cell contact

Gap junctions in animals and plasmodesmata in plants are tiny channels that directly connect

neighboring cells. These water-filled channels allow small signaling molecules,

called intracellular mediators, to diffuse between the two cells. Small molecules and ions are
able to move between cells, but large molecules like proteins and DNA cannot fit through the

channels without special assistance.

The transfer of signaling molecules transmits the current state of one cell to its neighbor. This

allows a group of cells to coordinate their response to a signal that only one of them may

have received. In plants, there are plasmodesmata between almost all cells, making the entire

plant into one giant network.

Image modified from "Signaling molecules and cellular receptors: Figure 1," by OpenStax

College, Biology (CC BY 3.0).

In another form of direct signaling, two cells may bind to one another because they carry

complementary proteins on their surfaces. When the proteins bind to one another, this

interaction changes the shape of one or both proteins, transmitting a signal. This kind of

signaling is especially important in the immune system, where immune cells use cell-surface

markers to recognize “self” cells (the body's own cells) and cells infected by pathogens\

[^{9}\].
_Image modified from "Adaptive immune response: Figure 7," by OpenStax College,

Biology (CC BY 3.0)._